Magneto-optic Modulator Research Articles

Rotational dynamics of colloidal hexaferrite nanoplates

Here, we report an experimental study on the rotational dynamics of hard magnetic hexaferrite nanoparticles in water. A stable aqueous colloid of SrFe12O19 was synthesized by the borate glass-ceramic dissolution technique and studied by TEM, small-angle X-ray scattering (SAXS), magnetometry, and optical transmission methods in applied DC and AC magnetic fields. The particles represent nanoplates with a mean diameter of 50 nm and a mean thickness of 5 nm having a coercive force of 4700 Oe and a saturation magnetization of 51.5 emu/g. According to magnetic field dependent SAXS data, a probability orientation function was suggested considering the colloidal particle rotation in the applied field as an activation-free process. The magnetization dynamics of the colloidal rotators was described by an interplay of magnetic torque and drag force in the frame of a non-interacting particle model. At frequencies below 100 Hz, the particles are able to fully rotate after the magnetic field. At higher frequencies, the complete following becomes impossible due to the energy dissipation and the particle movement changes to oscillations around randomly oriented axes. These vibrational axes can be aligned by a strong external permanent magnetic field, causing the coherent particle oscillations and correspondingly the rise of the high-frequency response of the colloid optical transmission. As a result, the efficient magneto-optical light modulation has been achieved at frequencies exceeding 5 kHz, revealing the fastest response rates among known colloidal magneto-optical media.

Structural Features of Mineral Crystalline Phases and Defectiveness of Bismuth Organosiliconate Crystals at High Temperatures

Structural features of mineral crystal phases and defectiveness of bismuth organosiliconate crystals are inspected at various temperatures of treatment (from 100 to 500°C). X-ray diffraction enables evaluation of crystal lattice periods from spectrograms recorded in СuKα radiation. The approximation analysis of the broadening of the most intense diffraction lines in crystals from (hkl) crystallographic indices allows one to determine the coherent domain sizes and microdistortions Δа/а of a crystal lattice. It is found that exposure of the Na2O–Bi2O3–SiO2 system (NBS material) to temperatures of 300–500°C leads to a decrease in amorphism, microdistortions, and the density of dislocations in a crystal lattice of Bi12SiO20 sillenite. The formation of a denser structure of sillenite crystal with higher X-ray density (9.210 g/cm3) and greater cubic crystal lattice parameter (a = 10.1335 A) is detected. The presented Bi12SiO20 material can be used as a gamma and protective filler of radiation protective polymers and in the design of electro-and magneto-optical laser radiation modulators.

Current-induced domain-wall motion in patterned nanowires with various Gd–Fe compositions for magneto-optical light modulator applications

We have fabricated nanowires with various compositions of Gdx–Fe1−x and evaluated current-induced domain-wall (DW) motion for magneto-optical light modulator application. Successful DW expansion and shrinkage according to the direction of current injection using two nanomagnets aligned in an anti-parallel configuration. The fastest DW velocity was obtained with Gd22.7Fe77.3, which is slightly Fe richer than the magnetization compensation composition. This composition that gave the highest DW velocity was attributed to the angular momentum compensation composition in Gd–Fe, which is slightly Fe richer than the compensation composition.

Measurement of Birefringence Using the Magneto-Optical Modulator in the Magnetomechanical Resonance Mode

In order to detect magnetic linear birefringence of new materials, we have developed an installation using the magneto-optical modulator in the mode of magnetomechanical resonance. We have theoretically substantiated and experimentally applied radiation with phase-frequency dependencies of the Stokes vector on magnetomechanical resonance frequency for measuring magnetic linear birefringence. Our results demonstrate that the radiation in which the variable components of the Stokes vector have different initial phases after passing through the sample with linear birefringence leads to additional phase delays of the signal at the output of the photopolarimeter. As an example, the results of measurements of the magnetic linear birefringence in the film of yttrium iron garnet have been introduced. The minimum value of the linear birefringence which was measured in this experiment 0.018 deg. The advantage of the installation is the independence of the measurements results from the changes of the intensity in the optical system.

Twofold unidirectional properties of parity-time symmetry structure under magneto-optical modulation

Twofold unidirectional properties of parity-time symmetry structure under magneto-optical modulation

Study on the spatial roll angle measurement based on magneto-optic modulation

Faraday rotation angle formula for polarized light propagating in magneto-optic medium is investigated theoretically. Based on the Biot-Savart Law and Maxwell's equations, a calculation model for electromagnetic field in solenoid paraxial region is deduced. Taking advantage of this model, together with the Bessel functions, the axial magnetic field in a typical medium is calculated and analyzed. In addition, to thoroughly analyze the influence mechanisms of the incident wavelength and temperature on the Verdet constant and then uncover its essence, a hypothesis which suggests the Faraday effect results from the combination of various factors is proposed. Furthermore, a theory of wave-transition contribution to the Verdet constant is deduced by adopting the theory of wave-particle duality. Taking the typical diamagnetic material ZF1 and paramagnetic material TGG as examples, the deduced theory together with the corresponding models is tested and verified by analyzing the relevant parameters and the test data. At last, a testing system for magneto-optical rotation together with its finite element model is introduced.

