Light loops around (and around) an optical Möbius strip
M. Wang et al., Phys. Rev. Lett. 129, 186101 (2022)
When light is trapped in a ring of material, it typically completes some integer number of wave cycles as it makes a round trip. That integer also sets the light’s angular momentum. Such light is called a whispering-gallery mode, and it has the advantage of having a long lifetime—that is, having a high quality factor (Q)—and being stored in a small volume.
But what if the material is not a circular ring? Kartik Srinivasan of NIST and his colleagues have now developed a gear-shaped photonic microring that, for the first time, offers light with fractional angular momentum and the benefits of a whispering-gallery mode. Such fractional-momentum light has a helical wavefront, which is useful for rotating atoms in optical traps and measuring and manipulating entangled photons in quantum information systems, among other applications.
The microgear, because of its repeating N notches, adds a symmetry on top of the rotational symmetry of a circular microring. That change in symmetry hybridizes optical modes with distinct angular momenta: Modes that have integer cycles per round trip partner up to create an N-fold symmetric pattern. Those superposition modes have fractional angular momenta when N is odd.
A fractional angular momentum means the light doesn’t return to its initial state after one round trip. Rather, it must make multiple round trips—in the case of this experiment, two. The angular momentum’s phase thus evolves in a way that can be represented as traveling around the surface of a Möbius strip.
Srinivasan and his colleagues’ microgear (shown above) was made of insulating silicon nitride fashioned into a ring about 50 µm across with 333 notches. After injecting light into the photonic ring, they took IR images of the light scattered from the device to measure the various optical modes.
The scattered light appeared in patterns of bright nodes and dark antinodes around the ring. The researchers saw the modes with half-integer angular momenta that were expected theoretically. Notably, those superposition modes lack mirror symmetry and include some modes in which the light is localized to the outer edge or even to one region of the microgear.
All the fractional modes still maintained the benefits of conventional whispering-gallery modes, including a high Q and easy fabrication. In the future, the microgear could serve as a microscale source of fractional-angular-momentum light that’s compatible with photonic chips. (M. Wang et al., Phys. Rev. Lett. 129, 186101, 2022 .)
