High-harmonic generation in quantum spin systems

Abstract

We theoretically study the high-harmonic generation (HHG) in one-dimensional spin systems. While in electronic systems the driving by AC electric fields produces radiation from the dynamics of excited charges, we consider here the situation where spin systems excited by a magnetic field pulse generate radiation via a time-dependent magnetization. Specifically, we study the magnetic dipole radiation in two types of ferromagnetic spin chain models, the Ising model with static longitudinal field and the XXZ model, and reveal the structure of the spin HHG and its relation to spin excitations. For weak laser amplitude, a peak structure appears which can be explained by time-dependent perturbation theory. With increasing amplitude, plateaus with well-defined cutoff energies emerge. In the Ising model with longitudinal field, the thresholds of the multiple plateaus in the radiation spectra can be explained by the annihilation of multiple magnons. In the XXZ model, which retains the Z2 symmetry, the laser magnetic field can induce a phase transition of the ground state when it exceeds a critical value, which results in a drastic change of the spin excitation character. As a consequence, the first cutoff energy in the HHG spectrum changes from a single-magnon to a two-magnon energy at this transition. Our results demonstrate the possibility of generating high-harmonic radiation from magnetically ordered materials and the usefulness of high-harmonic signals for extracting information on the spin excitation spectrum. arXiv:1901.07588v2 [cond-mat.str-el]

Tunable Magnon Interactions in a Ferromagnetic Spin-1 Chain

“…Depending on the field orientation these interactions are either attractive or repulsive (at large transverse field). We note that while our experimental work relied on thermal excitations to generate and subsequently probe two magnons within linear response, one can imagine utilizing non-linear THz spectroscopy with intense THz pulses to directly excite higher order states via two-photon absorption [41,42]. Subsequent interaction dynamics may be studied with the resulting non-linear response of the system.…”

High-harmonic generation in spin-orbit coupled systems

“…Which of these two processes dominates the emission has been debated for a long time, and a unified HHG mechanism applicable to a wide range of solids is still lacking [12]. Recently, the scope of HHG studies has been extended to strongly correlated systems [8,9,[13][14][15][16], disordered systems [17][18][19][20], the effects of spin-polarized defects [21], spin or multiferroic systems [22,23], HHG in graphene and transition metal dicalchogenides [24,25], to mention a few.…”

“…Prior to the Fourier transform, we apply a Blackman window to all quantities. This is given as f B (t ) = 0.42-0.5 cos( 2πt MT ) + 0.08 cos( 4πt MT ) [22]. Care must be taken with the spin current as it has a DC component [46].…”

High-order nonlinear optical response of a twisted bilayer graphene

“…This phenomenon has attracted interest not only for compact frequency converter applications [12,13] but also as a probe of electron dynamics in intense optical fields [14]. Among various systems such as semiconductors [15][16][17][18][19][20][21][22][23][24][25], superconductors [26][27][28], strongly correlated systems [29][30][31][32][33][34], quantum magnets [35][36][37], and topological insulators [38,39], Dirac materials have turned out to have extremely large nonlinear susceptibility from the mid-infrared [40][41][42] down to the teraherz [43][44][45] frequency regimes. In particular, nonlinear response of graphene has been studied extensively [46][47][48][49][50].…”