Two photon polymerization (TPP) has enabled three-dimensional microfabrication with sub-diffraction limited
spatial resolution. However, depth at which TPP could be achieved, has been limited due to the high numerical
aperture microscope objective, used to focus the ultrafast laser beam. Here, we report fiber-optic two photon
polymerization (FTP) for in-depth fabrication of microstructures from a photopolymerizable resin. A cleaved single
mode optical fiber coupled with tunable femtosecond laser could achieve TPP, forming extended waveguide on the
fiber itself. The length of the FTP tip was found to depend on the laser power and exposure duration. Microfabricated
fiber tip using FTP was employed to deliver continuous wave laser beam on to polystyrene microspheres
in order to transport and manipulate selected particles by scattering force and 2D trapping. Such microstructures
formed by TPP on tip of the fiber will also enable puncture and micro-surgery of cellular structures. With use of a
cleaved fiber or axicon tip, FTP structures were fabricated on curved surfaces at large depth. The required Power for
FTP and the polymerization rate was faster while using an axicon tip optical fiber. This enabled fabrication of
complex octopus-like microstructures.
Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest.
We report on the development of single fiber femto second optical tweezers and its use in two-photon fluorescence
(TPF) excitation of trapped fluorescent particles. Trapping of the floating objects led to stable fluorescence emission
intensity over a long period of time, suitable for spectroscopic measurements. Trapping depth of few cm was achieved
inside colloidal sample with TPF from the trapped particle being visible to the naked eye. Furthermore, the fiber optic
trapping was so stable that the trapped particle could be moved in 3D even by holding the fiber in hand and slow
maneuvering of the same. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber
tip, a relatively longer streak of fluorescence was observed along the microsphere length. The cone angle of axicon was
engineered so as to provide better trapping stability and high axial confinement of TPF. The theoretical simulation of
fiber optical microbeam profiles emerging from the axicon tip and trapping force estimations was found to be in good
agreement with the experimentally observed stiffness and TPF patterns. Apart from miniaturization capability into lab-on-
a-chip micro-fluidic devices, the proposed non-invasive micro axicon tipped optical fiber can be used in multifunctional
mode for in-depth trapping, rotation, sorting and ablation as well as for two-photon fluorescence excitation of
motile sample which will revolutionize biophysics and research in material science.
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