Magneto-optical Research Articles

Indirect measurement of atomic magneto-optical rotation via Hilbert transform

Abstract The Kramers–Kronig relations are a pivotal foundation of linear optics and atomic physics, embedding a physical connection between the real and imaginary components of any causal response function. A mathematically equivalent, but simpler, approach instead utilises the Hilbert transform. In a previous study, the Hilbert transform was applied to absorption spectra in order to infer the sole refractive index of an atomic medium in the absence of an external magnetic field. The presence of a magnetic field causes the medium to become birefringent and dichroic, and therefore it is instead characterised by two refractive indices. In this study, we apply the same Hilbert transform technique to independently measure both refractive indices of a birefringent atomic medium, leading to an indirect measurement of atomic magneto-optical rotation. Key to this measurement is the insight that inputting specific light polarisations into an atomic medium induces absorption associated with only one of the refractive indices. We show this is true in two configurations, commonly referred to in literature as the Faraday and Voigt geometries, which differ by the magnetic field orientation with respect to the light wavevector. For both cases, we measure the two refractive indices independently for a Rb thermal vapour in a 0.6 T magnetic field, finding excellent agreement with theory. This study further emphasises the application of the Hilbert transform to the field of quantum and atomic optics in the linear regime.

Magneto-optical chiral metasurfaces for achieving polarization-independent nonreciprocal transmission.

Nonreciprocal transmission, resulting from the breaking of Lorentz reciprocity, plays a pivotal role in nonreciprocal communication systems by enabling asymmetric forward and backward propagations. Metasurfaces endowed with nonreciprocity represent a compact and facile platform for manipulating electromagnetic waves in an unprecedented manner. However, most passive metasurfaces that achieve nonreciprocal transmissions are polarization dependent. While incorporation of active elements or nonlinear materials can achieve polarization-independent nonreciprocal metasurfaces, the complicated configurations limit their practical applications. To address this issue, we propose and demonstrate a passive and linear metasurface that combines magneto-optical and chiral effects, enabling polarization-independent isolation. The designed metasurface achieves a transmittance of up to 80%, with a high contrast between forward and backward propagations. Our work introduces a novel mechanism for nonreciprocal transmission and lays the foundation for the development of compact, polarization-insensitive nonreciprocal devices.

Enhancing terahertz magneto-optical effects in wafer-scale RIG single crystal thick films with anti-reflective coatings for improved transmittance

Wafer-scale rare-earth iron garnet (RIG) single crystal thick films were fabricated on 3-in. gadolinium gallium garnet (GGG) substrates using liquid phase epitaxy. The terahertz transmittance of the RIG crystals improved after removing the GGG substrate by polishing. The time-domain spectra at Terahertz (THz) frequencies indicate the existence of a magneto-optical effect in RIG samples. The results indicate that the RIG samples exhibit a high refractive index of ∼4.50 within the 0.1–1.0 THz frequency range, a transmittance of around 40%, and an absorption rate of only 10–50 cm−1. The Faraday rotation angles of the thick single-crystal films of the RIG samples were measured using a THz-TDS system. The RIG has a thickness of ∼330 μm. The Faraday rotation angles of RIG crystals at THz frequencies can reach up to 16° when an external magnetic field of 0.18 T is applied. The Verdet constants of the RIG sample were calculated to be ∼120°/mm/T. To improve the transmittance of the RIG sample, epoxy resin and polymethylpentene (TPX) were used as anti-reflective films. The transmittance of the RIG sample increased by ∼5% for the 80 μm thick epoxy and about 10% for the 320 μm thick TPX. Therefore, this RIG single crystal thick film can achieve a low loss, a high transmittance, and a strong magneto-optical effect in the terahertz region with the cooperation of a reflection-reducing film. It is expected to have wide applications in terahertz magnetic polarization conversion, non-reciprocal phase shifters, and isolators.

Broadband Enhancement of Magneto-Optical Effects in Hybrid Waveguide-Plasmonic Surfaces for Sensing.

