An apparatus may comprise an optical detector configured to detect an optical beam reflected from a cantilever. The apparatus may further comprise an optical fiber probe suspended from the cantilever and a piezotube configured to move a sample substance in proximity to the optical fiber probe. The cantilever may be configured to deflect in response to an interfacial force between the sample substance and the optical fiber probe. The apparatus may further comprise a feedback controller communicatively coupled to the optical detector and a semiconductive circuit element abutting the cantilever. In response to detecting movement of the optical beam reflected from the cantilever, the feedback controller may apply a voltage to the semiconductive circuit element, which may reduce deflection of the cantilever. The voltage applied by the feedback controller may indicate a strength of the interfacial force between the sample substance and the optical fiber probe.