Giant Optical Second Harmonic Generation in Two-Dimensional Multiferroics

Abstract

Nonlinear optical properties of materials such as second and higher order harmonic generation and electro-optic effect play pivotal roles in lasers, frequency conversion, electro-optic modulators, switches, and so forth. The strength of nonlinear optical responses highly depends on intrinsic crystal symmetry, transition dipole moments, specific optical excitation, and local environment. Using first-principles electronic structure theory, here we predict giant second harmonic generation (SHG) in recently discovered two-dimensional (2D) ferroelectric-ferroelastic multiferroics-group IV monochalcogenides (i.e., GeSe, GeS, SnSe, and SnS). Remarkably, the strength of SHG susceptibility in GeSe and SnSe monolayers is more than 1 order of magnitude higher than that in monolayer MoS, and 2 orders of magnitude higher than that in monolayer hexagonal BN. Their extraordinary SHG is dominated by the large residual of two opposite intraband contributions in the SHG susceptibility. More importantly, the SHG polarization anisotropy is strongly correlated with the intrinsic ferroelastic and ferroelectric orders in group IV monochalcogenide monolayers. Our present findings provide a microscopic understanding of the large SHG susceptibility in 2D group IV monochalcogenide multiferroics from first-principles theory and open up a variety of new avenues for 2D ferroelectrics, multiferroics, and nonlinear optoelectronics, for example, realizing active electrical/optical/mechanical switching of ferroic orders in 2D multiferroics and in situ ultrafast optical characterization of local atomistic and electronic structures using noncontact noninvasive optical SHG techniques.

Vibrational and dielectric properties of monolayer transition metal dichalcogenides

“…Comparisons to other literature results require care, as our calcu-lated values are the zero frequency limit of the frequencydependent non-linear susceptibility. Nevertheless, we find generally good agreement between our calculations and experimental measurements [130,131]. Our calculations indicate that the nonlinear susceptibilities of the h compounds change sign when going from S to Se to Te.…”

“…Our calculations indicate that the nonlinear susceptibilities of the h compounds change sign when going from S to Se to Te. The overall size of the tensor element decreases as the calculated band gap decreases, as was shown in other work [130].…”

First-principles Studies of Second-Order Nonlinear Optical Properties of Organic-Inorganic Hybrid Halide Perovskites

“…The linear optical dielectric function and NLO susceptibility are calculated based on the linear response formalism within the independent-particle approximation (IPA) [35][36][37][38]. The formulas are adapted from the reference [35] with slight modifications [39]. Because of the large unit cell of OHPs, the GW method [40], an established ab initio approach for obtaining reliable quasiparticle band gaps, is formidable for our current simulation capability.…”

“…Because of the large unit cell of OHPs, the GW method [40], an established ab initio approach for obtaining reliable quasiparticle band gaps, is formidable for our current simulation capability. Thus, we employ the hybrid functional and the scissor approximations to correct DFT band gaps [21,35,39,41], which have been widely used in previous works [4,27,33].…”

“…Because of the large unit cell of OHPs, the GW method [40], an established ab initio approach for obtaining reliable quasiparticle band gaps, is formidable for our current simulation capability. Thus, we employ the hybrid functional and the scissor approximations to correct DFT band gaps [21,35,39,41], which have been widely used in previous works [4,27,33].More sophisticated approaches, such as the "layer-by-layer" analysis [42], will be implemented in the future for improving the calculation of optical responses.Excitonic effects are not included in this work, due to the extremely high computational expense, especially for OHPs with a large number of electrons per unit cell. Moreover, electron-hole interactions are usually more significant in reduced-dimensional structures or large-gap W.S.…”