Conventional magnetophotonic nanostructures typically function within narrow wavelength and incident angle ranges, where resonance is observed and magneto-optical (MO) effects are amplified. Expanding these operational ranges may allow for improved applications, including in (bio)sensing devices. In this study, we describe a hybrid magnetoplasmonic waveguide grating (HMPWG) in which the coupling of plasmonic resonances and waveguide modes leads to enhanced MO effects and sensitivity, according to full-wave electromagnetic simulations. High transverse magneto-optical Kerr effect (TMOKE) signals were observed for the full range of wavelengths and angles investigated, i.e., for θinc ≥ 1° and 500 nm ≤ λ ≤ 850 nm. As a proof-of-concept we verified that using the HMPWG nanostructure with an aqueous solution as superstrate one may obtain a sensitivity in variation of the refractive index unit (RIU) of S = 166°/RIU and S = 230 nm/RIU in angle and wavelength interrogation modes, respectively. Upon comparing with conventional magnetoplasmonic gratings, which only enable excitation of plasmonic resonances, we demonstrate that HMPWG nanostructures can be further optimized to reach not only high sensitivity but also high resolution in sensing and biosensing.

Inverse Faraday Effect in Ferrite–Garnet Films in the Near-Infrared Range

The magneto-optical Faraday effect is determined by the electric dipole and magnetic dipole transitions in a transparent magnetic material. At the same time, the inverse Faraday effect is still described by an expression that includes only electric dipole transitions. The magnetic dipole contribution to the inverse Faraday effect is considered theoretically, and the dependence of the inverse Faraday effect on the wavelength in the near-infrared range (where the magnetic dipole contribution becomes significant) is obtained by the example of a ferrite–garnet film. It is shown that, although both contributions to the inverse Faraday effect always take place for homogeneous films, only the magnetic-dipole inverse Faraday effect manifests itself for films with a periodic nanostructure upon excitation by a TE waveguide mode, which may be useful for its experimental observation.

Effects of hydrostatic pressure, aluminium composition, and external magnetic field on intersubband and intrasubband transitions characteristics in symmetric potential quantum wells

Aluminium composition (λ), magnetic field (B), and hydrostatic pressure (P) influences on magneto-optical (MO) properties of intra- and inter-subband transitions in GaAs/AlλGa1−λAs symmetric QWs for four models of confined optic phonons are studied and compared. The findings indicate: the MO characteristics of intra- and inter-subband transitions are significantly influenced by B, λ, and P for phonon absorption and emission. The e–p interaction intensity decreases as P increases, whereas it increases as λ increases. The influences of λ and P on phonon absorption are more significant than those on emission. E–p interaction intensity dependence on λ and P for intra-subband transitions is stronger than that for inter-subband ones. The e–p interaction in QWs is well described by Knipp–Reinecke and Huang–Zhu models. To reduce the influence of P on MO properties, we can increase T and λ or decrease B. At T≈600 K, the influence of P is insignificant and can be neglected.

Optical fiber sensor based on magneto-optical surface plasmon resonance with ultra-high figure of merit

Compared to surface plasmon resonance (SPR), the sensors based on the magneto-optical SPR (MOSPR) technique have much higher figure-of-merit (FOM). However, there are no reports about applying MOSPR in the optical fiber structure now. In this work, a novel D-shaped optical fiber sensor based on the MOSPR technique is proposed. The D-shaped optical fiber is coated with a thin silver film and a magneto-optical (MO) material film of Cerium-doped Yttrium-Iron garnet (CeYIG). By applying a magnetic field on the sensing region, the magneto-optical Kerr effect (MOKE) of the CeYIG layer and the related MOSPR phenomenon could be excited when appropriate light is transmitted in the proposed optical fiber sensor. The influence of the structural parameters including the residual cladding thickness, silver and MO material film thicknesses are analyzed theoretically by the finite element method (FEM). With the optimal parameters, the sensor achieves the sensitivity of 5304 nm RIU−1. Since the peak width of MOSPR spectra is much narrower than that of the SPR spectra, the FOM of the sensor is largely enhanced to 3864 RIU−1 on average and 13260 RIU−1 in maximum, which surpasses the optical fiber SPR sensors vastly. The miniaturized and simple design of the D-shaped optical fiber MOSPR sensor, coupled with the ultra-high FOM, offers itself great potential in biochemical sensing applications.

