We present a terahertz quantum cascade laser operating on a thermoelectric cooler up to a record-high temperature of 210.5 K. The active region design is based on only two quantum wells and achieves high temperature operation thanks to a systematic optimization by means of a nonequilibrium Green's function model. Laser spectra were measured with a room temperature detector, making the whole setup cryogenic free. At low temperatures (∼ 40 K), a maximum output power of 200 mW was measured.Terahertz (THz) radiation is subject to a wide range of research and technological efforts 1 , from solid state fundamental physics to biomedicine and astrophysics. Specifically, since materials such as textiles, plastics, coatings and biological tissues are transparent to THz radiation, this spectral region is also promising for a variety of non-invasive imaging and non-destructive quality assessment applications such as airport security screening and thickness coating measurements. In order to unlock the full potential of these applications on a large scale, compact and powerful THz sources are needed. A promising candidate is the quantum cascade laser (QCL) 2,3 , a compact injection laser based on semiconductor heterostructures. THz QCLs have already shown high emitted powers (several hundred mW both in pulsed and continuous wave) 4-6 and spectral coverage throughout the 1-6 THz range 7 with single mode and broadband devices 8 . However, despite several efforts, to-date THz QCL operation is still restricted to cryogenic cooling and their maximum operating temperature is still below 200 K 9 . In this Letter we present a THz QCL operating up to 210 K, allowing the use of a small footprint, 4-stage Peltier cooler. In combination with a commercial DTGS detector, this constitutes a platform for THz spectroscopy completely free from any cryogenic and Helium-based (He-based) technology. For any given wavelength, resonator losses, and broadening lifetime, the ultimate temperature limit to QCL laser operation increases with the fraction of the electron population in the upper state 10 , as parasitic reabsorption by the free electrons participating to the transport and maintaining the electrical stability are the key source of optical losses 11 . This line of reasoning provides an explanation for the general trend of THz QCL designs that has been towards reducing the number of states per period. This qualitative result is supported by the quantitative gain computations shown in Fig. 1(a), where the computed active region population and gain at 200 K of a selection of THz QCLs using different design schemes show a relative increase in the upper laser state (ULS) population (n ULS ) as the number of active states is reduced. This leads to both an increase in population inversion a) Electronic mail: lbosco@phys.ethz.ch and a reduction of intersubband re-absorption and justifies using this quantity as a performance indicator 10 . Early THz QCLs used the "bound-to-continuum" (BtC) scheme 3,12-14 , where the upper laser state (ULS) is localize...

A general theoretical approach to study the quantum kinetics in a system coupled to a bath is proposed. Starting with the microscopic interaction, a Lindblad master equation is established, which goes beyond the common secular approximation. This allows for the treatment of systems, where coherences are generated by the bath couplings while avoiding the negative occupations occurring in the Bloch-Wangsness-Redfield kinetic equations. The versatility and accuracy of the approach is verified by its application to three entirely different physical systems: (i) electric transport through a double-dot system coupled to electronic reservoirs, (ii) exciton kinetics in coupled chromophores in the presence of a heat bath, and (iii) the simulation of quantum cascade lasers, where the coherent electron transport is established by scattering with phonons and impurities.

We present a two-quantum well THz intersubband laser operating up to 192 K. The structure has been optimized with a non-equilibrium Green's function model. The result of this optimization was confirmed experimentally by growing, processing and measuring a number of proposed designs. At high temperature (T > 200 K), the simulations indicate that lasing fails due to a combination of electron-electron scattering, thermal backfilling, and, most importantly, re-absorption coming from broadened states.Terahertz quantum cascade lasers (QCLs) 1 are interesting candidates for a wide variety of potential applications 2,3 . However, to date, their operation is limited to ∼200 K 4 and the necessity of cryogenic cooling hinders a widespread use of these devices. In the last decade, significant scientific effort has been directed towards identifying the main temperaturedegrading mechanisms 5-8 , as well as finding optimized QCL designs 9-14 . The degrading mechanisms include thermal backfilling 3,15 , thermally activated LO phonon emission [6][7][8]16 , increased broadening [17][18][19] , and carrier leakage into continuum states 20 . When numerically optimizing a design, it is important to take all of these effects into consideration, in order to ensure a close correspondence between the model and the real device. Combined with the fact that the optimization parameters are typically trade-offs for one another, the task is very complex. Here, typically simpler rate equation or density matrix models are used in order to more quickly sweep the parameter space 21-23 , while more advanced models, such as non-equilibrium Green's functions (NEGF) or Monte-Carlo, are used to validate and analyze the final designs 13,[24][25][26] . In contrast, in this work we will employ an advanced model directly at the optimization stage. Specifically, we shall use a NEGF model 27 , capable of accurately simulating experimental devices 13,26,28 and including the most general treatment of scattering, from all relevant processes.The goal of the optimization is to achieve the highest possible operating temperature. Thus, the gain of the active medium should be maximized at high lattice temperature, and simultaneously the external losses minimized. The key figures for gain are inversion, oscillator strength, and line width 29 . These are mainly controlled by the doping density, the energy difference E ex between the lower laser level ll and the extractor state e, and the width of the two barriers: the laser and injection barriers. Population inversion increases with doping, although a too high level promotes detrimental effects, such as electron-electron scattering. E ex , which is chosen to be close to the LO phonon resonance E LO in order to have a short ll lifetime, and the laser frequency ω are mainly determined by the well widths. The laser barrier width determines the oscillator strength, which at the same time affects inversion; a more vertical transition

