Time Resolved Four Wave Mixing (TR-FWM) is an excellent method for probing molecular vibrational dynamics on the femtosecond time scale [1].  Recently we discussed a 3-Dimensional forward propagating Boxcars geometry of three well collimated beams intersecting within a sample.  We have shown that the arrival time of each of the pulses to a particular location within the intersection region  determines the time delay between the pulses such that an image of the FWM signal generated in the different locations provides full information on the TR-FWM signal within a single laser shot [2]. Thus, each single shot experiment provides a time resolved signal, which in turn is Fourier Transformed to provide the frequency dependence of the FWM signal. We further showed [3] that the well collimated beams impose strict phase matching conditions on the generated FWM signal, thus enabling Phase Matching Spectral Filtering within the broad spectral width of the ultrashort pulses by varying the angle of the Stokes beam. A full spectrally and time resolved FWM signal was achieved when the experiment was repeated for different angles. 

The next step, presented here, is the amalgamation of these multiple experiments into a single one. The experimental geometry was modified, such that instead of changing the input Stokes angle δ, now the Stokes beam was gently focused by a cylindrical lens to the interaction region. Thus, the different directions (angles) were all present simultaneously, each giving rise to a signal centered at a different frequency and phase matched in a slightly different geometrical direction. The picture of the FWM signal as captured directly on the CCD camera contains the entire range of input frequencies hitherto necessitating a series of different experiments with individually tuned input angles. The method is demonstrated on liquid neat dibromomethane (CH₂Br₂).

The simultaneous observation of the entire spectrum allows not only the assignment of the observed beats to specific vibrational modes but also the identification of distinct contributions from various spectroscopic pathways.  In conclusion, the ability to carry out fully time and frequency resolved measurement within a single laser shot opens the road to the addressing and spectral interpretation of complex molecules undergoing rapid photo-bleaching as is the case for many biological molecules.

[1] S. Ruhman , A Joly, K. Nelson,  IEEE J. Quantum Elect. 24 (1988) 460

[2] Y. Paskover, I. S. Averbukh Y. Prior, OE 15 (2007) 1700

[3] Y. Paskover, A Shalit Y. Prior, Opt. Commun.  283 (2010) 1917