H. Shiku et al., Noncontact near-field scanning optical microscopy imaging using an interferometric optical feedback mechanism, LANGMUIR, 15(6), 1999, pp. 2162-2168
An interferometric optical feedback mechanism is explored for tip-sample di
stance control in near-field scanning optical microscopy. An optical signal
, based on the interference between the light exiting the tip aperture and
that reflecting off the sample surface, is used to implement a feedback sch
eme to regulate tip-sample distance. The noncontact nature of this feedback
mechanism may provide greater flexibility in imaging soft or fragile sampl
es. To characterize the performance of the optical feedback mechanism, imag
es are analyzed of a calibration standard, fluorescently doped lipid monola
yers, latex spheres, and fixed cells. Images taken of these samples using o
ptical feedback and the standard tapping-mode feedback are comparable in qu
ality. These samples also demonstrate the ability of optical feedback to fo
llow both large and small height changes and accurately reflect the sample
topography. In a nonscanning mode, the interferometric signal can be used t
o noninvasively probe small dynamic height changes of a sample? with nanome
tric spatial resolution. Using a piezo ceramic bimorph to simulate sample m
ovement, we show that nanometric height changes can be detected with millis
econd time resolution. This may provide a unique way to probe protein confo
rmational changes free of tip-sample interactions.