Light Magnetic Field Research Articles

Quantum Metamaterials with Magnetic Response at Optical Frequencies.

We propose novel quantum antennas and metamaterials with a strong magnetic response at optical frequencies. Our design is based on the arrangement of natural quantum emitters with only electric dipole transition moments at distances smaller than a wavelength of light but much larger than their physical size. In particular, we show that an atomic dimer can serve as a magnetic antenna at its antisymmetric mode to enhance the decay rate of a magnetic transition in its vicinity by several orders of magnitude. Furthermore, we study metasurfaces composed of atomic bilayers with and without cavities and show that they can fully reflect the electric and magnetic fields of light, thus, forming nearly perfect electric or magnetic mirrors. The proposed metamaterials will embody the intrinsic quantum functionalities of natural emitters such as atoms, ions, color center, or molecules and can be fabricated with available state-of-the-art technologies, promising several applications both in classical optics and quantum engineering.

Linear and nonlinear crystal optics using the magnetic field of light

We study the $^{7}F_{0}\ensuremath{\rightarrow}^{5}D_{1}$ magnetic-dipole transition in the $4f$ shell of europium ions at a wavelength of 527.5 nm and demonstrate spectral hole burning, ion class selection, and optical pumping to specific ground hyperfine states. We then use the narrow lines of the selected ions to observe linear group delay of a weak probe pulse with a group velocity of $c/3800$. We also demonstrate electromagnetically induced transparency by establishing a $\mathrm{\ensuremath{\Lambda}}$ scheme between the ground hyperfine levels and driving the ions to a dark state.

Controlling of light with electromagnons

Abstract Magnetoelectric coupling in multiferroic materials opens new routes to control the propagation of light. The new effects arise due to dynamic magnetoelectric susceptibility that cross-couples the electric and magnetic fields of light and modifies the solutions of Maxwell equations in media. In this paper, two major effects will be considered in detail: optical activity and asymmetric propagation. In case of optical activity the polarization plane of the input radiation rotates by an angle proportional to the magnetoelectric susceptibility. The asymmetric propagation is a counter-intuitive phenomenon and it represents different transmission coefficients for forward and backward directions. Both effects are especially strong close to resonance frequencies of electromagnons, i. e. excitations in multiferroic materials that reveal simultaneous electric and magnetic character.

Magnetic fields alter strong-field ionization

When a strong laser pulse induces the ionization of an atom, momentum conservation dictates that the absorbed photons transfer their momentum $p_{\gamma}=E_{\gamma}/c$ to the electron and its parent ion. Even after 30 years of studying strong-field ionization, the sharing of the photon momentum between the two particles and its underlying mechanism are still under debate in theory. Corresponding experiments are very challenging due to the extremely small photon momentum ($~10^{-4}$ a.u.) and their precision has been too limited, so far, to ultimately resolve the debate. Here, by utilizing a novel experimental approach of two counter-propagating laser pulses, we present a detailed study on the effects of the photon momentum in strong-field ionization. The high precision and self-referencing of the method allows to unambiguously demonstrate the action of the light's magnetic field on the electron while it is under the tunnel barrier, confirming theoretical predictions, disproving others. Our results deepen the understanding of, for example, molecular imaging and time-resolved photoelectron holography.

Magnetic spin\u2013orbit interaction of light

We study the directional excitation of optical surface waves controlled by the magnetic field of light. We theoretically predict that a spinning magnetic dipole develops a tunable unidirectional coupling of light to transverse electric (TE) polarized Bloch surface waves (BSWs). Experimentally, we show that the helicity of light projected onto a subwavelength groove milled into the top layer of a 1D photonic crystal (PC) controls the power distribution between two TE-polarized BSWs excited on both sides of the groove. Such a phenomenon is shown to be solely mediated by the helicity of the magnetic optical field, thus revealing a magnetic spin-orbit interaction of light. Remarkably, this magnetic optical effect is clearly observed via a near-field coupler governed by an electric dipole moment: it is of the same order of magnitude as the electric optical effects involved in the coupling. This opens up new degrees of freedom for the manipulation of light and offers desirable and novel opportunities for the development of integrated optical functionalities.

Tunable Spectral Ordering of Magnetic Plasmon Resonances in Noble Metal Nanoclusters

Experimental characterization of the optical and magnetic properties of noble metal nanoclusters has exposed a size regime in between single-ring and extended two-dimensional nanocluster networks where the behavior of magnetic plasmon resonances is not well understood. In this intermediate size regime, individual electric dipole plasmons on each nanoparticle within the cluster can hybridize into delocalized magnetic modes that couple to either the electric or magnetic field of light with seemingly arbitrary energy ordering. Here, using a coupled-dipole model that includes fully retarded interactions between electric dipole plasmons, we show that magnetic plasmon resonance energies can be controllably tuned and even made to cross as a function of nanocluster size. Experimental confirmation of this prediction is challenging because optical selection rules dictate the simultaneous excitation of many spectrally overlapping magnetic plasmon modes. However, based on analytic modeling and numerical simulation, w...

