耦合量子阱中激子凝聚研究新进展

玻色-爱因斯坦凝聚成为探索量子世界的一种新方法,而且在半导体纳米结构中激子的凝聚研究取得了很大进展.实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离,显著提高了激子的冷却速度和寿命,成功地把激子冷却到1 K以下,观察到了激子的准凝聚状态.着重介绍冷激子系统凝聚现象、发光图案和宏观有序的激子态.理解这些简并激子系统的形成机理,为其在半导体纳米结构中最终实现玻色-爱斯坦凝聚提供新的机会.     

Exciton Dynamics and Optically Pumped Lasing in 1-Naphthylmethylamine-Based Quasi-2D Perovskite Films

Films of the quasi-2D perovskite based on 1-naphthylmethylamine (NMA) are promising as the gain medium for optically pumped lasing and future electrically pumped lasing because of its low lasing threshold and small electroluminescence efficiency rolloff. However, reasons for the low threshold and small efficiency rolloff are still unclear. Therefore, exciton dynamics are investigated in NMA-based quasi-2D perovskite films. It is found that quenching of bright excitons by other excitons or charge carriers is unlikely in NMA-based quasi-2D perovskite films, which is one reason for the low lasing threshold and small efficiency rolloff. Moreover, thermally stimulated current measurements reveal that the defect levels inside the band gap of the NMA-based quasi-2D perovskite are shallow, with a depth of ≈0.3 eV, causing a decrease in nonradiative exciton recombination through the defects. Therefore, population inversion can be easily achieved, leading to the low lasing threshold as well. For fabrication of NMA-based quasi-2D perovskite laser devices with even lower lasing thresholds, a circular-shaped optical resonator, and small-molecule-based defect passivation are used. Optically pumped lasing can be obtained from these devices, with a threshold of ≈1 µJ cm⁻², which is one of the lowest values ever reported in any perovskite lasers.     

Room-Temperature Exceptional-Point-Driven Polariton Lasing from Perovskite Metasurface

Excitons in lead bromide perovskites exhibit high binding energy and high oscillator strength, allowing for a strong light-matter coupling regime in the perovskite-based cavities localizing photons at the nanoscale. This opens up the way for the realization of exciton-polariton Bose–Einstein condensation and polariton lasing at room temperature – the inversion-free low-threshold stimulated emission. However, polariton lasing in perovskite planar photon cavities without Bragg mirrors has not yet been observed and proved experimentally. In this study, perovskite metasurface is employed, fabricated with nanoimprint lithography, supporting so-called exceptional points to demonstrate the room-temperature polariton lasing. The exceptional points in exciton-polariton dispersion of the metasurface appear upon optically pumping in the nonlinear regime in the spectral vicinity of a symmetry-protected bound state in the continuum providing high mode confinement with the enhanced local density of states beneficial for polariton condensation. The observed lasing emission possesses high directivity with a divergence angle of 1° over one axis. The employed nanoimprinting approach for solution-processable large-scale polariton lasers is compatible with various planar photonic platforms suitable for on-chip integration.     

纳米结构中激子研究进展

回顾了近年来耦合量子阱中激子玻色-爱因斯坦凝聚的研究进展和主要实验成果,提供了耦合量子阱中间接激子凝聚的实验证据和高度简并冷激子系统形成的实验条件以及形成机理.介绍了量子点中激子在量子信息中的最新发展,在纳米结构中的准粒子激子已经成为极有应用前景的发展目标.     

Bose condensation of exciton polaritons in microcavities

The current status of research on exciton polaritons in semiconductor heterostructures with microcavities and the collective properties of polaritons under the conditions of Bose condensation are discussed.     

