Faster Than Can Be Explained – Photon Time Crystals Could Revolutionize Optics

Researchers have developed photonic time crystals in the visible spectrum, which could revolutionize the applications of light science. This breakthrough extends the previously recognized range of PTCs, which were only seen in radio waves.

Recent studies show oscillations in the refractive index that are faster than can be explained by current theories.

A study recently published in the journal Nanophotonics reveals that by rapidly modulating the refractive index – which is the ratio of the speed of electromagnetic radiation in a medium compared to its speed in a vacuum – it is possible to produce photonic time crystals (PTCs) in the near-visible part of the spectrum.

The authors of the study suggest that the ability to store PTCs in the optical domain could have a profound impact on the science of light, making it possible to disrupt applications in the future.

PTCs, devices in which the refractive index rises and falls rapidly in time, is equivalent to the time of photonic crystals in which the refractive index oscillates periodically in the area that causes, for example, the iridescence of precious minerals and insect wings.

Time-refraction test setup for single cycle control. Credit: Eran Lustig et al.

A PTC is stable only if the refractive index can be made to rise and fall in accordance with one cycle of the electromagnetic wave of the relevant electromagnetic frequency so, surprisingly, PTCs have so far been observed at the lowest-frequency end of the electromagnetic spectrum: by radio waves.

In this new study, lead author Mordechai Segev of the Technion-Israel Institute of Technology, Haifa, Israel, and collaborators Vladimir Shalaev and Alexandra Boltasseva from Purdue University, Indiana, USA, and their teams, sent very short pulses (5-6 femtosecond) . of laser light at a length of 800 nanometers by using a transparent conductive oxide material.

This caused a sudden change in the refractive index which was studied using a probe laser beam at a slightly longer (near infrared) wavelength. The probe beam was quickly redshifted (that is, its wavelength increased) and then turned blue (wavelength decreased) as the refractive index of the material returned to its normal value.

Spectrograms of the transmission of 44 fs probe pulses that passed through the ITO sample, in pulse modulations of different temporal widths. Credit: Eran Lustig et al.

The time taken for one of these refractive index changes was minuscule – less than 10 femtoseconds – and, therefore, within one cycle necessary to create a stable PTC.

“Electrons that enjoy high energy in crystals often need more than ten times to relax back to their ground states, and many researchers think that the rapid relaxation we see here is impossible,” Segev said. “Right now we don’t really understand how it happened.”

Co-author Shalaev further suggests that the ability to store PTCs in the optical domain, as shown here, “will open a new chapter in the science of light and enable truly disruptive applications”. However, we know as little of what these might be as physicists in the 1960s knew about the possible uses of lasers.

Reference: “Time-refraction optics and single-cycle modulation” by Eran Lustig, Ohad Segal, Soham Saha, Eliyahu Bordo, Sarah N. Chowdhury, Yonatan Sharabi, Avner Fleischer, Alexandra Boltasseva, Oren Cohen, Vladimir M. Shalaev and Mordechai Segev, 31 May 2023, Nanophotonics.
DOI: 10.1515/nanoph-2023-0126

The research was funded by the German Research Foundation.