Intersubband Transitions In Quantum Research Articles

Mid-Infrared Intersubband Cavity Polaritons in Flexible Single Quantum Well

Strong and ultrastrong coupling between intersubband transitions in quantum wells and cavity photons have been realized in mid-infrared and terahertz spectral regions. However, most previous works employed a large number of quantum wells on rigid substrates to achieve coupling strengths reaching the strong or ultrastrong coupling regime. In this work, we experimentally demonstrate ultrastrong coupling between the intersubband transition in a single quantum well and the resonant mode of photonic nanocavity at room temperature. We also observe strong coupling between the nanocavity resonance and the second-order intersubband transition in a single quantum well. Furthermore, we implement for the first time such intersubband cavity polariton systems on soft and flexible substrates and demonstrate that bending of the single quantum well does not significantly affect the characteristics of the cavity polaritons. This work paves the way to broaden the range of potential applications of intersubband cavity polaritons including soft and wearable photonics.

Intersubband transitions in quantum well mid-infrared photodetectors

A study of intersubband transitions in quantum well infrared detectors working at high temperatures has been reported. This study allows a greater tunability in the device designs, with the ability to control the peak wavelength, the absorption coefficient, the dark current, the quantum efficiency and the detectivity of the modeled structure operating around 3.3μm wavelength. The detection energy and absorption coefficient dependences with an applied electric field are given. Then, the electro-optic performances of the modeled mid-infrared detector are estimated, the dark current dependence with the applied voltage and temperature as well as the quantum efficiency and the detectivity are investigated and discussed. High detectivities were found at high temperatures revealing the good performances of the designed photodetector, especially at 3.3μm wavelength.

Modeling of intersubband transitions in quantum well infrared photodetectors with complex potential profiles

The subband energy dispersions and optical intersubband transitions in n-type InGaAs/AlxGa1-xAs quantum well infrared photodetector (QWIP) with linear-graded barriers are calculated using an 8-band k·p model combined with the envelope-function Fourier expansion. The relaxation of quantum confinement in the growth direction has been taken into reasonable consideration. This work is helpful for the analysis and the design of QWIPs with complex well and barrier structures.

Rabi flopping in a two-level system with a time-dependent energy renormalization: intersubband transitions in quantum wells.

We obtain pulse-driven Rabi oscillations guided by a generalization of the rotating-wave approximation to include, in the optical-Bloch equations, two-level systems with a time-varying transition energy. We achieve this by using chirped pulses with the central frequency given by the time-varying transition energy. Using this approach, we predict Rabi oscillations in intersubband transitions in a two-subband n-type modulation-doped quantum well by taking into account the time-dependent intersubband energy-gap renormalization due to depolarization-shift effects. We obtain Rabi oscillations for jpi (j=0,1,2, ) pulses in the presence of dephasing.

Optimized random/ordered grating for an n-type quantum well infrared photodetector

Coupling efficiency of infrared radiation to electron intersubband transition in quantum well infrared photodetector by a two-dimensional (2D) grooved grating is examined. A resonant enhancement of the coupling efficiency at a certain optimal groove depth is predicted and confirmed experimentally. It is argued that a detector with a properly designed ordered cross grating performs better than a detector with optimized 2D random grating. In a detector with ordered grating a quantum efficiency of 75% is obtained with a doping level of only 3.5×1017 cm−3 in each quantum well and dark current density of 0.34 mA/cm2 at 78 K.

Mechanisms of Intersubband Transition in n-Type III-V Quantum Well Superlattice and Improvement on Absorption for TE Polarized Field

AbstractIn this paper, the theoretical study of intersubband transitions in quantum well infrared photodetectors (QWIPs) applying the eight bands k.p model incorporated with envelope function approximation is described. The focus of the work is on the intersubband transition in n-type IIIV QWIP based on AIGaAs/GaAs and AlGaAs/InGaAs material system, with particular emphasis placed on the physics of TE excited transition and the improvement of resulted absorption. Various theoretical absorption spectra of the two material systems are compared, the distinct mechanisms of the intersubband transition for the two material systems are proposed. Possible ways of improving on the absorption for such excitation are also investigated and discussed.

