Anomalous Temperature Effect in Weakly Coupled Superlattices: Carrier Transport in a THz Quantum Cascade Laser.

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.

Flexible terahertz gas sensing platform based on substrate-integrated hollow waveguides and an opto-electronic light source.

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 (N2O). A fast frequency sideband modulation technique results in substantially reduced measurement times and increased accuracy compared to a standard wavelength tuning method.

Ultrabroadband Heterogeneous THz Quantum Cascade Laser.

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.

Flexible terahertz opto-electronic frequency comb light source tunable over 3.5 THz.

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 <10MHz by resolving the pressure-dependent collisional broadening of an ammonia molecule rotational mode.

Deep learning control of THz QCLs.

Artificial neural networks are capable of fitting highly non-linear and complex systems. Such complicated systems can be found everywhere in nature, including the non-linear interaction between optical modes in laser resonators. In this work, we demonstrate artificial neural networks trained to model these complex interactions in the cavity of a Quantum Cascade Random Laser. The neural networks are able to predict modulation schemes for desired laser spectra in real-time. This radically novel approach makes it possible to adapt spectra to individual requirements without the need for lengthy and costly simulation and fabrication iterations.

Barrier Height Tuning of Terahertz Quantum Cascade Lasers for High-Temperature Operation.

Terahertz quantum cascade lasers (QCLs) are excellent coherent light sources, but are still limited to an operating temperature below 200 K. To tackle this, we analyze the influence of the barrier height for the identical three-well terahertz QCL layer sequence by comparing different aluminum concentrations (x = 0.12-0.24) in the GaAs/Al x Ga1-x As material system, and then we present an optimized structure based on these findings. Electron injection and extraction mechanisms as well as LO-phonon depopulation processes play crucial roles in the efficient operation of these lasers and are investigated in this study. Experimental results of the barrier height study show the highest operating temperature of 186.5 K for the structure with 21% aluminum barriers, with a record k B T max/ℏω value of 1.36 for a three-well active region design. An optimized heterostructure with 21% aluminum concentration and reduced cavity waveguide losses is designed and enables a record operating temperature of 196 K for a 3.8 THz QCL.