Cascaded Bragg gratings in photonic crystal fiber for plasmonic cladding mode-based biosensing of HER2 protein

Spectral multiplexing of biosensors in a single optical fiber has been a long-standing challenge, which we address here for the first time by combining photonic crystal fibers (PCF) with fiber Bragg grating technology. We exploit the features of the optical transmission spectrum of a straight fiber Bragg grating written in a PCF that allows exciting cladding mode resonances within a spectral span of about 60 nm, which is significantly narrower than the width of the transmission spectra of tilted gratings in standard single-mode step-index fibers. More specifically, we consider the cladding mode resonances that feature effective index values close to the refractive index of phosphate buffered saline, and we demonstrate plasmonic label-free biodetection of HER2 (human epidermal growth factor receptor 2) protein. We report on the simultaneous monitoring of the wavelength shifts of said cladding mode resonances from two spatially separated biofunctionalized Bragg gratings and we find that the PCF sensor is able to detect the protein concentration of 8.62 nM with high reproducibility.