Losses of Slot Mode Devices
An array of 200 silicon-on-insulator slot waveguide devices of varying slot widths, ribs widths, taper lengths, and slot lengths were created in each cell of a wafer fabricated at a commercial foundry. The cells were cleaved into individual chips after fabrication. Some chips were coated with thin films of polymers that fully infiltrated the slots. Measurements of spectral loss were made on the grating coupler waveguide devices of both coated and uncoated chips. Individual devices exhibited insertion losses varying from several dB up to values so great that the response was below the noise floor of the optical spectrum analyzer (OSA) employed as a receiver. The chips that failed the transmission test were primarily uncoated ones. Nominally identical devices on different chips exhibited nominally identical behavior. A commercial software program was used to simulate each of the structures that were included in the 200 device test. The simulations were seen to agree quantitatively well with the experimental results and to show a degree of qualitative agreement. Comparison of the experiment and the simulations indicate that the loss inherent in a slot waveguide is quite low. Also near loss free couplers from ridges to slots are achievable. Use of a surface roughness model in comparison with analytical results for slow mode propagation indicates that the excess loss that slots exhibit with respect to a ridge mode counterpart arise almost solely from surface scattering off the surface roughness. The increased loss in the case of the slot guide arises from the higher electromagnetic energy density at the surface of the guide due to the electric field discontinuity that is employed as a guidance mechanism in slot modes in contradistinction to ridge modes that are index-guided. Conclusions include some speculation as to the limits on the loss that can be achieved by variation aware design of slot guides without any improvement in surface roughness over what is now available with fabrication in commercial foundries.