Magneto-optical Faraday Rotation Research Articles

Controlling Bi/Fe ratio in bismuth iron garnet thin films deposited by confocal magnetron sputtering for enhanced Faraday rotation

We have successfully deposited high-quality bismuth-iron-garnet (BIG; Bi3Fe5O12) epitaxial thin films on single crystal gadolinium-gallium-garnet (GGG; Gd3Ga5O12) (111) substrates through Radio frequency (RF) confocal sputtering, utilizing separate bismuth and iron oxide sputtering targets and optimized thermal treatments. The Bi/Fe ratio in the deposited BIG thin films can be varied by controlling the sputter process parameters. These deposited thin films exhibit homogeneity and surface root mean square (rms) roughness of less than 2 nm. The epitaxial film quality is confirmed by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Moreover, the films demonstrate low optical loss and a magneto-optical Faraday rotation as high as −34o±1o /μm at a wavelength of 535 nm for a BIG thin film with a Bi/Fe ratio 0.7 and annealing temperature of 510oC. Additionally, the coercivity of the deposited films is lower than that of those deposited via RF sputtering using a single composite target. This co-sputtering approach provides real-time flexibility for the deposition of high-quality BIG films.

Extraordinary enhancement of magneto-optical Faraday rotation angle in Bi-YIG/Pt/glass prepared by metal organic decomposition method

We achieve the extraordinary enhancement of magneto-optical Faraday rotation angle for bismuth-substituted yttrium iron garnet (Bi-YIG) using the metal-organic decomposition method, by introducing a Pt underlayer. The Faraday rotation angle reaches -25°/μm for the sample with a Pt thickness of 45 nm, which is 5 times higher than that of a bare thin film, setting a new record value for chemically synthesized Bi-YIG films. Through X-ray diffractometer, vibrating sample magnetometer, and energy dispersive X-ray spectroscopy analyses, we found that this enhancement is attributed to improved crystallinity, saturation magnetization, and the formation of a PtBi alloy phase, facilitated by Pt acting as an absorbing layer of Bi. We propose that the Bi-YIG/Pt film structure, with an optimum thickness of Pt underlayer, shows great promise for applications in various magneto-optical devices that require high detection sensitivity for small signals.

Exciton spin Hall effect in arc-shaped strained WSe2

Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a mechanism based on intervalley exciton dynamics in strained transition metal dichalcogenides (TMDs) to achieve the in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe2, carrying nonzero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately ∼mT, can be detected using techniques such as superconducting quantum interference magnetometry or spatially resolved magneto-optical Faraday and Kerr rotations. Published by the American Physical Society 2024

Faraday rotation enhancement due to proximity effect of surface plasmon resonance in magnetoplasmonic colloidal mixtures

The modification of Magneto-optic Faraday Rotation (MFR) based on Surface Plasmon Resonance (SPR) of noble metal nanoparticles (NP) has been an exciting field of research. The underlying physical mechanisms are intriguing and correspond to various observation patterns in composite magnetoplasmonic systems. Exploiting the immense potential of SPR phenomenon for excellent electromagnetic field confinement and energy transfer to a wide variety of dimensionally different nanostructures, we report MFR measurements in typical magnetoplasmonic colloidal mixtures of separately synthesized cobalt ferrite (CoFe2O4) and gold (Au) NP at wavelength of 532 nm where we have observed a spectacular possibility for “proximity effect” based plasmonic enhancement of MFR, for the very first time. The Langevin-type MFR patterns show a strong dependence on the concentrations of participating systems, where a typical optimum concentration produced a significant enhancement in MFR. For both the entities present in the liquid, a mutually dependent optimum concentration was necessary to obtain the Langevin pattern as well as MFR enhancement, confirming the influence from SPR due to the proximity effect. The observations open up exciting possibilities to apply SPR to discover novel magnetoplasmonic phenomena underlined by proximity effects, in addition to eluding constrained procedures to synthesize composite magnetoplasmonic systems.

