Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

Six years after their birth, terahertz quantum-cascade lasers can now deliver milliwatts or more of continuous-wave coherent radiation throughout the terahertz range — the spectral regime between millimetre and infrared wavelengths, which has long resisted development. This paper reviews the state-of-the-art and future prospects for these lasers, including efforts to increase their operating temperatures, deliver higher output powers and emit longer wavelengths.

Terahertz quantum-cascade lasers

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

nextnano is a semiconductor nanodevice simulation tool that has been developed for predicting and understanding a wide range of electronic and optical properties of semiconductor nanostructures. The underlying idea is to provide a robust and generic framework for modeling device applications in the field of nanosized semiconductor heterostructures. The simulator deals with realistic geometries and almost any relevant combination of materials in one, two, and three spatial dimensions. It focuses on an accurate and reliable treatment of quantum mechanical effects and provides a self-consistent solution of the Schrodinger, Poisson, and current equations. Exchange-correlation effects are taken into account in terms of the local density scheme. The electronic structure is represented within the single-band or multiband kldrp envelope function approximation, including strain. The code is not intended to be a ldquoblack boxrdquo tool. It requires a good understanding of quantum mechanics. The input language provides a number of tools that simplify setting up device geometry or running repetitive tasks. In this paper, we present a brief overview of nextnano and present four examples that demonstrate the wide range of possible applications for this software in the fields of solid-state quantum computation, nanoelectronics, and optoelectronics, namely, 1) a realization of a qubit based on coupled quantum wires in a magnetic field, 2) and 3) carrier transport in two different nano-MOSFET devices, and 4) a quantum cascade laser.

nextnano: General Purpose 3-D Simulations

A new temperature performance record of 199.5 K for terahertz quantum cascade lasers is achieved by optimizing the lasing transition oscillator strength of the resonant phonon based three-well design. The optimum oscillator strength of 0.58 was found to be larger than that of the previous record (0.41) by Kumar et al. [Appl. Phys. Lett. 94, 131105 (2009)]. The choice of tunneling barrier thicknesses was determined with a simplified density matrix model, which converged towards higher tunneling coupling strengths than previously explored and nearly perfect alignment of the states across the injection and extraction barriers at the design electric field. At 8 K, the device showed a threshold current density of 1 kA/cm2, with a peak output power of ∼ 38 mW, and lasing frequency blue-shifting from 2.6 THz to 2.85 THz with increasing bias. The wavelength blue-shifted to 3.22 THz closer to the maximum operating temperature of 199.5 K, which corresponds to ∼ 1.28ħω/κB. The voltage dependence of laser frequency is related to the Stark effect of two intersubband transitions and is compared with the simulated gain spectra obtained by a Monte Carlo approach.

Terahertz quantum cascade lasers operating up to ∼ 200 K with optimized oscillator strength and improved injection tunneling

We report the development of a quantum cascade laser, at λ=87.2 μm, corresponding to 3.44 THz or 14.2 meV photon energy. The GaAs/Al0.15Ga0.85As laser structure utilizes longitudinal-optical (LO) phonon scattering for electron depopulation. Laser action is obtained in pulsed mode at temperatures up to 65 K, and at 50% duty cycle up to 29 K. Operating at 5 K in pulsed mode, the threshold current density is 840 A/cm2, and the peak power is approximately 2.5 mW. Based on the relatively high operating temperatures and duty cycles, we propose that direct LO-phonon-based depopulation is a robust method for achieving quantum cascade lasers at long-wavelength THz frequencies.

http://www.seas.ucla.edu/~bwilliam/williams_laser.pdf

3.4-THz quantum cascade laser based on longitudinal-optical-phonon scattering for depopulation

We report lasing at ∼3.0 THz (λ≈98–102 μm) in a quantum-cascade structure in which mode confinement is provided by a double-sided metal waveguide. The depopulation mechanism is based on resonant phonon scattering, as in our previous work. Lasing takes place in pulsed mode up to a heat-sink temperature of 77 K. The waveguide consists of metallic films placed above and below the 10-μm-thick multiple-quantum-well gain region, which gives low losses and a modal confinement factor of nearly unity. Fabrication takes place via low-temperature metallic wafer bonding and subsequent substrate removal using selective etching. This type of waveguide is expected to be increasingly advantageous at even longer wavelengths.

Terahertz quantum-cascade laser at λ≈100 μm using metal waveguide for mode confinement

A density matrix approach is used in combination with a tight-binding model to describe electron transport in terahertz quantum cascade lasers and is incorporated into a Monte Carlo simulation. Scattering events, including LO-phonon, electron-electron, and ionized impurity scattering, are treated semiclassically but contribute to dephasing scattering. In addition, a phenomenological “pure dephasing rate” was introduced to take into account dephasing caused by interface roughness scattering. This model was used to investigate the influence of dephasing on electron transport through a barrier. Additionally, current densities, populations and electron temperatures were calculated for a simple three-level structure and a five-level structure that achieved lasing at 3.2THz, and the results were compared to a semiclassical simulation. We find that the inclusion of coherent transport and dephasing in the calculations is essential when transport is dominated by transitions between weakly coupled states.

Importance of coherence for electron transport in terahertz quantum cascade lasers

We report operation of a terahertz quantum-cascade laser at 3.8 THz (λ≈79 μm) up to a heat-sink temperature of 137 K. A resonant phonon depopulation design was used with a low-loss metal–metal waveguide, which provided a confinement factor of nearly unity. A threshold current density of 625 A/cm2 was obtained in pulsed mode at 5 K. Devices fabricated using a conventional semi-insulating surface-plasmon waveguide lased up to 92 K with a threshold current density of 670 A/cm2 at 5 K.

Terahertz quantum-cascade laser operating up to 137 K

Using an ensemble Monte Carlo simulation, including both electron–electron and electron–phonon scattering as well as electron-impurity scattering, the current density, population inversion, electron temperature, and gain in two THz quantum-cascade structures are investigated and compared to measurements. We find that the inclusion of electron-impurity scattering in the calculations is crucial when modeling the intersubband transport dynamics in these devices. However, the calculated gain is higher than inferred from experiments. This can be attributed to wavefunction localization caused by dephasing scattering, which is unaccounted for in the present model.

Hans Callebaut

Papers

3.4-THz quantum cascade laser based on longitudinal-optical-phonon scattering for depopulation

Terahertz quantum-cascade laser at λ≈100 μm using metal waveguide for mode confinement

Importance of coherence for electron transport in terahertz quantum cascade lasers

Terahertz quantum-cascade laser operating up to 137 K

Importance of electron-impurity scattering for electron transport in terahertz quantum-cascade lasers