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Silicon nitride waveguides offer a high nonlinear refractive index and tight mode confinement, ideal for efficient four-wave mixing (FWM) processes. We present a light source for broadband as well as narrowband coherent anti-Stokes Raman scattering (CARS), with the potential to be set up as an all-integrated device, based on FWM in silicon nitride waveguides. Signal and idler pulses are generated via FWM with only 4 nJ input pulse energy and stimulated using a tunable continuous-wave seed source, such that the idler and residual pump pulses can be used for CARS measurements, enabling chemically-selective label-free imaging across the entire fingerprint region.

Summary from only given. We report on the first fiber-pumped CW LiNbO/sub 3/ optical parametric oscillator (OPO). The OPO is singly resonant (SRO) and generates idler wavelengths in the range of 3.0 /spl mu/m to 3.7 /spl mu/m with a maximum output power of 1.9 watt.

Lamellar Multilayer Gratings (LMG) offer improved resolution for soft-x-ray (SXR) monochromatization, while maintaining a high reflection efficiency in comparison to conventional multilayer mirrors (MM). We previously used a Coupled-Waves Approach (CWA) to calculate SXR diffraction by LMGs and identified a single-order regime in which the incident wave only excites a single diffraction order. We showed that in this regime the angular width of the zeroth-order diffraction peak simply scales linearly with Γ (lamel-to-period ratio) without loss of peak reflectivity. However, the number of bi-layers must then be increased by a factor of 1/Γ. Optimal LMG resolution and reflectivity is obtained in this single-order regime, requiring grating periods of only a few hundred nm, lamel widths < 100nm and lamel heights > 1μm [1]. For the fabrication of LMGs with these dimensions, we use a novel process based on UV-NanoImprint Lithography (UV-NIL) and Bosch-type Deep Reactive Ion Etching (DRIE). Successful fabrication of LMGs with periods down to 200nm, line widths of 60nm and multilayer stack heights of 1μm is demonstrated. SXR reflectivity measurements were performed on these LMGs at the PTB beamline at the BESSYII synchrotron facility. The measurements demonstrate an improvement in resolution by a factor 3,5 compared to conventional MMs. Further analysis of the SXR reflectivity measurements is currently being performed.

Programmable waveguide circuits are crucial building blocks for integrated spectrometric applications and quantum photonic information processing. Amongst the dielectric material platforms in integrated photonics, silicon nitride stands out with highly attractive properties such as a large bandgap energy and a moderately high index contrast. This allows low propagation losses in a wide spectral range while simultaneously allowing a dense packing of components. The aforementioned properties, together with the inherent phase stability and phase programmability achievable in silicon nitride, enable the creation of complex photonic circuits. In this thesis we describe and demonstrate densely integrated programmable photonic circuits based on silicon nitride waveguides for wavelength metrology and quantum information processing. We concentrate on reconfigurable photonic integrated circuits based on silicon nitride waveguides with low-loss propagation, to explore interference in the spectral and temporal domain for advanced applications. We investigated two types of integrated interferometric devices featuring low loss in combination with programmability for classical and quantum photonic processing. The first is simple tunable microring resonator circuits in combination with neural network data processing for the analysis of classical light in the spectral domain as wavelength meter. The second is a complex tunable network of waveguide interferometers for controlling quantum correlations (coincidences) between single photons. Exploiting the long-term interferometric stability, low propagation loss and tight optical confinement of integrated silicon nitride waveguides, we have shown complex reconfigurable optical circuits both for classical and quantum photonic processing. For future development of integrated programmable photonic processors, various challenges need to be addressed such as compact and low-power phase shifters, a further increase of the component density and lower the propagation losses.

In this thesis, we address the interdisciplinary research field known as “microwave photonics” (MWP), which has attracted considerable interest in scientific and industrial communities, and we investigate how integrated photonic technologies can be exploited to realize integrated microwave photonic systems. In brief, microwave photonics explores and develops methods and technologies to generate, process and distribute microwaves, millimeter waves and terahertz radiation via a photonic approach, i.e. in the optical domain. The main motivation for this approach is that systems based on microwave photonic technology can benefit from several advantages that are inherent to optical systems, such as high speed, low and frequency-independent propagation loss and reduced electromagnetic interference. However, although there is an indisputable and significant potential of microwave photonics, it has not yet been applied in real contexts. The main reason is that microwave photonics so far had to rely mostly on discrete components, which render according microwave photonic systems bulky, unstable and fragile. In order to overcome these limitations, while still taking advantage of the named, great opportunities, research and technology have begun targeting integration of microwave photonic systems, with the goal to enable the processing of microwave and millimeter waves via photonic chips.

