共轭聚合物中的量子限制效应和能量传递

在共混的两种共轭聚合物中,采用分子自级装技术制备了嵌段共轭聚合物量子线的异质结构,通过室温下吸收光谱、发光光谱和激姨光谱的研究证实了在这种嵌段共轭聚合物量子线的异质结构中存在很强的激子限制效应以及链内和链间的有效能量传递。     

Determining the structure of energy in heterostructures with diffuse interfaces

A standard way of determining the type of energy structure in heterostructures, based on analyzing the spectral shift of the band of steady-state photoluminescence (PL) as a function of the excitation power density, is developed with allowance for the effect of the fluctuation tails of the density of energy states. The technique is validated for InAs/AlAs, GaAs/AlAs, and AlSb _y As_1–y /AlAs heterostructures with quantum wells.     

Circular photogalvanic effect at inter-band excitation in InN

The circular photogalvanic effect (CPGE) is observed in InN at inter-band excitation. The function of the CPGE induced current on laser helicity is experimentally demonstrated and illustrated with the microscopic model. A spin-dependent current obtained in InN is one order larger than in the AlGaN/GaN heterostructures at inter-band excitation. The dependence of CPGE current amplitude on light power and incident angle can be well evaluated with phenomenological theory. This sizeable spin-dependent current not only provides an opportunity to realize spin polarized current at room temperature, but also can be utilized as a reliable tool of spin splitting investigation in semiconductors.     

Laser diodes with a wide flat modal gain spectrum in the (1–3)-μm range

The gain spectra of multilayer quantum-well heterostructures on the basis of GaInAs/GaInPAs and GaInAsSb/AlGaAsSb compounds have been analyzed. The possibility of implementing of wide and almost flat spectrum of modal gain in the near IR range upon inhomogeneous excitation of quantum wells in such compounds is shown.     

Hot charge-carrier electroluminescence from laser nanostructures in the spontaneous and stimulated emission modes and absorption of IR radiation by hot electrons in quantum wells

Charge-carrier heating and the carrier concentration dependence on electric current in quantum-well laser heterostructures have been studied under conditions of spontaneous and stimulated emission. Intersubband absorption in tunnel-coupled quantum wells has been investigated under charge-carrier heating by a lateral electric field and under high-power optical excitation.     

Analysis of photoconductivity spectra with a strongly delayed photoresponse

The problem of recording the photoresponse spectra in semiconductors with the persistent photoconductivity effect is considered for heterostructures with ??slow?? charge states at the heterointerface. The proposed versions of the solution of this problem include the use of modulation of excitation or reconstruction of the true shape of the spectrum recorded in the direct-current mode.     

Ultrafast nonlinear absorption and reflection of ZnS/ZnSe periodic nanostructures

ZnS/ZnSe heterostructures under condition of ZnSe interband excitation by a 150 fs laser pulse exhibit strong narrow-band modification of absorption and wide-band modification of reflection. Mean decay time for nonlinear reflection in heterostructures ranges from 2 to 3 ps whereas in bare ZnSe monolayer it exceeds 5 ps. Possible physical processes responsible for nonlinear refraction in the transparency region include interplay of absorption driven nonlinear refraction via Kramers–Kronig relations and intrinsic dielectric properties of dense electron–hole plasma. For nonlinear absorption at ZnSe band edge, interplay of plasma screening effects and states filling effects are relevant.     

Accumulation of the excess of one type of charge carriers and the formation of trions in GaAs/AlGaAs shallow quantum wells

The dynamics of excitons and trions in GaAs/AlGaAs heterostructures with shallow quantum wells is studied in time-resolved photoluminescence experiments carried out with different repetition rates of picosecond pump pulses for the cases of intrawell, above-barrier, and“two-color” excitation. It is established that excess charge carriers of one type accumulated in the quantum wells under above-barrier excitation play a key role in the formation and dynamics of the exciton-trion system and determine its composition and kinetic properties. The lifetime of excess charge carriers in the quantum wells, estimated from the experimental data, exceeds 10 μs.     

