Asymmetric Coupled Quantum Wells Research Articles

Photoluminescence spectroscopy of intersubband population inversion in aGaAs/AlxGa1−xAstriple-barrier tunneling structure

We have used interband photoluminescence (PL) spectroscopy to demonstrate the occurrence of an intersubband population inversion in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ triple barrier structure containing asymmetric coupled quantum wells. The relative populations of the $n=1$ $(E1)$ and $n=2$ $(E2)$ electron subbands of the wider quantum well (QW1) are deduced from the relative intensities of the PL peaks arising from recombination of the $E1$ and $E2$ electrons. A significant population inversion is obtained between $E2$ and $E1$ when the structure is biased so that $E1$ is in resonance with the $n=1$ ${(E1}^{*})$ level of the narrower quantum well (QW2). The key importance of the interwell resonance in achieving population inversion is confirmed by comparison with PL results from structures in which ${E1\ensuremath{-}E1}^{*}$ alignment does not occur.

Spatially-Localized Band-Filling Effects and Band-Gap Renormalization in Growth-Interrupted Quantum Wells

We have observed a large excitonic linewidth broadening at low temperatures in growth-interrupted asymmetric-coupled quantum wells as irradiance increases. We attribute this broadening to the decrease of the exciton binding energy due to spatially-localized band-gap renormalization. We have also observed the stepwise saturation of the photoluminescence emission peaks as irradiance increases in growth-interrupted asymmetric-coupled quantum wells with various structures and doping profiles. We attribute this saturation to the result of spatially-localized band-filling effects. Based on the time-resolved photoluminescence measurements, we have determined the nature of the recombination processes and the exciton densities. In both undoped and modulation-doped samples, the small interface island area as a result of growth-interruption allows us to generate large carrier density in the islands; both band-gap renormalization and band-filling effects become stronger even at low irradiances. In compensation-doped asymmetric-coupled quantum wells, we observed anomalously large blue shift of donor-acceptor pair transition energy as the laser intensity increases.

Nonlinear dynamics of excitons in asymmetric coupled quantum wells

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for an excitonic wave packet in a coupled quantum well system. Considering an electronic many-body interaction and an external electric field, we have obtained a nonlinear dynamical evolution for the excitonic wave function. At large electronic sheet densities, it is found that the charge dynamically trapped in both wells produces a reaction field which modifies the system resonant condition.

Long-wavelength (≈15.5 μm) unipolar semiconductor laser in GaAs quantum wells

A unipolar semiconductor laser emitting in the mid-infrared spectral region is demonstrated. The laser scheme relies on a simple three-level system in GaAs/AlGaAs asymmetric coupled quantum wells. Population inversion between excited states is achieved by optical pumping of electrons from the ground state with a CO2 laser. Long-wavelength (≈15.5 μm) laser emission is demonstrated. The laser is operated in the pulsed regime up to a temperature of 110 K and with an output peak power ≈0.4 W at 77 K. Unipolar quantum well semiconductor lasers based on this principle are capable of covering the long wavelength mid-infrared spectral region above 12 μm.

Evidence of strong sequential band filling at interface islands in asymmetric coupled quantum wells

We report the observation of relative saturation among excitonic emission peaks at different sets of interface islands in growth-interrupted asymmetric-coupled quantum well GaAs/Al0.2Ga0.8As structures. The saturation is due to sequential filling of excitonic states at different sets of interface islands. In contrast to free excitonic states, the small total area density of excitonic states at the interface islands makes their filling observable at much lower excitation levels. As a result of the relative saturation, an effective blue shift of the apparent excitonic emission peak at the islands, with a magnitude as large as ∼6.1 meV is observed when the excitation intensity increases from ∼1.6 to ∼215 W cm−2. The highest intensity required to observe the effective blue shift is about two orders of magnitude lower than that needed to observe a similar effect in a free excitonic emission peak.

Polarization-independent large field-induced refractive index change in a strained five-step GaAs–InAlGaAs asymmetric coupled quantum well

A tensile-strained five-step asymmetric coupled quantum well (FACQW) structure is proposed for large field-induced refractive index change without polarization dependence and redshift of the absorption edge. A strong exciton absorption peak is caused by electron-hole transitions between symmetric wave functions and antisymmetric wave functions with a small applied electric field. The field-induced refractive index change of strained the FACQW is larger by one order of magnitude compared to that of a rectangular quantum well and the difference in the refractive index change of TE and TM modes is under 2% when the operation wavelength is not at the absorption edge.