In this work we explored three different nonlinear optical approaches to manipulate light propagating through stoichiometric silicon nitride waveguide. In particular, we have used nonlinear optical processes for extreme spectral broadening of a short pulse injected into a properly engineered waveguides for phase amplitude modulation of the injected light and for conversion to a second harmonic wavelength. In Chapter 3, we show our results on supercontinuum generation in low-loss, dispersion engineer Si3N4 waveguides using a commercial pulsed laser working at the C-band for optical communications. In Chapter 4, we study how the phase and amplitude of the propagating light can be changed via strain induced by surface acoustic waves in the region of the optical mode propagating through a buried waveguide. In Chapter 5, we use the coherent photogalvanic effect to create and effective second-order susceptibility grating in the amorphous, stoichiometric silicon nitride and use this grating to produce second-harmonic generation. The work presented in this thesis shows that nonlinear optical processes can be used to control to a greater extend the properties of light in stoichiometric silicon nitride waveguides. Making use of these processes enhances the functionalities available in this platform, which is promising for many different applications, for example, high precision metrology, broadband communications and optical quantum computing.

We propose and investigate a novel scheme for achieving quasi-phase matching (QPM) in high-order harmonic generation (HHG) via a density modulated cluster jet, as opposed to a modulated gas jet. A density modulation is expected to be easily achieved by placing an array of wires (grid) on the top of the nozzle. However, in our initial experiments, we did not observe any enhanced HH output using previously fabricated grids. These experiments suggest that, even if using a density modulated cluster jet remains promising for achieving QPM in HHG, an according demonstration requires a thorough investigation of several basic and essential aspects. In this thesis, we investigated three essential aspects. First, we presented a comprehensive modelling of cluster formation and systematically investigated all influences of various critical physical assumptions for the gas condensation in a supersonic nozzle using argon as an example. Using the proposed baseline model, we showed that the liquid mass fraction, g, is very insensitive with regards to the named variations in this model. The average cluster size, , was then retrieved from interferometric and Rayleigh scattering measurements by using the calculated g from the baseline model. Secondly, we performed a detailed experimental study on HHG in a supersonic argon jet. In order to identify and separate the contributions from clusters versus that of gas monomers in the jet to the generation of high-order harmonics, we characterized the harmonic spectra over a broad range of stagnation pressures at two different reservoir temperatures. From the measured spectra, we found that below an average cluster size, , of about 1000 atoms, HHG in clusters shows the same efficiency as in gas monomers. Only with larger clusters, HHG becomes less efficient. Lastly, we investigated QPM for HHG, specifically aiming at determining a proper quasi-phase matching modulation period for maximizing output pulse energy. We developed a one-dimensional, unified QPM model for HHG in a periodic-structured gaseous medium (argon). From the model, we showed that the optimum HH output pulse energy is obtained when transient QPM is provided in the leading edge of the drive laser pulse instead of at the peak intensity.

