P.-K. Wei

DIRECT MEASUREMENTS OF THE TRUE VIBRATIONAL AMPLITUDES IN SHEAR FORCE MICROSCOPY

A new method to measure the tip’s true vibrational amplitude in the shear force microscopy is demonstrated. The measurements are based on modifications of the beam diffraction method. In addition to the dither vibration, the equilibrium position of the tip is set to move along the direction of the vibration. The ratio of ac amplitude to the derivative of dc amplitude gives the tip’s vibration. The sensitivity of this method is determined by the size of the laser focal spot times the ratio of the ac to dc signals of the diffracted light.

Vibration dynamics of tapered optical fiber probes

The motion of tapered fiber probes was studied both theoretically and experimentally. A continuum-mechanical model, including both the intrinsic and the external loss, is proposed to account for the vibration dynamics of the probe. The intrinsic loss was found to be the dominant damping factor experimentally. Analytical solutions based on a realistic probe geometry were obtained for the model in the presence of intrinsic loss. The results are compared with the measured overall motions of the tapered probe. The calculations agree well with the experimental results.

The inhomogeneity in conjugated polymer blend films

Conjugated polymer blend films are studied by time-resolved fluorescence and near-field scanning optical microscopy. Phase separations were directly observed by NSOM. The domains are found to be composed of different fractions of the two constituent polymers. The composition of the domains and the domain structure alter as a function of blend ratio. Surface roughness and optical contrast exhibit similar trend at the change of blend ratio. At 4 : 1 ratio, both have higher contrasts. For low PdOPV ratio, the two polymers are mixed homogeneously. As the concentration of PdOPV increases, condensed phase of PdOPV is formed.

Nanometer scale mixing homogeneity in light emitting polymer blend thin films

Phase separation in polymer blend thin films is directly observed by near-field scanning optical microscopy. Time-resolved fluorescence spectra indicate that the phase domains are composed of different fractions of the two constituent polymers. The mixing homogeneity was found to vary with the blending ratio.

Two-dimensional near-field intensity distribution of tapered fiber probes

The near-field intensity distribution perpendicular to the light-propagation direction was measured by photochemical processes on conjugated-polymer thin films. The shape of the distribution is elliptical, with the long axis along the direction of the incident polarization. The results are compared with calculations based on the realistic tapered probe geometry. The asymmetry distribution is due to the simultaneous presence of horizontal and vertical electric fields in the near field.

Phase separation in polyaniline with near-field scanning optical microscopy

We report the studies of conjugated polymers, polyaniline thin films, with a near-field scanning optical microscope. Because of the absorption variation in different oxidation states, transmission-mode near-field scanning optical microscope images were employed to map out the distribution of the oxidation states on a submicrometer scale. When the near-field wavelength is varied (between 632.8 and 543.5 nm), the phase separation between the oxidized and the reduced repeated units, with domain sizes on a nanometer-length scale, is observed.

DUAL-OPTICAL-MODE NEAR-FIELD SCANNING OPTICAL MICROSCOPY

The simultaneous operation of near-field scanning optical microscopy (NSOM) in reflection and transmission modes is demonstrated. In the transmission mode, a low-noise, large-area silicon photodetector was mounted between the piezoelectric transducer scanning stage and the sample. In the reflection mode, either a photomultiplier tube or two large-area silicon detectors was used for signal collection. The reflection-mode setup consisting of two silicon detectors provides a large numerical aperture of 0.9 as well as symmetrical detection of emitting photons. The dielectric thin films and the light-emitting polymers were used to demonstrate the capability of these two modes of NSOM. A comparison between these two different setups is also presented.

Polarization dependence of light intensity distribution from nanometer metallic slits

In this work, the near-field and far-field electric magnetic (EM) wave distributions of metallic slits was observed using tapered fiber probe and modelled using finite difference time domain (FDTD) computer simulations. The EM wave field distribution from rectangular slits with widths 100 nm, 300 nm, and 500 nm was mapped with excitation wavelength /spl lambda/ = 532 nm. /spl lambda//2 can be considered a characteristic length for the problem. From the experiments and FDTD simulation, E/sub /spl perp// wave is found to be governed by the surface plasmon wave (SPW) which results in ill near field pattern and cannot be confined in the slit. On the contrary, E/sub /spl par// wave has a well-shaped field distribution and can be confined in the slit.

Method to increase brightness of the tapered fiber probe for scanning near-field optical microscope

We present a new method to improve the brightness of tapered fiber probes for near-field scanning optical microscope. The new probes are fabricated by adding high refractive index materials onto the pulled tapered fiber tips before coated with metal. With a tip size of 100 nm, the far-field optical power of the new tapered probe which has 25 nm thickness of zinc sulfide on tip end is about 5 times larger than the same sized traditional fiber probe.

Conjugated polymer studies by near-field scanning optical microscope

We have used the near-field scanning optical microscope (NSOM) to study the inhomogeneity as well as initiate photochemical processes in conjugated polymer films. A simple transmission-mode NSOM is constructed for these studies. A low noise, large area Si photo-detector is mounted directly between the PZT scanning stage and the sample. This method provides a simple way to covert the commercial AFM/STM scanning stage to a near-field optical microscope.