Self-holding magneto-optical switch integrated on silicon photonic platforms

We demonstrated what we believe to be a novel self-holding waveguide switch integrated on silicon photonic platforms utilizing the magneto-optical (MO) effect with electrical switching operation. In this study, we designed thin-film magnet arrays and electrodes positioned beside the waveguide. The switching states were changed by flipping the magnetization of thin-film magnets with an applied current of 700 mA, and the switching states were successfully maintained with zero current. The fabricated MO switch has an extinction ratio (ER) of 16.5 dB and an insertion loss (IL) of 7.3 dB at a wavelength of 1546.2 nm, with a phase shift of π/2. This study shows the potential of self-holding silicon photonic switches, enabling large-scale photonic integrated circuits with extremely low energy consumption.

Magneto-optical Particles in Isotropic Spinning Fields Mimic Magnetic Monopoles.

In contrast with the typical electric currents accelerated under the influence of a Coulombic force, there are only a few condensed matter examples of particles experiencing a force proportional to a constant, external magnetic field. In this Letter, we present a new alternative, based on an isotropic radiation spinning field and the magneto-optical effect, in which a particle is propelled by a magnetic field just like a magnetic monopole will do. This is a purely nonreciprocal effect as the reciprocal equivalent (a chiral dipole), despite presenting a dichroic response, does not experience any force when illuminated by the spinning field. The "magnetic charge" induced by impinging radiation on the magneto-optical dipole is found to depend linearly on the helicity of the field. In addition, this artificial monopole experiences a dichroic permanent optical torque and does not interact with an external electric field.

Magneto-Optical Ceramics with High Transparency for Highly Sensitive Magnetometer via Quantum Weak Measurement.

Sensitive magnetometer technology is desirable for biomagnetic field detection and geomagnetic field measuring. Signal amplification materials such as magneto-optical crystals or ceramics are crucial for enhancing detection sensitivity, but severe optical scattering and low Verdet constant further limit its application. To develop high-sensitivity magnetometers for quantum weak measurement schemes, we have conducted investigations on the powder calcining dynamics and prepared a series of high-optical-quality (Ho/Dy)2Zr2O7 transparent ceramic samples. The Verdet constant of magneto-optical materials was measured across a continuous wavelength spectrum, exhibiting a peak at 283 ± 5 rad/(T·m). We further established an electron transition mechanism to elucidate the exceptional magneto-optical attributes of dysprosium. In addition, samples demonstrated superior performance in weak-value amplification, reaching a low detectable magnetic field threshold of 3.5 × 10-8 T and continuously worked over 6 h with high stability. Our work developed a highly sensitive magnetometer using optimized magneto-optical ceramics and provided guidance on design, fabrication, and application for magneto-optical ceramics in quantum weak measurement.

High-Verdet Constant and Low-Optical Loss Tb3+ Doped Magnetite Nanoparticles.

Magneto-optical (MO) polymer nanocomposites have emerged as alternatives to conventional MO crystals, particularly in nanophotonics applications, thanks to their better processing flexibility and superior Verdet constants. However, a higher Verdet constant commonly comes with excessive optical loss due to increased absorption and scattering, resulting in a constant or reduced figure-of-merit (FOM) defined as the Verdet constant over optical loss. By doping magnetite (Fe3O4) nanoparticles with Tb3+ ions, we report a new strategy to enhance the Verdet constant without increasing the optical loss. The Fe3O4:Tb3+ nanocomposite is one of a kind that simultaneously achieves a state-of-the-art Verdet constant of 5.6 × 105 °/T·m and a state-of-the-art FOM of 31°/T in the near-infrared region.

Assembly of Homochiral Magneto-Optical Dy6 Triangular Clusters by Fixing Carbon Dioxide in the Air.

