Tethered light: Physicists have captured light with the help of a new type of trap – and in doing so have demonstrated a physical effect that has never been experimentally proven. In this case, a special structure inhibits wave propagation, even though the spectrum of the light is outside the spectral range of the interference pattern. It was previously unclear whether a light chain is even possible through the so-called Anderson localization with such a combination – now it has been proven.
Phenomena such as the refraction and diffraction of light demonstrate that light and other radiation can be influenced by certain materials. Such interactions can change the direction, phase, polarization or wavelength of the light. In special metamaterials and photonic crystals, physicists have even succeeded in stopping light or accelerating its phase velocity to infinity.
Anderson localization: pinned waves
Another way to manipulate light and other waves is what is known as Anderson localization. It is based on a theoretical prediction presented by US physicist Philip Anderson in 1958. According to this, the disruptive effect of certain structures – so-called disordered systems – can suddenly arrest freely moving electrons and other quantum particles. In the case of electrons, this suddenly turns a conductor into an insulator.
Since then, this Anderson localization has also been demonstrated for various forms of radiation and sound waves. However, there seemed to be a limitation: Such wave traps only work if the grating size of the manipulative structure matches the spectrum and thus the wavelength of the radiation. “Experimentally, Anderson localization was therefore always limited to the spectral range of the disorder,” explain Alex Dikopoltsev from the Technion in Haifa and his colleagues.
Is this also possible for an “invisible” trap?
But the physicists have now cracked this limit: they have proven that structures that are “invisible” to light because they lie outside its spectrum can also capture the waves. Dikopoltsev and his team had already predicted this theoretically in 2019, and now, together with colleagues from the University of Rostock, they have also succeeded in experimental proof.
For their experiment, the physicists first constructed a photonic structure that acts as a disordered disruptive system – as a light trap. “To do this, we linked kilometers of optical glass fibers in such a way that the propagation of light in these fibers mimics the movement of electrons in disordered materials,” explains co-author Sebastian Weidemann from the University of Rostock. The researchers then sent packets of radiation through this system, the wave numbers of which were significantly above or below the spectral size of the interfering structure.
Experimental proof successful
The result: “We could clearly see that light waves are limited to small spatial areas even if the disorder is practically invisible to them,” reports Weidemann. The wave packets were held in place, showing clear evidence of Anderson localization. “We have thus experimentally proven for the first time that an Anderson localization can also take place completely outside the spectral range of the disorder,” the physicists state.
According to their analyses, this novel effect comes about through virtual transitions: “By light waves interacting several times in succession with the almost invisible disorder, an unexpectedly strong effect can arise that even forces such light waves to Anderson localization,” explains Dikopoltsev. “As a result, waves of any wavenumber, even beyond the disorder spectrum, can experience Anderson localization.”
Also important for practical applications
These results not only expand the knowledge about wave propagation in disordered systems, they are also important for concrete technical applications. Because it leads to new possibilities for selectively suppressing currents through such disordered systems – regardless of whether they are light, sound or electrons. (Science Advances, 2022; doi:10.1126/sciadv.abn7769)
Source: University of Rostock
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