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Microresonators, with highly compact size, strong nonlinearity and the ability to tailor the dispersion, have attracted great attention in the last decade. Microresonator-based frequency combs (microcombs) have accelerated the progress in the next-generation applications, including range sensing, quantum commutation, spectroscopy, etc. The ring structure limits the light circulating in the cavity to the resonant frequencies. The microcombs, resulting from the cascaded frequency conversions, are generated with the dual balances between nonlinearity and dispersion, as well as the gain and loss. This thesis starts with an overview of the frequency comb development and mechanism. We then report our theoretical studies of the microcomb generation and manipulation under different setups and materials.

Quadratic solitons have ridden the wave of recent interest in applications of frequency conversion and supercontinuum generation in particular. In recent years advances in fabrication of smaller Χ(2) waveguides with lower loss has made experimental quadratic soliton research more feasible, stoking significant theoretical interest in them. Here we will start by studying solitons due to quadratic nonlinearity only. We derive soliton existence criteria which allow us to identify five regimes of soliton existence. Using perturbation theory we predict the emission of non-solitonic radiation from these solitons due to their interaction with the linear dispersion of the system and other dispersive waves. We investigate two distinct lithium niobate nanowaveguide structures and analyse their modelled dispersion to assess their potential for quadratic soliton research. We find our two candidate waveguides provide all five existence regimes making them ideal for experimental investigation. Our numerical simulations of pulse propagation in these structures show good agreement with our analytic predictions for soliton existence and their dynamics. We improve the generality of our work by including cubic nonlinear terms in our next model. We show that two-component solitons supported by quadratic and cubic nonlinearity may undergo soliton self-frequency shift due to the Raman effect in lithium niobate. Our analytic predictions of temporal and spectral shifts match up remarkably closely with numerical simulations of soliton propagation. In some cases we see that extreme Raman shifting brings about instabilities in the soliton not reported elsewhere. These instabilities are associated with the two-component soliton shifting towards the boundary of its existence, a process that we show does not happen in Kerr solitons. In our final model we highlight a particular feature of lithium niobate nanowaveguides that we have observed while researching them. The focus of this work is the avoided crossings found between certain mode pairs in these waveguides. We therefore introduce an additional mode to our model such that the second harmonic component of the soliton can exist in the avoided crossing between the two modes centred at this frequency. The key advantage that these avoided crossings offer is the opportunity for modal group velocity matching which is essential for low power soliton existence and efficient frequency conversion in short pulses. This model also automatically includes the dispersion of nonlinearity seen in the modelled system. Continuous wave solutions are found to exist in this system and exhibit modulation instability over the majority of their existence domain. We find soliton solutions in this system and make comparisons between their existence and that of the continuous wave solutions. In general the spectral structure of these solitons is not simple and we find the core and tail frequencies often differ.

Dispersive shock wave (DSW), sometimes referred to as an undular bore in fluid mechanics, is a non-linear dispersive wave phenomenon which arises in non-linear dispersive media for which viscosity effects are negligible or non-existent. It is generated when physical quantities, such as fluid pressure, density, temperature and electromagnetic wave intensity, undergo rapid variations as time evolves. Its structure is a non-stationary modulated wavetrain which links two distinct physical states. DSW's occurrence in nature is quite omnipresent in classical/quantum fluids and non-linear optics. The main purpose of this thesis is to fully analyse all regimes for DSW propagation in the non-linear optical medium of a nematic liquid crystal in the defocusing regime. These DSWs are generated from step initial conditions for the intensity of the optical field and are resonant in that linear diffractive waves (termed dispersive waves in the context of fluid mechanics) are in resonance with the DSW, leading to a resonant wavetrain propagating ahead of it. It is found that there are six hydrodynamic regimes, which are distinct and require different solution methods. In previous studies, a reductive nematic Korteweg-de Vries equation and gas dynamic shock wave theory were used to understand all nematic dispersive hydrodynamics, which do not yield solutions in full agreement with numerical solutions. Indeed, the standard DSW structure disappears and a ``Whitham shock'' emerges for sufficiently large initial jumps. Asymptotic theory, approximate methods or Whitham's modulation theory are used to find solutions for these resonant DSWs in a given regime. It is found that for small initial intensity jumps, the resonant wavetrain is unstable, but that it stabilises above a critical jump height. It is additionally found that the DSW is unstable, except for small jump heights for which there is no resonance and large jump heights for which there is no standard DSW structure. The theoretical solutions are found to be in excellent agreement with numerical solutions of the nematic equations in all hydrodynamic regimes.

The research presented in this thesis is in three parts. First is an investigation into the generation of supercontinuum using tapered optical fibre. Second, is the creation of side-holes in hollow-core fibre, using an oxygen-butane flame, with the aim to be used for gas sensing. Finally, variables affecting the amount of power in a near infrared idler, of a four-wave mixing fibre were explored. Supercontinuum generation is most commonly generated using photonic crystal fibre. The purpose of this research is not to provide alternative to photonic crystal fibre but to investigate some of the processes that affect supercontinuum generation that utilises soliton fission. Using a master oscillator power amplifier to deliver a 1064 nm pump, it was found that supercontinuum could be generated using tapered optical fibre with a waist length under 250 mm, even though the pulses were as long as 10 ps. The tapers were made using the flame brush technique which allows the creation of tapers with micrometer diameters. These tapers are connected at either end by standard/full size optical fibres. This presents an opportunity to investigate taper waists of many designs whilst keeping the input and output wavelength dependence of coupling independent of the taper structure, unlike in photonic crystal fibres. Investigating the limits of soliton trapping led to non-uniform designs of taper waists. The design with the broadest spectrum began with a 5 μm diameter that was connected directly to a 2.5 μm section. This allowed dispersive waves to be generated, with decent power, in the 5 μm section before the change in group velocity induced by the 2.5 μm diameter, coupled with soliton trapping, dragged the dispersive to low wavelengths. This design produced a supercontinuum, that maintained a power level above -30 dBm, over a range of 420 - 1550 nm. Using hollow-core fibres for gas sensing is an active area of interest. Side-holes allow gases to enter the core of the fibre in order to be detected. Ultrafast lasers are the most common tool when creating these holes. Here an alternate method of using an oxygen-butane flame on a pressurised hollow-core fibre was investigated. It was found that reducing the inner diameter of the burner, delivering gases to the flame, can only reduce the size of the flame to ≈300 μm. Side-holes were successfully created on a hollow-core fibre of 125 μm diameter. However the minimum loss from a hole created this way was 3 dB which makes this method unsuitable for detection of gases over large areas. A master oscillator power amplifier laser system was used to deliver a 1064 nm pump to a photonic crystal fibre with an phase matched idler of 1915 nm. The width of the pulses was varied between 37 - 90 ps and had their maximum peak powers kept around 2200 W. The long width of these pulses allowed a narrow bandwidth that produced an idler peak that had a full width at half maximum of ≈30 nm. Three pulse repetition rates of 40, 20 and 10 MHz were used and the maximum average power adjusted in order to keep the same peak power. Over 100 mW of idler power was generated. It was found that the relation of peak power to length of fibre needed to generate power in the idler, is complicated by confinement loss of the idler. This means that to generate the most power the fibre should be short and the peak power of the pulse large.