Shallow-impurity-assisted transitions in the course of submillimeter magnetoabsorption of strained Ge/GeSi(111) quantum-well heterostructures

The submillimeter (f=130–1250 GHz) magnetoabsorption spectra of strained Ge/GeSi(111) multilayer heterostructures with quantum wells are investigated at T=4.2 K upon band-gap optical excitation. It is found that the magnetoabsorption spectra contain lines associated with the excitation of residual shallow acceptors. The resonance absorption observed can be initiated by optical transitions between the impurity states belonging to two pairs of Landau levels of holes in germanium quantum-well layers.     

Solar-blind MSM-photodetectors based on Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>N heterostructures

Solar-blind MSM photodetectors based on the AlGaN heterostructures have been fabricated and investigated. The influence of material properties on device parameters is discussed. Effect of different buffer layers on the detector performances has been examined. Detectors exhibit low dark currents and high sensitivity within the range of 250–290 nm. Effect of optical excitation energy on GaN-based MSM-detector performance is analyzed and discussed. At high excitation level the detector speed of response is limited by the field screening caused by the space-charge of the holes. The impulse response of GaN-based MSM-detector is compared favorably with GaAs MSM-device.     

Kinetic effects in recombination of optical excitations in disordered quantum heterostructures: Theory and experiment

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.     

Luminescence of a quasi-two-dimensional electron-hole liquid and excitonic molecules in Si/SiGe/Si heterostructures upon two-electron transitions

The low-temperature photoluminescence of Si/Si_0.91Ge_0.09/Si heterostructures in the near-infrared and visible spectral ranges is investigated. For the structure in which the barrier in the conduction band formed by the SiGe layer is transparent to electron tunneling, the broad luminescence line observed in the visible range is analyzed by comparing its shape with the numerical convolution of the spectrum of near-infrared recombination radiation originating from the electron-hole liquid. The comparison demonstrates that, at high excitation levels, the visible-range emission is caused by two-electron transitions in a quasi-two-dimensional spatially direct electron-hole liquid. Furthermore, the combined analysis of the photoluminescence spectra in the near-infrared and visible ranges yields the binding energy of a quasi-two-dimensional free biexciton in the SiGe layer of these heterostructures. In the structures with a wide SiGe layer that is not tunneling-transparent to electrons, a spatially indirect (dipolar) electron-hole liquid is observed.     

Coherent Phonon-Mode Excitation in Submicron Single-Crystal Diamond Films with a Graphitized Layer Built-In

We report the first experimental observation of high-efficient phonon-mode coherent excitation using stimulated low-frequency Raman scattering (SLFRS) in submicron diamond{graphite{diamond heterostructure films with an ion-beam-induced graphitized layer. We show that the SLFRS process in submicron diamond heterostructures has a frequency shift in the gigahertz range and estimate experimentally the SLFRS conversion efficiency and threshold.     

亚单层InGaAs量子点-量子阱异质结构的时间分辨光致发光谱

测定了亚单层InGaAs/GaAs量子点-量子阱异质结构在5K下的时间分辨光致发光谱.亚单层量 子点的辐射寿命在500 ps 至 800 ps之间，随量子点尺寸的增大而增大，与量子点中激子的 较小的横向限制能以及激子从小量子点向大量子点的隧穿转移有关.光致发光上升时间强烈 依赖于激发强度密度.在弱激发强度密度下，上升时间为 35 ps，纵光学声子发射为主要的 载流子俘获机理.在强激发强度密度下，上升时间随激发强度密度的增加而减小，俄歇过程 为主要的载流子俘获机理.该结果对理解亚单层量子点器件的工作特性非常有用. 关键词： 亚单层 量子点-量子阱 时间分辨光致发光谱     

