Photonic jets produced by microspheres integrated with hollow-core fibers for ultraprecise laser surgery

Recently it was proposed to use sharply focused optical beams produced by dielectric microspheres, also termed “photonic nanojets”, in contact laser surgery applications. The proposed designs were based on using multimodal beams and suffered from limited efficiency. In this work we developed single-mode designs of such devices providing higher efficiency and significantly smaller focal spot sizes compared to multimodal systems. The proposed single-mode systems include: i) diode-pumped Er:YAG laser source operating at the wavelength corresponding to the maximal water absorption peak in the tissue (λ = 2.94 μm), ii) low-loss hollow-core microstructured fiber delivery, and iii) high-index (n~1.8) focusing barium-titanate glass microsphere integrated with the fiber. By testing the system in air we demonstrated the focal spot diameters to be less than 4λ. Our numerical modeling shows a principal possibility to achieve diffraction-limited spot sizes on the order of λ/2. Due to the fact that the location of the photonic jet at the sphere surface is not strongly affected by the presence of fluid such systems can find applications in ultraprecise contact intraocular, brain or cellular microsurgeries.