Tiny lasers, like inside-out noise-cancelling headphones

Monday 13 September 2021

Physicists at the ANU Research School of Physics have made extremely efficient microscopic lasers, smaller than the width of a hair - smaller, even, than the wavelength of the light they produce.

These nanolasers will power photonics - smaller, faster technology that uses laser light instead of electronics - and will have a huge variety of medical, surgical, industrial and military uses, covering everything from hair removal to laser printers and night-time surveillance, said lead research Professor Yuri Kivshar.

“It’s exciting to see how the concepts of meta-materials and photonics can be combined to create practical devices,” said Professor Kivshar.

“These nanolasers can be integrated on a chip, which is important for nanoscale photonics. For example, they could be mounted directly on the tip of an optical fibre to illuminate or operate on a particular spot inside a human body.”

The research is reported in Nature Communications [https://www.nature.com/articles/s41467-021-24502-0 ]

Professor Kivshar’s team used a clever trick to modify the conventional lasers, which traditionally comprises some form of light amplification device placed between two mirrors. As light bounces back and forth between the two mirrors, it is amplified during each pass through the device, becoming brighter and brighter.

Instead of mirrors, the team created a device similar to inside-out noise-cancelling headphones. Unlike noise-cancelling headphones, which prevent sound energy from getting in, the team’s device prevented energy from escaping. This was achieved with a periodic structure designed so that the different modes of vibration of the light waves cancelled out around the edges of the device. The light energy, trapped inside, can then build up into a strong, well-shaped laser.

This trick overcomes a well-known challenge of nanolasers, energy leakage. 

The idea was triggered when a student was experimenting with microwave radiation – a much larger scale version of the light in this final publication. He was setting up an array of tubes with water (which, in the final experiment with light, would become an array of tiny holes in a layer of indium phosphide). 

When analysing the microwave data, Kivshar’s team found the results were varying apparently randomly from day to day. Eventually they realised that the exact water level was an important factor, even though the experiments had been examining horizontal wave phenomena. 

The team realised that they were seeing interference between energy leakage from horizontal and vertical modes – in all directions, not just the direction of radiation propagation.

With this in mind, they were able to get the perfect radiation cancellation for a particular wavelength by tweaking the material array properties – radius and spacing of holes and the thickness of indium phosphide layer. 

“The leaky tails of the different modes are cancelling out almost perfectly,” Professor Kivshar said.

The result seems to be dark magic, because indium phosphide is transparent to light, and does not normally act as a mirror. Yet, with careful engineering Professor Kivshar and his team have succeeded in trapping the light in a resonator that is open, a phenomenon known as a bound state in a continuum (BIC).

Initial attempts to fabricate BICs into practical devices were not successful, as they found that the cancellation was easily disrupted by fabrication imperfections.

However, the team’s modelling of BICs showed that there were multiple ways to create them, and with more careful analysis they realised how to engineer a structure that had multiple BICs for a given wavelength – effectively overlaying even more leaky tails, all finessed for perfect cancellation. 

This super-BIC was much less sensitive to imperfections and constrained more than 99.9% of the light (Q factor more than two orders higher than any previous BIC laser cavities). 

To complete the laser, the team collaborated with the group of Professor Hong-Gyu Park from the Korea University and fabricated a device with seven quantum wells embedded in the indium phosphide layer. As hoped, the device’s efficiency was high - only a small amount of energy was required to start the laser shining (threshold around 50 times lower than any previously reported nanolaser) and the beam was narrow.

For Professor Kivshar it is gratifying to successfully employ the BICs concept in a practical device, nearly one hundred years after the quantum mechanical concept was discovered. 

“This mathematical solution was published by Wigner and von Neumann in 1929, in a paper that seemed very strange at the time - it was not explained for many years. 

“Now this 100 year old discovery is driving tomorrow's technology!” Professor Kivshar said.

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