Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.     

Intersubband absorption of light in selectively doped asymmetric double tunnel-coupled quantum wells

The spectrum of equilibrium intersubband absorption has been studied in selectively doped asymmetrical double tunnel-coupled quantum wells designed for research into the modulation of IR light in a longitudinal electric field. The comparison of calculated and experimental spectra at different temperatures is carried out. In calculations, the influence of the space charge on the energy spectra of electrons and the difference in the electron effective mass in different subbands are taken into account. The data obtained on the intersubband absorption spectra in equilibrium conditions and under electron excitation by high-power picosecond pulses of light in the mid-IR range allow us to refine the energy spectrum of electrons in the actual structure.     

Intersubband absorption of light in heterostructures with double tunnel-coupled GaAs/AlGaAs quantum wells

Intersubband absorption of mid-IR light was studied in heterostructures with asymmetrical tunnel-coupled quantum wells in equilibrium conditions and under high-power pumping by picosecond pulses of light. The energy spectrum of electrons in tunnel-coupled quantum wells was found from an analysis of equilibrium and nonequilibrium intersubband absorption spectra. The dynamics of intersubband absorption under high-power optical pumping was studied using the pump-and-probe picosecond technique. The experimental data are compared with the results of calculations based on solving rate equations. The intersubband relaxation times are determined.     

Modulation of optical absorption of GaAs/AlGaAs quantum wells in a transverse electric field

Results of experimental investigations of the transformation of intraband light absorption spectra by the quantum-well electrons in a transverse electric field are presented. In addition to the familiar Stark effect, absorption oscillations in the photoionization band are detected. These oscillations are caused by electron transitions from the ground state in a quantum well to quasidiscrete levels arising in the continuum of states above the well due to the linear potential of the external electric field. Fiz. Tekh. Poluprovodn. 32, 849–851 (July 1998)     

Drift velocity of electrons in quantum wells in high electric fields

It is experimentally found that the maximum drift velocity of electrons in quantum wells of differently arranged AlGaAs/GaAs heterostructures and pseudoamorphous Al_0.36Ga_0.64As/In_0.15Ga_0.85 As heterostructures is higher than the maximum drift velocity of electrons in bulk materials. It is established that no negative differential conductivity is exhibited by the field dependence of the drift velocity of two-dimensional electrons in GaAs and In_0.15Ga_0.85As. The drift velocity in the GaAs quantum well is saturated in fields several times higher than the field corresponding to the Γ-L intervalley transitions of electrons in bulk GaAs.     

Optically pumped intersubband lasers based on quantum wells

A far infrared (FIR) laser based on intersubband transitions in quantum wells is proposed where a pumping laser is used to create population inversion in the structure. The goal is to develop a structure which operates essentially as a 4-level laser, to minimize bottlenecking of the lower laser state. Multiple quantum wells can be used in the active laser of these structures to enhance the laser gain and the minimum required reflectivity in the cavity structure. The possibility of using both conduction and valence band quantum-well structures are investigated. Our study shows that, due to high intersubband scattering rates in the valence band structure, the creation of population inversion is more difficult and requires a high pumping power density while in the conduction band structure, population inversion can be achieved by a moderate pumping power density. The maximum population inversion in the conduction band structure is estimated to be 2.1×10¹¹ cm², which requires a pumping power density 2 kW cm^-2 for a single quantum well. The threshold power as well as the minimum required reflectivity of the cavity structure for the conduction band scheme are estimated for different well numbers     

All-optical switch utilizing intersubband transition in GaN quantum wells

Intersubband transition (ISBT) in GaN quantum wells (QWs) was investigated from the viewpoint of application to ultrafast all-optical switches. The effect of crystalline quality on the absorption saturation characteristics was examined and reduction of edge dislocations was found to be a crucial factor in decreasing operation energy. Then, the switching performance was investigated for devices with improved crystalline quality. Modulation of signal pulses with a pulse interval of less than 1 ps was confirmed. Another device displayed gate-switch operation with an extinction ratio of greater than 10 dB. Comparison of the absorption recovery process in both devices suggested that the process is strongly affected by the QW structure.     

