Abstract
We report on the first experimental demonstration of the suppression of spontaneous Raman scattering via ground state depletion. The concept of Raman suppression can be used to achieve sub-diffraction-limited resolution in label-free microscopy by exploiting spatially selective signal suppression when imaging a sample with a combination of Gaussian- and donut-shaped beams and reconstructing a resolution-enhanced image from this data. Using a nanosecond pulsed laser source with an emission wavelength of 355 nm, the ground state of tris(bipyridine)ruthenium(II) molecules solved in acetonitrile was depleted and the spontaneous Raman scattering at 355 nm suppressed by nearly 50 %. Based on spectroscopic data retrieved from our experiment, we modeled the Raman image of a scattering center in order to demonstrate the applicability of this effect for superresolution Raman microscopy.
Highlights
Achieving sub-diffraction-limited resolution is a highly important goal for the analysis of samples with optical microscopy in the life and material sciences
Restricted to fluorescence microscopy based on fluorophore labeling, several methods such as stimulated emission depletion (STED) [1], stochastic optical reconstruction microscopy (STORM) [2], and photoactivated localization microscopy (PALM) [3] were developed in the past few years
The effect could be increased by utilizing the simultaneous population of more than one excited state at the same time, which is an idea that was numerically investigated for ground state depletion (GSD) coherent anti-Stokes Raman scattering (CARS) microscopy in the past, with the result, that by using two instead of one excited state, the minimally achievable ground state population could be lowered from a half to a third [12]
Summary
Achieving sub-diffraction-limited resolution is a highly important goal for the analysis of samples with optical microscopy in the life and material sciences. If an experimental evidence of Raman suppression via GSD could be obtained, this effect could be employed for a superresolution technique in the following way: when using a beam of a single wavelength that generates the Raman scattering signal while depleting the ground state of the sample, the detected Raman signal would show a saturation behavior with increasing pulse energy In this case, a resolution-enhanced image could be obtained by subtracting two images from each other: one acquired by scanning the sample with a donut-shaped beam and one acquired with a beam that is a combination of the donut-shaped beam together with a ten-fold lower intensity Gaussian beam. GSD was successfully used to enhance the resolution in fluorescence microscopy [16] and in transient absorption microscopy [17] its experimental applicability for the resolution enhancement in Raman microscopy yet remained short of a proof
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