Terahertz Gas-Fiber Photonics

The terahertz (THz) part of the electromagnetic spectrum lies between the domains of microwaves and optics. In the last 20 years there has been enormous growth in research on exploiting this spectral window for materials research and practical applications like security and medical screening. Many applications rely on sources of extremely short pulses of THz radiation and are limited by their presently low peak and average power. The recently discovered phenomenon of terahertz supercontinuum generation during the formation of laser ionized plasmas in free space combines remarkably broad, continuous and controllable bandwidth, stretching from the far to the mid infrared, with pulse energies as high as a micro-Joule. It promises to be an important new source of high peak and average power terahertz radiation for scientific studies of materials in extreme THz fields and for imaging and sensing. Its potential exploitation is however constrained by the low optical to THz conversion efficency, which is limited by diffraction, and the cost and low pulse repetition rate of the high energy laser systems currently needed. Our primary aim is to overcome these limitations by spatially confining the THz generation to the hollow core of a gas filled waveguide based on a type recently developed in Bath known as photonic crystal fiber. The combination of small core area and long interaction length is expected to reduce the threshold pump energy for THz generation by orders of magnitude. The optical waveguide will be integrated with a terahertz guide. By engineering the velocities of the optical and teraherts waves by composite waveguide design to achive a condition called 'phase matching' we will at the same time increase the conversion efficiency and thus retain the high peak power of the THz radiation. A secondary aim is to perform sensitive detection in similar composite guides by exploiting an intinsic optical nonlinearitiy of gases, thus creating the essential building blocks of what could be, with the addition of powerful fiber amplifiers currently in commercial development, a cost effective and robust 'all-fiber' platform for THz science and technology with unprecedented power and flexibility. This is our long term vision and ambition.

In the short term (3 or less years), the beneficiaries of the proposed research are intended to be entirely academic users of pulsed THz spectroscopy, sensing and imaging systems. The applications of these cover many disciplines including physics, chemistry, biophysics, engineering and archaeology. On a 3 or longer year timescales there could be economic and possibly societal benefits. This is because the aim of the project is to develop a powerful new THz instumentation platform which will encourage the development of new applications and and facilitate existing ones. Although applications of THz technology outside the academic sphere are in their infancy, a 2008 market survey by BCC Research estimates that the global market in THz systems will rise from $77 million in 2008 to $500 million by 2018. This estimate is based on an assumed expansion in the sales of spectroscopy and imaging systems, particularly in the pharmaceutical and security sectors, and the eventual emergence of new markets and applications such as manufacturing quality control, chemical or bio-sensing, medical screening and art restoration. The applications of THz technology are often over sold because many of the triumphs of THz measurements, particularly time domain ones, in the lab are difficult to transfer to the real world because of the lack of intense enough and cheap enough pulsed sources. For the most part, applications of pulsed THz sources still rely on micro-Watt average power technology that has not significantly changed in the last 20 years. This bottleneck is addressed by the proposed advances in peak and average power and (on a 5 year timescale) affordability.
The UK has a strong commercial presence in both the pulsed and continuous wave technology sectors of THz instrumentation via companies such as Teraview, ThruVision and Terahertz.co.uk. Start-ups and SMEs presently play the most significant commercial role. Thus, we expect that successful innovations in THz technology resulting from our research could be quickly exploited and play a part in increasing the competitiveness of the UK and fostering global economic performance. To a large extent this will depend on parallel developments in fiber laser technology. If the later can be made sufficiently cheap (there is no fundamental constraint on doing this) then the envisaged THz systems could be used for closer to real time quality control in the pharmaceutical, plastics and semiconductor industries. In the public sector, applications as varied as security screening, melanoma and burns assessment and art/archaeology conservation might also become more practical and competitive. Therapeutic treatments analogous to the microwave treatments for menorrhagia and liver cancer (both pioneered by the medical microwave group in the physics department in Bath) are other possibilities since focused micro-Joule energy THz pulses are intense enough to disrupt tissue.
The project will have a major impact on the careers of the postdoc and, assuming recruitment, PhD students who will be working at the cutting edge of THz and fibre photonics. It should therefore provide a source of highly skilled potential employees.