Single-crystal structure, cation distribution, and optical/magnetic properties of Ce3+-doped Gd3Al2Ga3O12 crystal

Ce-doped Gd3Al2Ga3O12 (Ce3+:GAGG) scintillator finds practical application in industrial computed tomography and medical imaging, however a precise understanding of its single-crystal structure has been lacking. In this study, we grew a 1-inch-sized single crystal with the nominal composition 0.5%Ce3+:GAGG using the Czochralski method. To determine the crystal structure, we employed single-crystal XRD along with element analyses using ICP and XRF-WDS techniques. Our analysis revealed the precise formula of the crystal as 0.1% Ce3+:Gd3(Ga0.543Al0.457)2(Ga0.721Al0.279)3O12. Notably, Ga3+ cations exhibit a stronger preference at the tetrahedral sites compared to Al3+ cations. The optical absorption and luminescence resulting from Ce3+ 4f–5d transitions are cross-validated in the transmission and photoluminescence spectra. From 1.8 to 300 K, Ce:GAGG exhibits paramagnetic behavior and obeys the Curie-Weiss law. The 8S7/2 ground state of Gd3+ ion contributes to the overall effective magnetic moment on the one hand and plays a significant role in energy transfer from Gd3+ and Ce3+ upon excitation on the other hand. The magneto-optical Faraday rotation measurement of Ce:GAGG was performed to determine Verdet constants at 532 nm and 633 nm. These results may help a better understanding and the practical exploitation of Ce3+:GAGG as a multifunctional material in optic and magnetic fields.

Dual mid-infrared wavelength Faraday rotation spectroscopy NOx sensor based on NdFeB ring magnet array

Faraday rotation spectroscopy (FRS) exploits the magneto-optical Faraday rotation effect to achieve highly sensitive detection of paramagnetic molecules. Most of the currently reported FRS sensors suffer from high power consumption, while enabling detection of only a single component. In this work, a novel dual mid-infrared wavelength FRS sensor is proposed that enables the simultaneous detection of two paramagnetic molecules (nitrogen oxides, NOx = NO + NO2) in a single absorption cell. A neodymium-iron-boron (NdFeB) ring magnet array was designed to replace the solenoid coil to provide a stable, power-free magnetic field. The magnetic induction line distribution characteristics around the permanent magnets were analyzed based on the finite element method. Two mid-infrared quantum cascade lasers were used to detect the strongest fundamental ν2 rotational-vibrational band of NO (1875.81 cm−1) and the ν3 fundamental band of NO2 (1613.25 cm−1), respectively. A dual-wavelength Herriott cell (DWHC) is coaxially combined with a NdFeB ring magnet array to amplify the interaction of the laser beam with paramagnetic NOx molecules in an axial magnetic field. This low-power FRS NOx sensor achieves minimum detection limits of 0.58 ppb and 0.95 ppb for NO2 and NO, respectively, over an optical length of 23.7 m.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals.

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Investigating the Langevin behaviour of Faraday rotation in soft ferrimagnetic CoFe2O4 nanoparticles dispersed in PVA-water medium

Soft ferrimagnetic (FM) cobalt ferrite, CoFe2O4 (CoF) nanoparticles (NP) were synthesized using chemical co-precipitation technique. The constraint related to forming stable dispersion of the synthesized NP was successfully overcome by using an efficient dispersing medium namely, a 2 Wt% Polyvinyl Alcohol (PVA) - water solution, the PVA molecules of which played an important role in offering excellent stability by preventing the agglomeration of the NP considerably. Magneto-optic Faraday Rotation (MOFR) studies performed on the prepared stable dispersions at 532 nm laser wavelength for applied magnetic fields upto 2400 Oe indicated a Langevin behaviour consistent with the chain/cluster formation in them. The studies were carried out at two different concentrations, where the saturated value of MOFR, Bs , was higher for the more concentrated sample, while the saturation magnetic field, HS , followed the reverse trend. It was interesting to see that HS , was as large as 1800 Oe for the concentrated sample, and still higher for the less concentrated one, which may be due to an obstruction effect due to PVA molecules, which delay the formation and saturation of NP chains. The MOFR studies in soft FM NP dispersions open up an interesting domain, both for fundamental understanding of physical phenomena as well as for applications.

A femtosecond magnetic circular dichroism spectrometer.

We describe the development of a broadband magneto-optical spectrometer with femtosecond temporal resolution. The absorption spectrometer is based on a white-light supercontinuum (∼320 to 750 nm) using shot-to-shot temporal and spectral referencing at 1 kHz. Static and transient absorption spectra using circularly polarized light are collected in a magnetic field. The difference spectra with respect to the external field direction give the static and transient magneto-optical Faraday rotation (magnetic optical rotary dispersion) and ellipticity (magnetic circular dichroism) spectra. An achromatic quarter-wave plate is used, and the impact of the deviation from ideal retardance on the spectra is discussed. Results from solution-based and thin-film samples are used to demonstrate the performance and wide applicability of the instrument. The sensitivities for the static and time-resolved data were found to be 5 and 0.4 mdeg, respectively. The method presents a simple way to measure magneto-optical spectra using a transient absorption spectrometer and an electromagnet.