The goal of the work presented in this thesis is to realize a spectroscopic spatiotemporal mapping of selected species in laser-induced plasmas used in pulsed laser deposition (PLD), for a better understanding of the internal plasma dynamics and chemistry. We are especially interested in the influence of external parameters on the plasma dynamics and chemistry, such as the fluence of the ablation laser and the composition of the background gas and its pressure, as this can be the key to an improved understanding and control of stoichiometric film growth. We have chosen a combination of laser-induced fluorescence (LIF) and absorption spectroscopy (AS). LIF enables the detection of plasma constituents even in "dark" plasmas, i.e., also when the plasma plume has cooled down and no longer spontaneously fluoresces. AS provides a means to calibrate the relative density distribution maps obtained from LIF measurements for obtaining absolute density distributions. The combination of spectroscopic (also called chemical) in-situ spatiotemporal mapping of individual plasma constituents with a detailed analysis of the grown films has enabled an unparalleled insight in the influence of the external PLD parameters on the plasma propagation, chemical evolution and material deposition. In this thesis we present the results of three materials we have investigated, LaAlO3, SrTiO3 and YBiO3, of which we have spatiotemporally mapped the ground state populations of several plasma species. From these three materials it can already be seen that spatiotemporally resolved LIF is a highly valuable analytic approach for analyzing and understanding intricate details and features in PLD growth processes. Even when only mapping a select number of plasma constituents of a deposition material, much additional insight can be obtained. Having applied LIF in various different material systems has clearly shown that there are no universal plasma and growth dynamics. Instead, each material system, due to its individual chemical and physical properties, provides its own dynamics in the plasma, both at the substrate and at the target. This observed wide span in variety promises that there is much more to discover and understand through LIF and possibly also other types of spectroscopy.

Generating intense radiation in the wavelength band of the extreme ultraviolet (XUV, 100 to 10 nm) remains a very challenging goal in today’s scientific research. In this thesis, we describe a unique XUV source based on high harmonic generation. We have thoroughly investigated a number of intrinsic properties of high harmonic generation in a waveguided geometry. We conclude that using waveguides allows for spatial and spectral control of high-harmonic radiation. Our studies have also shown that waveguided high harmonic generation is particularly suited for producing radiation at XUV wavelengths with excellent beam coherence and unique spectral features. Future work, focused on using higher drive laser energy and larger waveguide diameters offer the potential of achieving higher XUV output energy.

In this thesis, we investigated nonlinear frequency conversion of optical wavelengths using integrated silicon nitride (Si3N4) waveguides. Two nonlinear conversion schemes were considered: seeded four-wave mixing and supercontinuum generation. The first—seeded four-wave mixing—is investigated by a numerical study and proposed as light source for coherent anti-Stokes Raman scattering (CARS). The compatibility of the silicon nitride-based integrated waveguide with microfluidic channels enables potential applications for on-chip CARS spectroscopy. Both four-wave mixing and supercontinuum generation require waveguides with a large core area to obtain the dispersion required for phase-matched nonlinear frequency conversion. A novel fabrication technique for manufacturing large-core Si3N4 waveguides was investigated. The main advantage of this novel technique is the ability to manufacture crack-free Si3N4 waveguides with sufficient thickness to phase match nonlinear optical processes, while simultaneously realizing a high device yield. To demonstrate that such waveguides can be dispersion engineered and, i.e., allow for phase matching for nonlinear frequency conversion, we experimentally investigated supercontinuum generation in these waveguides using two different pump wavelengths. For a pump wavelength of 1560 nm, the waveguide was dispersion engineered to have a zero-dispersion wavelength just above 1600 nm, such that the pump wavelength experiences anomalous dispersion. The generated supercontinuum spanned more than 700 nm (at -30 dB), limited by the available pump energy. Theoretical modeling showed an exceptionally good agreement with the measured spectrum, and that an octave-spanning supercontinuum is possible if the pump energy is increased. The pulse-to-pulse coherence was calculated for this case and we found that the supercontinuum was fully coherent over its bandwidth (-30 dB), showing that Si3N4 waveguides could be used to generate an optical frequency comb. For a pump wavelength of 1064 nm, the waveguide was designed to have a zero-dispersion wavelength just below 1000 nm to have, again, anomalous dispersion for the pump wavelength. At this pump wavelength, the generated supercontinuum spanned an ultrabroad-bandwidth of nearly 500 THz covering nearly the whole transparency window of Si3N4/SiO2 waveguides.

