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.

中文翻译：

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具有单周期调制的时间折射光学器件

我们提出了由短至单个光学周期的时间界面引起的光学时间折射的实验研究。具体来说，我们研究了探测脉冲通过样品的传播，该样品经历了由强调制器脉冲引起的大折射率变化。在这些系统中，突然增加折射率会导致时间折射，其中在介质中传播的所有波的光谱都发生红移，随后当折射率松弛回到其原始值时发生蓝移。我们在单周期状态下观察到这些现象。此外，通过将调制器的时间宽度缩短至 ~5-6 fs，我们观察到与时间折射相关的红移上升时间按比例缩短。实验是在充当 epsilon-near-zero 材料的透明导电氧化物中进行的。这些观察结果对发生在如此超短时间内的基础物理学提出了多个问题，并为在不久的将来用折射率的周期性超快变化产生的光子时间晶体进行实验开辟了道路。