Faraday Rotation Research Articles

Ab initio investigation of laser-induced ultrafast demagnetization of L10 FePt: Intensity dependence and importance of electron coherence

We theoretically investigate the optically induced demagnetization of ferromagnetic FePt using the time-dependent density functional theory (TDDFT). We compare the demagnetization mechanism in the perturbative and nonperturbative limits of light-matter interaction and show how the underlying mechanism of the ultrafast demagnetization depends on the driving laser intensity. Our calculations show that the femtosecond demagnetization in TDDFT is a longitudinal magnetization reduction and results from a nonlinear optomagnetic effect, akin to the inverse Faraday effect. The demagnetization scales quadratically with the electric field E in the perturbative limit, i.e., ΔMz∝E2. Moreover, the magnetization dynamics happens dominantly at even multiples nω0, (n=0,2,⋯) of the pump-laser frequency ω0, whereas odd multiples of ω0 do not contribute. We further investigate the demagnetization in conjunction to the optically induced change of electron occupations and electron correlations. Dynamical correlations within the Kohn-Sham local-density framework are shown to have an appreciable yet distinct effect on the amount of demagnetization depending on the laser intensity. Comparing the computed demagnetizations with those calculated from spin occupations, we show that electronic coherence plays a dominant role in the demagnetization process, whereas interpretations based on the time-dependent occupation numbers poorly describe the ultrafast demagnetization. Published by the American Physical Society 2024

Giant reduced magnetic anisotropy in magneto-optical garnet

Magneto-optical garnet shows great potential in integrated optical isolators due to its highly efficient non-reciprocity. In this work, we prepared magneto-optical lithium-modified terbium bismuth iron garnet films using the liquid phase epitaxy method and systemically investigated the effect of lithium modification on magnetic properties. We found a trace amount of lithium modification induced in-plane magnetization and a minimum in-plane driving field around 2 Oe. Meanwhile, high remanence over 70% was obtained in tensile-strain samples. Most importantly, in comparison to unmodified samples, the calculation of anisotropy demonstrated the giant reduction of growth-induced anisotropy in the Li: TbBiIG up to 105erg/cm3. It reveals a new rule of ions’ preferred occupation in dodecahedral sublattices indirectly. Furthermore, enhanced Faraday rotation in the near-infrared band was confirmed in the samples about 1 micrometer, up to 48% higher than unmodified samples. Our work shows Li:TbBiIGs enable a prime candidate material with substantial potential for integrated optical isolation applications.

Gravitational Faraday rotation of light propagation in the Kerr–Newman–Taub–NUT space-time

Gravitational Faraday rotation of light propagation in the Kerr–Newman–Taub–NUT space-time

Weak Faraday Effect Measurement in Anti-Resonant Fiber Based on Intermodal Interference Suppression

Anti-resonant fiber (ARF) works well in a relatively strong magnetic field due to its weak Faraday effect, which results from the fundamental mode mainly transmitting in the air core. Accurately measuring the Faraday effect strength, i.e., the effective Verdet constant, of an ARF determines its applicable scenarios. However, the effective Verdet constant of ARF is ~3 orders of magnitude lower than that of a standard single-mode fiber, which is very difficult to measure. In this paper, we reveal that intermodal interference is the main obstacle to measuring the ultralow effective Verdet constant of ARF and propose using a narrow-band low-coherence light to suppress it. The measured effective Verdet constant of ARF is 0.423 ± 0.005 mrad/T/m at 1550 nm.

Giant Faraday rotation in atomically thin semiconductors

Faraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices, such as optical isolators. Here, we demonstrate that the plane of polarization of light exhibits a giant Faraday rotation of several degrees around the A exciton transition in hBN-encapsulated monolayers of WSe2 and MoSe2 under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 107 deg T−1 cm−1 for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS2 exhibit a large Verdet constant (VIL ≈ +2 × 105 deg T−1 cm−2) of opposite sign compared to A excitons in monolayers. The giant Faraday rotation is due to the giant oscillator strength and high g-factor of the excitons in atomically thin semiconducting transition metal dichalcogenides. We deduce the complete in-plane complex dielectric tensor of hBN-encapsulated WSe2 and MoSe2 monolayers, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. Our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices.

