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.

Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schr\"odinger equation and Schr\"odinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function (NEGF) formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures

We report the observation of a coherent multimode instability in quantum cascade lasers (QCLs), which is driven by the same fundamental mechanism of Rabi oscillations as the elusive RiskenNummedal-Graham-Haken (RNGH) instability predicted 40 years ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.PACS numbers: 42.55. Px, 42.60.Mi, 42.65.Sf The fundamental coherent mechanism that can destabilize a single-mode laser was predicted in the early 60s [1] and was later extended to multi-mode lasers [2,3] where it became known as the RNGH instability. These instabilities became classic landmarks for the general field of nonlinear dynamics [4,5] because they emerge in conceptually the simplest laser model, which in the single-mode case was shown to be equivalent to the Lorentz model of deterministic chaos [6]. Another feature that makes these instabilities so interesting and unique is their coherent nature that involves the polarization of the medium as a nontrivial dynamical variable. Most other physical mechanisms that can drive a laser from a singlemode to a multi-mode regime, such as spatial and spectral hole burning, Q-switching, and saturable absorption [7,8], can be adequately described within the standard rate equation formalism, in which the polarization of the active medium is adiabatically eliminated. The RNGH instability and its single-mode twin cannot be explained by the rate equations. Such coherent effects can be only observed when the polarization is driven faster than or comparable to the dephasing time T 2 [9].The origin of both phenomena is the oscillation of the population inversion at the Rabi frequency Ω Rabi that takes place when the intracavity laser intensity becomes large. This results in a modification of the gain spectrum and the emergence of sidebands separated from the maximum of the gain curve by an amount corresponding to the Rabi frequency. These sidebands can be regarded as a manifestation of parametric gain. The instability sets in when the intracavity power is sufficiently large: the Rabi angular frequency Ω Rabi has to be greater than the relaxation timescales of the gain medium (more precisely, Ω Rabi is sufficiently greater than (T 1 T 2 ) −1/2 , where T 1 is the gain relaxation time). The instability threshold is often called the second laser threshold due to its universal nature.Pioneering theoretical works stimulated extensive experimental studies that finally resulted in the observation of the Lorenz-type chaos in a far-infrared singlemode laser [10]. However, despite almost 40 years of efforts, the experimental demonstration of the multimode RNGH instability has remained somewhat controversial [11,12,13,14,15,16].In lasers with long gain recov...

The generation of frequency combs in the mid-infrared and terahertz regimes from compact and potentially cheap sources could have a strong impact on spectroscopy, as many molecules have their rotovibrational bands in this spectral range. Thus, quantum cascade lasers (QCLs) are the perfect candidates for comb generation in these portions of the electromagnetic spectrum. Here we present a theoretical model based on a full numerical solution of Maxwell-Bloch equations suitable for the simulation of such devices. We show that our approach captures the intricate interplay between four wave mixing, spatial hole burning, coherent tunneling and chromatic dispersion which are present in free running QCLs. We investigate the premises for the generation of QCL based terahertz combs. The simulated comb spectrum is in good agreement with experiment, and also the observed temporal pulse switching between high and low frequency components is reproduced. Furthermore, non-comb operation resulting in a complex multimode dynamics is investigated.

We investigate the origin of high frequency noise in Fourier domain mode locked (FDML) lasers and present an extremely well dispersion compensated setup which virtually eliminates intensity noise and dramatically improves coherence properties. We show optical coherence tomography (OCT) imaging at 3.2 MHz A-scan rate and demonstrate the positive impact of the described improvements on the image quality. Especially in highly scattering samples, at specular reflections and for strong signals at large depth, the noise in optical coherence tomography images is significantly reduced. We also describe a simple model that suggests a passive physical stabilizing mechanism that leads to an automatic compensation of remaining cavity dispersion in FDML lasers.