S. Mito

Fabrication and characteristics of one-dimensional magnetophotonic crystals for magneto-optic spatial light phase modulators

We prepared the reflection type magnetophotonic crystals (R-MPC) to enhance the magneto-optic property of magneto-optic spatial light phase modulator (MOSLM). Magnetophotonic crystals have a localized wavelength by using a defect layer in which the magneto-optic property can be increased tremendously. By the calculation, it was revealed clearly that Kerr rotation can be enhanced with increasing the repetition number (k) which is the number of dielectric layer. In this study, Dy1.0Bi1.5Y1.0Fe3.8Al1.2O12 (DyBi:YIG) garnet material, of which Faraday rotation angle is about −3 deg/μm, was utilized as a defect layer. At the designed wavelength of 532 nm and k=5, SGGG/[SiO2/Ta2O5]2/DyBi:YIG/[SiO2/Ta2O5]k structure fabricated with garnet thickness of 347.6 nm showed Kerr rotation angle of 26° and reflectivity of 0.2% which is coincident considerably with the results (θk=30° and R=0.59%) calculated by the matrix approach method. The fabricated R-MPC-MOSLM structure exhibited the magneto-optic property of about 25...

Study on magnetophotonic crystals for use in reflection-type magneto-optical spatial light modulators

A magneto-optical spatial light modulator (MOSLM) is a real-time programmable device for modulating the amplitude and/or phase of two-dimensional optical signals at a high speed. Recently, we reported the development of the transmission-type MOSLM (T-MOSLM) based on a one-dimensional magnetophotonic crystal (MPC) with a high driving performance. However, further improvement of the T-MOSLM is required. The structure of the T-MOSLM has low optical efficiency due to weakening of transmittance through electrodes; furthermore, the fabrication of the driving electrodes is a very complicated process. In this study, we suggest a new design of the MPC for a reflection-type MOSLM (R-MOSLM). The idea of a reflection-type MPC (R-MPC) is to replace one of the Bragg mirrors with an aluminum reflection film. The structure is glass∕(Ta2O5∕SiO2)k∕Bi:YIG∕Al, where k is the repetition number. The approach of the optimal structure of the R-MPC was done by the calculation of the matrix approach method. In the calculation of t...

Magnetic property of polycrystalline magnetic garnet for voltage driven type magneto-optic spatial light phase modulator

In this study, we investigated the magnetization response of the polycrystalline magnetic garnet film for stress. The calculation suggests that the balance of the magnetic anisotropic energy under magnetic field is an essential factor to modulate the magnetization by stress. We fabricated the magnetophotonic crystal based voltage driven type magneto-optic spatial light modulator, and measured the magneto-optic response for stress. The experimental results had a good agreement with our calculation. In our results, the magnitude and linearity of the modulation by stress was depending on static bias field, stress, and magnetostriction constant. The obtained results implied that the calculation could estimate the device performance by deciding material composition and prepare stress inside garnet.

Magnetophotonic Crystals: Experimental Realization and Applications

The most striking feature of photonic crystals, compared with homogeneous optical materials, is the existence of photonic band gaps. The band gaps are responsible for resonant light coupling to constituents of photonic crystals in both the cases where periodicity is ideal or broken by defects introduced intentionally. What if photonic crystals are made of magnetic materials? May magnetism bring about new advances in the field of photonic crystals where not only amplitudes but also polarization states are controlled by the spin subsystem? Below we will discuss magnetophotonic crystals and show that light confinement in their non-reciprocal magnetic constituents results in new magneto-optical phenomena. This chapter reviews studies on magnetophotonic crystals with various designs; it focuses on their experimental realizations, theoretical analysis and application to spatial light modulators.

Polycrystalline magnetic garnet films comprising weakly coupled crystallites for piezoelectrically-driven magneto-optic spatial light modulators

We have investigated the magnetization process of the polycrystalline magnetic garnet films in order to determine the most suitable composition of garnet films for piezoelectrically-driven magneto-optic spatial light modulators (MOSLMs). For experiment, the bismuth–dysprosium–aluminum-substituted yttrium iron (Bi1.3Dy0.7Y1.0Fe3.1Al1.9O12) garnet films were deposited by an RF magnetron sputter and annealed at 700 °C in air. The annealing time was varied in a range of several minutes to control the grain size. The saturation magnetization, the remanent magnetization and the composition of the fabricated garnet films slightly changed versus the annealing time. Experiments showed that the coercivity and the grain size increased at longer annealing; the coercivity was larger for films with bigger grains. This work shows that garnet films with smaller coercivity are most suitable for controlling the magnetization of garnet and, correspondingly, the magneto-optical rotation of MOSLM pixels driven by piezoelectrics.

