Terahertz (THz)

The terahertz (THz or 1012 Hz) is the part of the electromagnetic spectrum where most molecules have rotational and vibrational absorption modes and therefore is a rich area for molecular spectroscopy. The frequency range is also termed the far-infrared but the terahertz term was originally used for the region between photonic devices and electronic transit-time devices where few practical sources of radiation were previously available, termed the THz gap (roughly 200 GHz to 10 THz - or 30 µm to 1.5 mm wavelength). In the last 5 years this situation has changed significantly and there are many THz sources available with differing power, bandwidth and operating temperatures (see below). The frequency range is particularly interesting for medical and security imaging and spectroscopy as the radiation is non-ionising and hence is far safer than X-ray or gamma camera techniques.

On the left is a graph showing output powers from a number of different sources in the literature up to June 2005. In particular we are investing Si/SiGe quantum cascade lasers and Si/SiGe impurity lasers which are predicted to have benefits over presently available sources. The Si/SiGe QCLs should operate at higher temperatures than equivalent GaAs/AlGaAs THz QCLs due to the lack of polar optical phonon scattering but also the cheap and mature silicon technology should allow significantly cheaper devices to be realised.

We also have a DTI MNT grant with Kodak and TeraView investigating polymer materials which can be cheaply and easily fabricated to form different terahertz optical components such as anti-reflective coatings, dielectric mirrors, laser facet coatings (for e.g. QCLs) and electro-optical materials for switching and modulation purposes.