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Register a new userHigher-Order Harmonic Generation caused by Elliptically Polarized Electric Fields in Solid-State Materials
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We theoretically investigated the dependence of higher-order harmonic generation (HHG) in solid-state materials on the ellipticity of the electric field. We found that in the multiphoton absorption and ac Zener regimes, the intensity of HHG monotonically decreases with increasing ellipticity of the incident electric field, while in the semimetal regime, the intensity reaches a maximum for finite values of ellipticity. Moreover, the characteristics of the polarization of the emitted HHG change depending on the field intensity; only parallel emissions with respect to the major axis exist in the multiphoton absorption and ac Zener regimes, while both parallel and perpendicular emissions exist in the semimetal regime. These peculiar characteristics of the semimetal regime can be understood on the basis of changes in the HHG mechanism and may be able to be identified in experiments utilizing solid-state materials such as narrow-gap semiconductors.
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High harmonic generation (HHG) has unleashed the power of strong laser physics in solids. Here we investigate HHG from a large system, solid C$_{60}$, with 240 valence electrons engaging harmonic generation at each crystal momentum, the first of this kind. We employ the density functional theory and the time-dependent Liouville equation of the density matrix to compute HHG signals. We find that under a moderately strong laser pulse, HHG signals reach 15th order, consistent with the experimental results from C$_{60}$ plasma. The helicity dependence in solid C$_{60}$ is weak, due to the high symmetry. In contrast to the general belief, HHG is unsuitable for band structure mapping in C$_{60}$. However, we find a window of opportunity using a long wavelength, where harmonics are generated through multiple-photon excitation. In particular, the 5th order harmonic energies closely follow the transition energy dispersion between the valence and conduction bands. This finding is expected to motivate future experimental investigations.
High-harmonic generation (HHG), a typical nonlinear optical effect, has been actively studied in electron systems such as semiconductors and superconductors. As a natural extension, we theoretically study HHG from electric polarization, spin current and magnetization in magnetic insulators under terahertz (THz) or gigahertz (GHz) electromagnetic waves. We use simple one-dimensional spin chain models with or without multiferroic coupling between spins and the electric polarization, and study the dynamics of the spin chain coupled to an external ac electric or magnetic field. We map spin chains to two-band fermions and invoke an analogy of semiconductors and superconductors. With a quantum master equation and Lindblad approximation, we compute the time evolution of the electric polarization, spin current, and magnetization, showing that they exhibit clear harmonic peaks. We also show that the even-order HHG by magnetization dynamics can be controlled by static magnetic fields in a wide class of magnetic insulators. We propose experimental setups to observe these HHG, and estimate the required strength of the ac electric field $E_0$ for detection as $E_0sim100$kV/cm--1MV/cm, which corresponds to the magnetic field $B_0sim0.1$T--1T. The estimated strength would be relevant also for experimental realizations of other theoretically-proposed nonlinear optical effects in magnetic insulators such as Floquet engineering of magnets.
We investigate high-order harmonic generation (HHG) in graphene with a quantum master equation approach. The simulations reproduce the observed enhancement in HHG in graphene under elliptically polarized light [N. Yoshikawa et al, Science 356, 736 (2017)]. On the basis of a microscopic decomposition of the emitted high-order harmonics, we find that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light. Furthermore, we reveal that contributions from different excitation channels destructively interfere with each other. This finding suggests a path to potentially enhance the HHG by blocking a part of the channels and canceling the destructive interference through band-gap or chemical potential manipulation.
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