Transient grating spectroscopy

Transient grating spectroscopy is an optical technique used to measure quasiparticle propagation. It can track changes in metallic materials as they are irradiated.

It is a pump-probe method in which short-lived standing waves are generated upon a sample surface. This is performed by combining two simultaneous pump laser beams with an angle (theta) between them, which creates an interference pattern on the sample, similar to the interference pattern generated by the well-known double slit experiment. The space between the regions of constructive interference is given by the following equation:

$${\displaystyle \Lambda ={\frac {\lambda }{2sin(\theta /2)}}}$$

where Λ is the distance between the interference stripes, λ, the wavelength of the pump pulse, and θ, the angle between the two incident overlapping beams. The regions of the sample in the constructive interference fringes become thermally/vibrationally excited, and in combination with the unexcited fringes, creates a standing wave of wavelength Λ, also known as a surface acoustic wave. The surface acoustic waves act as transient absorption or reflection gratings that can be probed with a continuous laser that is pulsed immediately after the pump beams. The probe beam is either diffracted through or reflected from the surface, depending on the nature of the sample, toward a detector. The surface acoustic wave fluctuations modulate the diffraction or reflection of the probe beam at the surface of the sample. Its intensity gets monitored by the detector as a function of time. The intensity of the diffracted or reflected probe beam will converge at a baseline level, where no surface acoustic wave is interfering with the diffraction or reflection of the probe.