A polariton laser based on a bulk GaN microcavity

Simulations of polariton relaxation in a bulk GaN microcavity are performed by solving semi-classical Boltzmann equations. The structure which is considered is that of a recently grown microcavity showing strong coupling at 300 K. A kinetic phase diagram is calculated. We show that such a cavity can operate as a low-threshold polariton laser at room temperature.     

Polariton thermalization in GaN microcavities in the strong light–matter coupling regime

We discuss the thermalization of the polariton population from a bulk GaN λ/2 microcavity at room temperature. Simultaneous optical measurements of reflectivity and photoluminescence (PL), as well as time resolved measurements, reveal strong light–matter coupling. Transfer matrix theory is used to calculate reflectivity, absorption, and transmission coefficients for the structure. The PL emission is found to be thermalized, despite its very short lifetime, suggesting the existence of very fast energy relaxation channels.     

Influence of disorder on microcavity polariton linewidths

Polariton linewidths have been measured in a series of microcavities with different excitonic and cavity inhomogeneous broadening in the weak-disorder regime. We show experimentally that the behaviour of the polariton linewidths as a function of the detuning depends on the asymmetric line shape of an inhomogeneously broadened exciton line and particularly the disorder effect can be modulated and cancelled around resonance. When the disorder contribution is minimal, the behaviour of the cavity polariton linewidths tends to one of the homogeneous broadening system.     

Optical spectra of organic quantum dots in the strong coupling regime

We analyze the emission spectrum of a single organic quantum dot coupled to a microcavity in the strong coupling regime. We take into account non-linearities arising from both exciton–exciton interactions and the Pauli exclusion principle. We apply the recently developed Pade approximation for regular truncation of Green functions to calculate the emission spectrum. We show that as the number of excitations in the quantum dot increases the Rabi doublet evolves into a more complicated triplet structure.     

Coherent response of a semiconductor microcavity in the strong coupling regime

We have studied the coherent dynamics of a semiconductor microcavity by means of interferometric correlation measurements with subpicosecond time resolution in a backscattering geometry. Evidence is brought of the resolution of a homogeneous polariton line in an inhomogeneously broadened exciton system. Surprisingly, photon-like polaritons exhibit an inhomogeneous dephasing. Moreover, we observe an unexpected stationary coherence up to 8 ps for the lower polariton branch close to resonance. All these experimental results are well reproduced within the framework of a linear dispersion theory assuming a coherent superposition of the reflectivity and resonant Rayleigh scattering signals with a well-defined relative phase.     

Optical absorption by atomically doped carbon nanotubes under strong atom–field coupling

We analyze optical absorption by atomically doped carbon nanotubes with a special focus on the frequency range close to the atomic transition frequency. We derive the optical absorption line-shape function and, having analyzed particular achiral nanotubes of different diameters, predict the effect of absorption line splitting due to strong atom–vacuum–field coupling in small-diameter nanotubes.     

Influence of two-photon absorption on bistable switching in a silicon photonic crystal microcavity

By employing a simplified nonlinear coupled mode theory, we discuss the influence of two-photon absorption (TPA) on the characteristics of bistable switching. It is revealed that the critical value of frequency detuning for bistability rises linearly with increasing TPA coefficient k (when k is less than 30), and eventually access to a saturated value. It is also found that TPA effect will be enhanced for a greater frequency detuning, especially when transmission reaches its peak value. As a result, the peak transmission will decrease monotonously with the increasing frequency detuning. Based on this simplified model, the TPA-induced temperature rise in microcavity is also estimated. The theoretical predictions show good agreement with the simultaneous results, as well as the proposed experimental phenomena.     

Effects of transverse mode coupling and optical confinement factor on gallium-nitride based laser diode

We have investigated the transverse mode pattern and the optical field confinement factor of gallium nitride （GaN） laser diodes （LDs） theoretically. For the particular LD structure, composed of approximate 4 μm thick n-GaN substrate layer, the maximum optical confinement factor was found to be corresponding to the 5^th order transverse mode, the so-called lasing mode. Moreover, the value of the maximum confinement factor varies periodically when increasing the n-side GaN layer thickness, which simultaneously changes and increases the oscillation mode order of the GaN LD caused by the effects of mode coupling. The effects of the thickness and the average composition of Al in the AlGaN/GaN superlat.tice on the optical confinement factor are also presented. Finally, the mode coupling and optimization of the layers in the GaN-based LD are discussed.     

柱状与孔状图形衬底对MOVPE生长GaN体材料及LED器件的影响

在柱状图形蓝宝石衬底（PSS-p）和孔状图形蓝宝石衬底（PSS-h）上外延了GaN体材料和LED结构并进行了详细对比和分析.X射线衍射仪（XRD）和原子力显微镜（AFM）测试结果表明,PSS-h上体材料的晶体质量和表面形貌都优于PSS-p上体材料的特性,通过断面扫面电子显微镜（SEM）照片看出PSS-h上GaN的侧向生长是导致这种差异的原因.另外,基于PSS-p和PSS-h上外延的LED材料制作而成的器件结果表明,其20?mA下光功率水平相比普通蓝宝石衬底（CSS）分别提高了46%和33%.通过变温光荧光 关键词： 蓝宝石图形衬底 氮化镓 发光二极管 侧向生长     

Exciton binding energy and excitonic absorption spectra in a parabolic quantum wire under transverse electric field

The effects of transverse electric field on the energy levels of electron and heavy hole, exciton binding energy and excitonic absorption spectra of GaAs parabolic quantum wire are theoretically investigated in detail. The results indicate that the electron and hole energy levels, exciton binding energy, excitonic absorption coefficient and absorption energy becomes smaller with the increase of electric field. That is more significant at the condition of weaker parabolic confinement potential. The phenomena can be explained by the separation of overlap integral of the electron and hole at the ground states.     

