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.
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