Stimulated emission processes in highly excited CdS at 80 K

The stimulated emission from CdS at 80 K under high excitation density is studied by means of quasi-resonant dye laser pumping. The evidence of exciton-exciton (P line) and exciton-electron (E line) scattering and, at the highest excitation level, of electron-hole plasma (EHP) recombination are reported and discussed also by means of optical gain measurements.     

Cooling of high density electron-hole plasma

We generalize for high density electron-hole plasma, the previous theories [12] of temperature cooling of non-equilibrium hot plasma. Especially we take into account the cooling by emission of mixed longitudinal optical phonon and plasmon modes, these quasiparticles described by a non-equilibrium distribution function. We show that a strong slowing of the plasma cooling occurs, at high electron-hole density. We calculate for CdSe the temperature kinetics of plasma created by Yag laser (pulse duration 30 ps).     

Role of electron-hole pairs in the mechanisms of desorption from semiconductor surfaces

In the presence of a dense electron-hole plasma (due to high power laser irradiation or resulting from a plasma driven phase transition or melting), it is shown that desorption is drastically enhanced. This permits us to propose a test for the validity of the hypothesis of the plasma driven melting in Si. Also the creation of surface excitons is shown to lead to enhanced desorption.     

射频磁控溅射法制备高质量ZnO薄膜的激光特性研究

用射频磁控溅射方法在SiO2衬底上制备ZnO薄膜。在室温下观测到了A、B激子吸收以及在19K下发现的A、B、C激子的反射表明所制备的ZnO薄膜具有很好的纤锌矿结构。我们获得了来自于电子空穴等离子体的受激发射。进一步研究我们发现由大量窄峰所组成的激光发射，窄峰的间距都为0.5nm左右。根据理论计算，产生激光发射的自成腔的长度为31.5 μm。我们认为ZnO薄膜中产生激光发射的自成腔的形成与其六角型结构有重要关系。     

Luminescence of high density electron-hole plasma in CdSe at elevated temperature

Luminescence of high density electron-hole plasma in CdSe is observed in the 77–300 K temperature range by picosecond pulse excitation. With increasing temperature from 77 K the stimulated emission band is replaced by the spontaneous emission band. Temperature changes of spectral features of these two bands and also their time dependence after pulse excitation are consistent with the theoretical consideration.     

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．     

Influence of the absorbed optical quanta energy on GHz ultrasound generation in GaAs

Experimental results of the investigation of the optoacoustic processes taetang place in GaAs semiconductor at ultrashort time scales are reported. Femtosecond laser has been used both for the generation (through the deformation potential mechanism by the interband absorption of laser radiation) and detection of GHz ultrasound waves. First experimental observation of an abrupt change in the phase of the photoexcited GHz ultrasound, when with increase of the energy of optical quanta direct generation of the electron-hole pairs in the side-valleys of GaAs becomes allowed by the energy and momentum conservation laws, is reported. We relate this observation, at least partially, to abrupt change in the ultrafast dynamics of photogenerated electron-hole plasma, in particular to deceleration of plasma diffusion when heavy carriers in the high energy side valley are photogenerated instead of light carriers in the lowest energy valley.     

Band gap collapse and ultrafast “cold” melting of silicon during femtosecond laser pulse

It is established experimentally that a high concentration of electron-hole plasma produced in silicon by femtosecond laser pulse induces a sequential “collapse” of the band gap in the [111] and [100] directions and leads to the formation of a “cold” metallic liquid phase during the pulse.     

磁控溅射制备ZnO薄膜的受激发射特性的研究

用射频磁控反应溅射法在二氧化硅衬底上制备ZnO薄膜。得到了在不同温度下ZnO薄膜的吸收与光致发光。观测到了纵光学波 (LO)声子吸收峰与自由激子吸收峰 ;室温 (30 0K)下 ,PL谱中仅有自由激子发光峰。这些结果证实了ZnO薄膜具有较高的质量。探讨了变温ZnO薄膜的发光特性。研究了ZnO薄膜的受激发射特性。     

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field,which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.     

