New Switchable Metamaterials Could Lead to Advanced Optical Devices

A cross-sectional scanning electron microscopy images of a 750nm period grating fabricated by focused ion beam milling in a 300nm thick amorphous germanium antimony telluride film on silica. (CREDIT: Karvounis/Gholipour/MacDonald/Zheludev, Optoelectronics Research Centre, University of Southampton)

Metamaterials play a bigger role in invisibility cloaks than magic. The materials are human-engineered and they have properties that are not seen in nature, enabling them to bend and manipulate light in unusual ways.

For example, some of these materials are capable of channeling light around an object so that at a certain wavelength it appears invisible. Metamaterials are also used in various applications, including faster, smaller, and highly energy efficient optics, sensors, light detectors, light sources, and telecommunications devices.

Recently, a team of researchers designed a new kind of metamaterial with properties that can be modified by flicking a switch. In their proof-of-concept experiment, they utilized germanium antimony telluride (GST) - a phase-change material used in DVDs and CDs - to create an enhanced switchable metasurface that can transmit or block specific wavelengths of light using light pulses.

The new metamaterial is described by the team in this week’s issue of Applied Physics Letters, from AIP Publishing. The paper also explains how the metamaterial’s ability to switch properties can be applied in a variety of advanced optical devices.

Technologies based upon the control and manipulation of light are all around us and of fundamental importance to modern society. Metamaterials are part of the process of finding new ways to use light and do new things with it -- they are an enabling technology platform for 21st century optics.

Kevin MacDonald, Researcher, University of Southampton

When the materials optical properties are dynamically controlled the light beams features can be modulated, choosen, or switched, such as phase, intensity, color and direction. This ability is important for several current and potential devices.

Generally, switchable metamaterials are not new. MacDonald and several others have created this type of material before by integrating metallic metamaterials with active media such as GST, which can act in response to external stimuli such as light heat, or an electric field.

The metal component in these hybrid materials is structurally engineered at the nanometer scale to offer the preferred optical properties. Adding the active medium provides a way to switch or tune those properties. The problem is that metals have a tendency to absorb light at infrared and visible wavelengths, rendering them inappropriate for several optical device applications.

Melting points in nanostructured metals are also suppressed, making them prone to damage from laser beams. A distinctive metal is gold, which is not compatible with the CMOS technology that is frequently used in making integrated devices today.

In the new research, MacDonald and his colleagues at Southampton’s Optoelectronics Research Centre & Centre for Photonic Metamaterials have created a switchable metamaterial without using any metal.

What we've done now is structure the phase-change material itself. We have created what is known as an all-dielectric metamaterial, with the added benefit of GST’s nonvolatile phase-switching behavior.

Kevin MacDonald, Researcher, University of Southampton

Laser light pulses can modify the GST’s structure between a random, amorphous one and a crystalline one. For GST, this characteristic is nonvolatile, which means it will remain in one state until another pulse switches it back. In rewritable DVDs and CDs, this binary laser-induced switching is the foundation for data storage.

The team developed metamaterial grating patterns in an amorphous GST film measuring only 300 nm in thickness, with lines 750 to 950 nm apart. The line spacing allows the surfaces to selectively block the light transmission at near-infrared wavelengths – from 1300 to 1600 nm. However, when a green laser transforms the surfaces into a crystalline state, they turn transparent at these wavelengths.

Currently, the research team is involved in the creation of metamaterials capable of switching back and forth over several cycles. They also plan to develop increasingly complex structures to offer highly advanced optical functions. For instance, this method could be applied to build switchable ultra-thin metasurface lenses and other flat, optical parts.

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