Birefringence and optical power confinement in horizontal multi-slot waveguides made of Si and SiO2

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Abstract
Through simulations and measurements, we show that in multi-slot thin film waveguides, the TM polarized modes can be confined mostly in the low refractive index layers of the waveguide. The structure consisted of alternating layers of a-Si and SiO2, in the thickness range between 3 and 40 nm, for which the slots were the SiO2 layers. Simulations were performed using the transfer matrix method and experiments using the m-line technique at 1.55 µm. The dependence of the birefringence and of the power confinement in the slots was studied as a function of the waveguide thickness, the Si and SiO2 layer thicknesses, and the SiO2/Si layer thickness ratio. We find a large birefringence—a refractive index difference between TE and TM modes—as large as 0.8. For TM polarized modes, up to ~85% of the total power in the fundamental mode can be confined in the slots.