Subwavelength on-chip light focusing with bigradient all-dielectric metamaterials for dense photonic integration

Photonic integrated circuits (PICs) provide a promising platform for miniaturized on-chip optical systems for communication, computation, and sensing applications. The dense integration of photonic components is one of the keys to exploit the advantages of PIC. Although light focusing is a fundamental and indispensable function in PICs, focusing light at the micro/nanometer-scale is challenging. Here, a bigradient on-chip metalens (BOML) is proposed to achieve ultrasmall focal lengths and spot sizes at the subwavelength scale for dense PICs. The design of BOML combines gradient geometry and gradient refractive index into one metalens by simultaneously engineering the length and width of subwavelength silicon slots. With a small device footprint of only 168 mu m, the BOML achieves efficient on-chip focusing with the record-breaking figure-of-merits, which are the ratio of wavelength to focal length/spot size (0.268 and 2.83) and numerical aperture (1.78). Leveraging on the Fresnel design, the footprint of BOML is further reduced by 55.1%, and the numerical aperture is enhanced to 1.9. The demonstration of mode conversion and beam steering with efficiency over 80% and a tilting range of 7.2 degrees holds the potential for highly dense on-chip photonic systems for optical communication, optical sensing, nonlinear optics, and neural networks for deep learning.

Automated summary

Photonic integrated circuits (PICs) offer a promising platform for miniaturized on-chip optical systems, with one key aspect being the dense integration of photonic components. This study introduces a bigradient on-chip metalens (BOML) to achieve ultrasmall focal lengths and spot sizes at the subwavelength scale for efficient on-chip focusing in PICs. The BOML design combines gradient geometry and refractive index to optimize performance metrics such as figure-of-merits and numerical aperture, enabling high-density on-chip photonic systems for various applications.