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Laser Physics

Coherent anti-stokes raman scattering microscopy in the presence of electromagnetic confinement

Michele Marrocco

Published Jul 1, 2007• 10 citations
Publisher

Abstract

The optical response used in studies of coherent anti-Stokes Raman scattering (CARS) microscopy can be enhanced or suppressed by means of electromagnetic confinement. In this paper, an approach based on planar mirrors is presented. Because of the classical origin of such a nonlinear optical effect, the theory is developed within a fully classical framework and it turns out that the CARS signal can be modified in a fashion similar to what has been extensively investigated for equivalent alterations of incoherent signals understood in terms of cavity quantum electrodynamics.

DOI
10.1134/s1054660x07070067

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Coherent anti-Stokes Raman scattering (CARS) interferometry is used to deplete the anti-Stokes radiation emerging from a tightly focused spot. Near-to-complete depletion of the anti-Stokes radiation is obtained when a phase-controlled local oscillator field at the anti-Stokes frequency is out of phase with the induced CARS field in the focal volume. Unlike in traditional interferometry, this depleted energy is not spatially redistributed. A theoretical analysis shows that the energy loss in the anti-Stokes channel is accompanied by an energy gain in the pump and Stokes channels. Interferometric switching of anti-Stokes radiation may offer a route toward developing high-resolution CARS microscopy.

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arXiv:1701.00207•Jan 1, 2017•1 citation
The optical response used in studies of coherent anti-Stokes Raman scattering (CARS) microscopy can be enhanced or suppressed by means of electromagnetic confinement. In this paper, an approach based on planar mirrors is presented. Because of the classical origin of such a nonlinear optical effect, the theory is developed within a fully classical framework and it turns out that the CARS signal can be modified in a fashion similar to what has been extensively investigated for equivalent alterations of incoherent signals understood in terms of cavity quantum electrodynamics.
Coherent anti-Stokes Raman scattering (CARS) interferometry is used to deplete the anti-Stokes radiation emerging from a tightly focused spot. Near-to-complete depletion of the anti-Stokes radiation is obtained when a phase-controlled local oscillator field at the anti-Stokes frequency is out of phase with the induced CARS field in the focal volume. Unlike in traditional interferometry, this depleted energy is not spatially redistributed. A theoretical analysis shows that the energy loss in the anti-Stokes channel is accompanied by an energy gain in the pump and Stokes channels. Interferometric switching of anti-Stokes radiation may offer a route toward developing high-resolution CARS microscopy.
In theoretical investigations, we review several nonlinear optical approaches towards subdiffraction-limited resolution in label-free imaging via coherent anti-Stokes Raman scattering (CARS). Using a density matrix model and numerical integration, we investigate various level schemes and combinations of the light fields that induce CARS along with additional control laser fields. As the key to techniques that gain far-field resolution below the diffraction limit, we identify the inhibition of the buildup of vibrational molecular coherence via saturation or depletion of population (Beeker et al. Opt Express 17:22632–22638, 2009) or the generation of Stark broadening and spatially dependent Rabi sideband generation (Beeker et al. Phys Rev A 81(1), 2010). Depending on the coherence and population decay rates offered by a particular molecular energy level scheme, we identify various different regimes. In the first case, where an additional state (called the control state) and a vibrational state are able to rapidly exchange population via incoherent processes, a prepopulation of the upper vibrational state inhibits the buildup of vibrational coherence. With increasing control laser intensity, this suppresses CARS emission via an incoherent, saturation type of nonlinear process. Using an intense, donut-shaped control laser beam, similar to stimulated emission depletion (STED) microscopy, this can suppress CARS emission from all sample locations except within a subdiffraction-sized range around the central node. Scanning the control beams across the sample provides subdiffraction-limited resolution imaging. An alternative, which does not require a rapid exchange of population with the control state, applies a control beam that only partially depletes the vibrational ground state. Thereby, a CARS point spread function containing a subdiffraction-limited component is generated. Subdiffraction images can then be retrieved through deconvolution. Further approaches are based on the coherent, nonlinear, resonant response of the sample. In this case, CARS signal depletion by Stark splitting of the weakly populated upper vibrational state or the observation of spatially dependent Rabi oscillation may increase the resolution beyond the diffraction limit.
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.
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 a scheme in which backward coherent anti-Stokes Raman scattering (CARS) is significantly enhanced by using slow light. Specifically, we reduce the group velocity of the Stokes excitation pulse by introducing a coupling laser that causes electromagnetically induced transparency (EIT). When the Stokes pulse has a spatial length shorter than the CARS wavelength, the backward CARS emission is significantly enhanced. We also investigated the possibility of applying this scheme as a CARS lidar with O2 or N2 as the EIT medium. We found that if nanosecond laser with large pulse energy (>1 J) and a telescope with large aperture (~10 m) are equipped in the lidar system, a CARS lidar could become much more sensitive than a spontaneous Raman lidar.