Population-inversion and gain estimates for a semiconductor TASER

We have investigated a solid-state design advanced (see Soref et al, in SPIE Proceedings, vol. 3795, p, 516, 1999) to achieve a terahertz-amplification-by-the-stimulated-emision-of-radiation (TASER), The original design was based on light-to heavy-hole intersubband transitions in SiGe/Si heterostructures, This work adapts the design to electron intersubband transitions in the more readily available GaAs/Ga1-xAlxAs material system. It is found that the electric-field induced anti-crossings of the states, derived from the first excited state with the ground states of a superlattice in the Stark-ladder regime, offers the possibility of a population inversion and gain at room temperature

Terahertz gain in a SiGe/Si quantum staircase utilizing the heavy-hole inverted effective mass

Modeling and design studies show that a strain-balanced Si1−xGex/Si superlattice onSi1−yGey-buffered Si can be engineered to give an inverted effective mass HH2 subband adjacent to HH1, thereby enabling a 77 K edge-emitting electrically pumped p–i–pquantum staircase laser for THz emission at energies below the 37 meV Ge–Ge optical phonon energy. Analysis of hole-phonon scattering, lifetimes, matrix elements, and hole populations indicates that a gain of 450 cm−1 will be feasible at f = 7.3 THz during 1.7 kA/cm2 current injection

Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1−xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm−1 at 300 K reducing to 14.5 cm−1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1−xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm−1 and 23.8 cm−1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings

Si/SiGe bound-to-continuum quantum cascade emitters

Si/SiGe bound-to-continuum quantum cascade emitters designed by self-consistent 6-band k.p modeling and grown by low energy plasma enhanced chemical vapour deposition are presented demonstrating electroluminescence between 1.5 and 3 THz. The electroluminescence is Stark shifted by an electric field and demonstrates polarized emission consistent with the design. Transmission electron microscopy and x-ray diffraction are also presented to characterize the thick heterolayer structure

Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 µm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 µm thick active gain region with silver metal of 21 cm-1 at 300 K reducing to 14.5 cm-1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1-xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm-1 and 23.8 cm-1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings. © 2020 OSA - The Optical Society. All rights reserved

Monte Carlo Modeling of Terahertz Quantum Cascade Detectors

We demonstrate an Ensemble Monte Carlo (EMC) modeling approach for robust and rigorous simulations of photovoltaic quantum cascade detectors (QCDs) in the mid-infrared (mid-IR) and terahertz (THz) range. The existing EMC simulation tool for quantum cascade lasers (QCLs) was extended to simulate the photovoltaic transport effects in QCDs at thermal equilibrium under zero bias. Here, we present the results of the EMC study of a THz detector design with a detection wavelength of 84 $\mu$m. The simulation results show good agreement with experimental data. For a temperature of 10 K we obtain a peak responsivity of 9.4 mA/W.Comment: 3 pages, 2 figures, has been accepted for 33rd URSI GAS

Ignition of quantum cascade lasers in a state of oscillating electric field domains

Quantum Cascade Lasers (QCLs) are generally designed to avoid negative differential conductivity (NDC) in the vicinity of the operation point in order to prevent instabilities. We demonstrate, that the threshold condition is possible under an inhomogeneous distribution of the electric field (domains) and leads to lasing at an operation point with a voltage bias normally attributed to the NDC region. For our example, a Terahertz QCL operating up to the current maximum temperature of 199 K, the theoretical findings agree well with the experimental observations. In particular, we experimentally observe self-sustained oscillations with GHz frequency before and after threshold. These are attributed to traveling domains by our simulations. Overcoming the design paradigm to avoid NDC may allow for the further optimization of QCLs with less dissipation due to stabilizing background current.Comment: 22 page

Modeling electronic and optical properties of III-V quantum dots – selected recent developments

Electronic properties of selected quantum dot (QD) systems are surveyed based on the multi-band k·p method, which we benchmark by direct comparison to the empirical tight-binding algorithm, and we also discuss the newly developed “linear combination of quantum dot orbitals” method. Furthermore, we focus on two major complexes: First, the role of antimony incorporation in InGaAs/GaAs submonolayer QDs and In1−xGax AsySb1−y/GaP QDs, and second, the theory of QD-based quantum cascade lasers and the related prospect of room temperature lasing.TU Berlin, Open-Access-Mittel - 2022EC/H2020/956548/EU/Quantum Semiconductor Technologies Exploiting Antimony/QUANTIMONYEC/H2020/731473/EU/QuantERA ERA-NET Cofund in Quantum Technologies/QuantER

Nanometer Probing of Operating Nano-Photonic Devices

The external performance of quantum optoelectronic devices is governed by the three-dimensional profiles of electric potentials determined by the distribution of charge carriers (electrons and holes) within the active regions of the devices. Charge carrier dynamics play a vital role in active photonic quantum/nano devices, such as electrically-pumped semiconductor lasers. As an example, in quantum cascade lasers (QCLs) the Electric Field Domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially non-uniform and temporally unstable. Until now, there are no experimental means of probing the inner potential profile directly and as a result the mechanisms responsible for sub-par device performance of QCLs remain the subject of speculation. Another example is interband cascade lasers (ICLs), in which the distribution of gain-providing charge carrier governs the operation and performance of the devices, but has not been experimentally measured prior to this study. This work presents a systematic experimental study of gain-providing charge carrier distribution in a lasing interband cascade laser and electric potential distribution in THz QCLs. The unique charge carrier distribution profile in the quantum-well active region is quantitatively measured at nanometer scales by using the non-invasive scanning voltage microscopy (SVM) technique. Experimental results clearly confirm the accumulation and spatial segregation of holes and electrons in the core of the ICL device. The measurement also shows that the charge carrier density is essentially clamped in the presence of stimulated emission in ICLs, thus conclusively differentiating the lasing from non-lasing devices. The SVM technique has been applied to lasing THz QCLs to verify the hypothesis of electric field domains in semiconductor quantum structures. The experimental results reveal that the multi-quantum-well active region is divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules are observed not to continuously increase in proportion to the gradual increase of the applied device bias, but rather jumps between discrete values related to tunneling resonances. Also in the THz QCLs the progression of the observed EFDs are carefully probed. Experimental evidences reveal that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region. The SVM findings in THz QCLs indicate the importance of quantum active region design for intrinsically more uniform and stable electric field profiles. The two showcase study examples demonstrate that the cryogenic-temperature SVM is an enabling technique, being able to measure and resolve nanometer scale features non-destructively on operating devices. This experimental approach allows directly mapping the electric field distribution as well as the charge carrier distribution inside operating semiconductor quantum devices at nanometer scales, thus connecting the inner workings with the external measures of the devices. The experimental approach is expected to facilitate a deeper understanding of fundamental processes that are governing the operation and performance of a wide range of nanoelectronic and nanophotonic devices.4 month