Researchers have made a breakthrough in the development of a new generation of terahertz quantum cascade lasers, which could lead to the transmission of data at the rate of 100 gigabits per second – an exponential improvement on present transfer speeds.
What distinguishes a terahertz quantum cascade laser from other lasers is the fact that it emits light in the terahertz range of the electromagnetic spectrum; this has opened up applications in the field of spectroscopy and the identification gases and chemical compounds. New applications might also be found in short-range data transmission. The lasers could eventually provide ultra-fast, short-range wireless links across hospitals and university campuses – or in military and satellite communications.
To be able to send data at these increased speeds, the lasers need to be modulated very rapidly: ideally they should switch on and off or pulse around 100 billion times every second.
Engineers and scientists have so far failed to develop a way of achieving this. A research team from the University of Leeds and University of Nottingham believe they have found a way of delivering ultra- fast modulation, by combining the power of acoustic and light waves. They have published their findings today in Nature Communications.
John Cunningham, Professor of Nanoelectronics at Leeds, said: "This is exciting research. At the moment, the system for modulating a quantum cascade laser is electrically driven – but that system has limitations.
"Ironically, the same electronics that delivers the modulation usually puts a brake on the speed of the modulation. The mechanism we are developing relies instead on acoustic waves."
A quantum cascade laser is very efficient. As an electron passes through the optical component of the laser, it goes through a series of 'quantum wells' where the energy level of the electron drops and a photon or pulse of light energy is emitted. One electron is capable of emitting multiple photons. It is this process that is controlled during the modulation.
Instead of using external electronics, the teams of researchers at Leeds and Nottingham Universities used acoustic waves to vibrate the quantum wells inside the quantum cascade laser. The acoustic waves were generated by the impact of a pulse from another laser onto an aluminium film. This caused the film to expand and contract, sending a mechanical wave through the quantum cascade laser.
Essentially, what we did was use the acoustic wave to shake the intricate electronic states inside the quantum cascade laser. We could then see that its terahertz light output was being altered by the acoustic wave. This result opens a new area for physics and engineering to come together in the exploration of the interaction of terahertz sound and light waves, which could have real technological applications
Professor Cunningham added: "We did not reach a situation where we could stop and start the flow completely, but we were able to control the light output by a few percent, which is a great start. We believe that with further refinement, we will be able to develop a new mechanism for complete control of the photon emissions from the laser, and perhaps even integrate structures generating sound with the terahertz laser, so that no external sound source is needed."
