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The formation of exciton-polaritons allows the transport of energy over hundreds of nanometres at velocities up to 10^6 m s^-1 in organic semiconductors films in the absence of external cavity structures.

Strong-coupling of monolayer metal dichalcogenide semiconductors with light offers encouraging prospects for realistic exciton devices at room temperature. However, the nature of this coupling depends extremely sensitively on the optical confinement and the orientation of electronic dipoles and fields. Here, we show how plasmon strong coupling can be achieved in compact robust easily-assembled gold nano-gap resonators at room temperature. We prove that strong coupling is impossible with monolayers due to the large exciton coherence size, but resolve clear anti-crossings for greater than 7 layer devices with Rabi splittings exceeding 135 meV. We show that such structures improve on prospects for nonlinear exciton functionalities by at least 10$^{4}$, while retaining quantum efficiencies above 50%, and show evidence for superlinear light emission., We acknowledge support from EPSRC grants EP/G060649/1, EP/L027151/1, EP/G037221/1, EPSRC NanoDTC, and ERC grant LINASS 320503. J.M. acknowledges support from the Winton Programme of the Physics of Sustainability. R.C. acknowledges support from the Dr Manmohan Singh scholarship from St John’s College, University of Cambridge. AIT and EMA acknowledge support from EPSRC grant EP/M012727/1, Graphene Flagship grant 696656, and ITN Spin-NANO 676108. CC acknowledges support from the UK National Physical Laboratory. CG acknowledges support by the A. v. Humboldt Foundation.

Gold nanostructure arrays exhibit surface plasmon resonances that split after attaching light harvesting complexes 1 and 2 (LH1 and LH2) from purple bacteria. The splitting is attributed to strong coupling between the localized surface plasmon resonances and excitons in the light-harvesting complexes. Wild-type and mutant LH1 and LH2 from Rhodobacter sphaeroides containing different carotenoids yield different splitting energies, demonstrating that the coupling mechanism is sensitive to the electronic states in the light harvesting complexes. Plasmon-exciton coupling models reveal different coupling strengths depending on the molecular organization and the protein coverage, consistent with strong coupling. Strong coupling was also observed for self-assembling polypeptide maquettes that contain only chlorins. However, it is not observed for monolayers of bacteriochlorophyll, indicating that strong plasmon-exciton coupling is sensitive to the specific presentation of the pigment molecules.

Light absorbers available at present provide far from optimal black-body performance. The need for more efficient absorbers is particularly acute on the microscale, where they can play a significant role in preventing crosstalk between optical interconnects, and also as thermal light-emitting sources. Several efforts have been made in this context to achieve near-total but directionally dependent absorption using periodic grating structures. However, the ability to absorb light completely for any incident direction of light remains a challenge. Here we show that total omnidirectional absorption of light can be achieved in nanostructured metal surfaces that sustain localized optical excitations. The effect is realized over a full range of incident angles and can be tuned throughout the visible and near-infrared regimes by scaling the nanostructure dimensions. We suggest that surfaces displaying omnidirectional absorption will play a key role in devising efficient photovoltaic cells in which the absorbed light leads to electron–hole pair production.

Laser-generated XUV and soft x-ray radiation sources using high-harmonic generation (HHG), typically producing 108 photons per pulse per harmonic (~5nm to ~50nm) in a coherent low-divergence beam [1], are a promising source for nanometer scale imaging [2]. Here we present XUV diffraction from a single-layer self-assembled hexagonal-close-packed (HCP) array of 200nm diameter PMMA nanospheres. Such a lattice is too small to diffract from using visible light, and hence XUV wavelengths are required. Figure (a) shows the far-field pattern from the single crystal at a 22µm focus, and overlaid with 100 mrad contours. The interference peaks are a convolution of a Bragg peak with both the distribution of lattice planes (due to crystal defects) and the harmonic spectra. However, as these convolutions are in orthogonal directions, they can be independently resolved (figures b & c), providing information on the crystal quality and generated wavelengths.The far-field diffraction pattern from a single-layer HCP array of spheres is the Fourier Transform (FT) of a hexagonal grid of delta functions multiplied by the FT of a the scattering function from a single sphere, which can be calculated using Mie theory. Figure (d) shows, (+) the measured intensity for each of the eighteen interference peaks for the harmonic at 27.6nm, (.) the theoretical intensity versus angle distribution for a single sphere, and (o) the predicted intensity at each interference peak. The excellent agreement indicates that Mie scattering predicts an appropriate form factor for this analysis and enables us to extract the complex refractive index of PMMA at this wavelength.

