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Numerous important processes in nature occur at THz frequencies: for example, many rotational and vibrational transitions of various liquid and gas molecules lie within the THz frequency band. In particular, the vibrational breathing modes of many large biomolecules occur at these low frequencies, giving a unique fingerprint in the THz region. While it is clear that the THz band is scientifically very rich, research in this frequency region is limited by technology: the so-called THz gap , occupying a large portion of the electromagnetic spectrum between the infrared and microwave bands, remains relatively unexplored due to a lack of efficient laboratory emitters/detectors and optical components compared to neighboring spectral regions.Here, we explore the potential for developing new THz components and sensors based on semiconductor surface plasmon-polaritons (SPPs). SPPs are electromagnetic waves that propagate along the interface between a conductor and insulator, bound to the surface by the free electrons in the conducting medium. To date, most research investigating the properties of SPPs has been limited to frequencies near metallic plasma frequencies (i.e. at visible and infrared frequencies) where SPP modes are strongly confined to metal surfaces. Semiconductors, with plasma frequencies in the THz range, offer the potential for sustaining SPPs at THz frequencies. Furthermore, semiconductors offer a unique and hugely beneficial advantage over metals: since the surface charge density can be modified by, for example chemical doping, plasma frequencies and SPP properties can be tailored within the THz frequency range. An extension of this is the exciting possibility of all-optical plasmon control, i.e. 'photo-doping' a semiconductor with visible frequency light, so that plasma frequencies may be tuned by a visible frequency light source. Using ultrafast laser sources for this purpose, the properties of THz SPP modes can therefore be tailored and switched on very fast (picosecond) timescales, something that is essential for high-bandwidth and/or time resolved applicationsBorrowing from the well established fields of microwave and optical photonics, and by utilizing the intrinsically tunable nature of semiconductors, we aim to manipulate THz light in new ways using semiconductor SPPs. Initially, we wish to explore the underlying physics of SPPs in the THz frequency range, before looking to develop new applications for this concept. Although there are many areas of potential application for semiconductor SPPs, we will concentrate on two specific areas: firstly, the design of optical components (such as tunable filters, modulators and beam steering systems) based on semiconductor SPPs and secondly, spectroscopy/sensing of biomolecules. The development of new optical components is essential for the continued expansion of scientific research in the THz frequency domain, and we will exploit the wealth of experience currently employed in Exeter to investigate similar applications at microwave and optical frequencies. The second of these potential applications involves what is thought to be possibly the future killer application of THz radiation, i.e. using the fingerprint of large molecules in the THz region for biosensing and biomedical applications. In an analogy to recently developed surface plasmon sensors operating at visible frequencies, employing SPPs for THz sensing will significantly improve sensitivity by concentrating THz radiation in a very thin region close to the semiconductor surface, allowing sensing of very low concentration samples.
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