artist sketch of moth eye

A moth’s eye provided inspiration for a 3-D-printed, angle-insensitive narrowband metamaterial absorber “lens” fabricated on a curved substrate. The lens can absorb specific electromagnetic frequencies from any direction. [Image: Hojat Nejad / Tufts University, Nano Lab]

Engineers from Tufts University, USA, have created metamaterial-embedded devices with interesting optical and electrical properties by combining 3-D printing with metal-coating and wet-etching techniques. The devices, which the researchers refer to as metamaterial embedded geometrical optics (MEGO), include a bio-inspired arrays of mushroom structures and a moth-eye-type lens, as well as an optical parabolic mirror with both reflecting and filtering functionalities (Microsyst. Nanoeng., doi: 10.1038/s41378-019-0053-6).

The Tufts team, led by Sameer Sonkusale, says the MEGO fabrication technique can create a variety of devices that absorb, enhance, reflect or bend light or electromagnetic waves in ways that would be difficult to achieve with conventional fabrication methods. Devices created using this simplified manufacturing process, according to the team, could be used in a variety of applications, including medical sensors, next-generation cloaking devices, and smaller, portable versions of common optical instruments like spectrometers.  

Simple approach, multiple devices

Conventional fabrication techniques for geometrically complex 3-D printed metamaterials require multiple steps and sometimes additional components—like using photolithography on a curved substrate, or printing metamaterials on a flexible “photomask” and draping it over a 3-D printed device. The Tufts engineers have simplified the approach by merging stereolithography (SLA), a type of 3-D printing that uses photocurable resins, with metal coating and wet etching to fabricate devices with unique optical and electromagnetic functionalities. Here are some that the team has already tested out:

A mushroom array: The array consists of metal disk resonators atop silver- or gold-coated resin “stalks.” It functions as a gigahertz (GHz) absorber or reflector, tunable to a range of frequencies depending on the spacing and composition of the millimeter-scale mushrooms.

This MEGO device could be used in medical sensors, telecommunications antennae or imaging applications. The engineers experimentally validated the mushroom-array design by extracting transmission spectra for both the gold and silver-coated arrays. The obtained resonant frequencies, at 248 GHz for gold devices and 222 GHz for silver, were consistent with the team’s simulation results.

Moth-eye lens: This MEGO device is an omni-directional hemispherical lens modeled after a moth’s eye. The lens functions as an antenna, absorbing and detecting electromagnetic signals from any direction for specified wavelengths. The team composed the lens by molding a mushroom array into a half-hemisphere shape and coating it with silver. The engineers say that, to their knowledge, this is the first-ever fabrication of an angle-insensitive narrowband metamaterial absorber fabricated on a curved substrate.

The moth-eye lens could be used to improve photodetector responsivity or for next-generation cloaking devices. The engineers validated the device’s performance by collecting transmission spectra for angles-of-incidence from −45 degrees to +45 degrees. The results showed only slight differences in spectra for each angle, which is consistent with computer simulations.

Optical parabolic mirror with frequency-selective transmissive filter: This instrument combines reflecting and filtering functions, which are traditionally performed by separate optical and metamaterial components, into a single MEGO device. The mirror/filter is composed of a slightly curved resin disk coated in a dielectric layer of Parylene-C. Layers of chromium and gold are added after laying down a stencil mask for patterning metamaterials on the mirror’s surface.

The mirror can selectively absorb and reflect a collimated beam at a single focal point. This MEGO device could be used to reduce the size of optical measuring devices to the point where they are portable enough for use in field studies.  The engineers’ validation studies showed that the mirror/filter’s reflection spectrum produced resonant frequencies around 91 GHz, which matched their simulation results.

Based on these results, the Tufts engineers believe their hybrid fabrication approach for MEGO devices could “bring a new toolkit to microwave and optical designers using conventional 3-D printers,” as well as simplify the manufacturability of such devices.