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In recent years, quantum cascade lasers (QCLs) have shown tremendous potential for the generation of frequency combs in the mid-infrared and terahertz portions of the electromagnetic spectrum. The research community has experienced success both in the theoretical understanding and experimental realization of QCL devices, capable of generating stable and broadband frequency combs. Specifically, it has been pointed out that four wave mixing (FWM) is the main comb formation process and group velocity dispersion (GVD) is the main comb-degradation mechanism. As a consequence, special dispersion compensation techniques have been employed, in order to suppress the latter and simultaneously enhance the former processes. Here, we perform a detailed computational analysis of FWM, GVD, and spatial hole burning (SHB), all known to play a role in QCLs, and show that SHB has a considerable impact on whether the device will operate as a comb or not. We therefore conclude that for a successful implementation of a quantum cascade laser frequency comb, one would need to address this effect as well.

We study the possibility for ultrashort pulse generation from THz quantum cascade lasers via the colliding pulse mode locking technique. Our analysis shows that this approach could enable sub-ps pulses from QCLs, even in devices with carrier recovery times as short as ~ 10 ps.

We present long-term dynamic simulations of the optical and carrier dynamics in quantum cascade lasers, obtained by numerically solving the Maxwell-Bloch equations without invoking the rotating wave approximation. We discuss the applied numerical methods and parallelization techniques used to obtain the required numerical efficiency and accuracy, and show results for a mode-locked terahertz quantum cascade laser.

Maxwell–Bloch equations are widely used to model the dynamics due to coherent light-matter interaction in quantum cascade laser (QCL) structures, which plays an essential role especially for the generation of frequency combs and mode-locked pulses. While the modest numerical complexity of the Maxwell–Bloch system allows for a full spatiotemporal treatment, its main disadvantage is the inclusion of dissipation by empirical dephasing rates and electron lifetimes. We present a self-consistent multi-domain approach which couples the Maxwell–Bloch equations to advanced carrier transport simulations based on a density matrix Monte Carlo technique, yielding the scattering and dephasing rates. In this way, the compact spatiotemporal modeling of the carrier-light dynamics by the Maxwell–Bloch system can be combined with the versatility and reliability of self-consistent carrier transport approaches. Simulation results are shown for a QCL-based terahertz frequency comb source, and good agreement with experiment is obtained.

Their inherently broadband gain and high value of the third order nonlinearity makes quantum cascade lasers (QCLs) ideal candidates for frequency comb generation in the terahertz (THz) and mid-infrared (midIR) spectral ranges. In recent years, it has been demonstrated that free running QCLs can produce broad comb spectra, even without any special endeavour to induce phase-locking of the lasing modes [1]. Unfortunately, such “coherent” regimes of multimode operation seem to span only a very narrow portion of the full dynamic range of the devices, which hinders the practical usability of this technology [1, 2].

Quantum cascade lasers (QCLs) are unipolar semiconductor devices emitting in the mid-and far-infrared spectral ranges. Due to their relatively narrow linewidths and short gain recovery times, these lasers have been deemed difficult, if not impossible, to passively mode lock (PML) [1, 2]. However, the successful mode locking of QCLs is of utmost importance as it could allow generation of ultrashort pulses and also formation of broadband frequency combs. As a consequence, the research community invested substantial effort and funds in devising methods for active mode locking of QCLs with some limited success achieved [1, 2]. In this submission, we revise the prospect of passively mode locking a THz QCL via a fast saturable absorber (FSA). Our results, based on the numerical solution of the Maxwell-Bloch equations, show that it could be possible to implement a “text-book” FSA-PML mechanism, robust over large variations of the experimental parameters.

We investigate the possibility of slowing down terahertz light via intersubband transitions in quantum well heterostructures. The slow-down effect is achieved via tunneling induced transparency [1] in a three level Λ-system by careful engineering of the wave functions [2]. The inherently short coherence times in such solid state media inevitably leads to strong absorption of the signal, which we propose to compensate for by incorporating a quantum cascade amplifier (QCA) into the system [3].

Graphene nanomaterials are actively used in electronics and materials science as elements of electric circuits and both structural and storage components. Their unique structure and electronic properties allow for a wide variety of applications (i.e., electron and thermal conductivity, ion transport, ion storage, and electric-current rectification). In this work, we investigate the electric-current-rectifying properties of mono- and bilayer two-terminal nanographene devices with the nonequilibrium Green’s function method combined with density functional theory. The diode-like properties are achieved by control of the nanoribbons’ edges. The sequential combination of armchair and zigzag domains leads to nanographene junctions with asymmetric current–voltage characteristics. The rectifying properties of the asymmetric armchair–zigzag carbon materials are derived from the nonequilibrium Green’s function theory. The electric-current rectification is explained by the interaction of the external electric field ...

We present a theoretical and computational investigation of the possibility of achieving slow terahertz light by exploiting the tunneling induced transparency (TIT) effect in suitably engineered quantum well heterostructure devices. We design such a meta-material and show how TIT could lead to large values of the group refractive index, unfortunately at the cost of strong field attenuation due to decoherence. As a suitable alternative, we propose a grating, consisting of a buffer and a quantum cascade amplifier regions, arranged in such a way as to achieve slow light and simultaneously compensate for the large signal losses. Our calculations show that a binary message could be reliably transmitted through this system, with non-critical reduction of the signal to noise ratio, as we achieve a slow-down factor of more than 70. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We investigate the temporal dynamics of terahertz (THz) quantum cascade laser (QCL) frequency combs with a strong injector anticrossing via numerical solution of the Maxwell-Bloch laser equations [1]. The presence of a strong anticrossing between the injector and upper laser levels of the device in [2] leads to a pronounced splitting of the emission spectra into high and low frequency lobe components around the central frequency of 3.5 THz. Moreover, such an effect also manifests itself in the time domain as a form of pulse switching between signals corresponding to the two lobes of the split gain, as experimentally observed in [3]. This process was explained as a form of temporal hole burning. Here we present a theoretical model, based on coupled density matrix/Maxwell equations, which can correctly account for that effect. Fig. 1 compares experimental results (left column) with simulation data (right column) for the instantaneous intensity of a terahertz QCL frequency comb with a strong injector anticrossing [2, 3]. In both plots the mentioned pulse switching behaviour is clearly visible, and we also notice a more prolonged pulse duration for the signal corresponding to the high frequency spectral component. Density matrix calculations reveal that this dynamics could be due to alternating saturation of the symmetric and anti-symmetric dressed states [4], which are responsible for the low and high frequency radiative transitions, respectively. The dependence of this process on different model parameters such as bias, anticrossing strength, chromatic dispersion etc. will be thoroughly discussed and possible methods for its utilization for frequency comb characterization will be considered.

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.