A 3D-Printed Dual-Fiber Probe for Background-Free Brillouin Spectroscopy

Brillouin spectroscopy is a powerful non-invasive method for mapping micromechanical properties of biological samples, with potential in disease diagnosis. Its in vivo endoscopic use is limited by two issues: single-fiber probes produce strong parasitic signals that mask the weak sample response, while dual-fiber designs eliminate this background but suffer from poor excitation–collection overlap and thus very weak signals, when using conventional optics. We developed a miniaturized dual-fiber Brillouin probe (285 µm tip) featuring a tailored freeform micro-lens fabricated via two-photon lithography (TPL) directly onto two singlemode fibers. This 3D-printed lens focuses both beams into a common focal spot, removing fiber background and boosting collection efficiency eightfold compared to a lensless dual-fiber setup. The probe achieves sub-10 µm lateral and 43 µm axial resolution and enables clear, background-free Brillouin signals from polymer microstructures. It also produced the first 2D Brillouin maps of phase-separated DDX4 protein droplets using a fiber probe. We further address improving measurability of steep-slope surfaces in TPL-fabricated optics. This work advances endoscopic Brillouin imaging for biomedical and materials science applications.