Phase analysis method of plasmonic tilted fiber Bragg grating based biosensors

Gold-coated tilted fiber Bragg gratings (Au-TFBG) are promising platforms leading to the development of highly sensitive biosensors. The spectral resonance of the surface plasmon polariton carried by the gold layer interface depends on the refractive index of the surrounding medium. Once covered with bioreceptors, the p-polarized mode spectrum can be used to detect the change in refractive index induced by the analyte-bioreceptor interactions. In practice, a polarization controller is used to extract its insertion loss spectrum. Usual demodulation techniques track the evolution of the insertion loss spectral attenuation instead of the phase evolution. Indeed, to extract the phase, the polarization controller needs to be removed, and the p-polarized mode must be retrieved by other means. In this paper, a new demodulation technique based on phase evolution with a sensitivity of several thousands of degrees/RIU is presented. Using the complete transfer matrix function (Jones’s matrix), the amplitudes and phases for both p and s polarized modes are mathematically exact for each wavelength considered. Thanks to this procedure, every characteristic like phase and amplitude field evolution can be retrieved. The physical platform sensitivity is measured using LiCl solutions and the fiber is functionalized with anti-HER2 (Human Epidermal Growth Factor Receptor-2,a breast cancer marker) aptamers. Phase demodulation is utilized to extract the refractive index modification induced by all the process. Biosensing experiments on specific antiaptamer/HER2 interactions are also tested for concentrations of HER2 proteins at 1μg/mL.