Coherent control of magneto-optical Faraday rotation at terahertz frequencies in graphene-based metasurfaces via electromagnetically induced transparency

We investigate theoretically the controlled magneto-optical Faraday rotation (MO-FR) through graphene-based metasurfaces placed on electromagnetically induced transparency (EIT) substrate at terahertz (THz) frequencies. MO-FR and transmission of the probe light are studied by changing the electric field that manipulates the group index of the substrate. The MO-FR can be controlled and enhanced by varying the periodic width of graphene ribbons. The polarization plane of MO-FR changes with 180 degrees while increasing the probe field detuning through the EIT substrate. The zero-crossing of MO-FR shifts to higher THz frequencies with increasing the magnetic field. The transmission of the probe light pulse at THz frequencies is investigated with the probe detuning and the applied magnetic field.

Tunable magneto-optical Faraday rotation with a five-level atomic system near the plasmonic nanostructure

We theoretically investigate the possibility of controlling the high magneto-optical rotation without suffering an accompanying loss in transmission via the presence of a plasmonic nanostructure near the atomic five-level system. The key to optimization is the introduction of an additional degree of freedom by the location of the plasmonic nanostructure. We show that placing the plasmonic nanostructure at different distances from the atomic system allows arbitrary magneto-optical rotations with virtually 100% transmission. We numerically and analytically show that the existence of the plasmonic nanostructure near the atomic system leads to a strong adjustment of the optical properties of the quantum systems. We present a number of controlling parameters with emphasis on realizing 45° and 90° rotations in the proposed structure. The introduced model can exhibit great performances in converting TE-TM polarization and isolator applications.

Tunable optical and magneto-optical Faraday and Kerr rotations in a dielectric slab doped with double-V type atoms

We theoretically investigate the optical and magneto-optical Faraday and Kerr rotations of a probe field that propagates through a nonmagnetic dielectric slab doped with double-V type atoms. Both rotations and corresponding ellipticities, as well as the intensities of transmitted and reflected beams, are modified by quantum coherence induced in the atomic system. We show that applying a control laser field makes the system optically active and simultaneously, transparent to one component of the probe field. We demonstrate that the response of the slab can be modified both electrically and magnetically. Applying the second control laser field with different Rabi frequencies improves the optical properties of the slab due to the induced coherent effects. We present analytical expressions for facilitating the detailed study of the system behaviors. Magneto-optical Faraday rotation 45° with transmission close to 100 % and large Kerr rotation with high reflection are significant results from the influence of both the control and magnetic fields on such a small structure. By prevailing over the tradeoff between reflection and rotation, the proposed model could be considered as a special candidate for rotating the polarization plane of the transmitted and reflected beams, simultaneously.

Finite-size effects in the dynamic conductivity and Faraday effect of quantum anomalous Hall insulators

We theoretically study the finite-size effects in the dynamical response of a quantum anomalous Hall insulator in the disk geometry. Semi-analytic and numerical results are obtained for the wavefunctions and energies of the disk within a continuum Dirac Hamiltonian description subject to a topological infinite mass boundary condition. Using the Kubo formula, we obtain the frequency-dependent longitudinal and Hall conductivities and find that optical transitions between edge states contribute dominantly to the real part of the dynamic Hall conductivity for frequency values both within and beyond the bulk band gap. We also find that the topological infinite mass boundary condition changes the low-frequency Hall conductivity to $ e^2/h $ in a finite-size system from the well-known value $ e^2/2h $ in an extended system. The magneto-optical Faraday rotation is then studied as a function of frequency for the setup of a quantum anomalous Hall insulator mounted on a dielectric substrate, showing both finite-size effects of the disk and Fabry-P\'erot resonances due to the substrate. Our work demonstrates the important role played by the boundary condition in the topological properties of finite-size systems through its effects on the electronic wavefunctions.