Controlled waveguide coupling for photon emission from colloidal PbS quantum dot using tunable microcavity made of optical polymer and silicon

A tunable microcavity device composed of optical polymer and Si with a colloidal quantum dot (QD) is proposed as a single-photon source for planar optical circuit. Cavity size is controlled by electrostatic micromachine behavior with the air bridge structure to tune timing of photon injection into optical waveguide from QD. Three-dimensional positioning of a QD in the cavity structure is available using a nanohole on Si processed by scanning probe microscope lithography. We fabricated the prototype microcavity with PbS-QD-mixed polymenthyl methacrylate on a SOI (semiconductor-on-insulator) substrate to show the tunability of cavity size as the shift of emission peak wavelength of QD ensemble.     

Luminescent properties of GaN films grown on porous silicon substrate

GaN films have been grown on porous silicon at high temperatures (800-1050 °C) by metal organic vapor phase epitaxy. The optical properties of GaN layers were investigated by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL spectra recorded at 5 K exhibit excitonic emissions around 3.36-3.501 eV and a broad yellow luminescence at 2.2 eV. CL analysis at different electron excitation conditions shows spatial non-uniformity in-depth of the yellow and the band-edge emissions. These bands of luminescence are broadened and red- or blue-shifted as the electron beam penetrates in the sample. These behaviors are explained by a change of the fundamental band gap due to residual strain and the local thermal effect. It was found that the use of AlN buffer layer improves the crystalline quality and the luminescence property of GaN.     

Influence of carrier correlations on the excitonic optical response including disorder and microcavity effects

A model study of the characteristic signatures of carrier-correlation effects on the excitonic differential absorption spectra in semiconductors is presented. Using an effectively one-dimensional tight-binding system Coulomb-induced carrier-correlations up to third-order in the optical field are treated without additional approximations. To illustrate the influence of the different many-body contributions excitonic differential absorption spectra are computed for various polarizations and pump-probe time delays. The simultaneous influence of energetic disorder and correlations on the differential absorption spectra is discussed. Also presented are numerical results for the case when the semiconductor system is placed inside a microcavity in the strong-coupling regime. It is shown that the correlations induce characteristic signatures in the normal mode spectra. Received 22 January 1999     

Comparison study of GaN films grown on porous and planar GaN templates

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition(MOCVD)-GaN template by halide vapor phase epitaxy(HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence(PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.     

Synthesis and optical nondestructive evaluation of GaN nanorods on silicon surfaces with gold catalyst

Nanometric gallium-nitride rods were grown on a silicon (1 1 1) substrate through a chemical vapor deposition process with gold particles as the catalyst. Randomly distributed gallium-nitride rods of 20–200 nm in diameter and of various densities and lengths were formed under different deposition conditions. Characterization analyses, such as scanning electron microscopy and optical reflection spectroscopy, have been carried out on samples containing gallium-nitride rods different in size, shape, length and density. While the scanning electron microscopy shows directly the images of the sample surfaces, the optical spectroscopy provides a nondestructive evaluation of the sample surfaces, especially helpful for checking the uniformity of the samples.     

Spectrum of orientifold QCD in the strong coupling and hopping expansion approximation

We use the strong coupling and hopping parameter expansions to calculate the pion and rho meson masses for lattice Yang–Mills gauge theories with fermions in irreducible two-index representations, namely the adjoint, symmetric and antisymmetric. The results are found to be consistent with orientifold planar equivalence, and leading order 1/Nc$1/N_c$ corrections are calculated in the lattice phase. An estimate of the critical bare mass, for which the pion is massless, is obtained as a function of the bare coupling. A comparison to data from the two-flavour SU(2)$\mathit{SU}(2)$ theory with adjoint fermions gives evidence for a bulk phase transition at βc∼2${\beta }_c\sim 2$, separating a pure lattice phase from a phase smoothly connected to the continuum.     

基于光纤耦合长光程LED-DOAS技术的大气NO_2测量研究

大气中NO2的含量在10-9量级。研究了一种以发光二极管(LED)为光源的光纤耦合长光程差分吸收光谱(DOAS)系统,用于测量大气NO2。对比了四种不同类型和波段的蓝光LED,确定以中心波长451nm的CREE宝蓝LED为测量光源。在0.8km的测量光程、2min的测量时间分辨率的情况下,在445nm~465nm光谱反演波段内得到NO2的探测限为3.36×10-9。利用系统对大气NO2的浓度进行了一整天的连续观测,通过对吸收光谱的分析计算,反演出的大气NO2浓度在(7~31)×10-9之间变化。测量结果表明,将光纤耦合技术与LED光源的长光程DOAS系统相结合后,可实现大气NO2的高灵敏、高时间分辨率探测。     

Modelling of strong coupling regime in bulk GaN microcavities using transfer matrix and quasiparticle formalisms

The evolution of the polaritonic Rabi splitting versus the excitonic broadenings is analyzed within two different formalisms: the transfer-matrix and the quasiparticle models. The splittings are deduced from angle-resolved reflectivity measurements performed at 5 K and 300 K on two GaN microcavities grown on silicon. Although the quasiparticle model allows a qualitative analysis when the quality factor of the cavity is high, the transfer-matrix formalism is more appropriate for the interpretation of the reflectivity spectra, whatever the configuration of the structure.