Energy relaxation of excitonlike polaritons in semiconductor microcavities: Effect on the parametric scattering of polaritons

Polariton emission in GaAs-based microcavities has been studied under variable conditions, which made it possible to excite (a) polaritons from the upper polariton branch and hot free polaritons and electrons, (b) polaritons from the lower polariton branch (LPB) and localized excitons, and (c) the mixed system. Variation of the excitation conditions leads to substantial differences in the energy distributions of polaritons and in the temperature dependences of polariton emission. It is established that the energy relaxation of resonantly excited LPB polaritons via polariton and localized exciton states at liquid helium temperatures is ineffective. Instead, the relaxation bottleneck effect is suppressed with increasing temperature by means of exciton delocalization (due to thermal excitation by phonons). The most effective mechanism of relaxation to the LPB bottom is via scattering of delocalized excitons on hot free carriers. It is found that the slow energy relaxation of polaritons excited below the free exciton energy can be significantly accelerated at low temperatures by means of additional weak generation of hot excitons and, especially, hot electrons. This acceleration of the energy relaxation of polaritons by means of additional overbarrier photoexcitation sharply decreases the barrier for stimulated parametric scattering of polaritons excited at an LPB inflection point. Therefore, additional illumination can be used to control the polariton-polariton scattering.     

非对称DBR-金属-DBR结构的光学Tamm态理论研究

作为一种特殊的金属表面态, 光学Tamm态 (OTS) 对光的控制和操作具有独到优势, 在新一代光子器件设计中备受青睐. 本文基于分布式Bragg反射镜(DBR)-金属-DBR(DMD)结构, 通过金属两侧 DBR中心频率的失配引入不对称机制, 设计和控制可见光区域OTS的产生; 通过分析反射谱及电场分布特性, 揭示了金属两侧OTS的相互作用及变化规律. 结果表明: DMD结构可支持两个不同本征波长OTS 存在, 失配量δ将影响两个OTS的强度及本征波长, 即随着δ变化OTS 出现上下两个分支; 同时, 入射光的偏振态、入射角等也对OTS的强度及本征波长具有明显影响. 关键词： 金属表面态 光学Tamm态 分布式Bragg反射镜     

Effects of the piezoelectric field on the modulation of exciton–polaritons by surface acoustic waves

Surface acoustic waves (SAWs) provide a powerful tool for the modulation of polaritons in GaAs-based microcavities. In this contribution, we compare the modulation introduced by SAWs propagating along piezoelectric and non-piezoelectric crystal directions of the sample surface. Strain calculations reveal that the type-I band-gap modulation induced by the strain field is comparable for both SAW types. Piezoelectric SAWs have, however, an intrinsic longitudinal electric field, which can dissociate quantum well (QW) excitons and, thus, degrade the modulation. Images of the polariton far-field photoluminescence reveal this behavior for different excitation conditions.     

平面金属等离激元美特材料对光学Tamm态及相关激射行为的增强作用

本文对具有类EIR色散特性的平面金属等离激元美特材料(planar plasmonic metamaterials, PPM)对光学Tamm态及相关激射行为的增强作用进行了研究. 我们首先运用传输矩阵方法分析了利用PPM结构的色散来增强光学Tamm态对应模式电磁局域密度的可能性. 其次, 我们将具有类EIR特性的PPM与一维光子晶体(photonic crystal, PC)合在一起设计了一种平面等离激元美特材料-光子晶体(PPM-PC)异质结构. 研究发现, 通过在电磁局域密度最高的PPM结构中(或附近)加入增益介质, 可观察到比通常光学Tamm态更强的激射增强效应及更明显的单色性响应. 这些特性使得这种PPM-PC结构有望被应用于低阈值激光器、荧光增强等方面.     

Polariton-polariton kinematic interaction modulation and kinematic bipolaritons in organic microcavities

We study polariton-polariton kinematic interactions in organic microcavities. Using the Agranovich-Toshich transformation, to transform the Frenkel excitons from Paulions into Bosons, the exciton-exciton kinematic interaction is derived. In the strong coupling regime, the polariton excitonic part results in the polariton-polariton kinematic interaction. The scattering amplitude is calculated and the effective potential is obtained for a scattering between two free polaritons. The effective potential can be modulated by changing the exciton-cavity photon detuning, and we show the crossover of the effective potential from attractive into repulsive one. A pole in the two-particle Green's function is the signature of the formation of polariton bound state, i.e. bipolariton. Due to the smallness of the polariton effective mass, the obtained bound state is very shallow and appears below the minimum of the lower polariton branch, and falls inside the natural bandwidth of the polariton branch.     

