A recent study reveals refractive index oscillations that are faster than current theories can explain.
A study recently published in the journal Nanophotonics reveals that by rapidly modulating the index of refraction – which is the ratio of the speed of electromagnetic radiation in a medium to its speed in a vacuum – it is possible to produce photonic time crystals (PTCs) in the nearly visible part of the spectrum.
The study authors suggest that the ability to maintain PTCs in the optical domain could have profound implications for the science of light, enabling truly breakthrough applications in the future.
PTCs, materials in which the index of refraction increases and decreases rapidly over time, are the temporal equivalent of photonic crystals in which the index of refraction periodically oscillates in space causing, for example, the iridescence of minerals precious and insect wings.
A PTC is only stable if the index of refraction can rise and fall with a single cycle of electromagnetic waves at the frequency of interest. So, unsurprisingly, PTCs have so far been observed at the lower 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, with collaborators Vladimir Shalaev and Alexndra Boltasseva of Purdue University, Indiana, USA, and their teams, sent extremely short pulses (5-6 femtoseconds). of laser light at a wavelength of 800 nanometers through transparent conductive oxide materials.
This caused a rapid change in refractive index which was explored using a probe laser beam at a slightly longer wavelength (near infrared). The probe beam was rapidly red-shifted (i.e. its wavelength increased) and then blue-shifted (the wavelength decreased) as the refractive index of the material returned to its normal value.
The time required for each of these refractive index changes was miniscule – less than 10 femtoseconds – and, therefore, within the single cycle needed to form a stable PTC.
“High-energy excited electrons in crystals typically take ten times longer to return to their ground state, and many researchers thought the ultra-fast relaxation we observe here would be impossible,” Segev said. “We don’t yet understand exactly how this happens.”
Co-author Shalaev further suggests that the ability to maintain PTCs in the optical domain, as demonstrated here, “will open a new chapter in the science of light and enable truly revolutionary applications.” However, we know as little about what it could be as physicists in the 1960s knew about the possible applications of lasers.
Reference: “Time Refractive Optics with 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 , May 31, 2023, Nanophotonics.
The research was funded by the German Research Foundation.
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