Stimulated emission of free excitons in Cd1−xMnxTe under nonresonant two-photon excitation

We report on free excitons coexisting with exciton magnetic polarons (EMPs) in bulk semimagnetic semiconductors of Cd_1−xMn_xTe for 0.04?x?0.36 at 2 K under nonresonant two-photon excitation. This two-photon excitation not only generates free excitons but also more efficiently creates EMPs compared with ordinary one-photon excitation. Stimulated emission from free excitons is demonstrated under strong two-photon excitation.     

A new emission band in the near band edge region in ZnSe single crystal

An emission band with FWHM of 2.7 meV has been observed at 2.8002 eV. Its emission intensity is proportional to the 1.2 power of the excitation intensity. Excitation spectrum and selective excitation spectrum do not show any additional spectral structure. It is proposed that the origin of this emission is the scattering of excitons by other free particles at a structural defect with slightly lower potential for free particles and free excitons.     

Mixing of singlet and triplet exciton states in highly excited CdS

Fluorescence spectra due to the free excitons have been studied in CdS at 4.2 K under various excitation levels. It has been found that the triplet-exciton emission intensity relative to the singlet line is enhanced remarkably with the excitation power density. This effect is reasonably explained by the mixing of the singlet with the triplet through the many body interaction. The density of optically generated excitons is determined from the magnetic field dependence of the triplet emission intensity.     

Photoluminescence from excitons in ZrS3

The photoluminescence spectrum of the layered ZrS₃ crystal shows several narrow emission lines under the excitation by an Ar⁺ laser in the wavelength range from 605–630 nm at 4.2 K. The excitation spectra for these emission lines and their temperature dependences suggest that the observed narrow emission lines originate from the radiative annihilation of indirect excitons bound to impurities. Some of these emission lines seem to be associated with the radiative annihilation of the indirect bound exciton with emission of phonons.     

Study of photoluminescence and electronic states in nanophase strontium titanate

Nanophase strontium titanate specimens were prepared by a stearic acid sol-gel process. The phase structures, photoluminescence, and electron states were investigated by X-ray diffraction and fluorescence spectroscopy. A visible emission band centered at 500 nm was observed in the nanosized particles under an excitation with energy lower than the band gap at room temperature. The peak intensity of the visible emission band increases with decreasing particle size. This visible emission is attributed to the recombination of the long-lived self-trapped excitons with forceful binding-energy formed in the nanosized SrTiO₃ mediated by the localized levels which are correlated to the intrinsic surface states and defect centers via a strong electron-phonon interaction. The dependence of the visible emission band on excitation wavelength was examined under both room and liquid nitrogen temperatures where there exist a series of levels of the self-trapped excitons within the forbidden gap and the properties of the self-trapped states are closely related to temperature.     

纳米结构ZnO晶体薄膜室温紫外激光发射

文章综述了纳米结构的氧化锌半导体薄膜在室温下自由激子的自发辐射以及由自由激子引起的受激发射的特性，阐述了在不同激发密度下室温紫外受激发射的机理．纳米结构氧化锌半导体薄膜是用激光分子束外延(L-MBE)技术生长在蓝宝石衬底上的．薄膜由密集而规则排列的纳米尺度的六角柱组成．这些纳米六角柱起着限制激子运动的作用，激子的量子尺寸效应，使激子的跃迁振子强度大幅度增强．同时六角柱之间的晶面组成了一个天然的激光谐振腔．室温下用三倍频的YAG脉冲激光激发，可从这些纳米结构的氧化锌薄膜中观测到很强的紫外激光发射．研究发现，在中等激发密度下，紫外受激发射是由于激子与激子间碰撞而引起的辐射复合．在高密度激发条件下，由于激子趋于离化，紫外受激发射主要由电子-空穴等离子体的辐射复合引起．由于纳米结构中激子的跃迁振子增强效应，在室温下测量到的光学增益高达320cm^-1，这比在同样条件下测量到的块状氧化锌晶体的光学增益要高一个量级以上．与传统的电子-空穴等离子体激光辐射相比，激子引起的受激发射可在较低的激发密度条件下实现．这在实际应用上很有价值．     

偏置电场对聚对苯乙烯激发态弛豫特性的影响

为探讨洞悉电场对有机发光二极管电致荧光量子效率的影响,通过激发-探测超快光谱技术研究了激子在电场下的瞬态行为.与单重态激子相应的激发态在230 μJ/cm2激发强度下,显示了快慢两个弛豫过程. 快慢组分的权重因子及快组分弛豫时间常数是电场相关的, 在6.4×10⁵ V/cm的电场下,与无偏置电场相比,激子的快组分弛豫时间加速,快组分的权重因子由22%增加为72%,约50%的初始激子又通过电场而离解. 慢组分是电场无关的,其弛豫时间常数为890 ps. 实验结果还揭示了由激发光所产生的长程声学声子,其声速为17 /ps. 关键词： 聚对苯乙烯 超快光谱 激子 有机发光二极管     

The luminescence of II–VI compounds with blende type structure under high excitation

The luminescence of ZnSe is investigated as a function of excitation intensity for temperatures between 5 K and 300 K. At low excitation we observe emission due to free and bound excitons and due to donor-acceptor pair recombination. At higher excitation, the emission is dominated by inelastic exciton-exciton and exciton-free carrier scattering at lower and higher temperatures, respectively. A “M-band” observed in ZnSe and ZnTe is tentatively ascribed to a biexciton decay. The biexciton binding energies are 2±1 meV for ZnSe and 1,5±1 meV for ZnTe.     