Laboratory evidence of Weibel magnetogenesis driven by temperature gradient using three-dimensional synchronous proton radiography.

The origin of the cosmic magnetic field remains an unsolved mystery, relying not only on specific dynamo processes but also on the seed field to be amplified. Recently, the diffuse radio emission and Faraday rotation observations reveal that there has been a microgauss-level magnetic field in intracluster medium in the early universe, which places strong constraints on the strength of the initial field and implies the underlying kinetic effects; the commonly believed Biermann battery can only provide extremely weak seed of 10-21 G. Here, we present evidence for the spontaneous Weibel-type magnetogenesis in laser-produced weakly collisional plasma with the three-dimensional synchronous proton radiography, where the distribution anisotropy directly arises from the temperature gradient, even without the commonly considered interpenetrating plasmas or shear flows. This field can achieve sufficient strength and is sensitive to Coulomb collision. Our results demonstrate the importance of kinetics in magnetogenesis in weakly collisional astrophysical scenarios.

Space Domain Awareness Observations Using the Buckland Park VHF Radar

There is increasing interest in space domain awareness worldwide, motivating investigation of the use of non-traditional sensors for space surveillance. One such class of sensor is VHF wind profiling radars, which have a low cost relative to other radars typically applied to this task. These radars are ubiquitous throughout the world and may potentially offer complementary space surveillance capabilities to the Space Surveillance Network. This paper updates an initial investigation on the use of Buckland Park VHF wind profiling radars for observing resident space objects in low Earth orbit to further investigate the space surveillance capabilities of the sensor class. The radar was operated during the Australian Defence “SpaceFest” 2019 activity, incorporating new beam scheduling and signal processing functionality that extend upon the capabilities described in the initial investigation. The beam scheduling capability used two-line element propagations to determine the appropriate beam direction to use to observe transiting satellites. The signal processing capabilities used a technique based on the Keystone transform to correct for range migration, allowing the development of new signal processing modes that allow the coherent integration time to be increased to improve the SNR of the observed targets, thereby increasing the detection rate. The results reveal that 5874 objects were detected over 10 days, with 2202 unique objects detected, representing a three-fold increase in detection rate over previous single-beam direction observations. The maximum detection height was 2975.4 km, indicating a capability to detect objects in medium Earth orbit. A minimum detectable RCS at 1000 km of −10.97 dBm2 (0.09 m2) was observed. The effects of Faraday rotation resulting from the use of linearly polarised antennae are demonstrated. The radar’s utility for providing total electron content (TEC) measurements is investigated using a high-range resolution mode and high-precision ephemeris data. The short-term Fourier transform is applied to demonstrate the radar’s ability to investigate satellite rotation characteristics and monitor ionospheric plasma waves and instabilities.

Direct observation of the transition from spin waves to the magnon Bose condensate

Bose-Einstein condensation occurs at an appropriate density of bosonic particles, depending on their mass and temperature. The transition from the semiclassical paradigm of spin waves to the magnon Bose-Einstein condensed state (mBEC) was obtained experimentally with increasing magnon density. We used the Faraday rotation effect to study the spatial distribution of the magnon density and phase far from their excitation region. A coherent magnetization precession was observed throughout the sample, which indicates the formation of a magnon BEC. It is shown that this result under experimental conditions goes beyond the applicability of the Landau-Lifshitz-Gilbert semiclassical theory.

