Ultra-high NA graphene oxide flat lens on fiber facet with near diffraction-limited focusing

• xiaoke Chen, Lin Ma, Zuyuan He, Guiyuan Cao, Han Lin, and Baohua Jia
• received 02/08/2024; accepted 04/20/2024; posted 04/22/2024; Doc. ID 521005
• Abstract: Realizing high numerical aperture (NA) fiber lens is critical to achieve high imaging resolution of endoscope and enable subwavelength operation in optical tweezers and high efficiency coupling between optical fibers and photonic chips. But it remains challenging with conventional design and fabrication. Here we propose an ultra-thin (400 𝑛𝑚) graphene oxide (GO) film lens in-situ fabricated on a standard single-mode fiber facet using the femtosecond laser direct writing technique. Extremely high NA of 0.89 has been realized with near diffraction-limited focal spot (FWHM=0.68 λ), which has been verified theoretically and experimentally.The diameter of the fabricated fiber GO lens is as small as 12 𝜇𝑚 with no beam expansion structure. The proposed fiber GO lens is promising in applications such as super-resolution imaging, compact optical tweezers, medical endoscopes and on-chip integration.