Analysis and research of polarization aberration in rapid space angle measuring system

The precise angle measurement and transmission technology have been widely used in the precision measurement, aerospace, military, biomedicine and other devices, which are based on the polarized light and magneto-optical modulation. This method has the characteristics of no rigid connection required, long distance transmission, high precision, etc. However, the azimuth information measurement method needs the assistance of complex servo tracking system according to the orthogonal extinction principle of polarization prism, meanwhile, the measurement time is longer, which reduces reliability and reaction sensitivity of the system. In order to improve the measurement accuracy and fast response capability of the system, a fast space goniometry method is proposed through the Wollaston prism polarizing beam splitter, with which the azimuth is directly calculated according to the two light intensities. The measurement time can be shortened, and the accuracy is improved by the use of magneto-optical modulation technology. The rapid space angle measuring system needs to realize the measurement function in a certain translation range, which requires the beam to have a certain coverage area in the receiving unit. However, the system is limited by size and volume of the device; we can only choose to expand the incident beam. Therefore, when the beam is incident onto the receiving unit, some incident angle and azimuth, that is, non-vertical incidence will be produced. However, the polarization of the non-vertically incident light passing through the system will change and polarization aberration exists, which will lead to measurement error. In this paper, the beam passes through the polarizing prism in a certain range of azimuths and incident angles, and the polarized light tracing method and the boundary condition of the electromagnetic field are used to study and simulate the polarization change and distribution of the outgoing beam. The changes of different incident azimuths and angles can be simulated through the translation of receiving unit, and the azimuths can be measured indirectly by using self-collimation theodolite and right angle prism. By comparing the measured azimuths under the translational and centering conditions, the influence of polarization aberration on the angle measuring system and the correctness of the theoretical analysis are verified. It is concluded that when the azimuth angle is 0, the measurement error is small; when the azimuth is 90, the measurement error is largest, meanwhile the measurement error will increase with the translation distance becoming longer (i.e., the incident angle). According to the comparison between the experimental data and the simulation results, the existing problems are pointed out, and the corresponding improvement measures are proposed. The results of this work have some significance in guiding the optimization of the system structure, and the further improvement inthe performance of the system.

Ultrafast polarization modulation of laser pulses at terahertz frequencies via optical Kerr effect

Polarized light has already been widely used for photography and display technologies. Magneto-optical Faraday effect, i.e., the light polarization rotates in the magnetic field applied to the material in the direction of light propagation, plays a crucial role in the interaction between light and spin. Faraday effect allow us to understand the nature of magnetization in condensed materials. As an effect opposite to the Faraday effect, the magnetization can be induced in a transparent medium exposed to a circularly polarized electromagnetic wave, which is called inverse Faraday effect. Knowledge of the mechanism provides the opportunities of modulation devices in photonics, ultrafast opto-magnetism and magnonics. In this paper, we experimentally demonstrate a proof-of-concept ultrafast polarization modulation by employing circularly polarized light to demonstrate a strengthened terahertz (THz) frequency Kerr modulation signal, at room temperature. By using the transient pumpprobe spectroscopy with the reflected geometry, we are able to demonstrate the feasibility of such an ultrafast magneto-optical polarization modulation at 0.19 THz in a paramagnetic Li:NaTb (WO<sub>4</sub>)<sub>2</sub> crystal with a thickness of 3 mm. The time-resolved modulation signal is explained by the interaction between two counter-propagating laser pulses (central photon energy of 1.55 eV) within the crystal via the optical Kerr effect. We find that the amplitude of the modulation increases with the pump fluence increasing, while the modulation frequency is dependent neither on the pump fluence nor on polarization of pump beam. However, it can further be found that the phase and amplitude of the transient Kerr modulation are strongly dependent on the helicity of the circularly polarized pump pulses. Indeed, these oscillating signals may be mistaken for spin excitation modes. The present findings allow us to get an insight into the transient magneto-optical dynamical process in transparent medium. In addition, the polarization modulation of ultrashort laser pulse on a picosecond time scale will facilitate all-optical data processing, as well as the polarization-dependent ultrafast dynamics in various material systems, which span from condensed matter to molecular spectroscopy. In this regard, our experimental results provide a possibility for designing novel all-optical (magneto-optical) modulators operating at THz clock frequencies. The magneto-optical polarization response modulated at THz frequencies may have new possibilities for designing all-optical devices, such as ultrafast modulators.

