Theoretical design of miniaturized two-photon STED micro-endoscopic probe based on double-cladding optical fiber and dual-wavelength confocal metalens

The free-space two-photon STED microscopy has previously shown benefits in high-resolution biological imaging. However, the bulky setup and complicated alignment of the doughnut-shaped STED beam with the Gaussian-liked excitation beam suffered from the light modulation and achromatic focusing prevent the STED endoscopy using widespreadly in the living tissue imaging. Here, the miniaturized all fiber two-photon STED micro-endoscopic probe is theoretically designed, in which the light transmission and beam modulation are obtained by the double-cladding fiber and achromatic focusing are achieved by the dual-wavelength confocal metalens. We theoretically optimize the structure parameters of the double-cladding fiber to achieve the 915 nm linearly polarized excitation beam and the 510 nm azimuthally polarized STED beam simultaneously. The dual-wavelength confocal metalens directly deposited onto the double-cladding fiber tip could focus the excitation and STED beams on the same spot. Then, a super-resolution effective fluorescent spot will be attained by the effective overlapping of excitation and STED spots. The lateral full width at half-maximum (FWHM) of the effective fluorescent spot is 50 nm theoretically calculated by the two-photon STED theory with a saturated case. Hence, the designed two-photon STED micro-endoscopic probe will be used for high resolution and minimally invasive imaging in deep brain regions.