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Using sound and light for ultra-fast data transfer

Quantum cascade laser on its mounting

Researchers have made a breakthrough in the control of terahertz quantum cascade lasers, which could lead to the transmission of data at the rate of 100 gigabits per second.

That ultra-fast data transfer would be around one thousand times quicker than a fast Ethernet operating at 100 megabits a second.

What distinguishes terahertz quantum cascade lasers from other lasers is the fact they emit light in the terahertz range of the electromagnetic spectrum. They have applications in the field of spectroscopy where they are used in chemical analysis.

The lasers could also eventually provide ultra-fast, short-hop wireless links where large datasets have to be transferred across hospital campuses or between research facilities on universities – or in satellite communications.

This result opens a new area for physics and engineering … which could have real technological applications,” said Professor Tony Kent from the University of Nottingham.

To be able to send data at these increased speeds, the lasers need to be modulated very rapidly: switching on and off or pulsing about 100 billion times every second.

Engineers and scientists have so far failed to develop a way of achieving this.

A research team of academics from the University of Leeds and University of Nottingham believes its has found a way of delivering ultra- fast modulation, by combining the power of acoustic and light waves. The findings were published in Nature Communications.

John Cunningham, Professor of Nanoelectronics in the School of Electronic and Electrical Engineering 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 researchers 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.

Tony Kent, Professor of Physics at Nottingham, said “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.”

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.”

Professor Kent said: “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.”

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