Optical properties of magnetic photonic crystals with an arbitrary magnetization orientation

We have studied the peculiarities of diffraction of light in magnetic photonic crystals at large values of magnetooptical activity parameter and modulation depth. We have considered the case of an arbitrary angle between the directions of the external static magnetic field and the normal to the layer. The problem has been solved by the modified Ambartsumyan layer summation method. It has been shown that the given system is nonreciprocal with respect to not only circular, but linear polarizations also. In this case, a new type of nonreciprocity is observed (namely, the relation R(α) ≠ R(–α) holds, where R is the reflection coefficient and α is the angle of incidence). It has been demonstrated that in the case of oblique incidence, there appears a new photonic forbidden band that is not selective relative to the polarization of incident light. We have detected strong dependences of reflectance, absorbance, transmittance nonreciprocity, and other characteristics on the angle between the direction of the external static magnetic field and the normal to the layer boundary. Such a system can be used as a controllable polarization filter and a mirror, as well as a source of circular (elliptic) polarization, a controllable optical diode, and so on.

Polarization research of YIG based two-dimensional Magneto Photonic Crystals

In this paper, the axial propagation of light in two-dimensional YIG based MPCs (Magnetic Photonic Crystals) was simulated by three dimensional FDTD (Finite Difference Time Domain) algorithm with Matlab and Rsoft tools. The effects of structure, filling rate of lattice, wavelength of incident light and the external magnetic field on polarization of the output light were discussed. According to the simulation, a specific structure with 25% filling rate, 1.2 μm wavelength and 2um lattice constant in the absence of defects in MPCs was validated as an ideal model to obtain a good magneto-optical response. When a central defect existed, the Faraday rotation angle increased and the ellipticity of polarization was suppressed to a certain extent for the broken periodicity of the original structure. The defect model can reach a rotation angle of 9.4 degrees, which is larger than the angle of 8.3 degrees of non-defect model when transmitting a distance of 0.5 μm under the condition of the same incident light wavelength of 1.2 μm and filling rate of 25%. A large Faraday rotation in short propagating distance was obtained when a certain polarization degree was maintained, which can be used in magneto-optical modulation devices.

Band structure peculiarities of magnetic photonic crystals

In this work we studied light diffraction in magneto-photonic crystals (MPC) having large magneto-optical activity and modulation large depth. The case of arbitrary angles between the direction of the external static magnetic field and the normal to the border of the MPC layer is considered. The problem is solved by Ambartsumian’s modified layer addition method. It is found that there is a new type of non-reciprocity, namely, the relation R(α)≠R(-α) takes place, where R is the reflection coefficient, and α is the incidence angle. It is shown the formation of new photonic band gap (PBG) at oblique incidence of light, which is not selective for the polarization of the incident light, in the case when the external magnetic field is directed along the medium axis. Such a system can be used as: a tunable polarization filter, polarization mirror, circular (elliptical) polarizer, tunable optical diode, etc.

Magnetization precession induced by picosecond acoustic pulses in a freestanding film acting as an acoustic cavity

We report the magnetization dynamics excited by picosecond acoustic pulses confined in the acoustic cavity of freestanding nickel films. By detecting both sides of the sample we show that acoustic pulses moving back and forth inside the cavity efficiently control the magnetization precession. The multiple round trips of acoustic pulses, which survive for 12 bounces, enhance the amplitude of the magnetization precession by a factor of 4.5. Even though the conditions of total reflection are met in the freestanding film, the acoustic pulses have a large attenuation as compared to similar Ni films on $\mathrm{Si}{\mathrm{O}}_{2}$ substrate, for which we speculate that it is related to a stress induced by the substrate. Our results constitute a guideline for simple designs of acoustic cavities and for achieving a very large amplification of the magnetization precession angle. It is potentially useful for high-frequency magneto-optical modulators.

Magneto-optic modulator for high bandwidth cavity length stabilization.

We propose a novel magneto-optical approach for the repetition frequency stabilization of optical frequency combs. We developed a Yb:fiber mode-locked laser with a fiber-based magneto-optic modulator used to stabilize one of the longitudinal modes to an optical reference with sub-hundred mrad residual phase noise. This modulator does not induce mechanical resonances and as such has the potential to achieve much broader feedback bandwidths than conventional modulators used for cavity length stabilization.

The application rule of multi-frequency sine wave compound modulation in measuring spatial roll angle

The purpose of the study is to summarize the application rule of multi-frequency sine wave compound magneto-optical modulation in measuring roll angle. Firstly, we establish the model of compound modulation signal composed of fundamental and multi-frequency sine wave signals, and then the modulated signal is analyzed with Bessel function. Secondly, we analyze the characteristic of AC signal extracted from the modulated signal, and discuss the feasibility of establishing angle measurement model with extremum in AC signal. Finally, we draw the conclusion that study on the fundamental sine wave signal superimposed on itself compound modulation in roll angle measurement is enough, the others is unnecessary. The study result has an important guiding significance on the application of multi-frequency sine wave compound magneto-optical modulation in measuring roll angle.

