Research Of High Sensitivity Biosensor Based On Symmetric Metal-Clad Waveguide

Optical biosensor is a general term for a wide range of device that measures the presence or concentration of biological molecules, biological structures, microorganisms etc., by translating a biochemical interaction into a quantifiable optical signal. The potential application fields of biosensors are very extensive, including medical and health care, biological research, chemical industries, drug development, food and water examine, environment monitoring and protection, etc.The rapid developments of integrated optics and waveguide technologies have boosted the research on optical biosensors. Evolved from the ABBE refractometer, biosensors based on surface plasmon resonance (SPR) and the leaky mode waveguide (LW) have attracted a lot of interest for the simple structure and relatively high sensitivity over previous technologies. Recently, the presence of reverse asymmetric leaky waveguide sensor has further improved the sensitivity. But still, as the technologies evolving, application demands for higher sensitivity have never stopped.By employing the theory on dielectric optical waveguide, characteristics of the symmetric metal-clad optical waveguide (SMCW) have been analyzed in this thesis. This kind of waveguide consists of two metal films and a guiding layer in between. The upper metal film is relatively thin to be around 40nm, the incident light will go through it to be coupled into the waveguide structure. On the contrary, the base metal film is usually thicker to block the light from leaking into the substrates. The dielectric media between the two metal films acts as the guiding layer, the guided light wave is confined to propagate in it.As the real part of the dielectric constants of the metal films is negative, the refractive index, RI, of the guiding layer can be very small. Besides, in the SMCW structure, the thickness of the guiding layer can be enlarged to submillimeter scale and can contain ultrahigh order guided modes with mode index of over 1000. The effective RI of those ultrahigh order modes is very small (near 0), and is extremely sensitive to the RI of the guiding layer. These ultrahigh order modes also behave polarization independence and can be excited thorough free coupling technology.These unique features enable us to fabricate an extremely sensitive biosensor based on the SMCW structure. By putting liquid or even gas samples (with small RI as low as to 1) in the sample cell sealed between two metal films, a typical three layer SMCW sensor is presented. The sample works as the guiding layer. By monitoring the coupling properties of the SMCW sensor structure, minute changes in the sample RI can be captured, and thus to identify sample property changes.Based on the theoretical analysis, sensitivity comparison of the proposed sensor with SPR sensor and LW sensor is conducted. The comparisons show that there is a huge sensitivity improvement of the proposed sensor over previous ones. The concentration of incident energy in the sample layer, the small effective RI and the narrow resonance dip of ultrahigh order modes, all the three factors contribute to the sensitivity enhancement of SMCW sensor.To verify the performance of the proposed SMCW sensor, two SMCW sensors with different coupling methods are fabricated (one with prism coupling, the other with free space coupling). Experiments are carried out using angular interrogation and intensity interrogation, respectively, to test the sensor's performance. The results reveal that by using angular interrogation, the sensitivity of proposed SMCW sensor is 1 to 2 orders to the magnitude higher than the previous sensor structure. What's more, by using the intensity interrogation, the sensitivity enhancement is even larger. The causes that have influence on the sensor's sensitivity and performance are also analyzed in the thesis.Based on the solid theoretical analysis and experimental proof, the SMCW proposed in this thesis is of profound potential in future applications for its extremely high sensitivity and simple configuration.