Resonant screening effect on the photon-drag current spectra in quantum wells

Influence of the resonant screening on the photon-drag effect connected with the resonant intersubband transitions in quantum well systems is discussed theoretically employing the self-consistent field method. The results obtained indicate that in systems where the momentum relaxation times for the ground and excited subband are different the photon-drag current can be written as a sum of two terms. These terms are affected by the resonant screening in a different way.

Two-photon intersubband transitions in quantum well infrared photoconductors

We demonstrate that a symmetric GaAs/GaAlAs quantum well photodetector with a peak responsivity centered at 8.1 μm shows a significant quadratic power response at 10.6 μm. This is due to intersubband two-photon transitions from the ground level to continuum states. The two-photon responsivity agrees with a theoretical prediction based on a perturbative approach and photoconductive gain estimations.

Anisotropy, birefringence, and optical phase retardation related to intersubband transitions in multiple quantum well structures

It is well known that the selection rules for intersubband transitions in quantum well structures require that the infrared light be polarized parallel to the growth direction. As a result, the induced intersubband susceptibility tensor becomes highly anisotropic and the crystal becomes birefringent. We have studied the effect of induced birefringence at the midinfrared range of the spectrum using a number of experimental techniques, including Fourier transform infrared absorption spectroscopy and optical phase retardation measurements using a tunable CO2 laser and a cross polarizer setup. We have observed that linearly polarized light becomes almost circularly polarized due to optical phase retardation between the ordinary and the extraordinary directions over a short optical path length of the order of 20 μm near the resonance (but not at the resonance). The real and the imaginary parts of the induced extraordinary refractive index were measured and have been found to be of the same order of magnitude. We also show that a solution of the Fresnel equation, modified to take into account both the imaginary and the real parts of the susceptibility tensor, is in good agreement with our experimental results.

Intersubband transitions in quantum well heterostructures with delta-doped barriers

A single quantum well doped by a negative delta-function potential (δ-potential) in the barrier regions is analyzed in terms of the optical transitions in between subbands. The first two states of the quantum well do not change at all as a function of the strength of the δ-potential up to a certain value, whereas the third one gets lowered almost exponentially. An important point is that the δ-potential brings a state from the continuum to the bound region. There is a range of the strength of the δ-potential during which the transition energy for the first to second state decreases rapidly, and at a certain strength the first and second states disappear, and the third and fourth, which have been brought from the continuum, take their places. The oscillator strengths of the allowed transitions have a kind of oscillatory behavior in that range.

Compositionally asymmetrical multiquantum wells: “Pseudo-molecules” for giant optical nonlinearities in the infrared (9–11 μm)

Intersubband transitions in quantum well have extremely large oscillator strengths and induce strong nonlinear effects in structures where inversion symmetry is broken, realized by growing AlGaAs quantum wells with asymmetrical A1 gradients. These compositionally asymmetrical multiquantum wells may thus be viewed as giant “quasimolecules” optimized for optimal nonlinearities in the mid infrared. Optical rectification as well as second harmonic generation have been measured in those structures using a continuous CO 2 laser. At 10.6 μm the nonlinear coefficients are more than 3 orders of magnitude higher in these samples than for bulk GaAs (i.e. χ 0 (2) = 5.3 × 10 −6m/V, χ 2 ω (2) = 7.2 × 10 −7 m/V) and are in good agreement with theoretical predictions. We present more complex “pseudo-molecules” involving weakly coupled quantum wells. The optical rectification effects in these devices are so large χ 0 (2) = 1.6 × 10 −3 m/V) that application to infrared detection may be envisioned.

Electric field effects on intersubband transitions in quantum well structures

We report on a Raman scattering study of the electric-field dependence of c 0 → c 1 intersubband transitions of photoexcited electrons in a 264 Å GaAs- Al 0.3Ga 0.7As quantum-well structure. The measured Stark shifts are in very good agreement with theoretical predictions. The intensity of the intersubband peak increases rapidly with applied field due to parity-mixing. In contrast to the enhanced broadening shown by excitation resonances, the width of c 0 → c 1 is nearly independent of the field. This feature is attributed to effects of structural disorder.