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The Journal of Chemical Physics

Selective excitation of femtosecond coherent anti-Stokes Raman scattering in the mixture by phase-modulated pump and probe pulses

Shian Zhang, Hui Zhang, Tianqing Jia, Zugeng Wang, Zhenrong Sun

Published Jan 26, 2010• 18 citations
Publisher

Abstract

In this paper, we present a feasible method to realize and improve the selective excitation of femtosecond coherent anti-Stokes Raman scattering (CARS) in a mixture. We theoretically show that, by shaping both the pump and probe pulses with the pi phase step, the CARS signal from one quantum system can be enhanced and simultaneously that from the other quantum system is effectively suppressed. Comparing with only shaping the probe pulse [D. Oron, N. Dudovich, D. Yelin, and Y. Silberberg, Phys. Rev. Lett., 88, 063004 (2002)], the selectivity of femtosecond CARS by shaping both pump and probe pulses can be greatly improved. Finally, we experimentally compare two shaping schemes by investigating the selective excitation of the femtosecond CARS in the mixture of dibromomethane (CH(2)Br(2)) and chloroform (CHCl(3)).

DOI
10.1063/1.3298859

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arXiv:2309.01232•Sep 3, 2023•0 citations
In this paper, we present a feasible method to realize and improve the selective excitation of femtosecond coherent anti-Stokes Raman scattering (CARS) in a mixture. We theoretically show that, by shaping both the pump and probe pulses with the pi phase step, the CARS signal from one quantum system can be enhanced and simultaneously that from the other quantum system is effectively suppressed. Comparing with only shaping the probe pulse [D. Oron, N. Dudovich, D. Yelin, and Y. Silberberg, Phys. Rev. Lett., 88, 063004 (2002)], the selectivity of femtosecond CARS by shaping both pump and probe pulses can be greatly improved. Finally, we experimentally compare two shaping schemes by investigating the selective excitation of the femtosecond CARS in the mixture of dibromomethane (CH(2)Br(2)) and chloroform (CHCl(3)).
Coherent anti-Stokes Raman scattering (CARS) microscopy has been a major tool of investigation of biological structures as it contains the vibrational signature of molecules. A quantum control method based on chirped pulse adiabatic passage was recently proposed for selective excitation of a predetermined vibrational mode in CARS microscopy [Malinovskaya and Malinovsky, Opt. Lett. 32, 707 (2007)]. The method utilizes the chirp sign variation at the peak pulse amplitude and gives a robust adiabatic excitation of the desired vibrational mode. Using this method, we investigate the impact of coupling between vibrational modes in molecules on controllability of excitation of the CARS signal. We analyze two models of two coupled two-level systems (TLSs) having slightly different transitional frequencies. The first model, featuring degenerate ground states of the TLSs, gives robust adiabatic excitation and maximum coherence in the resonant TLS for positive value of the chirp. In the second model, implying nondegenerate ground states in the TLSs, a population distribution is observed in both TLSs, resulting in a lack of selectivity of excitation and low coherence. It is shown that the relative phase and coupling between the TLSs play an important role in optimizing coherence in the desired vibrational mode and suppressing unwanted transitions in CARS microscopy.
A chirped-pulse quantum control scheme applicable to Coherent Anti-Stokes Raman Scattering spectroscopy, named as C-CARS, is presented aimed at maximizing the vibrational coherence in molecules. It implies chirping of three incoming pulses in the four-wave mixing process of CARS, the pump, the Stokes and the probe, to fulfil the conditions of adiabatic passage. The scheme is derived in the framework of rotating wave approximation and adiabatic elimination of excited state manifold simplifying the four-level model system into a ``super-effective'' two level system. The robustness, spectral selectivity and adiabatic nature of this method are helpful in improving the existing methods of CARS spectroscopy for sensing, imaging and detection. We also show that the selectivity of excitation of vibrational degrees of freedom can be controlled by carefully choosing the spectral chirp rate of the pulses.
Saturation of vibrational Raman coherence and coherent anti-Stokes Raman scattering (CARS) using femtosecond (fs) excitation pulses is investigated theoretically. The pump in a typical fs-CARS configuration has a bandwidth of a few hundred cm${}^{\ensuremath{-}1}$ that can couple tens of rotational states of room-air nitrogen molecules simultaneously, unlike in CARS with longer pulse durations. It is demonstrated that the vibrational coherence and also the vibrational CARS with fs excitation display saturationlike behavior once the rotational coherence is saturated. The Raman saturation threshold for the fs pump is numerically estimated to be at a peak intensity of $\ensuremath{\sim}$1022{10}^{22}1022 W/m2{}^{2}2, which is six to seven orders of magnitude higher than that in the nanosecond regime. The results are compared with the known saturation thresholds in different pulse-duration regimes and placed in perspective with other nonlinear thresholds reported in fs excitation regimes.
Abstract A chirped-pulse quantum control scheme applicable to coherent anti-Stokes Raman scattering spectroscopy, named as C-CARS, is presented aimed at maximizing the vibrational coherence in molecules. It implies chirping of three incoming pulses in the four-wave mixing process of CARS, the pump, the Stokes and the probe, to fulfill the conditions of adiabatic passage. The scheme is derived in the framework of rotating wave approximation and adiabatic elimination of excited state manifold simplifying the four-level model system into a ‘super-effective’ two level system. We demonstrate that the selectivity of excitation of vibrational degrees of freedom can be controlled by carefully choosing the spectral chirp rate of the pulses. The robustness, spectral selectivity and adiabatic nature of this method are advantageous for improving the existing methods of CARS spectroscopy for sensing, imaging and detection.
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.
A novel chirped pulse control scheme is presented based on Coherent Anti-Stokes Raman Spectroscopy (C-CARS) aiming at maximizing the vibrational coherence in atoms and molecules. The scheme utilizes chirping of the three incoming pulses, the pump, the Stokes and the probe, in the four-wave mixing process of C-CARS to fulfill the adiabatic passage conditions. The derivation of the scheme is based on simplifying the four-level system into a 'super-effective' two level system via rotating wave approximation and adiabatic elimination of the excited state manifold. The robustness, spectral selectivity and adiabatic nature of C-CARS method may prove useful for sensing, imaging, and detection. It is demonstrated that the selectivity in excitation of vibrational degrees of freedom can be controlled by carefully choosing the spectral chirp rate of the pulses. The C-CARS control scheme is applied to a surrogate methanol molecule to generate an optimal anti-Stokes signal backscattered from a cloud of molecules a kilometer away. The theory is based on the solution of the coupled Maxwell-Liouville von Neumann equations and focuses on the quantum effects induced in the target molecules by the control pulse trains. The propagation effects of pulses through the medium are evaluated and the buildup of the molecular-specific anti-Stokes signal is demonstrated numerically. A deep learning technique, using Convolutional Neural Networks (CNN), is implemented to characterize the control pulses and evaluate time-dependent phase characteristics from them. The effects of decoherence induced by spontaneous decay and collisional dephasing are also examined. Additionally, we present the technique of Fractional Stimulated Raman Adiabatic Passage (F-STIRAP) and demonstrate that it can be utilized for remote detection in a multi-level system by creation of a maximally coherent superposition state.