Ruthenium Oxide: A Near 0.8 µm Epsilon-Near-Zero Medium for Multipurpose Nonlinear Photonics

Abstract

The intriguing optical response associated with the vanishingly small dielectric primitivity in epsilon-near-zero (ENZ) materials has offered unprecedented opportunities for photonics. Due to the high-limit of doping concentration, current homogeneous ENZ materials based on heavily doped conductive oxide including ITO usually exhibit a zero-permittivity wavelength (λ₀) longer than 1200 nm. Here, this is identified with combined theoretical and experimental investigations that stoichiometric rutile-type RuO₂ (r-RuO₂) exhibits the shortest λ₀ of near 800 nm among conductive oxides, benefiting from the high free electron concentration with characteristic Drude-type response. The ENZ effect is manifested by the strong field enhancement and the large nonlinear optical (NLO) response in colloidal processed r-RuO₂ nanoparticles (NPs) and thin films. By ultrafast transient spectroscopy, the strong NLO response in r-RuO₂ NPs is revealed to be the result of rapid thermalization/cooling of free electrons in the strong hybridized Ru-4d/O-2p orbitals. The ultrafast optical nonlinearity is exploited further for the development of multi-purpose all-optical switches that enable stable pulse laser generation in four mode-locked and Q-switched fiber lasers. Our results underscore the potential of stoichiometric metallic metal oxides as alternative ENZ materials for nonlinear photonics applications in the visible and NIR regions.