An apparatus for assessing topology of a surface of a target including an optical source for generating a probe laser beam. The apparatus also includes a means for scanning the probe laser beam across at least a portion of the surface of the target and a beamsplitter for redirecting a return signal toward the means for detecting the return signal in a substantially quadrature condition. A quadrature interferometric method for determining the presence or absence of a target analyte in a sample comprising a laser probe beam having a wavelength .lamda. and a waist w.sub.o to probe at least a portion of a substrate having a reflecting surface that includes at least a first region having a layer of recognition molecules specific to the target analyte and a second region that does not include a layer of recognition molecules specific to the target analyte.