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Credit: University of Twente

Light that is sent into a photonic crystal, cant go deeper than the so-called Bragg length. Deeper inside the crystal, light of a certain color range can simply not exist. Still, researchers of the University of Twente, the University of Iowa and the University of Copenhagen managed to break this law. They steer light into a crystal, using a programmed pattern, and demonstrate that it will reach places far beyond the Bragg length. They publish their findings in Physical Review Letters.

Photonic crystals have a regular pattern of nano pores etched in silicon. They are typically designed to work as a mirror for a certain color range of light. Inside the crystal, light of those colors is forbidden. Even if youd be able to place an atom inside the crystal, that typically emits one color, it will stop emitting light. The so-called Bragg length is the maximum distance light is allowed to travel, according to a well-known physics law.

This property can be used for creating perfect mirrors for certain wavelengths, but it also helps improving solar cells. Still, if there is a sign that says forbidden anywhere, then it is always tempting to go there. This is what the researchers did, they proved that light can penetrate the photonic crystal, much deeper than the Bragg length.

They managed to do this by using light that was pre-programmed, and by using the small imperfections that always come with creating nanostructures. These imperfections cause light waves to be scattered randomly inside the crystal. The researchers program the light in such a way that every location inside the photonic crystal can be reached. They even demonstrate a bright spot at five times the Bragg length, where light is enhanced100 times instead of decreased 100 to 1000 times.

This remarkable result can be used for creating stable quantum bits, for a light-driven quantum computer. The forbidden effect can also be employed in miniature on-chip light sources and lasers.

The research was done in theComplex Photonics group of Professor Willem Vos. The group is part of UTs MESA+ Institute. The first author, Ravitej Uppu, who worked in this group earlier on, is now a Professor at the University of Iowa. The research collaboration was continued, also together with the University of Copenhagen. It was supported by Dutch Research Council (NWO) programs Stirring of Light, Free form scattering optics and Self-assembled icosahedral quasicrystals with a band gap for visible light, by the Applied Nanophotonics section of the MESA+ Institute and the Center for Hybrid Quantum Networks of the Niels Bohr Institute in Copenhagen.

Reference: Spatially Shaping Waves to Penetrate Deep inside a Forbidden Gap by Ravitej Uppu, Manashee Adhikary, Cornelis A.M. Harteveld and Willem L. Vos, 27 April 2021, Physical Review Letters.DOI: 10.1103/PhysRevLett.126.177402

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Breaking the Laws of Physics: Steering Light to Places It Isnt Supposed to Go - SciTechDaily