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...

We present a directly generated on-chip dual-comb source at terahertz (THz) frequencies. The multi-heterodyne beating signal of two free-running THz quantum cascade laser frequency combs is measured electrically using one of the combs as a detector, fully exploiting the unique characteristics of quantum cascade active regions. Up to 30 modes can be detected corresponding to a spectral bandwidth of 630 GHz, being the available bandwidth of the dual comb configuration. The multi-heterodyne signal is used to investigate the equidistance of the comb modes showing an accuracy of 10−12 at the carrier frequency of 2.5 THz.

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

The formation of a nano-porous structure in amorphous Ge thin film (sputter-deposited on SiO2) during ion irradiation at room temperature with 300 keV Ge+ has been observed. The porous film showed a sponge-like structure substantially different from the columnar structure reported for ion implanted bulk Ge. The voids size and structure resulted to be strongly affected by the material preparation, while the volume expansion turned out to be determined only by the nuclear deposition energy. In SiGe alloys, the swelling occurs only if the Ge concentration is above 90%. These findings rely on peculiar characteristics related to the mechanism of voids nucleation and growth, but they are crucial for future applications of active nanostructured layers such as low cost chemical and biochemical sensing devices or electrodes in batteries.

This paper reports on the formation of nanoporous Ge in polycrystalline and amorphous Ge thin films, grown by molecular beam epitaxy and implanted with Ge+ ions at 300 keV with different fluences (3×1015–2×1016 Ge/cm2). Implanted polycrystals show a more regular columnar structure with respect to smaller and disconnected voids of amorphous grown films. These results strongly rely on the film properties and mechanism of void nucleation. Our findings represent a goal for the technology transfer of the ion-induced nanoporosity from bulk Ge to Ge thin films and meet the requirements for future applications.

We introduce a double metal terahertz quantum cascade laser meant for astrophysical heterodyne measurements. The laser ridge is embedded in benzocyclobutene, and the device exhibits single mode, continuous wave operation around 4.745 THz with a peak power of almost 1.8 mW at 10 K and a power consumption of ≈1.6 W. Moreover, thanks to the integration of a top metal contact with a patch array antenna for light out-coupling the beam of the emitted light has a low-divergence single-lobe profile and an FWHM of ≈30°.

Avalanche photodetectors (APDs) are key components in optical communication systems due to their increased photocurrent gain and short response time as compared to conventional photodetectors. A detector design where the multiplication region is implemented in a large bandgap material is desired to avoid detrimental Zener tunneling leakage currents, a concern otherwise in smaller bandgap materials required for absorption at 1.3/1.55 µm. Self-assembled III-V semiconductor nanowires offer key advantages such as enhanced absorption due to optical resonance effects, strain-relaxed heterostructures and compatibility with main-stream silicon technology. Here, we present electrical and optical characteristics of single InP and InP/InAsP nanowire APD structures. Temperature-dependent breakdown characteristics of p + -n-n + InP nanowire devices were investigated first. A clear trap-induced shift in breakdown voltage was inferred from I-V measurements. An improved contact formation to the p + -InP segment was observed upon annealing, and its effect on breakdown characteristics was investigated. The bandgap in the absorption region was subsequently varied from pure InP to InAsP to realize spatially separate absorption and multiplication APDs in heterostructure nanowires. In contrast to the homojunction APDs, no trap-induced shifts were observed for the heterostructure APDs. A gain of 12 was demonstrated for selective optical excitation of the InAsP segment. Additional electron beam-induced current measurements were carried out to investigate the effect of local excitation along the nanowire on the I-V characteristics. Our results provide important insight for optimization of avalanche photodetector devices based on III-V nanowires.

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.