Terahertz Sources

Abstract

The terahertz (THz) region of the electromagnetic spectrum is commonly defined as that lying at frequencies between 0.1 and10 THz. The frequently-heard expression 'the THz gap' refers to the fact that at these frequencies generation and detection of radiation becomes very difficult using either electronic or optical means. Nevertheless, with the growth of interest in THz research and applications and a great expansion of activities in this area, a large number and variety of sources have been developed and have become widely available. These include THz lasers, laser-activated emitters and electronic devices. Broadly speaking, in applications requiring a continuous-wave narrow-line signal at frequencies below 1 THz, electronic sources predominate. Examples include local oscillators for astronomical instruments and security scanners. On the other hand, lasers and laser-activated emitters are mainly used for broadband THz spectroscopy covering a bandwidth of several THz. This division of functions is likely to persist since electronic THz sources are, by their nature, singlefrequency devices, whereas laser-based sources are either inherently broadband or widely tuneable. Nevertheless, both types of THz emitting devices have found numerous applications, and will be described here. Terahertz Generation from Laser SourcesThe recent rapid expansion of the field of terahertz research and applications owes much of its existence to the development of suitable laser sources and methods of THz generation. These developments fall into two categories: direct emission from far-infrared lasers and optical down-conversion of near-infrared lasers. Owing to the flexibility of the systems and the broad tuning range, down-conversion greatly predominates as the technique of choice, and accounts for the vast majority of work in the THz field.

Comparative study of by near-field heterodyne transient grating and continuously variable spatial frequency transient grating methods for measurements of terahertz reflection responses

“…[6][7][8] In terahertz spectroscopy, femtosecond optical pulses are used for generating terahertz fields that are projected from sources into free space and detected after transmission or reflection by a sample using a separate receiver. [9][10][11][12] However, an integrated platform that supports terahertz generation, propagation, transmission and characterization would be more versatile and convenient. For this purpose, a method using optically generated phonon polariton (PP) waves offer strong prospects.…”