Abstract
Biosensors are chemical sensors that use biological materials to detect the presence of the substance of interest. Optical methods to transduce the interaction between the analyte and the biological detection element are highly sensitive, non-invasive and are not subject to noise from electrical interference. Biosensors based on photonic devices such as lasers, optical fibres and photonic crystals offer the possibility of high sensitivity combined with small size, which is highly advantageous for in-the-field and remote applications. In this thesis, the feasibility of using semiconductor lasers as miniature biosensors has been investigated. Semiconductor laser chips contain a light source and a waveguide. Modification of the laser structure should allow the guided modes inside the laser to interact with material on the surface of the chip, thereby making the mode effective index sensitive to changes in the refractive index of material at the surface. This is the basis of an evanescent field sensor. Simulations have been carried out with the aim of finding an optimum structure for laser waveguides adapted for sensing purposes. Laser waveguides were modelled using RSoft Photonics CAD Suite and the BeamPROP simulation tool. For a ridge waveguide structure, two "windows" were placed either side of the ridge to form sensing areas. The optimum dimensions of these windows, and the optimum ridge width, were found by finding the structure with maximum sensitivity of effective index to a change in the cover index. A laser device was modified using focussed ion beam (FIB) milling to create sensing regions on the surface of the chip, in accordance with the computer model. The free spectral range (FSR) of the laser was measured before and after a polymer film was applied to the sensing regions of the chip, and a reproducible negative shift in the FSR was observed when the polymer was applied, indicating an increase in the effective index of the laser cavity. The sensitivity and detection limit (defined as the smallest measureable change in index of the sample) of the device were estimated as 1.5 x 10-3 and 0.15 respectively, although this could be improved significantly by measuring the heterodyne signal of two DFB lasers. The refractive index at a wavelength of lambda = 1550nm of spin-cast films of four different polymers have been measured by ellipsometry. These polymer films can now be used as standard test layers of known refractive indices for characterising the sensitivity of future laser-based biosensors. Finally an investigation into a new type of biosensor surface has been carried out. Antibody- conjugated latex particles are embedded in a cellulose acetate membrane, forming a biologically active surface to which antigens can selectively bind, without the need for complex surface chemistry to attach the antibodies. The membranes should also be reuseable by selectively dissolving the latex particles, and rebinding fresh particles into the imprints left in the membrane. It was found that the biological activity of the surface was difficult to preserve, but some selective rebinding of the latex particles into the imprinted membrane appeared to take place.