Current optical communication systems rely on the use of wavelength division multiplexing (WDM) to keep up with the increasing data rate requirements. The wavelength demultiplexer is the key component to implement WDM systems. In this Letter, we design and experimentally demonstrate a demultiplexer based on a curved grating waveguide geometry that separates eight channels with a spacing of 10 nm (1249 GHz) around the central wavelength of 1550 nm. The fabricated device shows very low insertion loss ( ∼ 1 d B ) and a crosstalk (XT) below − 25 d B . This device leverages metamaterial index engineering to implement the lateral cladding on one side of the waveguide. This makes it possible to design a waveguide grating with highly directional lateral emission by operating in a regime where diffraction into the silica upper cladding is frustrated, thus suppressing losses due to off-chip radiation.