International audience

High Harmonic Generation (HHG) in a gas filled capillary provides a convenient source of coherent soft x-rays, which can be focussed using glass fibre tapers to provide an x-ray source to record diffraction and scattering images of sub-micron particles, to demonstrate the feasibility of moving down in scale to image single molecules. We demonstrate the ability to control and scan the x-ray spectrum. Control over the tuning can be achieved by using different gases, by changing the average intensity of the laser pulse or by varying the quadratic spectral phase of the laser pulse. We observe an ionization-dependent blue-shift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. The harmonic tuning is shown to depend on nonlinear spectral shifts of the fundamental laser pulse that are due to the plasma created by ionization. We can spatially resolve the x-ray spectra from a single grid diffraction image.

We describe the first non-destructive measurements of the evolution of the optical continuum along a non-linear waveguide. By utilizing near-field microscopy, the spectral variation can be imaged along and across the waveguide with sub-wavelength resolution.

We describe spectral measurements using NSOM of the nonlinear propagation of femtosecond pulses in waveguides. Nonlinearity produces self phase modulation, which broadens the spectrum of the propagating light as it travels along the guide. This process is the basis for supercontinuum sources that span octaves in frequency, which are important for metrology[1] and carrier-envelope phase measurement[2]. Our experiments uniquely measure the spectral evolution of this continuum as the light propagates along the waveguide by sampling the evanescent field with an uncoated NSOM tip, allowing measurement of the spectrum as it develops over long (mm) length scales, and also allowing the study of the submicron evolution of the spectra both along and across the guide. The evolution in one example is shown in figure 1(a). We will compare the measured continuum generation with our nonlinear propagation modeling, and discuss how the observed spectra differ from predictions of simple models.

We report the fabrication of metallic metamaterials using microstructured optical fibres as templates. The resulting fibres serve as excellent substrates for surface enhanced Raman spectroscopy and represent an exciting platform for in-fibre plasmonic devices.

We report the incorporation of thin films of a cyanine dye J aggregate into a versatile, length tunable, optical microcavity. The dense J-aggregate layers give an optical response that can be modified by embedding them at specific positions within heterostructures of dielectric and metal layers. The microcavities are composed of separate gold mirrors, which can be individually nanopositioned, and give sharp resonant modes in the red/near-infrared region of the spectrum. With the dye layer favorably placed, anticrossing behavior is observed as the cavity modes are successively swept through the absorption resonance. Large Rabi splittings of up to 170 meV are achieved at room temperature, agreeing well with predictions from a transfer-matrix model. These strongly coupled microcavities pave the way for microelectromechanical systems-integrated microdevices with tailored nonlinear optical properties.

Short pulse X-ray radiation at nanometer wavelength provides a nanoscale source for a wide range of applications in nanoscience and technology. We demonstrate the generation of coherent ultrafast soft X-rays by high harmonic generation in a capillary filled with argon gas. Harmonics up to the 29th order, at wavelengths down to 25nm, are obtained. The enhancement of particular harmonic intensities and tuning of wavelength position is achieved by controlling the argon ionization level either by changing the average intensity of the laser pulse or by varying the chirp of the laser pulse. We observe an ionization dependent blue shift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. We now aim to focus the x-ray beam with a tapered fibre to obtain a nanometer size spot for nanoscale imaging of small objects and to directly observe dynamics with simultaneously high temporal and spatial resolution.

We demonstrate the synthesis of PbSe nanocrystal quantum-dots using colloidal chemistry techniques [1]. These nanocrystals are found to have highly efficient, infrared luminescence from intrinsic quantum-confined states[2]. Based upon the sharp exciton absorption and photoluminescence (PL) emission spectra, a particle size distribution with a standard deviation of approximately 5% may be inferred. The wavelength of the PL may also be conveniently size-tuned in order to access the 1.3-1.5 microns "telecommunications window"[3]. The characterisation of PbSe nanocrystals in a variety of different optical environments is also reported. In particular, we examine the incorporation of nanocrystals into photonic structures, such as planar waveguides, and also cylindrical microcavities. In solution, these nanocrystals may also have use in infiltrating photonic structures for the purpose of achieving "liquid gain". The possibilities for using PbSe nanocrystals in a wide range of optoelectronic and telecommunications applications are discussed in the context of this work.

Self-phase modulation induced spectral broadening has been observed for ultrashort pulses propagating through Ta2O5 rib waveguide. The associated nonlinear refractive index was estimated to be 7.23 x10-19 m2/W, which is higher by one order of magnitude than silica glass.