A high-speed intersubband modulator based on quantum interferencein double quantum wells

Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 μm is predicted for a device length of 18 μm and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect     

Electroabsorption modulator using intersubband transitions in GaN-AlGaN-AlN step quantum wells

We calculate the high-speed modulation properties of an electroabsorption modulator for /spl lambda/=1.55 /spl mu/m based on Stark shifting an intersubband resonance in GaN-AlGaN-AlN step quantum wells. In a realistic simulation assuming an absorption linewidth /spl Gamma/=100 meV we obtain an RC-limited electrical f/sub 3dB//spl sim/60 GHz at an applied voltage swing V/sub pp/=2.8 V. We also show that a small negative effective chirp parameter suitable for standard single-mode fiber is obtained and that the absorption is virtually unsaturable. The waveguide is proposed to be based on the plasma effect in order to simultaneously achieve a strong confinement of the optical mode, a low series resistance, and lattice-matched cladding and core waveguide layers. Extrapolated results reflecting the decisive dependence of the high-speed performance on the intersubband absorption linewidth /spl Gamma/ are also given. At the assumed linewidth the modulation speed versus signal power ratio is on a par with existing lumped interband modulators based on the quantum confined Stark effect.     

Understanding the ultra-low intersubband saturation intensity in InGaAs-AlAsSb quantum wells

We report a novel intersubband material system based on strained InGaAs-AlAs-AlAsSb quantum wells that exhibits a very low, 3 fJ//spl mu/m/sup 2/, saturation intensity and about 2-ps carrier-relaxation time at 1.68 /spl mu/m. We performed a density matrix calculation to estimate the saturation intensity by simulating the pulsed excitation conditions of the experiment. These studies indicate slow dephasing time and reduced inhomogeneity as the possible mechanisms for the observed large nonlinearity. A detailed dephasing time calculation shows that rather than the reduction in the electron effective mass in these strained quantum wells (QWs), the effect of enhanced concentration of doped carriers in these QWs with reduced inhomogeneity could be the origin of slow dephasing and the observed large nonlinearity.     

Phonon-assisted tunnelling of photoexcited carriers from InGaAs quantum wells in applied electric fields

We have measured photocurrent spectra and photoluminescnce decay times of InGaAs quantum well pin structures as a function of temperature under different conditions of illumination intensity and applied electric field. Our results give clear evidence that phonon-assisted tunnelling of carriers is the dominant escape mechanism. We have obtained direct evidence for the build up of holes and interpret the reduction of the hole concentration at high fields in terms of phonon-assisted tunnelling via the lowest light-hole subband.     

Light absorption and refraction due to intersubband transitions of hot electrons in coupled GaAs/AlGaAs quantum wells

Variation of the absorption coefficient and refractive index of a system of tunnel-coupled GaAs/AlGaAs quantum wells in a longitudinal electric field is discovered and investigated in the spectral region corresponding to intersubband electron transitions. The phenomena observed are explained by electron heating in the electric field and electron transfer in physical space. The equilibrium absorption spectra at lattice temperatures of 80 and 295 K are presented. Fiz. Tekh. Poluprovodn. 32, 852–856 (July 1998)     

On the optimization of resonant intersubband nonlinear opticalsusceptibilities in semiconductor quantum wells

A systematic procedure is proposed for the optimal design of quantum-well structures that provides maximal nonlinear optical susceptibility of second-order under conditions of double resonance. The method is based on the supersymmetric quantum mechanics. It starts from the linear harmonic oscillator potential, itself lacking any nonlinear susceptibility, to generate a family of isospectral asymmetric potentials-its supersymmetric partners-which are then varied to maximize the products of matrix elements relevant for nonlinear susceptibility. The best value of susceptibility obtained exceeds those reported in the current literature. Possibilities of practical realization of quantum-well structures having optimized potentials are also discussed     

Periodic formation of transient population inversion for intersubband laser transitions in quantum wells

