Photonic nanojet focusing for hollow-core photonic crystal fiber probes.

Large-pitch kagome-lattice hollow-core photonic crystal fibers (HC-PCFs) offer appealing optical properties for beam delivery and remote sensing. However, focusing their optical mode on a submicrometer spot can be challenging due to the large mode diameter and low numerical aperture of these fibers. Here, we demonstrate that a 30 μm latex microsphere directly set at the HC-PCF end-face provides an efficient means to focus the fiber mode down to a spot of 540 nm full width at half-maximum thanks to a photonic nanojet effect. The system is used for fluorescence imaging and direct laser writing on a thin absorbing layer. Potential applications include inspection of semiconductor wafers, photolithography, laser surgery, fluorescence sensing, or optical transfection.