We have obtained a closed-form expression for the threshold of Risken-Nummedal-Graham-Haken (RNGH) multimode instability in a Fabry-Pérot (FP) cavity quantum cascade laser (QCL). This simple analytical expression is a versatile tool that can easily be applied in practical situations which require analysis of QCL dynamic behavior and estimation of its RNGH multimode instability threshold. Our model for a FP cavity laser accounts for the carrier coherence grating and carrier population grating as well as their relaxation due to carrier diffusion. In the model, the RNGH instability threshold is analyzed using a second-order bi-orthogonal perturbation theory and we confirm our analytical solution by a comparison with the numerical simulations. In particular, the model predicts a low RNGH instability threshold in QCLs. This agrees very well with experimental data available in the literature.

A theoretical study on low-threshold multimode instabilities in quantum cascade lasers (QCLs) and laser diodes (LDs) is presented. Previously, low threshold Risken-Nummedal-Graham-Haken (RNGH) instabilities were reported in several experimental studies of QCLs. They were attributed so far to the combined effect of the induced grating of carrier population and of a built-in saturable absorption feature that may be present in the monolithic single-section cavity of these Fabry-Pérot lasers. Here we show that low-threshold RNGH instabilities in QCLs occur due to a combined effect of the carrier coherence grating and carrier population grating induced in the gain medium and not due to an intracavity saturable absorption. We find that QCLs with a few mm long cavity exhibit intermittent RNGH self-pulsations while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 µm or smaller. We examine a transient behavior to RNGH self-pulsations in short-cavity QCLs and find features that resemble cooperative superradiance. Our findings open a practical way of achieving ultra-short pulse production regimes in the mid-infrared spectral range. Applying same approach to semiconductor laser diodes (LDs) we explain the absence of RNGH selfpulsation in single-section LDs based on a quantum well gain media, while practically established method for reaching the ultrafast coherent emission regimes in LDs is to incorporate a separately contacted saturable absorber section in the LD cavity.

Methods for systematic optimization of step-graded and continuously graded ternary alloy based quantum wells (QW's), in respect to second-or third-order intersubband nonlinear susceptibilities at resonance, are discussed. The use of these methods is examplified on the design of Al x Ga 10x N and Al x Ga 10x As-based QW's intended for resonant second harmonic or third harmonic generation with h! = 116 meV or h! = 240 meV pump photon energies, the objective being the largest susceptibility achievable with the chosen material. The obtained results exceed those previously reported in the literature.

We have further improved our procedure for the optimization of multilayer semiconductor nanostructures, based upon diluted magnetic semiconductors, developed previously with the goal of maximizing their spin-filtering properties. The new optimization scheme relies on the application of a modern class of evolutionary algorithms for global optimization, specifically the genetic algorithm. Its fitness function is set to select the best possible spin-polarization properties within the chosen range of bias voltages and with a fixed value of external magnetic field. Numerical calculations are presented for the ZnSe/Zn1−xMnxSe based semiconductor system, and the obtained results predict an enhanced spin-diode performance over the existing designs.

Time delays for an intense transverse electric (TE) wave propagating through a Kerr-type nonlinear slab are investigated. The relation between the bidirectional group delay and the dwell time is derived and it is shown that the difference between them can be separated into three terms. The first one is the familiar self interference time, due to the dispersion of the medium surrounding the slab. The other two terms are caused by the nonlinearity and oblique incidence of the TE wave.It is shown that the electric field distribution along the slab may be expressed in terms of Jacobi elliptic functions while the phase difference introduced by the slab is given in terms of incomplete elliptic integrals. The expressions for the field intensity dependent complex reflection and transmission coefficients are derived and the multivalued oscillatory behavior of the delay times for the case of a thin slab is demonstrated.