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New Opportunities in Terahertz Engineering and Science

Final Report Summary - NOTES (New Opportunities in Terahertz Engineering and Science)

The last 15–20 years have witnessed a remarkable growth in the field of terahertz (THz) frequency science and engineering. The THz region of the electromagnetic spectrum lies between the microwave and mid-infrared regions but has remained under-exploited owing to the long-standing difficulty in developing convenient sources of THz radiation. The recent growth in interest arguably began with the development of a pulsed (single-cycle) THz emitter – the semiconductor photoconductive switch – and the subsequent development of THz time-domain spectroscopy (TDS). Since then, considerable success has been achieved in the use of THz-TDS for a wide range of imaging and spectroscopy studies. Recent progress in guided-wave systems, both via freestanding structures and on-chip, has extended the field, and the realization and subsequent development of the THz frequency quantum cascade laser (QCL), a solid-state device based on a layered semiconductor superlattice, through the EC WANTED programme in 2002, has opened the way for the development of the field of THz photonics.

The aim of the NOTES programme was to develop underpinning technologies based on THz frequency QCLs and guided-wave systems to provide future opportunities for THz science and engineering, and to investigate the THz frequency / picosecond timescale physics of low-dimensional semiconductor-based electronic systems; the high frequency response of nanoscale electronics is central to the microelectronics industry, as well as being of fundamental scientific interest.

QCLs have the potential to create a step-change in the uptake of THz technology but a current limitation is the amount of radiation that can be coupled out of the device and the quality of the beam profile. We investigated a number of methodologies to engineer low-divergence surface and edge emission, and in particular by etching an intricate pattern (a ‘photonic crystal’) into a QCL. We also demonstrated THz QCLs as optical amplifiers, in which weak pulses of THz radiation emitted by photoconductive switches were amplified; this will assist uptake of THz technology, and extend the application of time-domain spectrometers. Furthermore, we demonstrated a technique known as ‘mode-locking’ to create controlled pulsed emission from a THz QCL, which in addition to providing insight into the fundamental operational principles of these devices, supports development of high-power coherent THz QCL imaging and spectroscopy systems. Tuneable QCLs are beneficial for many applications, and we developed a series of methodologies to address this. We also pursued a number of QCL imaging and spectroscopy techniques, most noteworthy being a novel scheme enabling use of a QCL device for both generation and detection of the THz radiation, offering high sensitivity, a very fast response, and a simple and compact design.

To develop an understanding of the high frequency dynamic conductivity of low-dimensional electron systems, we pursued a number of complementary approaches. We built a THz-TDS transmission system capable of simultaneously measuring the orthogonal polarization components of a free-space propagating signal, and extracted the conductivity tensor components of a two-dimensional electron system (2DES). Also, using a 2DES patterned with a coplanar waveguide, we established the influence of short- and long-range impurity potentials on the 2DES high frequency response (to 20 GHz), and interpreted this in the context of theoretical models of magneto-conductivity. To extend these studies to THz frequencies, and also to investigate the electronic properties of one-dimensional (wires) and zero-dimensional (dots) systems on THz/picosecond timescales, we developed technology to introduce picosecond electronic pulses into low-dimensional systems at cryogenic temperatures. Our technology can also be used for studying biologically-relevant molecules (such as proteins), and a number of proving measurements were undertaken; potentially, understanding of the low-energy vibrational modes of such molecules could address fundamental biological questions related to protein dynamics and function.