Abstract
We report about a Bessel beam CARS approach for axial profiling of multi-layer structures. This study presents an experimental implementation for the generation of CARS by Bessel beam excitation using only passive optical elements. Furthermore, an analytical expression is provided describing the generated anti-Stokes field by a homogeneous sample. Based on the concept of coherent transfer functions, the underling resolving power of axially structured geometries is investigated. It is found that through the non-linearity of the CARS process in combination with the folded illumination geometry continuous phase-matching is achieved starting from homogeneous samples up to spatial sample frequencies at twice of the pumping electric field wave. The experimental and analytical findings are modeled by the implementation of the Debye Integral and scalar Green function approach. Finally, the goal of reconstructing an axially layered sample is demonstrated on the basis of the numerically simulated modulus and phase of the anti-Stokes far-field radiation pattern.
Highlights
Coherent anti-Stokes Raman scattering (CARS) microscopy is complicated due to the requirement for a second excitation source at a shifted wavelength and, more profoundly, by the coherent nature of the CARS process
The shown experimental implementation of pump Bessel beams operates without the requirement for high peak power laser sources and, in addition, requires only inexpensive passive optical elements
Taking the influence of the sample explicitly into account the phase-matching range of Bessel beams for a z-structured sample was investigated based on coherent transfer functions
Summary
CARS microscopy is complicated due to the requirement for a second excitation source at a shifted wavelength and, more profoundly, by the coherent nature of the CARS process. The present report addresses these issues under the confinement of isotropic, predominantly axial structured samples with negligible non-resonant background. Such requirements are met in reasonable approximation by biological and non-biological samples such as the layered structure of human skin or for layered dielectric media such as solar cells, respectively. The theoretically achievable axial resolution is visualized based on coherent transfer functions. Numerical results based on the Debye-Wolf integral and scalar Green function are compared to experimental data and the predicted axial resolution capacity is confirmed. As an outlook the possibility of a sample reconstruction from numerically generated far-field radiation data solving the inverse source scattering problem (ISCP) is exemplified
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