A method of periodic short-term inversion for intersubband laser transitions in a quantum well with a repetition frequency of ～1 GHz and inversion retention time ～1 ps is suggested. The method is based on fast population of the upper transition level as a result of resonance tunneling of charge carriers from neighboring quantum wells as the electric field applied to the heterostructure is varied. As a consequence, the peak value of the population inversion can be as large, for example, as 8 × 10¹⁰ cm^?2 for the transition corresponding to the wavelength λ ≈ 10 μm and 2 × 10¹⁰ cm^?2 in the case of λ ≈ 25 μm. This method can be implemented at room temperature, which makes it attractive for fabrication of amplifiers of picosecond pulses for the middle and far infrared optical regions and the terahertz band.     

Time-resolved raman scattering measurement of electron-optical phonon intersubband relaxation in GaAs quantum wells

We have measured the lifetime of electronic intersubband excitations by emission of longitudinal optical phonons in GaAs---AlGaAs quantum wells using time-resolved anti-Stokes Raman scattering. We find the lifetime of electrons in the N = 2 level of a 146Å well to be 1ps which is in excellent agreement with theoretical calculations of two-dimensional electron-photon interactions.     

Intersubband absorption saturation in InGaAs-AlAsSb quantum wells

We present intersubband absorption saturation studies in InGaAs-AlAsSb quantum wells. We carried out a density matrix calculation to simulate the pulsed excitation condition after including the dephasing time and the short pulse profile to estimate the saturation intensity (I/sub S/). We compare the calculated results with measurements for both resonant and nonresonant excitation. We also present our results on the effect of pulsewidth on Is estimation.     

双曲型量子阱中非线性光学吸收率的计算

量子阱中的非线性光学效应因其潜在的实用价值而引起了人们的广泛的关注,而量子阱内带间的光学吸收问题对研究远红外光学探测器等光电子器件具有重要的理论指导意义.用量子力学中的密度矩阵算符理论导出了双曲型量子阱中的线性与三阶非线性光学吸收系数的表达式.因双曲型量子阱中有一个可调参数,随着参数的增加,阱宽将相应增加,因此势阱的形状以及阱内的非线性光学吸收率将随参数变化而发生规律性的变化,并且当入射光强增强到一定程度会出现较强的饱和吸收现象,通过对这些规律的研究从而为实验研究提供了必要的理论依据.     

Characterizing electric fields in (111)B InGaAs quantum wells using electric field modulated photoluminescence and reflectance techniques

We have performed a series of electroreflectance, photoluminescence, and electric-field-modulated photoluminescence experiments to characterize the strain-induced electric fields in (111)B InGaAs/AlGaAs quantum well p-i-n diode structures. A 180° phase change in the lineshapes of electroreflectance spectra of these samples determines when the quantum well is biased to flatband. Using this bias and a depletion model for the diode, the polarization field in the quantum well can be determined. Contrary to expectations, this polarization field increases significantly with increasing temperature. In addition, at fixed temperature, the quantum well transition energies red-shift with increasing excitation intensity when excited by photons of energy higher than the lowest quantum well transition but lower than the AlGaAs diode's bandgap. When excited with photons of energy greater than the AlGaAs bandgap, the transition energy first red shifts then blue shifts with increasing excitation intensity.     

Inversion of the electron population in subbands of dimensional quantization with longitudinal transport in tunnel-coupled quantum wells

The scheme of a laser which can operate in the far-infrared range (λ ∼ 150 μm) is suggested. In order to attain the inversion of the subband population it was suggested that electron transport in three tunnel-coupled quantum wells in a strong electric field, which lies in the plane of quantum wells, be used. An important specific feature of the structure suggested is the presence of a single rough heterointerface. The electron trans-port was simulated by the Monte Carlo method for the Al_xGa_1−x As/GaAs (x=0.2–0.3) heterostructure. The simulation demonstrated that the population inversion in the first and second subbands of dimensional quantization is realized in the field above 1.2 kV/cm at T=4.2 and 77 K. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 724–729. Original Russian Text Copyright ? 2002 by Aleshkin, Dubinov.