The mass of a particle reflects its ability to respond to external forces, and it remains constant under the non-relativistic framework. In crystals, electrons are described by periodic Bloch waves, while the motion of their wave packets in an external field exhibits particle-like behavior. The effective masses of electrons and holes under laser excitation are jointly determined by their crystal momentum and the dispersion relation of the energy band. Their values are not constant and can span the entire real number range. Interestingly, when the effective mass changes from a positive value to a negative one, it does not undergo a gradual transition; instead, it abruptly jumps from positive infinity to negative infinity, giving rise to the so-called "effective-mass singularity". However, in previous studies on ultrafast electron dynamics and high-order harmonic radiation in solids, this important physical quantity has often been overlooked.
High-order harmonic generation is a nonlinear optical phenomenon that converts low-energy photons into high-energy photons, and it is of great significance for understanding and controlling ultrafast processes within solids. It is expected to generate ultrashort attosecond pulses in solids, thereby providing a platform for the development of novel ultrafast light sources. This harmonic radiation process is highly sensitive to electron motion and band structures, enabling us to probe the internal electronic behavior of materials and unveil their topological properties. It provides a theoretical basis for designing high-frequency, high-efficiency optoelectronic devices.
Schematic diagram of high-order harmonic generation in solids
Recently, the research group on laser-induced ultrafast electron dynamics at the Innovation Academy for Precision Measurement Science and Technology (APM) discovered, through simulating the high-order harmonic radiation phenomenon during the interaction between laser and a one-dimensional SSH chain crystal, that intracell and intercell harmonics undergo a π-phase jump at the effective-mass singularity. This transformation leads to a change from complete constructive interference in regions with positive effective mass to complete destructive interference in regions with negative effective mass. Meanwhile, when the polarization of the incident laser is aligned along the direction where mirror symmetry is broken in two-dimensional boron nitride, a similar phenomenon is also observed. However, this phenomenon is not seen when the polarization is along the direction where mirror symmetry is preserved. This study indicates that the effective mass plays a crucial role in the process of electron-hole pair recombination and harmonic radiation, and its significance is particularly prominent when considering the orientation-dependent characteristics of solid harmonics.
This achievement represents a significant theoretical advancement made by the laser-induced ultrafast electron dynamics team in the field of ultrafast dynamics in crystals under strong fields. The research findings were published in Physical Review Letters (Phys. Rev. Lett.) under the title "Mass-Singularity-Induced Phase Jump in High-Harmonic Generation from Symmetry-Breaking Crystals". Postdoctoral researcher CHEN Jiaxiang served as the first author, and Prof. BIAN Xuebin acted as the corresponding author.
This research was supported by grants from the Hubei Provincial Natural Science Foundation, the National Natural Science Foundation of China, and the CAS Project for Young Scientists in Basic Research.
Link to the article: https://doi.org/10.1103/PhysRevLett.134.186901