Scattering media have traditionally been seen as a hindrance to the controlled transport of light through media, creating the familiar speckle pattern. However such matter does not cause the loss of information but instead performs a highly complex deterministic operation on the incoming flux. Through sculpting the properties of the incoming wavefront, we can unlock the hidden characteristics of these media, affording us far more degrees of freedom than that which is available to us in traditional ballistic optics. These additional degrees of freedom have allowed for the creation of compact sophisticated optical devices based only on the deterministic nature of light scattering. Such devices include diffraction-limit-beating lenses, polarimeters, spectrometers, and some which can transmit entire images through a scattering substance. Additional degrees of freedom would allow for the creation of even more powerful devices, in new working regimes. In particular, the application of related techniques where the scattering material is actively modified is limited. This thesis is concerned with the use of optothermal nonlinearity in random media as a way to provide an additional degree of control over light which scatters through it. Specifically, we are concerned with silica aerogel as a platform for this study. Silica aerogel is a lightweight skeletal structure of silica fibrils, which results in a material which is up to 99.98 % by volume. This material exhibits a unique cocktail of properties of use such as near unitary refractive index, an order of magnitude lower thermal conductivity, and high optothermal nonlinearity. The latter two of these properties allow for the creation of localised steep thermal gradients, proportionally affecting the low refractive index significantly. Additionally through differing fabrication steps, the opacity, and as a result, we can adjust the scattering strength. In line with the development of light deterministic light scattering techniques in linear media, we develop through the use of pump-probe setups, a framework for the development of a similar line of techniques in nonlinear scattering media. We show that we can reversibly control the far-field propagation of light in weakly scattering silica aerogel. Following this, we show that nonlinear perturbation can be used to extend and modify the optical memory effect, where slight adjustments in scattering direction maintain the overall correlation of the scattered profile. Finally, we measure the nonlinear transmission matrix, a complete description of how any wavefront would pass through at a particular point in a scattering media, and how that scattering can be modified through the application of an optothermal nonlinearity. Extending the tool of scattering media into the nonlinear regime helps pave the way toward the next set of advances in the field of light scattering control.

Terahertz-over-Fibre (ToF) is a promising photonic technology for the next generation high-data-rate wireless communication. The key to a successful ToF system is an integrated, compact and powerful terahertz (THz) source. Quantum cascade lasers (QCLs) have this potential at a range of THz frequency bands. However, a major challenge for a high-capacity wireless scheme at this high frequency is to achieve a multitude of optical channels and also requiring spectral agility. This thesis investigates the ToF technology by exploiting a computer-generated hologram, here working as an aperiodic photonic lattice, integrated within a QCL, providing electronically switchable lasing modes at multiple, pre-defined THz frequencies. For the first time, a transfer matrix method (TMM) has been employed to study nonlinear optics, focusing on sum- and difference-frequency generation, within such aperiodic lattice (AL) microcavities. As a foundation, before performing the nonlinear calculations, the microcavity linear optical response is studied. The results reveal that these holographically designed microcavities possess a multi-band spectra, containing both defect and band-edge photonic states. Furthermore, under the influence of the gain, it has been observed that the lasing solutions of the defect modes are insensitive to variations of the grating coupling strength; this outcome is particularly useful, as exemplified elsewhere, in relation to the fabrication imperfections during the ion milling of photonic lattices within THz QCLs. Remarkable spectral control over a wide bandwidth is also maintained under the influence of cavity feedback when an AL is integrated within a Fabry-PÂ ́erot (FP) QCL. The introduction of the AL to FP cavities gives rise to: (i) electronic switching and (ii) mode-dependant electronic fine-tuning, as verified by numerical and previous experimental demonstrations. Specifically, TMM has been used to model the cavity pulling effect at multiple lasing modes by employing the Kramers-Kronig principle. Proceeding from THz to telecom wavelengths, a multimode of THz emission has been up-converted to the telecom carrier as experimentally demonstrated in FP QCLs. TMM study of this up-conversion process revealed the interference fringes of the background FP cavity, which significantly enhances the signal power. Analysis of the cavity dispersion shows that a single frequency near-infrared (NIR) laser can up-convert THz modes spanning a bandwidth of 220 GHz, only limited by the group-index mismatch between the NIR and THz waves. Finally, a numerical model is developed for the ToF system based upon the computer-generated hologram studied here. The results show an enhancement of the photonic density of mode (DOM) at designed THz frequencies, as expected; but crucially, it also caused an analogous enhancement of signal power in the NIR with a straightforward one-to-one correspondence between the signal spectra and the hologram response function. The photonic lattice dispersion study revealed that each THz mode has its own effective modal index, allowing us to engineer them to phase-match to different optical pump frequencies. As a result, an electronically switchable sum- and difference-frequency generation process, at optical wavelengths, has been demonstrated in an aperiodic lattice laser. The nonlinear TMM study of the THz-NIR up- conversion also reveals that the phase-matching process could be electronically tuned over the entire telecom band, along with the inherent multi-band hologram response at the THz band, within a single-selection laser. Compared to previously published work, where multiple double-metal QCL waveguides were needed, the integrated hologram technique could be a key breakthrough for tunable multi-channel THz wireless communications.

This thesis presents studies on a novel ‘meta-material’ as a potential candidate to replace the traditional Electro-Optic crystals (GaP, ZnTe) used in ultrashort bunch monitors for electron/positron accelerators. This study is aimed at showing the linear and non-linear optical properties of such materials, and creating a toolbox for both optical characterisation and manipulation of their properties using an ultra-short pulsed laser. The material studied throughout this thesis is a composite of silver nanoparticles (a “nanocomposite”) embedded within soda-lime glass. The Surface Plasmon Resonance (SPR) is a feature of these particles that is responsible for its unique optical properties. It is shown in this work how SPR is utilised for shape modification of silver nanoparticles with the use of a ps- pulsed laser with various laser beam polarisations. The impact of linear polarisation irradiation is investigated. It is found that multipulse irradiation has the effect of elongating nanoparticles to form prolate spheroids, which results in a dichroic effect on the composite as a whole, caused by the anisotropic SPR band shift. It is also shown that changing the laser polarisation from linear to radial and/or azimuthal changes the character of the reshaped nanoparticles. It was observed that a localised change of ellipsoid orientation is achieved, resulting in a non-directionally-dependent SPR band shift. Second Harmonic Generation (SHG) has been observed from reshaped nanoparticles embedded in soda-lime glass. A comparison of the effect was made between ps-pulsed reshaped, fs-pulsed reshaped and mechanically stretched samples containing silver nanoparticles. Multiphoton Absorption Induced Luminescence (MAIL) was observed along with the SHG and characterised for the various laser polarisation components. The dependence of the aforementioned effects on the elongated nanoparticle aspect ratio was shown to have a great impact. A novel method for reshaped nanoparticles characterisation is presented. It is based on the laser-induced SHG and MAIL signal and is proved to give a precise measurement of the nanoparticle shape and orientation. Frequency Resolved Optical Gating (FROG) measurement of a fs-pulse is measured with great accuracy, in the case where the BBO nonlinear crystal is replaced by the reshaped nanoparticle composite. This was demonstrated to be caused by the anisotropic SHG of the ellipsoidal nanoparticles. Preliminary THz based measurements were performed as a part of a feasibility study of the application of these composites in the EO-based detection of ultrashort electron bunches. Future work is suggested in order to achieve more efficient EO detection.

Quantum cascade lasers (QCLs) are high power, narrow linewidth mid-IR lasers that are increasingly popular for trace gas sensing and nonlinear spectroscopy. This thesis demonstrates QCL based spectroscopic techniques for multi-species detection, nonlinear optics and trace gas detection, highlighting the versatility of these lasers. Firstly, a Fabry-Perot QCL (FP-QCL) operating at 5.3 μm was used to perform multi-mode absorption spectroscopy (MUMAS) for sensing of NO and H2O. The laser provides a spectral coverage of ca. 50 cm-1 with a time resolution down to the ms timescale. It also exhibits a frequency resolution of < 50 MHz, limited by the longitudinal mode linewidth. This was the first demonstration of MUMAS with QCLs. Secondly, the optical purity of a single mode distributed feedback (DFB) QCL operating at 5.3 μm was determined using Lamb-dip spectroscopy and by analysing the frequency noise power spectral density (PSD). Both methods revealed the linewidth of the device is ca. 500 kHz, which can be reproducibly broadened up to ˜∼8 MHz by applying broadband uncorrelated radio-frequency current perturbations. Thirdly, the DFB-QCL was used in combination with an external cavity (EC) QCL to study the evolution of velocity selected coherent signals in vibrationally excited NO. The coherent signatures showed lifetimes up to 378 ± 23 ns. Maximum population transfer to the v = 1 level was achieved after pumping for 10 ± 0.5 μs, and the decay time of post-pumping coherent transient signals was 5.7 ± 0.3 μs at a pressure of 3 mTorr. The collisional relaxation rate of the coherent systems was determined for various buffer gases, reaching (4.28 ± 0:13) ×104 s-1 mTorr-1 with CO2. Lastly, a DFB-QCL was used to demonstrate intracavity Faraday modulation spectroscopy (INFAMOS), a technique for the sensitive and selective detection of radicals, using NO as a test molecule. A model that predicts the INFAMOS lineshape was presented, and the sensitivity for the detection of NO was 0.21 ppbv Hz1/2, corresponding to a minimum detectable rotation angle of 0.16 nrad Hz1/2, at a total pressure of 412 Torr. Optical saturation effects were observed and included in the INFAMOS model.

