Polarons are quasiparticles composed of excess electrons dressed with virtual phonon clouds. They are ubiquitous in materials and play a key role in various physicochemical properties, including superconductivity, photocatalysis and ferroelectricity. However, carrier mobility is reduced when polarons are formed due to strong carrier localization, which degrades the performance of electrical devices, while an effective strategy to solve this problem is still lacking.
Ultrafast photoexcitation is an efficient method to manipulate the dynamics of (quasi) particles under non-equilibrium conditions. However, most of the previous ultrafast experiments were limited to characterizing the polaron formation process, and only speculations about the polaron transport mechanism were presented. Theoretical studies based on the adiabatic approximation can only reveal the static and thermaldynamic properties of polarons.
WANG Huimin and LIU Xinbao et al. in Prof. MENG Sheng's group from the Institute of Physics of the Chinese Academy of Sciences have discovered that laser-driven coherent phonons enable an order-of-magnitude increase in polaron mobility.
They found that the selective excitation of specific vibrational modes effectively reduces the energy barrier of polaron hopping.
Due to the strong non-adiabatic couplings between electronic and ionic subsystems, phonon-phonon scattering in q space occurs rapidly within sub-picoseconds, triggering the migration of polaronic deformations. The carrier mobility in the prototypical polaronic material Li2O2 can be increased by eight orders of magnitude by tuning the laser parameters, much more efficiently than thermal effects.
These results extend the understanding of polaron dynamics to the non-equilibrium regime. For the first time, the non-thermal route to ultrafast control of polarons is proposed, which would innovate the design principles of optoelectronic devices with high on-off ratio and ultrafast responsibility.
The study, entitled "Giant acceleration of polaron transport by ultrafast laser-induced coherent phonons," was published in Science Advances.
This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.
Laser-controlled polaron transfer and its potential applications. (Image by Institute of Physics)
Effects of coherent phonon excitation on polaron transport. (Image by Institute of Physics)
Nonadiabatic effect during polaron transfer. (Image by Institute of Physics)
