Far Infra-Red Emission and Lasing in Doped Semiconductors

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology

Abstract

The terahertz band is located between the visible/near infrared frequencies and millimetre/microwave frequencies. Its physical properties bear some resemblance to light on one side and heat or microwaves on the other. It can be reflected and focused like light using special mirrors and lenses. It transfers energy/heat to materials in a similar way to microwaves, by causing the whole molecular structure to vibrate when radiation of the correct frequency is absorbed. This particular property makes terahertz radiation an ideal tool to study the properties of new materials because each material has a unique absorption signature. Why is this new and exciting? Until very recently there have been no practical sources of terahertz radiation, or indeed ways to detect it. So, in many ways this is uncharted territory. The situation changed radically with the invention (in the UK) of the first terahertz laser along with the development of a number of new techniques for producing powerful terahertz pulses.Current terahertz sources are broadly divided into two classes: broadband and single frequency. Terahertz radiation generated from photoconductive antennae and from surface fields is generally classed as broadband. The main limitation of this type of generation scheme is the low powers achieved. Lasers make up the second class, that is, single frequency terahertz sources. The III-V terahertz quantum cascade laser was first demonstrated in 2002 and considerable progress has been made since then. While the quantum cascade laser is undoubtedly an elegant device, its main disadvantage is that it requires complicated and time consuming epitaxial growth. The quantum cascade active region typically contains many hundreds of epilayers and growth times of 36 hours are not unusual.No practical materials exist with conventional bandgaps at terahertz frequencies and thus some other approach must be adopted. However, there is another fundamental energy gap in certain semiconductor materials where the energy separation lies in the terahertz frequency range. Doped semiconductors contain a series of quantized states either just below the bottom of the conduction band (donor levels) or just above the top of the valence band (acceptor levels). Under the right optical pumping conditions it has recently been shown that a population inversion can be achieved between states and stimulated emission at terahertz frequencies has been observed.The overall aim of this project is to re-visit the subject of shallow level impurities in the broad spectrum of semiconductor materials now available to us, and in doing so, open up a whole new field of terahertz laser research. Since most current commercial off-the-shelf terahertz lasers are cumbersome gas based systems, an optically pumped impurity doped semiconductor system would have an obvious size and weight advantage. Furthermore, an electrically pumped impurity based laser would have an additional advantage in that a CO2 pump laser would no longer be required. The technology, if successfully exploited, has the potential to result in a whole new breed of cheap reliable off-the-shelf sources of FIR radiation.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
EP/E061265/1 01/10/2007 01/05/2011 £519,900
EP/E061265/2 Transfer EP/E061265/1 01/08/2011 31/12/2012 £158,474
 
Description We discovered a method to control and manipulate the quantum state of certain donor atoms in silicon. This is a first step towards building a quantum bit (qubit) in silicon.
Exploitation Route N/A
Sectors Digital/Communication/Information Technologies (including Software),Electronics

 
Description The research has been cited my other scientists working in the field. Nature paper 'Coherent control of Rydberg states in silicon' has been cited 52 times since it was published in 2010.
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics
 
Title A PORTABLE APPARATUS FOR MEASURING THE EFFICIENCY OF A PHOTOCATALYST 
Description The present invention relates to a portable apparatus and methods for measuring the efficiency of a photocatalyst. The invention is also concerned with the use of the apparatus for measuring the efficiency of a photocatalyst and/or to determine the purity of a sample of water. The invention is further concerned with water purification systems comprising the apparatus according to the present invention. 
IP Reference WO2012098346 
Protection Patent granted
Year Protection Granted 2012
Licensed No
Impact n/a