Science 352, 795–797 (2016)

Any material with free charges described by the Drude model, including metals or sufficiently doped semiconductors, can exhibit permittivity near zero at the transition frequency between metal and dielectric behaviour. Applications for the epsilon-near-zero regime may include subwavelength waveguiding, enhanced third-harmonic generation and ultrafast optical switching. Now a team of researchers from Canada, Mexico and the US have observed an optical nonlinear response from indium tin oxide at epsilon-near-zero frequencies that is two orders of magnitude larger than for As2Se3 chalcogenide glass. The team achieved a 170% variation in the real part of the refractive index, due to a change in permittivity from 0 + 0.352i to 1.22 + 0.61i, and propose that the strong response is related to laser-induced electron 'heating' and modification of the energy distribution of the conduction band. In their experiment, the authors used commercially available indium tin oxide with an epsilon-near-zero wavelength of 1,240 nm. Near this wavelength they find a nonlinear index coefficient ∼43 times higher than at 970 nm. For transverse magnetic-polarized light and optimal angles of incidence, the nonlinear coefficient is even larger. The material recovers from the pulses on a timescale of just 360 fs, which the authors point out may enable modulation up to 1.5 THz.