Photonic gauge potentials are crucial for manipulating charge-neutral photons like their counterpart electrons in the electromagnetic field, allowing the analogous Aharonov-Bohm effect in photonics and paving the way for critical applications like photonic isolation. Normally, a gauge potential exhibits phase inversion along two opposite propagation paths. Here, we experimentally demonstrate an anomalous gauge potential for photons in a spatial-frequency space.

For this purpose, we consider an optomechanical system, where optical photon will also experience up- and down-conversion through the interaction with acoustic phonons. In our system, the phases associated with the two time-reversed up- and down-conversion processes are not equal in magnitude, which is caused by a near-phase-matching condition in the nonlinear Stokes/ anti-Stokes scattering processes. In contrast with previous work, therefore, we refer to such a scenario of having unequal up- and down-conversion phases as having an anomalous gauge potential for photons, and such an optomechanical system as a gauge-field modulator (GM) in the frequency space. Based on this anomalous gauge potential, we demonstrate a photonic isolator in the frequency space, constructed as an interferometer with a parallel-connected pair of an EOM and a GM, which can achieve unidirectional frequency conversion. Moreover, similar coherent controlled unidirectional frequency conversions are also illustrated.

The photonic isolator in the frequency space and its associated anomalous gauge potential is the essential ingredient for constructing an effective magnetic field in the synthetic frequency dimensions. More importantly, they can also be coherently controlled in directions offering more functionality such as signal routers or switches. Hence, these results may offer a unique platform for a compact, integrated solution to implement synthetic-dimension devices for on-chip optical signal processing.

The research was published in “Jianfan Yang, Luqi Yuan, Tian Qin, Fangxing Zhang, Yao Chen, Xiaoshun Jiang, Xianfeng Chen, Shanhui Fan, and Wenjie Wan,” Phonon-induced anomalous gauge potential for photonic isolation in frequency space” Optica 8(11), 1448-1457 (2021)
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Link： https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-8-11-1448&id=464681