Two exciting fields in modern optics are plasmonics and the optics of low permittivity materials, also known as epsilon-near-zero (ENZ) materials. On the one hand, plasmonics allows us to confine and manipulate light at deep subwavelength scales, while ENZ materials can exhibit truly remarkable properties, such as a phenomenal nonlinear optical response. Indeed, the prospects of these fields combined offer an exciting road ahead for research and development of novel nanophotonic devices. Of key importance for these future developments is the capacity to control the optical properties of plasmonic and ENZ materials — in particular, materials such as transparent conductive oxides, which have emerged as a novel CMOS-compatible material platform for nanophotonics. In this work, Wang and co-authors present a material fabrication approach for indium tin oxide (ITO) films, a transparent conductive oxide, which greatly extends the ENZ and plasmonic spectral region of the material from the near-infrared to the mid-infrared spectrum (from 1150 nm to 4270 nm). Their work includes a systematic study of various post-deposition annealing parameters and their effects on the optical properties of the material. In particular, they find that annealing in an oxygen atmosphere enables a large tunability of the material’s ENZ wavelength as well as a reduction of the material’s optical absorption. By fabricating arrays of ITO micro-disks, the authors also demonstrated that the plasmonic properties of such structures can be widely tuned in the mid-infrared spectral region, covering wavelengths ranging from 5 μm to 10 μm. The results by Wang and coworkers provide a simple and cost-effective method to control the ENZ and plasmonic properties of transparent conductive oxides like ITO, enabling the fabrication of novel mid-infrared nanophotonic devices.
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