Bose-einstein condensation of exciton polaritons in high-<Emphasis Type="Italic">Q</Emphasis> planar microcavities with GaAs quantum wells

Condensation of exciton polaritons in planar microcavities with GaAs/AlAs quantum wells in the active area has been studied. It has been found that an increase in the lifetime of polaritons up to ∼10–15 ps when the Q factor of a microcavity exceeds 7000 makes it possible to detect Bose-Einstein condensation of polaritons with a dominant (>90%) photon component. Condensation occurs under thermodynamically nonequilibrium conditions in lateral traps with diameters ∼10 μm formed due to long-range fluctuations of the polariton potential. The violet shift of the polariton emission line at the condensation threshold significantly exceeds the energy of the repulsive interaction between polaritons in the condensate. It has been shown that the shift is mainly due to a decrease in the oscillator strength of bright excitons in lateral traps, caused by the localization of photoexcited long-living dark excitons.     

Dynamics of microcavity polaritons in the Markovian limit

A master equation for the reduced density matrix of the microcavity polaritons coupled with the reservoir of high energy excitons is derived. It is allowed both the polaritons and the excitons to be self-interacting systems. Long time asymptotic properties of the polariton population is studied in the whole range of the reservoir temperatures and the corresponding decoherence effects are reported.     

Multiple optical Tamm states at a metal-dielectric mirror interface

The reflectivity map is theoretically investigated by applying the transfer matrix method in a metal-dielectric mirror structure at normal incidence. The existence of multiple optical Tamm states (OTSs) is demonstrated in this structure. It is found that an OTS is periodic resonance with the variation of the top-layer thickness of the dielectric mirror for a given wavelength. The energy for the corresponding OTS mainly depends on the thickness of the top layer. The appropriate thicknesses of metal film are proposed for the convenient observation of OTSs. This work may be useful in designing a new type of multichannel filter in optical communication systems.     

The interference of additional waves of forbidden polaritons excited by allowed polaritons in CuGaS2

Additional waves of exciton polaritons are studied in thin (1.5–1.8 μm) CuGaS₂ crystals at 9 K. The reflectivity spectra show a fine structure related to the interference of Fabry–Perot and additional waves which is a consequence of the polariton spatial dispersion. The main parameters of the exciton polaritons were determined from the spectra calculations. The Γ₄ excitons of big oscillator strength are shown to excite the additional polariton waves of the Γ₅ excitons of small oscillator strength, which interfere determining the fine structure in exciton resonance optical spectra.     

Crossover from exciton to biexciton polaritons in semiconductor microcavities

Pump-probe measurements in a microcavity containing a quantum well show that a population of circularly polarized ( sigma(+)) excitons can completely inhibit the transition to sigma(-) one-exciton states by transferring the oscillator strength to the biexcitonic resonance. With increasing pump intensity the linear exciton-polariton doublet evolves into a triplet polariton structure and finally into a shifted biexciton-polariton doublet. A theoretical model of interacting excitons demonstrates that the crossover from exciton to biexciton polaritons is driven by three-exciton Coulomb correlation.     

Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity

A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole–light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.     

Second-order time correlations within a polariton Bose-Einstein condensate in a CdTe microcavity

Second-order time correlations of polaritons have been measured across the condensation threshold in a CdTe microcavity. The onset of Bose-Einstein condensation is marked by the disappearance of photon bunching, demonstrating the transition from a thermal-like state to a coherent state. Coherence is, however, degraded with increasing polariton density, most probably as a result of self-interaction within the condensate and scatterings with noncondensed excitons and polaritons. Such behavior clearly differentiates polariton Bose condensation from photon lasing.     

