Chiral Metamaterial Causes Optical Shift in mW Range

A metamaterial with chiroptical properties in the nonlinear regime has shown the ability to produce a significant spectral shift with power levels in the milliwatt (mW) range. Two absorptive resonances were spectrally modified through incremental exposure to power intensities beyond the material’s linear optical regime. When a 15 mW change in excitation power was made, there was a 10-nm spectral shift in the material’s transmission resonances and a 14° polarization rotation.

Laser light shows the nanopatterned structure of a chiral metamaterial developed by researchers in the School of Electrical and Computer Engineering at the Georgia Institute of Technology. Courtesy of Rob Felt, Georgia Tech.

Researchers from Georgia Institute of Technology believe that this measurement may be the strongest nonlinear optical rotation ever reported for a chiral metamaterial. It is about 100,000 times larger than the current record measurement for this type of material.

The metamaterial was made by nano-patterning layers of silver, approximately 33 nm thick, onto glass substrates. A 45-nm thick layer of dielectric material was sandwiched between the layers of silver. An elliptical pattern was created using electron beam lithography. The structure was then encapsulated within a dielectric material to prevent oxidation.

The material operates in the visible to NIR spectrum, at approximately 740 to 1000 nm. 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 0.5 mW up to 15 mW. While that is comparatively low power for a laser system, it had a high enough energy flux to instigate change.

“The beam size is roughly 40 microns, so it is really focused,” said researcher Sean 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 showed that the power applications did not damage the metamaterial.

Chiral materials exhibit optical properties that differ depending on their opposing circular polarizations. The differences between the responses, which are created by the nanoscale patterning of absorptive materials, can be used to create chiroptical resonances. To be useful in applications such as all-optical switching, these resonances need to be induced by external tuning, such as modulations in power input.

Sean Rodrigues, a Ph.D. candidate in the Georgia Tech School of Electrical and Computer Engineering, adjusts a sample of a chiral metamaterial whose properties in the nonlinear regime produce a significant spectral shift with power levels in the milliwatt range. Courtesy of Rob Felt, Georgia Tech.

“When you increase the power, you shift the spectrum,” Rodrigues said. “In effect, you change the transmission at certain wavelengths, meaning you’re changing the amount of light passing through the sample by simply modifying input power.”

For optical engineers, that could be the basis for a switch.

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

The 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. These technologies could have applications in such areas as data processing, sensing and communications.

“Nanoscale chiral structures offer an approach to modulating optical signals with relatively small variations in input power,” said Rodrigues. “To see this kind of change in such a thin material makes chiroptical metamaterials an interesting new platform for optical signal modulation.”

The research was published in Nature Communications (doi: 10.1038/ncomms14602).