First Sagittarius A* Event Horizon Telescope Results. VIII. Physical Interpretation of the Polarized Ring

In a companion paper, we present the first spatially resolved polarized image of Sagittarius A* on event horizon scales, captured using the Event Horizon Telescope, a global very long baseline interferometric array operating at a wavelength of 1.3 mm. Here we interpret this image using both simple analytic models and numerical general relativistic magnetohydrodynamic (GRMHD) simulations. The large spatially resolved linear polarization fraction (24%–28%, peaking at ∼40%) is the most stringent constraint on parameter space, disfavoring models that are too Faraday depolarized. Similar to our studies of M87*, polarimetric constraints reinforce a preference for GRMHD models with dynamically important magnetic fields. Although the spiral morphology of the polarization pattern is known to constrain the spin and inclination angle, the time-variable rotation measure (RM) of Sgr A* (equivalent to ≈46° ± 12° rotation at 228 GHz) limits its present utility as a constraint. If we attribute the RM to internal Faraday rotation, then the motion of accreting material is inferred to be counterclockwise, contrary to inferences based on historical polarized flares, and no model satisfies all polarimetric and total intensity constraints. On the other hand, if we attribute the mean RM to an external Faraday screen, then the motion of accreting material is inferred to be clockwise, and one model passes all applied total intensity and polarimetric constraints: a model with strong magnetic fields, a spin parameter of 0.94, and an inclination of 150°. We discuss how future 345 GHz and dynamical imaging will mitigate our present uncertainties and provide additional constraints on the black hole and its accretion flow.

Deep Synoptic Array Science: Polarimetry of 25 New Fast Radio Bursts Provides Insights into Their Origins

We report on a full-polarization analysis of the first 25 as yet nonrepeating fast radio bursts (FRBs) detected at 1.4 GHz by the 110-antenna Deep Synoptic Array (DSA-110) during commissioning observations. We present details of the data-reduction, calibration, and analysis procedures developed for this novel instrument. Faraday rotation measures (RMs) are searched between ±106 rad m−2 and detected for 20 FRBs, with magnitudes ranging from 4 to 4670 rad m−2. Fifteen out of 25 FRBs are consistent with 100% polarization, 10 of which have high (≥70%) linear-polarization fractions and two of which have high (≥30%) circular-polarization fractions. Our results disfavor multipath RM scattering as a dominant depolarization mechanism. Polarization-state and possible RM variations are observed in the four FRBs with multiple subcomponents. We combine the DSA-110 sample with polarimetry of previously published FRBs, and compare the polarization properties of FRB subpopulations and FRBs with Galactic pulsars. Although FRB polarization fractions are typically higher than those of Galactic pulsars, and cover a wider range than those of pulsar single pulses, they resemble those of the youngest (characteristic ages <105 yr) pulsars. Our results support a scenario wherein FRB emission is intrinsically highly linearly polarized, and propagation effects can result in conversion to circular polarization and depolarization. Young pulsar emission and magnetospheric propagation geometries may form a useful analogy for the origin of FRB polarization.

Метод эффективных вычитаний: работа с данными Иркутского радара некогерентного рассеяния

For incoherent scatter measurements, the effective subtraction technique is to alternate the duration of amplitude-modulated signals between a pair of consequently radiated pulses. The resulting gain of spatial resolution enables us to steadily assess the electron density profile by the Faraday rotation method. The paper describes the electron density measurement technique, which involves analyzing narrow-band signals from Irkutsk Incoherent Scatter Radar, and proposes an automated method of determining the electron density for the problem in which the convolution of the radiated signal waveform with backscatter signal cannot be neglected. The inverse problem of electron density recovery is considered as a standard nonlinear optimization problem, which is solved using the algorithms for global and local optimization applied consequently. We compare the electron density profiles obtained by analyzing different pulse waveforms and from Irkutsk ionosonde data.

Effective subtraction technique: implementation for Irkutsk Incoherent Scatter Radar

For incoherent scatter measurements, the effective subtraction technique is to alternate the duration of amplitude-modulated signals between a pair of consequently radiated pulses. The resulting gain of spatial resolution enables us to steadily assess the electron density profile by the Faraday rotation method. The paper describes the electron density measurement technique, which involves analyzing narrow-band signals from Irkutsk Incoherent Scatter Radar, and proposes an automated method of determining the electron density for the problem in which the convolution of the radiated signal waveform with backscatter signal cannot be neglected. The inverse problem of electron density recovery is considered as a standard nonlinear optimization problem, which is solved using the algorithms for global and local optimization applied consequently. We compare the electron density profiles obtained by analyzing different pulse waveforms and from Irkutsk ionosonde data.

