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Development of an Interferometer with Subpicometer Resolution for dc and Low Frequency Displacement Measurement

NIST Nanomechanical Properties Group and Manufacturing Metrology Division researchers have developed a fiber-optic interferometer optimized for best performance in the frequency range from dc to 1 kHz, with displacement response linear to within 1% over a range of ±25 nm, and noise-limited resolution of 2 pm. The interferometer makes possible measurement of mechanical properties arising in single-atom contacts and the direct observation of atomic bonding interactions, and thus provides opportunities for development of “intrinsic” nano- and pico-scale force standards, traceable to the International System of Units via the wavelength of light, and for direct comparison of computational physics results with experiments.

When dealing with single atom contact mechanics, such as those realized by the interaction of an atomically sharp probe tip with an atomically flat surface, relative displacement of the tip with respect to the surface must be monitored with resolution well below the lattice spacing of the atoms, < 0.1 nm. Similarly, the interaction forces must be resolved to within at least a few percent of the rupture force of an atomic bond, typically < 2 nN. The requirements for atomic scale length metrology at such contacts are clear, and point to the need for an accurate sensor capable of recording full-scale relative displacements on the order of 10 nm with sub-percent level nonlinearity and a noise floor of picometers, in a bandwidth from dc to hundreds of hertz. The interferometer achieves these requirements using a tunable infrared laser source with nominal 1550 nm wavelength and great amplitude and wavelength stability, low spontaneous self-emission noise, high sideband suppression, and a coherence control feature that broadens the laser linewidth and dramatically lowers the low-frequency noise in the system. The amplitude stability of the source, combined with the use of specially manufactured “bend-insensitive” fiber and all-spliced fiber construction, results in a robust homodyne interferometer system that achieves a resolution of 40 fm Hz−1/2 above 20 Hz and approaches the shot-noise-limit of 20 fm Hz−1/2 at 1 kHz.

A description of the device and its performance has been published: “A fiber-optic interferometer with subpicometer resolution for dc and low-frequency displacement measurement,” D.T. Smith, J.R. Pratt, and L.P. Howard, Rev. Sci. Instruments 80 (2009) 035105.

Contact: Doug Smith, x5768