Photoluminescence in amorphous zirconium titanate

Photoluminescence at room temperature was observed in amorphous zirconium titanate obtained from the polymeric precursor method. This was the first time in which PL was noticed in an amorphous compound made of two network formers. The PL spectra could be deconvoluted into two bands, whose center 1 was located from 540 nm to 552 nm and center 2 from 625 nm to 641 nm. The co-existence of fivefold and sixfold oxygen coordination of titanium and/or zirconium could be the origin of the radiative recombination of electron-hole pairs in amorphous ZT, which may be responsible for the emission of the photoluminescence. PACS 78.55.m; 78.55.Hx; 78.66.W; 78.66.Jg; 81.20.Fw     

Pulsed laser deposition of chromium-doped zinc selenide thin films for mid-infrared applications

We have grown Cr doped ZnSe thin films by pulsed laser deposition on GaAs, sapphire and Si substrates through KrF excimer laser ablation of hot-pressed targets containing appropriate stoichiometric mixtures of Zn, Se, and Cr species and hot-pressed ceramic targets made of ZnSe and CrSe powders in vacuum and in an He background environment (10^-4 Torr). Deposited films were analyzed using X-ray diffraction to determine crystallinity and energy dispersive X-ray fluorescence to confirm Cr incorporation into the films. Photoluminescence measurements on the films show intracenter Cr²⁺ emission in the technologically important 2–2.6 μm spectral range. PACS 78.66.hf; 78.66.-w; 78.55.-m; 78.66.Bz; 78.20.-e     

硅中浅杂质的激光感应光电导谱

首次采用光子能量小于硅中浅受主杂质电离能的可调谐远红外激光器作为激发源,获得了硅中浅受主杂质的光电导谱.可调谐半导体远红外激光器的调谐范围为380～500cm~(-1),光子流密度约10~(18)/cm~2·sec,用双光子跃迁对光电导谱进行了解释.对于Si:Al样品,光电导谱中的双峰分别相应于2P~1和2P~2中间态的双光子共振跃迁.也观察到了双光子透明的反共振现象.     

Improvement of the direct optical transition probability in Si by wavefunction engineering

A model for improving the direct photon emission probability from electron-hole interaction in silicon is presented. Electron and hole wavefunctions are engineered for optimum overlap in SiGeC heterostructures by proper definition of alloy compositions, potentials and layer thickness dimensions. The direct no-phonon transition probability is increased by a factor of at least 10, compared to the non-optimized direct transition probability. The calibrated structure corresponds to a type II interband structure where photons are emitted with an energy lower than the bandgap energy of the constituent layers. PACS 78.55.-m; 78.66.-w; 78.67.De     

Laser-induced down-conversion parameters of singly and doubly doped ZnS phosphors

Singly and doubly doped ZnS phosphors have been synthesized using flux method. Laser-induced photoluminescence has been observed in ZnS-doped phosphors when these were excited by the pulsed UV N₂ laser radiation. Due to down-conversion phenomenon, fast phosphorescence emission in the visible region is recorded in milliseconds time domain for ZnS:Mn while in the case of ZnS:Mn:killer (Fe, Co and Ni) the lifetime reduces to microseconds time domain. Experimentally observed luminescent emission parameters of excited states such as, lifetimes, trap-depth values and decay constants have been reported here at room temperature. The high efficiency and fast recombination times observed in doped ZnS phosphors make these materials very attractive for optoelectronic applications.     

Femtosecond laser pulse irradiation of Sb-rich AgInSbTe films: Scanning electron microscopy and atomic force microscopy investigations

The single-layer and multilayer Sb-rich AgInSbTe films were irradiated by a single femtosecond laser pulse with the duration of 120 fs. The morphological feature resulting from the laser irradiation have been investigated by scanning electron microscopy and atom force microscopy. For the single-layer film, the center of the irradiated spot is a dark depression and the border is a bright protrusion; however, for the multilayer film, the center morphology changes from a depression to a protrusion as the energy increases. The crystallization threshold fluence of the single-layer and the multilayer films is 46.36 mJ/cm², 63.74 mJ/cm², respectively. PACS 79.20.Ds; 78.55.Qr; 78.66.Jg; 68.37.Ef; 68.37.Ps     

Nonclassical light generation in quasi-phase-matched parametric self-frequency conversion

We present a quantum theory of the parametric self-conversion of the laser radiation frequency in active nonlinear crystals with a regular domain structure. Such crystals feature simultaneous lasing and quasi-phase-matched parametric conversion of the laser radiation frequency. These processes are described using the Heisenberg-Langevin equations in two regimes of the subharmonic generation: super-and subthreshold. The spectral properties of the quadrature components of the laser frequency and its subharmonic and the photon statistics have been studied as dependent on the pump power, crystal length, and reflectance of the laser cavity output mirror. Using the obtained analytical expressions, these characteristics are calculated for a active nonlinear Nd:Mg:LiNbO₃ crystal with a regular domain structure. In the subthreshold regime, the maximum decrease in the spectral density of fluctuations in the subharmonic quadrature component relative to the standard quantum limit may reach 90%; in the above-threshold regime, these fluctuations are virtually not suppressed. A decrease in the spectral density of fluctuations of the laser frequency quadrature does not exceed 10%. In the subthreshold excitation regime, the subharmonic photons obey a super-Poisson statistics; in the above-threshold regime, the photon statistics is Poisson-like.     

