Multiexciton transients in a single quantum dot

The relaxation dynamics of a multiple exciton complex (multiexciton) confined in a semiconductor quantum dot has been investigated. Emission signals from a single self-organized GaAs/Al_0.3Ga_0.7As quantum dot are temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model describing the successive relaxation of multiexcitons.     

Room temperature intra-band lasing in quantum dot arrays placed in high photon density cavities—a theoretical study

Intersubband (intra-band) transitions are very attractive forlong wavelength lasers due to the high degree of tailoring possible in the emission spectra. In general, if intra-band population inversion is to be created in a conduction band quantum well by carrier injection at the barrier energy, it is necessary that the electron non-radiative intra-band energy relaxation times are long. Additionally, the extraction time for the electron from the lower state should be short. In a bipolar device studied here, this means the bandedge electron-hole recombination times should be short.The use of sub-two-dimensional (2D) structures (quantum dots) allows us to increase the intra-band energy relaxation time from about a picosecond for bulk or quasi-2D systems to several hundred picoseconds at room temperatures. Also, by placing these structures in a cavity with a high photon number, it is possible to decrease the bandedge electron-hole recombination times through stimulated emission. Our studies show that strong population inversion and lasing under d.c. conditions is possible at room temperature in such systems.     

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.     

Mott transition for picosecond all-optical nor gate in CdSe

Using picosecond time resolved spectroscopy, we study the extinction kinetics of the optical transmission of CdSe platelets induced by strong optical pumping. We investigate particularly the all-optical gate capabilities (resulting from the bandgap shrinkage due to the Mott transition) for laser beams the wavelength of which ranges from 676 to 678 nm. The electron-hole plasma density necessary to enable the optical switching is determined (ϱ ∼ 2 × 10¹⁷cm⁻³ at 20K). The switch-off time (i.e. the transparency recovery time) is also studied at different wavelengths by picosecond spectroscopy.     

噁嗪750激光染料分子在醇类溶剂中超快动力学研究

利用飞秒时间分辨荧光亏蚀光谱技术,研究了噁嗪750激光染料分子在典型的醇类溶剂中超快动力学过程.实验发现两个超快动力学过程:飞秒量级的快速弛豫过程和皮秒量级的慢速弛豫过程.快速弛豫过程来源于分子内振动能量再分配(IVR)和溶剂分子超快惯性弛豫动力学过程,而慢速弛豫过程对应于溶剂化的扩散分子弛豫动力学过程.实验结果表明慢速弛豫过程的时间常数随醇溶剂分子间氢键键能的增强而增大.     

Results of investigation of the langmuir and upper-hybrid plasma turbulence evolution by means of stimulated ionospheric emission

The results of investigations of the temporal evolution of high-frequency plasma turbulence obtained by measurements of stimulated ionospheric emission (SIE) features for different stages of interaction of powerful radio emission, with an ionospheric F region plasma are presented. It is found that the relaxation time decreased up to 2–4 times with increase in the pump duration and pumping power under the conditions of striction parametric instability (excitation of Langmuir plasma turbulence) over times t≤2.00 ms. A similar decrease of the SIE relaxation time was also observed under the conditions of thermal parametric instability (excitation of upper-hybrid plasma turbulence) in long-term quasicontinuous heating of ionospheric plasma. The possible mechanisms of collisionless dissipation of plasma turbulence energy are discussed to explain the observed dependence of the plasma wave damping rate on the temperal stage of development and intensity of plasma turbulence. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 3, pp. 313–347, March, 1998.     

