Epsilon-near-zero response in indium tin oxide thin films. Octave span tuning and IR plasmonics

Epsilon near zero (ENZ) materials exhibit strongly confined optical modes and plasmonic response around and beyond the ENZ wavelength (λENZ). In order to exploit the novel properties of ENZ materials for real-world applications, it is important to develop material platforms that offer continuous tuning of λENZ. We report octave span, controllable, and reversible tuning of λENZ from 1280 nm to 2900 nm in commercially available thin films of indium tin oxide (ITO), employing a low temperature annealing protocol. Electrical, spectroscopic, and optical measurements establish the physical basis of the observed tunability in free electron density by over an order of magnitude and quantify the real and imaginary components of the refractive index for ITO thin films. Excitation of surface plasmon polaritons (SPPs) in the metallic regime of ITO probes its infrared plasmonic response demonstrating continuous tunability of SPP frequency and crossover to the tunable ENZ plasmon mode in ultrathin films. Finally, dispersion tuning of optical fiber modes by optical coupling with a tunable λENZ platform is demonstrated by investigating modal interference in a tapered silica fiber in contact with various custom tuned ITO films.