Room temperature operation of terahertz quantum cascade lasers (THz QCLs) has been a long-pursued goal to realize compact semiconductor THz sources. In this paper, we report on improving the maximum operating temperature of THz QCLs to ∼ 261 K as a step toward the realization of this goal.

Perfect absorption of light by an optically thin metasurface is among several remarkable optical functionalities enabled by nanophotonics. This functionality can be introduced into optoelectronic devices by structuring an active semiconductor-based element as a perfectly absorbing all-dielectric metasurface, leading to improved optical properties while simultaneously providing electrical conductivity. However, a delicate combination of geometrical and material parameters is required for perfect absorption, and currently, no general all-dielectric metasurface design fulfills these conditions for a desired semiconductor and operation wavelength. Here, using numerical simulations, we demonstrate that Mie resonators with subwavelength-size interconnecting channels allow this combination of perfect absorption requirements to be satisfied for different wavelengths of operation and different levels of intrinsic material absorption. We reveal the underlying physics and show that interconnecting channels play a critical role in achieving perfect absorption through their effects on the resonant wavelengths and losses for the electric dipole and magnetic dipole modes in Mie resonators. By adjusting only the channel widths, perfect absorption can be achieved for an optically thin GaAs-based metasurface at a desired wavelength of operation in a range from 715 nm to 840 nm, where the intrinsic absorption level in GaAs varies by more than a factor of 2. Optical transmission experiments confirm that these metasurfaces resonantly enhance optical absorption. This work lays out the foundation and guidelines for replacing bulk semiconductors with electrically connected, optically thin, perfectly absorbing metasurfaces in optical detectors.

Waveguide-integrated photodiodes with InGaAs/ GaAsSb type-II quantum well absorption regions designed to absorb light at 2 μm are presented. A novel dual-integrated waveguide-depletion layer was used to maximize quantum efficiency in photodiodes designed with thin absorbers for high-speed optical response. Low dark currents (1 nA at -1 V) and an internal responsivity of 0.84 A/W along with a bandwidth above 10 GHz and an open eye diagram at 10 Gb/s have been demonstrated at 2 μm. The high-speed carrier dynamics within InGaAs/GaAsSb type-II quantum wells are explored for the first time and suggest that the transit time of the photodiode is limited by light hole escape times in the quantum wells.

A microscopic many-body theory driven design and optimization supports the experimental demonstration of sub-100 fs pulse duration directly from a semiconductor laser. A passively modelocked vertical external cavity surface emitting laser producing a pulse duration of 95 fs at a central wavelength of 1025 nm is demonstrated. The semiconductor gain and absorber structures used in the experiment are numerically optimized by modelling the pulse formation dynamic of the system. The resulting structure design is described in detail and the physical limitations in terms of pulse duration and power are discussed. Using a ring cavity geometry, a stable colliding pulse modelocking regime with an output power of 90 mW per beam at a repetition rate of 2.2 GHz is demonstrated. The output pulses are thoroughly characterized and are in good agreement with our predictive model.

We compare the performance of 10 and 5 μm thick metal–metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures.

The Galactic/Extra Ultra-Long-Duration Balloon Spectroscopic-Stratospheric Terahertz Observatory (GUSTO) is a NASA balloon-borne project and is scheduled for launch in late 2022. The balloon will carry a spectroscopic telescope that will detect three brightest emission lines from the interstellar medium. GUSTO measurements will shed light on the life cycle of the gases in the Milky Way and Large Magellanic Cloud. In this article, we will discuss the details of a quantum cascade laser used in the local oscillator for detecting the oxygen line at 4.74 THz.