InGaAs/GaAs量子链的光学特性研究

研究了InGaAs/GaAs量子链的稳态和瞬态光谱特性,特别是载流子的动力学过程.实验发现荧光寿命有很强的探测能量依赖关系,荧光寿命随发光能量的增加而减小;实验还发现,当激发功率较小时,荧光寿命随激发功率增大而增大,当激发功率足够大时,荧光寿命趋于饱和.这些结果清楚地表明,在量子链结构中,参与发光的载流子之间存在明显的耦合和输运现象,进一步分析表明,这种输运主要是由于载流子沿量子链方向的耦合造成的.发光的偏振特性研究进一步证实了载流子沿量子链方向输运过程. 关键词： InGaAs/GaAs 量子点 量子链     

Pulsed Optical Pumping as a Tool for the Determination of Population Mechanisms of Excited States in a Low-Pressure Mercury Discharge

Experiments are described in which a low-pressure mercury discharge is irradiated with a dye-laser pulse tuned resonant to one of the 6 3P-7 3S1 mercury transitions. The effects of this optical pumping process are monitored via fluorescence on many mercury transitions. The sign and time behavior of each fluorescence signal are different, and are determined by one or more of the following processes: a) radiative decay of the 7 3S1 level; b) a change in electron-impact excitation rate from the ground state induced by the optogal-vanic effect (OGE). This OGE results from the population redistribution in the 6 3P levels after radiative decay of the 7 3S1 level; c) a change in electron-impact excitation rate from the 6 3P levels; and d) a change in 6 3P-6 3P collisional excitation rate. It is demonstrated that the signal and time behavior of the fluorescence signals induced by the pulsed optical pumping process can be used to determine the population mechanism(s) of excited states qualitatively.     

Nonlinear Photoluminescence from ZnO Nanobelts

The nonlinear photoluminescence （PL） including second-harmonic generation （SHG） and the multiphoton luminescence （MPL） around 385 nm and 530 nm from ZnO nanobelts are investigated by using near-infrared excitations. The excitation wavelength dependence of MPL intensity reveals resonant energy transfer from SHG to MPL near the band gap excitation. The lifetime measurement of the MPL shows a much slower decay process of the defect emission, which results from the generation and recombination of both donors and acceptors on the disordered surface of the ZnO nanobelts.     

Photon Echo from Localized Excitons in Semiconductor Nanostructures

An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.     

Spin relaxation quenching in semiconductor quantum dots

We have studied the spin dynamics in self-organized InAs/GaAs quantum dots by time-resolved photoluminescence performed under strictly resonant excitation. At low temperature, we observe strictly no decay of both the linear and the circular luminescence polarization. This demonstrates that the carrier spins are totally frozen on the exciton lifetime scale.     

Intra-dot relaxation and dephasing rates from time-resolved photoluminescence from InAs quantum dot ensembles

An ensemble of InAs quantum dots with ground state transition energies centered at 1.216 eV and density 10¹¹dots/cm² has been studied by time-resolved photoluminescence (PL). The wavelength of the 100-fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon sideband) PL. The decay of the PL was time resolved with a streak camera in the interval 1.5–3 ns to avoid scattered laser light. The intensity of the PL was recorded with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal’s cleave axes); the ratio is 2.22 at low temperatures and low excitation. A phenomenological rate equation analysis is made, separating the excitations into two classes, one polarized along the excitation polarization and the other unpolarized (either that way immediately after the excitation pulse or scattered from the first class). Excellent fits to the data lead to the conclusion that both classes decay radiatively with a lifetime of 1 ns, and a transfer from the polarized to the unpolarized species takes place with a distribution time of 12 ns at low temperatures and low excitation, dropping rapidly toward zero for temperatures above 30 K and for intense excitation levels. The polarization of a coherently excited ground state exciton should dephase with a rate equal to the sum of the radiative rate plus the inverse of this distribution time.     