Optical properties of submonolayer CdSe-(Zn,Mg)(S,Se) structures

The optical properties of structures with submonolayer inclusions of CdSe in a Zn(S,Se) matrix are studied. The submonolayer coating consists of a group of nanosized (40 Å) islands with a height of one monolayer. The exciton oscillator strength of multiple submonolayer CdSe-ZnSSe structures is substantially increased compared to the case of a uniform quantum well of comparable thickness and composition. In submonolayer structures lasing takes place immediately next to the energy of the ground state of the heavy exciton, in contrast with ordinary quantum wells of ZnCdSe, where it is strongly shifted to longer wavelengths by the energy of a single optical phonon. This effect results from the removal of the momentum selection rules during radiative recombination of excitons in submonolayer structures.     

Possibility of coherent light emission from Bose condensed statesin quantum-well systems

The possibility of coherent light emission in systems with a pairing of electrons and holes in a double quantum well spatially separated by a thin barrier [separated electron hole pairs (SEHPs)] is predicted. Bose condensation of excitons formed from SEHPs in GaAs/Al _xGa_(1-x)As double quantum wells of ~10 Å size are possible at relatively high temperatures. Coherent recombination emission due to the transfer of coherence from the Bose-condensed SEHPs to the photon field is expected     

Scattering of screened excitons by free carriers in semiconductingquantum well structures

The theoretical treatment of the scattering of excitons by free electrons and holes in a two-dimensional semiconducting quantum-well system is extended to take into account screening by the free carriers. The scattering cross sections are calculated using the Born approximation for elastic scattering of the excitons by the free carriers. For the heavy-hole exciton, the screening by the free carriers reduces the cross section for free-carrier exciton scattering for all values of the energy of relative motion of the free carriers and the excitors. For the light-hole exciton, however, screening can actually lead to an enhancement of the scattering cross section for low values of the energy of relative motion when the density of free carriers is high. This is because screening not only reduces the interaction between the free carriers and the exciton, but also decreases the binding of the exciton, leading to a larger effective radius of the exciton. The results for the scattering cross sections are then applied to calculate the contribution of the exciton linewidth due to elastic scattering of the excitons by free carriers. It is found that this contribution to the exciton linewidth is decreased below its value in the absence of screening for both the heavy- and light-hole excitons     

Optical transmittance of thin GaAs wafers upon laser pumping in the region of exciton resonances and the continuum of states: Exciton-exciton interaction

The transmittance spectra of thin “pure” GaAs bulk wafers upon optical pumping corresponding to the band of the ground state of the exciton series are recorded at a temperature of T = 1.7 K. The wafers were grown by molecular-beam epitaxy and vapor-phase epitaxy. An increase in the line amplitude and width upon pumping is observed, with no noticeable changes in the spectral position of the line peak. The increase is similar to that observed upon pumping in the continuum of states, but occurs at a somewhat lower rate. Estimation of the concentration of excitons created by pumping provides a means for determining the exciton-exciton interaction constant and comparing the result with known data. The integrated absorption method makes it possible to refine the exciton polariton-free charge carrier and exciton polariton-impurity interaction constants. The differential photoabsorption of the samples at the pumping modulation frequency is measured. The resultant modulated absorption spectra demonstrate the connection between induced absorption and the formation of differential spectra.     

Radiative transitions between exciton sub-bands in semiconductors

It is shown both theoretically and experimentally, that radiative Auger-recombination of excitons and impact excitation of excitons by hot excitons or electrons in direct-gap semiconductors at appropriate conditions lead to inverse population of exciton and bi-exciton subbands and, consequently, to stimulated submillimeter (far infrared) radiation. In CdS crystals, excited by nitrogen laser at temperatures from 1.4–4.2 K to 300 K, a stimulated submillimeter radiation on exciton transitions was observed. At T=1.4–4.2 K a few milliwatts of total output in 50–120 mkm spectral interval was obtained by Hg- lamp excitation. The strong correlation between phenomena taken place in visible and submillimeter regions was established. Our data permit to obtain the most fulfil picture of exciton and bi-exciton energy spectrum and to explain the structure of P-band in luminescence spectra.     

