The waveguide sensors for bio-sensing that we are developing at Lehigh University consist of the following parts; a plano-convex lens for proper coupling of the light from an optical fiber into the waveguide, the waveguide, which consists of a core, cladding and a nano-porous layer, all made from chalcogenide materials, the out-coupling grating and the quantum dot infrared photodetector (QDIP) on the same substrate.

The waveguide sensors for bio sensing are fabricated in a multilayer configuration. We use a chalcogenide layer as the cladding and another layer to act as core of the waveguide. In the waveguide, the light is propagated in the core by the total internal reflection at the interface between the glass and the top layer (nano-porous layer in our case). Each reflection induces an evanescent field at the surface of the waveguide.

Although chalcogenide glasses transmit in the near, mid and far infrared region, they absorb very strongly in the visible region, especially with band gap illumination. Various studies have confirmed changes in the optical properties (band gap, refractive index, etc.,), etching properties by band gap illumination and these changes occur in the illuminated portion only. We use this peculiar property of chalcogenide glasses for creating channel waveguides from planer waveguides. After fabricating the core layer, the area where the channel waveguide is to be fabricated is illuminated with band gap light to induce a change in the refractive index in that particular illuminated area.

As the biological agents have their identification signatures in the IR range and these signatures are well known, a combination of FTIR and chalcogenide waveguides can prove to be very beneficial in developing sensors which can work over wider ranges of detection. Chalcogenide materials transmit in the mid and far IR region and this creates numerous applications in the civil, medical and military areas. These can be divided into two groups, namely the “passive” and “active” applications. The passive applications utilize chalcogenide fibers/waveguides as a light conduit from one location to another without changing the optical properties, other than that due to scattering, absorption and the end face reflection losses in fibers or coupling losses in waveguides. Active applications are where the initial light propagation is modified by a process other than that due to scattering, absorption etc.,. Chalcogenide waveguides are well suited for chemical sensing applications since practically most molecular species vibrate in the infrared region and chalcogenide materials transmit (no absorption) in the infrared region. Already various techniques employing FTIR spectroscopy and chalcogenide fibers have been considered by various groups for the use in the sensor development. Numerous systems have been studied and a plethora of species have been detected including aqueous, non-aqueous and toxic liquids as well as solids. In spite of these various systems being developed, the performance remains to be improved. The basic issues are the selectivity and sensitivity, which need to be increased. Clinicians, food technologists, environmentalists and military all have an interest in generally increases sensitivity and limits of detection for a range of agents. While the precise demands to meet today’s requirements may be modest in these respects, few would contest the longer term benefits of reliable detection of trace amounts of various indicators. Selectivity has to be improved to extend the range of detection of the biosensor. The improvement of selectivity is an issue which needs to be addressed in the future sensors as this will allow us to use a single sensor over a wide range.