Using sound and light to generate ultra-fast data transfer

The terahertz quantum cascade laser on its mounting. A pair of tweezers shows how small the device is. Credit: University of Leeds

https://phys.org/news/2020-02-ultra-fast.html

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—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 that 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.

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

Dr Aniela Dunn holds the laser and its mounting in the palm of her hand. 

Credit: University of Leeds

"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.

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."

Radical new graphene design operates at terahertz speed

http://www.kurzweilai.net/radical-new-graphene-design-operates-at-terahertz-speed?utm_source=twitterfeed&utm_medium=twitter

May 2, 2013

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Illustration of tunneling transistor based on vertical graphene heterostructures. Tunneling current between two graphene layers can be controlled by gating. (Credit: Condensed Matter Physics Group/University of Manchester)

A new transistor capable of revolutionizing technologies for medical imaging and security screening has beendeveloped by graphene researchers from the Universities of Manchester and Nottingham.

This is the first graphene-based transistor with bistable characteristics, which means that the device can spontaneously switch between two electronic states.

Such devices are in great demand as emitters of terahertz (trillions of oscillations per second, or thousands of gigahertz) electromagnetic waves in the high-frequency range between radar and infrared, which are relevant for applications such as security systems and medical imaging.

Bistability is a common phenomenon — a seesaw-like system has two equivalent states and small perturbations can trigger spontaneous switching between them. The way in which charge-carrying electrons in graphene transistors move makes this switching incredibly fast — trillions of switches per second.

Wonder material graphene is the world’s thinnest, strongest and most conductive material, and has the potential to revolutionize a huge number of diverse applications; from smartphones and ultrafast broadband to drug delivery and computer chips. It was first isolated at The University of Manchester in 2004.

How it works

Schematic diagram of graphene-BN resonant tunneling transistor (credit: Condensed Matter Physics Group/University of Manchester)

The device consists of two layers of graphene separated by an insulating layer of boron nitride just a few atomic layers thick. (conventional resonant tunneling devices are tens of nanometers

thick), allowing for ultra-fast transit times.

This feature, combined with the multi-valued form of the device characteristics, suggests potential applications in high-frequency and logic devices.

The electron clouds in each graphene layer can be tuned by applying a small voltage. This can induce the electrons into a state where they move spontaneously at high speed between the layers.

Because the insulating layer separating the two graphene sheets is ultra-thin, electrons are able to move through this barrier by quantum tunneling.

This process induces a rapid motion of electrical charge that can lead to the emission of high-frequency electromagnetic waves.

These new transistors exhibit the essential signature of a quantum seesaw, called “negative differential conductance,” whereby the same electrical current flows at two different applied voltages. The next step for researchers is to learn how to optimize the transistor as a detector and emitter.

One of the researchers, Professor Laurence Eaves, said: “In addition to its potential in medical imaging and security screening, the graphene devices could also be integrated on a chip with conventional, or other graphene-based electronic components to provide new architectures and functionality.

“For more than 40 years, technology has led to ever-smaller transistors; a tour de force of engineering that has provided us with today’s state-of-the-art silicon chips which contain billions of transistors. Scientists are searching for an alternative to silicon-based technology, which is likely to hit the buffers in a few years’ time, and graphene may be an answer.”

“Graphene research is relatively mature but multi-layered devices made of different atomically-thin materials such as graphene were first reported only a year ago. This architecture can bring many more surprises”, adds Dr Liam Britnell, University of Manchester, the first author of the paper.

REFERENCES:

• L. Britnell et al., Resonant tunnelling and negative differential conductance in graphene transistors, Nature Communications, 2013, DOI: 10.1038/ncomms2817 (open access)