1200^x broadband modal converter using a subwavelength self-focusing structure

Efficient modal interconversion between optical manipulation of cold atoms in free space and transmitted light within an integrated waveguide remains a challenge in the area of integrated atomic photonics. Here, a 1200^x modal converter with a footprint on the order of millimeters is proposed based on a Si₃N₄ subwavelength self-focusing structure. The 2.8µm×1.7µm subwavelength structure enables efficient single modal conversion. The transmission efficiency is 84% at a wavelength of 830 nm with a working bandwidth of 240 nm. The device can work in dual polarization states.