Simultaneous microscopic imaging of thickness and refractive index of thin layers by heterodyne interferometric reflectometry (HiRef)

Abstract: The detection of spatial or temporal variations in very thin samples has important applications in the biological sciences. For example, cellular membranes exhibit changes in lipid composition and order, which in turn modulate their function in space and time. Simultaneous measurement of thickness and refractive index would be one way to observe these variations, yet doing it noninvasively remains an elusive goal. Here we present a microscopic-imaging technique to simultaneously measure the thickness and refractive index of thin layers in a spatially resolved manner using reflectometry. The heterodyne-detected interference between a light field reflected by the sample and a reference field allows measurement of the amplitude and phase of the reflected field and thus determination of the complex reflection coefficient. Comparing the results with the simulated reflection of a thin layer under coherent illumination of high numerical aperture by the microscope objective, the refractive index and thickness of the layer can be determined. We present results on a layer of polyvinylacetate (PVA) with a thickness of approximately 80~nm. These results have a precision better than 10\% in the thickness and better than 1\% in the refractive index and are consistent within error with measurements by quantitative differential interference contrast (qDIC) and literature values. We discuss the significance of these results, and the possibility of performing accurate measurements on nanometric layers. Notably, the shot-noise limit of the technique is below 0.5~nm in thickness and 0.0005 in refractive index for millisecond measurement times.