A recent study published in the journal Nanophotonics reveals a fascinating development in the field of optics. Researchers have demonstrated the modulation of refractive index, which is the ratio of the speed of electromagnetic radiation in a medium to its speed in a vacuum, fast enough to generate photonic time crystals (PTCs) in the near-visible part of the spectrum. This breakthrough has the potential to revolutionize our understanding of light and lead to disruptive applications in the future.
Photonic time crystals are materials in which the refractive index rapidly rises and falls over time, analogous to how photonic crystals exhibit periodic oscillations in refractive index in space, resulting in the iridescent colors seen in precious minerals and insect wings. Until now, PTCs have only been observed at the lowest-frequency end of the electromagnetic spectrum, specifically with radio waves.
In this study, the research team led by Mordechai Segev from the Technion-Israel Institute of Technology, along with collaborators Vladimir Shalaev and Alexandra Boltasseva from Purdue University, utilized short pulses of laser light with a wavelength of 800 nanometers. These pulses were directed through transparent conductive oxide materials, inducing a rapid shift in refractive index. The researchers then used a probe laser beam at a slightly longer wavelength in the near-infrared range to explore the resulting refractive index changes.
Remarkably, the refractive index changes occurred within an incredibly short timeframe of less than 10 femtoseconds, which is within the duration of a single cycle of electromagnetic waves required for stable PTC formation. This rapid relaxation of the refractive index back to its normal value defied expectations, as typically, electrons excited to high energy in crystals require much longer to return to their ground states.
The ability to sustain PTCs in the optical domain, as demonstrated in this study, represents a significant advancement. Co-author Shalaev envisions this achievement as opening a new chapter in the science of light and paving the way for truly disruptive applications. Similar to how physicists in the 1960s had limited knowledge about the potential applications of lasers, we currently have limited understanding of the specific applications that may arise from this breakthrough. Nonetheless, the implications are expected to be profound.