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Metamaterial preserves optical properties while conforming to irregular surfaces

By Kendra Redmond August 1, 2019
Flexible ENZ
Fabrication process. (a) A sacrificial layer (Omnicoat) and a supporting layer of SU-8 (an epoxy-based polymer) are deposited on a rigid glass or Si substrate. (b,c) The first metal/dielectric (Ag/SU-8) bilayer is deposited. (d) The deposition of the bilayer is repeated to yield an epsilon-near-zero (ENZ) metamaterial of desired thickness. (e) The ENZ metamaterial is released and (f) transferred onto an irregular object. Credit: Li et al., APL Photonics, 4, 056107, (2019).

Researchers from the UK and Italy have fabricated a thin, flexible metamaterial that can be removed from its substrate and applied to another surface in order to change its photonic features. As reported in APL Photonics, the low-loss metamaterial can conform to irregular surfaces and maintain its optical properties even after thousands of bending cycles. This proof-of-concept work suggests that the photonic properties of surfaces can be engineered through prefabricated coatings.

Photonic surfaces have conventionally been patterned by lithography, a technique that requires flat interfaces. However, University of St Andrews researcher Andrea Di Falco envisions a world in which photonic “skins” can be fabricated to any specification that conform to any surface. “I can now take a photonic skin and apply it on top of any target that is too difficult to structure,” he says, “I can make something photonic out of anything.”

To this end, Di Falco led a research team in developing the new flexible metamaterial. The photonic properties of the material are determined not by lithography, but by engineering the material so that its dielectric permittivity approaches zero in response to visible light. This is known as vanishing effective permittivity or an epsilon-near-zero (ENZ) response.

Several materials naturally exhibit ENZ responses to specific frequency bands. In the last 10 years, researchers have become especially interested in designing materials with tailored ENZ responses. If the optical losses of the material are small, the refractive index of an ENZ material can also approach zero. Exploiting this could yield exciting possibilities in areas such as optical imaging beyond the diffraction limit and wavefront engineering.

ENZ metamaterials have been fabricated on flat, rigid materials using techniques such as spin coating and electron-beam evaporation or sputtering. However, flexible ENZ metamaterials have remained elusive until now.

In response to the work by Di Falco’s team, Marcello Ferrera, who leads the Advanced Structured Nanophotonics Lab at Heriot-Watt University, says, “The limited number of [scientific] articles dealing with flexible epsilon-near-zero (ENZ) materials is primarily due to fabrication complexity rather than lack of interest.”

To create the flexible ENZ material, Di Falco’s team first covered a glass substrate in a sacrificial layer of Omnicoat. Then the researchers fabricated an ENZ metamaterial on top of the sacrificial layer. The metamaterial consisted of alternating layers of the epoxy-based polymer SU-8 (85 nm thick) and germanium-coated silver (15 nm thick). The sacrificial layer was then removed to reveal a stand-alone flexible ENZ material.

Using a broadband spectral analysis of the metamaterial, the researchers characterized the optical properties of the ENZ metamaterial. Comparisons between their experimental measurements and theoretical models run by collaborators at the Institute for Superconductors, Oxides and Other Innovative Materials and Devices (SPIN) at the National Research Council in Italy showed that the permittivity values converged for samples with five bilayers.

After being released from the substrate, the five-bilayer sample showed promising flexibility, resilience, and conformity. It sustained 10,000 bending cycles from a motorized translational stage without measurable changes in transmission, reflection, and effective permittivity. In addition, scanning electron microscopy images revealed that the researchers were able to successfully transfer the membrane onto a silicon substrate with 6 μm features.

Looking forward, Di Falco says the challenge is to reduce optical losses in the material. “The refractive index isn’t zero yet,” he says. “The phase at which light propagates depends on the losses of the material itself. . . .The challenge now is to devise a scheme where the effect of the losses can be minimized or reduced in the first place.”

According to Ferrera, this work is likely to stimulate new avenues in experimental research. “Numerous photonic applications in the domain of transformation optics and hyperbolic metamaterials could benefit from the technical information contained in the manuscript,” he says. Ferrera expects the main challenges to be evaluating the boundaries within which the theoretical model is robust and consistent and moving toward all-dielectric devices while maintaining flexibility.

Read the article in APL Photonics.