630-631 (2008). 27. For the density matrix calculations, the electron temperature was chosen 90 K higher than lattice. Pure dephasing time constants of tunneling τ * = 0.35 ps, and of optical intersubband transition τ * ul = 1.1 ps were used. Intrawell intersubband scatterings by LO phonon, e-impurities and interface roughness were considered. The momentum dependance of scattering is averaged over the assumed Maxwell-Boltzmann distribution of carriers in the subbands. 28.

A theoretical and experimental study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes: One is spatial hole burning (SHB) and the other one is related to the Risken-Nummedal-Graham-Haken instability predicted in the 1960s. A model that can account for coherent phenomena, a saturable absorber, and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with a narrow active region and/or with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have a weaker saturable absorber, do not exhibit a Rabi splitting and their multimode regime is governed by SHB. This experimental phenomenology is well-explained by our theoretical model. The temperature dependence of the multimode regime is also presented

The design and projected performance of quantum-well infrared photodetectors (QWIP) for the terahertz (1–10 THz) or the very-far-infrared region are presented together with our initial demonstration of a GaAs/AlGaAs QWIP working at photon energies below the optical phonons. We point out the problem with this initial device, discuss possible causes, and suggest areas of improvement.

Taking into account the discrete nature of the quantum-well intersubband infrared detectors, we construct a model to calculate the photoconductive gain. It is shown that the photoconductive gain is inversely proportional to the number of quantum wells and that the detector-current responsivity is independent of the number of wells.

We report on the generation of picosecond self-mode-locked pulses from midinfrared quantum cascade lasers, at wavelengths within the important molecular fingerprint region. These devices are based on intersubband electron transitions in semiconductor nanostructures, which are characterized by some of the largest optical nonlinearities observed in nature and by picosecond relaxation lifetimes. Our results are interpreted with a model in which one of these nonlinearities, the intensity-dependent refractive index of the lasing transition, creates a nonlinear waveguide where the optical losses decrease with increasing intensity. This favors the generation of ultrashort pulses, because of their larger instantaneous intensity relative to continuous-wave emission.

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Photoresponse characteristics of InAs/GaAs self-assembled quantum-dot infrared photodetectors in a wide spectral region from the mid- to far-infrared are reported. Clear polarization behaviors with a dominant P-polarized response in the mid-infrared and a strong S-response in the far infrared are shown. These behaviors can be qualitatively understood in view of the quantum-dot shape of a large in-plane diameter and a small height in the growth direction. With a set of three samples, effects of the number of electrons per dot on the spectra are investigated.

We characterized a set of terahertz quantum-cascade lasers with identical device parameters except for the doping concentration. The δ-doping density was varied from 3.2×1010to4.8×1010cm−2. We observed that the threshold current density increased monotonically with doping. Moreover, the measured results on devices with different cavity lengths provided evidence that the free carrier absorption caused waveguide loss also increased monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping density of 3.6×1010cm−2.