I’m trying to reproduce the acousto-optic coupling simulation in the paper Frequency–angular resolving LiDAR using chip-scale acousto-optic beam steering.
LNOI(lithium niobate on insulator) is used as the substrate, and opposite acoustic and electromagnetic waves are applied to the LN layer. If the acoustic frequency is sufficiently high, the acoustic wavenumber K will be large enough to scatter an optical waveguide mode into the light cone, thereby steering a beam into free space. This is a kind of anomalous Bragg diffraction, and momentum matching needs to be satisfied.
To simulate this AO coupling, both moving boundary and photoelastic effect need to be introduced. The author gives the detailed steps as follows:
For the simulation, we used two components and two studies—one for acoustic mode simulation, and the other for beam steering. In the first Component and Study, we conducted a conventional acoustic eigenmode simulation. Then the displacement fields of the desired mode are imported into the second Component's geometry using a moving mesh and general extrusion (for a correct periodicity). Photoelastic effect-induced refractive index changes of LN are defined in the Variables in the second component, calculated by photoelastic coefficients and displacement fields. The material's refractive index is updated accordingly. By doing these, we introduced both moving boundary and photoelastic effect into the second component. The rest of the simulation is just as a conventional optical eigenfrequency study. Please note that a periodic boundary condition that matches the leaky mode's in-plane momentum is used in the propagation direction. With these set up correctly, you should be able to see the desired mode in the eigenmode results.
I made the model according to the above. The acoustic mode chooses the Rayleigh mode, but finally I do not know why can not get the same result as in the paper. The moving mesh setup may be where the problem is, but how to check whether it is correct? I didn’t find out a proper way.
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