Faraday Geometry Research Articles

Signal-to-noise trade-offs between magnet diameter and shield-to-coil distance for cylindrical Halbach-based portable MRI systems for neuroimaging.

To investigate the trade-off between magnet bore diameter and the distance between the conductive Faraday shield and RF head coil for low-field point-of-care neuroimaging systems. Electromagnetic simulations were performed for three different Faraday shield geometries and two commonly used RF coil designs (spiral and solenoid) to assess the effects of a close-fitting shield on the RF coil's transmit and receive efficiencies. Experimental measurements were performed to confirm the accuracy of the simulations. Parallel simulations were performed to assess the static magnet ( ) field as a function of the magnet bore diameter. The obtainable SNR was then calculated as a function of these two related variables. Simulations of the RF coil characteristics and transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the RF coil transmit efficiency was, as expected, higher when the gap between the shield and coil increased. The calculated intrinsic SNR showed that maximum SNR would be obtained for a cylindrical shield of diameter 310mm with an inner diameter of the magnet of 320mm (assuming 10mm for the gradient coils). This work presents an overview of the trade-offs in transmit efficiencies for RF coils used for POC MRI neuroimaging as a function of coil-to-shield distance and inner diameter of the Halbach magnet. Results show that there is a relatively shallow optimum between a magnet diameter of 290 and 330mm, with values falling more than 10% if either smaller or larger magnets are used.

Table-top laser-based terahertz high harmonic generation spectroscopy under magnetic fields and low temperatures.

We have developed a terahertz (THz) nonlinear spectrometer at low temperatures (1.5-300K) and under high magnetic fields (up to 10T) by combining the laser-driven table-top intense THz source with a superconducting magnet. The strong-field THz pump pulse was generated from LiNbO3 crystal using the tilted-pulse-front technique and tightly focused into the center of the magnet by an off-axis parabolic mirror and a THz lens. The electric fields at the focus can achieve 500kV/cm with a monocycle waveform and 30kV/cm with a multicycle waveform at 0.5THz. The sample was mounted on a low-temperature motorized rotation stage, which enables performing the polarization dependent measurements of the third harmonic generation (THG) intensity without rotating the incident THz pulses. The magnetic field direction can be rotated using a mechanical rotator, allowing for a convenient switch between Faraday and Voigt geometry. We demonstrate the excellent performance of our instrument by conducting THG measurements in the two-band superconductor MgB2 as a function of temperature, sample azimuth angle, as well as in-plane and out-of-plane magnetic fields. The successful combination of the strong field THz source with magnetic fields enables us to study a variety of materials with magnetic-field-dependent properties of interest.

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.

Magnetically Induced Near-Infrared Circularly Polarized Electroluminescence from an Achiral Perovskite Light-Emitting Diode

Circularly polarized electroluminescent devices are conventionally fabricated by incorporating an optically active chiral luminophore into their emission layer. Herein, we developed a circularly polarized perovskite light-emitting diode (PeLED) system with an optically inactive perovskite luminophore that can emit near-infrared circularly polarized electroluminescence (CPEL) upon application of an external magnetic field. The magnitude of the magnetic CPEL (gMCPEL) was in the order of 10−3 in the near-infrared wavelength range of 771–773 nm. Although the Pb perovskite quantum dots were achiral, the rotation direction of the CPEL of the magnetic circularly polarized PeLED system was successfully reversed by switching the Faraday geometry of the applied magnetic field. The use of achiral luminophores exhibiting magnetic-field-induced CPEL represents a new approach for the development of circularly polarized electroluminescent devices.

A device for magnetic-field angle control in magneto-optical filters using a solenoid-permanent magnet pair.

Atomic bandpass filters are used in a variety of applications due to their narrow bandwidths and high transmission at specific frequencies. Predominantly, these filters are in the Faraday (Voigt) geometry, using an applied axial (transverse) magnetic field with respect to the laser propagation direction. Recently, there has been interest in filters realized with arbitrary-angle magnetic fields, which have been made by rotating permanent magnets with respect to the k-vector of the interrogating laser beam. However, the magnetic field angle achievable with this method is limited as field uniformity across the cell decreases as the rotation angle increases. In this work, we propose and demonstrate a new method of generating an arbitrary-angle magnetic field, using a solenoid to produce a small, and easily alterable, axial field, in conjunction with fixed permanent magnets to produce a large transverse field. We directly measure the fields produced by both methods, finding them to be very similar over the length of the vapor cell. We then compare the transmission profiles of filters produced using both methods, again finding excellent agreement. Finally, we demonstrate the sensitivity of the filter profile to changing magnetic field angle (solenoid current), which becomes easier to exploit with the much improved angle control and precision offered by our new design.

