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Similar to the other physical dimensions of light, such as time, space, polarization, wavelength, and intensity, optical angular momentum (AM) is another physically-orthogonal dimension of light. Owing to an unbounded set of orbital angular momentum (OAM) modes carried by helically-phased beams, the availability of using AM-carrying beams as information carrier to generate, transport and detect optical signals has recently been largely explored in both classical and quantum optical communications, suggesting that AM is indeed a promising candidate to dramatically boost the optical multiplexing capacity. However, the extrinsic nature of OAM modes restricts conventional OAM multiplexing to bulky phase sensitive elements, imposing a fundamental limit for realizing on-chip OAM multiplexing. Recently, we demonstrate an entirely-new concept of nanoplasmonic multiplexing of AM of light, which for the first time enables AM multiplexing to be carried out by an integrated device with six orders of magnitude reduced footprint as compared to the conventional OAM detectors. We show that nanoring slit waveguides exhibit a distinctive outcoupling efficiency on tightly-confined plasmonic AM modes coupled from AM-carrying beams. More intriguingly, unlike the linear momentum sensitivity with a typical sharp resonance, the discovered AM mode-sorting sensitivity is nonresonant in nature, leading to an ultra-broadband AM multiplexing ranging from visible to terahertz wavelengths. This nanoplasmonic manipulation of AM of ultra-broadband light offers exciting avenues for future on-chip AM applications in highly-sensitive bio-imaging and bio-sensing, ultrahigh-bandwidth optical communications, ultrahigh-definition displays, and ultrahigh-capacity data storage.
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