The generation of photonic time crystals in the near-visible spectrum has been demonstrated under novel conditions, according to a team of researchers who say the breakthrough could have significant implications on the science of light, and may lead to the development of new disruptive technologies.
Time crystals are quantum systems consisting of particles in repetitive motion at their lowest energy states. Because of this, time crystals are unable to come to rest and lose energy to the surrounding environment since they are already in their quantum ground state, thereby representing a kind of “motion without energy” in the absence of kinetic energy.
Similar to the photonic crystals responsible for the iridescence in everything from precious stones to the shimmering colors of tropical fish and other organisms, photonic time crystals are their temporal counterpart, and represent a specific kind of time crystal whose refractive index (the ratio of the velocity of light in a given medium to its speed in a vacuum) fluctuates very quickly.
In a new study by an international team of researchers, modulating the refractive index allowed PTCs to be generated and sustained in the optical domain. However, maintaining PTC stability requires their refractive index to align with the cycle of electromagnetic waves at specific frequencies. Because of this, past observations of PTCs generally occur in radio waves since they represent the end of the electromagnetic spectrum with the lowest frequencies.
In research led by principal author Mordechai Segev with the Technion-Israel Institute of Technology at Haifa, Israel, pulses of laser light at very short durations of between 5 and 6 femtoseconds at 800-nanometer wavelengths were fired through conductive transparent oxide materials, allowing very sudden shifting of the refractive index.
Segev and the team studied the shifts in refractive index with a second laser operating at longer wavelengths closer to infra-red, which allowed them to observe a rapid increase in wavelengths with the second “probe” laser beam as it became red-shifted, then blue-shifted as wavelengths decreased in response to changes in the material’s refractive index.
The changes occurred during very short periods of less than 10 femtoseconds, constituting the cycle required for the formation of stable PTCs.
According to Segev and the team, this is unusual since when the energy level of electrons within time crystals are excited in this way, they usually require more than ten times that duration before they return to their normal ground states. Because of this, the sudden relaxation Segev and his team were able to demonstrate shouldn’t be possible.
“We don’t yet understand exactly how it happens,” Segev said in a statement.
The researchers now believe that optical sustainment of PTCs could not only lead to new insights into optics—the science of light, its behavior, and how it is perceived visually—but also may “enable truly disruptive applications,” according to Vladimir Shalaev, one of the study’s co-authors.