  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.
