Plasmon Enhanced Electro-Optical Reflectance and Hysteresis in Self-Assembled Gold Mesopyramids

Gold mesopyramids (MPs), formed by annealing a eutectic mixture of Au:Si:Au thin films, exhibit unique surface texturing making them an excellent platform for studying plasmonic activity. We report the observation of a pronounced, rate-dependent electro-optical hysteresis in the far-field diffraction from individual, self-assembled MPs on n-type Si (100) substrates. The hysteresis manifests as a distinct memory effect, where the reflectivity depends on the history and sweep rate of the applied bias. We attribute this phenomenon to charge trapping at the uniquely complex and highly textured Au-Si Schottky interface formed during the eutectic growth process of the MPs, where the free electron density at the surface of Au is modulated. This alters the gold's dielectric function and modulates the efficiency of Surface Plasmon Polariton (SPP) coupling modes, supported by the surface morphology of the MPs. The non-monotonic shape and DC persistence of the hysteresis loops point to the complex kinetics of shallow and deep-level traps at the interface, distinct from simple thermal effects or RC-time constant lags. These results demonstrate an effective strategy for plasmonic manipulation through electrical controls and suggest Gold MPs as potential platforms for optical memory elements.