Characterisation of spectroscopic and magneto-optical faraday rotation in Mn2+- doped CdS quantum dots in a silicate glass

We demonstrate the control of CdS and Mn2+-doped-CdS Q-dots in a silicate glass for magneto-optical applications. The microstructural properties of Q-dot glasses were investigated by X-Ray diffraction (XRD), Field Emission Transmission Electron Microscopy (FETEM) and the optical properties by UV–Visible–NIR and Photoluminescence (PL) spectroscopic techniques, respectively. The FETEM of the CdS QD–glass heat treated at 600oC reveals that the size of CdS and Mn2+-doped CdS Q-dots are in the range of 4-5 nm and 5-6 nm, respectively. The observed size distributions of Q-dots were in reasonable agreement with the data, derived from X-ray line broadening and estimated average Bohr radii using the UV–visible absorption data. Photoluminescence characteristics were investigated at room temperature by exciting the CdS and Mn2+-doped-CdS Q-dot glasses with a 420 nm excitation source, which yielded broad emission spectra in the visible and near-IR range (450-800 nm). We observed a red shift in the emission peak with increase in the Q-dot size, controlled by heat treatment temperature range (550-600oC). The room-temperature magneto-optical Faraday rotation measurements on Q-dots glasses were carried out using magnetic field strength up to 360 mT, and observed an increase in the value of Verdet constant, from 6.2 to 12.0°/T-cm, when comparing undoped CdS-Q-dot glass with Mn2+-doped CdS glass. The demonstration of enhanced Verdet constant in Q-dot silicate glasses with sub-Tesla field paves the path for engineering range magneto-optical devices for photonics, spintronics and sensors applications, in which the polarization of photons may be controlled with low-intensity magnetic field in optical waveguides.

RGB Magnetophotonic Crystals for High-contrast Magnetooptical Spatial Light Modulators

Magnetooptical spatial light modulators (MOSLMs) are photonic devices that encode information in photonic waveforms by changing their amplitude and phase using magnetooptical Faraday or Kerr rotation. Despite the progress on both MO materials and switching methods, significant improvements on materials engineering and SLM design are needed for demonstrating low-power, multicolor, analog and high-contrast MOSLM devices. In this study, we present design rules and example designs for a high-contrast and large figure-of-merit MOSLM using three-color magnetophotonic crystals (MPC). We demonstrate for the first time, a three-defect MPC capable of simultaneously enhancing Faraday rotation, and high-contrast modulation at three fundamental wavelengths of red, green and blue (RGB) within the same pixel. We show using 2D finite-difference time-domain simulations that bismuth-substituted yttrium iron garnet films are promising for low-loss and high Faraday rotation MOSLM device in the visible band. Faraday rotation and loss spectra as well as figure-of-merit values are calculated for different magnetophotonic crystals of the form (H/L)p/(D/L)q/(H/L)p. After an optimization of layer thicknesses and MPC configuration, Faraday rotation values were found to be between 20–55° for losses below 20 dB in an overall thickness less than 1.5 µm including three submicron garnet defect layers. The experimental demonstration of our proposed 3-color MOSLM devices can enable bistable photonic projectors, holographic displays, indoor visible light communication devices, photonic beamforming for 5 G telecommunications and beyond.

Interferometric Detection of Pinned Interactions in Bismuth-Substituted Iron Garnet

The utilization of a bismuth-substituted iron garnet as a magnetooptic Faraday rotator (MOFR) has been reported for all-optical networking purposes as well as for other applications. Our measurements and observations demonstrate that the MOFR saturates once a significantly large magnetic field (>225 G) is applied. After the applied magnetic field enters the saturation region, the material’s magnetic domains can become pinned at intermediate levels of magnetization. Pinning in this form has not been reported nor well studied for this application. In this paper, a method to detect and describe anomalous pinning in terms of Faraday rotation is presented. Measurements on the changes in the state of polarization that a pinned material produces are examined. This paper will also present practical methods for unpinning the MOFR material, which are traditionally considered to be challenging.

Ferromagnetism in 2D organic iron hemoglobin crystals based on nitrogenated conjugated micropore materials.

In this work we study a low-cost two-dimensional ferromagnetic semiconductor with possible applications in biomedicine, solar cells, spintronics, and energy and hydrogen storage. From first principle calculations we describe the unique electronic, transport, optical, and magnetic properties of a π-conjugated micropore polymer (CMP) with three iron atoms placed in the middle of an isolated pore locally resembling heme complexes. This material exhibits strong Fe-localized dz2 bands. The bandgap is direct and equal to 0.28 eV. The valence band is doubly degenerate at the Γ-point and for larger k-wavevectors the HOMO band becomes flat with low contribution to charge mobility. The absorption coefficient is roughly isotropic. The conductivity is also isotropic with the nonzero contribution in the energy range 0.3-8 eV. The xy-component of the imaginary part of the dielectric tensor determines the magneto-optical Faraday and Kerr rotation. Nonvanishing rotation is observed in the interval of 0.5-5.0 eV. This material is found to be a ferromagnet of an Ising type with long-range exchange interactions with a very high magnetic moment per unit cell, m = 6 μB. The exchange integral is calculated by two independent methods: (a) from the energy difference between ferromagnetic and antiferromagnetic states and (b) from a magnon dispersion curve. In the former case Jnn = 27 μeV. In the latter case the magnon dispersion is fitted by the Ising model with the nearest and next-nearest neighbor spin interactions. From these estimations we find that Jnn = 19.5 μeV and Jnnn = -3 μeV. Despite the different nature of the calculations, the exchange integrals are only within 28% difference.

