An interface device for performing mode transformation in optical waveguides includes an optical waveguide core for propagating light of a particular wavelength or a plurality of wavelengths. The optical waveguide core terminates in a subwavelength grating configured to change the propagation mode of the light. The subwavelength grating has a pitch sufficiently less than the wavelength of the light to frustrate diffraction. The device can thus serve as an optical coupler between different propagating media, or as an anti-reflective or high reflectivity device.