Molecular Beam Epitaxy

11Molecular Beam Epitaxial Growth of Terahertz Quantum Cascade Lasers

Harvey E. Beere and David A. Ritchie

Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK

11.1 Introduction

The terahertz (THz) frequency region of the electromagnetic spectrum (1–10 THz; ∼300–30 µm; 4–40 meV) has many important potential applications such as imaging, chemical spectroscopy and telecommunications [1]. This is a combination of the unique properties associated with this frequency regime. The penetration depth of THz waves through many materials in ordinary use, such as plastic and cloth, increases substantially with decreasing frequency. The vibrational modes of large molecules are in this frequency range, lending it to both chemical and biomedical sensing and imaging. In space, most molecules emit their rotational spectra in the THz, allowing processes in star formation and interstellar medium (gas clouds) to be observed. Communications over this frequency band are currently unallocated and have the potential for fast, terabit‐per‐second data link capacities.

Consequently, there has been a strong drive to develop compact, low‐cost, efficient THz sources. Although semiconductor devices traditionally account for a large share of the sources of electromagnetic waves, the THz range has remained substantially uncovered. The demonstration of the THz quantum cascade laser (QCL) in 2002 provided a potential solid‐state solution [2]. Significant progress has been made ...