In this thesis we describe the development of a new class of optical components to enhance the imaging performance by enabling adaptations of the optics. When used at extreme ultraviolet (EUV) wavelengths, such ‘adaptive optics’ offers the potential to achieve the highest spatial resolution in imaging known to date, i.e., a resolution down to several nanometers. These optics are discussed, e.g., in the field of photolithography for the fabrication of more energy efficient and faster electronic equipment. The topics in this thesis can be subdivided into two parts. The first part includes a mirror design that can be used to enhance the spectral quality of EUV light sources. Our design consists of a diffractive Fresnel zone plate patterned in the surface of a curved mirror to achieve spectral purification, i.e., removing undesired wavelengths of the EUV light sources. The removed light is recycled to produce more EUV light. This design achieves four orders of magnitude purification and provides a 35% improvement in efficiency. The second part of the thesis describes a new class of adaptive multilayer mirrors that can be used to correct wavefront distortions, reflectance degradations or wavelength deviations. These adaptive optics contain integrated crystalline piezoelectric thin films of which the film thickness can be adjusted by an externally applied voltage. Because the optics substrates typically have an amorphous structure, so far it was not possible to grow high quality crystalline piezoelectric films on them. We utilized crystalline nanosheets on these substrates to grow the highest quality, i.e., epitaxial, piezoelectric films (PbZr0.52Ti0.48O3, PZT). The PZT films show a record high piezoelectric coefficient of 98 pm/V, although the piezoelectric response in this case is still reduced due to clamping by the substrate. We minimized the clamping effect by depositing films with a columnar structure, which enhances the voltage-controlled change in film thickness. In these films we measured a piezoelectric response of 280 pm/V, the highest value reported on glass substrates. Importantly, the developed films can produce a stroke of 25 nm. This order of magnitude is useful when considering to correct EUV wavefront distortions.

A major challenge in the soft x-ray (SXR) and eXtreme UltraViolet (XUV) spectral ranges is the ability to manipulate the incident radiation using optical elements. By patterning conventional multilayer mirrors with nanoscale structures, novel optical elements with a variety of optical properties can be obtained. In this work, the design, fabrication and characterization of Lamellar Multilayer Gratings (LMG) was investigated. Such LMGs are a particular form of the general class of Bragg-Fresnel optics which combine Fresnel optics with Bragg reflection to provide unique dispersive and focusing optics. In particular, LMGs can be used to improve the spectral resolution of x-ray fluorescence techniques. A Coupled Waves Approach (CWA) was derived to simulate the optical performance, in terms of resolution and reflectivity, of LMGs. This CWA allowed to study the physical principles behind LMGs and resulted in the identification of an optimal LMG operating regime. In this regime, the incident beam is reflected in a single diffraction order and is hence referred to as the singleorder regime. Such single-order LMGs were fabricated using UV-NanoImprint Lithography and Bosch Deep Reactive Ion Etching, a process chosen to enable rapid practical development. Single-order operation was experimentally demonstrated and showed an improvement in spectral resolution of a factor of 3.8 with regard to convenventional multilayer mirrors. Singe-order excitation of higher diffraction orders was also measured and analyzed. Single-order operation often requires multilayer stacks and grating structures that cannot be fabricated with sufficient accuracy using current technologies. As this invalidates the semi-infinite multilayer approximation, we investigated the optical performance of LMGs with finite multilayer stacks. We determined the ratio between the absorber thickness and bi-layer period of the multilayer stack can be used to further tailor LMG optical performance to maximize bandwidth reduction or minimize peak reflectivity loss. We also investigated various degradation processes that could limit the lifetime of LMGs, which is important for the applicability of such elements. Oxidation of tungsten and silicon as well as changes to the sidewall composition were clearly seen. However, SXR reflectivity remained stable to within measurement accuracy for an extended storage period of 18 months in a 1 atm air environment. The possibility of applying a capping layer on LMG structures to reduce degradation was also studied.

We investigated spectral control of diode lasers using external waveguide circuits. The purpose of this work is to investigate such external control for providing a new class of diode lasers with technologically interesting properties, such as a narrow spectral bandwidth and spectrally tunable output in a hybrid integrated format. In more detail, they consist of an integrated optics waveguide circuit with micro-ring resonators, configured such that they reflect and spectrally filter light emitted by a diode laser gain material. This causes the diode laser to have a spectrally narrow bandwidth (25 kHz) and widely tunable output (over 40 nm). These lasers are of interest in a variety of scientific and industrial applications. Furthermore, we investigated the possibility of simultaneously control the spectrum and phase of entire arrays of diode lasers. The superimposed output may offer the generation of trains of ultrashort pulses, which is equivalent with the generation of a phase locked comb of equidistant frequencies. The standard method is called mode locking and uses only one gain medium. Because our method uses multiple separate gain media, our method is called separate gain (SEGA) mode locking. Such sources can, for example, be of interest for high-speed optical data storage.