金等离子体M带谱强度的定量测量

在"神光II"多束高功率激光装置上利用列阵透镜匀滑钕玻璃波长0.53μm的强激光幅照平面金(Au)靶时产生X射线,本文给出了X射线绝对转换效率ξx。研究了多束倍频激光叠加驱动靶形成X射线背景光源辐射金M壳层1.8—3.1Kev带谱的特性,获得了不同激光功率密度及不同角度驱动靶面等几种条件下X射线能谱的定量测量结果和能谱分布。     

Synthesis and stabilization of ZnS nanoparticles embedded in silica nanospheres

The silica-coated ZnS nanocomposites have been synthesized by a seeded-growth procedure in iso-propanol. The results of XRD, HRTEM and UV absorption show that the ZnS nanoparticles can be incorporated in the silica nanospheres without changing the particle size, and the composites are of multi-core structure. UV absorption and emission spectra have been performed to check the character of the composites, which show that the silica shell not only increases the PL intensity, but also greatly improves the anti-oxidation ability and thermal stability. PACS 81.05.Dz; 81.16.Be; 81.65.Rv; 78.55.Et; 78.66.Hf     

Photoluminescence spectra of doped GaAs films

We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2×10¹⁸ cm^-3 and the film thickness is in the range of the penetration length of the PL excitation laser. PACS 78.20.-e; 78.55.Cr; 78.66.Fd     

Lifetimes of bound excitons in CdSe

Lifetime measurements have been made on bound excitons in CdSe, by using the time-correlated single photon counting and a cw mode-locked dye laser. The lifetimes of the T₂ (an exciton bound to a neutral donor) and I₁ (an exciton bound to be a neutral acceptor) bound excitons have been determined to be 0.50±0.05 and 0.80±0.05 ns, respectively. These values are in good agreement with those predicted by the model of Rashba and Gurgenishvili.     

Numerical analysis of an injection laser with stripe geometry

A computer simulation of a stripe-geometry semiconductor laser is studied. The lasing properties of the laser are obtained from the drive current and several device parameters using this computer model. The model is made self-consistent by means of rate equations and by the requirement that the ratio of the power, incident on and departing from, each mirror obeys the law of reflection. Non-uniform gain distribution is also considered. The curves of the spatial distribution of the small-signal and large-signal gains, and the photon flux and photon density inside the cavity, are illustrated as well.     

Interaction between oxygen and dislocations in p-type silicon

The interaction between dislocations and impurities in silicon has been the subject of many studies over several years; nevertheless, many questions mainly relevant to the electronic states of complex defects containing impurities and dislocations are still unclear. In oxygen precipitated and plastically deformed p-type silicon deep levels were detected by deep level transient spectroscopy (DLTS). The comparison between differently treated samples allowed us to clarify the influence of oxygen on the defective states. One of the most prominent DLTS peaks usually observed in plastically deformed silicon was found to change substantially as a function of O precipitation. In particular, O precipitation increased the thermal stability of the peak, and also changed the capture and emission processes at the deep level. PACS 78.55.-m; 78.55.Ap; 71.55.-i; 71.55.Cn     

Determination of the saturation parameter of electronic transition in a uranium-neon hollow-cathode discharge by optogalvanic spectroscopy

Summary The optogalvanic signal (OGS) induced in a uranium-neon hollow-cathode discharge was measured as a function of the laser power density for the 0→16900 cm⁻¹ (591.5 nm) uranium transition. Theoretical relations derived by solving a two-level system rate equations showed the OGS dependence on the laser photon flux, for a modulated c.w. light and for stimulated transitions starting from the ground state. A fitting of the theoretical relations to the experimental measurements allowed the determination of the σ₀ τ product, that is, the saturation parameter of the transition. The results showed good agreement between the σ₀ τ values obtained by the optogalvanic and the usual optical absorption processes.     