This thesis covers work developing novel optical fibres for use as a delay fibre to decrease the pulse repetition rate of a short pulse mode-locked fibre laser operating at 1064 nm. The limits on the length of these laser cavities, and in turn the pulse repetition rate, was found to be the nonlinear phase shift accumulated by a pulse in one round trip of the laser cavity. The addition of extra fibre increases the cumulative nonlinear phase shift to the detriment of fundamental mode-locked pulses. Here, different optical fibres were developed with negligible nonlinear response to minimise additional nonlinearity when extending the laser cavity length. The fibres included large mode area (LMA) solid core fibres, hollow core photonic crystal fibres (PCF) and the more recently developed negative curvature anti-resonant hollow core fibre (NCF).The LMA fibres had an increased modal area which decreases the intensity of theguided light and thus decreases the nonlinear response. These fibres were used to reduce the repetition rate of a 20 MHz mode-locked fibre laser to 7.6 MHz operating at 1064 nm with a pulse duration of approximately 4 ps. The use of a hollow core fibre reduces the nonlinearity experienced by a pulse by changing the medium the light is propagating through. The low nonlinear response of air allowed for the reduction of a 4 ps pulse operating at 1064 nm and 37 MHz to be reduced to 27 MHz in PCF hollow core fibre and to a record 5.4 MHz in NCF.The PCF was found to be limited by polarisation effects and high fibre attenuation. However, the results using NCF showed no detriment to the near transform limited pulse. The use of hollow core fibre as a method of reducing the pulse repetition rate of mode-locked fibre lasers was demonstrated. The NCF developed for this work was also used as a means to induce spectralbroadening of a high power, amplified pulse pulse by the means of self phase modulation in a single pass through the fibre. A pulse was broadened by self phase modulation alone producing a symmetrical and broad spectrum that was controlled by the pressurisation of argon within the hollow NCF core. Using a grating pair on the fibre output a pulse of 10 ps at 1064 nm was compressed to 420 fs with an output pulse energy of 8μJ.

This thesis considers the problem of calculating and observing the mixing of modes of positive and negative frequency in inhomogeneous, dispersive media. Scattering of vacuum modes of the electromagnetic field at a moving interface in the refractive index of a dielectric medium is discussed. Kinematics arguments are used to demonstrate that this interface may, in a regime of linear dispersion, act as the analogue of the event horizon of a black hole to modes of the field. Furthermore, a study of the dispersion of the dielectric shows that five distinct configurations of modes of the inhomogeneous medium at the interface exist as a function of frequency. Thus it is shown that the interface is simultaneously a black- and white-hole horizon-like and horizonless emitter. The role, and importance, of negative-frequency modes of the field in mode conversion at the horizon is established and yields a calculation of the spontaneous photonic flux at the interface. An algorithm to calculate the scattering of vacuum modes at the interface is introduced. Spectra of the photonic flux in the moving and laboratory frame, for all modes and all realisable increase in the refractive index at the interface are computed. As a result of the various mode configurations, the spectra are highly structured in intervals with black-hole, white-hole and no horizon. The spectra are dominated by a negative-frequency mode, which is the partner in any Hawking-type emission. An experiment in which an incoming positive-frequency wave is populated with photons is assembled to observe the transfer of energy to outgoing waves of positive and negative frequency at the horizon. The effect of mode conversion at the interface is clearly shown to be a feature of horizon physics. This is a classical version of the quantum experiment that aims at validating the mechanism of Hawking radiation.

Terahertz (THz) technology stands to solve a number of problems in everyday life, from next generation wireless communication to spectroscopic identification and imaging. However it is technically challenging to make a high power, compact source for terahertz radiation. The Quantum Cascade Laser (QCL), which produces gain at THz frequencies by exploiting inter-sub-band transitions in quantum wells, offers one solution to this problem. However controlling and detecting the emission from such sources remains a major challenge. This thesis investigates the theory and measurement of emission frequencies from aperiodic lattice THz QCLs. Crucially, realising both frequency control and detection provides a complete system for coherent THz characterisation of devices at precise, user defined frequencies. The author starts by studying the emission frequencies and threshold of discretely tuned aperiodic lattice lasers. This is achieved using a numerical transfer matrix method (TMM), which allows the calculation of the aperiodic lattice threshold spectrum for the first time. Calculations reveal that the low threshold modes of aperiodic lattice lasers form at peaks in the electromagnetic density of modes. This shows that lasing in aperiodic lattices arises from slow light propagation induced by multiple photonic band gaps, leading to both band edge and defect laser modes. Frequency selective lasing is maintained even under the influence of external facet feedback, albeit at the cost of precise knowledge of the mode frequency. Importantly this framework allows the understanding of essentially any aperiodic lattice laser system. Most significantly, the TMM is exploited in order to understand how graphene can be used to control a THz laser. Graphene interacts strongly with THz waves, and can be easily integrated with semiconductor structures such as lasers and waveguides. Here, numerical calculations reveal that graphene can be introduced into the waveguide of a THz QCL, generating electrically tunable THz surface plasmons. Such surface plasmons couple into an aperiodic lattice to change the scattering strength of each individual grating element. The TMM reveals that this change in scattering strength controls the modal selectivity of an aperiodic lattice THz QCL. This hypothesis successfully explains both earlier experiments and those performed by the author. Crucially, this model was central to a publication in the journal Science. Finally, this thesis demonstrates a novel coherent detection system for the characterisation of THz QCL emission. The technique exploits non-linear up-conversion of THz waves to a telecoms frequency side-band, a process shown to be sensitive to THz waveguide dispersion. By mixing the up-converted THz wave with a near infra-red local oscillator laser, coherent detection of QCL emission using all fibre coupled components is demonstrated for the first time. This measurement allows for the characterisation of laser emission with high frequency and temporal resolution. Specifically sub-microsecond pulses of THz emission and transients can be detected. When taken as a whole, the work of this thesis constitutes a major step towards realising cost effective THz characterisation and spectroscopy using QCLs.

New mono-chelate complexes with ferrocenyl (Fc), octamethylferrocenyl (Me8Fc) and diaminophenyl (Dap) donor groups, connected through a conjugated bridge, to ZnCl2, Zn(OAc)2 or ReCl(CO)3 acceptor groups, are described. A thorough characterisation of the complexes is provided, including single crystal structures for one pro-ligand and three complexes. Visible d(FeII)→π* metal-to-ligand charge-transfer (MLCT) bands accompany π→π* intraligand charge-transfer absorptions in the UV region. TD-DFT calculations confirm the nature of these absorptions and indicate transitions at higher energies also contain some d(FeII)→π* character. Fc and Me8Fc containing chromophores display a fully reversible oxidation process when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined using hyper-Rayleigh scattering (HRS) and Stark spectroscopy. Larger β values are determined for complexes with Dap donors and ReICl(CO)3 acceptors. A family of novel fac-ReI(Lax)(CO)3(N-N) complexes, where N-N is 4,4′-dicyano-2,2′-bipyridyl (dcnbpy) or 4,4′-bis-(p-cyanophenyl)-2,2′-bipyridine (bbnbpy), with various axial ligands (Lax) are reported. The ReI complexes are useful precursors for metalation with electron-rich RuII ammine donor groups, to prepare novel tri-metallic V-shaped molecules. Single crystal X-ray structures are reported for five ReI complexes, confirming the fac geometry of the carbonyl ligands; the ReI complexes also display stretching frequencies typical of fac-ReI(Lax)(CO)3(N-N) complexes. The UV-visible spectra contain a low intensity band due to a d(ReI)→π*(bpy) transition, along with a more intense π→π* band. 1H NMR studies reveal the formation of trimetallic complexes, upon treatment of ReI complexes with a molar excess of [RuII(NH3)5(H2O)][PF6]2.The synthesis of octupolar heptametallic complexes containing RuII ammine donor groups has been investigated. The ligand 4,4′-bis-[(E)-2-(4-cyanophenyl)ethenyl]-2,2′-bipyridine (bbnpe) was used to prepare tris-chelate complexes of various transition metals, in order to understand its complexation behaviour. The ZnII tris-chelate BPh4– salt was treated with a 10 molar excess of a RuII ammine aquo complex, to produce the heptametallic complex as a mixed anion salt. HRS and Stark spectroscopy have been used to determine the quadratic NLO response for the heptametallic mixed anion complex salts, the latter gives large β0 values, approximately 10–27 esu. Density functional theory (DFT) calculations have been carried out on twelve cationic, 2D NLO chromophores with pyrazinylbis(pyridinium) electron acceptors with either 4-(methoxy/dimethylamino)-phenyl or pyridyl coordinated {RuII(NH3)5}2+/trans-{RuII(NH3)4(py)}2+ electron donor groups and the results compared with data previously obtained experimentally. The B3LYP/6-311G(d) level of theory was used to model the absorption spectra and to calculate static hyperpolarisability (β0) values, whilst the B3LYP/LANL2DZ/6-311G(d) level was used for the complexes. The extent of prediction of trends in ICT bands and β0 is partial, with the main discrepancies relating to the progression from one to two electron donor groups. The quantitative accuracy of predictions diminishes as the systems depart from a relatively simple one-dimensional (1D) dipolar motif.

