Femto-second single optical fiber tweezers enabled two-photon fluorescence excitation of trapped microscopic objects

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 labon-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.