Method and apparatus for evanescent filed measuring of particle-solid separation

Evanescent wave scattering by a scanning probe in a scanning probe microscope is utilized to determine and monitor separation between a scanning probe and a sample. A laser light is totally internally reflected at the interface between a more optically dense (incident) medium and less optically dense (transmitting) medium, exciting a decaying evanescent field in the less optically dense medium. A scanning probe, such as a colloidal probe, is dipped into the evanescent field, which scatters off the scanning probe. The portion of the scattered field propagates back into the incident medium and is then detected by a detector. A dependency between the intensity of the scattered evanescent field and the separation between the probe and the incident medium was measured and used in determining the separation. This dependency of intensity is used to prepare images or maps of interfaces. A particular application of determining the separation between the probe and the sample in an atomic force microscope is disclosed.