The goal of our investigations is to understand ultrafast molecular dynamics manifesting itself in absorption and emission spectra, and to develop new methods to control quantum systems.
Vibrational dynamics in CSRS and CARS spectroscopy. Development of control methods taking into account decoherence.
The CSRS and CARS techniques serve as an efficient tool to observe and coherently control vibrational dynamics in complex molecular systems. Generally, solvent environment increases complexity of the energy distribution and decreases coherence time owing to the coupling between the solute and solvent molecules. It significantly complicates light-matter interactions and makes it more difficult to find mechanisms of energy transfer, which requires high precision control.
Theoretical studies address (a) control of excitation of molecular vibrations taking into consideration phenomena originated in condensed phase, and (b) manifestation of dynamical changes in CSRS and CARS spectroscopy. We develop quantum theory of coherent stimulated Raman scattering (CSRS) and coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy in application to investigation of ultrafast dynamics of polyatomic molecular systems on real time scale and to adavance noninvasive imaging techniques.
A semiclassical theory is developed describing a multimode molecular system involved in the CSRS dynamics. A model system consists of a set of quantum, two-level systems describing normal vibrational modes in a molecule. External electromagnetic fields are treated classically. The theory is developed for an impulsive and non-impulsive stimulated Raman scattering, determined by the pulse duration with respect to a typical period of vibrational motion. Maxwell-Bloch equations are developed to determine evolution of Raman fields as a function of the induced polarization [NSP05]. Femtosecond pulse shaping is analyzed in terms of pulse amplitude and phase modulation as control parameters for selective excitation of two-level systems. In [JCP04,IJQC05] pulse shapes for the amplitude modulation are proposed to be used in impulsive Raman scattering. In [PRA06] a method for coherent control in non-impulsive regime is developed implementing a transform-limited pump pulse and a linearly-chirped Stokes pulse.
We analyzed a possibility to create a maximum coherence in a predetermined vibrational mode and optimize CARS signals at a given pulse intensity. We developed two new methods for adiabatic population transfer that maximizes state coherence by implementation of two linearly chirped pulses in the Raman configuration. One of the methods is particularly robust in experimental realization and led to a patent. We revealed the effects of relative phase and coupling between the vibrational modes on selective excitation in CARS microscopy.V. Patel, S.A. Malinovskaya, Phys. Rev. A 83, 013413 (2011); V. Patel, V. Malinovsky, S. Malinovskaya, Phys. Rev. A 81, 063404 (2010); S. Malinovskaya, Opt. Comm. 282, 3527 (2009); S.A. Malinovskaya, V.S. Malinovsky, J. Mod. Opt. 55, 3101 (2008); S.A. Malinovskaya, Int. J. Quant. Chem. 107, 3151 (2007); S.A. Malinovskaya, V.S. Malinovsky, Opt. Lett. 32, 707 (2007); S.A. Malinovskaya, Phys. Rev. A 73, 033416 (2006).
Prevention of decoherence in CARS using chirped pulse trains.
Based on the dressed state analysis originated from the Liouville von Neumann equation with relaxation, we developed a method to sustain high level of coherence in the selected vibrational mode in the presence of fast vibrational energy relaxation. The method implements two chirped pulse trains with the repetition rate close to the relaxation rate. S.A. Malinovskaya, J. Mod. Opt. 56, 784 (2009); S.A. Malinovskaya, Opt. Lett. 33, 2245 (2008).
Ultrafast control of the photo-induced reaction of isomerization reaction in the rhodopsin and bacteriophodopsin.
Retinal proteins, often called rhodopsins, are involved in a variety of responses of living cells to light, e.g., vision in higher organisms. All rhodopsins contain the molecule retinal as their chromophore. The activation of the rhodopsins is initiated by a photoinduced isomerization of retinal, which represents one of the fastest chemical reactions. Understanding the mechanism of the isomerization of the retinal in the visual pigment rhodopsin and development of the methods for control of the isomerization yield is the objective of our research . We develop a theory and perform numerical calculations of ultrafast dynamics in the rhodopsin molecule subject to interaction with external ultrafast electromagnetic fields.
