In a TG experiment, two infrared laser pulses, obtained dividing a single pulsed laser beam, interfere within the sample and produce an impulsive spatially periodic variation of the dielectric constant. The spatial modulation is characterized by a wave vector q which is given by the difference of the two pump wave vectors k1- k2. The relaxation toward equilibrium of the induced modulation can be probed by measuring the Bragg scattered intensity of a second cw laser beam. The time evolution of the diffracted signal supplies information about the dynamic of the relaxing TG and, consequently, on the dynamical properties of the studied sample. TG experiments fall within the framework of the four-wave-mixing theory. The heterodyne detected TG signal measures directly and linearly the relaxation processes defined by the tensor components of the response function. By selecting different polarizations of the fields, different elements of the response function tensor are probed.

A sketch of the TG optical set-up is reported in Fig. 2. The infrared pump pulses have a 1064nm wavelength, a temporal length of 20 ps and repetition rate of 10 Hz. They are produced by an amplified regenerated oscillator (Nd-YAG EKSPLA PL2143). The typically used pump pulse energy was 5 mJ. The probing beam, instead, is a continuous-wave laser at 532nm produced by a diode-pumped intracavity-doubled Nd-YVO (Verdi-Coherent).

The two laser beams are collinearly sent to a phase grating (PG) to get the two pump pulses and the probe and local field beams. These are obtained taking the +1 and .1 diffraction orders of the infrared and green lasers. A couple of confocal achromatic lenses, AL1 and AL2, collects all the four beams and focuses them on the sample. The phase grating directly supplies a probe at the right Bragg angle and a local field exactly collinear with the scattered field and phase locked with the probe. The HD-TG signal is optically filtered and measured by a fast avalanche silicon photodiode with a bandwidth of 1 GHz (APD, Hamamatsu). The signal is then amplified by a DC-800MHz AVTECH amplifier and recorded by a digital oscilloscope with a 7GHz bandwidth and a 20 Gs/s sampling rate (Tektronix). The instrumental function of our setup has a temporal full width half maximum of 1 ns and is mainly determined by the bandwidth of the detector and its amplifier.