Dynamic heterodyned polarization imaging: an electrostatic scanning probe technique for studying polarization dynamics in materials

Electric fields in dielectric materials produce dipoles related to the polarizability of the material. In this paper we present a technique that measures the polarization near a surface. The polarization is induced by a modulated signal applied to the conducting probe used for detection. The polarization dipoles in the surface layers of the material generate an electrostatic attraction between the probe and the dielectric material. Using techniques common in non-contact force microscopy these forces can easily be sensed. Remarkably, this measurement technique can be extended to frequencies well above the mechanical resonant frequency of the probe cantilever by utilizing amplitude modulation heterodyning. By rastering the probe over the surface, an image of the dielectric properties of the surface can be produced. We expect this technique to be useable up to frequencies of at least 20 GHz and time resolution of less than 100 ps. We present calculations of the forces generated, assuming simple probe geometries, and also thermal noise that compare favourably with experimental results. The technique has been used in the stroboscopic imaging of an operating 434 MHz surface acoustic wave device. The experiments already completed demonstrate that this technique may be employed to produce images that display the local polarizability of materials at a given frequency. In more detailed studies, regions of interest can be imaged repeatedly, with different frequencies used to produce each image.