Transient grating spectroscopy

Transient grating spectroscopy is an optical technique used to measure quasiparticle propagation.^[1] It can track changes in metallic materials as they are irradiated.^[2]

It is a pump-probe method ^[3] in which short-lived standing waves are generated upon a sample surface.^[1][4] This is performed by combining two simultaneous pump laser beams with an angle (theta) between them, which creates an interference pattern on the sample,^[5] 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.^[6] 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.

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