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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Two-Color Suppression of Stimulated Raman Scattering

Thomas Würthwein, Niels Irwin, Carsten Fallnich

Published Jun 1, 2019• 1 citation
Publisher

Abstract

Summary form only given. Sub-diffraction limited imaging schemes have become widely used in fluorescence microscopy [1]. Howaever, the need for labeling with fluorescent dyes remains a major downside of fluorescence microscopy. The size, availability, toxicity as well as photo-bleaching of the used dyes can complicate measurements [2]. In contrast, Raman imaging is inherently label-free. Unfortunately, the super-resolution schemes used in fluorescence microscopy are currently not transferable to Raman microscopy. This paper presents the development of a scheme for the suppression of coherent Raman scattering through the depletion of probe photons, inspired by the work of M. Cho et al. [3, 4], who showed the suppression of coherent Raman scattering using laser pulses at three different wavelengths and two Raman resonances. In our case, probe depletion is achieved through saturated stimulated Raman scattering in a three-beam setup with only two colors involved. Fig. 1 (a) describes the working principle: in the unsuppressed case, the combination of pump and probe pulse induces stimulated Raman scattering at the Raman resonances, leading to stimulated Raman loss (SRL) of the probe and and stimulated Raman gain (SRG) of the pump. The Raman spectrum then results from the difference between two spectra, recorded with pump on and off, respectively. In the suppressed case a second strong pump pulse, working as a depletion pulse, depletes the probe pulse via stimulated Raman scattering (SRLD) and induces a shortage of the probe photons at the Raman resonances, saturating the stimulated Raman scattering. Thus, only a small amount of additional SRLSup is induced by the original pump. If SRL is detected in addition to the spectral change, induced by the depletion pulse, the SRL will be reduced resulting in SRLSup<; SRL.