Nonlinear Bragg structures based on ZnS/ZnSe superlattices

Nonlinear optical response of periodic ZnSe/ZnS heterostructures is reported using interband excitation of a ZnSe sublattice by nano-, pico- and femtosecond laser pulses. A considerable shift of the reflection spectrum and large relative reflection changes were observed in a wide spectral range corresponding to the transparency region of ZnSe far from the intrinsic absorption onset. Evaluated refraction-index change is about −0.05 with the relaxation time being about 3 ps. In the case of femtosecond excitation, a wide-band nonlinear response is observed for both one-photon near-UV and two-photon near-IR excitation. The nonlinear refraction is supposed to be controlled by population-induced absorption changes in ZnSe single crystals and relevant refraction-index modification via Kramers–Kronig relations. The nonlinearity relaxation time is supposed to trace a transition from a non-equilibrium to a quasi-equilibrium distribution of electrons and holes within ZnSe conduction and valence bands, respectively, rather than the electron–hole recombination time. The nonlinearity mechanism does not reduce to just population-dependent absorption saturation, but essentially results from the specific distribution function in the first instance after excitation.     

Nanoscale ZnCdS/ZnSSe heterostructures for semiconductor lasers

We investigated cathode luminescence of ZnCdS/ZnSSe heterostructures and structures with quantum wells (QW) grown by vapor phase epitaxy from metalloorganic compounds depending on the excitation level, composition, and thickness of the ZnCdS layer. The observed dependences are explained by specific features of the energy band diagram of heterostructures with gaps in type II bands and composition nonuniformity over the ZnCdS layer thickness. A 478-nm laser with transverse optical pumping by N₂-laser was realized at T = 80 K on the ZnCdS/ZnS/ZnSSe structure with QW.     

Selective enhancement of the hole photocurrent by surface plasmon–polaritons in layers of Ge/Si quantum dots

It is found that the integration of Ge/Si heterostructures containing layers of Ge quantum dots with twodimensional regular lattices of subwave holes in a gold film on the surface of a semiconductor leads to the multiple (to 20 times) enhancement of the hole photocurrent in narrow wavelength regions of the mid-infrared range. The results are explained by the light wave excitation of the surface plasmon–polaritons at the metal–semiconductor interface effectively interacting with intraband transitions of holes in quantum dots.     

Sampling a terahertz dipole transition with subcycle time resolution

We present a time-resolved technique to measure optical excitation processes with a time resolution shorter than the oscillation period of the exciting light. Our terahertz (THz) experiments fully resolve the polarization dynamics of electrons in semiconductor heterostructures when they are excited by a THz pulse. The time resolution of the polarization enables us to deduce the population dynamics of the excited state, which includes the dynamics of a virtual population in the case of off-resonant excitation.     

Excitation intensity dependence of lateral photocurrent in InGaAs/GaAs dot-chain structures

Optical properties of multi-layer In_xGa_1 − xAs/GaAs dot-chain heterostructures are studied by means of lateral photoconductivity (PC). Effects of carrier generation and recombination on the photoresponsivity of deep defects and quantum dot arrays are considered. It is shown that the radiative recombination significantly affects the lateral PC spectra thus leading to a nonlinear dependence of photocurrent on excitation intensity. For ground state excitation of the quantum dots the photocurrent nonlinearities are determined by a competition of both generation and recombination processes which include thermal activation.     

Selective enhancement of green upconversion luminescence of Er-Yb: NaYF_4 by surface plasmon resonance of W_(18)O_(49) nanoflowers and applications in temperature sensing

The W_(18)O_(49) nanoflowers with a diameter of 500 nm are prepared by a facile hydrothermal method. The Er-Yb:NaYF4 nanoparticles are adsorbed on the petals(the position of the strongest local electric field on W_(18)O_(49) nanoflowers).With a 976 nm laser diode(LD) as an excitation source, the selectively green upconversion luminescence(UCL) is observed to be enhanced by two orders of magnitude in Er-Yb: NaYF4/W_(18)O_(49) nanoflowers heterostructures. It suggests that the near infrared(NIR)-excited localized surface plasmon resonance(LSPR) of W_(18)O_(49) is primarily responsible for the enhanced UCL, which could be partly reabsorbed by the W_(18)O_(49), thus leading to the selective enhancement of green UCL for the Er-Yb: NaYF4. The fluorescence intensity ratio is investigated as a function of temperature based on the intense green UCL, which indicates that Er-Yb: NaYF_4/W_(18)O_(49) nanoflower heterostructures have good potential for developing into temperature sensors.