Artist's impression of an optical photonic time crystal consisting of a metasurface made from silicon nanospheres that are exposed to a time-varying optical pulse [Image: Xuchen Wang / Aalto University]
Researchers in Finland, Germany and China have shown how a resonating artificial nanomaterial could be used to make photonic time crystals at optical frequencies (Nat. Photonics, doi: 10.1038/s41566-024-01563-3). They argue that such devices, once demonstrated experimentally, might find application in everything from advanced lasers to ultra-sensitive nanodetectors.
Rapid refractive-index modulation
Rather than having a repeating structure in space, as normal crystals do, time crystals instead are spatially uniform but oscillate in time. More specifically, photonic time crystals are artificial materials with a rapidly varying refractive index.
This periodic modulation of electromagnetic properties yields what is known as a momentum bandgap. Incident radiation whose momentum falls within this gap couples to complex frequency modes, causing it to be amplified many times over.
Last year, Viktar Asadchy of Aalto University, Finland, Xuchen Wang of Harbin Engineering University, China, and colleagues demonstrated photonic time crystals in the lab by exposing two-dimensional metasurfaces to microwave radiation. Doing the same thing at optical frequencies, however, is complicated by the need to modulate a material's refractive index at exceptional rates―usually at twice the frequency of the probing light―and simultaneously over almost 100% of its amplitude.
This can in principle be done by exploiting the nonlinear Kerr effect, but this effect is very weak in low-loss materials―generating a variation in the refractive index of no more than 1%. An alternative is to use transparent conductive oxides, which can yield refractive index changes of around 100%, but they need extremely intense pump sources that can damage the material.
A new approach
Simulating the optical response of such a metasurface to infrared radiation, the researchers found that its momentum bandgap ought to be about 350 times larger for resonant as opposed to non-resonant radiation.
In the latest work, Asadchy, Wang and coworkers put forward a different approach that involves modulating the resonance frequency of a material's basic building blocks. They point out that this could theoretically be done in bulk materials by using an external electric field to modulate the effective spring constants between nuclei and electrons in the material's atoms. But they say that this approach also requires very high optical pump powers, which might destroy the sample being studied. Instead they use artificial metamaterials consisting of arrays of meta-atoms, which have a number of parameters that can be varied to modulate the resonance frequency.
Their proposed photonic time crystal is a metasurface made up of dielectric silicon nanospheres whose electron density, and hence permittivity, would be modulated by an optical pulse, for example, with a time-varying intensity profile. Simulating the optical response of such a metasurface to infrared radiation, the researchers found that its momentum bandgap ought to be about 350 times larger for resonant as opposed to non-resonant radiation―implying that the necessary bandgap could be achieved with an amplitude modulation as small as 1%. They add that a stronger resonance could further reduce this required modulation.
What's more, Asadchy, Wang and colleagues say that the momentum bandgap in question can cover all of k-space (an abstract space associated with the Fourier transform of a spatial lattice). This means that it would encompass waves propagating through freespace as well as those confined to the crystal's surface, which, they say, suggests the possibility of designing “more complex photonic time and space−time crystals.”
As for applications, they reckon that such a time crystal would be ideal for amplifying the very weak fluorescence signals given off by tiny, excited particles such as viruses, pollutants or biomarkers. They also say that the crystal could be used to create a perfect lens by amplifying the evanescent modes that carry information about an object.
Actually building such a time crystal will involve the tricky task of maintaining ultrafast time modulation uniformly across the whole metasurface. Nevertheless, they believe that it should in future be possible to build devices even at visible wavelengths―assuming a suitable material with very low losses and ultrafast relaxation times can be found.