Broadband High-Precision Faraday Rotation Spectroscopy with Uniaxial Single Crystal CeF3 Modulator

We present a low-noise (&lt;10 µrad/Hz) broadband Faraday Rotation Spectroscopy method which is feasible for near-ultraviolet through near-infrared wavelengths. We demonstrate this in the context of a high-precision spectroscopy experiment using a heated Pb vapor cell and two different lasers, one in the UV (368 nm) and a second in the IR (1279 nm). A key element of the experimental technique is the use of a uniaxial single crystal CeF3 Faraday modulator with excellent transmission and optical rotation properties across the aforementioned wavelength range. Polarimeter performance is assessed as a function of crystal orientation and alignment, AC modulation amplitude, laser power, and laser wavelength. Crystal-induced distortion of the (6p2)3P0→(6p2)3P1 (1279 nm) and (6p2)3P1→(6p7s)3P0 (368 nm) spectral lines due to misalignment-induced birefringence is discussed and modeled using the Jones calculus.

High-sensitivity determination of ethanol concentration based on weak measurement system with Faraday effect

A high-sensitivity detection method is proposed for ethanol concentration based on weak measurement system with Faraday effect, and the improvement law of the sensitivity is studied. Different from chiral solution determination, the post-selection angle of the system is modified by the Faraday rotation angle of ethanol solution. The amplified photonic spin Hall effect acts as a probe for detecting the ethanol concentration in the system, and the sensitivity of ethanol concentration increases linearly with the increasing magnetic field intensity. Then we use the method to determine the ethanol concentration of commercial liquor, whose determination values are consistent with the label values and Raman spectroscopy results. Compared to other methods such as Raman spectrometry, the method has the advantages of real time, non-invasiveness, low cost and high sensitivity. Finally, the sensitivity of ethanol concentration can be further improved by designing of a suitable BK7-medium structure, and the sensitivity of 9.72 /vol% is achievable for the range 0 vol% ∼100 vol% of ethanol solutions.

Vibration-insensitive polarimetric fiber optic current sensor based on orbital angular momentum modes in an air-core optical fiber.

Current or magnetic field sensing is usually achieved by exploiting the Faraday effect of an optical material combined with an interferometric probe that provides the sensitivity. Being interferometric in nature, such sensors are typically sensitive to several other environmental parameters such as vibrations and mechanical disturbances, which, however, inevitably impose the inaccuracy and instability of the detection. Here we demonstrate a polarimetric fiber optic current sensor based on orbital angular momentum modes of an air-core optical fiber. In the fiber, spin-orbit interactions imply that the circular birefringence, which is sensitive to applied currents or resultant magnetic fields, is naturally resilient to mechanical vibrations. The sensor, which effectively measures polarization rotation at the output of a fiber in a magnetic field, exhibits high linearity in the measured signal versus the applied current that induces the magnetic field, with a sensitivity of 0.00128 rad/A and a noise limit of 1×10-5/H z. The measured polarization varies within only ±0.1% under mechanical vibrations with the frequency of up to 800 Hz, validating the robust environmental performance of the sensor.

Faraday rotation and transmittance as markers of topological phase transitions in 2D materials

We analyze the magneto-optical conductivity (and related magnitudes like transmittance and Faraday rotation of the irradiated polarized light) of some elemental two-dimensional Dirac materials of group IV (graphene analogues, buckled honeycomb lattices, like silicene, germanene, stannane, etc.), group V (phosphorene), and zincblende heterostructures (like HgTe/CdTe quantum wells) near the Dirac and gamma points, under out-of-plane magnetic and electric fields, to characterize topological-band insulator phase transitions and their critical points. We provide plots of the Faraday angle and transmittance as a function of the polarized light frequency, for different external electric and magnetic fields, chemical potential, HgTe layer thickness and temperature, to tune the material magneto-optical properties. We have shown that absortance/transmittance acquires extremal values at the critical point, where the Faraday angle changes sign, thus providing fine markers of the topological phase transition. In the case of non-topological materials as phosphorene, a minimum of the transmittance is also observed due to the energy gap closing by an external electric field.

