Hong-Ou-Mandel (HOM) interferometry, manifesting as photon bunching at a beamsplitter, can provide high precision optical path length sensing yet is robust to phase noise and immune to phase-wrapping. In this work we demonstrate how the sensitivity of HOM based imaging can be enhanced using dual wavelength illumination whilst maintaining its high level of robustness. We observe a beating in the second order correlation function due to the interference between the two wavelengths bands with a periodicity determined by the wavelength separation. These small-scale features can be used to precisely determine optical path length at the nanometre scale whilst a coarse (micron scale) estimation can be obtained by measuring only one of the bands.

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Ashley Lyons, Khaled Kassem, Daniele Faccio, and Jack Radford, "Dual wavelength Hong-Ou-Mandel imaging," Proc. SPIE PC12852, Quantitative Phase Imaging X, PC1285201 (Presented at SPIE BiOS: January 27, 2024; Published: 13 March 2024); https://doi.org/10.1117/12.3009777.dee29481-364b-ee11-a99c-00505691c5e1.