Large Kerr Effect Research Articles

Blue Phase-Polymer-Templated Ferroelectric Nematic Liquid Crystal.

Ferroelectric nematic liquid crystals (NFLCs) are promising electronic materials owing to their giant permittivity, polarization, and nonlinear optical coefficients. Although it is difficult to polymer-stabilize the NFLC orientation state due to their low affinity to polymers, in this study, we have succeeded in stabilizing the orientation into a blue phase (BP) three-dimensional structure by using BP polymer networks. This BP polymer-templated NFLC retains their ferroelectric properties and stabilizes with macroscopically canceled polarization due to the BP structure. The large polarization and dielectric constant of the NFLCs resulted in a large electro-optical Kerr effect. In addition, Curie point was observed in the BP polymer-templated NFLC, indicating the existence of multiple kinetic modes in the NFLCs themselves. This breakthrough expands the range of applications for both BPs and NFLCs.

High temperature ferrimagnetic semiconductors by spin-dependent doping in high temperature antiferromagnets

To realize room temperature ferromagnetic (FM) semiconductors is still a challenge in spintronics. Many antiferromagnetic (AFM) insulators and semiconductors with high Neel temperature TN are obtained in experiments, such as LaFeO3, BiFeO3, etc. High concentrations of magnetic impurities can be doped into these AFM materials, but AFM state with very tiny net magnetic moments was obtained in experiments because the magnetic impurities were equally doped into the spin up and down sublattices of the AFM materials. Here, we propose that the effective magnetic field provided by a FM substrate could guarantee the spin-dependent doping in AFM materials, where the doped magnetic impurities prefer one sublattice of spins, and the ferrimagnetic (FIM) materials are obtained. To demonstrate this proposal, we study the Mn-doped AFM insulator LaFeO3 with FM substrate of Fe metal by the density functional theory (DFT) calculations. It is shown that the doped magnetic Mn impurities prefer to occupy one sublattice of the AFM insulator and introduce large magnetic moments in La(Fe, Mn)O3. For the AFM insulator LaFeO3 with high TN = 740 K, several FIM semiconductors with high Curie temperature TC > 300 K and the band gap less than 2 eV are obtained by DFT calculations when 1/8 or 1/4 Fe atoms in LaFeO3 are replaced by the other 3d, 4d transition metal elements. The large magneto-optical Kerr effect (MOKE) is obtained in these LaFeO3-based FIM semiconductors. In addition, the FIM semiconductors with high TC are also obtained by spin-dependent doping in some other AFM materials with high TN, including BiFeO3, SrTcO3, CaTcO3, etc. Our theoretical results propose a way to obtain high TC FIM semiconductors by spin-dependent doping in high TN AFM insulators and semiconductors.

Brillouin Scattering Study of Electro-Optic KTa1-xNbxO3 Crystals.

The functionality enhancement of ferroelectrics by local polar clusters called polar nanoregions (PNRs) is one of the current interests in materials science. KTa1-xNbxO3 (KTN) with perovskite structure is a well-known electro-optic crystal with a large Kerr effect. The existence of PNRs in relaxor-like ferroelectric Nb-rich KTN with homovalent B-site cations is controversial. This paper reviews recent progress in understanding precursor dynamics in Nb-rich KTN crystals studied using Brillouin scattering. The intense central peak (CP) and significant softening of sound velocity are observed above the Curie temperature (TC) due to the polarization fluctuations in PNRs. The effects of Li-doping, defects, and electric fields on the growth and/or creation of PNRs are found using changes in acoustic properties. The electric-field-induced TC, which is shifted to higher values with increases in applied voltage, including critical endpoint (CEP) and field gradient by trapped electrons, are discussed as well. This new knowledge may give new insight into advanced functionality in perovskite ferroelectrics.

Strain effect on physical properties of the multiferroic Mn3Sn material: a first-principles calculations

ABSTRACT Magnetic Weyl semimetals Mn3Sn compound have appealing prospects for spintronic applications due to their various crystal structures and magnetic properties for the design of reliable high-density memories. This work presents the effect of tensile and compressive strains on the structural, electronic, magnetic, ferroelectric, optic, and magneto-optical properties of the Mn3Sn compound. Firstly, the first-principles calculations reveal an enormous ferroelectric effect with metallic behaviour. Secondly, we perform an analysis of the magneto-optical Kerr effect (MOKE) at stained thin-film, an important MOKE effect was found in an antiferromagnetic (AFM) ground state following along the direction [001] which is in good agreement with the experimental studies. Our results improve the theoretical understanding of the Dzyaloshinskii–Moriya interactions (DM) induced a large magneto-optical Kerr effect (MOKE). Also, the relationship between ferroelectric and ferromagnetic was investigated and should provide a basis for the experimental application of spintronic devices based on the anomalous Hall effect.

