PHOTONIC FORCE MICROSCOPE BASED ON OPTICAL TWEEZERS AND 2-PHOTON EXCITATION FOR BIOLOGICAL APPLICATIONS

A new scanning probe microscope, the photonic force microscope (PFM), based on optical tweezers and two photon absorption processes for biol ogical applications is described. Optical tweezers are used to trap a fluorescent latex bead with a diameter of 200 nm in an aqueous solutio n in all three dimensions. The fluorescent dye is chosen to fulfill th e two-photon absorption criterion for the 1064-nm line of a Nd:YVO4 la ser. The intensity of the fluorescence emission is utilized as a very sensitive position sensor along the optical axis. Two-dimensional imag es are formed by laterally scanning the trapped latex bead across biol ogical samples while recording the two-photon-induced fluorescence int ensity. A scanning probe image of the outer surface of a small neurite from a cultured rat hippocampal neuron is shown, which is hardly visi ble under differential interference contrast microscopy. The lateral r esolution is given by the bead diameter; the axial resolution is 40 nm . Under the experimental conditions the maximal imaging force applied by the probe is below 5 pN. (C) 1997 Academic Press.