The X-ray radiation from cathode spot fragments in laser-inducedvacuum discharges

The temporal evolution of X-ray emission of laser-induced discharges was investigated by means of a picosecond X-ray streak camera. Point-like structures with small divergence and thin hot layers of intense X-ray radiation with life time from 30 ps to 1.5 ns in a spectral range 100 eV-10 keV have been found in a comparatively low voltage vacuum discharge (U≈150 V-2.7 kV) initiated by picosecond laser beam. The radiation of laser induced breakdown was investigated just after breakdown as well as with a delay time (up to 20 ns) relative to the ignition point by the laser beam in order to eliminate the X-ray radiation coming from the laser-produced plasma     

Investigation of non-equilibrium electron-hole plasma in nanowires by THz spectroscopy

Efficient emission of THz radiation by AlGaAs nanowires via excitation of photocurrent by femtosecond optical pulses in nanowires was observed. Dynamics of photoinduced charge carrier was studied via influence of electron-hole plasma on THz radiation by optical pump THz probe method. It was found that characteristic time of screening of contact field is about 15 ps. Recombination of non-equilibrium occurs in two stages: fast recombination of free electron and holes (with relaxation time about 700 ps), and slow recombination (with relaxation time about 15 ns), which involves a capture of electrons and holes on the defects of crystalline structure of nanowires.     

Effect of parameters on Si plasma emission in collinear double-pulse laser-induced breakdown spectroscopy

Collinear dual-pulse laser-induced breakdown spectroscopy was carried out on Si crystal by using a pair of nanosecond Nd:YAG laser sources emitting at 1064 nm. The spectral intensities and signalto-noise ratios of selected Si atomic and ionic lines were used to evaluate the optical emission. The optical emission intensity was recorded while varying the interpulse delay time and energy ratio of the two pulsed lasers. The effects of the data acquisition delay time on the line intensity and signal-to-noise ratio have been investigated as well. Based on the results, the optimal interpulse delay time, energy ratio of the two pulsed lasers, and data acquisition delay time for achieving the maximum atomic and ionic line intensities were found for generation of Si plasma with the collinear dual-pulse laser approach. The dominant mechanism for the observed line intensity variation was also discussed. In addition, the plasma temperature and electron number density at different gate delay times and different interpulse delay times were derived. A significant influence of plasma shielding on the electron temperature and electron number density at shorter interpulse delay times was observed.     

激光诱导Al等离子体发射光谱特性的实验研究

本文从实验上研究了不同缓冲气体(He,Ar,N2和Air)中激光Al等离子体的时间分辨发射光谱,研究了原子发射谱线的强度和Stark展宽随延时、缓冲气体性质和压力变化的规律.结果表明原子谱线的强度在3μs左右达到最大值,随着延时的增加,谱线的Stark展宽减小,而缓冲气体压力的增大导致谱线的Stark展宽增大,在实验测定的四种缓冲气体中,Ar气体中谱线的Stark展宽最大.     

Modeling a distributed feedback dye laser pumped by a Nd-glass laser

A mathematical model has been developed to describe the dynamic emission of a distributed feedback dye laser (DFDL) pumped by a Nd-glass laser. The model is based on the coupled-wave theory. It allows the investigation of the temporal behavior of the Nd-glass pumping laser source and the DFDL pulses. The model allows studying the effect of the variation of the laser input parameters of the Nd-glass laser, such as maximum amplification coefficient, loss coefficient and pumping rate on the characteristics of DFDL pulses regarding the pulse width, delay time and separation time. The feedback process of the DFDL is provided either by changes of the refractive index or by optical gain or by both together. The model estimates the following: temporal behavior of the density of emitted radiation, energy densities of the first excited singlet and triplet states, DFDL output power, cavity decay time and the temperature of the produced grating. The numerical solution of the nonlinear coupled rate equation system predicts the generation of DFDL picosecond pulses. The calculated results are in good agreement with the available experimental data. The calculations were done using rhodamine 6G dissolved in ethanol as the investigated matrix.     

Evidence of the exciton-plasma transition in the emission spectra of GaSe

Summary We present luminescence spectra of gallium selenide at 2 K in which a slow and continuous evolution from excitonic recombination to an electron-hole plasma emission is observed when the excitation intensity is increased. We find a small red-shift and a broadening of the direct exciton emission line which is followed by its disappearing. We explain these results with a model which takes into account the electron-hole correlation. Part of this work was carried out during a stage at the Institut de Physique Appliquée of the Ecole Polytechnique Fédérale de Lausanne (Switzerland).     