Quantum memories are key components in photonics-based quantum information processing networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their very long storage times as well as their experimental simplicity and versatility. Operation with the Raman memory protocol enables high time-bandwidth products, which denote the number of possible storage trials within the memory lifetime. Since large time-bandwidth products enable multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing, the Raman memory is a promising tool for such tasks. Particularly, the broad spectral bandwidth allows for direct and technologically simple interfacing with other photonic primitives, such as heralded single photon sources. Here, this kind of light-matter interface is implemented using a warm caesium vapour Raman memory. Firstly, we study the storage of polarisation-encoded quantum information, a common standard in quantum information processing. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which constitutes a key technological capability for applications in temporal multiplexing. Prior to storage, single photons are produced in a waveguide-based spontaneous parametric down conversion source, whose bespoke design spectrally tailors the heralded photons to the memory acceptance bandwidth. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of approximately 0.3 in the memory output. Nevertheless, a clear influence of the quantum nature of an input photon is observed in the retrieved light by measuring the read-out signal's photon statistics via the g(2)-autocorrelation function. Here, we find a drop in g(2) by more than three standard deviations, from g(2) ~ 1.69 to g(2) ~ 1.59 upon changing the input signal from coherent states to heralded single photons. Finally, the memory noise processes and their scalings with the experimental parameters are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory. These experimental results and their theoretical description point towards practical solutions for noise-free operation.

This thesis investigates the coherent phase modulation of ultrashort pulses using vibrating hydrogen as a molecular medium. Self-phase modulation in a gas-filled hollow core capillary allows the generation of highpower few-cycle pulses in the NIR. Such pulses can be used to drive high harmonic generation (HHG) to deliver attosecond duration pulses in the extreme ultraviolet and soft X-ray spectral region. While reaching unrivalled pulse durations (down to 67 as), these sources have characteristically low efficiencies. The pump-probe spectroscopy community would greatly benefit from brighter short wavelength sources with sub-5 fs duration. In this work I apply Amplified RamaN Impulsive Excitation for Molecular Phase Modulation (ARNIEMPM), a multiple pulse scheme, to coherently prepare vibrating hydrogen molecules and exploit them for the phase modulation of ultrashort pulses. The preparation of the molecular motion is performed via impulsive stimulated Raman scattering and transient stimulated Raman scattering. The generated in-phase motion of molecules creates an oscillating optical polarizability in the medium which can be exploited by a probe pulse propagating through it, acting as a 125THz frequency phase modulator, the fastest among molecular media. This technique has the potential to provide bright, isolated subfemtosecond duration ultra-violet (UV) pulses via spectral broadening of broadband pulses. I experimentally investigate the preparation of the molecular motion against multiple experimental parameters. I then demonstrate the molecular phase modulation of ultrashort broadband probes in the near-infrared (NIR) and UV via a degenerate interferometric scheme. I used a waveguide to increase the interaction length of the process and reduce the energy requirements for the medium preparation. This allowed the use of a single laser system to generate all the required pulses, which are largely diverse in terms of wavelength, duration and power. Additionally, I present a novel technique named Attosecond Resolved Interferometric Electric-field Sampling (ARIES), which is capable of directly measuring the waveform of arbitrary pulses with attosecond resolution. This technique is based on high-harmonic generation (HHG) acting as a temporal gate for an applied secondary field, and tracking its electric field amplitude as a shift in the HHG cut-off frequency. I present experimental demonstration of a pulse waveform measurement by accurately retrieving a know inserted variation in dispersion and carrier-envelope-phase. A theoretical calculation of the technique applicability over a wide spectral range is also presented.

This thesis discusses nonlinear effects, such as modulation instability and solitons in nano-structured waveguides. The nanoscale optical waveguides have extremely small transverse dimensions, which can provide tight confinement of light. Therefore, by changing the waveguide geometry, the waveguide dispersion can be strongly altered. On the other hand, the confinement also enhances the nonlinear dispersion, allowing for nonlinear optical phenomena supported by dispersion of nonlinearity. The new models governing evolution of the amplitudes of components of the optical waves interacting in the waveguides are derived for continuous wave and pulse wave using perturbation expansion method. The new modulation instability condition is found, as we take into account the dispersion of nonlinearity which is enhanced through a strong variation of the modal profile with the wavelength of light in sub-wavelength waveguides. We demonstrate that this dispersion of nonlinearity can lead to the modulation instability in the regime of normal group velocity dispersion through the mechanism independent from higher order dispersions of linear waves for continuous wave. We address that the new mechanism highly associated with dispersion of nonlinearity in sub-wavelength semiconductor waveguide induces the modulation instability in picsecond regime together with the cascaded generation of higher-order sidebands. The impact of the dispersion of nonlinearity on spectral broadening of short pulses in a silicon waveguide also is considered. We study the temporal evolutions of fundamental and one-ring solitary waves with phase dislocation in dielectric-metal-dielectric waveguides with PT-symmetry and numerically analyze the properties of these nonlinear localized modes and, In particular, reveal different scenarios of their instability.

The primary goal of this thesis was to commission an instrument for vibrational sum frequency spectroscopy (VSFS), and exploit it for the study of solid/gas interfaces; of ultimate interest is characterisation of substrate surfaces in humid environments. Such effort is motivated by interest in understanding the potential for atmospheric corrosion in dry storage facilities of spent nuclear fuels or other nuclear-related wastes. VSFS is a non-linear, interface specific, vibrational spectroscopy, in which two photons of different energies (infrared (IR) and visible (VIS)) impinge upon a surface at the same point at the same time, leading to the generation of a third (sum-frequency generation (SFG)) photon. Features in VSFS spectra can be assigned to vibrational modes of interfacial species, and so enable details of interfacial structure and chemistry to be elucidated. An instrument for such measurements has been developed using laser facilitates located in the Photon Science Institute (PSI) of The University of Manchester. More specifically, an ultra-fast (femtosecond) laser has been employed as a light source, enabling acquisition of spectra (~250 cm-1 in width at a resolution of ~11 cm-1) without the need for scanning the energy of either IR or VIS beams, i.e. so called broad-band VSFS. To test performance, data have been acquired from self-assembled monolayers of alkanethiols (octadecanethiol) on gold substrates, which demonstrate the utility of the instrument. Subsequent to commissioning, the VSFS instrument was initially exploited to study the interaction of two organic molecules, acetonitrile and acetic acid, with a single crystal TiO2(110) substrate; measurements were performed with the sample exposed to the vapour of each organic species under ambient conditions. Surface adsorption was identified through the appearance of the CH3 symmetric stretch. Furthermore, spectra as a function of light (IR/VIS/SFG) polarization combinations have been recorded to explore adsorbate angular geometry. Finally, VSFS measurements have been undertaken from a number of substrates (GaAs, Au, Zn, Fe, Cr, stainless steel), as a function of relative humidity; D2O was employed to overcome the issue of loss of IR beam intensity due to interaction with atmospheric H2O. Signal quality varies significantly with substrate, with the most insight being gained for the interaction of D2O with polycrystalline Zn. Clear vibrational resonances due to both hydroxyls (OD) and molecular water (D2O) are observed, which vary with relative humidity, indicating that there are significant changes in interface structure with relative humidity.