Photoluminescence of water-soluble CdSe/ZnS nanoparticles in complexes with cationic and anionic polyelectrolytes

The data on the influence of polyelectrolytes on the photon emission probability of water-soluble CdSe/ZnS nanoparticles are obtained. The decrease in the photoluminescence quantum yield of nanoparticles occurring upon their transfer to aqueous solutions from toluene (in the course of solubilization) depends on the ionic nature of an agent applied for the replacement of trioctylphosphine oxide residues on the surface of nanoparticles. It turns out that such a cationic modifying agent as cysteamine leads to an insignificant (～10%) decrease in the photoluminescence quantum yield of nanoparticles. The use of such an anionic agent as mercaptoacetic acid causes a significant (～80%) decrease in the quantum yield and the average decay time of photoluminescence. For nanoparticles modified by mercaptoacetic acid (anionic nanoparticles), this decrease is partially compensated if these particles interact with polyelectrolytes whose backbone is oppositely charged (cationic polyelectrolytes), such as polyallylamine and polydiallyldimethylammonium chloride. In this case, the photoluminescence quantum yield shows a reverse increase by 40%, remaining the same within a matter of months or longer. In contrast to this, cationic nanoparticles, only slightly quenched by cysteamine at the stage of solubilization, are appreciably degraded in complexes with anionic polyelectrolytes in solutions and upon immobilization of complexes on a substrate, so that their photoluminescence quantum yield irreversible decreases to zero within a few days. Possible mechanisms of the effects observed are discussed and their consideration in polyelectrolyte-based molecular lithography.     

水溶性CdTe量子点的稳态和纳秒时间分辨光致发光光谱

报道了以飞秒脉冲激光为激发光源的水溶性CdTe量子点(QDs)的稳态荧光光谱和纳秒时间分辨荧光光谱.实验发现CdTe量子点的荧光光谱峰值位置随激发波长变化发生明显移动，激发脉冲波长越长，荧光峰位红移越大.荧光动力学实验数据显示，在400nm和800nm脉冲激光激发下，水溶性CdTe量子点的荧光光谱中均含有激子态和诱捕态两个衰减成分，两者的发射峰相距很近，诱捕态的发射峰波长较长.在800nm脉冲激光激发下的诱捕态成分占总荧光强度的比重比400nm激发下的约高3倍，其相对强度的这种变化导致了稳态荧光发射峰位的红移. 关键词： CdTe 量子点 时间分辨 荧光光谱 上转换荧光     

Observation of long-lived excitons in InAs quantum dots under thermal redistribution temperature

We study the temperature-dependent time-resolved photoluminescence (TRPL) of self-assembled InAs quantum dots (QDs). Under low excitation power, a surprisingly long PL decay time is observed at about 60 K, under the thermal redistribution temperature. The long decay time decreases with increasing excitation power but is nearly independent of the detection energy of TRPL measurements. A model considering the spin relaxation through the excited excitonic state is proposed to quantitatively explain the unusual phenomena. The rate equation analysis indicates that the observation of long-lived excitons is caused by the shortened spin-flip time.     

Optical properties of InGaN quantum dots

We present time-resolved and spatially-resolved photoluminescence (PL) measurements of InGaN inclusions in a GaN matrix. The structures were grown by metal-organic chemical vapor deposition on sapphire and Si(111) substrates. Nonresonant pulsed excitation yields a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot (QD) nature of luminescence in InGaN–GaN samples. Cathodoluminescence (CL) and micro-PL measurements demonstrate sharp emission lines from single QD states. The recombination dynamics of single QD’s and the whole QD ensemble were investigated. Monoexponential decay was observed for the PL of single QD’s. For similar transition energies different time constants were obtained. Therefore the nonexponential decay observed for the whole ensemble is attributed to the coexistence of QD’s having similar ground-state transition energies, but significantly different electron–hole overlap.     

Mechanoluminescence and high pressure photoluminescence of (Zn,Cd) S phosphors

The mechanoluminescence spectra of (Zn, Cd)S phosphors shift towards shorter wavelength side as compared to the photoluminescence spectra, however, the photoluminescence spectra shift towards shorter wavelength side with increasing pressure with less pressure coefficient. This finding eliminates the thermal population mechanism and suggests the electrical excitation mechanism for the mechanoluminescence excitation. The decay of mechanoluminescence after the deformation (Zn, Cd)S phosphors may be controlled by the recombination rate of holes and electrons, i.e. by the finite times required for the liberation of the electrons from the traps and for the electron transport, and consequently the decay of mechanoluminescence may be similar to the decay of photoluminescence.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

