Bessel beam CARS of axially structured samples

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.

Bessel-Beam Hyperspectral CARS Microscopy with Sparse Sampling: Enabling High-Content High-Throughput Label-Free Quantitative Chemical Imaging

“…Interestingly, their opposite use was discussed in ref. 24,25 to retrieve the axial structure of a laterally homogeneous sample from phase matching considerations and the directionality of the CARS emission. Most experimental implementations reported use a crystal phase shaper array to create the Bessel beam.…”

“…However, their implementation for high-throughput hyperspectral CARS microscopy has not yet been shown. Interestingly, their opposite use was discussed in ref. , to retrieve the axial structure of a laterally homogeneous sample from phase matching considerations and the directionality of the CARS emission.…”

“…This method is very flexible, as Bessel beams of different cone angle can be created, but is also rather lossy (60% transmission was reported at 840 nm), complex, costly, and requires active control. We have opted here for a simple solution using a single axicon (instead of two in refs and ) to create the Bessel beam, which allows for high transmission (>90% over a broad wavelength range of 650–1100 nm) and passive stability. Three simple elements (an aperture, an axicon, and a lens) are mounted on flip mounts, so that they can be easily switched in and out of the beam path, allowing access to both the Gaussian and Bessel beam excitation without realignment (see Figure a and Methods).…”

Volumetric chemical imaging by stimulated Raman projection microscopy and tomography

“…Several methods were reported to generate a Bessel beam 31 32 33 . Here we used two axicons and an objective lens to convert a Gaussian beam into a Bessel beam ( Fig.…”

Efficient second-harmonic imaging of collagen in histological slides using Bessel beam excitation

“…There is a need for images revealing both the fibrosis microstructure and its extent over scales of several millimeters. In this context, Bessel beam excitation, recently proposed for multiphoton microscopy applications 13 14 15 16 , appears as an interesting alternative to Gaussian beam excitation in SHG microscopy of tissue sections. Indeed, a focused Bessel beam can in principle provide a nonlinear probe volume with an axial extension encompassing the thickness and height heterogeneity of a histological section (i.e.…”