We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83THz (∆f /f = 50%) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low CW threshold (115 A· cm −2 ) and broadband comb operation (∆f /f = 25%) make these sources extremely appealing for on-chip frequency comb applications.arXiv:1912.00890v1 [physics.optics]

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We study the impact of interface roughness on the operation of mid-IR and THz quantum cascade lasers. Particular emphasis is given towards the differences between the Gaussian and exponential roughness distribution functions, for which we present results from simulation packages based on nonequilibrium Green's functions and density matrices. The Gaussian distribution suppresses scattering at high momentum transfer which enhances the lifetime of the upper laser level in mid-IR lasers. For THz lasers, a broader range of scattering transitions is of relevance, which is sensitive to the entire profile of the interface fluctuations. Furthermore we discuss the implementation of interface roughness within a two band model.

We report a device that provides coherent emission of phonon polaritons, a mixed state between photons and optical phonons in an ionic crystal. An electrically pumped GaInAs/AlInAs quantum cascade structure provides intersubband gain into the polariton mode at λ = 26.3 μm, allowing self-oscillations close to the longitudinal optical phonon energy of AlAs. Because of the large computed phonon fraction of the polariton of 65%, the emission appears directly on a Raman spectrum measurement, exhibiting a Stokes and anti-Stokes component with the expected shift of 48 meV.

Abstract. In this work we include electron-electron interaction beyond Hartree-Fock level in our non-equilibrium Green's function approach by a crude form of GW through the Single Plasmon Pole Approximation. This is achieved by treating all conduction band electrons as a single effective band screening the Coulomb potential. We describe the corresponding self-energies in this scheme for a multi-subband system. In order to apply the formalism to heterostructures we discuss the screening and plasmon dispersion in both 2D and 3D systems. Results are shown for a four well quantum cascade laser with different doping concentration where comparisons to experimental findings can be made. IntroductionSince the quantum cascade laser (QCL) was introduced more than 20 years ago [1] it has continuously been improved and redesigned to operate from the mid-infrared all the way down to the Terahertz (THz) range. Compact laser sources at these wavelengths are valuable for spectroscopic applications [2] but a major problem is that THz QCLs do not operate at room temperature. This can be overcome via difference frequency generation which recently has been demonstrated at powers in the milli-watt range [3]. Direct THz-QCLs still generate a lot of interest in the research community due to the promise of higher wall-plug efficiency and the prospects of miniaturization of cryo-coolers.The main temperature degrading mechanism of the THz QCLs is currently not fully understood and is still debated by the community [4]. This is a challenge to theory and there is a great need of realistic modeling tools that are able to treat all important quantum effects on the same footing. A summary of different methods for simulating these structures is found in [5]. Monte Carlo simulations have shown that electron-electron interaction, beyond the meanfield or Hartree approximation can influence the dynamics of THz QCLs [6,7,8].In this work we include and study the effects of a simple electron-electron scattering via the Single Plasmon Pole Approximation (SPPA) [9,10]. In this approximation we capture both the static limit as well as dynamic effects. This gives an energy dependent (non-local in time) interaction beyond the Hartree-Fock approximation. This has been studied in a similar model with promising results [11], and with this work we want to adapt the idea into our model described in Ref. [12]. In other methods based on Non-Equilibrium Green's Functions (NEGF)