Chiral metamaterial produces record optical shift under milliwatt-level power changes

Researchers at Georgia Institute of Technology (Atlanta, GA) have demonstrated an optical metamaterial whose chiral optical properties in the nonlinear optical regime produce a significant spectral shift with power levels in the milliwatt range.¹

The researchers recently 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 15 mW change in excitation power, they measured a 10 nm spectral shift in the material's transmission resonances and a 14° polarization rotation. The metamaterial itself was less than λ/7 (a seventh of a wavelength) thick.

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

"Nanoscale chiral structures offer an approach to modulating optical signals with relatively small variations in input power," says 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 chiral optical metamaterials an interesting new platform for optical signal modulation."

All-optical switching for data-processing possible

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

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

"When you increase the power, you shift the spectrum," Rodrigues says. "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.

The material demonstrated by Cai’s lab are made by nanopatterning layers of silver approximately 33 nm thick onto glass substrates. Between the silver layers is a 45 nm 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-IR spectrum at approximately 740 to 1000 nm. The optical rotation and circular dichroism measurements were taken with the beam entering the material at a normal-incidence angle.

The researchers induced the change in circular dichroism by increasing the optical power applied to the material from 0.5 to 15 mW. 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 µm, so it is really focused," says Rodrigues.

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.

Source: Georgia Institute of Technology

REFERENCE:

1. Rodrigues, S.P. et al., Nature Communications (2017); http://dx.doi.org/10.1038/ncomms14602.