Direct Refractive Index Retrieval with Spectral Interferometry in Time-Refraction Experimen

Time-refraction is the phenomena where a wave's frequency is altered in a time-varying medium. Transparent conducting oxides have become the leading platform for time-refraction experiments in optics. Recently, Lustig, et al. Nanophotonics 2023, 12 (12), 2221–2230, found a new ultrafast relaxation mechanism at 10-20 fs along with the more typical relaxation at ~0.5-1 ps. Thus, a direct measurement of the refractive index is crucial. Various indirect methods, like comparing with simulations and phase retrieval algorithms, have been used to infer the refractive index. More direct approaches of measuring modulated reflection and transmission and extracting the Fresnel coefficients can suffer from surface terms affecting results. We employed self-reference spectral interferometry to directly measure temporal phase shifts. In our modified pump-probe setup, the probe beam (100 fs, λ=1200 nm) is split, delayed, and recombined, to intersect with the pump beam (<40 fs, λ=800 nm) at a 240 nm thick ITO film. The delay between pump and probe is varied. The temporal evolution of the refractive index is directly recovered from a frequency domain hologram showing a slow increase from the pump-laser pedestal, followed by a ~40 fs rise and relaxes over ~150 fs, consistent with the observed spectral redshifts and weaker blueshifts.