Time-refraction optics with single cycle modulation

We present an experimental study of optical time-refraction caused by time-interfaces as short as a single optical cycle. Specifically, we study the propagation of a probe pulse through a sample undergoing a large refractive index change induced by an intense modulator pulse. In these systems, increasing the refractive index abruptly leads to time-refraction where the spectrum of all the waves propagating in the medium is red-shifted, and subsequently blue-shifted when the refractive index relaxes back to its original value. We observe these phenomena in the single-cycle regime. Moreover, by shortening the temporal width of the modulator to similar to 5-6 fs, we observe that the rise time of the red-shift associated with time-refraction is proportionally shorter. The experiments are carried out in transparent conducting oxides acting as epsilon-near-zero materials. These observations raise multiple questions on the fundamental physics occurring within such ultrashort time frames, and open the way for experimenting with photonic time-crystals, generated by periodic ultrafast changes to the refractive index, in the near future.

Automated summary

We experimentally study optical time-refraction caused by time-interfaces as short as a single optical cycle. By observing the propagation of a probe pulse through a sample with a large refractive index change induced by an intense modulator pulse, we find that increasing the refractive index abruptly leads to red-shifted waves while decreasing it back to the original value causes a subsequent blue-shift. Shortening the temporal width of the modulator pulse leads to a proportionally shorter rise time of the red-shift associated with time-refraction. These experiments are conducted in transparent conducting oxides acting as epsilon-near-zero materials. The findings stimulate questions about the fundamental physics in ultrashort time frames and pave the way for future experiments with photonic time-crystals generated by periodic refractive index changes.