Array detector for the atomic force microscope

We present a method for measuring the deflection of the optical beam in an atomic force microscope (AFM) that yields an increased signal-to-noise ratio, compared to the conventional two-segment detection. This increase is achieved by distributing the optical power from the beam across an array of photodetector segments and splitting it into multiple channels. Each channel has an adjustable gain factor that is set dynamically to weigh the contribution from each channel. We find a mathematical condition for the gain factors that allows detection of cantilever deflections with maximum signal-to-noise ratio and demonstrate this for the case of a 12-mu m-long cantilever in an AFM for small cantilevers. (C) 2000 American Institute of Physics. [S0003-6951(00)05024-5].