Photonic bandgap (PBG) structures have remarkable optical properties that can be exploited for biosensing applications. We describe the fabrication of 1-D PBG biosensors using porous silicon. The optical properties of porous silicon PBGs are sensitive to small changes of refractive index in the porous layers, which makes them a good sensing platform capable of detecting binding of the target molecules to the bioreceptors. The material nanostructure and device configuration that lead to optimum performance of the devices are investigated in detail by modeling the optical response. It is shown that porous silicon based PBG sensors are useful for detecting biological matter, from small molecules to larger proteins.
The sensitivity of photonic bandgap (PBG) structures to the environment makes them suitable for sensing applications. In this study, we describe how 1-D and 2-D PBG devices can be used for sensing biological matter, from small DNA segments to larger proteins. Our work focuses on using the tunability of silicon PBGs upon binding of the desired target on the internal surface of the air holes. Modeling of the optical response is performed to identify the material nanostructure and device configuration that lead to optimum performance (e.g., sensitivity).
The optical properties of photonic bandgap (PBG) structures are highly sensitive to the geometry and refractive index. This makes PBG structures a good host for sensor applications. The binding of target species inside the PBG structure changes the refractive index of the material, which can be detected by monitoring the optical response of the device. One-dimensional PBG biosensors based on porous silicon (PSi) have been fabricated. The device is a microcavity, made of a symmetry breaking PSi layer (defect layer) inserted between two PSi Bragg mirrors. Narrow resonances are introduced in the photoluminescence and reflectance spectra. The large internal surface of the sensor is functionalized for the capture of target biological materials. When the sensor is exposed to the target, binding to the internal surface increases the effective optical thickness of the microcavity and thus causes a red shift of the optical spectrum. The sensor's sensitivity is determined by the morphology and geometry of the device. We will present the details of the materials science, sensor fabrication and optimization, and also describe experiments performed with biological targets.
As photonic bandgap (PBG) technology matures and practical devices are realized, the effects of environmental factors, such as ambient temperature, on PBG device operation must be considered. The position of a PBG is determined by the geometry and refractive index of the constituent materials. Therefore, a thermally induced material expansion or refractive index change will alter the location of the PBG and affect the operation of PBG devices. In order to achieve faster switching times for PBG optical interconnects, enhanced sensitivity for PBG sensors, and smaller channel spacing for PBG-based wavelength division multiplexing, increasingly narrow PBG resonances are required. The drawback for the improved device operation is increased sensitivity to small changes in environmental conditions. A method to control and eliminate thermally induced drifts of silicon-based PBG structures has been developed based on a simple oxidation treatment. Oxide coverage of the silicon matrix provides a counterforce to the effect of the temperature dependent silicon refractive index. Depending on the degree of oxidation achieved, a redshift, no shift, or a blueshift of the PBG resonance results when the silicon-based PBG structure is heated. Control over the effects of thermal fluctuations has been demonstrated for two different PBG structure designs. Extensive reflectance and x-ray diffraction measurements have been performed to understand the mechanism behind this oxidation procedure as it relates to one-dimensional silicon-based PBG microcavities.