Recently, the Laser-induced Ultrafast Electron Dynamics (LUED) Group from the Innovation Academy for Precision Measurement Science and Technology (APM) has made significant progress in the study of high-order harmonic generation (HHG) in quasicrystals. The research team dismantled the barriers of studying quasicrystals by ultrafast and strong-field physics. They theoretically investigated the mechanism and advantages of quasicrystals as an ultrafast light source. The work has been published in Physical Review Letters.

The extreme nonlinear frequency up-conversion HHG process in the laser-matter interaction is of great value in producing extreme ultraviolet light sources and ultrashort attosecond pulses. In addition, HHG encodes electron dynamic information on the attosecond timescale and can reflect the microstructure of matter. Previous studies have shown that gases, plasmas, crystals, and liquids are all capable of producing HHG. Though with high photon energies, gas is inefficient in producing harmonics due to its low density. In recent years, crystal HHG has attracted much attention, but the low damage threshold of crystals limits the yield of high-energy photons. Liquids can withstand higher luminous intensities, but their disordered structure limits the yield of high-energy harmonic photons. Quasicrystals are special ordered systems without translational symmetry, which have not been studied with ultrafast and strong-field lasers to produce HHG.

The LUED group of APM has been engaged in the theoretical research of HHG in gases, crystals, and liquids. So far, the group has made a series of original and influential results of this concern. The lack of translational periodicity stands both a chance and a challenge for harmonic radiation, where the energy band theory of crystals is no longer conveniently applicable as the way it was. In such a situation, the research group simulated the Fibonacci quasicrystal (FQ) HHG for the first time and revealed that the acceleration theorem is approximately applicable in extracting the electron dynamics information of FQ. Moreover, from the sharp Bragg diffraction peaks of quasicrystals, the group found that the fractal energy band structure of quasicrystals can approximately be reconstructed by translating the energy bands of effective crystals, which facilitates the extraction of electron dynamics information from quasicrystals. The study shows that the quasicrystal fractal band structure leads to more electron excitation channels and more frequent backscattering, which offer the quasicrystals with advantages including higher HHG yield, higher cutoff energy, and more sensitivity in wavelength-dependence. Quasicrystals show the potential to become new and integratable solid-state ultrafast light sources and inspire a brand new research field.

Ph.D. student Jia-Qi Liu is the first author and Professor Xue-Bin Bian is the corresponding author.

This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China and the LU JIAXI International team program supported by the K.C. Wong Education Foundation and CAS.

Full text link：https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.213901