Suppression of resonance Raman scattering via ground state depletion towards sub-diffraction-limited label-free microscopy

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.

Density matrix study of ground state depletion towards sub-diffraction-limited spontaneous Raman scattering spectroscopy

“…We like to briefly recall how GSD can be applied to achieve a resolution enhancement in Raman scattering spectroscopy as proposed in Ref. 10. The key of this method is the irradiation of sample molecules with pulsed UV laser light, which generates a Raman scattering signal, while simultaneously depleting the ground state of the sample.…”

“…In another recent work, the depletion of the ground state of a molecule by an ultraviolet (UV) pulse was used for the suppression of spontaneous Raman scattering. 10 It was estimated that the observed suppression, if used in a STED-like Raman microscopy approach, should lead to a resolution enhancement by a factor of three. An even higher resolution enhancement should generally be possible using a higher pulse energy for stronger ground state depletion (GSD).…”

“…Here, we report on density matrix calculations on the suppression of spontaneous Raman scattering due to GSD in a level system based on the molecule tris(bipyridine) ruthenium(ii) (Ru(bpy) 2+ 3 ), which was employed in the experimental demonstration of GSD reported in Ref. 10. There, the spontaneous Raman scattering was suppressed by up to 50% via depletion of the ground state of the molecules using nanosecond UV laser pulses at a wavelength of 355 nm.…”

“…This is the reason for the extensively studied spectral differences between the ground and the excited state Raman scattering of Ru(bpy) 2+ 3 . 10,22,23 Both the ground and the excited state Raman spectrum feature various vibrational resonances. However, the GSD as well as the Raman scattering into each of the individual vibrational states should both be independent from the number of modeled vibrational states as long as those states itself are only populated to a small extent (in total up to approximately 10% in the presented calculations).…”

“…In order to determine appropriate values, calculations were performed with various different coherence lifetimes and compared to the experimentally determined Raman scattering suppression in Ru(bpy) 2+ 3 as reported in Ref. 10 (see Sec. IV D).…”