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We demonstrate terahertz quantum cascade lasers realized in "ideal" ring resonators without discontinuities from, e.g., contacting pads. We realize this by mounting rings episide-down on a silicon substrate by a die-bonding technique. This technique allows one to realize ideal conditions for optical confinement as well as heat dissipation and provides the basis for future Si integrated THz devices. The lasers emit light around 3.8 THz and show much reduced threshold current densities. When operated in continuous-wave operation, frequency comb formation with a spectral bandwidth of 70 GHz is observed. Frequency comb operation is indicated by a narrow beat note signal at 8.55 GHz with a signal-to-noise ratio up to 40 dB. The experimentally measured spectral behavior of ring devices is described accurately by the results obtained from numerical simulations based on the Maxwell–Bloch formalism. [ABSTRACT FROM AUTHOR]

We present an investigation into the vertical transport through 13 different superlattice structures, where the well and barrier widths, doping concentration, dopant position, and contact layers were varied. Although superlattices have been extensively studied since 1970, there is a lack of publications on transport through superlattices similarly low doped as THz quantum cascade lasers (QCLs), for which the doping is in the 3-5×10^{10}  cm^{-2} range. The superlattices presented are doped in the same range as THz QCLs, with contact layers and fabrication comparable to high-temperature THz QCLs. The temperature-dependent current-voltage characteristics were measured starting from 5 K and an anomalous temperature effect was observed at the first plateau. The measured current through the superlattice first decreases before increasing again with increasing temperature, resulting in the lowest current occurring at 75-110 K. This behavior is also observed in some THz QCLs. The effect disappears for thinner barriers, higher quantum well doping, or modified contact layers, indicating a strong dependency on band bending, due to the large difference in the doping of the contact layers and the superlattice, which is confirmed with multiscattering Büttiker simulations.

We report on a flexible platform for molecular sensing in the terahertz range. Merging the well-established technologies of near-infrared electro-optic modulation and photomixing realizes a spectrally adaptable terahertz source, which is combined with a new generation of compact gas cells, so-called substrate-integrated hollow waveguides (iHWGs). iHWGs have been developed in the mid-infrared and provide flexibility in the design of the optical absorption path. Here, we demonstrate its suitability for the terahertz domain by presenting its low propagation losses and by measuring rotational transitions of nitrious oxide (N 2 O). A fast frequency sideband modulation technique results in substantially reduced measurement times and increased accuracy compared to a standard wavelength tuning method.

Broadband emission in the terahertz spectral region is a prerequisite for applications such as spectroscopy or white light sources. Appropriate signal powers and a compact design are advantageous for this use. A technology which meets these requirements are terahertz quantum cascade lasers. These electrically pumped, on-chip semiconductor lasers provide high output powers and the freedom of tailoring their emission wavelength by bandstructure engineering. By combining multiple active region designs emitting at different wavelengths in a single structure, one can obtain broadband emission from a single device. Here, we present a heterogeneous terahertz quantum cascade laser consisting of five individual active regions based on a three-well, LO-phonon depopulation design. The devices lase in pulsed and continuous-wave operation and emit in a spectral range from 1.9 to 4.5 THz, covering a bandwidth of 1.37 octaves. The use of the three-well design, which was optimized for high temperature operation, leads to a maximum operating temperature in the pulsed operation of 143 K., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

We demonstrate a terahertz (THz) frequency comb that is flexible in terms of its frequency range and the number and spacing of comb lines. We use a combination of near-infrared laser diodes, phase modulation, and opto-electronic frequency conversion. The THz comb lines are characterized to be <10 M H z by resolving the pressure-dependent collisional broadening of an ammonia molecule rotational mode.