Effects of phase and coupling between the vibrational modes on selective excitation in coherent anti-Stokes Raman scattering microscopy

Abstract

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.

Nonadiabatic effects induced by the coupling between vibrational modes via Raman fields

“…When modeled, the coupling is fulfilled by the effective Rabi frequency 3 , the strength of which is determined, in particular, by the excitation fields [15]. Studying the effects of coupling may play an important role in interpreting the results observed in the laboratory.…”

Simple model of coherent anti‐Stokes Raman scattering signals generated by means of linearly chirped ultrashort Gaussian laser pulses

“…In recent years, CARS with chirped femtosecond (fs) laser pulses is increasingly attracting the interest of many groups at the experimental level and, following the progress of the experimental effort, many researchers have tried to formulate theoretical models that incorporate chirp effects . The nonlinear optical problem is rather intricate and, owing to the difficulty of treating chirped laser pulses, the current models are based on numerical methods, which are often limited to chirp in one single laser pulse and rarely consider chirp in more than one pulse .…”

Vibrational phase imaging in wide-field CARS for nonresonant background suppression

“…The anti-Stokes radiation, resulting from inelastic scattering of the probe beam, is resonantly enhanced if the difference frequency of pump and Stokes pulses is tuned to a specific vibrational mode. Inevitably, the Raman resonant anti-Stokes radiation is superimposed by a nonresonant contribution arising from the electronic part of the polarization [3][4][5][6][7][8][9][10][11][12][13][14][15]. The latter can overwhelm a small Raman resonant CARS signal; in particular, if the samples are composed of a significant amount of water, where a strong nonresonant background over the whole image is obtained.…”

“…The latter can overwhelm a small Raman resonant CARS signal; in particular, if the samples are composed of a significant amount of water, where a strong nonresonant background over the whole image is obtained. To date, various approaches including Epi [3], polarization sensitive [4], time-resolved [5], spatial phase control CARS [6], quantum control based pulse chirping [7][8][9] and CARS phase imaging [10][11][12][13][14] have been implemented in CARS microscopy to obtain the CARS image without the undesirable nonresonant background. Among CARS phase imaging, optical heterodyne detection schemes [10][11][12][13][14] are of particular interest due to their intrinsic ability to retrieve Im( (3) χ ), which is proportional to the Raman resonant signal.…”