Effective control over the dual-color emitted light waves from one-dimensional random media pumped by Femtosecond-Lasing Pulses

The spectral property of dual-color random laser is investigated by simultaneously solving Maxwell’s equations and rate equations of electronic population in which the pumping rate is described by a time function with duration of hundreds of femtoseconds. Results show that the delay time and peak intensity of the emitted light depends strongly on the pumping process. An effective optimization method is proposed to exercise control over the dual-color laser emission from the dye solution with scattering nano-particles. It is demonstrated numerically that the controllable single mode operation of a dual-color random laser by choosing the proper pump profile and pumping region. The present work enriches the knowledge about random lasers and offers more guidance for relevant experiments.     

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。     

Picosecond studies of highly excited CdS

Results on picosecond luminescence and excite-and-probe transmission as well as transient grating measurements for highly excited CdS measured at a bath temperature of 5 K will be presented. The luminescence and optical gain both due to electron-hole plasma and excitonic molecule recombination are observed. The electron-hole plasma decays very fast by bimolecular recombination of electrons and holes in the plasma and diffusion of the carrier toward the low density regions, and transforms into excitons and excitonic molecules within 100–200 ps. The possibility of electron-hole liquid formation is definitely excluded. The exciton and excitonic molecule decay rather slowly and govern the optical properties for times longer than 200 ps.     

Generation of picosecond optical pulses from single heterostructure GaAs diode laser and their emission characteristics

Emission characteristics of a single heterostructure GaAs diode laser are reported using a simple driver circuit. It provides a single picosecond time duration optical pulse, a pulse train or a broad optical pulse depending on the amplitude and time duration of the electrical pump pulse. Results show that relaxation oscillation frequency depends on the amplitude of pumping current pulse as well as on some inherent property of diode laser, which seems to be the level of impurity in lasing medium. Variation of relaxation oscillation frequency with amplitude of current pulse shows only the qualitative agreement with the reported theoretical predictions.     

Transient interband light absorption by quantum dots: Degenerate pump-probe spectroscopy

The optical pump-probe method, which makes it possible to determine the energy relaxation rate for excited electron-hole pairs and excitons in semiconductor quantum dots (QDs), is theoretically described. A scheme in which the carrier frequencies of optical pump and probe pulses are close to resonance with the same interband transition in the QD electron subsystem (degenerate case) is considered. The pump-induced probe energy absorption is analyzed as a function of the delay time between the pump and probe pulses. It is shown that under certain conditions this dependence is reduced to monoexponential, whose exponent is proportional to the energy relaxation rate for the considered state of electron-hole pairs and excitons. The size dependence of the energy relaxation rate of the electron-hole pair states is modeled by the example of PbSe-based QDs, whose electron subsystem is in the strong-confinement regime.     

Emission dynamics of a GaAs microcavity with embedded quantum wells under intense nonresonant excitation

In a GaAs-based microcavity with embedded quantum wells, the dynamics of emission processes under high levels of nonresonant picosecond laser-pulse excitation is studied. For pump levels above the stimulation threshold, the kinetics of the intensity, spectral position, and linewidth of the emission are measured. Upon the arrival of an excitation pulse, the emission line shifts to higher energies over a time interval comparable to the time it takes for the emission intensity to attain its peak value, and then shifts in the opposite direction towards its position at low polariton densities. The width of the emission line is largest immediately after the excitation pulse and attains a minimum when the stimulated-emission intensity is maximum. It is shown that after the excitation pulse, the system is initially in a weak exciton-photon coupling regime, and a transition to a strong-coupling regime occurs with time.     

X-ray generation in low-voltage vacuum discharges

The dynamics of x-ray emission from a low-voltage laser-induced discharge was studied with the aid of a picosecond x-ray streak camera. Directed x-ray emission in the spectral range from 100 eV to 10 keV in the form of point sources and thin layers with lifetimes ranging from 30 ps to 1 ns was observed in a low-voltage vacuum discharge (U=150 V) initiated by a picosecond laser beam. X-ray emission from a discharge was detected with a time delay (1–20 ns) relative to ignition by the laser beam in order to prevent the radiation of the laser plasma from entering the detector. Detection of directed x-ray emission in a low-voltage vacuum discharge is demonstrated. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 622–627 (10 May 1998)