A sensor apparatus and method includes a sensor head with at least two surfaces separated by a gap. One surface is mechanically fixed, a second surface is free to move and deflections of the second surface relative to the first surface are monitored by optical interferometry. In one embodiment, an optical fiber is used to direct light from a light source to the sensor and collect light reflected by the sensor. In alternate embodiments the sensor apparatus includes integrated optical elements, free-space optics, and direct laser-diode sensing. In operation, interaction of molecules or other objects in the sample with the second surface is detected as a change in amplitude and/or phase of deflection the second surface in response to an applied driving signal. A layer of binding molecules may be immobilized on the second surface and this surface exposed to a sample. The invention includes a method for detecting an analyte in a sample, including detecting the presence of analyte, the amount of analyte or the rate of association and/or dissociation of the analyte with a binding partner.