Transient grating scattering

The description of pump-probe signals presented in the preceding section can be immediately generalized to heterodyne-detected transient grating spectroscopy as well as to other four-wave mixing techniques. Heterodyne detection involves mixing the scattered field with an additional heterodyne field 4(r). The signal in the ks direction can then be written in terms of the polarization Ts(t) as... 

Figure 5 presents the frequency ranges that, by optimistic estimates, can be covered by the optical techniques discussed above. We have intentionally left out of the discussion several other optical spectroscopic methods, such as coherent anti-Stokes resonant scattering (CARS), or induced transient grating (TG) spectroscopy, mainly because they are rarely used in the present context and/or are much less straightforward to interpret. 

We have reviewed the EOM-PMA method for the calculation of two-pulse-induced (spontaneous emission, pump-probe, photon echo) and three-pulse-induced (transient grating, photon echo, coherent anti-Stokes-Raman scattering, four-wave-mixing) optical signals. In the EOM-PMA, the interactions of the system with the relevant laser pulses are incorporated into the system Hamiltonian and the driven system dynamics is simulated numerically exactly. 

In a heterodyne-detected transient-grating (HD-TG) experiment [5,8-11], two infrared laser pulses, typically obtained dividing a single pulsed laser beam, interfere within the sample producing an impulsive spatially periodic variation of the material optical properties. The spatial modulation is characterized by a wave vector which is given by the difference of the two pump wave vectors. The relaxation toward equilibrium of the induced modulation is probed by measuring the Bragg scattered intensity of a second continuous wave laser beam. A sketch of the experimental set-up and details on the laser systems can be found in ref 5 and ref. 10, respectively. 

Forced Rayleigh scattering [16] has been used to probe the dynamics of a conducting polymer, poly(3-octylthiophene), in solution. The optical labels which lead to the transient grating are the photoexcited states on the chains... 

Voehringer, R and Scherer, N.F. (1995). Transient grating optical hetero-d5me detected impulsive stimulated Raman scattering in simple liquids. J. Phys. Chem. 99 2684-2695. 

The transport coefficients have been measured by the transient holographic grating technique of Thermal Diffusion Forced Rayleigh Scattering (TDFRS) that has already been described in more detail in previous works [85-87] and will only be briefly sketched in the following (Fig. 1). 

Shibamoto K, Katayama K, Sawada T (2007) Ultrafast charge transfer in surface-enhanced Raman scattering (SERS) processes using transient reflecting grating (TRG) spectroscopy. Chem Phys Lett 433 385-389... 

Figure 1. Laser-induced ultrasonic wave excitation and detection using ISLS. Two 100 picosecond infrared pulses converge spatially and temporally within a transparent sample medium. The time-dependant intensity or strength of the optical transient diffraction grating is monitored by a third frequency doubled pulse which, in the case for relatively low scattering strength, is systematically delayed in time. The time response of stronger gratings can be monitored using a CW laser.

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