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UCSD creates flexible photonic crystal from liquid thin-film metasurface

06 Jan 2020

Development promises all-optical transistors that consume little power and which could enable more powerful computers.

Shimon Rubin and Yeshaiahu Fainman from the Ultrafast and Nanoscale Optics Group at the University of California San Diego have shown how it might be possible to create a flexible yet durable photonic crystal from a liquid. To achieve this they have performed a series of calculations to predict the formation and performance of a photonic crystal based on very localized heating in liquid thin films.

The work is reported in the SPIE journal Advanced Photonics.

Liquids are generally not considered a suitable medium for a photonic crystal because they lack a fixed structure. The optical properties of a photonic crystal depend on light being able to reflect millions of precisely placed structures. But liquids ebb and flow, so any structures are quickly washed away.

However, Rubin and Fainman noted that at the interface between a thin liquid film and a solid or gas, the interplay between the liquid's surface tension and the local temperature can create a small structure (for example, the liquid can pile up to create a little hill). However, it was not previously known if such structures were significant enough to function as a metasurface (a type of photonic crystal) and to modify light propagation.

The researchers investigated several arrangements of liquid films that readily allow light to be guided (at least partially) within the liquid. To obtain a structure, the researchers considered how light absorption might heat the liquid. By using light waves that cross each other at different angles inside the film, a pattern of bright and dark patches is created-this pattern is called a standing wave pattern. The liquid absorbs energy only from the bright patches, hence, the liquid will only heat up at very specific locations.

Flexible fluids

The researchers used the optical and thermal properties of the liquid, combined with fluid dynamic equations and light propagation to calculate the heat absorbed by the fluid, and how that would cause it to locally deform.

They showed that periodic arrangements of hills and valleys in the liquid film could be obtained by crossing between two and four light waves. Two light waves create lines of hills and valleys, three light waves create hexagonal arrangements of hills and valley, while four light beams create a chess-board arrangement. Optical properties were then calculated from these spatial arrangements.

To demonstrate the usefulness of their proposed metasurface, the researchers calculated the threshold of a laser. If a gain media like a dye is added to the fluid, the periodic deformation of the liquid as described above can lead to formation of resonators, capable to support lasing modes. Modifying the symmetry of the photonic liquid crystal then enables control of the frequency and emission direction of the lasing mode.

Liquid photonic crystals seem to have some very nice properties. Because light is used to create the pattern in liquid, the pattern forms naturally and without errors. And, the pattern can be changed on the fly by changing the angle between light waves, or wavelength of the light used to create the pattern.

Even moving patterns can be created by modulating one of the light waves. This inherent flexibility should enable many interesting applications in, for instance, computation and health care. However, the success of this approach will depend on a physical demonstration of the basic concept.

• The Ultrafast and Nanoscale Optics Group is an integral part of the Applied Optics-Photonics and Nanoscale Devices and Systems Programs of the Electrical and Computer Engineering Department at the University of California, San Diego. The research group is directed by Prof. Yeshaiahu Fainman.

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