Da. Higgins et al., POLARIZATION-MODULATION NEAR-FIELD SCANNING OPTICAL MICROSCOPY OF MESOSTRUCTURED MATERIALS, Journal of physical chemistry, 100(32), 1996, pp. 13794-13803
A new polarization-modulation near-field scanning optical microscope (
PM-NSOM) is described and demonstrated. Linearly polarized light is ro
tated through an angle of 180 degrees at a frequency of 2 kHz with an
electro-optic modulator and quarter-wave plate combination and is then
coupled into the near-field optical-fiber probe. The sample is positi
oned in the near field of the probe and the near-field Light coupled t
hrough 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 o
f the magnitude of the anisotropy and (ii) its phase, which yields the
characteristic direction of the anisotropy. For strongly absorbing di
chroic samples the amplitude and phase of the modulated signal give th
e spatially resolved anisotropic extinction coefficient and transition
dipole orientation, respectively. A more complex contrast mechanism i
s proposed for nonabsorbing samples, involving the effects of bath sam
ple birefringence and anisotropic spatial variations in the refractive
index. Nanoscopic characterization of optical materials with the PM-N
SOM is demonstrated through resonant imaging of dichroic single crysta
ls of rhodamine 110. Its application to nonabsorbing materials is also
demonstrated through nonresonant imaging of the rhodamine crystals, a
s well as through imaging of defects in fused-quartz cover slips. With
PM-NSOM, material defects such as cracks and pits are imaged with hig
h sensitivity, shot-noise-limited signal-to-noise, and better than 100
nm spatial resolution.