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Author

J.S. Roberts

Other affiliations: University of Southampton
Bio: J.S. Roberts is an academic researcher from University of Sheffield. The author has contributed to research in topics: Quantum well & Laser. The author has an hindex of 49, co-authored 486 publications receiving 10178 citations. Previous affiliations of J.S. Roberts include University of Southampton.


Papers
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TL;DR: This work experimentally demonstrates resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100 and utilizes boson amplification induced by stimulated energy relaxation.
Abstract: We experimentally demonstrate resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100. This new parametric phenomenon appears as a sharp angular resonance of the incoming pump beam, at which the moving excitonic polaritons undergo very large changes in momentum. Ultrafast stimulated scattering is clearly identified from the exponential dependence on pump intensity. This device utilizes boson amplification induced by stimulated energy relaxation.

653 citations

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TL;DR: A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity.
Abstract: A massive redistribution of the polariton occupancy to two specific wave vectors is observed under conditions of continuous wave excitation of a semiconductor microcavity. The “condensation” of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.

363 citations

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TL;DR: In this article, a strain-balance multiquantum well (MQW) was used to enhance the GaAs solar cell efficiency using a p-i-n diode structure.
Abstract: A strain-balance multiquantum well (MQW) approach to enhance the GaAs solar cell efficiency is reported. Using a p-i-n diode structure, the strain-balanced GaAsP/InGaAs MQW is grown on a GaAs substrate and equals a good GaAs cell in terms of power conversion efficiency. The cell design is presented together with measurements of the forward bias dark current density, quantum efficiency, and 3000 K light-IV response. Cell efficiencies under standard air mass (AM) 1.5 and AM 0 illumination are projected from experimental data and the suitability of this cell for enhancing GaInP/GaAs tandem cell efficiencies is discussed.

270 citations

Journal ArticleDOI
TL;DR: In this paper, a comprehensive theoretical and experimental study of linear exciton-light coupling in single and coupled semiconductor microcavities is presented: emphasis is given to angular dispersion and polarization effects in the strong-coupling regime.
Abstract: A comprehensive theoretical and experimental study of linear exciton-light coupling in single and coupled semiconductor microcavities is presented: emphasis is given to angular dispersion and polarization effects in the strong-coupling regime. The phase delay in the dielectric mirrors carries a nontrivial angle and polarization dependence. The polarization splitting of cavity modes increases with internal angle as ${\mathrm{sin}}^{2}{\ensuremath{\theta}}_{\mathrm{eff}}.$ Comparison with experimental results on a GaAs-based cavity with ${\mathrm{In}}_{0.13}{\mathrm{Ga}}_{0.87}\mathrm{As}$ QW's shows that a quantitative understanding of polariton dispersion and polarization splitting has been achieved. Coupling of two identical cavities through a central dielectric mirror induces an optical splitting between symmetric and antisymmetric modes. When QW excitons are embedded in both cavities at antinode positions, the system behaves as four coupled oscillators, leading to a splitting of otherwise degenerate exciton states and to separate anticrossing of symmetric and antisymmetric modes. These features are confirmed by experimental results on coupled GaAs cavities with ${\mathrm{In}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As}$ QW's. Finally, the polarization splitting in a coupled cavity is analyzed in detail and is in good agreement with the experimental findings.

242 citations

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TL;DR: In this article, the authors proposed an optical parametric oscillator based on quasiparticle engineering, which is a nonlinear resonator in which a coherent pump wave is converted into coherent "signal" and "idler" waves of different frequency.
Abstract: 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. Parametric oscillators are nonlinear resonators in which a coherent pump wave is converted into coherent ‘‘signal’’ and ‘‘idler’’ waves of different frequency, thus forming the basis for broadband tunable sources and mixers. 1 They have found widespread application in both microwave and optical frequency regions, as well as providing a ‘‘quantum testbed’’ for some of the most profound demonstrations of nonclassi

226 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, the authors present a comprehensive, up-to-date compilation of band parameters for the technologically important III-V zinc blende and wurtzite compound semiconductors.
Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

6,349 citations

Journal ArticleDOI
28 Sep 2006-Nature
TL;DR: A comprehensive set of experiments giving compelling evidence for BEC of polaritons of bosonic quasi-particles are detailed, which indicate the spontaneous onset of a macroscopic quantum phase.
Abstract: Phase transitions to quantum condensed phases—such as Bose–Einstein condensation (BEC), superfluidity, and superconductivity—have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 109 times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase. Bose–Einstein condensation (BEC), a form of matter first postulated in 1924, has famously been demonstrated in dilute atomic gases at ultra-low temperatures. Much effort is now being devoted to exploring solid-state systems in which BEC can occur. In theory semiconductor microcavities, where photons are confined and coupled to electronic excitations leading to the creation of polaritons, could allow BEC at standard cryogenic temperatures. Kasprzak et al. now present experiments in which polaritons are excited in such a microcavity. Above a critical polariton density, spontaneous onset of a macroscopic quantum phase occurs, indicating a solid-state BEC. BEC should also be possible at higher temperatures if coupling of light with solid excitations is sufficiently strong. Demokritov et al. have achieved just that, BEC at room temperature in a gas of magnons, which are a type of magnetic excitation. This paper presents a comprehensive set of experiments in which polaritons are excited in a semiconductor microcavity. Above a critical density of polaritons, massive occupation of the ground state at 19 K is observed and various pieces of experimental evidence point to a spontaneous onset of a macroscopic quantum phase.

2,527 citations

Journal ArticleDOI
TL;DR: The underlying self-imaging principle in multimode waveguides is described using a guided mode propagation analysis and it is shown that multimode interference couplers offer superior performance, excellent tolerance to polarization and wavelength variations, and relaxed fabrication requirements when compared to alternatives such as directional coupling.
Abstract: This paper presents an overview of integrated optics routing and coupling devices based on multimode interference. The underlying self-imaging principle in multimode waveguides is described using a guided mode propagation analysis. Special issues concerning the design and operation of multimode interference devices are discussed, followed by a survey of reported applications. It is shown that multimode interference couplers offer superior performance, excellent tolerance to polarization and wavelength variations, and relaxed fabrication requirements when compared to alternatives such as directional couplers, adiabatic X- or Y-junctions, and diffractive star couplers. >

2,477 citations