Near-field optical and atomic-force microscopy by use of the laser trapping technique of a probe

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We developed a scanning friction-force microscope which uses a small sphere trapped and scanned by laser beam. The force can be detected as position shift of the sphere, during the sample scanning. The system consists of an inverted microscope, a scanning stage, a near-infrared laser (Nd : YAG,2W), illumination optics, a position detector, and a computer for control. The detection limit of the friction-force with the system was 1.0(]SY.+-。[)0.2pN when we observed the surface of glass or PMMA.We added detection optics for near-field optical signal to the system in order to realize a near-field optical microscope. Since the microscope uses a laser trapped probe, which is trapped by pico-Newton-order force in water, the system has advantages as follows : 1) it gives little damage to the sample, 2) the distance between the sample surface and the probe is constant, and 3) the spatial resolution can be controlled by choosing an appropriate size of the probe. By using this system we trapped n … More ot only dielectric spheres but also metallic spheres. The metallic sphere has high scattering efficiency and provides high signal-to-noise ratio. Moreover small sphere (phi40nm) attains high spatial resolution.We developed three-dimensional microfabrication technique using two-photon absorption. Two-photon absorption process makes it possible to stiffen only small spot near the focal point. We estimated that lateral and axial resolutions are 0.6mum and 1.0mum respectively, by optimizing pulse-width and real power of mode-locked Ti : Sapphire laser (790nm). This system provides one-order higher resolution than conventional systems. Therefore this technique is available to make a probe, that can be stably and precisely trapped by a laser beam.We also showed that finite-difference time-domain (FDTD) method is effective to amalyze the photon force in near-field. We succeeded in calculation of the irradiation forces (=trapping forces) of evanescent or propagation light field to any shapes or materials probes. We investigated the size, the material, and the shape of the probe which is suitable for the microscope we developed. Less