Coherent electromagnetic waves are extensively used in various fields of research and many applications. Almost every part of the electromagnetic spectrum, ranging from radio waves to hard X-rays, has been put to work for the benefit of mankind. Therefore, it is not surprising that, despite a huge variety of existing sources of electromagnetic waves, there is a continuous demand for novel sources with improved properties tailored to particular needs.

This thesis reports on laser wakefield acceleration, a radically new approach for particle acceleration that builds on the huge electric fields that a plasma wave can provide. In this approach, an ultra-short laser pulse of high intensity is sent through a plasma. At sufficient intensity, the radiation pressure of the pulse expels a significant number of plasma electrons from the beam path while the ions remain at an almost fixed position due to their higher mass. This leads to a traveling charge separation wave. Associated with this wave is a traveling electric field distribution called the laser wakefield, that provides huge field strengths of hundreds of GV/m, which offer the potential to accelerate particles thousands of times more quickly than that which is currently possible with conventional accelerators. However, current experiments demonstrating the basic working of laser wakefield acceleration suffer from a fundamental lack of control, large shot-to-shot fluctuations and poor scalability. This is due to the fact that in all of the current schemes the injection of electrons and their subsequent acceleration is intrinsically coupled, because they are based on nonlinear dynamics such as wavebreaking in order to inject electrons from the plasma itself. This is why there is now a growing perception that electron bunches need to be injected from a separate external accelerator, so that the injection can be controlled. This thesis provides a theoretical investigation into the injection of electron bunches from an external accelerator. We investigate three methods in which the relatively long bunches from rf accelerators can be injected into a wakefield in such a way that the accelerated bunches attain a high quality. These schemes differ in their timing and thus in the position of the injected bunch with regard to the laser pulse. We investigated bunch injection behind the drive laser pulse, at an angle with the laser's propagation direction, and in front of the pulse. The final part of the thesis describes novel, ultrafast dynamics in laser wakefield accelerators. Previous experimental observations and theoretical investigations have shown that electron bunches can be generated with durations as short as a few femtoseconds. However, we predict that wakefield acceleration can generate significantly shorter bunches, with durations in the attosecond range.

In this thesis we have presented theoretical and experimental investigations of some fundamental properties and applications of nonlinear interaction between ultrashort optical pulses and matter.

This thesis reports on research performed on high harmonic generation and ion acceleration, processes that are based on the interaction of gaseous and solid media with high-intensity laser pulses. High harmonic generation leads to the production of beams of ultrashort, laser-like radiation in the wavelength range reaching from the extreme ultraviolet (XUV) to soft X-rays. Ion acceleration generates highly energetic protons and ions in the form of highly directional and pulsed beams with an ultrashort duration. The scope of this thesis is to present novel investigations and methods towards an enhancement of the flux and maximum energy of the accelerated ions and the harmonic radiation via an improved understanding of the underlying physics and, to provide a more complete identification of the actual requirements of the laser and the target for optimum output. The first part of this work concerns the construction and testing of a setup to generate high harmonics in gaseous media. The setup employs a capillary waveguide in which we implemented a novel manner of differential pumping. We performed an experiment on an enhancement technique, called Harmonic Excitation, and demonstrated for the first time that this technique can be applied in a capillary waveguide. The second part of the research concerns ion acceleration and high-order harmonic generation from solid-state targets using high-intensity laser pulses with ultra-high contrast. We investigated the coherence properties of high-order harmonic generation via coherent wake emission. Special attention was given to the preparation and investigation of extremely thin foils, i.e. freestanding nanofoils, to enter a very promising, but largely unexplored, regime. We experimentally investigated ion acceleration in this, so-called, transparent regime for the first time, and compared the results with predictions from improved numerical calculations. We also investigated for the first time high-order harmonic generation in the transparent regime, experimentally and via numerical modeling.