In the framework of quantum-chemical methods (Restricted Hartree-Fock, Moller-Plesset perturbation theory and Density Functional Theory) we analyzed the charge transfer in the rhodopsin and the bacteriorhodopsin as a mechanism for photoinduced isomerization reaction. We demonstrated that the isomerization reaction is accompanied by the substantial charge transfer within the isomerization region. Understanding the mechanism of the retinal in the visual pigment rhodopsin provides vital information for the development of methods for control of the isomerization yield. B. Corn, S.A. Malinovskaya, Int. J. Quant. Chem. 109, 3131 (2009).
Ultrafast control of Raman transitions using frequency combs: Prevention of decoherence
The project is devoted to theoretical studies of the optical frequency comb interaction with molecules. The excitation of two-photon Raman transitions is investigated induced by two pulse trains with the locked phase and also by crafted femtosecond pulse trains. Possibilities of selective excitation of predetermined Raman transitions are investigated taking into account effects of decoherence. The objectives of the project are to gain insight into the mechanisms of pulse train interactions with matter, to learn about factors that govern molecular states time evolution, and to develop new control methods of molecular dynamics in the presence of fast decoherence.
The optical frequency combs were implemented for rovibrational cooling of molecules from the Feshbach states. We developed a semiclassical theory of an optical frequency comb interaction with a three-level system and taking into account decoherence. We proposed a novel method for cooling of internal degrees of freedom in molecules from Feshbach states using a single, phase modulated optical frequency comb. The mechanism is based on the adiabatic, coherent accumulation of the population in the target, ultracold molecular state. S. Malinovskaya, W. Shi, J. Mod. Opt. 57, 1871 (2010); S. Malinovskaya, V. Patel, T. Collins, Int. J. Quant. Chem. 110, 3080 (2010); W. Shi, S. Malinovskaya, Phys. Rev. A 82, 013407 (2010); S.A. Malinovskaya, T. Collins, V. Patel, Adv. Quant. Chem. (2012).
Dynamics and Control of Core-Excited and Core-Ionized Molecules
In light of newest advancements in x-ray pulsed radiation, theoretical investigations of electron and nuclear dynamics in molecules following core-electron excitation or ionization have become of particular interest. In this project we study molecular dynamics following core-shell interaction with x-ray pulses and focus on the development of time-dependent picture of the resonant x-ray emission, Auger emission and resonant x-ray stimulated Raman scattering. We develop coherent control methods applicable to x-ray pulsed radiation to govern core-excitation and the induced dynamics. The objectives of investigations include time-dependent formulation of dynamical symmetry breaking and core-hole localization in highly symmetrical molecules, control of dynamical distortions caused by vibronic coupling, and the study of dissociation dynamics of the core-excited molecules. Work on this project helps us in understanding of the nature of the x-ray pulse interaction with matter, explains many features present in the x-ray and Auger spectra originated from the vibronic coupling of the core-excited state manifold, and will result in new methods of quantum control of molecular dynamics.
Controlling coherence in multilevel systems using Optimal Control Theory (OCT).
This project is devoted to the implementation of OCT to study dynamics and to develop methods to control coherence in multilevel systems. We implement the Krotov method to find the control fields to maximize coherence between predetermined states for the resonance and off-resonance case. P. Kumar, S.A. Malinovskaya, V.S. Malinovsky, J. Phys. B: At. Mol. Opt. Phys. 44, 154010 (2011).
We performed studies of the quantum dynamics using OCT in the presence of laser field noise. We investigated the dynamics in the HF molecule and the OH radical induced by noisy control fields and learned the extent to which noise is tolerable without a loss of controllability. Robust noisy control fields are obtained for the field-to-noise ratio ranging from 1 to 10. We demonstrated that in the presence of small amplitudes of noise, noise cooperates with the field following the stochastic resonance mechanism. P. Kumar, S. A. Malinovskaya, J. Mod. Opt. 57, 1243 (2010).