Exciton‐Charge Annihilation in Organic Semiconductor Films

Time‐resolved optical spectroscopy is used to investigate exciton‐charge annihilation reactions in blended films of organic semiconductors. In donor–acceptor blends where charges are photogenerated via excitons, pulsed optical excitation can deliver a sufficient density of temporally overlapping excitons and charges for them to interact. Transient absorption spectroscopy measurements demonstrate clear signatures of exciton‐charge annihilation reactions at excitation densities of ≈10¹⁸ cm^?3. The strength of exciton‐charge annihilation is consistent with a resonant energy transfer mechanism between fluorescent excitons and resonantly absorbing charges, which is shown to generally be strong in organic semiconductors. The extent of exciton‐charge annihilation is very sensitive not only to fluence but also to blend morphology, becoming notably strong in donor–acceptor blends with nanomorphologies optimized for photovoltaic operation. The results highlight both the value of transient optical spectroscopy to interrogate exciton‐charge annihilation reactions and the need to recognize and account for annihilation reactions in other transient optical investigations of organic semiconductors.     

Photoluminescence excitation spectroscopy of the dense exciton gasinvolved in the lasing transition in ZnSe epitaxial layers

The lasing transition in ZnSe epitaxial layers has been investigated at 77 K. The lowest lasing threshold (I_th) was achieved when the layers were resonantly excited at the photon energy of the exciton level. It was found that the exciton level at the excitation intensity just above the I_th was red-shifted by about 16 meV compared with the free exciton line (2.792 eV) under weak excitation condition. The energy difference between the exciton line and the lasing peak was about 19 meV at I=I_th and increased with increasing excitation intensity up to I=8×I_th. This suggests that the stimulated emission occurs due to the inelastic exciton-exciton scattering process at this temperature     

Structural and Optical Properties of ZnO Nanotips Grown on GaN Using Metalorganic Chemical Vapor Deposition

ZnO nanotips are grown on epitaxial GaN/c-sapphire templates by metalorganic chemical vapor deposition. X-ray diffraction (XRD) studies indicate that the epitaxial relationship between ZnO nanotips and the GaN layer is (0002)ZnO||(0002)GaN and (101̄0)ZnO||(101̄0)GaN. Temperature-dependent photoluminescence (PL) spectra have been measured. Sharp free exciton and donor-bound exciton peaks are observed at 4.4 K with photon energies of 3.380 eV, 3.369 eV, and 3.364 eV, confirming high optical quality of ZnO nanotips. Free exciton emission dominates at temperatures above 50 K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. The thermal activation energies of the bound excitons at 3.369 eV and 3.364 eV are 11 meV and 16 meV, respectively. Temperature-dependent free A exciton peak emission is fitted to the Varshni’s equation to study the variation of energy bandgap versus temperature.     

High‐Quality Whispering‐Gallery‐Mode Lasing from Cesium Lead Halide Perovskite Nanoplatelets

Semiconductor micro/nano‐cavities with high quality factor (Q) and small modal volume provide critical platforms for exploring strong light‐matter interactions and quantum optics, enabling further development of coherent and quantum photonic devices. Constrained by exciton binding energy and thermal fluctuation, only a handful of wide‐band semiconductors such as ZnO and GaN have stable excitons at room temperature. Metal halide perovskite with cubic lattice and well‐controlled exciton may provide solutions. In this work, high‐quality single‐crystalline cesium lead halide CsPbX₃ (X = Cl, Br, I) whispering‐gallery‐mode (WGM) microcavities are synthesized by vapor‐phase van der Waals epitaxy method. The as‐grown perovskites show strong emission and stable exciton at room temperature over the whole visible spectra range. By varying the halide composition, multi‐color (400–700 nm).WGM excitonic lasing is achieved at room temperature with low threshold (~ 2.0 μJ cm^?2) and high spectra coherence (~0.14–0.15 nm). The results advocate the promise of inorganic perovskites towards development of optoelectronic devices and strong light‐matter coupling in quantum optics.     