We report ultrabroadband measurements on waveguides of photoluminescent silicon-rich silicon dioxide produced by plasma enhanced chemical vapor deposition. Material absorption below 700 nm and waveguide loss above 1300 nm leave a broad spectral region of good transmission properties, which overlaps with the photoluminescence spectrum of the core material. Proposed mechanisms for the material absorption and photoluminescence are discussed based on our findings.

The use of lasers to directly pattern optoelectronic devices primarily utilizes direct irradiation by UV light. We present here an alternative route using multi-photon absorption within a spherical focus in 3D space, thus allowing complex embedded structures to be directly written. In wide-bandgap materials such as chalcogenide, fluoride and silica glasses, our observations suggest free electrons are produced within the focus of a high-power infrared ultrashort pulse. The anisotropic interaction of this plasma with the incident pulse produces micron-sized DBR gratings of a 150nm pitch. An amplified Ti:S laser with 250kHz repetition rate, 150fs pulse duration, and wavelength tuned from 800-850nml is used to write embedded diffraction gratings and arrays of dots. The laser beam is focussed with a 50x objective into transparent polished samples, with pulse energies ranging from 0.1-1.1pJ (Fig.la). During the writing process broadband sub-bandgap UV light is emitted from a micron-sized spot at the sample focus. The written structures are permanent, typically with large refractive index changes on the order of Delta.n = +0.01 depending on the material.

Thermal poling of silica glass modified by femtosecond laser irradiation has been demonstrated. Evidence of second-order nonlinearity modulation between the treated and non-treated regions has been observed. The technique was used for χ(2) grating fabrication.

In recent years, the use of a focused femtosecond laser to directly write structures deep within transparent media has attracted much attention due to its ability to write in three-dimensions [1]. By utilizing an amplified Ti:sapphire laser (pulse duration 150fs, repetition rate 250kHz, λ=850nm), we have developed a novel technique that enables us to write grating structures within the bulk of an optical fiber through its cleaved face, allowing control over light subsequently exiting the fiber. Fig 1(a) shows a microscope image of an embedded diffraction grating having a 5µm pitch and buried 25µm below a fiber's cleaved face. Fig 1(b) displays a far-field pattern created by a single-mode fiber which has a two-dimensional grating written 5µm below its cleaved face. The controllability of the power and direction of diffracting orders offers tantalizing opportunities for new methods of optical routing. A further grating of 20µm period and 4µm line width has been directly-written into a silica glass plate (Herasil 1). Thermal poling was carried out on the plate in air for 15 minutes at 280°C with 4kV applied [2]. After poling the grating was investigated using a Nd:YAG laser (λ=1064nm), with a focused spot sue of 20µm. The second harmonic was subsequently imaged by CCD camera. Fig 2(a) shows the second harmonic produced in a Gaussian beam away from the grating region as expected. However, Fig 2(b) shows the second harmonic produced at the position of the irradiated lines to be much stronger indicating that the χ(2) is larger in the Ti:Sapphire irradiated regions. The incident beam is large enough to encompass two grating lines as indicated by the arrows. In Fig 2(a) the brightness is enhanced compared to Fig 2(b) to make the spot visible. By direct-writing periodic structures into silica fibers from the side and subsequent poling, work is currently underway to investigate the feasibility of achieving efficient quasi-phase-matching in silica fibres [3]. Fig.1. (a) A 5µm period diffraction grating written 25µm below a fiber's cleaved face. (b) Far-field pattern of single-mode light exiting a fiber containing an embedded 2-d grating. Fig.2. Imaging of the SHG (a) Inclined incidence and well away from the grating. (b) Inclined incidence and on the grating.

Recent work on polariton-polariton scattering in semiconductor microcavities under continuous wave excitation conditions is reviewed. For weak non-resonant laser excitation, a marked bottleneck in the polariton distribution is observed, but which is suppressed by polariton-polariton scattering as the laser intensity is increased. However, the high excitation conditions necessary to observe stimulated emission lead to loss of strong coupling and conventional lasing in the weak coupling regime, By contrast for resonant excitation, polaritons are created directly in the polariton trap formed by the microcavity dispersion curve. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the centre of the Brillouin zone, and to a companion state at high wavevector. The stimulation phenomena lead to condensation of the bosonic quasi-particles to two specific regions of k-space, and to the formation of a new state with macroscopic coherence. The prospects to achieve a polariton laser under conditions of non-resonant excitation are discussed.

