V. V. Pavlov

Second-harmonic generation as a tool for studying electronic and magnetic structures of crystals: review

Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed. The symmetry of such crystals is determined by the arrangement of both the charges and the spins, so their contributions to the crystallographic and the magnetic structures, respectively, must be distinguished. Magnetic SHG is introduced as a probe for magnetic structures and sublattice interactions. The specific degrees of optical experiments - including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism. These include local or hidden phase transitions, interacting magnetized and polarized sublattices and domain walls, and magnetic interfaces. SHG in various centrosymmetric and noncentrosymmetric crystal classes of antiferromagnetic oxides such as Cr2O3, hexagonal RMnO3(R=Sc,Y,In,Ho-Lu), magnetic garnet films, CuB2O4, CoO, and NiO, is discussed.

Optical properties and electronic structure of multiferroic hexagonal orthoferrites RFeO3 (R = Ho, Er, Lu)

We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO3 (R = Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the range of 0.6–5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe3+ ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices.

Linear and nonlinear optical spectroscopy of gadolinium iron borate GdFe3(BO3)4

The optical spectra and the second-harmonic generation (SHG) are studied in a noncentrosymmetric GdFe3(BO3)4 magnet. In the region of weak absorption (α∼20–400 cm−1) below ∼3 eV, three absorption bands are distinguished, which can be unambiguously assigned to forbidden electronic transitions from the ground 6A1 state of the Fe3+ ion to its excited states 4T1(∼1.4 eV), 4T2(∼2 eV), and 4A1, 4E(∼2.8 eV). Intense absorption begins in the region above 3 eV (α∼2–4×105 cm−1), where two bands at ∼4.0 and 4.8 eV are observed, which are caused by allowed electric dipole charge-transfer transitions. The spectral features of SHG in the 1.2–3.0-eV region are explained by a change in the SHG efficiency caused by a change in the phase mismatch. It is shown that in the weak absorption region, phase matching can be achieved for SHG.

Determination of the magnetic structure of hexagonal manganites RMnO3 (R=Sc, Y, Ho, Er, Tm, Yb) by second-harmonic spectroscopy

Hexagonal manganites RMnO3 (R=Sc, Y, Ho, Er, Tm, Yb) are compounds in which a ferroelectric and an antiferromagnetic order may coexist. The ferroelectric crystal structure is well known but theoretical studies show that there are different possibilities for the spin structure. By means of polarization- and temperature-dependent second-harmonic spectroscopy it is possible to determine the magnetic structure of RMnO3 compounds.

Linear magnetoelectric effect in magnetic garnet thin films

Abstract Linear magnetoelectric effect (ME) in epitaxial iron garnet films of (YBiPrLu)3 (FeGa)5O12 composition has been studied by means of optical polarimetric method with spatial resolution ∼2.5 μm. The measurements were carried out in both longitudinal H∥E and transversal H⊤E geometry for films grown on the substrates of (111) and (210)-orientation. The magnitude of ME effect within individual magnetic domain and originated from domain wall movement is evaluated. The strong anisotropy of ME effect in transversal geometry in (210)-films was observed in the regime of rotation of magnetization. The electric field induced changes of magnetic anisotropy parameters responsible for the observed effects are evaluated.

Second-harmonic generation spectroscopy of excitons in ZnO

Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report results of a comprehensive experimental and theoretical study of the three-photon process of optical second harmonic generation (SHG) involving the exciton resonances of the noncentrosymmetric hexagonal wide-band-gap semiconductor ZnO in the photon energy range of 3.2-3.5 eV. Resonant crystallographic SHG is observed for the 1s(A,B), 2s(A,B), 2p(A,B), and 1s(C) excitons. We show that strong SHG signals at these exciton resonances are induced by the application of a magnetic field when the incident and the SHG light wave vectors are along the crystal z-axis where the crystallographic SHG response vanishes. A microscopic theory of SHG generation through excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation. Depending on the particular symmetry of the exciton states SHG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. The importance of each mechanism is analyzed and discussed by confronting experimental data and theoretical results for the dependencies of the SHG signals on photon energy, magnetic field, electric field, crystal temperature, and light polarization. Good agreement is obtained between experiment and theory proving the validity of our approach to the complex problem of nonlinear interaction of light with ZnO excitons. This general approach can be applied also to other semiconductors.

Optical second harmonic generation in the centrosymmetric magnetic semiconductors EuTe and EuSe

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spin-induced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4 eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.

Magnetization-odd nonreciprocal reflection of light from the magnetoelectric—ferromagnet LiFe5O8

Circular dichroism, which changes sign under magnetization reversal and exhibits a 120° periodicity, is observed in the reflection of light from the (111) plane of a noncentrosymmetric LiFe5O8 crystal in the transverse geometry k⊥M, where the linear Kerr effect is forbidden. It is shown that this phenomenon is due to the manifestation of optical magnetoelectric susceptibility and a Kerr effect of third order in the magnetization. The spectral dependences of the circular dichroism in the range 1.4–3.1 eV show that this phenomenon is of a resonance character.

Second-harmonic generation in the magnetic semiconductor (Cd, Mn)Te

A phenomenological analysis and experimental observations of second-harmonic generation (SHG) in magnetic semiconductors with zinc blende crystal structure ¯43m in zero and nonzero magnetic fields are presented. A magnetic field reduces symmetry and gives rise to new SHG components. SHG transmission spectra were studied in Cd_1−xMn_xTe with x = 0.24, 0.35, and 0.40. At low temperatures a narrow SHG band at 1.98 eV in the vicinity of the fundamental bandgap was observed in a (110) sample with x = 0.24. The band disappears with a temperature increase when short-range magnetic ordering vanishes. Below the bandgap a SHG signal is found that varies strongly as a function of magnetic field. We relate this phenomenon to magnetic linear birefringence and the shift of the bandgap in a magnetic field.

Nonlinear optical spectroscopy of epitaxial magnetic garnet films

Second and third harmonic optical spectra are studied in epitaxialmagnetic thin films in the spectral ranges 1.7–3.2 eV and 2.4–4.2 eV, respectively. No significant increase of the intensity of the nonlinear spectra is found above the bandgap near 3.2 eV, where the linear absorption increases by two orders of magnitude. Large magnetic contributions to the second harmonic spectra and magnetic contrast as high as 100% are observed at selected photon energies. Contrary to that, no magnetic contribution to the third harmonic spectra is found.