Semiconductor diode lasers are being used increasingly widely because they emit light at a single frequency from a very small sized and relatively cheap device. The most widely available and used devices have outputs at visible wavelengths or a little longer. Obtaining devices that will work conveniently at longer wavelengths in the infrared and submillimeter region has proved much more difficult requiring the use of new and less developed semiconductor materials.
The work of F. Capasso, R. F. Kazarinov, J. Faist, and R. A. Suris has led to the demonstration of a completely new type of laser, which will operate at infrared wavelengths and which can be fabricated from well established, wide-gap semiconductor materials. The operation differs in a fundamental way from previous semiconductor lasers. It relies on only one type of charge carrier and electron transitions between quantum states arising from carrier confinement between thin alternating layers of different alloy composition, known as superlatticies. A single electron travelling through the structure sequentially emits many photons, hence the name - quantum cascade laser. The fundamental idea of the amplification of electromagnetic waves in a semiconductor superlattice structure was first put forward in a theoretical paper by R. F. Kazarinov and R. A. Suris in 1971, (Soviet Physics- Semiconductors 5 (4) p.707, 1971 ). However, it was not until 1995 that F. Capasso and J. Faist were able to devise a practical structure in which lasing could be achieved (US patent 5.457.709). Using molecular beam epitaxy and band structure engineering, they were able to make the first experimental demonstration of the new type of laser device. This has now been operated wavelengths between 3 and 8 microns in a pulsed mode at room temperature and at wavelengths as long as 13 microns at reduced temperatures. Similar devices have important potential applications, for example as simple spectrometers for chemical analysis and remote sensing of atmospheric pollutant gases. As such they will be of great benefit to mankind.
The Chairman of the Trustees
THE RANK PRIZE FUND
London, 8th December 1998