Doubly-resonant metal ridge array for enhanced acousto-optical modulation

International audienceAcousto-optical interaction describes the modification of the optical properties of a material system under the action of an acoustic wave that moves the equilibrium positions of the atoms composing the medium of propagation. The effect is usually considered in the frame of perturbation theory and limited to the first Born approximation. Hence, acousto-optical interaction can produce significant effects only if diffracted optical waves from a large number of periods of the acoustic wave add coherently. Can we escape the small perturbation regimeand design a system for which the AO interaction is strong even though the interaction length is smaller than the wavelength? This is the question we consider in this communication. Specifically, we exhibit a nano-optical system whose transmission can be strongly modulated when sustaining a resonant acoustic wave excited thanks to piezoelectricity of the substrate. The system is composed of a periodic array of π-resonant optical cavities formed by metal ridges on a lithium niobate substrate. As such, the array forms a one-dimensional phoxonic crystal structure. The array supports a Γ-point optical resonance appearing in the light cone that creates a dip in the transmission for normal incidence of optical plane waves. The array simultaneously supports localized mechanical resonances that are efficiently excited by a radio-frequency electric signal, creating a large amplitude flexural motion of the metal ridges. Indeed, those ridges are metal electrodes that naturally constitute a transducer for surface acoustic waves. As we show based on a combination of finite-element time-domain (FDTD) and finite element analysis, flexural motion of the metal ridges at a resonant acoustic frequency produces a strongly nonlinear modulation of the optical transmission when the vibration amplitude increases [5]. Numerical simulations performed for silver ridges on a lithium niobate substrate suggest that transmission modulation approaching 100% at GHz frequencies should be feasible for flexural vibrations of about 10 nm of amplitude and for a thickness of the nano-optical system of only 300 nm