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We present an approach to the design of an RGB coupler for miniature projection and near-eye display devices using a photonic crystal-like sub-wavelength (PhC-SW) structure based on the SiN-SiO2 photonic integration platform. The proposed device was optimized using an inverse design method. By etching pixelated air holes on the SiN core layer, light propagation can be controlled by modulating the equivalent refractive index. This approach results in a device with an ultra-compact size of 3×4μm2, which is one of the smallest reported design size for the RGB coupler. In addition, the proposed design could achieve transmission efficiencies of 72.10%, 84.41% and 90.16% insertion losses (IL) of -1.42dB, -0.74dB and -0.45dB, and 3dB bandwidth of -270.73dB, -248.20dB and -224.32dB at the RGB lights, respectively. This new method would provide a compact and efficient solution for the miniaturization of RGB couplers.
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