Exciton emission of crystalline Zn(S)Se thin films arranged in microcavities based on amorphous insulating coatings

A technology for the production of fully hybrid microcavities on the basis of Zn(S)Se films and amorphous insulating SiO₂/Ta₂O₅ coatings is proposed. The influence of all stages of the manufacturing cycle on the structure of exciton states in the Zn(S)Se films is demonstrated. This influence is reduced to four main effects: the appearance of a fine structure of emission lines related to free excitons; a decrease in the relative contribution of excitons bound at neutral acceptors to the exciton-emission spectrum; a shift of the emission lines related to exciton–impurity complexes and free excitons to lower frequencies; and a decrease in the splitting between emission lines related to heavy and light excitons. Samples of fully hybrid microcavities, in which the high structural and optical quality of Zn(S)Se films is retained, are fabricated.     

Interaction of coherent confined optical modes in neighboring 3D cylindrical ZnO microcavities

Luminescent properties of pairs of neighboring variously spaced 3D cylindrical ZnO microcavities 1.8 μm in diameter, produced by electron-beam lithography and reactive ion etching, are studied. Narrow luminescence peaks in the ZnO exciton spectral region, related to single-mode lasing, were observed. The energy of exchange interaction of coupled modes of two microcavities was calculated as a function of the intercavity distance. Broadening of the line of stimulated UV luminescence associated with coupled photonic modes of two microcavities was observed as the intercavity distance decreased.     

单层二硫化钨的荧光特性研究

单层二硫化钨的介电屏蔽效应弱,具有大的激子束缚能和强的荧光发射,是直接带隙 半导体。利用自制荧光显微光谱成像实验装置对单层二硫化钨进行了荧光性能的测试 和分析。结果表明,单层二硫化钨的荧光发射峰处于620 nm 附近。随着激光功率的 增大,单层二硫化钨的荧光强度增大,其荧光发射峰发生明显的红移,且荧光峰位红 移量和荧光强度受衬底类型调控。激子行为分析表明,中性激子和带电激子的峰值强 度比是衬底依赖的。     

Effect of surface on the excitonic characteristics of semiconductors

The effect of surface treatments on the main characteristics of excitons in the subsurface region of semiconductors (for GaAs), as well as the spatial distribution of main characteristics of excitons (for CdS), was studied. An analysis of experimental data showed that the deposition of insulator layers with a lower dielectric constant on the surface of the semiconductor resulted in an enhancement of the exciton-phonon interaction and an increase in the exciton binding energy. The appearance of the surface layer with a higher defect concentration increasing after some surface treatments results in the lowering of the exciton binding energy in the subsurface region and also in the weakening of the exciton-phonon interaction.     

On the condensation of exciton polaritons in microcavities induced by a magnetic field

The photoluminescence spectra of exciton polaritons in microcavities under conditions of three-dimensional quantization are studied as a factor of the density of the optical excitation and magnetic field. The behavior of the degree of circular polarization of the exciton luminescence in a magnetic field shows that, when the concentration of excitons increases, they condense at the lowest Zeeman sublevel.     

Exciton characteristics and exciton luminescence of silicon quantum dot structures

The exciton binding energy, the energies of the basic radiative exciton transition, and the zerophonon radiative lifetime of excitons in silicon quantum dots embedded in the SiO_x matrix are calculated in effective mass approximation with quadratic dispersion relation. In addition, the spectra of steady-state photoluminescence and of time-resolved photoluminescence of excitons in the silicon quantum dots are calculated, and the kinetics of the photoluminescence relaxation is considered. The theory is compared with the experiment. It is shown that, for nanostructures involving silicon quantum dots with diameters smaller than 4 nm, the governing factor in the broadening of the spectral photoluminescence bands is the effect of mesoscopic quantum fluctuations. In this case, either an even one dangling bond at the interface, or one intrinsic point defect, or one foreign atom located inside the small-sized nanocrystallite or in its close surroundings produces a pronounced effect on the energy of the exciton transition.