Faster than can be explained: Photonic time crystals could revolutionize optics

A recent study revealed faster refractive index oscillations than can be explained by current theories.

A study recently published in the journal nanophotonics He reveals that by rapidly adjusting the index of refraction—which is the ratio of the velocity of electromagnetic radiation in a medium compared to its velocity in a vacuum—it is possible to produce photonic time crystals (PTCs) in the near-visible part of the spectrum.

The study authors suggest that the ability to preserve PTCs in the visual field could have profound implications for the science of photonics, enabling truly disruptive applications in the future.

PTCs, materials whose refractive index rises and falls rapidly with time, are the temporal equivalent of photonic crystals in which the refractive index oscillates periodically in space causing, for example, the iridescence of precious metals and the wings of insects.

Experimental setup for temporal refractometer in a single-cycle system. Credit: Iran Lustig et al.

PTCs are only stable if the refractive index can be made to rise and fall in line with one cycle of electromagnetic waves at the frequency in question. Therefore, it is not surprising that PTCs have so far been observed at the lower frequency end of the electromagnetic spectrum: with radio waves.

In this new study, lead author Mordechai Segev of the Technion-Israel Institute of Technology, Haifa, Israel, along with collaborators Vladimir Shalev and Alexandra Boltseva of Purdue University, Indiana, USA, and their teams sent extremely short (5-6 femtoseconds) pulses of light Lasers with a wavelength of 800 nm through transparent conductive oxide materials.

This caused a rapid shift in the refractive index that was explored with a probe laser beam at a slightly longer (near-infrared) wavelength. The probe beam was rapidly redshifted (increasing its wavelength) and then blue (wavelength decreasing) as the material’s refractive index fell back to its normal value.

Transmission spectrograms of 44 fs probe pulses passed through an ITO sample, modulating pulses of different time widths. Credit: Iran Lustig et al.

The time taken for each of these refractive index changes was minimal—less than 10 femtoseconds—thus, within the single cycle needed to form a stable PTC.

“High-energy excited electrons in crystals generally need more than ten times as long to relax back to their ground state, and many researchers believe that the ultra-fast relaxation we observe here would be impossible,” Segev said. “We don’t understand exactly how that happens.”

Co-author Shalev also notes that the ability to preserve PTCs in the optical domain, as shown here, will “open a new chapter in the science of photonics and enable truly disruptive applications.” However, we know little about what that might be, since in the 1960s physicists knew about the potential applications of lasers.

Reference: “Temporal Refractive Optics with Single-Cycle Modulation” by Iran Lustig, Ohad Segal, Soham Saha, Eliyahu Bordo, Sarah N. Chowdhury, Yonatan Sharabi, Avner Fleischer, Alexandra Boltseva, Oren Cohen, Vladimir M. Shalev, and Mordechai Segev, May 31, 2023, nanophotonics.
DOI: 10.1515/nanov-2023-0126

The research was funded by the German Research Foundation.