Bose condensation of exciton polaritons in an optical microcavity

Two methods are considered for producing traps for exciton polaritons in an optical microcavity with an embedded quantum well. The first method for controlling polaritons consists in producing a polariton trap governed by the longitudinal confinement of photons. Traps of this type can be created using an optical microcavity with a variable width. In traps of the second type, the exciton confinement is ensured by a weak potential that is applied to a quantum well with excitons or when this well is subjected to an inhomogeneous deformation. The behavior of a two-component Bose condensate of photons and excitons is analyzed theoretically. The Bose condensate is described by the coupled system of equations of the Gross-Pitaevskii type. The approximate wave functions and the spatial profiles of coupled photon and exciton condensates are obtained.     

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.     

Spin-dependent coexistence of weakly coupled and strongly coupled modes in semiconductor microcavities

The dependence of the transition from the strong-coupling regime to the weak-coupling regime on the polariton spin orientation in a InGaAs semiconductor microcavity is experimental studied by means of time-resolved photoluminescence. Polaritons are created by non-resonant circularly-polarized optical excitation and the power intensity that breaks the strong coupling is found to be much lower for co-polarized polaritons than that for cross-polarized polaritons. Coulomb screening effects alone cannot explain the stronger loss of oscillator strength for majority excitons (co-polarized) and spin-dependent mechanisms are required to justify such behaviour.     

Spin-dependent polariton–polariton scattering in planar microcavities

We present a microscopic theory of polariton–polariton (PP) scattering in quantum microcavities, which is developed with allowance for the composite nature of polaritons. Analytical estimations of the effective scattering rate for PP scattering with parallel spin configuration are presented, and the role of dark excitons in the opposite spin configuration is discussed.     

Exciton polariton in an organic–inorganic multiple-quantum-well crystal (C4H9NH3)2PbBr4

Exciton polariton in an organic–inorganic multiple-quantum-well (MQW) single crystal (C₄H₉NH₃)₂PbBr₄ at low temperature has been investigated by photoluminescence excitation (PLE), reflection, and time-resolved photoluminescence measurements. Since (C₄H₉NH₃)₂PbBr₄ has ideal two-dimensional excitons with an extremely large oscillator strength and forms self-organized MQW with a very short well-period (d～10 Å), polaritonic coupling among excitons is strong and extends over a large number of wells. Therefore, observed MQW polariton features were the same as those of bulk polaritons. We have also investigated relaxation dynamics of the MQW polariton in the same framework as discussions on bulk polaritons.     

Spatial condensation of trapped polaritons in graphene and semiconductor structures

The theoretical and experimental status of the Bose–Einstein Condensation (BEC) of trapped quantum well (QW) polaritons in a microcavity is presented. The results of recent experiments that have shown the possibility to create an in-plane harmonic potential trap for a two-dimensional (2D) exciton polaritons in a cavity are discussed. We report the theory of BEC and of the trapped QW exciton polaritons in a microcavity. In addition, we study the BEC of trapped magnetoexciton polaritons in a graphene layer (GL) embedded in an optical microcavity in high magnetic field. In both cases the polaritons are considered to be in a harmonic potential trap. We compare the theoretical results with the existing experiments and discuss the experimental observation of predicted phenomena.     

Polarization selection rules in scattering of cavity polaritons

We theoretically discuss scattering of polaritons in semiconductor microcavities by means of a microscopic model. Taking into account the composite character of excitons (formed by an electron and a hole), we analyze the relation between polarizations of incoming and outgoing polariton states under resonant excitation by linearly polarized laser beams with opposite in-plane momenta. In addition to these polarization selection rules, we investigate the nonlinear processes up to the sixth order and we show the origin of an induced anisotropy due to the excitation beams which is responsible for the operation of an optical gate based on polariton–polariton scattering in a microcavity.     

Exciton warming in III–V semiconductors and microcavities

We present a time resolved experiment in which we dynamically tailor the occupation and temperature of a photogenerated exciton distribution in QWs by excitation with two delayed picosecond pulses. The modification of the excitonic distribution results in ultrafast changes in the PL dynamics. Our experimental results are well accounted by a quasiequilibrium thermodynamical model, which includes the occupation and momentum distribution of the excitons. We use this model and the two-pulse experimental technique to study the polariton dynamics in InGaAs-based microcavities in the strong coupling regime. In particular, we demonstrate that resonantly injected upper polaritons mainly relax to the lower polariton branch via scattering to large momentum polariton states, producing the warming of the polariton reservoir.