Dilute noble metal films for infrared optics and plasmonics [Invited]

The infrared optical response of noble metals is traditionally considered perfect electrical conductor (PEC)-like due to the noble metals’ exceptionally large electron concentrations, and thus large (and negative) real permittivity. While PEC-like behavior is ideal for a broad range of applications, for instance mirrors, gratings, and wavelength- (and macro-) scale resonators and antennas, the utility of noble metals for nanoscale (sub-diffraction-limit) physics at long wavelengths is limited. However, in ultra-low volume (dilute) metal films, such as those with nanometer-scale thicknesses or lithographic dilution (subwavelength perforation), the thin films’ sheet conductivity is massively reduced, enabling light to penetrate and interact with the films much more efficiently. This avails the infrared of a host of opportunities for noble-metal-based plasmonics, with the potential for nanoscale (deep subwavelength) confinement and strong light-matter interaction, otherwise prohibited with noble metals in this wavelength range. In this perspective, we review the recent advances in dilute metal films for near- and mid-infrared photonics and plasmonics, and discuss the advantageous properties of these optical thin films for potential applications in sensors, detectors, sources, and nonlinear and quantum optics.