An optical sensor, sensing system and method of sensing employ a half-core hollow optical waveguide adjacent to a surface of an optical waveguide layer of a substrate. The half-core hollow optical waveguide and the adjacent optical waveguide layer cooperatively provide both an optical path that confines and guides an optical signal and an internal hollow channel. The optical path and channel extend longitudinally along a hollow core of the half-core hollow optical waveguide. The system further includes an optical source at an input of the optical path and an optical detector at an output of the optical path. A spectroscopic interaction between an analyte material that is introduced into the channel and an optical signal propagating along the optical path determines a characteristic of the analyte material.