Manipulation of light propagation in non-traditional ways is possible today in nanophotonics due to plasmonic metasurfaces, a class of planar devices based on subwavelength metallic elements arranged into periodic arrays. Typically, the spacing between these subwavelength units and their dimensions is much smaller than the wavelength of light with which they interact. Researchers at Northwestern University and the US Air Force Research Laboratory have now developed a method for designing and prototyping titanium nitride (TiN) metasurfaces, opening new opportunities for imaging applications. This can provide the basis for compact optical components.

When it comes to integrated optoelectronics and durable miniature photonic devices for high-temperature applications, the materials for fabricating metasurfaces need to have high mechanical and thermal stability. TiN is one such unconventional plasmonic material which, in contrast to the traditionally used gold and silver, offers potential compatibility with current silicon-based processing techniques used in semiconductor manufacturing.

As Alexandra Boltasseva of Purdue University, an expert in plasmonic materials who was not involved in the study, puts it, “TiN and similar durable metal nitrides [represent] the holy grail of practical plasmonics, promising to bring breakthrough advances to high-temperature optical technologies.” But these advances are hindered, mostly because existing patterning techniques for TiN films cannot realize the required nanoscale features.

Teri Odom and her team have demonstrated a computational approach and a prototyping technique to realize TiN metasurfaces based on nanohole lattices, for wavelengths in the near-infrared range. In parallel, the team introduced a masked-etching method to produce the TiN lattices, with the patterning process comprising a combination of focused-ion beam (FIB) milling and a wet-etching technique.

By using an object-oriented lattice evolution algorithm (OLEA), the researchers designed metasurfaces with different nanohole sizes able to produce complex optical patterns in three dimensions. Evolutionary algorithms mimic natural processes that govern how nature evolves and optimizes biological species to maximize survival of the fittest. An optimization process based on an evolutionary algorithm can provide the best design, without examining the entire set of solutions, the size of which can be overwhelming. OLEA simulations are also more advantageous at dealing with more than one criteria simultaneously.

“What was most surprising to us was the robustness of the properties of the TiN metasurfaces,” Odom says. “The designed light patterns showed negligible changes with up to 30% variation in nanohole sizes and 20% random noise in the nanohole arrangements,” she adds. Since the performance of OLEA-designed metasurfaces can tolerate structural defects, the prototyping process does not require strict control over nanohole size and shape.

For the patterning of the nanohole array, single-crystalline epitaxial TiN films were first prepared on sapphire substrates by reactive magnetron sputtering. A sacrificial 5 nm chromium layer was then deposited on the TiN films by means of thermal evaporation and was patterned with FIB milling. Using wet etching for 5 min at room temperature, the pattern of the chromium etch mask was transferred onto the TiN layer. Finally, the chromium layer was removed.

The researchers succeeded in prototyping an array of 120 lattice lenses in 30 minutes, when, with the same current conditions, direct FIB milling in TiN required over five hours to pattern the same area. Despite wet etching being an isotropic process, the researchers produced lattices with different hole sizes and anisotropic holes by changing the FIB milling patterns and wet etching times.

Ritesh Agarwal, an expert in nanoscale photonics at the University of Pennsylvania, who also was not part of the study, says, “The results of [Odom’s team] extend the type of materials that can be utilized for fabricating metasurfaces, which is important to advance the field with the hope that they can lead to useful and practical devices.”

Boltasseva says that “this work represents an important step toward rapid prototyping and cost-effective fabrication of TiN flat-optics devices for robust, high-intensity stable on-chip optics, harsh-environment sensors and other heat-assisted optical technologies.”

Odom’s team has already planned the next steps. “We are now interested in improving the directionality of the chemical etching process to more finely tune the features in TiN,” Odom says, adding that they also try to further optimize the design algorithm for better efficiency, user-experience, and accuracy. In the long term, the researchers aim to apply the design strategy for novel materials systems in order to achieve reconfigurable metasurfaces using phase-change materials. 

Read the abstract in ACS Photonics.