Single Shot Spectroscopy

 Most time domain 2D experiments require scanning of at least two delays, leading to long measurement times, and imposing severe requirements on the laser system and molecular long term stability. Recently in our laboratory a new experimental technique was developed that enables single-shot measurements of time resolved DFWM. The technique relies on mapping of time-delay to spatial coordinates in the intersection region of the three input beams. In this technique, various delay time combinations are realized all together, and the entire measurement is performed in a single shot, enabling the tracking of intramolecular dynamics and coupling between different degrees of freedom.
Here we will briefly demonstrate the principles of the new technique. Consider a Degenerate Four Wave Mixing experiment in a three-dimensional Folded Boxcars configuration. Three broad input beams pass through three corners of a square as scathed at fig. 1, and the signal beam is emitted toward the fourth corner due to phase matching conditions
 

Figure 1: Configuration of the three incoming beams in single-shot CARS experiment. The signal beam is collected on a camera.

 Pulses propagating along different directions arrive to each point of the beams’ intersection at different times. Those different arrival times may be utilized as controllable time delays between the individual pulses. In fact, FWM signal emerging from each point of the beams’ interaction region may be attributed to particular pair of inter pulse delays. Thus the image of the non-linear signal provides full two dimensional information on molecular dynamics.
 If the fundamental beams are not in resonance with electronic transitions of the molecule, the signal will be observed only when two excitation pulses arrive simultaneously to establish coherence in the sample, to be probed later by the delayed third pulse. The schematic representation of the expected signal is shown in fig. 2. Note that due to phase matching considerations, the dump pulse should be provided by the beam with k₂ direction, while each of the two other pulses can serve as the pump or the probe.

Figure 2: Sketch of the expected form of the non-resonant FWM image

 The non resonant single shot DFWM signal from dichloromethane is shown below in fig 3. The signal shows periodic pattern situated along the zero pump-dump delay line (depicted as dashed line on the image). The coherence pick which is not shown here is significantly stronger than the vibrational signal and therefore is moved out of the picture by delaying the probe pulse. The profile of this pattern is shown at the inset at the fig. 3b. The  power  spectrum of this profile shows a dominant peak at 285cm^-1 (fig. 3b), which is in good correspondence to the 283cm^-1 asymmetric stretching mode of dichloromethane.

Figure 3 a) Single Shot FWM image of neat CH₂Cl₂. b) The Power spectrum of the spatial profile of the FWM image. The spatial profile is provided in the inset.

For further details please see our paper: Y. Paskover, I. S. Averbukh Y. Prior, OE 15 (2007) 1700.