• Lamb waves
• GaAs patterned membrane
• Fano resonances
• optomechanical light modulator
• photonic-phononic metamaterial

The purpose of this work is the investigation of nanostructured metamaterials in which light interacts with the mechanical motion through optomechanical effects; in particular, it aims to design, fabricate and characterize a device based on the mechanical and photonic properties of a patterned dielectric membrane, in order to realize an optomechanical light modulator, with features that advance well-established technologies.
This is precisely the case of the object considered in this work: a phononic-photonic metamaterial represented by a Gallium Arsenide (GaAs) slab of 220 nm thickness, patterned with a square lattice of square air holes.
From a photonic point of view, the dielectric membrane can support electromagnetic modes called leaky modes, which can couple with the external radiation, leading to guided-mode resonances. According to the coupled-mode theory, in the transmission and reflection spectra, the system exhibits a Fano asymmetric line shape, which is typical of a scattered field that is a sum of the non-resonantly scattered field and the field scattered out of the resonator. From a phononic point of view, instead, also the mechanical vibrations are affected by the periodicity of the lattice, similarly to electromagnetic waves in photonic crystals. The metasurface can sustain mechanical modes which are referred to as Lamb waves and excitable by an inter-digital transducer (IDT). For our purpose, we have chosen to work with the first order band-edge mode, originating from the antisymmetric Lamb mode. The final goal is to observe a wavelength shift of the optical resonance of the metamaterial when the transducer excites this mechanical resonance in the metasurface, so as to obtain a modulation of the incident light on the membrane.
In principle, it is possible to change the modulator work frequency by modifying the resonance frequency of the mechanical Lamb mode trough which the GaAs slab is excited. This possibility makes the device under consideration the first step towards the development of high frequency (GHz) optomechanical modulators.