Ultrafast All-optical Modulation Exploiting the Vibrational Dynamic of Metallic Meta-atoms

Optical control over elementary molecular vibration establishes fundamental capabilities for exploiting the broad range of optical linear and nonlinear phenomena. However, experimental demonstration of the coherently driven molecular vibration remains a challenge task due to the weak optical force imposed on natural materials. Here we report the design of "meta-atom" that exhibits giant artificial optical nonlinearity. These "meta-atoms" support co-localized magnetic resonance at optical frequency and vibration resonance at GHz frequency with a deep-sub-diffraction-limit spatial confinement (${\lambda}^2/100$). The coherent coupling of those two distinct resonances manifests a strong optical force, which is fundamentally different from the commonly studied form of radiation forces, the gradient forces, or photo-thermal induced deformation. It results in a giant third-order susceptibility $\chi^{(3)}$ of $10^{-13}$ $m^2$/$V^2$, which is more than six orders of magnitude larger than that found in natural materials. The all-optical modulation at the frequency well above 1 GHz has thus been demonstrated experimentally.