The work of Professor Zhang Yong's team on "Ultra-broadband mode size converter using on-chip metamaterial-based Luneburg lens" has recently been accepted and published by ACS Photonics as a cover paper. This work was partially supported by the National Key R&D Program of China (2019YFB2203601), the National Natural Science Foundation of China (61975115/61835008/62035016), and the Shanghai Science and Technology Committee (2017SHZDZX03). The author proposed an integrated ultra-broadband mode size converter based on a Luneburg lens for converting TE/TM mode sizes in waveguides with different widths at extremely low loss. This work not only achieved an ultra-low loss of 1.5 dB in a single device but also solved the problem of the narrow bandwidth of previous mode size converters, enabling the conversion of mode sizes over an ultra-broadband range of 740 nm. This provides a promising solution for integrated devices such as ultra-broadband waveguide crossings, end-face coupling, and large-scale integration applications.
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.