Topological interface states - A possible path towards a Landau-level laser in the THz regime

Volkov-Pankratov surface bands arise in smooth topological interfaces, interfaces between a topological and a trivial insulator, in addition to the chiral surface state imposed by the bulk-surface correspondence of topological materials. These two-dimensional bands become Landau-quantized if a magnetic field is applied perpendicular to the interface. I show that the energy scales, which are typically in the 10-10010−100 meV range, can be controlled both by the perpendicular magnetic field and the interface width. The latter can still be varied with the help of a magnetic-field component in the interface. The Landau levels of the different Volkov-Pankratov bands are optically coupled, and their arrangement may allow one to obtain population inversion by optical pumping. This could serve as the elementary brick of a multi-level laser based on Landau levels. Moreover, the photons are absorbed and emitted either parallel or perpendicular to the magnetic field, respectively in the Voigt and Faraday geometry, depending on the Volkov-Pankratov bands and Landau levels involved in the optical transitions.

Zeeman Splitting of Excitons in GaAs/AlGaAs Quantum Wells in the Faraday Geometry

Zeeman Splitting of Excitons in GaAs/AlGaAs Quantum Wells in the Faraday Geometry

External magnetic field-induced circularly polarized luminescence and electroluminescence from optically inactive thermally activated delayed fluorescence material 4CzIPN.

An achiral optically inactive organic luminophore, 4CzIPN, exhibits circularly polarized thermally activated delayed fluorescence when photoexcited under an external magnetic field. By embedding this luminophore in an active emission layer, an external-magnetic-field-induced circularly polarized electroluminescent device is developed in this study. The Faraday geometry of the applied magnetic field completely controls the direction of rotation of 4CzIPN-derived circularly polarized luminescence and electroluminescence.

Theoretical investigations on Kerr and Faraday rotations in topological multi-Weyl Semimetals

Motivated by the recent proposal of giant Kerr rotation in WSMs, we investigate the Kerr and Faraday rotations in time-reversal broken multi-Weyl semimetals (mWSMs) in the absence of an external magnetic field. Using the framework of Kubo response theory, we find that both the longitudinal and transverse components of the optical conductivity in mWSMs are modified by the topological charge (nn). Engendered by the optical Hall conductivity, we show in the thin film limit that, while the giant Kerr rotation and corresponding ellipticity are independent of nn, the Faraday rotation and its ellipticity angle scale as nn and n^2n2, respectively. In contrast, the polarization rotation in semi-infinite mWSMs is dominated by the axion field showing nn dependence. In particular, the magnitude of Kerr (Faraday) angle decreases (increases) with increasing nn in Faraday geometry, whereas in Voigt geometry, it depicts different nn-dependencies in different frequency regimes. The obtained results on the behavior of polarization rotations in mWSMs could be used in experiments as a probe to distinguish single, double, and triple WSMs, as well as discriminate the surfaces of mWSMs with and without hosting Fermi arcs.

Optical Alignment and Optical Orientation of Excitons in CdSe/CdS Colloidal Nanoplatelets.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron-hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Tunable magnetless optical isolation with twisted Weyl semimetals

Abstract Weyl semimetals hold great promise in revolutionizing nonreciprocal optical components due to their unique topological properties. By exhibiting nonreciprocal magneto-optical effects without necessitating an external magnetic field, these materials offer remarkable miniaturization opportunities and reduced energy consumption. However, their intrinsic topological robustness poses a challenge for applications demanding tunability. In this work, we introduce an innovative approach to enhance the tunability of their response, utilizing multilayered configurations of twisted anisotropic Weyl semimetals. Our design enables controlled and reversible isolation by adjusting the twist angle between the anisotropic layers. When implemented in the Faraday geometry within the mid-IR frequency range, our design delivers impressive isolation, exceeding 50 dB, while maintaining a minimal insertion loss of just 0.33 dB. Moreover, the in-plane anisotropy of Weyl semimetals eliminates one or both polarizers of conventional isolator geometry, significantly reducing the overall dimensions. These results set the stage for creating highly adaptable, ultra-compact optical isolators that can propel the fields of integrated photonics and quantum technology applications to new heights.

