Thin material shows spectral shift with 15mW power

Article By : Georgia Institute of Technology

A modulation of chiroptical responses from metamaterials by manipulating input power paves way for new types of active optics, researchers say.

An optical metamaterial developed by researchers at the Georgia Institute of Technology features chiroptical properties that produce a significant spectral shift in the nonlinear regime with power levels in the milliwatt range.

The researchers have demonstrated properties of their chiral metamaterial, in which they spectrally modified two absorptive resonances by incrementally exposing the material to power intensities beyond its linear optical regime. With a 15mW change in excitation power, they measured a 10nm spectral shift in the material’s transmission resonances and a 14° polarisation rotation.

The researchers believe that may be the strongest nonlinear optical rotation ever reported for a chiral metamaterial, and is about a hundred thousand times larger than the current record measurement for this type of structure. The research, supported by the National Science Foundation and the Air Force Research Laboratory, was reported in the journal Nature Communications.

“Nanoscale chiral structures offer an approach to modulating optical signals with relatively small variations in input power,” said Sean Rodrigues, a Ph.D. candidate who led the research in the laboratory of Associate Professor Wenshan Cai in Georgia Tech’s School of Electrical and Computer Engineering. “To see this kind of change in such a thin material makes chiroptical metamaterials an interesting new platform for optical signal modulation.”

This modulation of chiroptical responses from metamaterials by manipulating input power offers the potential for new types of active optics such as all-optical switching and light modulation, researchers said, noting that such technologies could have applications in areas as data processing, sensing and communications.

The material demonstrated by Cai’s lab are made by nano-patterning layers of silver—approximately 33nm thick—onto glass substrates. Between the carefully-designed silver layers is a 45nm layer of dielectric material. An elliptical pattern is created using electron beam lithography, then the entire structure is encapsulated within a dielectric material to prevent oxidation.

The material operates in the visible to near-infrared spectrum, at approximately 740 to 1,000nm. The optical rotation and circular dichroism measurements were taken with the beam entering the material at a normal incident angle.

The researchers induced the change in circular dichroism by increasing the optical power applied to the material from 0.5mW up to 15mW. While that is comparatively low power for a laser system, it has a high enough energy flux (energy transfer in time) to instigate change.

“The beam size is roughly 40 microns, so it is really focused,” said Rodrigues. “We are putting a lot of energy into a small area, which causes the effect to be fairly intense.”

The researchers don’t yet know what prompts the change, but suspect that thermal processes may be involved in altering the material’s properties to boost the circular dichroism. Tests show that the power applications do not damage the metamaterial.

“It is the engineering of these structures that gives us these chiral optical properties,” Rodrigues explained. “The goal is really to take advantage of the discrepancy between one circular polarisation versus the other to create the broadband resonances we need.”

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