Time-refraction optics with single cycle modulation

TLDR

In optical systems, abrupt refractive index changes cause time‑refraction, red‑shifting the spectrum and later blue‑shifting it as the index returns, raising questions about ultrashort‑time physics and enabling future photonic time‑crystal experiments. The study experimentally investigates optical time‑refraction induced by single‑cycle time‑interfaces. The experiment probes a pulse propagating through epsilon‑near‑zero transparent conducting oxides while a strong modulator pulse induces a large refractive index change. The authors observe time‑refraction and its subsequent blue‑shift in the single‑cycle regime, and find that reducing the modulator width to 5‑6 fs shortens the red‑shift rise time proportionally.

Abstract

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 ∼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.