The evanescent wave birefringence change in a polymer optical waveguide with high refractive index layers of nanometer scale thickness for optical sensing

We investigate the optical birefringence properties of evanescent field excited in the polymer waveguide where local engineering of refractive index is applied for evanescent mode enhancement for optical sensor purpose. Additional cladding layers of nanometer scale thickness comprising high index materials such as TiO₂, SiO₂ and their combination are deposited to enhance the evanescent mode interaction with the analyte solution on the waveguide sensing surface. We perform the measurement of the time-dependent polarization change caused by analyte solution injection and demonstrated that the device differential sensitivity stemming from analyte-induced birefringence change increases with increasing evanescent mode interaction with analyte. It is also seen that the analyte layer formation over a long term period can induce an additional birefringence change even in the case of no concentration change of analyte solution. The temporal response of evanescent mode birefringence change is also discussed using the different types of analyte solutions for the cases of deposition of various high-index materials on the wave-guide.