Physicists at the ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS) at the Australian National University have demonstrated a metasurface whose optical response can be switched entirely by light—no electrodes, no magnetic components, no heating elements. The device returns to its initial state in less than a second after the control beam is switched off.

The work, published in Nanoscale (DOI: 10.1039/d6nr00501b), addresses a structural gap in photonic chips. Optical chips promise faster, more energy-efficient signal processing with less noise than electronic chips, but they still lack key signal-processing functions. The new technique could supply some of that missing functionality, including optical switching and dynamic beam shaping.

"We can change the optical response of the metasurface optically—without any electrodes, magnetic components or heating elements, just using light," said project leader Dr. Yana Izdebskaya of ANU's Department of Fundamental and Theoretical Physics. "It's entirely reversible, which has been a challenge until now."

How it works

Metasurfaces generate optical effects that natural materials cannot, using surface arrays of nanoscopic shapes smaller than the wavelength of light. Their behavior depends on the refractive-index contrast between the metasurface material—in this case hydrogenated amorphous silicon, a dielectric—and the surrounding medium, typically air. Air's refractive index is not easily changed; a liquid crystal layer with a tunable index solves that problem and makes the static device dynamically controllable.

The TMOS team had previously pioneered magnetic-field control of liquid-crystal-infiltrated metasurfaces, but the goal was always to eliminate electromagnetic actuation in favor of purely optical control.

The missing piece came from Dr. Andrey Iljin, a dye specialist at the University of Münster in Germany, during a visit to the group. Iljin helped the team dope the liquid crystal with methyl red, a large photoresponsive dye molecule. Under green light at 532 nm, methyl red switches between straight and bent forms depending on the light's polarization. That molecular shape change reorients the surrounding nematic liquid crystals—long, skinny molecules—and shifts their polarization response at the metasurface's operating wavelength in the infrared.

Seconds, not heat cycles

The experimental metasurface consists of elliptical silicon nanocylinders engineered to sustain resonances through bound states in the continuum (BIC), supporting both electric and magnetic dipole resonances. The infiltrating liquid crystal carried a 1% concentration of methyl red dye.

By rotating the polarization of the green control beam, the researchers changed the relative strength of the two infrared dipole resonances. Increasing the green light's intensity strengthened the effect. When the beam went dark, the response relaxed back to its initial state in under a second—a substantial improvement over earlier liquid-crystal systems, which required heating to reset their molecular orientation, a far slower process.

"We were happy with this fast response in our initial experiments, but I think if we optimize the concentration or try other dyes we can further improve this control," Izdebskaya said.

Toward pixel-scale photonics

Izdebskaya and colleagues in the group led by Professor Ilya Shadrivov now aim to cut the response time further and shrink the controlled metasurfaces to pixel-sized elements. The potential device scope is broad.

"It's important because this approach can be useful for many different devices—metalenses, imaging systems and holography," Izdebskaya said. "In the future these very small nanoscale components could replace bulky devices such as lenses in smartphones and cameras."

If the team can push response times down and pattern these dye-doped elements into arrays, electrode-free metasurfaces would compete directly with conventional actuated optics in switching speed and form factor—while holographic video remains the most demanding application on the roadmap.