Researchers develop tiny but tough lasers

Wednesday 22 July 2020

Researchers from The Australian National University (ANU) have developed a new robust type of light technology that could lead to cheaper and faster devices.

The team report in Light: Science & Applications the creation of a laser that is immune to fabrication imperfections and external disturbances, such as unwanted light reflections and scattering.

“What’s interesting about this laser is that it’s quite difficult to destabilise or destroy its operation,” said PhD student Aditya TrIpathi, from the ANU Research School of Physics’ Nonlinear Physics Centre, led by Professor Yuri Kivshar.

The lasers are so small that around a hundred of them could fit within the cross-section of a human hair. Their size could enable a wide range of light-based technology to be miniaturised cheaply and employed in the quest to replace current electronic circuitry in our computers with faster, laser-based circuits.

Co-author Dr Sergey Kruk, also from the Nonlinear Physics Centre, says lasers are the backbone of the internet.

“When you send an email to a person in another country, the information gets converted into short pulses of light produced by semiconductor lasers, which then travel long distances via optical fibres,” he said.

“Conventional lasers have been tremendously successful for long-range communications – between buildings and between continents.

“However, scientists and engineers have run into challenges when it came to applying this same technology within tiny and very cramped spaces, such as inside an integrated circuit.

“In tiny circuitry the operation of lasers gets compromised by device imperfections and undesirable light reflections.”

The researchers drew on recent discoveries of materials that are have similar robustness in their electrical properties. These properties were first developed as abstract mathematical concepts, and have only recently been successfully created around the edges of particular materials, with asymmetry or unusual shape (topology), hence they are named topological states. 

“We are all familiar with various physical states, such as the gas, liquid or solid states,” said Dr Kruk.

“Physics and modern high-tech also operate within more exotic states, such as superconductivity or Bose-Einstein condensates. Recently, our toolbox of various physical states has been enriched by topology.”

The robust properties of topological states is great addition to the physicists’ toolbox, said Mr. Tripathi.

“An optical device operating in a topological state will keep working under conditions that would interrupt the operation of a conventional device.”

The ANU team theorised that they could create topological properties in light, and devised a lattice with symmetry resembling honeycomb. The design comprised an array of triangular holes within a thin piece of conventional laser material, indium gallium arsenide. Triangles of different sizes were paired up, one of 174 nanometers and one of 269 nanometer side length, with each pair 460 nanometres apart

To introduce an asymmetric element that could generate topologically protected light, they made a larger triangular area (size eight pairs a side), in which the arrangement of the small and large triangles was reversed.

Next they turned to Professor Hong-Gyu Park’s research group at Korea University, who employed their nanofabrication expertise to create the design.

Sure enough, the new device behaved as predicted, showing strong laser operation around the topologically protected triangular shape, especially from its vertices.  Mathematical modelling showed that even distorting the size of the cavity by 30 percent would barely impair the device’s operation.

The success gives the researchers confidence to move forward, said Mr Tripathi.

“We can now start looking at how to harness this technology in practical ways, for example, for high-speed information transfer inside microchips.

“It could pave the way for other optical devices, such as sensors or transistors that use light instead of electricity to be made small enough to fit on a silicon chip”

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