Molecular Design Principles for Magneto-Electric Materials: All-Electric Susceptibilities Relevant to Optimal Molecular Chromophores

Magneto-electric (M-E) response at the molecular level arises from the interaction of matter with the electric and magnetic fields of light, and can manifest itself as nonlinear M-E magnetization (MNL) or M-E rectification (PNL). However, there is presently a limited understanding of how molecular material properties impact M-E response. Here we investigate the relationship between M-E nonlinear coefficients and the third-order electric susceptibility, χ(3), finding that MNL is proportional to χxxxx(3) while PNL scales with χzzxx(3) due to a cascaded nonlinearity. By applying a sum-over-states (SOS) expression for the elements of χ(3) to valence-bond charge-transfer (VB-CT) models, we formulate practical guidelines for the design of materials expected to exhibit enhanced M-E properties. On this basis, we predict that many conventional nonlinear optical chromophores with large values of χxxxx(3) may be suitable for generating optical magnetism at low intensities. In the case of M-E rectification, analysis ...

Coherent Magnetic Response at Optical Frequencies Using Atomic Transitions

Developing exotic optical devices such as super-resolution lenses and cloaks requires atoms that interact strongly with the magnetic field of light. Such an interaction between a laser and an ensemble of europium atoms is shown for the first time.

Index of refraction changes under magnetic field observed in La_066Sr_033MnO_3 correlated to the magnetorefractive effect

The magnetorefractive effect is a change in a sample’s reflectivity with applied magentic field. This is caused by the material’s index of refraction’s sensitivity to magnetic fields. However, measurements of the index of refraction as a function of applied magnetic field have not been previously published. This experimental study measures both the magnetorefractive effect and the index of refraction as a function of applied magnetic field and temperature in a 70 nm thick film of lanthanum strontium manganite La0.66Sr0.33MnO3. Index of refraction characterizations were performed with 633/nm light in magnetic fields ranging from −3 kOe to 3 kOe and near the Curie point, at temperatures from 278 K to 308 K.

All-dielectric metamaterials.

The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

Magnetoelectric resonance with electromagnons in a perovskite helimagnet

Maxwell’s equations describe the interrelation between temporally changing electric (E) and magnetic (H) fields in a given medium. In materials that exhibit relativistic spin–orbit interactions, we also expect their polarization (P) and magnetization (M) to be dynamically coupled. This in turn could enable greater control over the cross-coupling between the electric and magnetic fields of light in the development of photonic devices1. Such magnetoelectric phenomena are expected to be enhanced within materials that support electromagnons—fundamental excitations that exhibit both electric and magnetic dipole moments. Here we report the discovery of electromagnons in the perovskite (Eu,Y)MnO3, which arise from fluctuations in the spontaneous polarization generated by cycloidal spin order2,3,4,5. The resulting dynamical M–P cross-coupling causes the material to exhibit colossal directional dichroism—a difference in the absorption of light propagating in opposite directions—at the resonance frequency (sub-THz) associated with these excitations.

Glimpsing the weak magnetic field of light

Glimpsing the weak magnetic field of light

Persistent photoinduced magnetization and hole droplets inLa0.9Ca0.1MnO3films

Photoinduced magnetization following a stretched exponential growth with time is observed in ${\mathrm{La}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_{3}$ films exposed to near infrared light in magnetic fields of $\mathit{B}\ensuremath{\geqslant}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$. The magnetic irreversibility observed below 56 K between zero-field-cooled and field-cooled films is strongly influenced by illumination, giving space to a persistent magnetic state with increased ferromagnetic interactions, modified magnetic anisotropy and decrease of the blocking temperature to 5 K. When the illumination and the magnetic field are removed the magnetization of the films decays very slowly after a short period of fast relaxation but recovers the original level when the field is applied again. Such behavior obeys predictions for domain pinning in narrow-wall random-field Ising systems. The results suggest trapping of photogenerated electrons by magnetic disorder while the holes contribute to growth of ferromagnetism in the films.

Manifestation of Classical Non-integrability in Magneto-oscillatory Spectra in Cuprous Oxide

The magneto-oscillatory spectra in cuprous oxide can be interpreted as reflecting classical non-integrability of the Hamiltonian of an electron–hole pair generated by the incident light in magnetic fields. The spectra have multi-oscillatory structure, and three periods are extracted from the spectra. It is found that the three periods coincide with three recurrence periods associated with corresponding three types of unstable classical trajectories. The instability is due to non-integrability of the system with spherical Coulomb and planar diamagneticpotential of magnetic fields. This type of analysis opens the way to get an evidence of “quantum chaos” in solid state.

