Fig. 1: Concept of time refraction.
From: Broadband frequency translation through time refraction in an epsilon-near-zero material
a A spatial boundary defined by a refractive index change from n1 to n2 leads to a change in the wavevector of a light beam as it passes through the boundary and is described by n1λ1 = n2λ2 (left panel). A refractive index boundary defined in time leads to time-refraction effect of a light beam as it passes through the boundary and is described by n1f1 = n2f2 (right panel). Here f is the frequency of light waves in the medium. b The permittivity of an ITO film used in the experiment. The inset shows the simplified experimental setup and the shaded region shows the spectral range of interest in this work. c Simplified illustration of the temporal index change Δn(t) of ITO excited by a pump pulse. d The frequency of the probe redshifts (blueshifts) if the pump beam lags (leads) the probe. At near-zero delay both redshift and blueshift can occur.
