Scanning pulsed CARS

Common to all narrow-bandwidth excitation schemes is sequential scanning of an experimental parameter in order to adjust the Raman shift in CRS detection. In order to obtain an entire CRS spectrum, this is not only time consuming but also prone to sources of noise induced by fluctuations in laser pulse parameters. As a consequence, dynamical changes in a CRS spectrum are difficult to follow. This problem can be circumvented by use of multiplex CRS spectroscopies [48, 49], which will be discussed in combination with CARS and SRS microscopy in Sects. 6.3 and 6.4, respectively. 

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. 

Fig. 16. Schematic of experiment to study shock-induced nanopore collapse in real time, (a) One element of a shock target array. A near-IR laser pulse generates shock waves by ablation of an absorbing surface layer. The shock front steepens up to <25 ps in the buffer layer. CARS spectroscopy is used to probe dye molecules in the nanoporous layer, which monitor strain and temperature, and a thin anthracene downstream gauge which monitors changes in the risetime of the shock front caused by pore collapse, (b) Scanning electron micrograph of the surface of the nanoporous layer. The pore size distribution is 100 nm 10%. The distribution appears broader in the image, since it sees only the pore cross-section in the surface plane. Reproduced from ref. [120].

In the above analysis, the pump frequency (cOp) and the Stokes frequency (coj have been assumed to be ideally monochromatic, which is applicable when each vibration-rotation line is scanned. In a broadband CARS system as described here the Stokes (dye) laser has a broad spectral profile so that the multiplexed CARS spectral profile of the probed species, is generated with each laser pulse. 

By scanning the delay time between the pump and the probe pulses, the delay in 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. 

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