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Precession damping in [Co <sub/>60</sub> Fe <sub/>40</sub> /Pt] <sub/>5</sub> multilayers with varying magnetic homogeneity investigated with femtosecond laser pulses
摘要： We report on the ultrafast magnetization dynamics of [Co60Fe40/Pt]5 multilayers studied with femtosecond laser pulses. The samples were grown at room temperature by DC magnetron sputtering with Ta capping and Pt buffer layers and present the same thickness and perpendicular magnetic anisotropy as determined by vibrating sample magnetometry. Controlled growth rate of the Pt buffer layer modified the anisotropy fields and magnetic domain sizes as measured by magnetic force microscopy (MFM). An estimation of the average magnetic domain sizes was obtained from the profile of the self-correlation transform of the MFM images. For multilayers having an average magnetic domain size of 490 nm, we report a damped precession of the magnetization which decays with a time constant of ～100 ps and which has a frequency which varies from 8.4 GHz to 17.0 GHz as the external field increases from 192 mT to 398 mT. Fitting the precession dynamics with the Landau-Lifshitz-Gilbert equation we evaluated the damping α, which decreases from 0.18 to 0.05 with increasing magnetic domain sizes (127 nm to 490 nm). These α values are higher than for single layers suggesting an enhanced scattering and spin pumping effects from the Pt adjacent layers. In addition, the precession frequency increases from 2.04 GHz to 11.50 GHz as the anisotropy field of the multilayers increases from 6.5 kOe to 13.0 kOe. Finally, a comparative analysis between micromagnetic simulations and MFM images allowed us to determine the exchange stiffness (Aex) in the [Co60Fe40/Pt]5 multilayers.
关键词： exchange stiffness,femtosecond laser pulses,Landau-Lifshitz-Gilbert equation,[Co60Fe40/Pt]5 multilayers,damping,magnetic force microscopy,ultrafast magnetization dynamics,perpendicular magnetic anisotropy
更新于2025-09-12 10:27:22
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Influence of MBE growth parameters on GaSb/GaAs quantum dot morphology
摘要： In this paper, growth of self-assembled type-II GaSb/GaAs quantum dots by molecular beam epitaxy technique is presented. The effect of various growth parameters such as substrate temperature, Sb/Ga beam equivalent pressure ratio, growth rate and total mono-layer coverage on surface morphology is analyzed. The GaSb quantum dots exhibit high anisotropy along <110> and <1-10> directions under specific growth conditions. The anisotropy is discussed on the basis of incorporation of background Arsenic and anisotropic diffusion of Gallium adatoms on the GaSb surface. The low temperature photoluminescence measurements in GaAs capped samples show recombination in the type-II quantum dots and the wetting layer.
关键词： Self assembled process,Anisotropy,Molecular beam epitaxy,Type-II quantum dots,Stranski-Krastanov growth
更新于2025-09-12 10:27:22
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Effect of the electric field induced birefringence on the slab waveguide evanescent-wave sensor sensitivity
摘要： We have investigated the potential of using the E-field induced birefringence for improving the sensitivity of uniaxial anisotropic slab waveguide sensor based on evanescent wave interactions. LiNbO3 waveguide core was used as an example. We have calculated the sensor sensitivity formulas for the two kinds of modes propagating simultaneously in the waveguide sensor. In our study, we have distinguished between two different cases. The first case when the electric field is applied along the optic axis (+c) of the LiNbO3 wave guiding film (positive electric field); the second case when the electric field is applied opposite to the optic axis (?c) of such uniaxial crystal (negative electric field). The obtained results showed that, for positive electric field, increasing the electric field induces an increasing of the total anisotropy which causes decreasing on sensor sensitivity. However, for negative electric field, the increase of absolute values of negative electric field induces a decrease of the total anisotropy, the latter increases the sensor sensitivity. On the other hand, the study of the physical parameters on the sensor sensitivity showed that, to maximize the sensor sensitivity, it is advisable to use isotropic substrate that has a refractive index as closer as possible to the measurand index.
关键词： Evanescent wave,Uniaxial anisotropy,Planar waveguide sensor,Induced birefringence,Sensor sensitivity
更新于2025-09-12 10:27:22
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A Study of the Effect of Anisotropic Gold Nanoparticles on Plasmonic Coupling with a Photosensitizer for Antimicrobial Film.
摘要： Development of antimicrobial surfaces for sterilization is much needed to avoid the spreading of drug resistant bacteria. Light can activate antimicrobial surfaces by an interaction between nanoparticles and a photosensitizer dye, to produce a steady and efficient killing of bacteria. The film studied in this work contains gold nanorods (AuNRs) of 32 nm length and 16 nm diameter and gold nanostars (AuNSs) of 50 nm of diameter, in combination with crystal violet dye (CV). The surface plasmon resonance (SPR) of the nanoparticles used in the film was mathematically simulated and characterized to understand difference SPR between the particles. Their effects on plasmonic coupling with the dye, thus the production of reactive oxygen species (ROS) and consequently the activity of the film against bacteria were studied. The films showed great antimicrobial activity against gram negative bacteria (E. coli) in 4 h of light exposure, when modified with AuNSs, it could kill E. coli in 5 orders of magnitudes (5-log) and the one modified with AuNRs could kill with 4 order of magnitudes (4-log). While maintaining partial activity against gram positive bacteria (S. aureus), i.e. being able to kill in 2.5 orders of magnitudes by the film containing AuNSs and 3 orders of magnitudes by those containing AuNRs. The differential response of gram (-) and gram (+) bacteria to the ROS generated by the films, would allow more targeted approach for specific bacterial species, for example, surfaces of bedpans or common contact surfaces (handles, handrails etc.) that are contaminated principally by gram (-) or gram (+) bacteria, respectively.
