Nanoporous Silicon Waveguide-based Microcavities for Chemical and Biological Sensing

Biological and chemical sensors have an important and growing role in diverse fields including medical diagnosis, drug development, homeland security, and manufacturing. Common to all of these fields is the desire for sensors with high sensitivity, small size, and low cost. The goal of this new project is to develop a new architecture for optical sensing of biological and chemical substances that could provide unprecedented sensitivity in a device that can be readily fabricated with conventional silicon materials and techniques.

Nanoporous silicon is a unique and versatile material that has several features that make it especially attractive for chemical and biological sensors, including a very high surface area to volume ratio, simple and inexpensive fabrication techniques, and suitability for integration with silicon electronics.

The proposed sensor is designed to measure small changes in the refractive index of porous silicon that occur when exposed to chemical vapors or when biological molecules attach to the internal surfaces. Travelling-wave ring resonators provide a sensitive way to measure such small changes.

The goals of this exploratory project are to (1) develop the technology for designing, fabricating and testing porous silicon waveguides and resonators, and (2) assess the feasibility of using these devices for chemical and biological sensing. Preliminary estimates predict that the proposed devices could be orders of magnitude more sensitive than established methods that do not take advantage of guided-wave structures.

Fig. 1. A comparison of porous-silicon optical sensors. (a) A single layer (d = 5 µm) of porous silicon is illuminated with a broadband light source, and the reflection spectrum is measured. Small changes in the refractive index lead to a shift in the observed spectrum. Data from Lin et al. 1997. (b) Multilayer Bragg mirrors are used instead to improve the resolution of the fringes. Data taken from Chan, 2000. (c) We propose to improve the resolution by replacing the vertical Fabry Perot cavity with a travelling-wave ring resonator, as depicted here. Because of the longer interaction length, the device is more sensitive to small changes in refractive index..

Fig.2. Concept of porous silicon biosensor. The internal surface of the porous silicon is coated with an interactant (e.g. biotin molecule) that selectively binds to a target substance (e.g. streptavidin protein) to be detected. When the target material is immobilized on the surface, the macroscopic optical properties of the porous silicon film change in a measurable way.

Fig. 3. (a) Cross-sectional diagram of porous silicon waveguide and (b) the corresponding calculated TE mode profile. In this case, the structure was optimized for single-mode operation at 1550 nm.

References

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