Photonic time crystals triggered by laser pulses may open the door to a new branch of optics

When scientists discovered that laser pulses can rapidly cause refractive index changes in the medium, resulting in "photonic time crystals (PTC)" in the near-visible light band, the door to a disruptive new application in optics seemed to quietly open.

Scientists have a certain degree of understanding of photonic crystals and time crystals, the two have almost nothing in common, the basic common point is that both will produce structures over time.

Photonic crystals are artificial periodic dielectric structures, which are periodically arranged by media of different refractive indices, and can block photons of specific frequencies and then affect photon motion. In other words, the periodic dielectric structures with "photonic frequency bandgap" are called photonic crystals. In addition, there are photonic crystals in nature, which can be seen from the flickering of insect wings or precious minerals.

A time crystal is a quantum system composed of repeated motions of particles in the lowest energy state. Compared with regular crystals that repeat periodically in space, time crystals will repeat periodically in time and show a permanent state, for example, they change with time, but they will always return to the original state.

Photonic time crystals are materials whose electromagnetic properties change greatly with time, or represent a specific type of time crystal, whose refractive index rises and falls rapidly with time, and there is more than one type of photonic time crystal.

It is proved that stable photonic time crystals can also exist in near visible wavelengths

In order to maintain the stability of photonic time crystals, the refractive index must be made to rise and fall in line with the single period of electromagnetic waves of a specific frequency, so far scientists have only observed photonic time crystals at the lowest frequency end of the electromagnetic spectrum (radio waves), and it is quite challenging to find photonic time crystals in the optical field.

But according to new research led by Technion Israel scientist Mordechai Segev, Purdue University scientists Vladimir Shalaev, AlexndraBoltasseva and others, the team sent laser pulses with a wavelength of 800 nanometres through transparent conductive oxides (TCO), It was found that the time required for each refractive index change was very short (less than 10 femtoseconds), constituting the single period required to form a stable PTC.

Normally, electrons excited to high energies in a time crystal take 10 times more time to return to the ground state, but this experiment found that the relaxation time of light (the time required for the refractive index to return to normal) is extremely short, basically "impossible things."

It is not yet clear why this happens or how it will eventually be applied, but it could lead to breakthroughs in optics, just as physicists in the 1960s began to discover what practical applications laser beams could bring, perhaps in highly efficient laser-based particle accelerators or highly sensitive particle detectors with adjustable angular resolution.

The new paper is published in the journal Nanophotonics.

Source: Laser Network