Intersubband stimulated emission in GaAs/AlGaAs quantum wells: Pump-probe experiments using a two-color free-electron laser

Intersubband stimulated emission under optical pumping has been observed in the conduction band of GaAs–AlGaAs quantum wells. The asymmetric coupled quantum wells which exhibit three conduction bound levels are designed to exhibit population inversion under optical pumping. An optical excitation at λ=9.2 μm is used to bleach the absorption between the ground and second excited subband. The population inversion between excited subbands is pumped and probed on a picosecond time scale by a tunable two-color free-electron laser. The stimulated amplification is studied at low temperature in infrared waveguides as a function of the waveguide length and of the probe wavelength. A stimulated gain ≈80 cm−1 is measured at 12.5 μm in agreement with calculations.

Mid-infrared intersubband emission from optically pumped asymmetric coupled quantum wells

Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 μm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved.

Large field-induced refractive index change without red shift of absorption edge in five-step asymmetric coupled quantum wells with modified potential

A structure composed of five-step asymmetric coupled quantum wells (FACQWs) is first proposed for large field-induced refractive index change without red shift of absorption edge. A strong exciton absorption peak is caused by e1hh2 and e2hh1 transitions with a small applied electric field. The field-induced refractive index change of FACQWs is larger by one order compared to that of rectangular quantum well when the operation wavelength is apart from absorption edge.

Chaotic behavior of hole mixing tunneling in asymmetric coupled quantum wells

Coherent oscillatory motion of hole mixing tunneling in asymmetric coupled quantum well structures collapses if the heavy hole state in the first well is aligned with the light hole state in the second well under a significant in-plane vector k ∥. The tunneling probability obtained by the time-dependent Schrödinger operator with the Luttinger Hamiltonian in the oscillation collapse region is shown to be a chaotic time series using the simplex projection method. This effect is attributed to the nonlinear interaction of mixing tunnelings between HH to the LH state in the first well and HH to the LH state in the second well.

Engineering of the nonradiative transition rates in modulation-doped multiple-quantum wells

The feasibility of the enhancement of the acoustic-phonon limited intersubband transition rate through impurity scattering has been theoretically investigated in double asymmetric-coupled quantum wells. The dependence of the acoustic-phonon rate on the barrier thickness and the effect of the position of the /spl delta/-doped region on the impurity rate have been treated rigorously. A 10-/spl Aring/-doped region with a 10/sup 10/-cm/sup -2/-sheet density can enhance the acoustic-phonon transition limit by more than an order of magnitude. This allows for the design of intersubband lasers in which population inversion between acoustic-phonon limited discrete conduction-band states is achieved by impurity scattering and control of barrier thickness.

All-optical waveguide-type gain switch utilizing intersubband transition

All-optical switching performance of waveguide-type device utilizing intersubband transition is investigated by analyzing the position-dependent change of optical power densities and carrier densities in the waveguide. In the device, the three lowest energy-level subbands in the conduction band structure of asymmetric coupled quantum wells are used. The optical excitation pulse excites the electrons in the first subband to the third subband and the stimulated emission between the third subband and second subband amplifies the optical signal pulse. We find that the signal pulse is notably amplified when the excitation pulsewidth is less than the relaxation time of the third subband. The calculated results show that wavelength conversion from 1.35 to 1.6 /spl mu/m with a gain of more than 10 dB is achieved by the excitation using an optical pulse with 1.5-ps width. This demonstrates the effectiveness of using a waveguide-type device structure with pulsed operation to make devices with a high-optical gain. We also derive a simple approximation for the optical gain. We show that the approximation gives results in good agreement with the results of the rigorous simulation using a finite difference method and the approximation is useful for designing devices.

Continuous wave half-gap second-harmonic generation in [formula omitted] asymmetric coupled quantum wells

We report continuous-wave interband second-harmonic generation (SHG) at 460–480 nm in strained Zn1 − xCdxSeZnSe asymmetric quantum wells grown by molecular beam epitaxy. The dependence of the SHG intensity on the fundamental frequency allowed us to identify the individual quantum-well exciton resonances in the second-harmonic signal. The strongest contribution to SHG associated with the light-hole exciton is discussed and related to confinement properties of the light-hole. Numerical calculations of the SHG intensity that take into account exciton and continuum states have been performed as a function of the valence-band contribution Qv to the band-gap difference in the heterostructures. Best agreement with experiment is found for Qv = 0.33.

Photoluminescence Investigation of Doped Asymmetric Coupled Quantum Wells

We report in this paper the photoluminescence (PL) investigation of a partly doped asymmetric coupled quantum well (ACQW) structure. The dependence of non-resonant tunneling rate on excitation power for different Al0.28Ga0.72As interbarrier thicknesses is studied in detail. The PL intensity from a 20 nm Al0.2Ga0.8As well is much larger than that from a 5.5 nm Si-doped GaAs well when the interbarrier is thick (>12 nm). This result indicates that photo excited carriers are preferentially relaxed toward the wide well, which has larger state density than the doped narrow well. The competition among different processes of intersubband relaxations is discussed in connection with the PL spectra.

