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Terahertz electromagnetic radiation is one of the last remaining unexplored regions of the electromagnetic spectrum. Until relatively recently, the THz region (0.1-3 THz, 0.1-3 mm), occupying a large portion of the electromagnetic spectrum between the infrared and microwave bands, has remained in relative obscurity due to a lack of efficient laboratory emitters, detectors and optical components compared to neighboring microwave and optical bands. This is despite the fact that many important processes in nature occur at THz frequencies: for example, the vibrational breathing modes of many large molecules occur at these low frequencies, giving rise to a unique _fingerprint' for many biomolecules in the THz region. Indeed, it has been proposed that one of the most important future applications of THz radiation will be in biomedicine. In the future the THz region will be as useful as the microwave and infrared frequency bands are today.
Here, we explore the potential for developing new THz components and sensors to fill this _gap', borrowing from the well established fields of microwave and optical photonics. We employ ultrafast laser sources to generate and detect our THz radiation directly in the time domain. These methods of generating and detecting THz offers several key advantages over conventional spectroscopic measurements: firstly, a short (picosecond) single cycle pulse of THz energy is generated (fig. 1(a)) which allows dynamical studies. Secondly, such a pulse contains a broad spectral bandwidth, so that spectral information about a sample may be easily obtained over a very large range of frequencies, typically covering almost three orders of magnitude in frequency (0.05 - 3 THz _ see fig 1). Thirdly, unlike common optical spectroscopes which only measure the intensity of light at specific frequencies, the Fourier transform of THz time-domain measurement gives amplitude and phase information (see fig. 1(b)), providing the real and imaginary parts of the THz response without the use of the Kramers-Kronig relations.