Spectral tunability of laser-induced spin dynamics in the ferromagnetic semiconductor CdCr2Se4

Here we report that femtosecond laser pulses are able to trigger oscillations of the magneto-optical Faraday rotation in the ferromagnetic semiconductor $\mathrm{CdC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{e}}_{4}$ in the presence of an applied magnetic field. The frequency of these oscillations is a linear function of the magnetic field and corresponds to the ferromagnetic resonance (FMR). Tuning the photon energy of the pump pulses we reveal two different mechanisms, which induce FMR precession in this material. In the case of pumping from the valence band deep into the conduction band (photon energy 3.1 eV), the phase of the spin oscillations is not sensitive to the polarization of the pump, but can be reversed over 180 deg by changing the polarity of the applied magnetic field. We assign these oscillations to the coherent spin precession triggered by ultrafast laser-induced heating. This mechanism requires a strong optical absorption in the material and becomes inactive if the pump photon energy is below the band gap. Tuning the photon energy in a wide range from 0.88 to 2.1 eV reveals the second mechanism of optical excitation of coherent spin oscillations with a maximum around 1.2 eV, i.e., very close to the energy of the band gap in the semiconductor. Contrary to the laser-induced heating, this excitation mechanism is pump polarization dependent, being the most efficient if the pump is circularly polarized. The phase of the spin oscillations is independent of the polarity of the applied magnetic field, but changes by 180 deg under reversing the helicity of light. We suggest that the effect can be interpreted in terms of spin transfer torque experienced by the network of the ordered $\mathrm{C}{\mathrm{r}}^{3+}$ spins as a result of excitation of electrons from the top of the $p$-type valence band to the bottom of the $s$-type conduction band. In particular, a strong spin-orbit interaction experienced by the carriers in the valence band is responsible for the coupling of the spins of the photogenerated carriers and the polarization of light. Due to strong $pd$- and $sd$-exchange interactions the spins of the photocarriers appear to be coupled to the network of ordered spins of the $\mathrm{C}{\mathrm{r}}^{3+}$ ions.

Tamm plasmon boosting Faraday rotation in a coupled resonator magneto-plasmonic structure

The present study aimed to evaluate the magneto-optic Faraday rotation of one-dimensional coupled resonator magnetoplasmonic structure by metallic cover layer in each resonator. To this purpose, transfer matrix method was used where SiO2 and Bismuth substitute garnet thin films playing main building block and the gold or silver layer use to reach ((SiO2/Bi:YIG)n/(Au or Ag)/SiO2)m structure; where n and m are considered as the repetition and resonator numbers, respectively. Tamm plasmons related to the phase cancelation in photonic band gap are identified by optical and magneto-optical spectra and accordingly the phase change in the structure. Based on these modes, a wide range of wavelengths is detected by which the figure of merit increases due to the interaction of light with Tamm plasmons and surprisingly the flat optical window in this region in addition to the main resonance. These structures can open a new gate for enhancing performance of the magneto-optic devices.

Weak enhanced resonant Faraday rotation in pure cobalt plasmonic lattices: Thickness dependent Faraday rotation studies

The thickness dependent magneto-optical Faraday rotation in Co ferromagnetic thin films and perforated periodic Co nano-hole arrays were investigated. It was observed that the optical response and Faraday rotation of Co nano-hole arrays were strongly dependent on the thickness of the film, and the Faraday rotation reached the maximum amplitude at an optimal thickness value of 30–50 nm, which contrasted with the Co thin films where the Faraday rotation increased with the film thickness. In addition, the Faraday rotation spectra of Co nano-hole arrays showed a weak resonantly enhanced peak at the anti-resonance modes (or Wood’s anomalies) on the transmission spectra, which correspond to the enhanced local electric field at the film and surrounding medium interfaces. All experimental results were confirmed by the finite-difference time domain calculations.