Magnetic-field induced second harmonic generation in CuB2O4

Giant Modulation of the Second Harmonic Generation by Magnetoelectricity in Two-Dimensional Multiferroic CuCrP2S6

Multiferroic materials have attracted considerable attention owing to their unique magnetoelectric or magnetooptical properties. The recent discovery of few-layer van der Waals multiferroic crystals provides a new research direction for controlling the multiferroic properties in the atomic layer limit. However, research on few-layer multiferroic crystals is limited and the effect of thickness-dependent symmetries on those properties is less explored. In this study, the symmetries and magnetoelectric responses of van der Waals multiferroic CuCrP2S6 are investigated by optical second harmonic generation (SHG). Structural and magnetic phase transitions are successfully probed by the temperature-dependent SHG signals, revealing significant changes by applying the magnetic field reflecting the magnetoelectric effect. Moreover, it is found that symmetries and resultant magnetoelectric responses can be modulated by the number of layers. These results offer a new principle of controlling the multiferroicity and indicate that 2D van der Waals multiferroic material is a promising building block for functional nanodevices.

Helicity-dependent resonant X-ray scattering in CuB2O4

Exploitation of X-ray circular polarized beams to study forbidden Bragg reflections and new information that could be obtained in these experiments are discussed. It is shown that the intensities of such reflections can be different for the right- and left-circular polarizations (i.e. exhibiting circular dichroism) even for the dipole-dipole resonant transitions involved in the scattering process. This difference can be observed only in crystals having no center of inversion. Here, this approach is used to study helicity-dependent resonant diffraction in copper metaborate CuB₂O₄ single crystal, which is non-centrosymmetric but achiral. Nonetheless, a strong circular dichroism has been observed for hh0 forbidden reflections in the vicinity of the Cu K-edge. This effect is shown to originate from dipolar transitions in Cu atoms occupying the 8(d) Wyckoff position only.

Nonreciprocal second harmonic generation in a magnetoelectric material

Mirror symmetries are of particular importance because they are connected to fundamental properties and conservation laws. Spatial inversion and time reversal are typically associated to charge and spin phenomena, respectively. When both are broken, magnetoelectric cross-coupling can arise. In the optical regime, a difference between forward and backward propagation of light may result. Usually, this nonreciprocal response is small. We show that a giant nonreciprocal optical response can occur when transferring from linear to nonlinear optics, specifically second harmonic generation (SHG). CuB₂O₄ exhibits SHG transmission changes by almost 100% upon reversal of a magnetic field of just ±10 mT. The observed nonreciprocity results from an interference between magnetic-dipole and electric-dipole SHG. Although the former is inherently weaker than the latter, a resonantly enhanced magnetic-dipole transition has a comparable amplitude as a nonresonant electric-dipole transition, thus maximizing the nonreciprocity. Multiferroics and magnetoelectrics are an obvious materials platform to exhibit nonreciprocal nonlinear optical functionalities.

Nonreciprocal Refraction of Light in a Magnetoelectric Material

In magnetoelectric materials, where the time-reversal and space-inversion symmetries are simultaneously broken, optical properties can differ between the opposite propagation directions of light. We report on an experimental observation of nonreciprocal trajectory of a light ray in magnetoelectric material CuB_{2}O_{4}. The light is refracted in different ways between the opposite propagation directions of light. We find a nonreciprocal refraction at the interface between a matter with macroscopic toroidal moment and vacuum. The resultant nonreciprocal deflection of the light is 0.005 deg, which is quantitatively explained using Fermat's principle.

Ab initio study of the enantio-selective magnetic-field-induced second harmonic generation in chiral molecules

We present a systematic ab initio study of enantio-selective magnetic-field-induced second harmonic generation (MFISHG) on a set of chiral systems ((l)-alanine, (l)-arginine and (l)-cysteine; 3,4-dehydro-(l)-proline; (S)-α-phellandrene; (R,S)- and (S,S)-cystine disulphide; N-(4-nitrophenyl)-(S)-prolinol, N-(4-(2-nitrovinyl)-phenyl)-(S)-prolinol, N-(4-tricyanovinyl-phenyl)-(S)-prolinol, (R)-BINOL, (S)-BINAM and 6-(M)-helicene). The needed electronic frequency dependent cubic response calculations are performed within a density functional theory (DFT) approach. A study of the dependence of the property on the choice of electron correlation, on one-electron basis set extension and on the choice of magnetic gauge origin is carried out on a prototype system (twisted oxygen peroxide). The magnetic gauge dependence analysis is extended also to the molecules of the set. An attempt to analyze the structure-property relationships is also made, based on the results obtained for biphenyl (in a frozen twisted conformation), for prolinol and for some of their derivatives. The strength of the effect is discussed, in order to establish its measurability with a proposed experimental setup.