Assignment of quasi-Landau levels in magneto-oscillatory spectra in cuprous oxide to classical unstable trajectories of hydrogen type atoms

It is found, for the first time in the field of solid state spectroscopy, that quasi-Landau levels in magneto-oscillatory spectra in cuprous oxide reflect “classical non-integrability.” Cuprous oxide is well known to exhibit excitonic absorption spectra of typical wannier type near the absorption edge in yellow spectral region. Magneto-oscillatory spectra are observed in the region above the limiting energy of the exciton series in magnetic fields. The spectra are measured at liquid helium temperature with right and left circularly polarized light in magnetic fields up to 4.5 T generated by a superconducting magnet. The observed spectra look like “Landau levels” corresponding to the optical transitions between states of the hole in a valence band and those of the electron in a conduction band without Coulomb attraction between them qualitatively, but never coincide with the Landau levels quantitatively. By calculating inverse Fourier transform of the observed spectra (IFFT spectra), three peaks are found in the auto-correlation function of the excited excitonic state. These spectra are interpreted as those of a hydrogen type atom with the effective masses of the electron–hole pair in homogeneous magnetic field, which is known to be a typical non-integrable system in classical mechanics. Instead of obtaining the quantum mechanical motions of wave packet, the classical trajectories are numerically calculated. First peak of the IFFT spectra is assigned to the trajectories on which the wave packet circulates and returns to approximately to the starting point after the duration corresponding to the first peak. Immediately after that, the trajectories are rapidly apart from the starting point on account of their instability, which reflects the classical non-integrability of the system.

Magneto-oscillatory spectra in cuprous oxide

Optical absorption spectra of yellow series excitons in cuprous oxide were measured with right and left circularly polarized light in magnetic fields up to 4.5 T generated by a superconducting magnet. Magneto-oscillatory spectra were observed in the high magnetic field and short wavelength region above the hydrogenic series. The Zeeman splittings of the hydrogenic series are equal to those of the magneto-oscillatory spectra. Energies of the diamagnetic term of the hydrogenic series are also equal to the cyclotron energy of the magneto-oscillatory spectra. It is concluded from these facts that the two absorption spectra are due to transitons to the same excitonic states.

Magneto-optical Investigation of the Free-Exciton Reflectance from High-Purity Epitaxial GaAs

The free-exciton reflectance structure at 1.5149 \ifmmode\pm\else\textpm\fi{} 0.0001 eV (2 K, zero field) in GaAs has been studied with polarized light in magnetic fields up to $H\ensuremath{\approx}250$ kG. Large shifts, up to six times the exciton binding energy, and complex splittings of the eightfold-degenerate exciton ground state have been observed. These effects have been analyzed with a simple hydrogenic model as well as with a low-field perturbation model that includes the anisotropy and degeneracy of the zinc-blende valence band at $k=0$. The shifts are satisfactorily accounted for over the whole range, whereas only qualitative agreement between the predicted and observed splitting pattern is achieved. Reasons for this discrepancy are discussed.

Magnetic Circular Dichroism Spectra of Divalent Lanthanide Ions in Calcium Fluoride

The broad-band spectra of most of the divalent lanthanide ions in calcium fluoride were studied with circularly polarized light in magnetic fields at liquid-helium temperatures. Large field- and temperature-dependent anisotropies were found in the bands of ions having an odd number of electrons, all of whose states have Kramers degeneracy. A variety of effects are seen for ions having an even number of electrons, ranging from zero as in Ca${\mathrm{F}}_{2}$:${\mathrm{Er}}^{2+}$ to the large, complex effects in Ca${\mathrm{F}}_{2}$:${\mathrm{Dy}}^{2+}$. The magnetic linear dichroism (MLD) of both ${\mathrm{Dy}}^{2+}$ and ${\mathrm{Eu}}^{2+}$ were also measured. The magnetic circular dichroism (MCD) effects of the photochromic ions ${\mathrm{La}}^{2+}$, ${\mathrm{Ce}}^{2+}$, ${\mathrm{Gd}}^{2+}$, and ${\mathrm{Tb}}^{2+}$ are similar to each other and are only briefly described. This study is primarily concerned with the nonphotochromic ions, whose spectra are due to transitions between the opposite-parity configurations $4{f}^{n}$ and $4{f}^{n\ensuremath{-}1}5d$. These ions all have the ground configuration $4{f}^{n}$. The magnetic dichroism effects are discussed in terms of the symmetry properties of the lanthanide ion. The large magneto-optic effects are the result of large spin-orbit coupling and predominantly totally symmetric vibronic coupling. Since the MCD and MLD effects are large throughout entire bands, they may be conveniently used in a variety of optical pumping experiments and may serve to detect changes in level populations.

Two Electron Models of a Constant-Frequency Relativistic Cyclotron

Two model constant-frequency cyclotrons based on the principle of L. H. Thomas, as extended by David L. Judd, are described. Both accelerated electrons to speeds of half that of light in magnetic fields of three-fold azimuthal periodicity. Three 60°-wide wedge-shaped electrodes, driven 120° out of phase, provided an energy gain per revolution of 3 eV0, where V0 is the peak electrode-to-ground voltage. Electrons were accelerated to 75 kv with V0=23 v, implying a minimum of one thousand revolutions in the cyclotron. The beam reached full energy without axial loss and it was demonstrated that essentially all of the circulating current will emerge from this type of accelerator without the use of additional deflecting systems. The success of this development program has shown the feasibility of a high-current, high-energy cyclotron based on the Thomas principle.