Magneto-optical Circular Dichroism Research Articles

Optical isolator based on chiral light-matter interactions in a ring resonator integrating a dichroic magneto-optical material

Nonreciprocal optical devices based on magneto-optical ferrites in their low-loss regimes have been widely investigated as a promising platform for integrated photonics. Nonreciprocity in such devices originates from circular birefringence, leading to frequency splitting of forward and backward modes and, as a result, nonreciprocal transmission. In this paper, we propose an alternative approach to realize nonreciprocal devices based on magneto-optical circular dichroism and relying on the very presence of optical absorption. Our approach relies on the phenomenon of spin-Hall effect of light, which gives rise to chiral near field interactions of light carrying transverse angular momentum with matter, which, in lossy regimes, yields a disparate absorption for forward and backward optical modes. As an example of practical application, we design an optical isolator based on ring resonator integrating Ce:YIG ferrite, and we demonstrate isolation near 880 nm absorption line due to the ionic electric dipole transition. A Ce:YIG film asymmetrically placed on the inner side of the ring yields different critical coupling conditions due to the chiral nature of evanescent light for forward and backward waves, leading to nonreciprocal absorption and transmission. The proposed approach to nonreciprocity may significantly broaden the possible choice of magneto-optical materials for nonreciprocal devices, enabling operation even in lossy regimes.

Switching the Optical Chirality in Magnetoplasmonic Metasurfaces Using Applied Magnetic Fields.

Chiral nanophotonic devices are promising candidates for chiral molecule sensing, polarization of diverse nanophotonics, and display technologies. Active chiral nanophotonic devices, where the optical chirality can be controlled by an external stimulus has triggered great research interest. However, efficient modulation of the optical chirality has been challenging. Here, we demonstrate switching of the extrinsic chirality by applied magnetic fields in a magnetoplasmonic metasurface device based on a magneto-optical oxide material, Ce1Y2Fe5O12 (Ce:YIG). Due to the low optical loss and strong magneto-optical effect of Ce:YIG, we experimentally demonstrated giant and continuous far-field circular dichroism (CD) modulation by applied magnetic fields from -0.6 ± 0.2° to +1.9 ± 0.1° at 950 nm wavelength under glancing incident conditions. The far-field CD modulation is due to both magneto-optical circular dichroism and near-field modulation of the superchiral fields by applied magnetic fields. Finally, we demonstrate magnetic-field-tunable chiral imaging in millimeter-scale magnetoplasmonic metasurfaces fabricated using self-assembly.

The Quest for Nanoscale Magnets: The example of [Mn12] Single Molecule Magnets.

Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects.

Epitaxial growth and physical properties of a room temperature ferromagnetic semiconductor: Anatase phase Ti1−xCoxO2

There has been considerable debate regarding the origin of ferromagnetism in the ferromagnetic semiconductor anatase phase TiO2 doped with Co, ranging from carrier induced ferromagnetism in a framework of diluted magnetic semiconductor to merely the precipitation of ferromagnetic and metallic Co precipitates in the TiO2 matrix. This paper reports on a systematic investigation of the structures, together with the electronic and magneto-optical properties, of thin films of anatase phase TiO2, doped with Co. Films of anatase Ti1−xCoxO2 (x=0–0.10) were grown epitaxially on LaSrAlO4 (001) substrates, using pulsed laser deposition method. In order to control n-type carrier concentration, the oxygen pressure during the growth was systematically varied, yielding films with insulative, semiconductive (carrier concentration n∼1×1017 cm−3), and metallic (n∼2×1019 cm−3) properties. Reflection high energy electron diffraction patterns, atomic force microscopy, and transmission electron microscopy show no evidence of Co segregation in these films. Magneto-optical response was examined by measuring magneto-optical circular dichroism (MCD) spectra at room temperature. A large MCD peak appears at a photon energy close to the band gap of TiO2. The magnetic field dependence of MCD signal indicates ferromagnetic behavior, which is similar to that of the magnetization. The MCD intensity increases systematically with the increase of n or x. Since MCD response originates from charge carriers at the band edge, one can conclude that interaction between the charge carriers and Co impurities is essential to realize ferromagnetism.

Optical and magneto-opticalproperties of the simple cubic phase of the C60 crystal

Using the Hartree-Fock and single configuration interactionmodel in conjunction with the local field method, the dielectrictensor, refractive index, circular dichroism, birefringencecoefficient and the effects of spatial dispersion on thespectrum of the transverse normal polariton waves of the simplecubic phase of the C60 crystal are calculated. Our resultson magneto-optical circular dichroism in the energy range3-4 eV almost reproduces the reported experimental results.Also, our results on the effect of spatial dispersion, comparedwith the non-dispersive case, indicate that the first branch ofthe transverse normal polariton in the C60 crystal is dueto the coupling of the second excitonic state of the C60molecule with the electromagnetic field and the first excitonicstate has very weak coupling.

Magnetic Domain Imaging with Scanning Near-field Optical Microscopy

An apparatus combining various scanning probe microscopy techniques is presented. It is dedicated to near-field magneto-optical (MO) and optical transmission imaging, and simultaneously gives topographical information. This MO scanning near-field optical microscope (SNOM), working in transmission-collection mode, measures the magneto-optical circular dichroism (MOCD) effect of the samples, using a tapered and metal-coated optical fiber probe. The performance of the apparatus is mainly determined by this fiber probe. Resolution of about 200 nm was obtained on 200 A thick Co/Pt and CoNi/Pt multilayer samples. MOCD-SNOM imaging artifacts are presented and discussed.

Imaging of magnetic domains in thin Co/Pt and CoNi/Pt multilayers by near-field magneto-optical circular dichroism

A scanning near-field optical microscope (SNOM) was adapted to image magnetic domains by magneto-optical circular dichroism (MOCD), and was used to study magnetization reversal in sputtered Co/Pt and CoNi/Pt multilayers. Furthermore, local near-field MO hysteresis loops could be measured using the MOCD-SNOM technique. Artifacts due to structural defects and topography are discussed.