Noncontact near-field scanning optical microscopy imaging using an interferometric optical feedback mechanism

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.