High Time-Resolved Near-Field Scanning Optical Microscopy

High time-resolved near-field scanning optical microscopy is a new high technology that combines near-field scanning optical microscopy (NSOM) and high time-resolved spectroscopy. Its aim is to develop a measurement with high temporal resolution and high spatial resolution simultaneously. It will be a valuable tool for elucidating photophysical, photochemical and photobiological dynamics occurring on nanometer scale. In this paper, we have studied all related techniques widely and deeply for this purpose. First, we described a simple chemical etching technique to increase a fiber tip抯 cone angle at a proper ratio of buffered hydrofluoric acid for a high-transmission fiber probe. After aluminum coating by vacuum evaporation, a fiber probe with diameter of 100 nm, cone angle of 30~ and transmission of 5.5x ~ had been achieved. We also developed a new technique to fabricate a bent fiber tip, in which a fiber was first bent by a resistant wire heater and then chemically etched. The bent tips at a bend angle of 600, with a radius of 300 ~m and in length of 500 ~tm were achieved. Based on piezoelectric effect, a non-optical detection for shear force of a straight tip and normal force of a bent tip was realized. By full digital feedback control, the separation between tip and sample was maintained within 10 nm. Especially, the control distance in normal force detection was more than 40 tim, the double in the case of shear-force detection. And the other techniques in NSOM such as anti-vibration, automatically coarse approach, PZT single tube scanner and its electronics, control program, data acquisition and image processing were accomplished. Finally, an experimental transmission NSOM with the resolution of less than 125 nm was set up. Then, we established a transmission-mode picosecond- and nanosecond- resolved NSOM setup, using a home-made picosecond infrad streak camera and photo multiplier tube (PMT) respectively. Coupled a ultrashort laser pulses into the fiber probe to serve as near-field optical excitation and acquired their response optical pulse, images of each moment that happened in a few picoseconds or nanoseconds could be obtained only once. The time broaden properties for a 50 ps pulse through a fiber probe were measured by streak camera. To demonstrate the performance of the experimental setup, we employed nanosecond-resolved NSOM to detect and image the infrad stimulated luminescence of a powder CaS:Eu,Sm, a kind of infrad upconversion materials. The local distributions of the powder were mapped out by their falltime constants in near-field optics. In addition, we calculated the field distribution from a fiber probe with a certain cone angle but different diameters using a two-dimensional finite difference time domain (FDTD) computational analysis. The relationships between their spot size, intensity and working distance were given. The results described a guideline to the application of NSOM.