Enhancement of magnetic circular dichroism in bi-layered ZnO-Bi:YIG thin films

Bi-layered zinc oxide (ZnO) and bismuth substituted yttrium iron garnet (Bi:YIG) was fabricated and magneto-optically investigated. Enhancement of Faraday rotation and magnetic circular dichroism (MCD) was observed. The wavelength of MCD enhancement was in good agreement with exciton wavelength of ZnO. This enhancement was only observed in the bi-layer, and implies that the exciton generated in ZnO interacted with Bi:YIG. Because the exciton wavelength of ZnO can be controlled by electro-optic effect, this result has the potential for realizing voltage control of magneto-optic effect.

Spatial light phase modulators with one-dimensional magnetophotonic crystals driven by piezoelectric films

We demonstrate a novel spatial light phase modulator composed of a one-dimensional magnetophotonic crystal driven by a piezoelectric film. With the application of a constant magnetic bias field perpendicular to the film plane, the magneto-optic Kerr rotation angle of the modulator was subject to continuous change from 3.8–2.4° with the voltage up to 50 V and hence a continuous phase modulation for rays with circular polarization. Surprisingly, the magneto-optical response of the modulator showed nonvolatile behavior and the light phase was maintained at a designated value without applying any stress. Based upon the above experimental results, magneto-optic spatial light phase modulators with a 64 × 64 array of magnetic pixels were constructed. The fabricated device had a dimension of 15 μm2 and demonstrated two-dimensional optical modulation of the phase of reflected rays.

Multiferroic behavior of disordered bismuth-substituted zinc ferrite

In this study, we reported the multiferroic behavior of disordered bismuth-substituted zinc ferrite. Bi:ZnFe2O4 films were deposited by using the RF-magnetron sputtering method. As-deposited films exhibited a ferromagnetic hysteresis and a large Faraday rotation angle in the short wavelength range (∼400 nm): a saturation magnetization of 120 G and the Faraday rotation angle of −1.2 deg/μm were observed. Ferroelectric hysteresis was measured using the Sawyer-Tower method. Dielectric constant of Bi:ZnFe2O4 was measured with a LCR meter in a frequency range from 120 Hz to 100 kHz. For 120 Hz, the dielectric constant of 800 was observed at room temperature. By contrast, the dielectric constant of disordered zinc ferrite was 5.5 at 120 Hz. Our studies showed that bismuth substitution in ZnFe2O4 enhanced the dielectric constant drastically. Furthermore, in the films, voltage-induced alteration of the magneto-optic Kerr rotation angle was identified.

Stress induced modulation of magnetic domain diffraction of single crystalline yttrium iron garnet

Stress induced modulation of the diffraction angle and efficiency of the light reflected from a stripe-domain magnetic garnet was demonstrated. The spacing of the magnetic domain was changed using the inverse magnetostriction effect. The sample structure was a piezo actuator/Al reflection layer/magnetic garnet substrate. A diffraction angle between the 0th and 1st ordered light was changed from 9.12 deg. to 10.20 deg. This result indicates that the domain spacing was changed from 3.3 μm to 3.0 μm. The change of the diffraction angle was irreversible for the voltage. However, reversible, linear and continuous change of the diffraction efficiency was observed. These results could be applicable for a voltage-driven optical solid state light deflector with low power consumption and high switching speed.

Electroless plated maghemite for three-dimensional magneto photonic crystals

Three-dimensional magneto photonic crystals (3D-MPCs) are promising material for manipulating light in 3D space. In this study, we fabricated 3D-MPC that is filling the air-gap of opal photonic crystal with magnetic material by electroless plating. The electroless plating is an attractive film-forming method which provides magnetic material films on various substrates in aqueous solution at 24-90 °C. As magnetic material for filling the air-gap, maghemite (γ-Fe2O3) film was plated in opal photonic crystal. The plated maghemite film showed a Faraday rotation of 0.6 deg./μm at 440 nm and significantly lower absorption than magnetite. The plated opal showed photonic band gap and magneto-optic response. Faraday rotation of the plated opal was enhanced at the band edge. The photonic band gap and the Faraday rotation spectra were changed as a function of incident angle of light. Electroless plating of maghemite could be promising technique for fabricating 3D-MPCs.