A refractive index sensor is provided for analyzing an analyte, the sensor including: a strip waveguide for receiving an input light signal therein and transmitting the light signal, subject to manipulation as it propagates through the strip waveguide, to a detector for analysis with respect to the analyte; and a slot waveguide for sensing the analyte disposed thereon and for receiving a sensing signal, corresponding to said manipulation of the light signal, from the strip waveguide, wherein a grating is formed on a surface of the strip waveguide to enable coupling of the sensing signal from the strip waveguide to the slot waveguide, and the sensor is configured with enhanced sensitivity based on a sensitivity difference between the slot waveguide and the strip waveguide, and/or a group index difference between the slot waveguide and the strip waveguide.