Observation of infrared intersubband emissionin optically pumped quantum wells

Mid-infrared intersubband emission is investigated in GaAs/AlGaAs asymmetric coupled quantum wells exhibiting an energy separation between the ground and first excited subbands close to the LO-phonon energy. The authors show that an infrared emission occurs between excited subbands under intersubband optical pumping.

Optically pumped intersub-band emission in quantumwells

Mid-infrared intersub-band emission is investigated in asymmetric coupled quantum wells. The authors predict that stimulated emission can occur between excited sub-bands under either intersub-band or band-to-band optical pumping when the energy separation between the ground and first excited sub-bands equals the LO phonon energy.

Electric-field enhancement and extinguishment of optical second-harmonic generation in asymmetric coupled quantum wells

The second-harmonic susceptibilities of asymmetric coupled quantum wells (ACQW) and compositionally asymmetric coupled quantum wells (CACQW) under the influence of the applied electric field are investigated theoretically. Analytic forms of the second-harmonic susceptibility are derived by using the density matrix formalism. Coupled one-dimensional Schrodinger equation and Poisson's equation are solved self-consistently to find the subband eigenenergies and the envelope wavefunctions for the ACQW and CACQW structures. Dipole moment matrix elements for the ACQW and CACQW are evaluated from the resulting envelope wavefunctions and large nonlinear optical effects are predicted for these two structures. Based on the theoretical calculations, the second-harmonic susceptibilities of 50 nm/V and 35 nm/V can be achieved for the ACQW and the CACQW, respectively. This is more than two orders of magnitude enhancement as compared to that of the bulk GaAs. By a suitable choice of the composition of CACQW, a novel structure which the second-order nonlinear optical effect can be turned on or off by the applied electric field is proposed based on the results of the theoretical calculation. This phenomenon is attributed to the symmetry restoration of envelope wavefunctions of the CACQW under the quenching electric field F/sub off/. A simple physical model to estimate the F/sub off/ has also been developed successfully. >

Electric-field-induced refractive index changes in InGaAs-InAlAs asymmetric coupled quantum wells

Two kinds of InGaAs-InAlAs asymmetric coupled quantum wells (QW's) were numerically analyzed in an effort to enhance change in the refractive index (/spl Delta/n) at longer wavelengths where absorption coefficient is small. The analysis reveals the operating voltage, /spl Delta/n, chirp parameter, /spl Delta/n//spl Delta/k, (/spl Delta/k: change in the extinction coefficient) and figure of merit, /spl Delta/n//spl alpha/, (/spl alpha/: absorption coefficient) can be improved by these two kinds of the QW's. This improvement is caused by an abrupt electric-field-induced increase or decrease in the oscillator strength for the lowest heavy hole exciton.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of the photoluminescence-linewidth broadening in periodic multiple narrow asymmetric coupled quantum wells.

Photoluminescence spectra were measured as a function of temperature and excitation intensity in two periodic multiple narrow asymmetric coupled quantum-well structures. The peaks of the spectra have been identified and the temperature-dependent natures of the recombination processes have been determined. The half-widths at half maximum of the line shapes were deduced by fitting the low-energy sides to Gaussian functions. The excellent fits indicate that inhomogeneous broadening is dominant. The rate at which the linewidth broadens with temperature is more dramatic below 140 K due to the sharp changes of the ionized impurity and of the longitudinal-optical-phonon scattering mechanisms. Above 140 K, the linewidth has a linear dependence on temperature due to the thermal excitation of the longitudinal optical phonons. By using the model proposed by Lee et al. we have determined the scattering rates of the longitudinal acoustic and optical phonons, of the impurities, and of the inhomogeneous broadening. Our results show that unless the growth is interrupted at each interface, the inhomogeneous broadening remains dominant up to room temperature. Measurement of the temperature dependence of the photoluminescence linewidth proves to be a simple and effective technique to investigate the scattering mechanisms of the photogenerated carriers and the origins of the emission peak's linewidth broadening in these structures.

Triply resonant enhancement of third-order nonlinear optical susceptibility in compositionally asymmetric coupled quantum wells

The third-order nonlinear optical susceptibility χ(3)(3ω) due to the intersubband transitions in the four-level AlInAs/GaInAs compositionally asymmetric coupled quantum well (CACQW) is investigated theoretically. The subband eigenenergy En of the CACQW structure could be designed to form an equally spaced energy-level ladder. Since the eigenenergy spacing could be designed to resonate with the pumping source, the third-order nonlinear optical susceptibility could be greatly enhanced through the triple resonance. Based on the theoretical calculations, a magnitude of ‖χ(3)(3ω)‖ as high as 2.2×105 (nm/V)2 can be achieved for the CACQW structure. This is a more than eight orders of magnitude enhancement as compared to that of the bulk value in GaAs. In addition to the design of CACQW structure, the triple resonance can also be achieved by biasing the CACQW under a proper electric field due to the large Stark effect of the CACQW structure.