Coherent Phonon Dynamics in Solids

A femtosecond laser pulse can initiate collective, in-phase atomic motions in solids called Coherent Phonons. Because of the pulse width, it can always pick two photons in one pulse to generate one phonon and the coherence of the photons will transfer to the phonons, thus we can generate coherent phonons. We can use pump probe experiment to capture the phonon dynamics, where the pump is for excitation and probe is sensitive to the refractive index change due to the modulation in the mean displacement as well as mean square displacement of the atomic positions. Either reflection or transmission change will be recorded. The generation and detection of coherent phonons provide an opportunity to understand the fundamental physics between light and matter interaction, as well as a path to manipulate other physical processes, for applications such as sound amplification stimulated emission (SASER), phonon mode manipulation, ultrafast phase switching, superconductivity enhancement and manipulation of magnetism.

A complete phase transformation in bulk CdSe may be reached when the absorbed laser energy can be localized for long enough time, as observed in nanocrystalline CdSe [1-2].

Figure 1 Reversible ultrafast melting in bulk CdSe. (a) Transient reflectivity (-dR⁄R) signals of bulk CdSe measured under a wide range of pump fluences; (b) Coherent phonon signal (red) under the fluence of 17.49mJ/cm²after removing a slowly varying background and the fitting results (black); Change of (a) (-dR⁄R)_max, (b) coherent phonon amplitude, and (c) (-dR⁄R)_5pswith pump fluences. Dashed lines mark three different regions.

Thermal Transport and Coherent phonons in Superlattices

Not all phonons contribute to the heat transport equally. Phonons that are thermally important are called ‘thermal phonons’. Conventionally, we will not consider the coherence of the phonons in the bulk materials when we calculate the scattering rate using the Boltzmann equation. However, in some special material systems, like superlattices, phase information can be important. In comparison with bulk Bi₂Te₃, the spectrum of mode-wise thermal conductivities of longitudinal acoustic phonons along the Γ-Z direction in the Bi₂Te₃/Sb₂Te₃ SL shows a shift to higher frequencies. Our results suggest that it is possible to use the SL structure to manipulate coherent phonon propagation and to tailor thermal conductivity [3].

Figure 2 Normalized mode-wise thermal conductivities of longitudinal acoustic phonons along Γ-Z direction. Comparing with bulk Bi₂Te₃, the spectrum of mode-wise thermal conductivities in Bi₂Te₃/Sb₂Te₃ SL shifts to higher frequencies. The line and the shaded area are just for eye guidance.

Surface Plasmon Enhanced Coherent Phonon Detection

Surface plasmon resonance (SPR) has been widely used for gas detection and biosensing since last decade. People are also trying to implement SPR into other spectroscopy to enhance the signal to noise ratio and detect the optical properties of the materials, especially in the Raman scattering technique. SPR enhanced technique is applicable to enhance the coherent phonon signal.

Figure 3 (a) SPR imbedded pump-probe system; (b) Enlarged sample position using Kretschmann configuration. The prism will be put on a rotational stage.

References:

1. W. Wu and Wang, Y., “Ultrafast carrier dynamics and coherent acoustic phonons in bulk CdSe,” Optics Letters, vol. 40, no. 1, pp. 64-67, 2015.
2. W. Wu, He, F., and Wang, Y., “Reversible ultrafast melting in bulk CdSe,” Journal of Applied Physics, vol. 119, pp. 055701, 2016.
3. F. He, Wu, W., and Wang, Y., “Direct measurement of coherent thermal phonons in Bi₂Te₃/Sb₂Te₃ superlattice,” Applied Physics A, vol. 122, pp. 777, 2016.