Colossal magneto-optical modulation at terahertz frequencies by counterpropagating femtosecond laser pulses in Tb3Ga5O12.

Single-frequency terahertz modulation of the magneto-optical Faraday effect with a record amplitude of the polarization rotation of ∼0.5° is achieved using a slab of the etalon Faraday rotator crystal Tb3Ga5O12. The modulation is the result of the interaction of two counterpropagating laser pulses via the optical Kerr effect. The frequency of the modulation is determined by the applied magnetic field and is continuously tunable in a terahertz frequency range between 0 and 0.7THz.

Transverse magnetic field impact on waveguide modes of photonic crystals.

This Letter presents a theoretical and experimental study of waveguide modes of one-dimensional magneto-photonic crystals magnetized in the in-plane direction. It is shown that the propagation constants of the TM waveguide modes are sensitive to the transverse magnetization and the spectrum of the transverse magneto-optical Kerr effect has resonant features at mode excitation frequencies. Two types of structures are considered: a non-magnetic photonic crystal with an additional magnetic layer on top and a magneto-photonic crystal with a magnetic layer within each period. We found that the magneto-optical non-reciprocity effect is greater in the first case: it has a magnitude of δ∼10-4, while the second structure type demonstrates δ∼10-5 only, due to the higher asymmetry of the claddings of the magnetic layer. Experimental observations show resonant features in the optical and magneto-optical Kerr effect spectra. The measured dispersion properties are in good agreement with the theoretical predictions. An amplitude of light intensity modulation of up to 2.5% was observed for waveguide mode excitation within the magnetic top layer of the non-magnetic photonic crystal structure. The presented theoretical approach may be utilized for the design of magneto-optical sensors and modulators requiring pre-determined spectral features.

Composition-tuned magneto-optical Kerr effect in L10-MnxGa films with giant perpendicular anisotropy

We report the large polar magnetooptical Kerr effect in L10-MnxGa (0.76 ⩽ x ⩽ 1.29) epitaxial films with giant perpendicular magnetic anisotropy. The Kerr rotation is enhanced by a factor of up to 2.5 by decreasing Mn atomic concentration, which most likely arises from the variation of the effective spin–orbit coupling strength, compensation effect of magnetic moments at different Mn atom sites, and overall strain. A significant tuning effect of composition is also observed on Kerr ellipticity and complex Kerr angle (including the magnitude and phase angle). The good epitaxial compatibility with semiconductors, moderate coercivity of 4.6–9.7 kOe, large Kerr rotation of up to 0.10°, high reflectivity of 35%–55% in a wide wavelength range of 400 ~ 850 nm, and giant magnetic anisotropic field of up to 140 kOe together make these L10-MnxGa films promising for scientific and technological applications in spintronics and terahertz-frequency magnetooptical modulators.

Optical modulators with 2D layered materials

Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that two-dimensional layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this review, we cover the state-of-the-art of optical modulators based on two-dimensional layered materials including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as two-dimensional heterostructures, plasmonic structures, and silicon/fibre integrated structures. We also take a look at future perspectives and discuss the potential of yet relatively unexplored mechanisms such as magneto-optic and acousto-optic modulation.

Refractive index sensor based on magnetoplasmonic crystals

A magneto-optical surface plasmon resonance (MOSPR) sensor based on a magnetoplasmonic crystal trilayer structure is presented. The sensitivity of the MOSPR sensor is studied as a function of ferromagnetic layer thickness and at the different modes of operation. The enhancement of the sensitivity caused by using the MOSPR sensor in magneto-optical modulation regime in comparison with reflection regime is observed.

Nonlinear Surface Magnetoplasmonics in Kretschmann Multilayers

The nonlinear magneto-plasmonics aims to utilize plasmonic excitations to control the mechanisms and taylor the efficiencies of the non-linear light frequency conversion at the nanoscale. We investigate the mechanisms of magnetic second harmonic generation in hybrid gold-cobalt-silver multilayer structures, which support propagating surface plasmon polaritons at both fundamental and second harmonic frequencies. Using magneto-optical spectroscopy in Kretschmann geometry, we show that the huge magneto-optical modulation of the second harmonic intensity is dominated by the excitation of surface plasmon polaritons at the second harmonic frequency, as shown by tuning the optical wavelength over the spectral region of strong plasmonic dispersion. Our proof-of-principle experiment highlights bright prospects of nonlinear magneto-plasmonics and contributes to the general understanding of the nonlinear optics of magnetic surfaces and interfaces.