  We report that flat substrates such as glass coverslips with surface
roughness well below 0.5 nm feature notable speckle patterns when observed with
high-sensitivity interference microscopy. We uncover that these speckle
patterns unambiguously originate from the subnanometer surface undulations, and
develop an intuitive model to illustrate how subnanometer non-resonant
dielectric features could generate pronounced interference contrast in the far
field. We introduce the concept of optical fingerprint for the deterministic
speckle pattern associated with a particular substrate surface area and
intentionally enhance the speckle amplitudes for potential applications. We
demonstrate such optical fingerprints can be leveraged for reproducible
position identification and marker-free lateral displacement detection with an
experimental precision of 0.22 nm. The reproducible position identification
allows us to detect new nanoscopic features developed during laborious
processes performed outside of the microscope. The demonstrated capability for
ultrasensitive displacement detection may find applications in the
semiconductor industry and super-resolution optical microscopy.
