High-harmonic generation from spin-polarised defects in solids

Abstract

The generation of high-order harmonics in gases enabled to probe the attosecond electron dynamics in atoms and molecules with unprecedented resolution. Extending these techniques to solids, which were originally developed for atomic and molecular gases, requires a fundamental understanding of the physics that has been partially addressed theoretically. Here, we employ timedependent density-functional theory to investigate how the electron dynamics resulting in high-harmonic emission in monolayer hexagonal boron nitride is affected by the presence of vacancies. We show how these realistic spin-polarised defects modify the harmonic emission and demonstrate that important differences exist between harmonics from a pristine solid and a defected solid. In particular, we found that the different spin channels are affected differently by the presence of the spin-polarised point defect. Moreover, the localisation of the wavefunction, the geometry of the defect, and the electron-electron interaction are all crucial ingredients to describe high-harmonic generation in defected solids.npj Computational Materials (2020) 6:10 ; https://doi.

Effects of vacancies on high-order harmonic generation in a linear chain with band gap

“…5, the defect-state orbitals, located in the figure between the two vertical black lines, contribute significantly to the HHG yield compared to, e.g., the VB orbitals with indexes around 220. We note that the importance of the defect states for the HHG process was recently reported in a study on hexagonal boron nitride [37].…”

“…For example, interband HHG is produced in a process resembling the atomic three-step model [11,12] and involves the following steps: (i) an electron tunnels from the valence band to the conduction band, producing a hole in the valence band; (ii) the electron and its corresponding hole propagate in the solid, driven by the external field; and (iii) they recombine and emit HHG radiation with a frequency that corresponds to the band-gap energy at the crystal momentum at which the recollision occurs [13]. More generally, it has emerged that several aspects of the HHG process in solids can be captured by solving the time-dependent Schrödinger equation for Bloch electrons in single-active-electron models [14][15][16][17][18][19][20][21] by applying many-electron approaches such as the semiconductor Bloch equations [22][23][24][25], by time-dependent densityfunctional theory (TDDFT) [26][27][28][29][30][31][32][33][34][35][36][37], and by time-dependent Hartree-Fock theory [38].…”

“…Defects and vacancies are inevitable present in real solids and may also be engineered and controlled to modify properties of materials in a desired manner (see, e.g., the review [40]). Very recently it was investigated by TDDFT calculations how a single vacancy affects the HHG yield in hexagonal boron nitride [37]. The HHG spectra were found to be sensitive to whether a boron or a nitrogen atom was removed.…”

Imperfect Recollisions in High-Harmonic Generation in Solids

“…As a concrete example, we study HHG in monolayer hexagonal boron nitride (h-BN) driven by infrared pulses linearly polarized in the crystal plane, and we compare the ERM results to solutions of the semiconductor Bloch equations (SBEs) [33,34]. In addition to the recent intense interest in HHG from two-dimensional materials [8,28,[35][36][37][38][39], h-BN is interesting due to its lack of an inversion center which leads to non-zero Berry connections and TDPs. Also, pulse propagation effects [40,41] can be neglected in monolayer materials.…”

“…We found that the spatial width of the electron and hole wave packets can be almost one order of magnitude larger than the lattice constant, allowing for the imperfect recollisions. This suggests that the harmonic emission can probe the degree of spatial homogeneity of the periodic structure [25,39,[53][54][55] as well as the temporal dephasing introduced by e.g. electron correlation.…”