Measurement of Rubidium Vapor Density Based on Spin‐Exchange Rate

AbstractAn efficient method is proposed for quantifying rubidium vapor density by analyzing the spin‐exchange rate between alkali metals. This method effectively addresses two primary challenges in measuring alkali vapor density. First, it overcomes the limitation of absorption spectroscopy to measure vapor density under optically thick conditions, typically restricted to temperature higher than 420K, equivalent to alkali vapor density exceeding . Second, it mitigates the risk of disrupting shielding functionality due to the application of a strong magnetic field, often in the range of several tens of Gauss or higher, when employing the Faraday rotation for vapor‐density measurement. The spin‐exchange rate between atoms, inherently related to the vapor density, is evident in the electron‐paramagnetic‐resonance spectrum of spin‐polarized , thereby providing a possibility for measuring alkali vapor density. To eliminate overlap between two Lorentzian curves resulting from two decomposed components of an oscillating field, a small rotating magnetic field with identical amplitude and frequency but a 90‐degree phase shift along both the x and y axes is applied. This method is successfully employed to measure vapor density in the temperature range of 373–433 K, with the measured outcomes closely matching the saturation vapor curve.

Proposal to Observe Transverse Sound in Normal Liquid $$^3$$He in Aerogel

AbstractIn the Fermi liquid metallic state, a static local magnetic moment is induced on the application of a circularly polarized electromagnetic wave, via the inverse Faraday effect (IFE). The direction of this moment is along the direction of propagation of light, and the magnitude of the moment depends on the frequency of light, the temperature and various material parameters characteristic of the metal. I propose an analogous effect in the Fermi liquid state of $$^3$$ 3 He. A static circulating current is induced when liquid $$^3$$ 3 He is driven by a circularly polarized transverse acoustic wave. For liquid $$^3$$ 3 He filled into aerogel, the coupled system supports a low-attenuation transverse sound mode. I estimate the magnitude of induced circulating currents for this system and find that these are within the range of experimental measurement in the low-attenuation regime. The axis of circulation is along the direction of propagation of the acoustic wave. I propose this analogue of the inverse Faraday effect as a scheme to experimentally demonstrate the propagation of transverse sound in $$^3$$ 3 He-aerogel.

Distributed optical fiber magnetic field sensor based on polarization-sensitive OFDR.

A distributed optical fiber magnetic field sensor based on a polarization-sensitive optical frequency domain reflectometer (POFDR) is proposed. It extracts the accumulated Faraday rotation by combining the Stokes vectors and the backward Mueller matrices from the measured states of polarization (SOPs) and obtains the magnetic field component. This method avoids adjusting the input polarization during the magnetic field sensing process. It overcomes the drawback of the conventional POFDR scheme, which requires at least two sets of different input SOPs for each sensing. Finally, the aforementioned effectiveness has been experimentally verified by using a single-mode sensing fiber. The results show that the sensor has good repeatability and linearity. The measurement error of the magnetic field sensor is 19.4 mT. The measured magnetic field variations agree with the applied ones with similarities higher than 0.98.

Time-resolved FRS detection of OH radical for chemical reactions rate measurement

The time-resolved Faraday rotation spectroscopy method with a DFB diode laser at 1434 nm was used to detect the hydroxyl radical OH in the first overtone region. Experiments were made at the most intense Q(3/2) line of the 2Π3/2 electronic term. The sufficient concentration for radical measurement is 2 × 1012 cm−3, with the resolving time approximately 10−6 s. This resolution is sufficient for studying fast chemical reactions, which was demonstrated by measuring the rate constant of the OH and dimethylsulfide reaction.