Nonresonant CARS signals

The main source of contrast in FE-CARS is based on differences in the amplitude and phase of The spectral phase plays an important role in FE-CARS. While the phase of the nonresonant CARS signal is independent of co, the resonant part of exhibits a characteristic r-jump in the vicinity of a vibrational resonance Or. In the presence of a spatial r-step in focus, the nonresonant background destructively... 

The nomesonant background prevalent in CARS experiments (discussed above), although much weaker than the signals due to strong Raman modes, can often obscure weaker modes. Another teclmique which can suppress the nonresonant background signal is Raman induced Kerr-efifect spectroscopy or RIKES [96, 97]. 

The nonresonant background in CARS spectroscopy originates from instantaneous four-mixing processes, while the resonant contribution involves real vibrational states. This provides a basis for possible discrimination against the nonresonant background. To do so, one has to come up with a pair of pulses that excite the vibrational state, and the third, time-delayed pulse will only contribute to the resonant part of the CARS signal. However, to make this scheme work efficiently, one has to overcome certain obstacles. To achieve high spectral resolution, the bandwidth of the third pulse should... 

FIGURE 9.12 (a) Calculated FE-CARS radiation profile when a HGOl excitation field overlaps with a lateral interface between a resonant and a nonresonant material. Note that the intensity along the optical axis is no longer zero due to partial lifting of the phase step by the interface. The inset shows the excitation field relative to the orientation of the interface, (b) Comparison of the calculated spectral dependence of CARS in a bulk material with a weak resonance and FE-CARS measured at an interface similar to the one considered in (a). Note the Raman-like spectral dependence of the FE-CARS signal. 

Recently, Silberberg and coworkers have introduced a different approach for active phase control in CARS [44 47]. By tailoring the spectral phase of a single ultrashort laser pulse, phase-sensitive detection of the resonant signal has been demonstrated where the strong nonresonant CARS background of... 

On the contrary, a way to image small objects embedded in a nonresonant solvent with high contrast is provided by the epi-detection geometry [60]. Because the E-CARS signal intensity from an isotropic bulk medium is negligible the signal from the microscopic object with an effective... 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

Fig. 6.11. Temporally and spatially resolved CARS signal from a l- lm polystyrene sphere embedded in water at a Raman shift centered at 3054 cm 1 where aromatic C-H stretching vibrations reside. A Measured and simulated decay curves when focused on the bead and into bulk water. B RFID images and the lateral intensity profiles along the lines indicated by the arrows at time 0 and r 370 fs, demonstrating the complete removal of nonresonant background contributions from both the object and the solvent to the image contrast at r w 370fs (Adapted from [64])...

Figure 3 Femtosecond nondegenerate CARS in liquids (a) Coherent probe scattering signal versus delay time open circles, dashed curve nonresonant scattering of CCU yielding the instrumental response function and the experimental time resolution of 80 fs full points, solid line resonant CARS signal from the CH3-mode of acetone at 2925 cue1, obtaining T2/2 = 304 3 fs. (b) Ratio of experimental and calculated scattered data of (a) for acetone versus delay time the small experimental error of the data points extending over 6 orders of magnitude is noteworthy.

We shall see that (3.5) predicts dramatically different decay times for the resonant and nonresonant contributions to the CARS signal. This effect has been illustrated beautifully in the experiments reported by Zinth et al.15 and Kamga and Sceats.16... 

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