A Space-Time Knife-Edge in Epsilon-Near-Zero Films for Ultrafast Pulse Characterization

Epsilon-near-zero (ENZ) materials have shown strong refractive nonlinearities that can be fast in an absolute sense. While continuing to advance fundamental science, such as time varying interactions, the community is still searching for an application that can effectively make use of the strong index modulation offered. Here, the effect of strong space-time index modulation in ENZ materials is combined with the beam deflection technique to introduce a new approach to optical pulse characterization that is termed a space-time knife edge. It is shown that in this approach, temporal and spatial information of a Gaussian beam can be extracted with only two time resolved measurements. The approach achieves this without phase-matching requirements (<1 µm thick) and can achieve a high signal to noise ratio by combining the system with lock-in detection, facilitating the measurement of weak refractive index changes (Δn $\approx$ 10⁻⁵) for low intensity beams. Thus, the space-time knife edge can offer a new avenue for ultrafast light measurement and demonstrates a use case of ENZ materials. In support of this, temporal dynamics for refractive index changes in non-colinear experiments opening avenues are outlined for better theoretical understanding of both the spatial and temporal dynamics of emerging ENZ films.