This thesis describes the development of several spectroscopic methods based on impulsive vibrational spectroscopy as well as of the technique itself. The first chapter describes the ultrafast time domain Raman spectrometer including the development of two noncollinear optical parametric amplifiers for sub-10 fs pulse generation with 343 or 515 nm pumping. In the first spectroscopic study we demonstrate, for the first time, that impulsive vibrational spectroscopy can be used for recording transient Raman spectra of molecules in excited electronic states. We obtain spectra of beta-carotene with comparable, or better, quality than established frequency domain based nonlinear Raman techniques. The following two chapters address the questions on the fate of vibrational coherences when generated on a reactive potential energy surface. We photoexcite bacteriorhodopsin and observe anharmonic coupling mediated vibrational coherence transfer to initially silent vibrational modes. Additionally, we are able to correlate the vibrational coherence activation with the efficiency of the isomerisation reaction in bR. Upon generation of vibrational coherence in the second excited electronic state of beta-carotene, by excitation from the ground electronic state, we are able to follow the wavepacket motion out of the Franck-Condon region. We observe vibrationally coherent internal conversion, through a conical intersection, into the first excited electronic state and are hence able to demonstrate that electronic surface crossings can occur in a vibrationally coherent fashion. Additionally, we find strong evidence for vibronic coupling mediated back and forth crossing between the two electronic states. As a combination of this work we develop a IVS based technique that allows for the direct recording of background and baseline free Raman spectra in the time domain. Several proof of principle experiments highlight the capabilities of this technique for time resolved Raman spectroscopy. In the final chapter we present work on weak-field coherent control. Here, we address the question of whether a photochemical reaction can be controlled by the phase term of an electric excitation field, in the one photon excitation limit. We study the systems rhodamine 101, bacteriorhodopsin, rhodopsin and isorhodopsin and, contrary to previous reports, find no evidence for one photon control.

Broadband spectral content is required to support ultrashort pulses. However this broadband content is subject to dispersion and hence the pulse duration of corresponding ultrashort pulses may be stretched accordingly. I used a commercially-available adaptive ultrashort pulse shaper featuring multiphoton intrapulse interference phase scan technology to characterise and compensate for the dispersion of the optical system in situ and conducted experimental and theoretical studies in various inter-linked topics relating to the light-matter interaction. Firstly, I examined the role of broadband ultrashort pulses in novel light-matter interacting systems involving optically co-trapped particle systems in which inter-particle light scattering occurs between optically-bound particles. Secondly, I delivered dispersion-compensated broadband ultrashort pulses in a dispersive microscope system to investigate the role of pulse duration in a biological light-matter interaction involving laser-induced cell membrane permeabilisation through linear and nonlinear optical absorption. Finally, I examined some of the propagation characteristics of broadband ultrashort pulse propagation using a computer-controlled spatial light modulator. The propagation characteristics of ultrashort pulses is of paramount importance for defining the light-matter interaction in systems. The ability to control ultrashort pulse propagation by using adaptive dispersion compensation enables chirp-free ultrashort pulses to be used in experiments requiring the shortest possible pulses for a specified spectral bandwidth. Ultrashort pulsed beams may be configured to provide high peak intensities over long propagation lengths, for example, using novel beam shapes such as Bessel-type beams, which has applications in biological light-matter interactions including phototransfection based on laser-induced cell membrane permeabilisation. The need for precise positioning of the beam focus on the cell membrane becomes less strenuous by virtue of the spatial properties of the Bessel beam. Dispersion compensation can be used to control the temporal properties of ultrashort pulses thus permitting, for example, a high peak intensity to be maintained along the length of a Bessel beam, thereby reducing the pulse energy required to permeabilise the cell membrane and potentially reduce damage therein.

Developing a greater understanding of multiferroic materials, particularly those in which a strong coupling is exhibited between magnetic and electrical orderings, is of great importance if potential applications are to be realised. This thesis reports new experimental findings on several multiferroics using the techniques of X-ray and neutron diffraction together with nonlinear optical experiments. Spherical neutron polarimetry measurements on RbFe(MoO)2 show how this system's chiral magnetic structure can be controlled by an external electric field. Consideration is given to the axial distortion that the crystal structure makes, and the effect that this has on the stabilised magnetic structures. A ferroaxial coupling is invoked to explain, from a symmetry point of view, the spin driven multiferroicity in this proper screw system. The charge ordering in YbFe2O4 is examined by a detailed imaging of reciprocal space measured by elastic X-ray diffraction. Continuous helices of scattering are observed above the three-dimensional ordering transition temperature, whereas the intensity is concentrated onto separated maxima below this. The low temperature data are modelled using a simple oxygen displacement pattern, generalised to an incommensurate structure. The observed incommensurability implies that YbFe2O4 cannot be truly ferroelectric. The low field magnetic structures of a Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22 are observed in a resonant soft X-ray diffraction study. In zero field the system is helimagnetic, and with small applied fields peaks corresponding to a new phase appear. Energy calculations are used to suggest a suitable magnetic structure for the new phase and to show how this relates to the known commensurate phases that are present in low fields. Finally, an experimental setup designed to measure second harmonic generation from non-centrosymmetric crystals is presented, along with static measurements on the multiferroic system MnWO4. An optical pump / second harmonic probe study is then undertaken, with the result that a pump induced enhancement in the efficiency of the second harmonic generation is observed.

This thesis describes the experimental implementation of novel intersecting cavity terahertz optical parametric oscillators based on bulk and periodically-poled magnesium oxide-doped lithium niobate. Both collinear and non-collinear phase-matching geometries have been demonstrated and injection-seeding has been implemented in devices using periodically-poled material to reduce threshold and increase the down-conversion efficiency. A comprehensive characterisation of the original intracavity terahertz OPO was undertaken, which revealed the parameters having the greatest impact on OPO efficiency (idler mirror reflectivity and cavity length) and led to a better understanding of the losses in the system. During the characterisation process, generation of further terahertz radiation at the same frequency as that generated by the parametric process was observed and identified as being a result of difference frequency generation (DFG) between the parametrically-generated idler and terahertz waves. This phenomenon had previously only been observed when periodically-poled materials were employed in the system. The effect of this additional DFG process has been analysed in terms of the enhancement of the terahertz field on the basis of the coupled wave equations and physically measured quantities. The use of periodically-poled lithium niobate has been a major part of the research presented in this thesis. A comprehensive study of the modified phase-matching conditions was carried out and both collinear and novel hybrid non-collinear phase-matching geometries were identified. Several computer models were developed to assess the performance of any given grating design in these different geometries and the effects of temperature tuning and pump wavelength variation were also investigated using the models. Experimental studies confirmed the viability of the modelling approach but material limitations (particularly the early onset of crystal damage) limited the outcomes of the experiments. A detailed comparison of the poled and bulk materials was made to highlight the present drawbacks of the poled material. Finally, injection seeding was used to improve the efficiency of the collinear phase-matched PPLN OPOs. When seeding was used the depletion of the pump pulse was increased to the point of being measurable, reaching an upper level of 10%. Coupling constraints placed on the seed laser limited the amount of depletion attained. The potential for injection seeding to be used in the hybrid non-collinear phase-matching scheme was also identified but not realised during the course of this work. Were this technique successful, the tuning range of the intersecting cavity terahertz OPO could be extended to encompass the sub-1THz region, something that has previously been limited by the available idler cavity angles.

