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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Optimizing CARS signal using coherent control methods

Vladimir S. Malinovsky

Published Feb 12, 2009• 2 citations
Publisher

Abstract

We demonstrate the adiabatic passage based method to maximize CARS coherence and present numerical results of the roof scheme implementation without making an assumption of adiabatic elimination of detuned excited electronic states. Also we analyze influence of fast decoherence in the molecular samples on the results of proposed scheme. It is shown that the adiabatic method allows achieving chemical sensitivity with high resolution and can be used to obtain CARS signal with efficiently suppressed background in molecular systems with coherence times of several hundred of femtoseconds.

DOI
10.1117/12.809691

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We discuss a novel method to create maximum coherence in a two-level system that is selectively excited among many having similar frequencies by making use of two specifically chirped pulses in the Raman configuration. The feasibility of the method is analyzed in the presence of decoherence caused by vibrational energy relaxation and collisional dephasing. It has been demonstrated that a high value of coherence may be sustained by the implementation of two chirped pulse trains prepared in accordance with the proposed method and having the same period as the vibrational energy relaxation time. Maximum coherence between predetermined states in atoms and molecules is an important aspect of many problems in quantum optics. One possible example is the generation of optimal CARS signal in the CARS microscopy aiming to reveal structures constituted of particular molecular groups. Presently, in experiments on biological imaging, the CARS signal is generated with implementation of picosecond laser pulses (1), a narrow spectral bandwidth of which allows one to address a particular molecular group with a good accuracy. Aiming to improve chemical sensitivity and to get optimal CARS signal for given pulse intensity we propose to use the quantum control methods for selective excitation of required vibrations in complex molecular systems. We developed a novel method that allows one to achieve maximum coherence in a predetermined vibrational mode in a multimode mole- cular system via the Raman transitions induced by the pump and Stokes chirped pulses. According to the Maxwell-Bloch equations, maximum coherence is the condition for the CARS signal to acquire the highest intensity upon propagation through a molecular medium (2). The mechanism of creation of maximum coherence underlying the proposed method is based on the adiabatic passage. An important aspect in the investigations asso- ciated with a condense phase is decoherence. Two main sources of decoherence, the vibrational energy relaxa- tion and collisional dephasing, may significantly affect the CARS signal intensity. The phase control of molecular processes in the presence of collisions was
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.
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.
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.