Design of Nanoarchitectures for Magnetoplasmonic Biosensing with Near-Zero-Transmittance Conditions.

Nanostructures exhibiting large transverse magneto-optical Kerr effect (TMOKE) are required for magnetoplasmonic biosensing if the aim is the minituarization and integration into microfluidic devices. In this work, we present a general strategy to design nanoarchitectures with enhanced TMOKE, which consist of an arrangement of gold ribs deposited on an magneto-optical (MO) dielectric slab of Bi:YIG (bismuth-substituted yttrium iron garnet) with a SiO2 substrate surrounded by water. Using the finite element method (FEM), we demonstrate numerically that the near-zero-transmittance condition is the most important requirement for high TMOKE values. This can be reached through geometric optimization of the nanoarchitecture by tuning the period, height, and width of the grating, thus leading to resonances at wavelengths where the MO dielectric slab has high MO activity. We also show that the TMOKE amplitude can be further increased if losses in metal ribs are reduced. For a magnetoplasmonic grating with optimized geometry, we demonstrated the potential detection of biologically relevant analytes with sensitivity in the order of 102 nm/RIU (refractive index unit). Since the nanoarchitecture proposed is experimentally feasible with, e.g., nanolithography techniques, one may expect that the design strategy may inspire the development of efficient magnetoplasmonic sensing platforms.

Tunable topological valley Hall edge state based on large optical Kerr effect

Most of the photonic valley-Hall edge states were constructed by changing structures to break the spatial inversion symmetry, restricting the practical application potential. In this paper, we construct a tunable topological valley-Hall edge state based on the large optical Kerr effect. It is demonstrated that topological phase transition happens by engineering the intensity of the injected pump and that a valley-Hall edge state can be generated at the interface between two regions with different topological properties. In addition, eigenfrequency and transmission characteristics of the edge state as a function of applied pump intensity are investigated. The topological protected valley-dependent transmission is studied under non-uniform distributed pump intensity. This work may open a new path toward designing reconfigurable all-optical metadevices.

Anomalous Magneto-Optical Effects in an Antiferromagnet–Topological-Insulator Heterostructure

We show that large magneto-optic Kerr effects (MOKEs) emerge when an antiferromagnet (AFM) is proximately coupled to a topological insulator (TI) film---where neither the perfect collinear N\'eel ordered single-domain AFM nor the unmagnetized TI individually shows any MOKE. Because of the lack of macroscopic magnetization, the AFM only couples to the spin of one of the TI's surfaces breaking time-reversal and inversion symmetry---which leads to a small microdegree MOKE signal. This small MOKE can be easily enhanced by 5 orders of magnitude, via cavity resonance, by optimizing the AFM and TI film thicknesses on the substrate. For slightly off-resonant structures, a ${6}^{\ensuremath{\circ}}$ Kerr rotation can be electrically switched on by varying the Fermi energy. This requires less than 20 meV, which is encouraging for low-power spintronics and magneto-optic devices. We further show that this simple structure is easily resilient to 5% material growth error.

Characterization of short-range molecular interaction by empirical solvent polarity scale to analyze the Kerr effect of nematic liquid crystals in the isotropic phase

It is well known that a relatively large Kerr effect in the isotropic phase of nematic liquid crystals (NLCs) is related to the existence of short-range ordering (SRO). Thus, the conversion of the long-range ordering (LRO) behavior in the nematic phase to the SRO in the isotropic phase of NLCs can characterize the nonlinear electro-optic Kerr effect. To show conversion from LRO to SRO, we employ an empirical solvent polarity scale (ETN). Interestingly, ETN originates from the solvatochromic polarity (SP) parameters, which demonstrates the quantitative value for describing the molecular interaction in each temperature point, even in the isotropic phase of NLCs. Our results show that the higher ETN value leads to a larger Kerr constant (B) owing to the larger short-range molecular interaction between rigid rod-like NLCs. To demonstrate a linear relationship of the B value with the ETN and SP parameters, a new correlation is introduced for the first time. Our investigations and newly introduced parameters strongly indicate that short-range molecular interactions still exist in the isotropic phase of NLCs, even when the macroscopically observable LRO behaviors disappear.

Linear and Nonlinear Magneto-optical Phenomena in Epitaxial Films of Europium Chalcogenides EuX (X = O, Se, Te)

The studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu2+ ions, is revealed.

Линейные и нелинейные магнитооптические явления в эпитаксиальных плeнках халькогенидов европия EuX (X=O, Se, Te)

AbstractThe studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu^2+ ions, is revealed.

