Nonlinear Magneto-optical Effect Research Articles

Giant and controllable nonlinear magneto-optical effects in two-dimensional magnets

The interplay of polarization and magnetism in materials with light can create rich nonlinear magneto-optical (NLMO) effects, and the recent discovery of two-dimensional (2D) van der Waals magnets provides remarkable control over NLMO effects due to their superb tunability. Here, based on first-principles calculations, we reported giant NLMO effects in CrI3-based 2D magnets, including a dramatic change of second-harmonics generation (SHG) polarization direction (90°) and intensity (on/off switch) under magnetization reversal and a 100% SHG circular dichroism effect. We further revealed that these effects could not only be used to design ultra-thin multifunctional optical devices but also to detect subtle magnetic orderings. Remarkably, we analytically derived conditions to achieve giant NLMO effects and proposed general strategies to realize them in 2D magnets. Our work not only uncovers a series of intriguing NLMO phenomena but also paves the way for both fundamental research and device applications of ultra-thin NLMO materials.

Second Harmonic Generation Versus Linear Magneto-Optical Response Studies of Laser-Induced Switching of Pinning Effects in Antiferromagnetic/Ferromagnetic Films

Composite magnetic nanostructures are a subject of high research interest, as they provide a number of exciting effects absent in live nature. Among others, much attention has been paid to the studies of exchange coupling in antiferromagnetic/ferromagnetic (AFM/FM) films, which leads to the pinning effect. It manifests itself as a widening and shift of the magnetic hysteresis loop with respect to zero value of the external magnetic field oriented along the pinning direction. In this work, we report on comparative studies of linear and nonlinear magneto-optical effects under the laser-induced switching of the pinning effect in IrMn/CoFe films of various thickness of the ferromagnetic CoFe layer. We found that the magneto-optical response of the pinned AFM/FM nanofilms appears with different hysteresis loop parameters in the transverse magneto-optical Kerr effect (MOKE) and interface-sensitive magnetization-induced second harmonic generation (SHG), indicating the diversity of the magnetic effects at interfaces compared to the bulk of the films.

Ultrafast light-driven magneto-optical nonlinearity in ferromagnetic heterostructures.

The dynamic control of magnetization by short laser pulses has recently attracted interest. The transient magnetization at the metallic magnetic interface has been investigated through second-harmonic generation and the time-resolved magneto-optical effect. However, the ultrafast light-driven magneto-optical nonlinearity in ferromagnetic heterostructures for terahertz (THz) radiation remains unclear. Here, we present THz generation from a metallic heterostructure, Pt/CoFeB/Ta, which is ascribed to an ∼6-8% contribution from the magnetization-induced optical rectification and an ∼94-92% contribution from both spin-to-charge current conversion and ultrafast demagnetization. Our results show that THz-emission spectroscopy is a powerful tool to study the picosecond-time-scale nonlinear magneto-optical effect in ferromagnetic heterostructures.

Theoretical investigation of the nonreciprocal bistable absorption of electromagnetic waves in one-dimensional photonic crystals combined with the nonlinear magnetized plasma defect layer

In this paper, we investigate the nonreciprocity bistable absorption of a one-dimensional photonic crystals combined with a novel nonlinear magnetized plasma defect layer. It is figured out that the structure has apparent nonreciprocity, bistable positive and negative absorption of the forward and backward incidences under the nonlinear magneto-optical effect. In addition, the tuning effects of the external magnetic fields and angles are studied. The consequences indicate that the two have significantly opposite working modulation effects on the bistable working range and absorptance. Specifically, the external magnetic field leads to individually controlling the bistability switch-down threshold, and the angle can affect the switch-up and switch-down thresholds concurrently. In the simulation of the electric field diagrams of the proposed structure, the incident side is larger than the transmitted to verify these negative absorption properties. The external magnetic fields and angles will form an alternating conversion between positive and negative absorption with different transmitted intensities. Considering the significance of theoretical experimental verification, concise and feasible experimental schemes are given. This work provides a constructive proposal for the design of magnetization modulated isolators for the nonlinear magnetized plasma materials.

A simple setup for in situ alkali metal electronic spin polarimetry.

Faraday rotation is considered a gold standard measurement of the electronic spin polarization of an alkali metal vapor produced under optical pumping. However, during the production of large volumes of hyperpolarized xenon gas, transmission monitoring measurements, otherwise known as field cycling measurements, are generally employed to measure the spin polarization of alkali metal atoms in situ as this method is easier to implement than Faraday rotation on standard polarizer setups. Here, we present a simple, low-cost experimental setup to perform Faraday rotation measurements of the electronic spin polarization of alkali metal atoms that can be easily implemented on standard polarizer setups. We then compare Rb polarization measurements obtained with the Faraday rotation method to those obtained with the transmission monitoring method. To our knowledge, a direct comparison of these methods has never been made. Overall, we found good agreement between the two methods, but at low Rb density and high laser power, we found evidence of nonlinear magneto-optical effects that may prevent Faraday rotation from being used under these conditions.

