Nonlinear Optics

3 Spectroscopy: CARS
Figure 1. Shown here is a photon diagram, corresponding to the CARS process. An arrow pointing up, correspond to absorption of a photon, and arrow down to stimulated emission, and the wavy arrow signifies the generated CARS signal.

In order to investigate the dynamics of molecules in solution, transient absorption is often useful for getting an an overall view of what is going on, since a very broad energy range of states are addressed. The absorptions spectrum of larger molecules in solution is however often very congested, characterized by broad overlapping lines, which can be difficult to analyze and assign. In these cases using a higher order technique, such as e.g. Coherent Antistokes Raman Scattering (CARS), can be a better approach for interrogating the dynamics.

All third order techniques involve 3 interactions with an electric field, generating a third order polarization in the sample, which then emits the signal. The CARS process corresponds to the process, depicted in Fig. 1, where the energy different between two of the laser pulses (Pump and Stokes) is tuned to the difference between two vibrational levels. The CARS probe is thus selective to a specific vibrational mode, making it possible to monitor vibrational dynamics of molecules upon photo excitation.

A schematic of the CARS setup can be seen in Fig. 2. In order to be able to time resolve very fast processes, we use ~ 20fs long laser pulses, from the Non collinear OPA’s, and limit the dispersion, polarization rotation and astigmatism using a Cassegrain setup for focusing onto the sample. Taking advantage of the coherent nature of these processes, we overlap the beams in a non collinear arrangement, known as box-CARS, on the sample, in which case the signal comes out in a direction dictated by phasematching. The signal can thus easily be separated from the background of the input beams (background free), and directed to an imaging CCD spectrometer for detection.

Figure 2. Experimental setup for the time and frequency resolved CARS. The regenerative Ti:sapphire amplifier producing 80fs pulses is used to pump two Non-collinear Optical Parametric Amplifiers (NOPA). Three optical delay lines control the relative timing of the pulses. The beams are arranged in a folded-BOXCARS configuration, generating a 3’rd order response at their common focus, leading to emission of Antistokes radiation the the phasematched direction. This CARS signal is dispersed in an imaging spectrometer, where its full spectrum can be collected by the computer for each delay.