This paper presents a modification of the refractive-index (RI) response of an intermodal interferometer based on photonic crystal fiber (PCF) using a thin plasma-deposited silicon nitride (SiN<sub>x</sub>) overlay with a high refractive index. We show that the film overlay can effectively change the distribution of the cladding modes and thus tune the RI sensitivity of the interferometer. Thanks to the nano-coating we were able to increase the RI sensitivity eightfold in the range required for biosensors (n<sub>D</sub> ~1.33). Due to the extreme hardness of SiN<sub>x</sub> films and their excellent adhesion to the fiber surface, we believe that after the film deposition the device will still maintain its advantages, i.e., lack of degradation over time or with temperature.
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