Si/SiO<Subscript>2</Subscript> superlattice based optical planar microcavity

Si/SiO₂ Fabry–Pérot microcavities with a silicon nanocrystal (Si-nc) active spacer have been realized using a novel process based on a reactive magnetron sputtering of a pure silica target. Spectral, spatial and temporal behaviours of the quantum dots confined inside the resonator are detailed. Compared with a reference sample, the spectral and spatial emission distributions are significantly narrowed and the forward emission intensity is enhanced. Time resolved photoluminescence measurements also revealed an increase of the spontaneous emission rate. PACS 42.70.Qs; 78.55.-m; 78.66.-w     

Modelling of Tm3+-doped LiNbO3 Waveguide Lasers

In this work, CW laser operation of Tm³⁺-doped LiNbO₃ channel waveguides has been modelled. The model is based on time dependent laser rate equations coupled with the laser signal and pump photon flux equations. Steady state solutions for the population densities, pump and signal powers are obtained by using finite difference discretization of the active volume. The effects of spectroscopic parameters such as concentration dependent cross-relaxation and excess waveguide loss have been analyzed. We demonstrate good agreement with experimental data previously reported in Zn-diffused LiNbO₃:Tm³⁺ channel waveguide lasers. It is shown also that laser performance can be substantially improved by optimizing the cavity length.     

Observation of exciton localization in PbSrSe thin films grown by molecular beam epitaxy

Detailed temperature and excitation-intensity dependence of radiative recombination characteristics in PbSrSe thin films grown by molecular beam epitaxy has investigated. For the first time, localized excitonic structure due to the alloy disorder has been observed at low temperature, and is gradually delocalized as wave-vector-nonconserving band-to-band transitions at high temperature in PbSrSe with low Sr composition. Furthermore, we are able to observe the evolution of the exciton localization, due to the strong alloy fluctuations and lattice distortions, into either free or trapped (strongly localized) excitons at low temperature by studying a PbSrSe thin film with the Sr composition as high as 0.276. The lattice deformation has been shown to play a key role in the composition-dependent excitonic peak broadening in PbSrSe at low temperature. This gives clear evidence for the observation of excitonic effects in lead salts due to their low carrier concentration and enhanced exciton binding energy. The results are discussed in the framework of theories taking into account the carrier migration and exciton trapping due to the alloy fluctuations and lattice vibrations . PACS 78.55.Hx; 78.66.Li     

Pulse compression in AO Q-switched diode-pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber

By simultaneously using both an acoustic-optic (AO) modulator and a Cr⁴⁺:YAG saturable absorber in the cavity, for the first time, a diode-pumped doubly Q-switched Nd:GdVO₄ laser has been realized. The pulse duration is obviously compressed in contrast to the actively acoustic-optic Q-switched laser. By considering the Gaussian transversal distribution of the intracavity photon density and the longitudinal distribution of the photon density along the cavity axis as well as the influence of turnoff time of the acoustic-optic (AO) Q-switch, we provide the coupled rate equations for a diode-pumped doubly Q-switched Nd:GdVO₄ laser with both an acoustic-optic (AO) modulator and a Cr⁴⁺:YAG saturable absorber. These coupled rate equations are solved numerically, and the dependence of pulse width, pulse energy and peak power on the incident pump power at different pulse repetition rates is obtained. The numerical solutions of equations agree well with the experimental results.This revised version was published online in August 2005 with a corrected cover date.     

Theoretical and experimental study of a laser-diode-pumped actively Q-switched Nd:YVO4 laser with acoustic–optic modulator

By considering the Gaussian spatial distribution of the intracavity photon density and initial population-inversion density as well as the longitudinal distribution of the photon density along the cavity axis and turnoff time of the acoustic–optic Q-switch, the coupled equations of an LD-pumped actively Q-switched Nd:YVO₄ laser with acoustic–optic modulator are given. These coupled rate equations are solved numerically on a computer, and the dependences of pulse width, single-pulse energy and peak power on incident pump power are obtained. In the experiment, a laser-diode-pumped actively Q-switched Nd:YVO₄ laser with acoustic–optic modulator is realized, and the experimental results are in fair agreement with the numerical solutions.     

Plasma-based accelerator diagnostics based upon longitudinalinterferometry with ultrashort optical pulses

All-optical ultrafast time-resolved plasma diagnostics of plasma-based accelerators (PBA's) are described, with emphasis on the laser wakefield accelerator (LWFA). Specifically, the diagnostic techniques involve replacing the trailing particle bunch in the LWFA with a trailing photon bunch: a weak ultrashort laser pulse. Since this photon pulse is derived directly from the intense pump pulse, practical difficulties such as synchronization and dephasing are eliminated. The interaction of the photon bunch with the plasma wake is essentially a simple time-domain shift in optical phase, which can produce both “DC” phase shifts and frequency blue/red-shifting of the probe pulse spectrum. These phase/frequency shifts are recorded in frequency domain interferograms, which are formally equivalent to time-domain holograms. Experimental results of longitudinal plasma density profiling are presented in which plasma density oscillations (Langmuir waves) in the wake of an intense (I_peak~3×10 ¹⁷ W/cm²) laser pulse (~100 fs) were measured with ultrafast time resolution. Phase shifts consistent with large amplitude (~80%) density oscillations at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of ~10 GV/m. Strong radial ponderomotive forces enhance the density oscillations. Finally, proposed single-shot schemes for simultaneous transverse and longitudinal profiling are discussed