Recently, the use of a focused femtosecond laser to directly write devices into the bulk of glass has become increasingly prevalent [1]. The physical damage created by such lasers, especially in the high power regime has been identified to display anisotropic properties such as anomalous anisotropic light scattering [2] and uniaxial birefringence [3]. So far, the microscopic processes underlying such anisotropies remain unclear. By writing embedded structures with a Ti:sapphire laser (repetition rate 250kHz pulse duration 150fs, λ=850nm, and beam width ~1.5µm), we have identified a further anisotropic property in silica - strong reflection from the modified region occurring only along the direction of polarization of the writing laser. Our analysis suggests that this effect is also the primary cause of all previously reported anisotropic phenomena. Fig. 1, which shows a schematic of embedded diffraction gratings and an array of 'dots' written into a silica plate, demonstrates the geometry of the phenomenon. We believe that the anisotropic reflectivity can only be explained as a consequence of Bragg reflection from a self organized periodic structure, produced by interference between the incident laser radiation and a plasmon-polariton wave generated within the sample. This is demonstrated in the magnified region of Fig. 1 which shows the structure within a single 1.5µm 'dot'. Figure 2(a) shows a microscope image of the edge of an array of 'dots'. Here, the writing polarization is directed out of the page. The observed anisotropic reflectivity (1% per 1.5µm 'dot') can be compared to Figure 2(b) which shows no reflection from the same object, viewed from an orthogonal direction. The spectrum given in Fig 2(c) shows the peak of reflection to be 460nm, indicating that the self organized structures have a characteristic period of ~150nm, and modulating refractive index variation of amplitude Δn~10. Fig. 1. Anisotropic reflection from embedded structures. The magnified region illustrates the self-organized periodic structuring Fig.2. Reflection from a 2-d array of 'dots' in directions (a) parallel and (b) perpendicular to writing laser. (c) Spectrum of reflection.

It is likely that the commercialisation of Photonic Crystal (PC) technology will require the high-volume capabilities provided by semiconductor nanofabrication processes. To this end, a Silicon-compatible PC waveguide could form the basic building block of an all-optical circuit. This presentation will discuss the development of PC research at Southampton University (UK) which began with deep photoelectrochemically etched holes in single-crystal Silicon. This initial work progressed to an all dielectric waveguide structure on a Silicon substrate, with the materials utilised being compatible with a CMOS process. PC nanofabrication and optical characterisation using these waveguiding structures will be described in detail.

Semiconductor microcavities can support quasiparticles which are half-light and half-matter with interactions possessed by neither component alone. We show that their distorted dispersion relation forms the basis of a quasiparticle "trap" and elicits extreme enhancements of their nonlinear optical properties. When driven by a continuous wave laser at a critical angle, the quasiparticles are sucked into the trap, condensing into a macroscopic quantum state which efficiently emits light. This device is thus an optical parametric oscillator based on quasiparticle engineering. In contrast to a laser, macroscopic coherence is established in the electronic excitations as well as the light field. This paves the way to new techniques analogous to those established in atomic and superconducting condensates, such as ultrasensitive solid-state interferometers.

GaAs microdisk lasers with holes pierced through the disk surface are investigated for their threshold characteristics. Disks are fabricated with either a single hole or two diametrically opposite holes at various distances from the disk outer edge. Even though the disk area is reduced by only 1%, we find that the lasing threshold for a disk with one hole is reduced by up to 50% compared to a disk with no hole. We attribute this reduction to the perturbation of nonlasing modes, which decreases the amplification of spontaneous emission in these modes and makes more carriers available to contribute to lasing.

We create a reservoir of hole traps in a resonant tunneling light emitting diode by etching the p-type contact into an array of nanometer scale pillars. In the off state, the charge reservoir keeps the light output extremely low, even at relatively high currents. The device can be switched on to produce light by raising the electron emitter past a confined electron state allowing holes to escape from the nonradiative region. The resulting electro-optic switch has an on/off ratio of at least 1000: 1, a large improvement over conventional resonant tunneling light emitting diodes.

Using a two-color ultrafast surface deflection spectroscopy, we demonstrate the time-resolved observation of acoustic phonon wave packets emitted from a single buried GaAs quantum well. A longitudinal acoustic phonon pulse is generated at a preselected depth within a highly confined region (>=10 nm thick) through the coherent nonequilibrium deforation potential which was previously unobserved in such structures. Subsequent detection with subpicosecond resolution at the surface resolves propagating high-wave-vector ballistic phonons and quasiballistic or "snake" phonons which subsequently merge into a quasidiffusive phonon pulse.