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

Nahmad-Rohen, Alexander, Regan, David, Borri, Paola and Langbein, Wolfgang 2021. Simultaneous microscopic imaging of thickness and refractive index of thin layers by heterodyne interferometric reflectometry (HiRef). Journal of Physics D: Applied Physics 55 (5) , 054001. 10.1088/1361-6463/ac22d4
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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. A coherent laser beam, focussed by a high-numerical-aperture microscope objective and reflected by the sample, is measured using its heterodyne interference with a reference in order to determine the amplitude and phase of the reflected field and thus the complex reflection coefficient. Comparing the results with the theoretically calculated reflection of a thin layer under coherent illumination of high numerical aperture by the microscope objective, the refractive index and thickness of the layer are determined. We present results on a layer of polyvinylacetate 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. The measured refractive index is consistent with literature values, and the measured thickness is close to measurements of the same sample by quantitative differential interference contrast. 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.

Item Type: Article
Date Type: Published Online
Status: Published
Schools: Physics and Astronomy
Publisher: IOP Publishing
ISSN: 0022-3727
Date of First Compliant Deposit: 22 November 2021
Date of Acceptance: 1 September 2021
Last Modified: 30 Nov 2021 06:04

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