Abstract
We theoretically investigate a scheme to obtain sub-diffraction-limited resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We find using density matrix calculations that the rise of vibrational (Raman) coherence can be strongly suppressed, and thereby the emission of CARS signals can be significantly reduced, when pre-populating the corresponding vibrational state through an incoherent process. The effectiveness of pre-populating the vibrational state of interest is investigated by considering the excitation of a neighbouring vibrational (control) state through an intense, mid-infrared control laser. We observe that, similar to the processes employed in stimulated emission depletion microscopy, the CARS signal exhibits saturation behaviour if the transition rate between the vibrational and the control state is large. Our approach opens up the possibility of achieving chemically selectivity sub-diffraction-limited spatially resolved imaging.
Highlights
Optical microscopy is the workhorse of biology, providing high contrast images, often in real time, of biological processes at sub-cellular distance scales
These calculations show that coherent anti-Stokes Raman scattering (CARS) emission can be suppressed to a maximum of 99.8% compared to the normal intensity of CARS emission
We found that the saturation intensity, which we define as the intensity of the control beam at which the CARS signal drops to one half of its maximum value [25], corresponds to a 25% reduction of the ground state population density achieved by the control pulse prior to
Summary
Optical microscopy is the workhorse of biology, providing high contrast images, often in real time, of biological processes at sub-cellular distance scales. Far-field sub-diffraction-limited resolution has been achieved by a few imaging techniques, such as: stimulated emission depletion (STED) microscopy [2, 3], photoactivated localization microscopy (PALM) [4], and stochastic optical reconstruction microscopy (STORM) [5, 6]. It was shown that the CARS emission can be suppressed through an interference technique [17], due to the linear nature of interference, this cannot be used to obtain sub-diffraction-limited resolution This leads in to an important point: sub-diffraction-limited resolution requires nonlinearity in the optical emission process. For this situation we identified a mechanism that should enable sub-diffraction-limited resolution CARS: incoherently populating the CARS vibrational state suppresses the build-up of the vibrational coherence and reduces the CARS emission This pre-population process is saturable and results in system behavior that is analogous to STED
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