Tailoring optical forces in waveguides through radiation pressure and electrostrictive forces

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Abstract
Radiation pressure is known to scale to large values in engineered micro and nanoscale photonic waveguide systems. In addition to radiation pressure, dielectric materials also exhibit strain-dependent refractive index changes, through which optical fields can induce electrostrictive forces. To date, little attention has been paid to the electrostrictive component of optical forces in high-index contrast waveguides. In this paper, we examine the magnitude, scaling, and spatial distribution of electrostrictive forces through analytical and numerical models, revealing that electrostrictive forces increase to large values in high index-contrast waveguides. Similar to radiation pressure, electrostrictive forces increase quadratically with the optical field. However, since electrostrictive forces are determined by the material photoelastic tensor, the sign of the electrostrictive force is highly material-dependent, resulting in cancellation with radiation pressure in some instances. Furthermore, our analysis reveals that the optical forces resulting from both radiation pressure and electrostriction can scale to remarkably high levels (i.e., greater than 104(N/m 2)) for realistic guided powers. Additionally, even in simple rectangular waveguides, the magnitude and distribution of both forces can be engineered at the various boundaries of the waveguide system by choice of material system and geometry of the waveguide.This tailorability points towards novel and simple waveguide designs which enable selective excitation of elastic waves with desired symmetries through engineered stimulated Brillouin scattering processes in nanoscale waveguide systems.

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