A highly drift-stable atomic magnetometer for fundamental physics experiments

We report the design and performance of a nonmagnetic drift stable optically pumped cesium magnetometer with a measured sensitivity of 35 fT at 200 s integration time and stability below 50 fT between 70 and 600 s. The sensor is based on the nonlinear magneto-optical rotation effect: in a Bell–Bloom configuration, a higher order polarization moment (alignment) of Cs atoms is created with a pump laser beam in an anti-relaxation coated Pyrex cell under vacuum, filled with Cs vapor at room temperature. The polarization plane of light passing through the cell is modulated due the precession of the atoms in an external magnetic field of 2.1 μT, used to optically determine the Larmor precession frequency. Operation is based on a sequence of optical pumping and observation of freely precessing spins at a repetition rate of 8 Hz. This free precession decay readout scheme separates optical pumping and probing and, thus, ensures a systematically highly clean measurement. Due to the residual offset of the sensor of <15 pT together with negligible crosstalk of adjacent sensors, this device is uniquely suitable for a variety of experiments in low-energy particle physics with extreme precision, here as a highly stable and systematically clean reference probe in search for time-reversal symmetry violating electric dipole moments.

Surface plasmon driven enhancement of linear and nonlinear magneto-optical Kerr effects in bimetallic magnetoplasmonic crystals in conical diffraction

Nowadays, control of light based on the manipulation of surface plasmon polaritons (SPPs) in plasmonic crystals becomes a cornerstone of photonics. The authors study the SPP-driven enhancement of linear and nonlinear magneto-optical effects in magnetoplasmonic crystals (MPCs) based on corrugated gold and ferromagnetic films. A signature of the considered MPCs is the magnetic anisotropy mediated by the surface profile of a thin ferromagnetic layer, which allows to intertwine plasmonic effects with nonuniform magnetization, providing an extra venue for control of magneto-optical effects.

Basic formulation and first-principles implementation of nonlinear magneto-optical effects

First-principles calculation of nonlinear magneto-optical effects has become an indispensable tool to reveal the geometric and topological nature of electronic states and to understand light-matter interactions. While intriguingly rich physics could emerge in magnetic materials, further methodological developments are required to deal with time-reversal symmetry breaking, due to the degeneracy and gauge problems caused by symmetry and the low-frequency divergence problem in the existing calculation formalism. Here we present a gauge-covariant and low-frequency convergent formalism for the first-principles computation. Remarkably, this formalism generally works for both non-magnetic and magnetic materials with or without band degeneracy. Reliability and capability of our method are demonstrated by studying example materials (i.e., bilayers of MnBi$_2$Te$_4$ and CrI$_3$) and comparing with published results. Moreover, an importance correction term that ensures gauge covariance of degenerate states is derived, whose influence on physical responses is systematically checked. Our method enables computation of nonlinear magneto-optical effects in magnetic materials and paves the way for exploring rich physics created by the interplay of light and magnetism.

Генерация магнитоиндуцированной второй гармоники в тонких пленках с интерфейсами ферромагнетик/антиферромагнетик

We experimentally studied the generation of optical second harmonic (SH) and magneto-optical effects at the SH frequency in thin films consisting of ferromagnetic and antiferromagnetic materials, including exchange-bonded layers, one of which is pinned with an antiferromagnetic layer. As expected, the loops of the linear magneto-optical Kerr effect are shifted relative to the zero magnetic field value, which reflects the presence of a pronounced exchange interaction in such structures. It is shown that in the case of the nonlinear Kerr magneto-optical effect this effect is also present, but it is significantly smaller than in the nonlinear case, and leads to the observation of only one hysteresis loop instead of two. The studied dependence of the magneto-optical response at the TH frequency on the probing laser radiation power demonstrates that its growth leads to a decrease in the magnetic hysteresis loop shift of the TH.

