Despite the recent incorporation of metasurfaces into a wide range of dielectric applications (some of which include axicon lenses, sub-diffraction focusing devices, beam deflectors and converters, holograms, and antireflection coatings), they are often limited to a specified bandwidth of operation. To further expand on the potential use of metasurfaces, it is imperative that their functionality magnifies their activity and reconfigurability.
To this end, a 2019 Nanophotonics paper examined the role of thermal tunability in metasurfaces, and how knowledge on these properties could expand the tunability and reconfigurable functions of metadevices.
Thermo-Optic (TO) Tunability and Reconfigurable Metadevices
One of the main challenges that researchers have faced in attempting to reconfigure metasurfaces is the ability to obtain large and continuous modulations to the optical properties of these planar optical structures; within both subwavelength and low-Q meta-atom resonators. Several different approaches have looked into the potential of several techniques, including ultrafast free-carrier injection, coupling active tuning to liquid crystals, and MEMS, to confront this challenge.
Despite these attempts, no viable solution to fully reconfigure metasurface devices has been achieved. The current study investigated the thermal-optic (TO) effects of high-index silicon (Si) and germanium (Ge) semiconductor resonators over a large temperature range, in an effort to elucidate any available reconfigurable properties.
Both the Si and Ge resonators used in this study were spherical and fabricated by femtosecond laser ablation. To characterize the optical properties of both metasurface devices, single particle spectroscopy was conducted at various temperatures, using a Fourier-transform infrared spectroscopy (FTIR) device that was coupled to an infrared microscope.
The thermal tuning capabilities of both the Si and Ge single resonators and their metasurfaces were examined over a temperature range of 80 to 873 Kelvin (K).
The University of California researchers found that by modifying the traditional TO effect, a temperature-dependent resonance frequency shift occurred. Furthermore, at both low and intermediate temperatures, the researchers discovered that all resonances exhibited a red-shift, which followed a normal positive thermo-optic coefficient.
When exposed to higher temperatures and longer wavelengths, the thermal excitation of the free carriers (FCs) were found to exhibit significant bandgap shrinkage, which ultimately caused the TOC value to become negative and yield a dn/dT value that was less than 0. This transition is believed to result from a continuous change in the resonance shift from red-shift at the low and moderate temperature to a blue-shift at the higher temperatures.
By discovering the significant TOC that exists at short near-infrared (NIR) wavelengths within the Si resonators, the researchers of this study identified both amplitude modulators and tunable metafilters that exist at the Si metasurfaces. This discovery highlights the thermally reconfigurable functionality that exists within Si metasurfaces.
As a result, the research discussed here provides an opportunity for semiconductor engineers to continue to investigate the thermally tunable properties of semiconductor metasurfaces in order to one day develop high-Q reconfigurable metadevices.
- Lewi, T., Butakov, N. A., & Schuller, J. A. (2019). Thermal tuning capabilities of semiconductor metasurface resonators. Nanophotonics 8(2); 331-338. DOI: 10.1515/nanoph-2018-0178.
The research discussed in this article was conducted by researchers from the Department of Electrical and Computer Engineering at the University of California Santa Barbara. This work was also supported by the Air Force Office of Scientific Research and the University of California Office of the President Multi-campus Research Programs and Initiatives.