Spatiotemporal refraction of light in an epsilon-near-zero ITO layer

When light travels through a medium in which the refractive index is rapidly changing with time, the light will undergo a shift in its frequency. Large frequency shifting effects have recently been reported for transparent conductive oxides. These observations have been interpreted as emerging from temporal changes to the propagation phase in a bulk medium resulting from temporal variations in the refractive index, an effect referred to as temporal refraction. Here, we show that the frequency shift in an epsilon-near-zero (ENZ) layer made of indium tin oxide (ITO) originates not only from this bulk response, but includes a significant effect resulting from temporal changes to the spatial boundary conditions. This boundary effect can lead to a dominant opposing shift to the bulk effect for certain angles. Hence, this process gives rise to a frequency shift that can be tailored through the angle of incidence, decoupling the amplitude and phase modulation.