Acoustoelectric Methods for the Generation Manipulation and Detection of THz Radiation

The conversion of acoustic signals (sound) to electrical signals, and vice-versa, is a technology that has found widespread practical applications. These include, for example: microphones and loudspeakers for sound recording and reproduction at audio frequencies (approx 20 Hz to 20 kHz); transducers for ultrasonic pulse-echo measurement and ultrasonic imaging systems (approx 20 KHz to 100s of MHz); and surface acoustic wave devices for signal processing in mobile communication devices (100s of MHz to a few GHz). The aim of this project is to develop a new technology for conversion between acoustic and electromagnetic (EM) signals, which works at much higher frequencies (10s of GHz to a few THz) and exploits acoustoelectric and piezojunction effects in semiconductor nanostructures and devices.

Acoustoelectric effects in semiconductors are due to the electrons "riding" the acoustic wave as it travels through the crystal. The electrons are effectively dragged along by the sound wave from one electrical contact to the other, giving rise to an electrical current. We have recently obtained experimental evidence for acoustoelectric effect at acoustic wave frequencies up to 100s of GHz in semiconductor nanostructures which points to the feasibility of the proposed project to reach the THz range. The piezojunction effect is a related phenomenon, where the sound wave modulates the electrical conduction across an interface, or junction, between semiconductors such as found in, for example, diodes and transistors. Again, recent experimental evidence obtained by us shows that the piezojunction effect works to very high (THz) frequencies. In the project we will investigate a number of the most promising devices for acoustoelectric applications, and optimise their sensitivity and speed in response to the THz acoustic waves generated by ultrafast laser techniques or saser (sound laser).

Potential applications, which we will explore in this project, include: new and improved methods of generation, manipulation and detection of THZ EM waves, e.g. heterodyne mixing of THz sound with THz EM waves, which have applications is scientific research, medical imaging and security screening; and the generation and detection of nanometre wavelength hypersound, which may be used to extend the established ultrasonics measurement and imaging techniques to the study of materials and structures at the nanoscale.

This research programme addresses a new area of science and technology, that of terahertz (THz) acousto-electronics. The research objectives are concerned with reaching a fundamental understanding of THz acousto-electric phenomena in semiconductor nanodevices and, through the integration of THz acoustics with THz electromagnetic technologies, developing a brand new class of electronic devices for high-frequency applications, which is an area identified for growth in the EPSRC "Shaping Capability" exercise. The project will impact on a number of the EPSRC thematic areas including: Information and communication technologies; Engineering; Physical sciences; and Healthcare technologies.
In the medium to long term, the project has the potential for significant economic and societal impact, through the development of new high-frequency devices and techniques. For example, applications of the THz acousto-electronic technologies in Information and Communications technologies include:

(1) single-chip low phase noise THz sources based on conversion of sound laser (saser) acoustic radiation to electromagnetic radiation;

(2) bulk acoustic wave filters working at frequencies up to 100s of GHz. The bandpass filter element in this case would be a superlattice structure sandwiched between the acoustoelectric devices used for conversion between sub-THz electrical and acoustical signals. Such devices could potentially be integrated with the acoustic/THz mixer, which will be developed in this project, and also conventional SAW IF filters to provide a complete sub-THz receiver front end including pre-selection, mixing and bandpass filtering elements on a single device.

Related to these potential applications, we have identified a UK-based international company, e2v, that is directly interested from the outset in the knowledge generated by this project (see letter of support).

In the shorter term, the project will impact on research and spectroscopy: THz acoustics extends ultrasonic measurements to the nanoscale. Applications include the non-destructive probing of optically opaque nanostructures in research and for industrial quality control and to probe biological samples with sub-cellular resolution. Currently THz acoustics requires specialized ultrafast optical setups, but the development of convenient and low-cost acousto-electric devices will enable more widespread application of THz acoustics in research and industry. As THz spectroscopy techniques have become more established, they have found many applications, e.g. in materials and biomedical research. The sensitivity to specific materials which have resonances in the THz range has led to applications in security screening, e.g. identifying explosive materials, and in pharmaceutical research. THz acousto-electric devices described in this proposal could be utilized for the manipulation of THz signals in high-resolution THz spectroscopy systems, for example, a "single-chip" spectrometer using acousto-electric THz generator and mixer both driven by a saser oscillator.