A new hydrazone Schiff base bridging ligand (H2LSchiff (E)-N'-((1-hydroxynaphthalen-2-yl)methylene)pyrazine-2-carbohydrazide) and L/D-proline were used to construct a pair of homochiral Dy6 cluster complexes, [Dy6(CO3)(L-Pro)6(LSchiff)4(HLSchiff)2]·5DMA·2H2O (L-1, L-HPro = L-proline; DMA = N,N-dimethylacetamide) and [Dy6(CO3)(D-Pro)6(LSchiff)4(HLSchiff)2]·5DMA·2H2O (D-1, D-HPro = D-proline), which show a novel triangular Dy6 topology. Notably, the fixation of CO2 in the air formed a carbonato central bridge, playing a key role in assembling L-1/D-1. Magnetic measurements revealed that L-1/D-1 displays intramolecular ferromagnetic coupling and magnetic relaxation behaviours. Furthermore, L-1/D-1 shows a distinct magneto-optical Faraday effect and has a second harmonic generation (SHG) response (1.0 × KDP) at room temperature. The results show that the immobilization of CO2 provides a novel pathway for homochiral multifunctional 4f cluster complexes.

Spin-related excited-state phenomena in photochemistry.

The spin of electrons plays a vital role in chemical reactions and processes, and the excited state generated by the absorption of photons shows abundant spin-related phenomena. However, the importance of electron spin in photochemistry studies has been rarely mentioned or summarized. In this review, we briefly introduce the concept of spin photochemistry based on the spin multiplicity of the excited state, which leads to the observation of various spin-related photophysical properties and photochemical reactivities. Then, we focus on the recent advances in terms of light-induced magnetic properties, excited-state magneto-optical effects and spin-dependent photochemical reactions. The review aims to provide a comprehensive overview to utilize the spin multiplicity of the excited state in manipulating the above photophysical and photochemical processes. Finally, we discuss the existing challenges in the emerging field of spin photochemistry and future opportunities such as smart magnetic materials, optical information technology and spin-enhanced photocatalysis.

Nonreciprocal Pancharatnam-Berry metasurface for unidirectional wavefront manipulations

Optical metasurfaces employing the Pancharatnam-Berry (PB) geometric phase, called PB metasurfaces, have been extensively applied to realize spin-dependent light manipulations. However, the properties of conventional PB metasurfaces are intrinsically limited by the Lorentz reciprocity. Breaking reciprocity can give rise to new properties and phenomena unavailable in conventional reciprocal systems. Here, we propose a mechanism to realize nonreciprocal PB metasurfaces of subwavelength thickness by using the Faraday magneto-optical (FMO) effect of yttrium iron garnet (YIG) material in synergy with the PB geometric phase of spatially rotating meta-atoms. Using full-wave numerical simulations and multipole analysis, we show that the metasurface composed of dielectric cylinders and a thin YIG layer can achieve high isolation of circularly polarized lights, attributed to the enhancement of the magneto-optical effect by the resonant Mie modes and Fabry-Pérot (FP) cavity mode. In addition, the metasurface can enable unidirectional wavefront manipulations of circularly polarized lights, including nonreciprocal beam steering and nonreciprocal beam focusing. The results contribute to the understanding of the interplay between nonreciprocity and geometric phase in light manipulations and can find applications in optical communications, optical sensing, and quantum information processing.

1.01-W narrow-linewidth ultra-violet laser by Pr: YLF

We presented the first report of a high-power narrow-linewidth 261 nm laser. This laser relies on a diode-pumped Pr: YLF crystal and employs a ring cavity design that integrates TSAG (Terbium Scandium Aluminum Garnet) as the magneto-optical crystal. A double-ended pumping configuration is established through the utilization of two 10-W 444-nm blue diode laser arrays as pump sources. Through intracavity frequency doubling facilitated by a BBO (Beta-Barium Borate) crystal, we have achieved a final output of 1.01 W narrow-linewidth laser radiation at 261 nm.

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.

Holmium-doped TbVO4 as a highly potential magneto-optical crystal for Faraday isolator

Tb1-xHoxVO4 (THV) magneto-optical crystal with diameter of 27 mm and length of 40 mm was grown by the Czochralski (Cz) technique for the first time. The structural perfection of THV crystal was studied by the high-resolution X-ray diffraction rocking curve measurement, proving the high crystallinity quality of THV. THV shows high transmittance of greater than 75 % within the wavelength range of 665–1600 nm. The Verdet constants of THV at 532, 633 and 1064 nm are 370, 226 and 56 rad·m−1T−1, 40 %–95 % higher than these of commercial TGG crystal. Additionally, THV exhibits lower absorption coefficient (0.05 cm−1), higher magneto-optical figure of merit (175 deg/dB) and extinction ratio (43 dB) at 1064 nm compared with TGG. Evaluation of thermal performance and laser-induced damage threshold of THV were also made to promote the potential application in high-power optical isolators.

