Ultra-compact single-mode fiber optical tweezer based on a 3D printed hybrid micro-optical structure (Conference Presentation)

Abstract

In 2018 Arthur Ashkin shared half of the Nobel prize physics for the optical tweezers and their application to biological Indeed, Optical Tweezers (OT) have found relevant applications in bioscience, thanks to the capability of trapping and manipulating small entities like viruses, bacteria, and DNA strands, in a non-destructive way. The balance of optical forces creating a stable all-optical trap is usually achieved by focusing a Gaussian beam with a high-NA microscope objective. Nevertheless, more efficient optical traps can be obtained by focusing a donut annular beam instead of a Gaussian beam [1], particles larger than the focused beam size. Additionally, the need bulky high-NA optics such as microscope objective may limit the use of OT in potential applications, like in-vivo trapping, and prompts the development of optical fiber-based OT. Here, we 3D printed a hybrid micro-optical structure on a single-mode optical fiber to create an ultra-compact annular beam fiber tweezer [2], with long working distance and a small probe diameter. A cascade of refractive, reflective, and waveguiding optical elements are microfabricated in a single step to transform the beam output from the fiber. The sequence is constituted by a gaussian to annular waveguide converter, a reflective annular beam expander, and a lensed prism deflector. The gaussian to annular waveguide converter transforms the single-mode fiber output into an annular beam with an inner radius of 4.5 µm and an outer radius of 9 µm. Then, a reflective inverted truncated cone expands the annular beam to an inner radius of 47 µm and an outer radius of 57 µm, by two total internal reflections. Afterward, a total internal reflection prism focuses the annular beam along its radius with a numerical aperture of 1 in water immersion. The output surface has a rounded profile with a radius of curvature of 7 µm to create a lens with a focal length of 50 µm that focuses the annular beam at the trapping point. All the total internal reflection surfaces are designed to work in water immersion, where the critical angle is around 60deg. The final result of this cascaded micro-optical structure is a sharply focused annular beam which forms an optical trap with a working distance of 30 µm. The microstructure is 275 µm long with a diameter of 116µm, smaller than the single-mode optical fiber which has a diameter of 125 µm. The structure has been 3D printed with a commercial Two-photon lithography system (Photonic Professional GT, Nanoscribe GmbH), by using the proprietary material Ip-Dip (Nanoscribe). We demonstrate the all-optical 3D trapping capabilities of this device by trapping polystyrene beads with different diameters. This device demonstrates the potential of 3D printing through two-photon lithography as a flexible tool to realize complexly integrated micro-optical systems. [1] O'Neil, A.T., Padgett, M.J., 2001. Axial and lateral trapping efficiency of Laguerre–Gaussian modes in inverted optical tweezers. Optics Communications, 193(1-6), pp.45-50. [2] Liberale, C. et al. Miniaturized all-fibre probe three-dimensional optical trapping and manipulation. Nature Photonics, 1(12), p. 723.