Femtosecond CARS

After the introduction of frequency resolved CARS by Maker and Terhune [1], time resolved experiments became possible with the invention of high power lasers with femtosecond resolution. Leonhardt [2] and for example Hayden [3] performed femtosecond CARS experiments in liquids. A first femtosecond time resolved CARS experiment in gas phase was performed by Motzkus et. al. [4] where the wave packet dynamics of the dissociation of Nal was monitored. The first observation of wave packet dynamics in gaseous iodine was reported by Schmitt et al. [5]. They were able to observe dynamics in both, the ground and excited state with the same experiment. A summary of high resolution spectroscopy in gas phase by nonlinear methods is given by Lang et al. [6]. 

The basis of the experimental femtosecond CARS apparatus developed by Okamoto and Yoshihara (1990) which is reproduced in Fig. 3.6-10 is essentially the same as that of Leonhardt et al. (1987) and Zinth et al. (1988) with the addition of the possibility to change the polarization of the laser radiation. The main parts of the system are two dye lasers with short pulses and high repetition rates, pumped by a cw mode-locked Nd YAG laser (1064 nm, repetition rate 81 MHz). The beam of the first dye-laser which produces light pulses with 75-100 fsec duration is divided into two parts of equal intensities and used as the pump and the probe beam. After fixed (for the pump beam) and variable (for the probe beam) optical delay lines, the radiation is focused onto the sample together with the Stokes radiation produced by the second laser (DL2), which is a standard synchronously pumped dye laser. The anti-Stokes signal generated in the sample is separated from the three input laser beams by an aperture, an interference filter, and a monochromator, and detected by a photomultiplier. For further details we refer to Okamoto and Yoshihara (1990). 

Using a three-colour femtosecond CARS apparatus, Fickenscher et al. (1992) were able to measure very precisely the dependence of the dephasing time T2 of the mode of... 

T Lang, KL Kompa, M Motzkus. Femtosecond CARS on H2. Chem Phys Lett 310 65-68, 1999. 

Meyer S and Engel V 2000 Femtosecond time-resolved CARS and DFWM spectroscopy on gas-phase I, a ... 

Developments in laser sources have been essential for the advances in CARS microscopy (Cheng and Xie 2004). Recent CARS microscopy studies have relied on solid-state ultrafast lasers, and have evolved from low-repetition-rate femtosecond... 

The experiments have been performed on a setup that used the ps-OPO-based CARS system described above and a femtosecond Tiisapphire laser in conjunction with a commercial laser scanning microscope (Carl Zeiss, model LSM-510). The peripheral nerve samples were gained from C57/B6 wild-type mice. After removing the skin from the lower extremities from freshly sacrificed mice, the saphenous nerve is exposed as it runs very conveniently for excision along the saphenous vein, without too much additional fatty tissue and a favorable tissue thickness of less than 20 m. A 500- m long piece is excised and freed from additional fatty tissue as well as the collagenous nerve sheath. The myelinated nerve tissue is fixed for 3-5 hr in 4% PEA or 10% formalin and mounted on 100-pm thick coverslips that are treated with 3-aminopropyltriethoxysilane or a chromium potassium sulfate solution. After... 

T Advanced Multiphoton and CARS Microspectroscopy with Broadband-Shaped Femtosecond Laser Pulses... 

Pastirk, I., DelaCruz, J. M., Walowicz, K. A., Lozovoy, V. V., andDantus, M. 2003. Selective two-photon microscopy with shaped femtosecond pulses. Opt. Exp. 11(14) 1695-1701. Potma, E. O., Evans, C. L., and Xie, X. S. 2006. Heterodyne coherent anti-Stokes Raman scattering (CARS) imaging. Opt. Lett. 31(2) 241M 3. 

The chemical specificity of CARS microscopy is readily combined with other nonlinear optical image contrast mechanisms, such as two-photon fluorescence (TPF), SHG, and THG, resulting in a multimodal CARS microscopy [88, 118, 117, 43]. In multimodal nonlinear optical imaging, TPF, SHG, and THG signals all benefit from the use of femtosecond laser pulses of high peak intensities, whereas the contrast and chemical selectivity of CARS benefits from the use of picosecond (narrow-bandwidth) pulses (see discussion in Sect. 6.2.3). As demonstrated by Pegoraro et al. [43], this apparent... 

