The relationship between the design parameters and small molecule detection sensitivity of porous silicon waveguides is theoretically and experimentally analyzed. Perturbation theory calculations suggest that the sensitivity asymptotically approaches infinity as the porosity of the waveguide approaches a critical porosity for a given mode and the resonant coupling angle of light into the waveguide approaches 90 degrees. Experimental measurements confirm the trend of the porosity-dependent sensitivity for multiple waveguide modes. Given the limitations of the available measurement apparatus that restricts the maximum coupling angle to 68 degrees, a high sensitivity of 120 degrees/RIU was demonstrated for the detection of 0.8 nm molecules attached inside a polymer-cladded nanoscale porous silicon waveguide. Optimized porous dielectric waveguides enable enhanced small molecule detection sensitivity due to their large available surface area for molecular binding.
Copyright 2009 Elsevier B.V. All rights reserved.