Large Electro‐Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

The electro-optic Kerr effect in simple dipolar fluids such as nitrobenzene has been widely applied in electro-optical phase modulators and light shutters. In 2005, the discovery of the large Kerr effect in liquid-crystalline blue phases (Y. Hisakado et al., Adv. Mater. 2005, 17, 96-98.) gave new directions to the search for advanced Kerr effect materials. Even though the Kerr effect is present in all transparent and optically isotropic media, it is well known that the effect can be anomalously large in complex fluids, namely in the isotropic phase of liquid crystals or in polyelectrolyte solutions. Herein, it is shown that the Kerr effect in the isotropic phase of ionic liquid crystals combines the effective counterion polarization mechanism found in polyelectrolytes and the unique pretransitional growth of the Kerr constant found in the isotropic phase of nematic liquid crystals. Maximum Kerr constants in the order of several 10-11 m V-2 (ten times higher than the Kerr constant of the toxic nitrobenzene and less temperature sensitive than Kerr constants of nematic liquid crystals) make ionic liquid crystals attractive as new class of functional materials in low-speed Kerr effect applications.

Room-temperature chiral charge pumping in Dirac semimetals

Chiral anomaly, a non-conservation of chiral charge pumped by the topological nontrivial gauge fields, has been predicted to exist in Weyl semimetals. However, until now, the experimental signature of this effect exclusively relies on the observation of negative longitudinal magnetoresistance at low temperatures. Here, we report the field-modulated chiral charge pumping process and valley diffusion in Cd3As2. Apart from the conventional negative magnetoresistance, we observe an unusual nonlocal response with negative field dependence up to room temperature, originating from the diffusion of valley polarization. Furthermore, a large magneto-optic Kerr effect generated by parallel electric and magnetic fields is detected. These new experimental approaches provide a quantitative analysis of the chiral anomaly phenomenon which was inaccessible previously. The ability to manipulate the valley polarization in topological semimetal at room temperature opens up a route towards understanding its fundamental properties and utilizing the chiral fermions.

Composition-tuned magneto-optical Kerr effect in L10-MnxGa films with giant perpendicular anisotropy

We report the large polar magnetooptical Kerr effect in L10-MnxGa (0.76 ⩽ x ⩽ 1.29) epitaxial films with giant perpendicular magnetic anisotropy. The Kerr rotation is enhanced by a factor of up to 2.5 by decreasing Mn atomic concentration, which most likely arises from the variation of the effective spin–orbit coupling strength, compensation effect of magnetic moments at different Mn atom sites, and overall strain. A significant tuning effect of composition is also observed on Kerr ellipticity and complex Kerr angle (including the magnitude and phase angle). The good epitaxial compatibility with semiconductors, moderate coercivity of 4.6–9.7 kOe, large Kerr rotation of up to 0.10°, high reflectivity of 35%–55% in a wide wavelength range of 400 ~ 850 nm, and giant magnetic anisotropic field of up to 140 kOe together make these L10-MnxGa films promising for scientific and technological applications in spintronics and terahertz-frequency magnetooptical modulators.

Mechano-optic logic gate controlled by third-order nonlinear optical properties in a rotating ZnO:Au thin film

Measurements of the third-order nonlinear optical properties exhibited by a ZnO thin solid film deposited on a SnO2 substrate are presented. The samples were prepared by a spray pyrolysis processing route. Scanning electron microscopy analysis and UV–Vis spectroscopy studies were carried out. The picosecond response at 1064 nm was explored by the z-scan technique. A large optical Kerr effect with two-photon absorption was obtained. The inhibition of the nonlinear optical absorption together with a noticeable enhancement in the optical Kerr effect in the sample was achieved by the incorporation of Au nanoparticles into the ZnO film. Additionally, a two-wave mixing configuration at 532 nm was performed and an optical Kerr effect was identified as the main cause of the nanosecond third-order optical nonlinearity. The relaxation time of the photothermal response of the sample was estimated to be about 1 s when the sample was excited by nanosecond single-shots. The rotation of the sample during the nanosecond two-wave mixing experiments was analyzed. It was stated that a non-monotonic relation between rotating frequency and pulse repetition rate governs the thermal contribution to the nonlinear refractive index exhibited by a rotating film. Potential applications for switching photothermal interactions in rotating samples can be contemplated. A rotary logic system dependent on Kerr transmittance in a two-wave mixing experiment was proposed.

Large magneto-optical Kerr effect in noncollinear antiferromagnetsMn3X(X=Rh,Ir,Pt)

Magneto-optical Kerr effect, normally found in magnetic materials with nonzero magnetization such as ferromagnets and ferrimagnets, has been known for more than a century. Here, using first-principles density functional theory, we demonstrate large magneto-optical Kerr effect in high temperature noncollinear antiferromagnets Mn$_{3}X$ ($X$ = Rh, Ir, or Pt), in contrast to usual wisdom. The calculated Kerr rotation angles are large, being comparable to that of transition metal magnets such as bcc Fe. The large Kerr rotation angles and ellipticities are found to originate from the lifting of the band double-degeneracy due to the absence of spatial symmetry in the Mn$_{3}X$ noncollinear antiferromagnets which together with the time-reversal symmetry would preserve the Kramers theorem. Our results indicate that Mn$_{3}X$ would provide a rare material platform for exploration of subtle magneto-optical phenomena in noncollinear magnetic materials without net magnetization.

