Piezoresistive Cantilevers as Optical Sensors for Scanning Near--Field Optical Microscopy

To combine the high lateral resolution of AFM with near--field optical measurements, we propose the use of microfabricated piezoresistive cantilevers as miniaturized photosensitive elements. The silicon--based sensors consist of a $p$-doped piezoresistive path, which also includes the tip. The resistance of this path can {\sl be changed either by pressure on the lever or by light}. For combined optical and topographical measurements, an evanescent field is created. Because the AFM tip is the only part of the cantilever that is exposed to the evanescent field, the tip can be used as a near--field optical probe. It is possible to extract the exponential decay of the evanescent field from combined force/optical measurements. To decouple optical and topographical information, the intensity of the evanescent field is modulated and the optical signal is measured with the lock-in technique.