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Nonlinear optics

Asymmetry from symmetry

Nature Physics volume 10, pages 336–337 (2014)Cite this article

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An unusual form of symmetry breaking in coupled microresonators with balanced optical gain and loss is now exploited to realize a novel type of optical isolator.

Each mode of an optical resonator has a finite lifetime: the confined light eventually leaks away either through radiation loss or intrinsic material absorption. The properties of a long-lifetime mode — particularly its frequency and spatial profile — are typically almost exactly the same as in the lossless case. However, this seemingly self-evident behaviour can break down if optical gain is also present. An example of this situation that has been of particular recent interest is the 'PT-symmetric' structure. These structures feature spatially balanced regions containing equal and opposite amounts of gain and loss — PT refers to the combination of spatial-parity (P) and time-reversal (T) symmetry operations, where the latter swaps gain and loss processes1,2. The simultaneous presence of gain and loss in a PT-symmetric structure can alter both the frequencies and spatial profiles of the optical modes. This occurs through an unusual version of spontaneous symmetry breaking: modes with unbroken PT symmetry are distributed evenly across both halves of the structure in such a way that the gain and loss compensate for each other; PT-broken modes, on the other hand, become spontaneously concentrated in either the amplifying or lossy half and undergo either net gain or loss.

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Figure 1: A nonlinear optical isolator based on parity–time symmetry.

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  1. Chong Yidong is at the School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore,

    Chong Yidong

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  1. Chong Yidong
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Correspondence to Chong Yidong.

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Yidong, C. Asymmetry from symmetry. Nature Phys 10, 336–337 (2014). https://doi.org/10.1038/nphys2941

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