Longitudinal Light Profile Microscopy: A New Method for Seeing below the Surfaces of Thin-Film Materials

Abstract

Longitudinal light profile microscopy (LLPM) is introduced in this work as a new optical method for depth profiling the properties of thin-film materials. The method uses irradiation of an optically polished cross section of a prepared thin film sample, by a laser beam propagating along the depth (longitudinal) axis of the material. An observation microscope, aligned along an orthogonal axis to the sample cross section, transfers an image of the light profile propagating along the material's longitudinal (depth) axis to a recording camera. While depth attenuation of the optical beam in the sample is dominated by light absorption, and turbid scatter (which our analysis neglects), light profile images recovered by the microscope use contrast mechanisms based on luminescence and elastic or inelastic light scatter. Blur contributions to the images arising from axial thickness of the light profile “object” are shown to be minimal in our microscope. Our experimental setup, moreover, is constructed from relatively inexpensive, easily available components. A number of different image contrast mechanisms, including luminescence and elastic scattering contrast, were demonstrated on materials with known optical properties, including continuous media and laminates. The sample dimensions and depth-dependent image features were directly observable and unambiguous to identify. Images recovered on the basis of elastic scattering showed unusual contrast for optical interfaces in materials which were transparent at the analysis wavelength. The method holds promise for providing a plethora of new depth-resolved imaging mechanisms.