Abstract
We theoretically and numerically investigate Stimulated Brillouin Scattering generated mode conversion in high-contrast suspended silicon nanophotonic waveguides. We predict significantly enhanced mode conversion when the linked effects of radiation pressure and motion of the waveguide boundaries are taken into account. The mode conversion is more than 10 times larger than would be predicted if the effect of radiation pressure is not taken into account: we find a waveguide length of 740 μm is required for 20dB of mode conversion, assuming a total pump power of 1W. This is sufficient to bring the effect into the realm of chip-scale photonic waveguides. We explore the interaction between the different types of acoustic modes that can exist within these waveguides, and show how the presence of these modes leads to enhanced conversion between the different possible optical modes.
Highlights
Stimulated Brillouin Scattering (SBS) is a strong nonlinear interaction between the electromagnetic and mechanical waves in an optical waveguide [1]
We find that there exist classes of mode conversion—for example, between differently-polarized optical modes—that do not appear in the conventional scalar SBS theory
We examine the classes of acoustic modes that lead to mode conversion between the different classes of optical modes, and investigate how the mode conversion depends on the presence or absence of the radiation pressure effect
Summary
Stimulated Brillouin Scattering (SBS) is a strong nonlinear interaction between the electromagnetic and mechanical waves in an optical waveguide [1]. The first involves the use of materials that possess a high refractive index n (SBS gain scales as n8), while simultaneously confining the acoustic mode in order to achieve good overlap with the optical field This strategy has been demonstrated in a number of recent experiments [18]. The acoustic modes in SBS are generated by optical forces arising from the pump, and for small waveguides the forces arising from radiation pressure at the waveguide boundary can be at least as large as the forces from electrostriction throughout the waveguide These two effects can reinforce, leading to large SBS gain. We find that there exist classes of mode conversion—for example, between differently-polarized optical modes—that do not appear in the conventional scalar SBS theory These interactions involve hybrid acoustic modes, such as quasi-flexural and quasi-torsional modes, which become important in small suspended waveguides and which induce waveguide boundary motions that scatter the optical field strongly. We explore the parameter space for high index-contrast waveguides, and explain the underlying physics of the mode conversion process in these waveguides
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