Resonant CARS signals

A spontaneous Raman spectra is shown in Figure 2.8d in which the on- and off-resonant frequencies are indicated. The DNA bundles are observed at the resonant frequency, as shown in Figure 2.8a, while they cannot be seen at the off-resonant frequency in Figure 2.8b. This indicates that the observed contrast is dominated by the vibrationally resonant CARS signals. Figure 2.8c shows a cross-section of Figure 2.8a denoted by two solid arrows, which were acquired with a 5 nm step. The FWHM of... 

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.

Figure 12. Resonant CARS intensity versus delay time r = 0.07 psec 1, T, = 0.75 psec 1. Comparison with Fig. 11 shows that the resonant CARS signal is insensitive to the rate of IVR on the excited-state potential surface.

Detection sensitivity is one of the key issues in CARS microscopy. This is an especially acute problem in applications where chemical selectivity of CARS perfectly suits the tracking of small changes in cells related to specific protein and DNA distributions, external drug delivery/distribution, etc. There is, however, a component in CARS signal that is not associated with a particular vibration resonance and therefore does not carry chemically specific information. Unfortunately, in many cases, it can distort and even overwhelm the resonant signal of interest. In modeled approach, the CARS response originates from the third-order nonlinear susceptibility, which... 

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... 

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... 

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. 

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]. 

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