Second Harmonic Generation of Yttrium Orthochromite with Magnetic Origin

Abstract

Some of perovskite-type oxides composed of rare-earth and transition metal ions have been attracting interests due to their magnetic, optical, magneto-electric and magnetooptical effects caused by their spin ordering. Among them yttrium orthochromite (YCrO3) is important as a fundamental material because it includes only spins of transition metals so its magnetic structure is relatively simple compared to other crystals in this family. Therefore it is possible to study the effects from the electronic and magnetic structure of chromium ions directly, due to the lack of spin interactions between rare-earth and transition metal ions and optical absorption from rare-earth ions. Recently, nonlinear optical spectroscopy has been widely employed as a tool for probing the magnetic structures of which details cannot be elucidated by conventional magnetic or magneto-optical methods. Optical nonlinearity of the second order as the second harmonic generation (SHG) is the most powerful tool for this purpose because it is very sensitive to the symmetry of magnetic structures as well as crystal structures. There have been found several types of SHG with magnetic origins. Elimination of space inversion symmetry by anti-ferromagnetic ordering induces SHG. For example, spin structure in ordered phase and their reorientation transition has been widely investigated in rareearth manganite crystals. Magnetization induced SHG can be also observed in symmetry broken surfaces of ferromagnets. SHG signals due to magnetic dipole transitions have been observed in chromium oxide crystals. Distorted perovskite-type crystal yttrium orthochromite (YCrO3) belongs to mmm point group at room temperature, and shows antiferromagnetism and weak ferromagnetism below Neel temperature 141K and belongs to mmm magnetic point group. Because these point groups has space inversion element, the crystals should not give the SHG via conventional processes. We observed SHG from the crystal and assigned the origin to axial nonlinear optical tensor components relating to magnetic-dipole transition from its spectral and polarization characteristics. The temperature dependence of SHG intensity was also measured in order to investigate the relation of optical nonlinearity and magnetic ordering properties. Single crystals were grown by a flux method and their thicknesses were reduced to about 100 mm by polishing. Absorption spectrum of the samples is given in Fig. 1, which shows absorption peaks due to d–d transitions in visible wavelength region. The peak at 450 nm is assigned to the quadrupole allowed transition to T1g state of trivalent chromium ions in octahedral site, and the other at 600 nm is to the magnetic-dipole allowed transition to T2g state. 14) The giant absorption in the UV region is caused by the charge-transfer transition from oxygen to chromium. The SHG spectrum was studied by the irradiation of an infrared pulsed laser light on the sample in the range of 450– 700 nm for SHG. An optical parametric oscillator pumped by a Q-switched frequency-tripled Nd:YAG laser (Continuum, Surelite II and Surelite OPO, 10Hz, 5 ns) was used as a laser source. The fundamental beam was focused with a lens ( f 1⁄4 50mm) on a polished plane of the sample located at the focal point with normal angle. The transmitted SHG signals filtered by a monochromator and color filters were detected by a photomultiplier tube and a boxcar integrator. Intensity of the incident pulse was kept at 0.3mJ in all wavelength range by a variable attenuator. The SHG intensity was proportional to the square of the input intensity at least up to 0.5mJ. The spectrum of SHG intensity is also depicted in the Fig. 1 which shows a peak resonant to the magnetic-dipole allowed transition at 600 nm and no signal resonant to the absorption at 450 nm. The polarization direction of the generated harmonic light was parallel to that of the fundamental light. When the polarization of incident light was rotated (actually the samples were rotated), the SHG intensity gave two-fold symmetry and it can be well fit to the function of sin . From these experimental results, apparently nonlinear optical processes including magnetic-dipole transition is known to contribute to the SHG signals. There are four types of third-rank tensors contributable to the SHG, and these are distinguished by polar and axial characteristics and also by i (time invariant) and c (time non-invariant) types. All components of odd-ranked polar tensors and even-ranked axial tensors vanishes if spacial inversion is included in the symmetry elements of the point group to which the material belongs. Only six components ð2Þi;a xyz , ð2Þi;a xzy , ð2Þi;a yzx , ð2Þi;a yxz , ð2Þi;a zyx and ð2Þi;a zxy of axial i-type tensor has non-zero values for SHG tensor of mmm crystals. In addition to these components, several axial c-tensor components are non-zero in mmm magnetic point group, that is, ð2Þc;a zxx , ð2Þc;a zyy , ð2Þc;a zzz , ð2Þc;a yyz , ð2Þc;a yzy , ð2Þc;a xzx and ð2Þc;a xxz .