Supercontinuum in a Crystal

A wide range of colours from a single laser is possible with a new process for achieving supercontinuum generation. (Source: Heriot-Watt U.)

Super­continuum generation is when intense laser light of one colour travels within a material, like glass, and broadens into a spectrum of colours. The effect lets scientists produce light at colours tailored to particular applications in sectors like bioimaging, optical communi­cations and funda­mental studies of materials. Until now, there were two ways to create a super­continuum. A special optical fibre could be used to concen­trate light to a very high intensity, over lengths of a few metres. Alternatively, even more powerful light from an amplified laser could be tightly focused into ordinary glass.

These traditional approaches come with disad­vantages, associated either with the size, complexity and cost of using an extremely high-energy laser, or with the precise and fragile alignment needed to force light into an optical fibre only two-thousandths of a millimetre in diameter. Photonics experts from Heriot-Watt have demons­trated a new method that combines the best of both worlds: a colourful super­continuum from a bulk material using only moderate-energy lasers.

Derryck Reid from the Institute of Photonics and Quantum Sciences said, “We’ve shown that combining a simple laser with a special, nonlinear crystal can create a super­continuum directly. We’ve removed the need for either a high-power laser or delicate coupling of light into tiny optical fibres. There’s a funda­mentally new mechanism at work here: our specially engineered gallium phosphide crystal creates a cascade effect.”

“We illu­minate the crystal with light from an infrared laser, some of which is converted to visible green light. This in turn generates more green light at a slightly longer wave­length, becoming first yellow, then orange and working all the way out to the red. The weaker edges of the light can generate green at longer and longer wave­lengths. This has never been reported before“, Reid added.

Reid and his team say further work is required to determine whether the effect is specific to the special gallium phosphide crystal they used and whether it can be further amplified.  He said: “This is really promising. We think we can make the spectrum of the light wider and more intense by optimising the properties of the crystal. Visible super­continua are already widely used in life sciences imaging and spectro­scopy, but are limited by the properties of special optical fibres. Our new technique could offer a convenient and compact alter­native to these existing light sources.” (Source: Heriot Watt U.)

Reference: M. Rutkauskas et al.: Supercontinuum generation in orientation-patterned gallium phosphide, Optica 7, 172 (2020); DOI: 10.1364/OPTICA.385200

Source: Institute of Photonics and Quantum Sciences, Heriot–Watt University, Edinburgh, UK

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