Synchrotrons are amazing devices that use magnetic fields to get particles racing around kilometer-long tracks until they emit powerful X-rays. Now a team of scientists from the University of Michigan have performed the same feat, but they’ve done it on a tiny microchip, pushing a beam of light along a circular path to create terahertz radiation.
The researchers used a laser to emit a pulse of red light—“a little light bullet”—and altered its path from a normal straight line using a metasurface, which has a series of tiny, periodic structures about the same size as the wavelength of the light striking it. The metasurface in this case was a series of millions of gold antennas, shaped like boomerangs with different bends and different orientations, on a crystal of lithium tantalate. “We manage to get the light bullet to go around in a circular path,” says Roberto Merlin, a professor of physics at the University of Michigan who led the work, in a recent issue of Science.
To travel in a circle, a beam of light must accelerate, in this case surpassing the normal speed of light in the material. (Nothing can go faster than the speed of light in a vacuum, but light moves more slowly when it’s passing through matter.) That acceleration generates an electromagnetic field in the crystal, which emits the extra energy as radiation at around 1 terahertz.
So-called T-rays exist in the far infrared part of the electromagnetic spectrum, just below microwaves. They’re useful in airport security, for example, because they can spectrographically identify substances, and they penetrate many materials without causing harm because, unlike X-rays, they’re not ionizing radiation.
The researchers only managed to get the red light pulse to travel in a half a circle, and only 5 percent of the red light was converted into T-rays. The terahertz emission was broadband, which is useful for spectroscopy, but a narrower frequency would be preferable for other applications. “It would be fantastic if we could get the light to go around many, many, many times,” Merlin says.
That may require using different materials, he says. Gold is probably not the best choice for the antennas, but it was good for the prototype. As it was, it took Meredith Henstridge, the then-graduate student who did most of the work, nearly four years to figure out how to build the antenna array. Henstridge is now a postdoc at the Max Planck Institute for the Structure and Dynamics of Matter, in Germany.