Magnetization induced second harmonic generation in thin films with ferromagnet/antiferromagnet interfaces

We experimentally studied the generation of optical second harmonic (SH) and magneto-optical effects at the SH frequency in thin films consisting of ferromagnetic and antiferromagnetic materials, including exchange-bonded layers, one of which is pinned with an antiferromagnetic layer. As expected, the loops of the linear magneto-optical Kerr effect are shifted relative to the zero magnetic field value, which reflects the presence of a pronounced exchange interaction in such structures. It is shown that in the case of the nonlinear Kerr magneto-optical effect this effect is also present, but it is significantly smaller than in the nonlinear case, and leads to the observation of only one hysteresis loop instead of two. The studied dependence of the magneto-optical response at the TH frequency on the probing laser radiation power demonstrates that its growth leads to a decrease in the magnetic hysteresis loop shift of the TH. Keywords: magnetization-induced SHG, ferromagnet/antiferromagnet interface, pinned structure, thin films

Tailored Magnetic Linear Birefringence in Wedge-Shaped Co Nanocluster Assemblies

The purpose of this paper is to present an experimental method to induce strong magnetic linear birefringence in two-dimensional assemblies of Co nanoclusters grown on glass plates. Additionally, we have also correlated the magnitude and characteristics of that nonlinear magneto-optical effect with the thickness and profile of those disordered nanostructures. For those aims, we have grown Co nanocluster assemblies on amorphous substrates, by means of pulsed laser ablation in off-axis geometry. This approach enabled us to obtain magnetic media with an intended and pronounced thickness profile, i.e., wedge-shaped assembly, to investigate the orientation and behavior of surface magnetization regarding both the thickness gradient direction and in-plane magnetic field. That study was accomplished by measuring the magneto-optical effects in reflection and transmission configurations, unveiling an out-of-plane magnetization whose magnitude depends closely on the thickness gradient direction. That component, arising from a graded magnetic anisotropy along the wedged nanostructure, adds a reversal mechanism to the surface magnetization, thus being responsible for the magnetic linear birefringence in our ultrathin Co assemblies.

Nonlinear magneto-optic effects in doped graphene and in gapped graphene: A perturbative treatment

The nonlinear magneto-optic responses are investigated for gapped graphene and doped graphene in a perpendicular magnetic field. The electronic states are described by Landau levels, and the electron dynamics in an optical field is obtained by solving the density matrix in the equation of motion. In the linear dispersion approximation around the Dirac points, both linear conductivity and third order nonlinear conductivities are numerically evaluated for infrared frequencies. The nonlinear phenomena, including third harmonic generation, Kerr effects and two photon absorption, and four wave mixing, are studied. All optical conductivities show strong dependence on the magnetic field. At weak magnetic fields, our results for doped graphene agree with those in the literature. We also present the spectra of the conductivities of gapped graphene. At strong magnetic fields, the third order conductivities show peaks with varying the magnetic field and the photon energy. These peaks are induced by the resonant transitions between different Landau levels. The resonant channels, the positions, and the divergences of peaks are analyzed. The conductivities can be greatly modified, up to orders of magnitude. The dependence of the conductivities on the gap parameter and the chemical potential is studied.

A cascading nonlinear magneto-optical effect in topological insulators

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometers, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-TI to a 2D insulator occurs. Although such a crossover is generally accessible by scanning tunneling microscopy, or by angle-resolved photoemission spectroscopy, such measurements require clean surfaces. Here, we demonstrate that a cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling can examine such crossovers. The helicity dependence of the time-resolved Kerr rotation exhibits a robust change in periodicity at a critical thickness, from which it is possible to predict the formation of a Dirac cone in a film several quintuple layers thick. This method enables prediction of a Dirac cone using a fundamental nonlinear optical effect that can be applied to a wide range of TIs and related 2D materials.

Anisotropy of magnetic properties in 2D arrays of permalloy antidots

Anisotropy of linear and nonlinear magnetooptical (MO) effects in a regular array of Py antidots is studied by means of the MO Kerr effect at the fundamental and second harmonic wavelengths. We have demonstrated that the value of the MO effect, coercivity, and magnetization distribution depend substantially on the azimuthal orientation of the antidots array relatively to the external magnetic field.

Giant nonlinear magneto-optical response of magnetoplasmonic crystals

Magnetoplasmonics provides unique possibilities for magnetic field control over light flow. A special interest in this field is attracted to magnetoplasmonic crystals (MPCs) that are nanostructured metal-dielectric materials with rich resonant spectra that can be tuned by a magnetic field. We show that MPC demonstrates enhanced nonlinear-optical effects such as optical second harmonic generation (SHG) and a giant nonlinear magneto-optical effect. Contrary to the linear-optical case, the main mechanism underlying the observed effects is the resonant enhancement of the local field achieved both for the surface-plasmon-polariton modes at metal/dielectric interfaces and for the waveguide modes in a dielectric slab, which provides an up to 25% magnetic field controlled SHG intensity variation. These effects are treated as an interplay of excitation of different modes that leads to Fano-type resonances in the nonlinear-optical response.

