Discovery of a new physical phenomenon: photonic time crystals

In recent years, scientists have been actively experimenting with the basic laws of physics, and they have recently discovered further evidence of what they call photonic time crystals (PTCs): materials in which the speed of light (refractive index) fluctuates very rapidly.

We already know much more about photonic crystals, where a repeating pattern in a material creates a change in the refractive index in a particular region of space; you can see this in the iridescence of insects or precious minerals. With PFCs, the element of change over time is also introduced.

In this study, the scientists aimed lasers at two materials known as transparent conductive oxides, which allow light to pass through themselves while conducting electricity. They used lasers to rapidly change the refractive index over periods of less than 10 femtoseconds (that’s 10 quadrillionths of a second).

The researchers observed significant changes in the frequency of light and the relaxation time of light (the time it takes for the refractive index to return to normal), depending on the thickness of the material and the rate of change in refractive index.

“Electrons excited to high energy in crystals typically take ten times longer to relax back to their ground states, and many researchers assumed that the ultra-fast relaxation we observe here would be impossible,” says physicist Mordechai Segev of the Technion-Israel Institute of Technology.

“We still don’t understand exactly how it happens.”

If your head already hurts from high-level physics, it’s worth knowing that there are several types of photonic time crystals. The research done here differs from photonic time crystals, which can manipulate light for a variety of advanced purposes.

So far, these types of FVCs have only been observed using radio waves, which have a much lower frequency than light waves – the faster the waves travel, the higher the frequency. The stability of FVCs depends on the refractive index rising and falling during a single cycle of an electromagnetic wave.

This is much more difficult in the ultra-fast world of light waves, which is what makes these experiments so interesting. Scientists have not yet been able to observe FVC in the visible spectrum of light, but they have come close.

As Segev says, it’s not yet clear why this is happening or how it might be used in the future, but we’re talking about a breakthrough in physics – “a new chapter in the science of light,” according to Vladimir Shalaev, an electrical engineer at Purdue University in the US.

“Our results … open the way to the observation of photonic time crystals at optical frequencies and many other phenomena related to time boundaries,” the researchers wrote in their published paper.

The study was published in the journal Nanophotonics.

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