An ultra-sensitive photonic crystal fiber magnetic field sensor based on Vernier effect using two parallel Sagnac loops

Magnetic field detection is of significant importance in various fields, including military, industrial, and power transmission systems. In this paper, we propose a novel ultra-sensitive photonic crystal fiber (PCF) magnetic field sensor based on the Vernier effect, employing two parallel Sagnac loops. Since magnetic field detection relies on the magneto-optical effect of magnetic fluids, all air holes in the PCF are assumed to filled with magnetic fluids. By inserting two slightly different lengths of PCFs into two parallel Sagnac loops, the Vernier effect can be excited to improve the sensitivity of magnetic field detection. The sensing characteristics of the PCF magnetic field sensor are theoretically studied using the finite element method (FEM). Moreover, the influences of the wavelength and magnetic field intensity on the sensing performance are also analyzed. The results show that the sensitivity and resolution of the PCF magnetic field sensor can reach 11.9 nm Oe−1 and 8.4 × 10−3 Oe, respectively, within the magnetic field intensity range of 80–150 Oe. To our best knowledge, the proposed magnetic field sensor exhibits the highest sensitivity among existing magnetic field sensors based on optical fiber interferometers. The proposed magnetic field sensor possesses ultra-high sensitivity and resolution, which exhibits good application prospects in the field of magnetic field detection.

Magneto-optical efficiencies combined with surface-plasmon resonance in FeSi/Au system

We designed and fabricated our original laminated materials that simultaneously exhibited different properties: magneto-optical (MO), i.e., the transverse MO Kerr effects (T-MOKE) and surface-plasmon resonance (SPR). The material design was composed of dielectric, magnetic, and noble-metal layers. We selected the soft-magnetic FeSi thin film as a T-MOKE magnetic layer, while an Au thin film was chosen as a SPR-source layer, creating an FeSi-/Au-based “MO-SPR material.” Strong interactions between T-MOKE and SPR were demonstrated. When the material is irradiated with a laser beam of wavelength 660 nm, at the SPR angle to the material, θR, the highest T-MOKE value was attained. The T-MOKE was markedly enhanced at θR: ∼32 to ∼84 times higher compared with the FeSi single layer (reference). The T-MOKE was amplified by a strong interaction between MO activities and electromagnetic field distributions. The FeSi (5.0 nm)/Au (14.8 nm) specimen achieved the best signal-to-noise ratio (SNR). The sample was then tested for its sensing efficiency by measuring the T-MOKE using distilled water and a glucose solution, respectively: It was possible to distinguish between two different solutions. Our MO-SPR materials utilizing both magnetism and near-field light are thus sufficiently sensitive to be applicable as sensing materials. Furthermore, the polarity of the T-MOKE signal is flipped under the application of a small, external magnetic field owing to the soft magnetism of the FeSi T-MOKE layer. This is highly advantageous to create high-frequency AC-magnetic synchronized T-MOKE sensing systems with low-power consumption.

Design of a high-resolution magneto-plasmonic biosensor for analyte detection

This paper introduces the design of a magneto-plasmonic refractometric sensor aimed at achieving high resolution. This sensor consists of arrays of gold nanowires and layers of SiO2 and Co6Ag94, where the analyte is placed on the gold nanowires. A p-polarized optical field with a wavelength of 631 nm is used to excite the structure, which is applied in the range of 1° to 45°. A magnetic field is applied to z-axis to create the magneto-optical effect. The reflected optical field of the samples is used to calculate the signal of the transverse magneto-optical Kerr effect, which shows significant changes in the refractive index of the samples and the direction of the magnetic field. The highest displacement is 4°. The highest value of the figure of merit is 3611 RIU−1, and the maximum sensitivity is obtained as 71 °/RIU.