The field of nanophotonics is a major research topic, as it offers potential solutions to important challenges, such as the creation of low power, high bandwidth interconnects or optical sensors. Within this field, resonant structures and slow light waveguides are used to improve device performance further. Photonic crystals are of particular interest, as they allow the fabrication of a wide variety of structures, including high Q-factor cavities and slow light waveguides. The high scattering loss of photonic crystal waveguides, caused by fabrication disorder, however, has so far proven to be the limiting factor for device applications. In this thesis, I present a detailed study of propagation loss in slow light photonic crystal waveguides. I examine the dependence of propagation loss on the group index, and on disorder, in more depth than previous work by other authors. I present a detailed study of the relative importance of different components of the propagation loss, as well as a calculation method for the average device properties. A new calculation method is introduced to study different device designs and to show that photonic crystal waveguide propagation loss can be reduced by device design alone. These “loss engineered” waveguides have been used to demonstrate the lowest loss photonic crystal based delay line (35 dB/ns) with further improvements being predicted (< 20 dB/ns). Novel fabrication techniques were investigated, with the aim of reducing fabrication disorder. Initial results showed the feasibility of a silicon anneal in a nitrogen atmosphere, however poor process control led to repeatability issues. The use of a slow-fast-slow light interface allowed for the fabrication of waveguides spanning multiple writefields of the electron-beam lithography tool, overcoming the problem of stitching errors. The slow-fast-slow light interfaces were combined with loss engineering waveguide designs, to achieve an order of magnitude reduction in the propagation loss compared to a W1 waveguide, with values as low as 130 dB/cm being achieved for a group index around 60.

In the past decade, great research effort was inspired by the need to realise active optical functionalities in silicon, in order to develop the full potential of silicon as a photonic platform. In this thesis we explore the possibility of achieving tunable delay and optical gain in silicon, taking advantage of the unique dispersion capabilities of photonic crystals. To achieve tunable optical delay, we adopt a wavelength conversion and group velocity dispersion approach in a miniaturised engineered slow light photonic crystal waveguide. Our scheme is equivalent to a two-step indirect photonic transition, involving an alteration of both the frequency and momentum of an optical pulse, where the former is modified by the adiabatic tuning possibilities enabled by slow light. We apply this concept in a demonstration of continuous tunability of the delay of pulses, and exploit the ultrafast nature of the tuning process to demonstrate manipulation of a single pulse in a train of two pulses. In order to address the propagation loss intrinsic to slow light structures, with a prospect for improving the performance of the tunable delay device, we also investigate the nonlinear effect of stimulated Raman scattering as a means of introducing optical gain in silicon. We study the influence of slowdown factors and pump-induced losses on the evolution of a signal beam along the waveguide, as well as the role of linear propagation loss and mode profile changes typical of realistic photonic crystal structures. We then describe the work conducted for the experimental demonstration of such effect and its enhancement due to slow light. Finally, as the Raman nonlinearity may become useful also in photonic crystal nanocavities, which confine light in very small volumes, we discuss the design and realisation of structures which satisfy the basic requirements on the resonant modes needed for improving Raman scattering.

The preparation of single photons in a pure quantum state is a subject of great interest in physics, enabling the control of light at an unprecedented level. The ease with which certain degrees of freedom of photon states, such as polarisation, can be manipulated, along with the inherent resilience of photons to decoherence, makes them well suited for use as qubits. Recent rapid developments in the transmission and processing of quantum information, as well as the likely technological impact of potential real-world applications such as quantum cryptography and quantum computation, mean that the demand for high performance single photon sources is likely to increase in the near future. One approach to producing single photon states, which are known to be in a well-defined spatio-temporal mode without destructively measuring them, is to take advantage of nonlinear optics. Nonlinear processes can be used to realise frequency conversion by generating a single, correlated pair of photons from an intense pump laser source. The detection of one of the photons from a pair can then be used to indicate the presence of the other photon in the pair, a procedure known as heralding. This thesis describes the development of a source of heralded single photons at 1550 nm, generated directly in the core of a photonic crystal fibre (PCF). By taking advantage of low loss fibre components for the required spectral filtering of the generated photon state, a heralding fidelity of 52% was achieved. The source was designed to be used with a picosecond pulsed fibre laser, making it relatively low cost and maintenance free. With 148 mW of average pump power a heralded output photon rate of 6.4 × 104 s-1 was observed, demonstrating the brightness of the source. The purity of the generated single photons was established by measuring non-classical interference, with a visibility of 70%, between the photons output from this source and a source based on a PPLN waveguide. The fabrication of a series of birefringent PCFs for the generation of spectrally pure state photons at 1550 nm is also discussed. These PCFs will be useful for incorporation in the next generation of high performance, fibre-based photon sources.

The development of compact and low-cost coherent sources in visible and infrared wavelength range can provide indispensible tools for a variety of scientific, technological and industrial applications. Great progress over the last years in material science, crystal growth and semiconductor material processing in combination with recent advances in some of the more traditional technologies, in particular nonlinear frequency conversion and parametric sources, have led to the realisation of a new generation of laser sources. Furthermore, the advent of a new generation of quasi-phase-matched, waveguided and semiconductor nonlinear materials together with novel semiconductor lasers have led to the development of new frequency conversion and parametric sources with previously unattainable performance capabilities. The research described in this thesis relates to the development and characterisation of novel semiconductor based laser sources tunable in the broad spectral ranges which are unattainable for conventional lasers due to a lack of suitable laser gain materials. In the first part of the thesis the subject matter is concerned with the direct emission from laser devices. In particular, a broadly tunable InGaAs/InP strained multi-quantum well external cavity diode laser, operating in the spectral range of 1494 nm – 1667 nm with a maximum CW output power in excess of 81 mW and side-mode suppression ratio higher than 50 dB is demonstrated. This represents the highest output power and side-mode suppression ratio ever to be generated in this spectral region. A record broadly tunable high-power external cavity InAs/GaAs quantum-dot diode laser with a tuning range of 202 nm (1122 nm - 1324 nm), a maximum output power of 480 mW and a side-mode suppression ratio greater than 45 dB, is also demonstrated. This represents a promising achievement for the development of a high-power fast swept tunable laser and compact nonlinear frequency generation schemes for the green-yellow-orange-red spectral range. The second part of the thesis relates to induced nonlinear processes, focusing on frequency doubling and optical parametric oscillation. In particular, an all-room-temperature CW second harmonic generation at 612.9 nm and 591.5 nm in periodically poled potassium titanyl phosphate waveguides pumped by a broadly-tunable quantum-dot external cavity diode laser with a conversion efficiency of 10.5% and 7.9%, respectively, is demonstrated. For the first time, a green-to-red tunable laser source with tunability of over 60 nm (567.7 nm – 629.1 nm) based on frequency doubling in a single periodically poled potassium titanyl phosphate waveguide pumped by a single broadly-tunable quantum dot laser is demonstrated. These results are an important step towards a compact tunable coherent visible light source, operating at room temperature. The possibility of nonlinear frequency conversion in orientation-patterned GaAs waveguides is also investigated. The technology of low-loss periodically poled GaAs waveguided crystals is developed and such crystals are fabricated. Second harmonic generation at 1621 nm in low-loss periodically poled GaAs waveguide is demonstrated. An optical parametric oscillator system used as the pump source for GaAs devices and based on the periodically poled 5 mol% MgO-doped Congruent Lithium Niobate crystal, generating light in the wavelength range between 1430 nm and 4157 nm, is presented. The obtained results show a great promise for realisation of efficient quasi-phase-matched optical parametric oscillator devices based on orientation-patterned GaAs waveguides, which enables the extending generated wavelength up to 16 µm.

