Recently, the work was completed by the group of Prof. Sheng Meng from the Institute of Physics, Chinese Academy of Sciences, and Beijing National Research Center for Condensed Matter Physics. By using the non-adiabatic time-dependent density functional molecular dynamics method, independently developed by the group software (TDAP), they studied the ultrafast melting process in insulator magnesium oxide (MgO) materials, confirming that longer-wavelength laser-induced tunnel ionization is an important reason for accelerating ultrafast melting in wide-bandgap insulator materials, which provides the universal microscopic mechanism explanations for the laser-induced phase transitions.
The relevant research results, titled "How does a ceramic melt under laser? Tunnel ionization dominant femtosecond ultrafast melting in MgO", were recently published in the Science sub-journal Ultrafast Science.
Background
In the past decades, the rapid advancements of ultrafast laser technologies have enabled the ultrafast nonequilibrium dynamics and controllable phase transitions in condensed matter. The photoinduced electronic excitations not only manipulate the electronic properties of materials, but also instantaneously modulate lattice stabilities via changing the potential energy surfaces of lattice configurations and ultrafast heating effects. Laser-induced melting plays a crucial role in advanced manufacturing technology and ultrafast science. Recently, the photoinduced ultrafast melting and subsequent welding was experimentally demonstrated in wide-gap hard ceramic materials with high melting temperatures, extending the melting objects from soft metals/semiconductors to hard insulating ceramics. However, its atomic processes and microscopic mechanisms, especially in a wide-gap ceramic, remain elusive due to complex interplays between many degrees of freedom within a timescale of ~100 femtoseconds, specific to the ultrafast mechanisms of photoionization, energy transfer and atomistic dynamics of laser-induced ultrafast melting is lacking for these wide-gap crystalline materials. In this study, we take magnesium oxide (MgO), one of the prototypical ceramic materials exhibiting a high melting temperature (~3200 K) and a wide band gap (~6.0 eV), as an example to study the nonequilibrium mechanism of photoinduced ultrafast melting in wide-gap insulator materials.
Highlights of Innovation
We report here that laser melting is greatly accelerated by intense laser-induced tunnel ionization in the MgO. The tunneling processes generate a large number of photocarriers and result in intense energy absorption, instantaneously altering the potential energy surface of lattice configuration. The strong electron-phonon couplings and fast carrier relaxation enable efficient energy transfer between electrons and the lattice. These results account well for the latest ultrafast melting experiments and provide atomistic details. Our work offers a first-step in-depth microscopic understanding of ultrafast homogeneous melting of ceramics in the ultimate time (~100 femtoseconds) and spatial (~1 nanometer) scale and presents an effective approach for rationally triggering ultrafast phase transitions using parameter-optimized laser pulses.
Figure 1 displays schematically the microscopic dynamics of MgO ultrafast melting process under intense laser irradiations. The lattice structure receives a large amount of heat energy under the irradiations of long-wavelength (LW) 1028 nm laser pulses, while the heating effect is relatively weak under the short-wavelength (SW) 191 nm laser irradiation. It is clear that the LW laser pulses lead to severe structural distortions and amorphization, whereas the variations of the crystal structure are negligible under SW laser pulses. The distinct heat accumulations and lattice dynamics correspond to the diverse photoexcitation mechanisms and energy transfer pathways. For the intense LW laser pulses used here, strong-field tunnel ionization gives rise to significant electronic excitations and energy absorptions (Fig. 2), which also instantaneously changes the potential energy surface of lattice structures. The continuous carrier distribution and Bloch oscillation of photocarriers by tunnel ionization accelerate the carrier-carrier scattering processes, which promotes the fast equilibrium process within electronic subsystems. Then, the energy transfer to atomistic systems leads to ultrafast melting. The photoinduced ultrafast melting features the nonequilibrium electronic and atomic states, ultrafast timescales, and homogeneous nucleation.
Besides MgO, we also perform simulations for other ceramic materials such as aluminum nitride (AlN), which shows similar melting processes under the SW and LW laser pulse irradiations. The fluence thresholds for melting or damage decrease with the decrease of photon energies for a variety of wide-gap materials, and various laser wavelengths are employed. The above evidence implies a universal mechanism of photoinduced ultrafast melting for wide-gap materials.
In addition, a detailed characterization of MgO phase diagram is obtained on a wide range of pressure-temperature conditions (Fig. 3). A nonequilibrium phase boundary between B1 (NaCl-type) and liquid phase can be obtained under laser irradiations with intensity exceeding the damage threshold. Moreover, the shock curve under the laser irradiations can be calculated by recording the temperature and pressure at the maximum amplitude of laser pulse and extend it to moderate pressure regimes (P = 2 GPa). The inclusion of the nonequilibrium states makes the shock from curve prediction more realistic, and the predicted shock curve exhibit better consistency than estimations from empirical Hugoniot curve.
Conclusion
In conclusion, the coupled electron-lattice dynamics of MgO under laser illuminations are investigated based on first-principles quantum dynamics simulations. The lower intensity threshold of ultrafast melting under longer wavelength laser pluses is attributed to the strong-field-induced tunnel ionization process, and a large number of valence electrons are excited to the conduction bands leading to intense energy absorption. The instantaneous changes of potential energy surface and fast energy transfer from electrons to the lattice via strong EPC lead to ultrafast lattice response and subsequent melting of crystal structure. This work not only offers new insights into the nonequilibrium mechanisms of laser-induced phase transition in ceramics, but also demonstrate the possibility of manipulating phase transition at ambient conditions via laser irradiations. Our study is of great significance to ultrafast physics, extreme condition physics and advanced material manufacturing.
This work was supported from National Key Research and Development Program of China (No. 2021YFA1400200), National Natural Science Foundation of China (No. 12025407,11934003, 12204513), and "Strategic Priority Research Program (B)" of Chinese Academy of Sciences (Grant No. XDB330301 and YSBR047).
About the Team
The work team is affiliated with the Institute of Physics, Chinese Academy of Sciences, and Beijing National Research Center for Condensed Matter Physics. Dr. Hui Zhao, who has graduated from the Institute of Physics, Chinese Academy of Sciences, is the first author of this work, and Prof. Sheng Meng and Associate Prof. Jiyu Xu from the Institute of Physics, Chinese Academy of Sciences are the corresponding authors. Other co-authors include Dr. Shiqi Hu, Dr. Mengxue Guan, Dr. Xinbao Liu and Dr. Daqiang Chen, who have graduated from the Institute of Physics, Chinese Academy of Sciences. Among them, Associate Prof. Jiyu Xu main research focuses on non-equilibrium phase transitions and first-principles computational simulations of complex systems. Prof. Sheng Meng main research is the interaction between light and matter, the development of time-dependent density functional theory, condensed matter theoretical calculations and materials science.
