Interferometry at the onset of total internal reflection

We report that nanometric axisymmetric surface deformations probed near the onset of total internal reflection produce a strongly anisotropic phase response. The local surface slope modifies the incidence angle, and the steep angular dependence of the Fresnel reflection phase near the critical angle converts these slope variations into measurable phase modulation. As a result, an axisymmetric bulge or dimple generates elliptical interference fringes aligned with the plane of incidence. When the deformation is sufficiently large, the local incidence angle falls below the critical angle, leading to a breakdown of total internal reflection and the appearance of a dark central region. Analytical relations linking the deformation profile, angular deviation, and the radius of the dark region are derived, enabling quantitative measurement of nanoscale surface deformation at fluid interfaces. This approach is useful for non-contact metrology of soft interfaces, thin-film mechanics, and biofluid diagnostics.