‘Light from sound’ could spot cancers and terrorists
Summary
Scientists at the University of Nottingham, UK, have developed a theoretical model with potential for converting sound waves into light waves at terahertz frequencies using a small piece of semiconductor material. Terahertz radiation, which occupies the region between microwaves and infrared frequencies on the electromagnetic spectrum, is able to enter fabrics and plastics while reflecting off explosives and metallic weapons hidden beneath clothing. This allows researchers to imagine a future use of their model in privacy-protecting scan-security. And, since terahertz waves can also identify density differences in skin tissue, a terahertz radiation devise could detect skin cancers before they are visible.
To get to their conclusions, the Nottingham team used a crystal composed of alternating layers of two semiconductors such as gallium arsenide and aluminum gallium arsenide. When a strong high-frequency sound wave hits the crystal, it generates a compression wave that propagates through the crystal and traps the semiconductor’s free electrons in the wave-troughs. Yet, when the sound wave’s amplitude rises above a certain point, the model showed that the electrons are freed from the troughs and begin to oscillate – Bloch oscillations, which are usually induced by applying a voltage and not a sound - around the lattice structure.
What happens is that the oscillating electrons act as dipoles emitting photons at terahertz frequencies. Such a device would need an intense source of sound waves, a saser – acoustic equivalent of a laser -, a technology already developed at Nottingham. Besides, the semiconductor crystal would have to be cooled to 4.2 kelvin to devoid the lattice of any thermal vibrations able to destroy Bloch oscillations.
The work is still theoretical but, in principle it could find uses in medical imagining and security.
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