Templated blue phases.

Cholesteric blue phases of a chiral liquid crystal are interesting examples of self-organised three-dimensional nanostructures formed by soft matter. Recently it was demonstrated that a polymer matrix introduced by photopolymerization inside a bulk blue phase not only stabilises the host blue phase significantly, but also serves as a template for blue phase ordering. We show with numerical modelling that the transfer of the orientational order of the blue phase to the surfaces of the polymer matrix, together with the resulting surface anchoring, can account for the templating behaviour of the polymer matrix inducing the blue phase ordering of an achiral nematic liquid crystal. Furthermore, tailoring the anchoring conditions of the polymer matrix surfaces can bring about orientational ordering different from those of bulk blue phases, including an intertwined complex of the polymer matrix and topological line defects of orientational order. Optical Kerr response of templated blue phases is explored, finding large Kerr constants in the range of K = 2-10 × 10(-9) m V(-2) and notable dependence on the surface anchoring strength. More generally, the presented numerical approach is aimed to clarify the role and actions of templating polymer matrices in complex chiral nematic fluids, and further to help design novel template-based materials from chiral liquid crystals.

Transverse magneto-optical Kerr effect in subwavelength dielectric gratings

We demonstrate theoretically a large transverse magneto-optical Kerr effect (TMOKE) in subwavelength gratings consisting of alternating magneto-insulating and nonmagnetic dielectric nanostripes. The reflectivity of the grating reaches 96% at the frequencies corresponding to the maximum of the TMOKE response. The combination of a large TMOKE response and high reflectivity is important for applications in 3D imaging, magneto-optical data storage, and magnonics.

Optically-isotropic nanoencapsulated liquid crystal displays based on Kerr effect

An innovative liquid crystal display (LCD), incorporating in-plane switching mechanism with LC nanoencapsulation, is developed and its electro-optic (EO) performances are characterized. The nanoencapsulated LCDs exhibit electric-field induced transitions from an optically-isotropic state to an anisotropic one based on Kerr effect. Extremely small size of LC droplets embedded in the nanoencapsulated layer, less than a quarter of wavelength of incident light, appear transparent in the voltage-off state and gradually becomes birefringent in response to applied voltage, exhibiting the optical behavior of uniaxial retarder. Due to optically-isotropic nature and a large Kerr effect, they show wide viewing angle characteristics and good EO performances.

Electro-optical detection based on large Kerr effect in polymer-stabilized liquid crystals

In this Letter, polymer-stabilized liquid crystals with experimentally observed large electro-optic effect are introduced to the electro-optical detection to improve the voltage sensitivity. The Kerr constant of materials prepared in this study reached as high as 7.2×10(-9) m/V(2), increasing by 1000 times the sensitivity of the conventional electro-optical materials. The noncontact detection configuration, using a laser beam as a probe, enables quick two-dimensional scanning measurements.

Magnetic tunnel junctions using Co/Pt multilayered free layers with perpendicular magnetic anisotropy

Co/Pt multilayered films with perpendicular magnetic anisotropy have a large magneto-optical Kerr effect. To use the films with a submicron magneto-optical light modulator driven by spin transfer switching, we fabricated two types of magnetic tunnel junctions (MTJs) with Co/Pt multilayered films for the free layers. One is an fcc-based MTJ, another is a bcc-based MTJ with CoFeB/MgO/CoFeB junction. The fcc-based MTJ with a Ag buffer layer on the bottom electrode showed a large coercive force of the pinned layer, a large Kerr rotation angle of 0.3 degree in the free layer and a tunnel magnetoresistance (TMR) ratio of 3.8%. In the CoFeB/MgO/CoFeB junction, an X-ray diffraction pattern of an MgO layer showed a large MgO(002)-orientation. However, the TMR ratio was less than 3 %. An MTJ with a Ta buffer layer between the CoFeB layer and the Co/Pt multilayered films in the free layer was prepared. The Ta buffer was used to alleviate a lattice mismatch between bcc-CoFeB/MgO/CoFeB and fcc-Co/Pt multilayer. The peak intensity of the MgO(002)-orientation was increased up to 2 times. This result suggests that the crystalline texture of the bcc-CoFeB/MgO/CoFeB junction is strongly influenced by the fcc-Co/Pt multilayered films.