Spectral behavior of the emission around 3.31 eV (A-line) from ZnO nanocrystals

The emission at around 3.31 eV (A-line) from three types of ZnO nanocrystals with different particle sizes (10-1000 nm) was studied. The photoluminescence (PL) measurements were performed under different excitation densities and at different temperatures. The A-line emission exhibited a strong dependence on temperature and excitation power density. With increasing excitation density and temperature overlapping of the closely spaced first longitudinal optical (LO) phonon replica of free excitons by the A-line emission was observed.     

Quenching by electric fields of the luminescence of As2Se3 single crystals

High electric fields reduce the luminescence of As₂Se₃ at 77 K and increase simultaneously the photoconductivity. Comparison of both effects points to excitons as the source for emission of light whereas free carriers contributing to photoconductance recombine non radiatively. The field affects free excitons and their thermalization but does not ionize excitons trapped in radiative centres. At low excitation energy, in the range of indirect transitions, the field quenching of luminescence obeys a Poole-Frenkel behaviour. Such interpretation yields an exciton binding energy of 50 meV. The quantum efficiency of luminescence decreases to higher photon energy. Part of this decrease is related to an increase of the photoconductivity.     

Exciton luminescence and many-body effect of monolayer WS2 at room temperature

Monolayer transition metal dichalcogenides favor the formation of a variety of excitonic quasiparticles, and can serve as an ideal material for exploring room-temperature many-body effects in two-dimensional systems. Here, using mechanically exfoliated monolayer WS₂ and photoluminescence (PL) spectroscopy, exciton emission peaks are confirmed through temperature-dependent and electric-field-tuned PL spectroscopy. The dependence of exciton concentration on the excitation power density at room temperature is quantitatively analyzed. Exciton concentrations covering four orders of magnitude are divided into three stages. Within the low carrier concentration stage, the system is dominated by excitons, with a small fraction of trions and localized excitons. At the high carrier concentration stage, the localized exciton emission from defects coincides with the emission peak position of trions, resulting in broad spectral characteristics at room temperature.     

Bosonic stimulation of cold 1s excitons into a harmonic potential minimum in Cu2O

The density distribution of cold exciton clouds generated into a strain-induced potential well by two-photon excitation in Cu₂O is studied at 2 K. We find that an anomalous spike, which can be interpreted as accumulation of the excitons into the ground state, emerges at the potential minimum. The accumulation can be due to stimulated scattering of cold excitons, mediated by acoustic phonon emission. The possibility of the formation of the thermodynamic Bose-Einstein condensate of paraexcitons is discussed.     

Magnetoluminescence studies of excitonic scattering processes in high purity GaAs

The broad emission line at about 1.512 eV, which dominates the emission spectra of pure GaAs at intermediate excitation levels (1–300 kW/cm²) and low temperatures (<40 K) is investigated in magnetic fields up to magnetic flux densities of 10 T. The shift of the emission maximum in the magnetic field is exactly the same as recently reported for the free exciton. This demonstrates that at high excitation levels exciton-electron scattering is the dominant mechanism for the radiative decay of free excitons.     

Time-resolved luminescence spectra of AgBr at high excitation densities

Time-resolved luminescence measurements in AgBr at 1.9K and cw excitation densities up to 100 kW/cm² reveal new emission bands with strong vibrational structure close to the free exciton. Besides the emission of excitons bound to isoelectronic I^- traps two different luminescence systems are separated by making use of their different lifetime and excitation behaviour. The results are discussed in view of the electron-hole-liquid recombination recently found in the investigated spectral range.     

Femtosecond dynamics of secondary radiation formation from quantum well excitons

The time-resolved secondary emission of resonantly created excitons in GaAs quantum wells is studied using femtosecond up-conversion spectroscopy. The behaviour of the rise and decay of the secondary emission and reflectivity in quantum wells is strongly dependent upon the disorder at the interfaces, the exciton density and the temperature. In the case of low densities and temperatures the emission is independent of the exciton density and rises quadratically in time, in excellent agreement with recent theory for Rayleigh scattering from two-dimensional excitons subjected to disorder. These rise times are compared directly with times measured by time-integrated four-wave mixing (FWM). The comparison of the dynamics displayed in time-resolved secondary radiation and time-integrated FWM provide a clear understanding of the coherence properties of QW excitons in the first few picoseconds after excitation. High-contrast oscillations that are due to quantum beats between the heavy- and light-hole 1s-states are seen. The visibility decay at very low densities is long ps and is related to the action of potential fluctuations on the scattering of heavy-hole and light-hole excitons.     

