In coherent phonon spectroscopy, we use ultrafast lasers in a pump-probe setup (see What is pump-probe? in the right panel) to study the phonons, or vibrations, of a material. With the rise of femtosecond lasers, it is now possible to achieve resolution down to tens of femtoseconds, allowing the investigation of the interaction between charge and phonons, and the propagation of sound waves with a spatial resolution below the nanometer.
When a laser pulse is absorbed, it deposits its energy in the material. The temperature of the electrons suddenly increases and can reach thousands of degrees. This change in the electron bath creates a stress that leads to a change in the atoms' position. The electrons then give this excess of energy to the lattice by emitting phonons. Consequently, the lattice's temperature increases, leading to a displacement of the atoms through thermal expansion. Thermal expansion is commonly observed in a mercury thermometer: when the temperature increases, mercury expands and rises within the thermometer's tube. With coherent phonon spectroscopy, we can monitor all these processes, as shown in the figure Signal. A sharp decay of the signal is induced by the absorption of the light and the signal slowly rise back to its original value through different interactions. On top of that, oscillations can be seen: they are cause by the interaction of light with coherent phonons.
With the pump-probe technique, we can study coherent phonons, but what do we mean by coherent? In the figure Coherent vs. Incoherent in the right panel, we observe two types of crowds. In the upper part, everyone is walking in-line in a similar direction: there is a correlation between their movement. In the lower part, this correlation has disappeared, and each person travels independently. Coherence for waves follows the same principle. Their amplitude, frequency, and phase define waves. Coherent waves correspond to a group of waves whose phases are correlated. In the case of acoustic phonons, this leads to different behavior: coherent acoustic phonons, also called sound waves, are used for ultrasonic imaging, while incoherent acoustic phonons are responsible for heat propagation in semiconductors and insulators.
One of the main applications of coherent phonon spectroscopy is the characterization of material properties. This technique allows generating sound waves that propagates within a sample and are reflected at the numerous interfaces of the sample. By measuring the round-trip time of these sound waves within the layer of the material, we can get access to their thickness. This technique is thus a sonar at the nanoscale.
Coherent phonons are a result of the interaction of light with electron and lattice. Therefore, they offer a window into the fundamental principle ruling these processes. Numerous material properties can be accessed through the study of coherent phonons:
•Thickness measurement (thin film)
•Dimensions measurement (nanostructures)
•Mechanical properties (elastic constants, mass density…)
•Optical properties (refractive index, photoelastic coefficient…)
•Surface and interface roughness characterization
•Charge diffusion, charge mobility…