Wide tuning of epsilon-near-zero plasmon resonance in pulsed laser deposited ITO thin films

Abstract

Oxygen vacancies in indium tin oxide (ITO) thin films provide a direct route to effectively tune the free electron density and thereby, controlling the epsilon-near-zero (ENZ) cut-off wavelength, the wavelength at which the real part of permittivity crosses zero of the permittivity axis. In this report, oxygen vacancies in pulsed laser deposited ITO thin films are systematically tuned using different background gases (O2, N2, Ar, and He). ENZ cut-offs are observed for the films deposited under He and Ar gases. In contrast, no such cut-offs are observed in the case of other two gases. An ITO thin film deposited under He gas exhibits deeper resonance signal than the one deposited under Ar gas. As expected, no such dip in the resonance spectra is observed for the films deposited under O2 and N2 gases. This observation is directly correlated to the change in the number of oxygen vacancies under different ambient gases. A modified transfer matrix method which incorporates surface roughness as an effective medium layer is developed to describe the experimentally observed resonance spectra numerically. Angular invariancy of ENZ plasmon resonance and the difference in absorption values for ITO films deposited under different gases is understood in terms of local field intensity enhancement factor. The study presented here will certainly be very useful in understanding the ENZ plasmon resonance phenomena as a whole. Additionally, ITO films deposited under an inert gas environment could be excellent material platforms for realizing several exotic ENZ applications.