A recent study published in the journal Nanophotonics shows that the refractive index, which is the ratio of the speed of electromagnetic radiation in a material to that in a vacuum, can be adjusted quickly enough to produce photonic time crystals (PTCs) in the near-visible region of the spectrum.
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According to the study’s authors, maintaining PTCs in the optical domain could have significant effects on the study of light and open the door for impactful future applications.
PTCs are materials whose refractive index changes rapidly over time, similar to photonic crystals, whose refractive index fluctuates sporadically in space and is responsible for phenomena like the iridescence of precious minerals and insect wings.
Unsurprisingly, PTCs have thus far been detected at the lowest-frequency end of the electromagnetic spectrum. A PTC is only stable if the refractive index can be induced to rise and fall in line with a single cycle of electromagnetic waves at the frequency concerned.
In a recent study, lead author Mordechai Segev of the Technion-Israel Institute of Technology in Haifa, Israel, along with collaborators Vladimir Shalaev and Alexandra Boltasseva of Purdue University in Indiana, USA, and their teams, sent incredibly brief (5–6 femtosecond) pulses of laser light through transparent conductive oxide materials.
This resulted in a quick shift in refractive index, which was investigated using a probe laser beam with a slightly longer (near infrared) wavelength. The probe beam was rapidly red-shifted (wavelength raised) and then blue-shifted (wavelength dropped) when the material’s refractive index relaxed back to its normal value.
Each of these tiny changes in refractive index took fewer than 10 femtoseconds to complete, within the single cycle required to create a stable PTC.
Electrons excited to high energy in crystals generally need over ten times as long to relax back to their ground states, and many researchers thought that the ultra-fast relaxation we observe here would be impossible. We don’t yet understand exactly how it happens.
Mordechai Segev, Study Lead Author and Professor, Physics and Electrical Engineering, Technion-Israel Institute of Technology
The study’s co-author Shalaev adds that it will “open a new chapter in the science of light and enable truly disruptive applications” if PTCs can be maintained in the optical domain. But, similar to physicists in the 1960s, little is understood about the potential uses of lasers.
Lustig, E., et al. (2023) Time-refraction optics with single cycle modulation. Nanophotonics. doi:10.1515/nanoph-2023-0126.