We show spontaneous symmetry breaking (SSB) in a nonlocal metasurface laser. The system is a hexagonal plasmonic distributed feedback laser that lases at the six K-points in momentum space, or more exactly at the K and K’ modes. A unique properties is that these modes are exactly degenerate in both spatial distribution and energy. By simultaneous real-space and Fourier-space measurements, we map both the relative amplitude (parity symmetry breaking) and phase (rotational symmetry breaking ) of the two symmetry-broken modes. Our results open new perspectives on studying SSB and emergence of spatial coherence in photonic systems.
Controlling near-field in space and time is crucial to applications of high-Q nonlocal metasurfaces, for instance for their use in nonlinear frequency conversion. We discuss near-field interferometric autocorrelation (IAC) measurements that reveal the dynamics of optical fields of quasi-bound states in the continuum (quasi-BICs) in plasmonic-dielectric metasurfaces. Using two-photon excited luminescence (TPEL) from quantum dots as local probes, our IAC measurements probe resonant near-field enhancement. Femtosecond laser pulse excitation of quasi-BICs produces coherent oscillations visible in TPEL, offering insights into resonances and their temporal beating. We discuss application scenarios for frequency-converting nonlinear metasurfaces in XUV generation and wafer metrology, as well as strategies for achieving high metasurface Q factors in metallic and dielectric systems.
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