Strong magneto-optical effects due to surface states in three-dimensional topological insulators.

We show that a thin film of a three-dimensional topological insulator such as Bi(2)Se(3)or Bi(2)Te(3) can exhibit strong linear and nonlinear magneto-optical effects in a transverse magnetic field. In particular, one can achieve an almost complete circular polarization of an incident terahertz or mid-infrared radiation and an efficient four-wave mixing.

Transient Optical Phenomenon in Ferrofluids

In this paper, we report the experimental observation of diffraction patterns in a ferrofluid comprising of Fe3O4 nanoparticles in hexane by a 10 mW He-Ne laser beam. An external dc magnetic field (∼1.7kG) was applied perpendicular to the beam. Transient phenomenon in diffraction patterns is studied with and without magnetic field. Diffraction patterns showed a variation in both zero and applied magnetic field. This provides some information of non-linear optical properties and magneto-optical effects. Laser beam passed through a sample establishes refractive index gradient modifying the original Gaussian beam of the laser. Analysis of transient diffraction patterns can be used to study the parameters of ferrofluid like viscosity and thermal conductivity. This gives a new insight to understand the dynamics of the magnetic fluid and its potential use for the development of ferrofluid based sensors.

Quadratic magnetooptical effects in two-dimensional permalloy particles investigated by scanning X-ray microscopy

We have investigated the magnetization structure and magnetization curves of individual rectangularity shaped permalloy particles using scanning X-ray microscopy in the ultrasoft X-ray regime. Magnetic contrast originates from X-ray magnetic circular dichroism and from the transverse magnetooptical Kerr effect. We studied magnetization curves in dependence on the field direction for particles of different shapes and sizes. Adjacent particles cause a significant dipole interaction. Asymmetric magnetization loops indicate the presence of non-linear magnetooptical effects.

Quantum-enhanced magnetometer with low-frequency squeezing

We report the demonstration of a magnetometer with noise-floor reduction below the shot-noise level. This magnetometer, based on a nonlinear magneto-optical rotation effect, is enhanced by the injection of a squeezed vacuum state into its input. The noise spectrum shows squeezed noise reduction of about 2 dB spanning from close to 100 Hz to several megahertz. We also report on the observation of two different regimes of operation of such a magnetometer: one in which the detection noise is limited by the quantum noise of the light probe only, and one in which we see additional noise originating from laser noise which is rotated into the vacuum polarization.

파라핀이 코팅된 Rb 증기 셀에서 비선형 광자기 효과 신호

본 연구에서는 파라핀이 코팅된 Rb원자 증기 셀을 이용하여 <TEX>$^{87}Rb$</TEX> <TEX>$D_1$</TEX>전이선의 Hanle스펙트럼과 비선형 광자기 효과 신호를 조사하였다. 파라핀 코팅 증기 셀의 효과에 의한 좁은 스펙트럼 폭을 가진 영역과 순수 증기 셀에서 얻을 수 있는 넓은 스펙트럼폭을 가진 이중구조의 Hanle 스펙트럼을 관측할 수 있었다. 스펙트럼에 나타난 이중구조에서 좁은 선폭 영역의 선폭은 1 kHz로 측정되었고, 스펙트럼 크기는 전체 스펙트럼에 대해서 10%로 측정되었다. 또한, Hanle 스펙트럼에 대응되는 이중 미분형태의 비선형 광자기 신호를 관측하였으며, 기울기가 급격하게 변하는 비선형 광자기 신호의 기울기는 레이저의 출력이 <TEX>$200{\mu}W$</TEX>일 때 대략 10 mV/<TEX>${\mu}T$</TEX>였다. In our study, the Hanle spectrum and nonlinear magneto-optic effect (NMOE) signals were observed as a function of magnetic field on <TEX>$D_1$</TEX> line of <TEX>$^{87}Rb$</TEX> atoms contained in a paraffin coated vapor cell. We observed the double structure from the Hanle and the NMOE spectrum in the paraffin coated Rb vapor cell. The narrow spectral width of the narrow resonance signal is approximately 1 kHz and the magnitude is approximately 10 percent of the total spectrum. Also, the NMOE signals corresponding to the Hanle configuration consisted of two different dispersion-like features. At the near zero magnetic field, a sharp slope signal was centered, and its value was 10 mV/<TEX>${\mu}T$</TEX> with laser power was <TEX>$200{\mu}W$</TEX>.