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

TL;DR

This paper reveals that frequency shifts of light in epsilon-near-zero ITO layers are influenced by both bulk temporal refraction and boundary effects, allowing for angle-dependent control of light modulation.

Contribution

It demonstrates that boundary effects significantly contribute to frequency shifts in ENZ materials, offering new ways to tailor light-matter interactions.

Findings

Boundary effects can oppose bulk frequency shifts.

Frequency shift can be controlled by the angle of incidence.

Both bulk and boundary effects influence light in ENZ layers.

Abstract

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.