The growing speed and bandwidth requirements of telecommunication systems demand all-optical on-chip solutions. Microphotonic devices can deliver low power nonlinear signal processing solutions. This thesis looks at the slow light photonic crystals in chalcogenide glasses to enhance low power nonlinear operation. I demonstrate the development of new fabrication techniques for this delicate class of materials. Both, reactive ion etching and chemically assisted ion beam etching are investigated for high quality photonic crystal fabrication. A new resist-removal technique was developed for the chemical, mechanical and light sensitive thin films. I have developed a membraning method based on vapor phase etching in combination with the development of a save and economical etching tool that can be used for a variety of vapour phase processes. Dispersion engineered slow light photonic crystals in Ge₃₃As₁₂Se₅₅ are designed and fabricated. The demonstration of low losses down to 21±8dB/cm is a prerequisite for the successful demonstration of dispersion engineered slow light waveguides up to a group index of around n[subscript(g)] ≈ 40. The slow light waveguides are used to demonstrate highly efficient third harmonic generation and the first advantages of a pure chalcogenide system over the commonly used silicon. Ge₁₁.₅As₂₄24Se₆₄.₅ is used for the fabrication of photonic crystal cavities. Quality factors of up to 13000 are demonstrated. The low nonlinear losses have enabled the demonstration of second and third harmonic generation in those cavities with powers up to twice as high as possible in silicon. A computationally efficient model for designing coupled resonator bandpass filters is used to design bandpass filters. Single ring resonators are fabricated using a novel method to define the circular shape of the rings to improve the fabrication quality. The spectral responses of the ring resonators are used to determine the coupling coefficient needed for the design and fabrication of the bandpass filters. A flat top bandpass filter is fabricated and characterized as demonstration of this method. A passive all-optical regenerator is proposed, by integrating a slow-light photonic crystal waveguide with a band-pass filter based on coupled ring resonators. A route of designing the regenerator is proposed by first using the dispersion engineering results for nonlinear pulse propagation and then using the filter responses to calculate the nonlinear transfer function.

This portfolio of work contributes to the remit of the National Physical Laboratory (NPL) to develop the underpinning expertise and tools for validating nascent and future optical quantum technologies based on the discrete and quantum properties of photons. This requirement overlaps with the requirement to provide validation for devices operating in the photon-counting regime. A common theme running through the portfolio is photon pairs generated through spontaneous parametric downconversion (SPDC). A Hong-Ou-Mandel (HOM) interferometer sourced with visible wavelength photon pairs from an SPDC process in beta-barium borate (BBO) was designed, built and characterised. The visibility of the HOM interference is dependent on the indistinguishability of the interfering photons, but is also influenced by imperfections of the interferometer; therefore an investigation was carried out to quantify the effects of the interferometer imperfections on the measured visibility so that the true photon indistinguishability could be measured with a quantified uncertainty. A bright source of correlated pair photons in the telecoms band based upon a pump enhanced SPDC process in periodically-poled potassium titanyl phosphate (PPKTP) was designed, built and characterised. From the characterisation measurements the source brightness was estimated to be 6.2×10⁴ pairs/ s/ mw pump. The photon pairs were further characterised through their incorporation as a source in a HOM interference experiment. The developed correlated photon pair source was at the heart of a novel scheme for the generation of polarisation entangled photon pairs, for which the design, build and characterisation work is presented. The source was demonstrated to produce two of the four maximally entangled Bell states with quantum interference visibilities of around 0.95. The generated states were also shown to break a form of Bell's inequality by around six standard deviations. The polarisation entangled photon pair source was originally built at the University of St Andrews and was later transferred to the NPL where it will extend NPL's capabilities to this key spectral region. Finally a study was carried out to investigate the possibility of a wavelength tuneable device for the absolute measurement of single photon detector quantum efficiencies based upon an established SPDC technique.

This thesis describes a number of studies on organic semiconductors focused around using them as gain media for lasers. The photophysical properties of organic semiconductors are studied using a wide range of experimental techniques, allowing the evaluation of new materials and novel excitation schemes for use in organic semiconductor lasers. Polyfluorene is a well-established conjugated polymer laser gain medium and in this thesis its excellent lasing properties are combined with its two photon absorption properties to demonstrate a tunable two-photon pumped solid-state laser based on a commercially available organic semiconductor. A family of bisfluorene dendrimers was studied using a number of photophysical techniques to evaluate their potential as laser materials. Distributed feedback lasers based on one of the dendrimers are demonstrated with lasing thresholds comparable to polyfluorene. The same materials were found to have enhanced two-photon absorption properties in comparison to polyfluorene, leading to the fabrication of tunable two-photon pumped dendrimer lasers. A member of a novel family of star-shaped oligofluorene truxenes was evaluated as a laser gain material and the distributed feedback lasers made from them show some of the lowest lasing thresholds reported for organic semiconductors, partly as a consequence of exceptionally low waveguide losses in comparison to other single-material thin films. Finally, an organic laser dye is blended with a conjugated polymer, where the dye molecules harvest the excitation light of a GaN laser diode and transfer its energy to the polymer molecules. This is the first time such a scheme is used in an organic laser and in combination with a novel surface-emitting distributed Bragg reflector resonator allows the demonstration of a diode-pumped organic laser, a significant step towards simplifying organic lasers.

I present several new methods for the characterisation of ultrashort pulses using interferometry. A generalisation of the concatenation algorithm for spectral shearing interferometry enables interferograms taken at multiple shears to be combined. This improves the precision of the reconstructed phase in the presence of detector noise, and enables the relative phase between disjoint spectral components to be obtained without decreasing the spectral resolution. The algorithm is applied to experimental data from two different implementations of spectral shearing interferometry for ultrashort optical pulses. In one, the shears are acquired sequentially, and in the other they are acquired simultaneously. I develop a form of spatio-temporal ultrashort pulse characterisation which performs both spatial and spectral shearing interferometry simultaneously. It requires a similar geometrical setup to common implementations of spectral phase interferometry for direct electric-field reconstruction, but provides complete amplitude and phase characterisation in time and one spatial dimension. I develop the theory of lateral shearing interferometry for spectrally resolved wavefront sensing of extended ultraviolet and soft x-ray pulses generated using high-harmonic generation. A comprehensive set of wavefront measurements of harmonics 13-25 in Krypton show good agreement with theory, validating the technique. I propose and numerically demonstrate quantum-path interferometry mediated by a weak control field for high harmonic generation. This is a general technique for measuring the amplitude and relative phases of each contributing quantum path. The control field perturbatively modulates the phase of each path. The differing sensitivity of each path to the parameters of the control field allows their contributions to be distinguished from one another.

Fiber optical parametric amplifiers (OPAs) are based on a highly-efficient four-wave mixing process. Their capability to give very high gain and large bandwidths have made them an attractive candidate for providing higher bandwidths for future telecommunication systems, such as wavelength-division multiplexed (WDM) photonics networks. In dynamic photonic networks a where number of channels are dropped and/or added all the time, the OPA gain for the other channels is affected. In this thesis we employed a well-known gain control technique, all-optical gain clamping (AOGC), and reduced the gain variation of fiber OPAs below 0.5 dB, under varying input conditions. We also showed an improvement in power penalties o at the bit-error rate of 10-8, from 2.5 dB to 0.5 dB for on/off keying modulation. We also investigated fiber optical parametric oscillators (OPOs). Using fiber OPAs as gain medium we realized two different continuous-wave (CW) OPOs, centred at 1561 nm and 1593 nm. One gave us watt-level output power from 1600 nm to 1670 nm, with overall tuning range of 211 nm. The output linewidth of signal and idler was measured to be 0.08 nm and 0.15 nm respectively. The OPO centred at 1593 nm gave us a record tuning range of 254 nm, and with 3 dB output coupling fraction, it gave us large output powers (20-27 dBm) from 1610 nm to 1720 nm. Using a large seed generated by a watt-level fiber OPO in the U-band, and using 3 W of CW pump source in the C-band for Raman amplification, we generated 3 W of CW output power. This gave us nearly 100% conversion efficiency. Launching a high-power CW pump with narrow linewidth into a fiber makes stimulated Brillouin scattering (SBS) a major problem. We investigated an SBS suppressor, based on a common technique of phase dithering of the pump to suppress the SBS. We compared a multitone modulation technique to modulation with a pseudo-random bit sequence (PRBS), and we showed that it can increase the SBS threshold by 4.18 dB, and is less expensive to implement.

