ABSTRACT The penetration depth of light within biological tissue is typically restricted by the optical diffusion limit to one transport mean free path primarily due to the multiple scattering. While whispering gallery mode (WGM) microlasers are promising internal light sources in tissue, their application has been limited to depths of a few hundred , likely due to the low signal intensity of small gain volume. Here, we demonstrate robust WGM lasing at depths significantly beyond the diffusion limit using a functionalized commercial nylon fiber with a diameter of 260 . The fiber's high refractive index () and mechanical flexibility enable efficient optical confinement and tissue integration. Benefiting from an enlarged gain volume, high‐quality WGM laser emission with discrete periodic resonances was maintained at depths of 5 mm in scattering phantoms (up to 53) and 3 mm beneath porcine skin. This work establishes the dye‐functionalized nylon fiber as a scalable architecture for high‐quality light generation deep in optically challenging biological environments. Integrating such lasing capabilities into existing medical filaments such as sutures in the future may pave the way toward long‐term in situ deep‐tissue therapeutics and diagnostics.
Ni et al. (Fri,) studied this question.