SPECTROSCOPIC INVESTIGATIONS AND IDENTIFICATION OF THE OCCURRENCE OF MULTIPHOTON ABSORPTION PROCESSES IN CADMIUM IODIDE

Spectroscopic investigations for cadmium iodide (CdI₂) crystal have been carried out at 80 K under laser excitation. Due to the self-trapped excitons of CdI₂, a broad-band stimulated emission spectrum with its peak emission around 520 nm was achieved from the crystal under two-photon pumping by means of ruby laser. The photoluminescence (PL) studies in the crystal show overall nonlinear dependence of the PL peak intensity on input laser energy. The occurrence of slopes about 2 and 3 obtained in the desired energy range of interest for ruby and neodymium laser excitations, respectively, indicate corresponding photon absorption process. The photon absorption coefficients for the processes were calculated.     

A study of non-thermalized luminescence spectra: the case of Cu2O

The emission spectra of a non-thermalized distribution of excitons in Cu₂O have been studied experimentally and theoretically. The emission spectra were found to exhibit interesting dependence on both the excitation frequencies and on the sample temperatures. These experimental results are explained quantitatively by a simple model calculation of the exciton distribution in Cu₂O under continuous excitation. Using this model the exciton non-radiative lifetime was deduced from the emission spectra. In addition, the present theory accounts for the lineshape of the resonant Raman peaks in Cu₂O more satisfactorily than the existing theory.     

二维电子气等离激元太赫兹波器件

固态等离激元太赫兹波器件正成为微波毫米波电子器件技术和半导体激光器技术向太赫兹波段发展和融合的重要方向之一。本综述介绍AlGaN/GaN异质结高浓度和高迁移率二维电子气中的等离激元调控、激发及其在太赫兹波探测器、调制器和光源中应用的近期研究进展。通过光栅和太赫兹天线实现自由空间太赫兹波与二维电子气等离激元的耦合,通过太赫兹法布里-珀罗谐振腔进一步调制太赫兹波模式,增强太赫兹波与等离激元的耦合强度。在光栅-谐振腔耦合的二维电子气中验证了场效应栅控的等离激元色散关系,实现了等离激元模式与太赫兹波腔模强耦合产生的等离极化激元模式,演示了太赫兹波的调制和发射。在太赫兹天线耦合二维电子气中实现了等离激元共振与非共振的太赫兹波探测,建立了太赫兹场效应混频探测的物理模型,指导了室温高灵敏度自混频探测器的设计与优化。研究表明,基于非共振等离激元激发可发展形成室温高速高灵敏度的太赫兹探测器及其焦平面阵列技术。然而,固态等离激元的高损耗特性仍是制约基于等离激元共振的高效太赫兹光源和调制器的主要瓶颈。未来的研究重点将围绕高品质因子等离激元谐振腔的构筑,包括固态等离激元物理、等离激元谐振腔边界的调控、新型室温高迁移率二维电子材料的运用和高品质太赫兹谐振腔与等离激元器件的集成等。     

Photoluminescence and time-resolved photoluminescence of star-shaped ZnO nanostructures

Optical properties of star-shaped ZnO nanostructures were studied. The temperature-dependent photoluminescence (PL) was examined up to fourth-order longitudinal optical (LO) phonon assisted emissions of free excitons and confirmed that the nature of the room temperature PL in ZnO is 1-LO phonon assisted emission of free excitons. Low threshold ultraviolet stimulated emissions (SE) were obtained for our powder samples at room temperature. Picosecond time-resolved PL measurements detected a bi-exponential decay behavior which is strongly dependent on the excitation intensity: the slow decay term decreased faster than the fast decay term as the excitation intensity increased and the emission decays were dominated by the fast one. We also found that the emission decays decreased super-linearly before the appearance of the SE. This behavior may be used to deduce the threshold of SE or lasing.     

Low-temperature photoluminescence in AgGaSe<Subscript>2</Subscript> single crystals

Photoluminescence spectra from a single-crystalline AgGaSe₂ ternary compound grown by the Bridgman-Stockbarger method from a nonstoichiometric melt are studied in the temperature interval 8–300 K under various excitation levels. The spectra contain emission bands associated with donor-acceptor recombination, as well as with bound and free excitons. The exciton binding energy and the energy gap of the AgGaSe₂ crystals are evaluated. The temperature dependence of the energies of bound and free excitons, as well as of the energy gap of the crystals, is constructed.