Polarization-multiplexing metafiber for dual-mode bright-field and dark-field microscopy.

Bright-field and dark-field microscopy are typically used together as complementary techniques to provide comprehensive information about biological specimens with different optical absorption properties. However, switching between these two modes usually involves replacing several bulk optical components, which inevitably increases system complexity, introduces alignment challenges, and results in longer switching times. Herein, we propose a new, to the best of our knowledge, polarization-multiplexing metafiber device for dual-mode bright-field and dark-field microscopy. Utilizing a polarization-multiplexing metalens, two tailored beams (i.e., Gaussian and OAM beam) can be generated, simply by changing the handedness of the incident circularly polarized light. By integrating such metalens onto the tip of a large-mode-area photonic crystal fiber, we experimentally demonstrated that this compact and flexible metafiber can realize the dual-mode bright-field and dark-field microscopy using raspberry trichomes and pine stem, without the need to replace any optical components. The ultra-compact and flexibility features of the proposed metafiber-based dual-mode microscopy pave the way for promising applications in portable and biological imaging.