Design of acousto-optical devices by topology optimization

In resent years it has been shown experimentally that it is possible to modulate optical waves in waveguides by surface acoustic waves and a theoretical study shows that the modulation can be improved by changing the waveguide geometry. Here a method to improve the acousto-optical interaction by means of topology optimization is presented. The surface acoustic waves are generated by interdigital transducers in a 2D piezoelectric model, which is coupled to an optical model where the optical mode in the waveguide is found by solving the time-harmonic wave equation for the magnetic field. Only the piezoelectric model is used in the optimization and the objective function is the squared absolute value of the strain in the vertical direction in the waveguide. The objective function is maximized by distributing air and solid material in an area below the waveguide. The optical model is solved for the initial and the optimized design and the measure of the acousto-optical interaction is the difference in effective refractive index for the fundamental mode between the two cases where a surface acoustic wave crest is in the waveguide and a trough is in the waveguide, which is then normalized with the squared applied electric power. It is here shown that the acousto-optical interaction can be increased almost 10 times by redistribution of solid material and air in the design domain.