By Dr Manas Kumar Haldar
(Published in’Campus & Beyond’, a weekly column written by Swinburne academics in the Borneo Post newspaper)
Lasers have been with us for 50 years. The first lasers were gas and solid state lasers (such as ruby laser) which were bulky. The next invention was the semiconductor laser. Its portability made many applications possible. These lasers can emit visible as well as invisible (infrared) light. Infrared light of wavelength around one micrometer (one thousandth of a millimetre) is used in optical fibre communications. On the other hand, the range of wavelengths between three and twenty micrometers is also very important. It is called the molecular fingerprint region. Many molecules absorb light at these wavelengths. The wavelength of the light absorbed by a molecule uniquely identifies it. The conventional semiconductor laser based on the compounds of gallium arsenide can not emit these wavelengths. Lasers based on lead salts can emit these wavelengths. They are so named because many of them consist of semiconducting compounds containing lead. However, lead-salt lasers require cooling by liquid nitrogen. This confines most of their applications to the laboratory. The recently developed quantum cascade lasers are also made of gallium arsenide compounds. However, because of a completely different method of operation, they can emit wavelengths from a few micrometers to well above 10 micrometers. Moreover, quantum cascade lasers operate at room temperature. This allows portable applications and has made lead salt based lasers almost obsolete.
The operation of all lasers is based on exciting charge carriers to a high energy state, such as by driving a current in a semiconductor laser. Unlike in gases, charge carriers in a semiconductor can have a range of energies called a band. The highest and mostly unoccupied energy band is called the conduction band and the next lower band is called the valence band. There is a range of energies between these two bands which an electron can not have. This is known as the bandgap. When the excited electron from the conduction band drops into the valence band it emits light.
Hence, conventional semiconductor lasers are called interband lasers. The wavelength of the emitted light depends on the bandgap. The desired wavelength can be obtained by adjusting the bandgap, which is quite limited for the conventional gallium arsenide based lasers.
The theoretical basis of the cascade laser was first proposed by two Russian scientists in 1971. However, the technology to make them was perfected more than 20 years later. One of the reasons is that the scheme requires alternate layers of semiconductors with different bandgaps, each layer only about 30 to 100 atoms thick. There may be hundred or so layers and the arrangement is called a super lattice. Two higher bandgap layers enclosing a lower bandgap layer present a barrier to the motion of electrons from the lower bandgap layer. This is called a quantum well. So the super lattice consists of a succession of quantum wells. Imagine falling in a well. You will not be able to escape without somebody’s help. The electron, unlike you, can tunnel out of the quantum well. Moreover, the electrons can only have certain energies in the well. When a voltage is applied, the successive wells are tilted like a staircase. An electron falls from a higher energy state to a lower energy state in a well generating light. It then tunnels out to the higher energy level of the next well, drops to the lower energy level emitting light and so on. It is like water falling in a cascade. The wells are all in the same energy band. The cascade laser action occurs inside the wells of a band not between bands. Hence, the cascade laser is called an intraband laser. The difference between the energy levels inside a well determines the wavelength. This can be varied over a wide range by suitable design of the well.
As mentioned earlier, the range of wavelengths is useful for detecting molecules. The cascade laser therefore constitutes a simple portable device for many applications. Some of the examples are detection of water content, monitoring plastics, monitoring the environment (e.g. automobile emissions) and even medical diagnosis. But this is not the complete story. Recently (2008), quantum cascade lasers have been developed to emit terahertz (1THz consists of 12 zeros after 1) frequencies at room temperature. This opens up a whole load of interesting applications yet to be found, because there has been no useful source in the 1 to 10 THz range. At the Swinburne University of Technology’s Sarawak campus, there are plans to study the operation and applications of quantum cascade lasers. At present, some theoretical studies are being made.
Dr Manas Kumar Haldar is Associate Professor with the School of Engineering, Computing and Science, Swinburne University of Technology Sarawak Campus. He can be contacted firstname.lastname@example.org