Nanostuctures in the form of thin layers of semiconductor can be engineered to make electrons cascade down an energy staircase emitting quanta of light (photons) as they lose energy. From these nanostructures quantum cascade lasers are formed.
The layers have to be really thin - approximately the same wavelength as the electrons - so for electrons in semiconductors that’s less than 10nm. When the nanostructures are approximately the same size as the electron waves the waves are confined and form bound energy states. Several of these nanostructures close to each form a staircase of bound energy states and as the electron cascades down the structure it moves from energy level to energy level – sometimes when it jumps down a step in the staircase it emits a photon.
The Quantum Cascade Laser image also shows a photonic band gap structure – the pillars that surround the central square section. These pillars are 1000nm in diameter and 12000nm high. The photonic band gap structure is designed to strongly reflect just one wavelength of light and can make quantum cascade laser more efficient and more compact.
It turns out that with the usual types of semiconductor that are easy to employ then the photons emitted have wavelengths that cover a part of the spectrum called the mid-infrared region where it’s possible to detect gases that are invisible to our eyes - gases such carbon dioxide, the well known greenhouse gas, pollutant gases such as Nox and Soxs and hydrocarbon gases important to industry such methane and ethane.
This is an example of nanotechnology applied to monitor the environment for pollutants and for safety.
Data & SEMs: Prof. C. Ironside, Electronics & Electrical Engineering, Glasgow University.
© 2007 M. Robertson/Nanovisions
|
|