DOI
10.1109/cleoe-eqec.2019.8872415

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Summary form only given. Sub-diffraction limited imaging schemes have become widely used in fluorescence microscopy [1]. Howaever, the need for labeling with fluorescent dyes remains a major downside of fluorescence microscopy. The size, availability, toxicity as well as photo-bleaching of the used dyes can complicate measurements [2]. In contrast, Raman imaging is inherently label-free. Unfortunately, the super-resolution schemes used in fluorescence microscopy are currently not transferable to Raman microscopy. This paper presents the development of a scheme for the suppression of coherent Raman scattering through the depletion of probe photons, inspired by the work of M. Cho et al. [3, 4], who showed the suppression of coherent Raman scattering using laser pulses at three different wavelengths and two Raman resonances. In our case, probe depletion is achieved through saturated stimulated Raman scattering in a three-beam setup with only two colors involved. Fig. 1 (a) describes the working principle: in the unsuppressed case, the combination of pump and probe pulse induces stimulated Raman scattering at the Raman resonances, leading to stimulated Raman loss (SRL) of the probe and and stimulated Raman gain (SRG) of the pump. The Raman spectrum then results from the difference between two spectra, recorded with pump on and off, respectively. In the suppressed case a second strong pump pulse, working as a depletion pulse, depletes the probe pulse via stimulated Raman scattering (SRLD) and induces a shortage of the probe photons at the Raman resonances, saturating the stimulated Raman scattering. Thus, only a small amount of additional SRLSup is induced by the original pump. If SRL is detected in addition to the spectral change, induced by the depletion pulse, the SRL will be reduced resulting in SRLSup<; SRL.
Two-beam stimulated Raman scattering with pump and Stokes beams is manifest in both the Raman loss of the pump beam and the Raman gain of the Stokes beam, and it has been used in various label-free bioimaging applications. Here, a three-beam stimulated Raman scattering that involves pump, Stokes, and depletion beams is considered, where two stimulated Raman gain-loss processes are deliberately made to compete with each other. It is shown that the three-beam Raman scattering process can be described by coupled differential equations for the increased numbers of Stokes and depletion beam photons. From approximate solutions of the coupled differential equations and numerical calculation results, it is shown that a highly efficient suppression of the Stokes Raman gain is possible by using an intense depletion beam whose frequency difference from that of the pump beam is identical to another acceptor Raman mode frequency. I anticipate that the present work will provide a theoretical framework for super-resolution stimulated Raman scattering microscopy.
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We theoretically investigate a scheme in which stimulated Raman scattering (SRS) can be suppressed by coherently controlling the coupling between molecular states. In conventional SRS, two laser beams at different frequencies interact resonantly with molecular vibration to induce a gain and a loss for the two beams, respectively. In our scheme, a third beam is introduced to couple the vibrational state to another coupling state. As a result, SRS is suppressed in a way analogous to electromagnetically induced transparency. We calculated the SRS signal analytically by the density matrix approach, and investigated the feasibility of this scheme for real molecular imaging. In SRS microscopy, a donut-shaped coupling laser can be used to suppress the SRS signal from the rim part of the focal spot, leading to super-resolution. Based on our numerical studies, the lateral resolution starts to be enhanced when the coupling laser intensity exceeds 0.1TW/cm20.1\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/\mathrm{c}{\mathrm{m}}^{2}0.1TW/cm2 at picosecond pulse duration.
We present a theoretical investigation of stimulated emission pumping to achieve sub-diffraction-limited spatial resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. A pair of control light fields is used to prepopulate the Raman state involved in the CARS process prior to the CARS signal generation. Efficient prepopulation is achieved by employing a path via an electronic or vibrational state. Thereby, the buildup of a coherence between the ground and Raman state during irradiation by the light fields for CARS signal generation is prevented, resulting in the suppression of the CARS intensity by more than 99.8%. Two-dimensional spatial excitation profiles have been calculated using donut-shaped spatial profiles for the control light fields, thus, an intensity-dependent narrowing of the CARS excitation profiles below the diffraction limit. Using computer-generated test images we demonstrate a resolution beyond the diffraction limit for CARS microscopy, which is scalable by the control light field intensity, similar to stimulated emission depletion microscopy.
We theoretically investigate ground-state depletion for subdiffraction-limited spatial resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We propose a scheme based on ground-state depopulation, which is achieved via a control laser light field incident prior to the CARS excitation light fields. This ground-state depopulation results in a reduced CARS signal generation. With an appropriate choice of spatial beam profiles, the scheme can be used to increase the spatial resolution. Based on the density matrix formalism we calculate the CARS signal generation and find a CARS signal suppression by 75% due to ground-state depletion with a single control light field and by using two control light fields the CARS signal suppression can be enhanced to 94%. Additional control light fields will enhance the CARS suppression even further. In case of a single control light field we calculate resulting CARS images using a computer-generated test image including quantum and detector noise and show that the background from the limited CARS suppression can be removed by calculating difference images, yielding subdiffraction-limited resolution where the resolution achievable depends only on the intensity used.
Stimulated Raman scattering (SRS) microscopy is a powerful technique that allows for the visualization of molecular vibrational images for label-free imaging, metabolic imaging, and supermultiplex imaging. However, its sensitivity is mainly limited by the shot noise of laser pulses, and long acquisition times are sometimes required to detect weak SRS signals hidden by the shot noise. To overcome this limitation, quantum-enhanced (QE-) SRS microscopy has been demonstrated, while the low optical power of squeezed pulses limits the sensitivity. In this work, we present QE-SRS microscopy using quantum-enhanced balanced detection (QE-BD) scheme, where the sensitivity of balanced detection SRS microscope is enhanced by injecting squeezed vacuum, allowing for QE-SRS imaging with high power SRS pump pulses (typically several tens of milliwatt), while balanced detection causes 3 dB drawback in the signal-to-noise ratio. We experimentally demonstrate QE-SRS imaging with 2.6 dB noise reduction compared with shot-noise-limited balanced detection SRS. We also demonstrate hyperspectral QE-SRS imaging by fast wavelength tuning of Stokes pulses. These results show the potential feasibility of high-power QE-SRS whose sensitivity is beyond that of classical shot-noise-limited SRS microscopes.