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.     

Monolithic spectrometer based on intersubband transitions inquantum wells

We present a new device based on the association of a quantum-well infrared photodetector and an asymmetric quantum-well infrared modulator in a three-terminal monolithic structure. This device is shown to allow the spectral analysis of an infrared signal. A large tunability and a constant responsivity make this device an ideal candidate for a solid-state infrared spectrometer     

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     

发光量子阱对波长上转换红外探测器效率的影响

波长上转换红外探测器在实现大面阵、低暗电流红外探测方面具有很大的发展潜力.短波长光子的发光效率是影响上转换器件效率的重要因素.设计、制作了具有不同发光阱个数的波长上转换红外探测器件,结合器件的红外响应和仿真计算,分析了发光阱个数对波长上转换效率的影响规律.研究结果表明,选择单个发光阱,有利于提高器件的发光效率,从而提高波长上转换效率.     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

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.     

High-responsivity intersubband infrared photodetector usingInGaAsP/InP superlattice

A 4-7-μm infrared detector made of an InGaAsP/InP short-period superlattice is demonstrated with materials grown by metalorganic chemical vapor deposition (MOCVD). A single current blocking layer of InP is used to reduce the dark current. At 40 K, the detector shows a low dark current of less than 4 pA at a bias voltage of 4 V. At 35 K, a peak responsivity of 4.0 A/W is obtained at 5.6 μm at a bias of 1 V     

Instabilities in optically pumped infrared lasers

The unstable emission of an optically pumped FIR gas laser is theoretically analysed through numerical resolution of the associated Maxwell-Bloch equations. The instability domain in the main control parameters space is determined and an example of chaotic behaviour is examined.     

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     

Room-temperature intersubband emission from GaInAs-AlAsSb quantum cascade structure

Room-temperature (300 K) intersubband electroluminescence has been obtained from a quantum cascade structure adopting triple-well vertical-transition active regions, based on Ga/sub 0.47/In/sub 0.53/As/AlAs/sub 0.56/Sb/sub 0.44/ heterostructures grown lattice-matched on InP substrate by molecular beam epitaxy. The emission peak wavelength varies from 4.3 /spl mu/m at 77 K to 4.5 /spl mu/m at 300 K.     

Observation and assignment of torsional transitions in FIR emission from optically pumped CH3OH

Using output coupling by reststrahlen reflection from NaCl we have observed 14 new FIR lines in the wavelength range between 30 μm and 60 μm. Two of them have been identified as torsional n=1→0 transitions in the CO stretch vibration. Together with lines previously reported in the literature they are the basis for an identification of 41 FIR lines associated with torsionally excited states.     

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.     

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     

Hole mixing and strain effects in the conduction intersubband transitions of undoped quantum wells

We study the effects of hole dispersion and mixing in the conduction intersubband transitions and infrared dressing of undoped quantum wells (QWs). This is done considering transitions of photo-excited electrons from one conduction subband (e1) to another (e2) in the presence of Coulomb interaction with the photo-excited holes. We show that, when the dispersion of the hole subband hh1 (lh1) is mainly parabolic, these transitions occur mainly between the s-states of e1-hh1 (e1-lh1) and e2-hh1 (e2-lh1) excitons with the same principal quantum numbers (allowed transitions). When the hole subbands have nonparabolic dispersions, however, such transitions are suppressed while another type of intersubband transitions in which the initial and final exciton states have different principal quantum numbers (nonallowed transitions) are enhanced. We show the enhancement and suppression processes reach their maxima when hh1 and lh1 are about to cross over, allowing multilevel mixing of excitons to occur when the undoped QW interacts with a single intense infrared field polarized along its growth. We associate these results with spinor mixing of hh1 and lh1 and illustrate how via changing the spinor contributions in these subbands one can employ strain to manipulate the dipole moments of the intersubband transitions.     

The effect of resonant sublevel coupling on intersubband transitions in coupled double quantum wells

The effect of resonant sublevel coupling on intersubband transitions in double quantum wells is investigated using far-infrared spectroscopy. We study widely tuneable parabolic double quantum wells in which potential spikes of different energetic height and thickness provide tunnel barriers for the electron systems on either side of the barrier. The use of gate electrodes enables us to tune both the carrier densities as well as the respective sublevel spacings and allow for a manifold degeneracy scheme like in an artificial molecule where the atomic number of both partners can be intentionally changed. Depending on the actual experimental condition, we observe pronounced level anticrossings into a symmetric and antisymmetric state. This single-particle sublevel coupling manifests itself in a rich spectrum of the observed collective intersubband transitions which occur at the depolarization shifted intersubband energy.     

Control of the interband and intersubband transition energy in quantum wells using localized isoelectronic perturbations

Within the scope of the simple one-band approximation, the effect of built-in attractive and repulsive δ-shaped potentials on the energy spectra of carriers and oscillator strength was studied for optical transitions in quantum wells. The effect of δ-shaped perturbation on the magnitude and sign of the energy level shift was shown to depend heavily on the isoelectronic probe position and the effective mass inhomogeneity. Calculation of the oscillator strength for intersubband transitions confirms the possibility of developing coupled quantum-well structures with interesting nonlinear properties.     

Nonlinear level crossing in the infrared transitions of optically pumped far infrared laser media

In conjunction with the Stark field induced power enhancement in optically pumped far infrared lasers, absorption transitions in a degenerate two level system under the influence of an weak static Stark field have been analyed. A third order theory to the density matrix formalism leads to zero field level crossing signals for absorption in rate equation approximation and in Doppler limit. For fast relaxation among Zeeman sublevels, the zero field level crossing signal arises from population effect. Optoacoustic measurements have been carried out on the ν₅,^qQ(16,8) absorption line of CH₃OH which produces the familiar 119-μm laser with the CO₂ 9P(36) pump line. The optoacoustic absorption signals observed are well reproduced by the theoretical expression.     

Mid infrared and far infrared studies of the propyne optically pumped laser

An extensive study of the propyne optically pumped laser is presented. 32 new far infrared and 8 new mid infrared outputs are reported, together with pump frequency offset data. Complete assignments of some far infrared transitions and partial assignments of the mid infrared outputs are given. Very strong interactions occur between the mid and far infrared laser transitions. These interactions are of vital importance in obtaining high output energies.