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

Abstract

The suppression of Raman scattering is of high interest for the achievement of sub-diffraction-limited resolution in Raman scattering spectroscopy and microscopy. We present density matrix calculations of the suppression of spontaneous Raman scattering via ground state depletion in a level system based on the molecule tris(bipyridine)ruthenium(ii). This particular molecule has been earlier used for an experimental demonstration of the suppression of spontaneous Raman scattering, allowing us to successfully verify the validity of our numerical calculations by a comparison to the experimental results. We investigate the required level of detail of the molecule model as well as the influence of certain molecule and pulse parameters on the Raman scattering suppression. It was found that pulses with a duration longer than the lifetime of the electronic states allow for a high suppression of the Raman scattering. Pulses shorter than the coherence lifetime between the ground state and electronic states lead to a similarly high suppression but also accomplish the suppression with more than one order of magnitude lower pulse energy fluence. Additionally, using a laser wavelength that is in resonance with one of the electronic transitions of the sample should allow suppressing the Raman scattering with four to six orders of magnitude lower pulse energy fluence.

Saturated Raman scattering for sub-diffraction-limited imaging

“…However, the energy difference of the three used wavelength-components had to match the energy difference of two molecular vibrations in the sample under investigation. In contrast to ground state depletion (GSD), [22][23][24][25] the saturation of the FSRS process enabled the suppression of the Raman signal using pulse energies which are at least one order of magnitude lower.…”

“…Ground state depletion (GSD) to a higher electronic state can be accomplished by using a laser at 355 nm to deplete the electrons in the molecular ground state and to excite the (near-) resonanceenhanced Raman signal at the same time. [22][23][24][25] For laser pulse energies in the order of 1.4 μJ, a reduction of the ground state Raman signal by up to 50% could be shown, but the experiment was fundamentally limited using only a single laser for both Raman scattering and GSD. 24 The Journal of Chemical Physics ARTICLE scitation.org/journal/jcp Alternatively, for option (ii), the SRS and CARS signal can be suppressed by using an additional competing SRS process, which leads to photon depletion of the pump beam.…”

Far-Field Super-Resolution Vibrational Spectroscopy

“…Another potential approach to resolution improvement is using a spatially shaped beam to selectively suppress signal generation in a defined area. Recent advances have included both theoretical descriptions − and experimental implementations , of approaches to obtain subdiffraction-limit resolution with Raman microscopy. They all propose the use of a doughnut-shaped beam to suppress Raman signal (Figure e), which is inspired by the fluorescence-based STED method …”

“…Two scans of the region of interest are taken using different pulse combinations, and their difference results in a subdiffraction image (Figure ). Although imaging results have yet to be published with this technique, simulations illustrating imaging applications are promising. , However, the ground state depletion process depends linearly on laser power and it is challenging to achieve excited state populations above 50%, limiting resolution improvements to at most a factor of 2. It is also worth noting that this technique is not suitable for all systems as the suppression efficiency and thus possible resolution enhancement depends strongly on molecular properties such as excited state lifetime and absorption cross section.…”

Stimulated Raman imaging below the diffraction limit with a MHz laser

“…[18,19] To avoid the use of fluorophores, label-free superresolution techniques have been the focus of significant research efforts. [4,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Techniques based on Raman spectroscopy have drawn interest due to the fact that it probes vibrational modes that are inherent in a given molecule. Even if an analyte of interest has a Raman spectrum that makes it difficult to isolate in a matrix containing similar vibrations, a very small vibrational tag can be used to provide contrast.…”