Giant enhancement of plasmonic response and epsilon-near-zero signature in refractory transition metals (Ta, W, and Mo) deposited at high-temperature

Abstract

In recent years, the plasmonic materials with high optical loss ushered many innovations, such as loss-induced heating, plasmon-induced hot carrier, and electro-thermo-plasmonic nanotweezers. Similarly, epsilon-near-zero (ENZ) materials that exhibit near-zero real-components of the dielectric permittivity (ε1) with high loss are promising for efficient absorbers, hot-electron generation, photo-catalysis, local heating, nanoparticle trapping, etc. Traditionally, the plasmonic and ENZ materials based on noble metals such as Au and Ag are not stable at high-temperatures, are CMOS incompatible, and exhibit ENZ behavior in a narrow wavelength range, whereas though transition metal nitrides such as TiN and ZrN exhibit ENZ in the visible spectrum, their spectral width is rather small. Therefore, ENZ materials that are broadband in nature, refractory, oxidation resistant, and CMOS compatible should exhibit better device performance and are highly desired. In this letter, we demonstrate epitaxial, highly crystalline refractory transition metals (RTMs) such as Ta, W, and Mo deposited at high-temperatures (700 °C) in ultra-high vacuum (∼2 × 10−9 Torr), which exhibit broadband ENZ characteristics with relatively flat ε1 in the visible-to-near-IR spectral ranges (200 nm–1000 nm). Phase-pure high crystalline-quality and smooth surfaces result in more metallicity, which leads to a large negative ε1 in long-wavelength (1000 nm–2500 nm) ranges that are comparable to the ε1 of alternative plasmonic materials such as TiN and ZrN. Plasmonic and ENZ characteristics are found to be robust at high-temperatures (∼700 °C) and result in enhanced optical absorption in the RTM metasurface that would be useful for thermophotonic energy conversion, nonlinear optics, and nonreciprocal optical devices.