Theory for the laser-induced femtosecond phase transition of silicon and GaAS

We analyze he femtosecond instability of the chamond lattice of silicon and GaAs, which is induced by a dense electron-hole plasma after excitation by a very imense laser pulse. We obtain that the electron-hole plasma causes an instability of both transverse acoustic and longitudinal optical phonons. So, within less than 200fs, the atoms are displaced more than 1 Å from their equilibrium position. The gap between the conduction and the valence band then vanishes and the symmetries of the diamond structure are destroyed, which has important effects on the optical reflectivity and second-harmonic generation. After that, the crystal melts very rapidly because of the high kinetic energy of the atoms. Note that mis is in good agreement with recent experiments done on Shand GaAs using a pump laser to excite a dense electron hole plasma and a probe laser to observe the resulting changes in the atomic and electronic structure.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Laserlike vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.     

Plasma induced optical nonlinearities and optical bistability in CdS

The transition from a low density gas of excitons to an electron-hole plasma is connected with strong variations of the optical properties. In this contribution we investigate both the optical nonlinearities produced in CdS by an intense laser beam in the spectral region close to the absorption edge and at the position of the laser itself. In a second part it is shown that the nonlinearities at the photon energy of the incident laser may lead to absorptive and dispersive optical bistability.     

High density photoluminescence induced by laser pulse excitation in InSe under pressure

Abstract

We report on high density photoluminescence (HDPL) measurements in undoped indium selenide under pressure at 300 K. Direct electron-hole plasma (DEHP) stimulated emission, induced by high density excitation, has been observed in InSe from room pressure up to 5.1 GPa. Spontaneous and stimulated emission bands observed in the spectra have been analyzed within the framework of the band gap renormalization theory (BGR) in a multi-valley scenario. The pressure coefficients of the spontaneous and stimulated emission bands have led us to attribute these bands to transitions from different minima in the conduction band, which show different renormalization energies determined by the effective masses and electron densities in each valley. Under high excitation, the direct to indirect crossover is shown to occur at a lower pressure than that observed in absorption measurements, as a result of the different renormalization energies of each transition.     

Formation of excitons in semiconductor nanostructures in the presence of electron-hole plasma

We propose a mechanism of increase in the binding energy of an exciton in wide band-gap semiconductors in the presence of optically pumped electron-hole plasma. These excitons with relatively high binding energy (>150 meV) can exist at room temperature when the dielectric constant of semiconductor in the infrared region of spectrum approaches zero. Calculations for CdS show that the density of electron-hole plasma should be higher than 10¹⁹ cm^?3 for formation of such excitons. We show that there exist a considerable number of close-lying energy levels of excitons with high binding energy in the forbidden band of the semiconductor. We guess that these excitons participate in the process of laser generation in optically pumped semiconductor nanocrystals.     

Nonthermal pulsed laser annealing of Si; plasma annealing

In the accompanying paper we have given evidence that pulsed laser annealing of Si does not involve normal thermal melting and recrystallization. Here we argue the importance of the electron-hole plasma produced by the laser to the annealing process.     

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of ～10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.     

Fine Structure of the Photoluminescence Spectrum of Diamond under the Multiple Emission of an Optical Phonon during the Autolocalization of Photoexcited Electrons

JETP Letters - The autolocalization of electrons photoexcited by laser pulses with a wavelength of 525 nm in the visible range and with a duration of 150 fs in an electron-hole plasma (density...     

Laser-induced sputtering of ZnO,TiO2, CdSe and GaP near threshold laser fluence

Sputtering of ZnO, TiO₂, CdSe and GaP induced by 10 ns laser pulses from a tunable dye laser has been investigated. It is shown that all of these materials exhibit the following characteristics: (1) non-linear dependence of sputtering yield on the excitation laser fluence, (2) existence of the threshold laser fluence, (3) dependence of the threshold laser fluence on laser wavelength, and (4) non-Maxwellian velocity distribution of sputtered particles. These results indicate that the laser-induced sputtering near threshold fluences is not a thermal effect but an effect of dense electron-hole plasma.     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.