Field-induced chiral transitions in the Weyl semimetal TaP

The chiral anomaly effect has been regarded as a hallmark of Weyl semimetals. While most research focuses on the change of electrical conductivity due to the chiral charge-pumping effect, these transport measurements are accompanied by other effects such as weak antilocalization which are difficult to exclude. Also, the magnetic field intensity at which the energy dispersion of the Landau level starts to display the chiral anomaly effect is hard to identify. Here, we report the evolutionary process of the energy dispersion of the zeroth Landau level under increasing magnetic fields, and we elucidate the relationship between the chiral Landau level and the Fermi level via magneto-optical measurements in Weyl semimetal TaP. Two series of Landau-level transitions have been extracted and fitted well to a massless Weyl fermion model and a classical semiconductor model, respectively. The field-induced chiral Landau-level transition is obtained from a comparison between the spectra under Voigt geometry and Faraday geometry in an increasing magnetic field. The chiral nature of the zeroth Landau level is found to enhance with the increase of magnetic field and clearly manifests in the magneto-optical spectrum above 13 T. The state transition of the zeroth Landau level can be explained by the strength of the field-induced chiral anomaly effect and the position of the Fermi level. These magneto-optical results unveil the chiral Landau-level transitions in TaP, which serves as an ideal material platform to analyze the chiral anomaly effect.

Spin noise of magnetically anisotropic centers

Spin noise spectroscopy, as a sort of magnetic resonance technique, uses, for detection of spin precession, spontaneous fluctuations of magnetization revealed as a peak in the Faraday-rotation (FR) noise spectrum at Larmor frequency. In the model of precessing magnetization, the FR noise signal should be the greatest in the Voigt geometry (with magnetic field aligned across the light propagation), and should vanish in the Faraday geometry (with the field along the probe beam). This reasoning employs, implicitly or explicitly, the so-called Van Vleck theorem that establishes, within the limits of certain assumptions, a direct relation between the FR and magnetization of the spin system. We show that violation of these assumptions in crystals with anisotropic paramagnetic centers may qualitatively change the conventional laws of spin noise detection, making, in particular, the FR noise detectable in the Faraday geometry. These conclusions are confirmed by experimental studies of spin-noise spectra of ${\mathrm{CaF}}_{2}$ crystals with tetragonal centers of ${\mathrm{Nd}}^{3+}$ ions.

Anomalous low-energy carrier dynamics of stripe-type charge ordered Pr0.5Sr0.5MnO3 thin film revealed by magneto-terahertz spectroscopy

The perovskite manganites host a range of charge- and orbital-ordered phases wherein the intrinsic and extrinsic controls of magnetic field, doping, epitaxial strain, chemical pressure, etc., induce subtleties of phase transition. The Pr0.5Sr0.5MnO3 (PSMO) is one such system that possesses a unique stripe-type charge-order manifesting in anisotropic transport. It exhibits a Drude-type and charge-density-wave (CDW) type of low-energy phases; the magnetic field is contemplated as a key control for both these phases; however, it has not yet been explored. Here, we have investigated the magnetic-field dependence of Drude-type terahertz (THz) carriers dynamics and the CDW type of low-energy excitations along two non-identical orthogonal in-plane axes of the (110) epitaxial thin film in the energy range of 1–7 meV using magneto-THz time-domain spectroscopy in Faraday geometry. While THz Drude conductivity expectedly increases with the increasing magnetic field, the CDW type of resonance absorption peak anomalously strengthens with the field. The origin of this unusual field-induced stabilization of the CDW-type mode is expected to lie in the d(x2–y2) orbital ordering in the A-type antiferromagnetic ordered state and anisotropic charge ordering. This study brings out a unique facet of the effect of magnetic field on orbital ordering of anisotropic stripe-type charge-ordered PSMO, which has potential THz modulation and switching applications.

Unconventional Landau level transitions in Weyl semimetal NbP

Weyl semimetals were experimentally discovered as a new quantum phase of matter that exhibits topologically protected states characterized by separated Weyl points. Similar to other topological materials, the research of Landau level transitions can provide abundant information on Fermi surfaces. Extensive experimental efforts on low-temperature transport and optical properties have been dedicated to investigating the nontrivial topology structure. However, there are some theoretically predicted unconventional Landau level transitions in Weyl semimetals that have not been observed experimentally. And, their contribution to the electromagnetic response of Weyl semimetal remains elusive. Here, we report the magneto-optical study of Landau quantization and selection rules in Weyl semimetal NbP under Voigt geometry. By changing the direction of the electric vector of the incoming light under Voigt and Faraday geometry, abundant Landau level transition modes which contain $\mathrm{\ensuremath{\Delta}}=0$, \ifmmode\pm\else\textpm\fi{}1, \ifmmode\pm\else\textpm\fi{}2 selection rules are obtained. The richness of the optical spectra, particularly the ones from the Voigt geometry, allows us to determine the location of Fermi energy and its evolution with magnetic fields. We further extract the transitions between Landau levels and Fermi energy and deduce the Landau index. Our results reveal the track of Fermi energy under varying magnetic fields based on the Landau level transitions under Voigt geometry and suggest that a combination of the magneto-optical spectra under Voigt and Faraday geometry can effectively determine the key properties of topological materials that are otherwise inaccessible.