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]. 

The implementation of time-resolved CARS for microspectroscopy and its application for vibrational imaging based on RFID was first demonstrated by Volkmer et al. [64] using three incident pulses that are much shorter than the relevant material time scale. Here, a pair of temporally overlapped pump and Stokes femtosecond pulses was used to impulsively polarize the molecular vibrations in the sample. Impulsive excitation with a single ultrashort pulse is also possible provided that the spectral bandwidth of the pulse exceeds the Raman shift of the molecular vibration of interest [152]. The relaxation of the induced third-order nonlinear polarization is then probed by scattering of another pulse at a certain delay time, r. A measurement of the RFID consists of the CARS signal collected at a series of delay times. 

In conclusion, although the increased propensity for photodamage by femtosecond pulses and the requirement for an additional delayed laser pulse can be disadvantageous, time-resolved CARS microspectroscopy not only provides a means for efficient and complete nonresonant background suppression but also offers the prospect for monitoring ultrafast processes of molecular species inside a sub-femtoliter sample volume [64, 152-154]. 

Analogous to the principal concept of multiplex CARS microspectroscopy (cf. Sect. 6.3.5), in multiplex SRS detection a pair of a broad-bandwidth pulse, eg., white-light femtosecond pulse, and a narrow-bandwidth picosecond pulse that determine the spectral width of the SRS spectrum and its inherent spectral resolution, respectively, is used to simultaneously excite multiple Raman resonances in the sample. Due to SRS, modulations appear in the spectrum of the transmitted broad-bandwidth pulse, which are read out using a photodiode array detector. Unlike SRS imaging, it is difficult to integrate phase-sensitive lock-in detection with a multiplex detector in order to directly retrieve the Raman spectrum from these modulations. Instead, two consecutive spectra, i.e., one with the narrow-bandwidth picosecond beam present and one with that beam blocked, are recorded. Their ratio allows the computation of the linear Raman spectrum that can readily be interpreted in a quantitative manner [49]. Unlike the spectral analysis of a multiplex CARS spectrum, no retrieval of hidden phase information is required to obtain the spontaneous Raman response in multiplex SRS microspectroscopy. 

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.

In CARS two ultrashort pulses of laser light (from femtoseconds to picoseconds in duration) arrive simultaneously at the sample of interest (Mukamel, 2000 Fourkas, 2001 and references herein). The difference between the frequencies (W) - w2) matches the frequency of a Raman active vibrational mode in the sample. A probe pulse (w3) emits a signal pulse of frequency Wj - w2 + w3 in a unique special direction. By scanning the delay time between the pump and probe pulses, the delay of the vibrational coherence can be measured. The distinct advantage of CARS is that it is a background free technique, since the signal propagates in a unique direction. 

Figure 3.6-10 Schematic diagram of a femtosecond time-resolved CARS apparatus. YAG, cw mode-locked Nd YAG laser ML, mode locker PL, polarizer A s, apertures LP, laser pot DM, dichroic mirror DLl, femtosecond dye laser SA, saturable absorber CLFB, cavity-length feedback system DL2, picosecond dye laser W, tuning wedge E, etalon FD, fixed delay VD, variable delay BS, beam splitter P s, half-wave plates (when necessary) F s, filters S, sample MC, monochromator PMT, cooled photomultiplier. (Okamoto and Yoshihara, 1990).

Since the pulse time is so short (see Sec. 3.6.2.2.3) one can coherently excite many vibrational modes at a time and monitor relaxation processes in real time. The first reported femtosecond time-resolved CARS experiments (Leonhardt et al., 1987 Zinth et al., 1988) showed beautiful beating patterns and fast decays of the coherent signal for several molecular liquids. The existence of an intermolecular coherence transfer effect was suggested from the analysis of the beating patterns (Rosker et al., 1986). Subsequent studies by Okamoto and Yoshihara (1990) include the vibrational dephasing of the 992 cm benzene mode. A fast dephasing process was found that is possibly related to... 

Another example of the observation of femtosecond time-resolved CARS is that of Inaba et al. (1993a) who studied the C=C stretching vibration of alkynes (monoalkyl-... 

---