Author：Yong Zhang，Yu He，Hongwei Wang，Lu Sun，and Yikai Su*
Abstract：A Luneburg lens with a gradient index distribution is an aberration-free and coma-free spherical lens. It has wide applications ranging from invisibility cloaks, illusion optics, and superlensing. However, it is challenging to realize an on-chip Luneburg lens with superior performance. In this paper, an on-chip Luneburg lens is implemented through the integration of gradient metamaterial structures and silicon waveguides. The filling ratio mapping of silicon nanorods is used to achieve the gradient index distribution. As an example of the application, a general and scalable approach for a mode size converter is demonstrated to match two waveguide modes with arbitrary widths. Benefiting from the aberration-free property, a 740 nm bandwidth is achieved in simulation. Limited by our measurement setup, the measured device bandwidth is 220 nm, covering the wavelengths of 1.26–1.36 and 1.507–1.627 μm. To the best of our knowledge, this silicon mode size converter exhibits the largest bandwidth.
Publishing source：ACS Photonics