Magneto-optical nonreciprocity without chirality: Archimedean spirals on InSb.

We report the observation of magneto-optical nonreciprocity in Faraday geometry in a hybrid metamaterial consisting of an Archimedean spiral metasurface and semiconductor InSb that serves as the magneto-optical medium. None of the metamaterial constituents possesses chirality, which is usually a necessary ingredient for optical nonreciprocity in natural materials when the light travels along the magnetic field direction. We also find that our metamaterial can serve as an optical element for polarization control via magnetic field. Another significant property of our hybrid metamaterial is the emergence of the four different transmittance states, which are observed for the four combinations of the positive and negative magnetic field and the direction of the wavevector of light.

Spin-Selective Currents of Tamm Polaritons

We propose an approach for the excitation of polariton Tamm states with a controllable nontrivial topology. The Tamm polaritons emerge at the interface of two binary one-dimensional photonic crystals belonging to a ${C}_{3v}$ point group, with an exciton resonance within their matching band gaps. The external magnetic field applied in the Faraday geometry endows the dispersion of the Tamm polaritons with the nonreciprocity: it lifts the degeneracy between opposite propagation directions in the interface plane. The phenomenology of Tamm polariton currents closely resembles one of a ${Z}_{2}$ topological insulator. The proposed structure acts as an optical spin splitter controlled by the magnetic field magnitude.

External Magnetic Field Driven, Ambidextrous Circularly Polarized Electroluminescence from Organic Light Emitting Diodes Containing Racemic Cyclometalated Iridium(III) Complexes

AbstractThis work reports the development of external magnetic field (EMF) driven, ambidextrous, circularly polarized electroluminescence (CPEL) devices, by embedding racemic mixtures of phosphorescent organoiridium(III) emitters in the active emitting layer. Homoleptic tris‐cyclometalated Ir(III)(ppy)3 and heteroleptic bis‐cyclometalated Ir(III)(ppy)2(acac) (ppy=2‐phenylpyridinate, acac=acetylacetonate) were used as the representative organoiridium(III) compounds. Chiroptical inversion of CPEL was influenced by the ligand environment in the Ir(III) complexes (homoleptic or heteroleptic), as well as the Faraday geometry.

Red‐Green‐Blue‐Yellow (RGBY) Magnetic Circularly Polarised Luminescence (MCPL) from Optically Inactive Phosphorescent Ir(III) Complexes

AbstractThe development of phosphorescent organometallic molecules, capable of full‐colour circularly polarised luminescence with a controlled polarised sign in the absence of any chiral chemical influences, presents a significant scientific and engineering challenge. The current study describes the first red‐green‐blue‐yellow (RGBY) coloured, nearly mirror‐image magnetic circularly polarised luminescence (MCPL) in dimethyl sulfoxide and dichloromethane, from four optically inactive Iridium(III) complexes viz. Ir(III)(piq)3, Ir(III)(ppy)3, Ir(III)(F2‐ppy)2(pic), and Ir(III)(BT)2(acac), bearing achiral organic cyclometalated and ancillary ligands. The absolute value of the anisotropy factors, |gMCPL|, ranging from 0.9×10−3 to 2.5×10−3 in the presence of an external 1.6 T magnetic field was nearly identical to the enantiomerically pure Iridium (III) complexes at zero‐magnetic fields. Unexpectedly, when the same north‐up/south‐up Faraday geometries were used, the MCPL signs were inverted due to the achiral ligands.

Mirror symmetric Green–Color magnetic circularly polarized luminescence from TbIII–containing inorganics under North-up and South-up Faraday geometries

The present paper reports on three new achiral terbiumIII inorganics that emit strong green-color magnetic circularly polarized luminescence (MCPL) (|gMCPL| ≤ 5.4 × 10−3 T−1 at 490–550 nm) when embedded in a KBr pellet in the presence of a 1.7 T external magnetic field. The signs of the green MCPL spectra are controllable by altering the S-up and N-up directions of the external magnetic field. Divalent manganeseII and trivalent terbiumIII emit similar green luminescence; however, their wavelengths and MCPL characteristics are very different. Hence, we anticipate that these green-color MCPL substances can be used as new security pigments.