TERAHERTZ SOUND LASERS (SASERS): RECENT DEVELOPMENTS AND APPLICATIONS

TERAHERTZ SOUND LASERS (SASERS): RECENT
DEVELOPMENTS AND APPLICATIONS
CL Poyser School of Physics and Astronomy, University of Nottingham, UK
AV Akimov School of Physics and Astronomy. University of Nottingham. UK
W York School of Physics and Astronomy, University of Nottingham, UK
RP Campion School of Physics and Astronomy. University of Nottingham. UK
AJ Kent School of Physics and Astronomy. University of Nottingham. UK
1 INTRODUCTION
Acoustic waves with frequencies in the sub-terahertz range (~O.1 - 1 THz) are sometimes called
nanoacoustic waves because their wavelength is typical solids is of order a few nanometers. Such
acoustic waves are used to extend the ultrasonic testing techniques, which are widely used at lower
(up to ~ 1 GHz) frequencies, to the realm of nanoscience. e.g. non-destructive probing of
nanostructures'. Currently. the so♥called picosecond acoustics? techniques are-used to study
nanoacoustic waves in a variety of nanostructuresJ. However. picosecond acoustics requires the
use of large and costly ultra-fast (femto/pico second) laser setups for the optical generation and
detection of the acoustic waves. As such, picoseccnd acoustics measurements are largely
restricted to laboratories which possess the necessary specialized equipment and expertise. More
widespread application of nanoacoustics in science and technology would befacilitated by the
development of low-cost electrical transducers for generation and detection of THz sound.
We have shown that various semiconductor devices. such as transistors and diodes, can be used to
detect nanoacoustic waves by converting the strain directly to an electrical signa|☁-5-°. What is also
required is a device that can generate nanoacoustic waves when driven electrically. In the 1950's
the requirement for a source of coherent high-frequency electromagnetic radiation stimulated the
development of the maser and laser. It was soon realized that, because phonons (the quanta of
sound radiation) are. just like photons. Bosons. the process of amplication by stimulated emission
should be theoretically possible for sound as well as light. An acoustic oscillator working on the
same principles as a maser or laser has been named a saser. for: sound amplication by the
stimulated emission of (phonon) radiation. In a saser gain medium, transitions of a quantum system.
eg. electronic or spin systems. between energy levels brought about by the stimulated emission of
phonons can. if the levels are population inverted, give rise to an increment in the phonon
occupation number at a particular frequency. i.e. phonon amplication.
Previously, evidence for acoustic amplification by stimulated emission of phonons had been
experimentally observed in ruby7-5-9 in vanadium doped Alea☁°; in BK 7 glass☜; and CdS".
However, none of these could be considered a true saser oscillator. as they lackedan efcient
acoustic cavity to provide phonon feedback at the frequency of operation. it has been relatively
recently demonstrated, however. that modern epitaxial semiconductor growth technologies enable
the production of high-quality acoustic mirrors and cavities for THz frequencies using the Ga(AI)As
semiconductor materials system☁av☜. A saser oscillator could be made by the incorporation of a
device or medium able to amplify phonons into such a cavity. However. the materials mentioned
above are incompatible with the materials used for THz acoustic mirror manufacture. Ideally. the
amplifying device would be based on the same semiconductor technology as the cavity.
Phonon amplication in semiconductor heterostructures has been studied in several theoretical
works. including: transverse acoustic (TA) phonon amplification via the Cerenkov effect in AlGaAs _
quantum well structures☝; a source of THz coherent phonons based on resonant tunnelling in a
Ga(Al)As double barrier resonant tunnelling diode (DBRTD)☜☁; and acoustic phonon amplication in
GaAs/AlAs superlattice (SL) structures"☜. A SL-based phonon amplier is the perfect complement
to the SL-based mirrors and cavities, enabling the entire saser structure to be grown by techniques