Metamaterial shows non-linear optical activity

Georgia Tech researchers have created a non-linear optical metamaterial that produces a spectral shift under only milliwatts of radation, suspecting it to have the strongest non-linear optical rotation ever reported for a chiral metamaterial – about a hundred thousand times larger than the current record measurement for this type of structure, said the university.

GaTech meta material Credit Rob Felt

A 500μW to 15mW change in excitation over a 40μm area in material only one seventh of a wavelength thick, creates a 10nm spectral shift in transmission resonance and a 14° polarisation rotation. All-optical switching and light modulation are potential applications.

“The goal is really to take advantage of the discrepancy between one circular polarisation versus the other to create the broadband resonances we need. Nanoscale chiral structures offer an approach to modulating optical signals with relatively small variations in input power,” said researcher Sean Rodrigues. “To see this kind of change in such a thin material makes chiroptical metamaterials an interesting new platform for optical signal modulation.”

The researchers don’t yet know what prompts the change, but as power density is high, they suspect thermal processes may be involved – tests show no lasting damage.

In detail

The structure operates in near-infra red between 740 and 1,000nm.

It is made using electron beam lithography to pattern two 33nm thick layers of silver separated by a 45nm layer of dielectric – all on a glass substrate. A regular two-row rectangular array pattern of oval holes (visible in photo) with 350nm pitch is made in both silver layers, with holes in one layer above holes in the other.

The long axes of the ovals are aligned within each sheet, but in one sheet they are skewed by 22.5° to the other sheet.

The work is described in ‘Intensity-dependent modulation of optically active signals in a chiral metamaterial‘ in Nature Communications.



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