Optical Vortex Beams Emit on Silicon

The smallest emitter of orbital angular momentum (OAM) light — light that travels in a twisting, corkscrew-like manner — was created on silicon. The chip is one thousand times smaller than any other such device reported to date and could be used in optoelectronics and communications technology.

OAM light, which can be used to encode and transmit information, has been generated for the past few decades using bulk optics such as lenses and plates, but only recently have scientists tried to produce it using chips.

An international group of universities in England, Scotland and China generated twisted light on a silicon chip one thousand times smaller than any previously reported device. Illustrated is an array consisting of three identical emitter; the three-dimensional emission pattern is calculated using a dipole-emission-based semianalytical model. Courtesy of Miss Yue Zhang, based on data from the paper’s authors.

The new emitters, invented by an international group led by the University of Bristol and the universities of Glasgow, Sun Yat-sen and Fudan, are based on silicon optical waveguides and can be made using standard integrated circuit fabrication technologies.

“Our microscopic optical vortex devices are so small and compact that silicon microchips containing thousands of emitters could be fabricated at very low costs and in high volume,” said Siyuan Yu, professor of photonics information systems in the Photonics Research Group at the University of Bristol. “Such integrated devices and systems could open up entirely new applications of optical vortex beams previously unattainable using bulk optics.”

SEM of a silicon integrated orbital angular momentum device. The radius of the microring resonator is 8 um. Courtesy of Michael J. Strain.

Light in optical vortex beams does not propagate in straight rays. Rather, its energy travels spirally in a hollow conical beam shape, appearing much like a vortex or cyclone, with light rays “twisting” either left- or right-handedly. Theoretically, no limit exists to how twisted the light rays can be.

In quantum mechanics, this property is associated with a photon’s ability to orbit around such a beam’s axis, or OAM.

Upon interaction with matter, such light asserts a rotational force on the matter, making it possible to rotate and trap microscopic particles or droplets by means of optical tweezing or spanning. Different degrees of twist also can be used to transmit information — allowing more information to be carried by a single optical signal, and increasing the capacity of optical communications links.

Interference patterns of the generated optical vortex with left or right hand circularly polarized reference beam (LHCP or RHCP). The numbers of the spiral arms in the interference pattern is related to the states of the polarization of the reference beams. The interference pattern with LHCP is l+1, while the interference pattern with RHCP is l-1. l is the topological charge of the optical vortex (l=-4 in picture). Courtesy of Xinlun Cai, Jianwei Wang, Mark G. Thompson, and Siyuan Yu.

Different streams of information can be transmitted by tweaking the OAM of light beams at the same frequency. Photons can use these different degrees of twist to represent quantum information, where a single photon can be simultaneously twisting clockwise and counterclockwise. Applications are being developed to use this light for imaging and sensing.

“Perhaps one of the most exciting applications is the control of twisted light at the single-photon level, enabling us to exploit the quantum mechanical properties of optical vortices for future applications in quantum communications and quantum computation,” said Dr. Mark Thompson, deputy director of the university’s Center for Quantum Photonics.

The study will appear in Science.

For more information, visit: www.bristol.ac.uk