Gold saturable metasurface for building a wavelength-tunable optical spiking neuron.

Plasmonic resonant metasurfaces have found many applications in nonlinear optics, such as harmonic generation, all-optical modulation, saturable absorption, etc. A saturable absorber, as a key device for pulsing emission, plays an important role in building passively Q-switched or mode-locked fiber lasers. Recently, excitable fiber lasers have attracted much attention in the area of neuromorphic photonics. In this work, a plasmonic metasurface consisting of periodic gold nanorods resonant near 1550 nm is designed and fabricated, which exhibits saturable absorption with a modulation depth of about 2.6%. The saturable metasurface is, for the first time, utilized in an excitable erbium-doped polarization-maintaining fiber laser, acting as a crucial nonlinear term for the dynamics of the optical spiking neuron. Compared to biological neurons, the artificial optical neuron possesses shorter a refractory period, faster pulse encoding capability, and changeable firing rate as a function of cavity length (up to 20 kHz in our experiment). In addition, the optical neuron is tunable in emission wavelength within the range from 1526.3 nm to 1568.2 nm, beneficial to wavelength-division multiplexing in photonic neural networks. The trial of the nonlinear plasmonic metasurface for an excitable laser could inspire new perspectives in constructing optical neurons and extend applications of metasurfaces from conventional nonlinear optics to neuromorphic computing.