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

A theoretical study of electrically pumped unipolar lasers exploiting intrawell or interwell intersubband radiative transitions in multiple quantum‐well heterostructures for the generation of IR and FIR radiation is presented. The feasibility of these coherent sources critically depends on the non‐radiative intersubband transition rates. Numerical simulations of acoustical phonon, optical‐phonon, and electron‐electron scattering were implemented, including their temperature dependence. For far‐infrared coherent sources, electron‐electron scattering emerges as the dominating non‐radiative relaxation mechanism. Interwell schemes offer distinctive advantages such as simplicity in design, greater tolerance in design and fabrication errors, field tunability of the emission frequency, improved internal quantum efficiency and aid in establishing population inversion. Design rules are put forward for such long wavelength sources.

Intrawell and interwell intersubband transitions in multiple quantum wells for far‐infrared sources

We have theoretically studied the field‐dependent optical properties of a step quantum‐well structure, which consists of a small well inside a big well. The wave functions of the small and big wells overlap and their eigenenergies have different field dependencies. The transition from a small‐well state to a big well state thus gives a large Stark shift at low applied field without degrading the oscillator strength. Another advantage is to allow optical transitions that are forbidden in simple single‐well structures. This relaxation of the selection rule makes it possible to provide optical pumping for intersubband lasing. Also, since the step well is asymmetric, many nonlinear properties are expected.

Optical transitions in a step quantum well

Infrared spectroscopy is a powerful tool for revealing some key properties of electrons in the minibands of semiconductor superlattices. Some theoretical foundations of superlattice physics as well as the experimental work on infrared absorption will be reviewed in this article. After a discussion of the energy levels (minibands, donor states) and the effect of external electric and magnetic fields on the miniband structure, a framework for the description of optical transitions between and within minibands is developed. Experiments on inter- and intraminiband as well as on cyclotron resonance absorption are presented in some detail. The most striking phenomena observed in this context are the van Hove singularities in the miniband joint density of states, the thermal quenching of miniband conductivity, and magnetic miniband breakdown. Finally, transport and infrared emission experiments are summarized.

Infrared spectroscopy and transport of electrons in semiconductor superlattices

The feasibility of optically pumped infrared laser based on the intersubband transitions in multiple quantum wells is studied theoretically. The criteria for population inversion and efficient lasing operations are established. A system consisting of a superlattice with four quantum wells per period is proposed and shown to satisfy the criteria. The relaxation processes are investigated, giving the electron-phonon interaction a rigorous theoretical treatment. It is shown that in the proposed structure, optical gain in excess of 300 cm/sup -1/ can be realized under practical pumping conditions. Modification and optimization of the proposed laser are discussed. >

Optically pumped four-level infrared laser based on intersubband transitions in multiple quantum wells: feasibility study

A novel infrared laser is proposed which uses the intersubband optical transition in a band‐aligned superlattice. In this band‐aligned superlattice laser, the miniband discontinuity within the conduction of valence band functions as a band offset in the heterojunction structure, and the population inversion is achieved by current injection as in the conventional heterojunction laser. It is more flexible than a heterojunction laser of a quantum well laser since one may tailor the bandwidth and band structures as well as the band gap of the minibands. Also indirect band‐gap materials like Si and Ge can be used for lasing in the intersubband transitions. The intersubband optical transition is similar to an atomic two‐level system which exhibits low threshold current, and a gain coefficient with weak temperature dependence and a narrow spectrum which is determined only by the line‐shape function. These special features make the band‐aligned superlattice laser competitive with and perhaps superior to the quantum well dot laser which is not presently feasible.

K. L. Wang

Papers

Novel infrared band‐aligned superlattice laser