A theoretical study of silicon-on-insulator (SOI) photonic wires was conducted. These nanoscale optical waveguides can have transverse dimensions substantially smaller than the wavelength of the infrared light they carry. This extreme confinement provides very strong dispersion, which can be greatly controlled by specifying the waveguide geometry. The confinement also enhances silicon's already considerable Kerr nonlinearity (whereby refractive index increases with optical intensity), allowing for nonlinear optical phenomena with record-breakingly small powers. The most notable of these phenomena (and the prime subject of this report) is the soliton, which is a self-sustaining localised pulse held together by a balance between dispersion and nonlinearity. A variety of other phenomena are also considered, including continuum generation, in which the spectral width of a pulse is greatly increased, and modulational instability, in which deviations from an optical waveform are reinforced by nonlinearity. Light propagation through a single wire is modelled numerically, and the results compared to third-party experimental data. The analysis reveals that the experimental results are consistent with soliton evolution, thus strengthening the base of evidence for the existence of solitons in silicon wires. Light propagation through arrays of multiple waveguides is also modelled. It is shown that interwire diffraction is intimately linked to dispersion, and that by exploiting this it is possible to realise both solitons and modulational instability in arrays of wires that individually would not be able to support these phenomena. It is also shown that silicon nanowires are an excellent medium for realising "optical bullets" in which a pulse of light is self-localised both in and transverse to the direction of propagation. A distinctive pattern of radiation emitted by these bullets is predicted. Solitons supported by the Raman effect (rather than the Kerr effect) are also considered. A novel class of soliton solutions are derived, which have the novel property of existing even when the frequency components comprising the soliton are not phase matched.

The two-photon absorption (TPA) coefficient is measured in indium phosphide (InP) using femtosecond pulses to be 45cm/GW at 1.32μm. Nanosecond pulses are subsequently used to find the free-carrier refractive index cross-section, σ_r, and the free-carrier absorption coefficient, σ_fca. The quantity βσ_r is measured to be -113x10⁻²ºcm⁴/GW at 1.064μm and -84x10⁻²ºcm⁴/GW at 1.534μm. At 1.064μm, with β assumed to be 22cm/GW, the value suggested by theory, σ_r is -5.1x10⁻²ºcm³. Similarly, at 1.534μm, assuming β to be 20cm/GW gives a σ_r value of -4.1x10⁻²ºcm³. Due to refraction affecting the measurements of σ_fca, only an upper limit of 1x10⁻¹⁵cm² can be put on its value. The free-carrier experiments are repeated on two samples of cadmium mercury telluride (CMT) having bandgaps of 0.89eV and 0.82eV. For the first sample, β_σr is measured to be -148x10⁻²ºcm⁴/GW. Assuming β to be 89cm/GW gives a σ_r value of -1.7x10⁻²ºcm³ whilst σ_fca is found to be at most 3x10⁻¹⁵cm². Significant linear absorption occurs in the second sample which generates a large free-carrier population. It is shown that this significantly enhances the nonlinearities. Finally, the results of the work are tested by modelling a nonlinear transmission experiment, and the results found in this work give a closer fit to experimental results than the result of theory. Four-wave mixing (FWM) in a photonic crystal fibre is exploited to create a high output power optical parametric amplifier (OPA). To optimise the OPA conversion efficiency, the fibre length has to be increased to 150m, well beyond the walk-off distance between the pump and signal/idler. In this regime, the Raman process can take over from the FWM process and lead to supercontinuum generation. The OPA exhibits up to 40% conversion efficiency, with the idler (0.9μm) and the signal (1.3μm) having a combined output power of over 1.5W.

The research described in this thesis relates to the design, fabrication and testing of novel semiconductor-based light sources that have been designed for the generation of infra-red light. The thesis is formatted to account for two distinct components of my work, where the first part concerns sources producing coherent light by direct laser emission, notably, ultrashort-pulse quantum-dot lasers. These types of lasers continue to show considerable promise as efficient, compact sources of ultrashort pulses with durations of hundreds of femtoseconds, while giving rise to unique and interesting electronic properties such as low lasing thresholds through the quantum nature of their density of states. At the outset a study of the most relevant aspects of the lasing dynamics of an optically pumped quantum-dot laser is outlined. Pumping of the device with intense discrete optical pulses leads to output from multiple electronic states, each having a characteristic wavelength and temporal properties. I show that pulses produced by excited-state emission have shorter durations (24 ps) and arrive earlier in time than those due to transitions from the ground state, which themselves have durations of around 180 ps. Investigations are then made on two different mode-locked quantum-dot laser systems. One is an all-quantum-dot external-cavity laser that is mode locked using a quantum-dot SESAM device at a repetition frequency of 860 MHz with output power approaching 20 mW. This is followed by a study of a monolithic two-section quantum-dot laser that is mode locked stably in a wide temperature range of 20°C to 70°C. The excellent performance characteristics presented serve to demonstrate both the versatility of quantum-dot material as components in mode-locked laser systems and the temperature stability of such laser devices. The second part of the thesis relates to structures that are designed to take advantage of nonlinear frequency conversion in GaAs-based semiconductors. This material system possesses a nonlinear coefficient of ~170 pm/V and is transparent from around 0.9 μm through to 17 μm, making it attractive for the realisation of a new class of efficient, integrable, quasi-phase-matched, optical parametric oscillator devices. Initially, ion implantation is utilised as a vector to create a periodically-switched nonlinear ridge waveguided device. The observation is made that in the course of implantation the transmissive properties of the device are severely degraded. Unfortunately, the high losses incurred, which reached 250 dB/cm, could not be removed without also destroying the modulation in nonlinearity. During the course of this investigation, significant technological advances were made in the production of orientation-patterned GaAs structures. By recognising the elegance and potential of this new orientation-patterned (OP) methodology, a study of its implications and applicability in the context of my project is initiated.

This thesis presents several advances in the technology and applications of photonic crystal fibres achieved over the last three years. Chapters 1 and 2 give the background material important to understand the results presented in chapters 3, 4 and 5. In chapter 1, linear properties of optical fibres are described. This chapter focuses particularly on how the engineering of the cladding structure of solid core photonic crystal fibres can be used to vary the fibre properties, most importantly the group index and dispersion. Propagation in all-solid photonic bandgap fibres is also discussed in terms of the anti-resonant reflecting optical waveguide model. Chapter 2 introduces the nonlinear optical effects that are important to understand the work presented in chapters 4 and 5. In chapter 3, a method to reduce bend losses in all-solid photonic bandgap fibres is outlined. The reduction of these losses is achieved by redesigning the high-index inclusions in the cladding structure to suppress cladding modes that strongly couple to the fundamental core-guided mode when the fibre is bent. In chapter 4, a method of tapering photonic crystal fibres in order to decrease the dispersion along their length is described. The tapers are used to compress solitons via adiabatic soliton compression and a combination of adiabatic soliton compression and soliton effect compression, achieving a factor of 15 compression of a transform-limited pulse to below 50 fs. Chapter 5 describes how engineering the cladding structure of photonic crystal fibres can be used to generate shorter frequencies in supercontinuum generation. The method by which this achieved is experimentally verified and then exploited to generate a continuum incorporating the entire visible spectrum using low cost, low maintenance pump sources.

Semiconductor diode lasers are successful tools in atomic spectroscopy. They are routinely used in frequency conversion applications to develop devices that access regions of the spectrum not directly available. This thesis describes the practical application of novel violet diode laser systems and their possible inclusion in spectroscopic systems. The design, assembly and successful operation of a doubly resonant optical parametric oscillator is described. There is discussion of the spectral behaviour of the device and the potential for pumping with a violet diode laser. Methods to adapt the output from the solitary diode devices are demonstrated with the use of microlensed diode lasers and extended cavity configurations. Details of the current tuning, linewidth and smooth tuning characteristics of a number of the lasers used are given. A commercial violet diode laser is used within an extended cavity to measure the hyperfine structure of atomic indium from a hollow cathode galvatron source at room temperature. Stabilisation of the diode laser to a line from the indium spectrum is attempted. The remainder of the thesis is concerned with the development of techniques to deliver clearer and more precise spectral information about trace species. Microlensed red and violet diode lasers are used to generate light at 254nm via sum frequency generation for the direct detection and modulation spectroscopy of mercury vapour, with microlensed lasers with modulation allowing more accurate discrimination between spectral features than direct absorption measurements. In addition Raman tweezers modulation spectroscopy is undertaken to investigate polymer microspheres and biological cell samples where the use of the modulation technique demonstrated improvements in the acquisition time and clarity of spectra through increased signal to noise and rejection of background fluorescence effects. A comparison between the direct and modulation techniques for all the systems indicates the greater sensitivity of the modulation technique.