Boston University

Optical Characterization and Nanophotonics Laboratory








Evanescent Waveguide Biosensors

Members: Anna K. Swan, M. Selim Ünlü


Alumni: Bennett B. Goldberg, Samuel H. Lipoff, Colin Worth, Yan Yin


Light from a polarized lazing source is launched into a thin, high index, high contrast optical waveguide, thus creating an evanescent field. The high contrast of a silicon waveguide results in the penetration of the evanescent field far into the cladding layers, thus the propagation of the light is extremely sensitive to changes of the index of refraction on the surface of the waveguide. The guiding layer of waveguides used in this biosensor are on the order of 2000Å (0.2µm) thick, thus only the first transverse electric (TEo) and transverse magnetic (TMo) modes are propagated. Changing the index of refraction on the surface of the waveguide results in a relative phase difference between the TEo and TMo modes, which may be understood by the Goos-Hänchen effect. This phase difference is directly dependent upon the material on the surface of the waveguide. We have designed a flow chamber which can pump fluid over an etched well on the surface of the waveguide. Having been emitted from the opposite facet of the waveguide interfere the TEo and TMo polarized beams, creating an interference pattern which may be measured by an ultra-sensitive detector. Thermal and acoustic interference is minimized by a dual-channel configuration that provides a control area on the waveguide to enable the noise floor to be substantially lowered. Particular pathogenic biological molecules can be detected at low concentration by attaching them to the surface of the waveguide by means of a correct antibody.

Having found that simple end-fire coupling (coupling light into the waveguide by simply focusing the laser onto a polished facet) yields low efficiency in the power throughput, and thus substantially raises the noise floor of the sensing device, we have explored alternative coupling techniques. We are currently exploring both prism and grating coupling for coupling of the TEo and TMo guided modes in silicon waveguides at a wavelength of 1.3µm as well as in Si3N4 waveguides at a wavelength of 0.6328µm. To facilitate our use of grating coupling, we are currently devising an innovative technique to fabricate the grating couplers by means of laser-assisted etching, which may circumvent a number of drawbacks of conventional etching techniques.

We have also prepared some Quicktime movies that demonstrate the sensitivity of our biosensing apparatus. While the interference pattern created by the TEo and TMo polarized waves emitted from the waveguide is quantitatively measured with an ultra-sensitive detector, we can also qualitatively measure it with a CCD array type camera. The files below contain some short movies of the movement of these fringe patterns as the waveguide is subjected to slight physical stimuli, including human breath and a sugar solution of low concentration.


C. Worth, B. B. Goldberg, M. F. Ruane, and M. S. Ünlü, "Surface Desensitization of Polarimetric Waveguide Interferometers," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 7, No. 6, November/December 2001, pp. 874-877


Ronald J. Rieder, Ph.D., Senior Electro-Optics Engineer, SatCon Technology Corporation

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