Metamaterials Observe Photonic Spin Hall Effect

A metamaterial surface constructed of V-shaped gold nanoantennas was used to obtain the strongest signal yet of the photonic spin Hall effect — a quantum mechanical optical phenomenon that could play a prominent role in the future of computing.

Researchers from Lawrence Berkeley National Laboratory amplified the naturally weak effect by measuring polarized light incident on a 2-D sheet of gold nanoantennas whose geometry could be configured by adjusting the length and orientation of the arms of the V’s. The findings could have profound implications for information processing and encoding, said Xiang Zhang, a faculty scientist with Berkeley Lab’s Materials Science Division.

Light propagating through a metamaterial follows a curved trajectory that drags light with different circular polarization in opposite transverse directions to produce a giant photonic spin Hall effect. Courtesy of Zhang group.

The spin Hall effect, named in honor of physicist Edwin Hall, describes the curved path that spinning electrons follow as they move through a semiconductor. The curved movement arises from the interaction between the physical motion of the electron and its spin — a quantized angular momentum that gives rise to magnetic moment. Think of a baseball pitcher putting spin on a ball to make it curve to the left or right.

“Light moving through a metal also displays the spin Hall effect, but the photonic spin Hall effect is very weak because the spin angular momentum of photons and spin-orbit interactions are very small,” said lead author Xiaobo Yin, a member of Zhang's research group. “In the past, people have managed to observe the photonic spin Hall effect by generating the process over and over again to obtain an accumulative signal, or by using highly sophisticated quantum measurements. Our metamaterial makes the photonic spin Hall effect observable even with a simple camera.”

Metamaterials have received significant attention in recent years for their unique electromagnetic properties not found in nature. For this study, the group fashioned metamaterial surfaces about 30 nm thick.

Ziliang Ye, Xiang Zhang and Xiaobo Yin of Berkeley Lab used metamaterials to create a giant photonic spin Hall effect, an optical phenomenon that could play a prominent role in the future of computing. Courtesy of Roy Kaltschmidt, Berkeley Lab.

“We chose eight different antenna configurations with optimized geometry parameters to generate a linear phase gradient along the X direction,” Yin said. “This enabled us to control the propagation of the light and introduce strong photon spin-orbit interactions through rapid changes in direction.” They observed that a sharper change in propagation direction made the effect stronger.

Since the entire metasurface sample used in the study measured only 0.3 mm, a 50-mm lens was used to project the transmission of the light through the material onto a CCD camera for imaging. From the CCD images, the investigators determined that both the control of the light propagation and the giant photonic spin Hall effect were the direct results of the designed metamaterial.

"The controllable spin-orbit interaction and momentum transfer between spin and orbital angular momentum allows us to manipulate the information encoded on the polarization of light, much like the 0 and 1 of today's electronic devices," Yin says. "But photonic devices could encode more information and provide greater information security than conventional electronic devices."

A scanning electron microscopy image of a metasurface comprising V-shaped antennas with a variety of arm configurations. Courtesy of Zhang group.

The ability to control left and right circular polarization of light in metamaterial surfaces should allow for the formation of optical elements, like highly coveted "flat lenses," or the management of light polarization without using wave plates, Yin said.

"Metamaterials provide us with tremendous design freedom that will allow us to modulate the strength of the photonic spin Hall effect at different spatial locations," he said. "We knew the photonic spin Hall effect existed in nature, but it was so hard to detect. Now, with the right metamaterials, we can not only enhance this effect, we can harness it for our own purposes."

The work appears in Science (doi: 10.1126/science.1231758).

For more information, visit: www.lbl.gov