Opening a new chapter in optics! Scientists succeed in maintaining photon time crystals

A new study published in the journal Nanophotonics shows that the refractive index-the ratio of the velocity of electromagnetic radiation in a medium to that in a vacuum-can be modulated fast enough to produce photonic time crystals (PTCs) in the near-visible part of the spectrum.

The authors of the study believe that the ability to sustain PTCs in the optical domain could have far-reaching implications for light science, enabling truly disruptive applications in the future.

Time-refraction optics with single cycle modulation

PTCs are materials in which the refractive index rises and falls rapidly in time, the temporal equivalent of photonic crystals, in which the refractive index oscillates periodically in space, leading to, for example, the iridescent colors of precious minerals and insect wings.

PTCs are only stable if the refractive index can be aligned with a single cycle of electromagnetic waves of the relevant frequency. Not surprisingly, therefore, PTCs to date have been observed at the lowest frequency end of the electromagnetic spectrum: radio waves.

In this new study, lead author Mordechai Segev of the Technion-Israel Institute of Technology, along with collaborators Vladimir Shalaev and Alexndra Boltasseva of Purdue University in the United States and their team, sent very short (5-6 femtoseconds) pulses of laser light at a wavelength of 800 nanometers through a transparent conducting oxide material.

This caused a rapid change in the refractive index; and this was explored using a probe laser beam with a slightly longer (near-infrared) wavelength. As the refractive index of the material relaxed to its normal value, the probe beam was rapidly redshifted (i.e., its wavelength increased), and then a blueshift (a decrease in wavelength) occurred.

The time required for these refractive index changes was negligible: less than 10 femtoseconds, thus forming the single period required for a stable PTC.

Experimental setup for measuring temporal refraction in a single-cycle system.

Transmission spectra of the 44 fs probe pulse through the ITO sample are plotted for modulator pulses of different time widths. For each panel, the zero delay time was arbitrarily set to a redshift of 20% of the maximum redshift value in each experiment.

Segev said, "Electrons excited to high energies in crystals typically take more than ten times as long to relax to their ground state, and many researchers believe that the ultrafast relaxation we observe here is impossible; we also do not yet understand exactly how it occurs."

The measurements show that in both ITO and AZO, the refractive index can be raised to a much larger exponent within 5-10 fs: 0.5 for ITO and 0.15 for AZO.

And, surprisingly, ITO can also exhibit extremely fast refractive index relaxation times: 10-20 fs. This finding, in particular the extremely fast relaxation of the refractive index change in ITO, as well as the process of raising the refractive index and the subsequent relaxation to the original value, paves the way for the observation of photonic time-crystals at the frequency of the light, as well as many other phenomena involving time boundaries.

Further, co-author Shalaev says that the ability to maintain PTCs in the optical domain will "open a new chapter in light science, enabling truly disruptive applications." However, we know very little about these possible applications - just as physicists in the 1960s knew about the possible applications of lasers.

Reference link:

[1]https://phys.org/news/2023-07-photonic-crystals-door-optics.html

[2]https://www.degruyter.com/document/doi/10.1515/nanoph-2023-0126/html