“At just the right distance from the center of a black hole, light can orbit the black hole in a circular orbit called a photon sphere. The radius of the photon sphere is 1.5 times larger than the Schwarzschild radius, inside which nothing can escape. This image shows the paths of light rays from a point source near a black hole.” Via.
Rainbow trapping in light pulses
Over the past decade, scientists have succeeded in slowing pulses of light down to zero speed by letting separate frequency components of the pulse conspire in such a way that a receptive medium through which the pulse is passing can host the information stored in the pulse but not actually absorb the pulse’s energy.
Trapping light means either stopping the light temporally or confining the light in space. Scientists have also been able to trap a light pulse in a tiny enclosure bounded by metamaterials; the light pulse retains its form but is kept from moving away.
Previously only light of a short frequency interval could be trapped in this way. Now a group of scientists at Nanjing University in China have shown how a rather wide spectrum of light — a rainbow of radiation — can be trapped in a single structure.
They propose to do this by sending the light rays into a self-similar-structured dielectric waveguide (SDW) — essentially a light pipe with a cladding of many layers.
Light of different colors propagates separately in different layers, each being tailored by color. They replace the conventional periodically-spaced, identical cladding layers with a non-periodic, self-similar pattern of successive layers made from two materials, A and B, with slightly different thicknesses and indices of refraction.
Self similarity, in this case, means that the pattern of layers successively outwards would be as follows: A, AB, ABBA, ABBABAAB, and so forth.
The effect might be applied for on-chip spectroscopy or on-chip ‘color-sorters.’ It might also be used for photon processing and information transport in optical communications and quantum computing.” Scientists expect that they can create trapped “rainbows” for light in many portions of the electromagnetic spectrum, including microwave, terahertz, infrared, and even visible.
Image: Different frequency components of a guided wave packet stop at correspondingly different thicknesses inside a tapered left-handed heterostructure (LHH). [+]