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 nonresonant 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 superresolution optical microscopy.

Institute of Photonics and Electronics of the Czech Academy of Sciences Chaberská 1014 57 18251 Prague Czech Republic

School of Physics and Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Luoyu Road 1037 Wuhan 430074 People's Republic of China

School of Physics and Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Luoyu Road 1037 Wuhan 430074 People's Republic of China and Institute for Quantum Science and Engineering and Hubei Key Laboratory of Gravitation and Quantum Physics Huazhong University of Science and Technology Luoyu Road 1037 Wuhan 430074 People's Republic of China

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