One-Way Transparency of Light in Multiferroic CuB(2)O(4)

We experimentally demonstrate one-way transparency of light in multiferroic CuB(2)O(4). The material is rendered transparent for light propagating in one direction, while opaque for light propagating in the opposite direction. The novel transparency results from a destructive interference of the electric dipole and magnetic dipole transitions. The realization of the effect has been accomplished by the application of a high magnetic field and the proper selection of the propagation direction of light in agreement with our quantum mechanical formulation of nonreciprocal directional dichroism.

Theory of optically forbidden d-d transitions in strongly correlated crystals

A general multiband formulation of the linear and nonlinear optical response functions for realistic models of correlated crystals is presented. Dipole-forbidden d-d optical transitions originate from vertex functions, which we consider assuming the locality of an irreducible four-leg vertex. The unified formulation for second- and third-order response functions in terms of the three-leg vertex is suitable for practical calculations in solids. We illustrate the general approach by consideration of intra-atomic spin-flip contributions, with an energy of 2J, where J is a Hund exchange, in the simplest two-orbital model.

Probing surface and interface structure using optics

Optical techniques for probing surface and interface structure are introduced and recent developments in the field are discussed. These techniques offer significant advantages over conventional surface probes: all pressure ranges of gas-condensed matter interfaces are accessible and liquid-liquid, liquid-solid and solid-solid interfaces can be probed, due to the large penetration depth of the optical radiation. Sensitivity and discrimination from the bulk are the two challenges facing optical techniques in probing surface and interface structure. Where instrumental improvements have resulted in enhanced sensitivity, conventional optical techniques can be used to characterize heterogeneous adsorbed layers on a substrate, often with sub-monolayer resolution. Nanoscale lateral resolution is possible using scanning near-field optics. A separate class of techniques, which includes reflection anisotropy spectroscopy, and nonlinear optical probes such as second-harmonic and sum-frequency generation, uses the difference in symmetry between the bulk and the surface or interface to suppress the bulk contribution. A perspective is presented of likely future developments in this rapidly expanding field.

Comment on 'Calculated chiral and magneto-electric dichroic signals for copper metaborate (CuB(2)O(4)) in an applied magnetic field'

Contrary to a claim by Lovesey and Staub (2009 J. Phys.: Condens. Matter 21 142201), a careful treatment of symmetry shows that the application of a magnetic field along a twofold axis can induce the crystallographic chirality in a tetragonal system with the point group [Formula: see text] like CuB(2)O(4). The chirality is reversed by a 90° rotation of the magnetic field around the c axis.

Bound excitons in Sr2CuO3

We investigated temperature dependent optical spectra of the one-dimensional chain compound Sr2CuO3. The charge transfer transition polarized along the chain direction shows a strongly asymmetric line shape as expected in one-dimensional extended Hubbard model. At low temperature, the charge transfer peak shows a large blueshift and reveals additional sharp peaks at the gap. Even though many spectroscopic studies suggest that this material cannot have a bound exciton based on the one-dimensional extended Hubbard model, we attribute the additional sharp peaks to excitons, which come to exist due to the long-range Coulomb interaction.

Magnetic control of crystal chirality and the existence of a large magneto-optical dichroism effect in CuB2O4

The possibility of a magnetic field controlling the chirality of matter has been debated for a long time. Here, we report the successful induction of chirality in the noncentrosymmetric canted antiferromagnet, CuB2O4, by application of a low intensity static magnetic field. The chirality is reversed by a 90 degrees rotation of the direction of the magnetic field. The induction of chirality by a magnetic field gives rise to a gigantic enhancement of magnetochiral dichroism in this material. The ability to switch handedness in combination with this large magnetochiral optical effect allows us to design new optical devices such as a magnetically controllable isolator.

Spin and orbital quantization of electronic states as origins of second harmonic generation in semiconductors

Basically different mechanisms of optical second harmonic generation (SHG) in semiconductors, induced by an external magnetic field H, have been identified experimentally by studying the diluted magnetic semiconductor (Cd,Mn)Te. For paramagnetic (Cd,Mn)Te the SHG response is governed by spin quantization of electronic states, in contrast with diamagnetic CdTe with its dominating orbital quantization. The mechanisms can be identified by the distinct magnetic field dependence of the SHG intensity which scales with the spin splitting in the paramagnetic case as compared to the H2 dependence observed for the diamagnetic case.

Magnetic-field-induced second-harmonic generation in semiconductor GaAs

We show that application of a magnetic field induces optical second-harmonic generation (SHG) in GaAs. This phenomenon arises from field-induced symmetry breaking causing new optical nonlinearities. A series of narrow SHG lines is observed in the spectral range from 1.52 to 1.77 eV that we attribute to Landau-level quantization of the band energy spectrum. The rotational anisotropy of the SHG signal distinctly differs from that of the electric-dipole approximation. Model calculations reveal that nonlinear magneto-optical spatial dispersion that comes together with the electric-dipole term is the dominant mechanism for this nonlinearity.