The realization of a high numerical aperture (NA) fiber lens is critical for achieving high imaging resolution in endoscopes, enabling subwavelength operation in optical tweezers and high efficiency coupling between optical fibers and photonic chips. However, it remains challenging with conventional design and fabrication. Here we propose an ultrathin (400 nm) graphene oxide (GO) film lens fabricated in situ on a standard single-mode fiber facet using the femtosecond laser direct writing technique. An extremely high NA of 0.89 is achieved with a near diffraction-limited focal spot (FWHM=0.68λ), which is 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 for applications such as super-resolution imaging, compact optical tweezers, medical endoscopes, and on-chip integration.

中文翻译：

---

光纤面上的超高数值孔径氧化石墨烯平面透镜，具有近衍射极限聚焦


高数值孔径（NA）光纤透镜的实现对于在内窥镜中实现高成像分辨率、实现光镊中的亚波长操作以及光纤与光子芯片之间的高效耦合至关重要。然而，传统的设计和制造仍然具有挑战性。在这里，我们提出了一种超薄（400 nm）氧化石墨烯（GO）薄膜透镜就地使用飞秒激光直写技术在标准单模光纤面上进行。通过接近衍射极限的焦点实现了 0.89 的极高数值孔径 (半高宽=0.68 λ ），并得到了理论和实验的验证。所制造的光纤GO透镜的直径小至12μm，并且没有扩束结构。所提出的光纤 GO 透镜有望用于超分辨率成像、紧凑型光镊、医用内窥镜和片上集成等应用。