Carrier recombination kinetics in (3 1 1)A InGaAs sidewall quantum wires

We present a study of the optical properties and carrier dynamics in strained InGaAs sidewall quantum wires (QWR) on patterned GaAs (3 1 1)A substrates by means of picosecond time-resolved photoluminescence (PL). A pronounced dynamical red shift of the QWR-PL band when increasing the delay time after the pulse excitation is observed. In addition, time-resolved data show a significant shortening of the PL decay time from the wire at short delay and when high excitation power is used. The data are compared with theoretical predictions. The results, i.e. the dynamical red shift observed in the wire emission and the shortening of the PL decay with increasing the excitation density, are interpreted in terms of a dynamical screening effect of the piezoelectric field.     

Time-resolved photoluminescence study of Ga In P alloys

The time-resolved photoluminescence spectra of ordered and disordered Ga_0.52In_0.48P alloys were studied at room temperature and at 77 K liquid nitrogen, respectively. The ordered samples have well fitted two exponential processes decay curves and the time constants are sample dependent and have little relationship with the ordering degree. The decay curve of disordered sample shows that it has single exponential process and its lifetime has a tendency of reduction with the decrease of excitation intensity. The photoluminescence spectra with different delay time at 77 K show that the ordered samples exhibit about 6 ∼ 10 meV blue-shift of PL peak energy with the delay time. Received 13 December 2001 Published online 19 July 2002     

Decay kinetics of the defect-based visible luminescence in ZnO

Photoluminescence spectra and decays under pulsed N₂ (337 nm) laser excitation were measured for hydrothermally grown bulk and liquid-phase epitaxy (LPE)-grown film ZnO samples within 9-300 K. Temperature dependence of integrated spectra over the exciton and visible spectral regions was evaluated using a model involving standard energy barrier processes. Decay curves measured within a broad time window (10 ns-1 ms) and with extreme signal/background ratio (five orders of magnitude) point to complex decay mechanism in which the exponential and inverse power-law processes can coexist. There is no straightforward interconnection between the observed temperature dependence of integrated visible photoluminescence intensity and its decay shape over the 9-300 K temperature interval.     

Anisotropy of optical absorption and luminescent properties of CaMoO4

Optical absorption, excitation and emission spectra, as well as photoluminescence decay time of CaMoO₄ single crystals have been measured. It has been revealed that annealing of crystals in oxygen atmosphere leads to the disappearance of their anisotropic behavior in optical absorption, as well as in X-ray luminescence and decay time. Moreover, it has been found that annealing can significantly affect the value of the decay time. The relative intensity of the long-wavelength emission band increases under excitation in the region of indirect band-to-band transitions.     

Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.     

Properties of resonant interatomic Coulombic decay in Ne dimers

Properties of the interatomic Coulombic decay (ICD) process in Ne dimers have been obtained by tracking the formation of energetic Ne+ ions. The double photoionization cross section, deduced from the Ne+/Ne+ coincidence signal, is dominated by the ICD process and presents a threshold 280 meV below the atomic Ne+2s(-1) threshold. Rydberg excitation of a 2s electron in the dimer creates molecular Rydberg states whose Sigma and Pi symmetries have been resolved. These excited states decay by a resonant ICD process releasing an energetic Ne+ ion and a neutral excited Ne* fragment. Subsequent autoionization of the Ne* fragment explains a double photoionization threshold below the dimer 2s ionization threshold.     

Time-resolved OSL studies of YAlO3:Mn2+ crystals

Time-Resolved Optically Stimulated Luminescence (TR-OSL) from single crystalline YAlO₃:Mn²⁺ samples was investigated using a green light emitting diode (λ ∼ 525 nm) as stimulation light source. The TR-OSL decay curve of the material can be described with a single exponential decay function with a lifetime about 80 ms that does not depend on irradiation dose in the range from 50 mGy to 1 kGy. This OSL decay is superposed on a photoluminescence signal with a much shorter (3.5 ms) decay lifetime. The Mn²⁺ photoluminescence decay with a lifetime of 3.5 ms can be easily eliminated by corresponding time resolution using pulsed OSL readout. Dose response and thermal stability of the OSL signal are consistent with the previous thermoluminescent (TL) studies of the material.