Polarization-Modulation Near-Field Scanning Optical Microscopy of Mesostructured Materials

Abstract

A new polarization-modulation near-field scanning optical microscope (PM-NSOM) is described and demonstrated. Linearly polarized light is rotated through an angle of 180°at a frequency of 2 kHz with an electro-optic modulator and quarter-wave plate combination and is then coupled into the near-field opticalfiber probe. The sample is positioned in the near field of the probe and the near-field light coupled through the sample to the far-field is detected. A 2 kHz modulation is observed in the intensity of the light reaching the detector when the probe is positioned over an optically anisotropic region of the sample. The modulated signal is shown to result from anisotropic absorptions in the sample and from polarization-dependent nearfield to far-field coupling. With lock-in detection of the signal, two optical images are recorded simultaneously as (i) the amplitude, which gives a measure of the magnitude of the anisotropy and (ii) its phase, which yields the characteristic direction of the anisotropy. For strongly absorbing dichroic samples the amplitude and phase of the modulated signal give the spatially resolved anisotropic extinction coefficient and transition dipole orientation, respectively. A more complex contrast mechanism is proposed for nonabsorbing samples, involving the effects of both sample birefringence and anisotropic spatial variations in the refractive index. Nanoscopic characterization of optical materials with the PM-NSOM is demonstrated through resonant imaging of dichroic single crystals of rhodamine 110. Its application to nonabsorbing materials is also demonstrated through nonresonant imaging of the rhodamine crystals, as well as through imaging of defects in fusedquartz cover slips. With PM-NSOM, material defects such as cracks and pits are imaged with high sensitivity, shot-noise-limited signal-to-noise, and better than 100 nm spatial resolution. IntroductionNear-field scanning optical microscopy (NSOM) is a rapidly evolving, high-resolution imaging technique that provides subdiffraction-limited spatial resolution in visible-light images of materials. 1-21 The tremendous improvement in the spatial resolution of NSOM images over those recorded by conventional far-field techniques is obtained via the use of a sub-wavelengthsized light source and by holding the sample within the near field of that light source (i.e., at a distance much less than the wavelength of light employed). The most common NSOM light source is a tapered, aluminum-coated, single-mode optical fiber. 6 The sample is raster-scanned at a constant distance beneath the NSOM probe, and images are collected one pixel at a time in a manner analogous to the other scanned-probe microscopies.Since visible light is employed as the imaging mechanism in NSOM, a number of different contrast mechanisms are available. Fluorescence represents by far the most sensitive and most easily interpreted contrast mechanism; however, the most general methods simply involve transmission of light through the sample. Contrast in transmission NSOM may ...

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“…For example, Figures 13d±f show a study of polarizationdependent NSOM transmissions of a sample of a collection of tens of nanometer±thick micrometer-length crystals of an organic dye. [120] The topographic image of the sample is shown in Figure 13d. Figures 13e and 13 f were recorded by a new technique called polarization modulation NSOM.…”

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