关键词： anisotropy,light-activated,non-contact sterilization,antimicrobial surface,simulations,Plasmonic coupling
更新于2025-09-12 10:27:22
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Ultrafast coherent exciton dynamics in size-controlled perylene bisimide aggregates
摘要： For H-aggregates of perylene bisimide (PBI), it has been reported that upon photoexcitation, an initially delocalized Frenkel exciton is localized by excimer formation. However, in recent studies, the beforehand exciton dynamics preceding the excimer formation was suggested in larger aggregates consisting of at least more than 10-PBI subunits, which was not observed in small aggregates comprising less than four-PBI subunits. This feature implies that the size of molecular aggregates plays a crucial role in the initial exciton dynamics. In this regard, we have tried to unveil the initial exciton dynamics in PBI H-aggregates by tracking down the transient reorientations of electronic transition dipoles formed by interactions between the PBI subunits in systematically size-controlled PBI H-aggregates. The ultrafast coherent exciton dynamics depending on the molecular aggregate sizes can be distinguished using polarization-dependent femtosecond-transient absorption anisotropy spectroscopic measurements with a time resolution of ~40 fs. The ultrafast decay profiles of the anisotropy values are unaffected by vibrational relaxation and rotational diffusion processes; hence, the coherent exciton dynamics of the PBI H-aggregates prior to the excimer formation can be directly revealed as the energy migration processes along the PBI H-aggregates.
关键词： exciton dynamics,femtosecond-transient absorption anisotropy,perylene bisimide,coherent exciton dynamics,H-aggregates
更新于2025-09-12 10:27:22
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Isotropic silicon etch characteristics in a purely inductively coupled SF <sub/>6</sub> plasma
摘要： The characteristics of isotropic etching of silicon in a purely inductively coupled SF6 plasma are quantitatively studied. Since the etch results are strongly dependent on mask features, the authors investigated both large area and narrow trench etch characteristics. Circles of diameter 500 μm were used as a proxy for unpatterned surfaces and etched for different durations to establish the material etch rate and surface roughness. The average etch rate using the chosen recipe was found to be 2.27 μm/min. Arrays of narrow trenches ranging from 8 to 28 μm were also etched to analyze the effect of trench size on etch rate and degree of anisotropy. The etch rate of the trenches was found to strongly decrease with decreasing trench width. The results demonstrate that isotropic SF6 etch can be readily used as a replacement for more exotic silicon vapor phase etch chemistries such as XeF2.
关键词： etch rate,silicon,isotropic etching,SF6 plasma,inductively coupled plasma,anisotropy
更新于2025-09-12 10:27:22
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Magnetization reversal in NiFe <sub/>2</sub> O4/SrTiO <sub/>3</sub> nanoheterostructures grown by laser molecular beam epitaxy
摘要： NiFe2O4/SrTiO3(001) nanoheterostructures have been fabricated by laser molecular beam epitaxy method. Surface morphology and crystal structure of Ni-ferrite films were analysed by atomic force microscopy (AFM), reflection of high energy electron diffraction (RHEED), X-ray diffraction (XRD). X-ray methods prove the presence of inverse spinel crystal structure of films that was confirmed by measurements of spectral dependence of optical polar Kerr (PMOKE) effect. Study of magnetization reversal for different orientations of magnetic field carried out by vibration sample magnetometry (VSM) and longitudinal magneto-optical Kerr effect (LMOKE) are presented. In-plane magnetization loops exhibit 90 period indicating presence of biaxial magnetic anisotropy. Asymmetry of LMOKE hysteresis loops is related to manifestation of quadratic in magnetization effects in reflection of light.
关键词： nanoheterostructures,NiFe2O4/SrTiO3,biaxial magnetic anisotropy,laser molecular beam epitaxy,magnetization reversal
更新于2025-09-12 10:27:22
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[IEEE 2019 21st International Conference on Transparent Optical Networks (ICTON) - Angers, France (2019.7.9-2019.7.13)] 2019 21st International Conference on Transparent Optical Networks (ICTON) - Controlled Oxidation of III-V Semiconductors for Photonic Devices
摘要： The oxidation of III-V semiconductors is a crucial technological process in the mass-volume manufacturing of (singlemode) Vertical-Cavity Surface-Emitting Lasers or in the fabrication of more-recently-demonstrated integrated photonic devices. To facilitate the deployment of VCSELs in their up-rising markets but also to sustain the above-mentioned emerging uses, the shape and size of the optical/electrical apertures resulting from the selective lateral oxidation of buried layers of aluminum-containing III-V semiconductors needs to be controlled with an ever-increasing accuracy and reliability. In this paper, we will review the recent experimental investigations and model developments we have carried out to analyse and describe in detail this oxidation process. In particular, we will show that its degree of anisotropy depends on the oxidation process conditions and that the detrimental shape distortion induced by this anisotropy may be mitigated by a careful design of the etched mesas used to enable the oxidation of the buried layers.
关键词： oxidation,VCSEL,waveguides,AlOx,anisotropy
更新于2025-09-12 10:27:22
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Zero-Birefringence Silicon Waveguides Based on Tilted Subwavelength Metamaterials
摘要： Polarization independent silicon-on-insulator nanowires are highly sought after, due the inherent high birefringence of this material platform. State-of-the-art designs of non-birefringent waveguides include ridge waveguides and square nanowires, which either imply large dimensions, multiple etching steps, low fabrication tolerances or high wavelength dependence. In this work, we overcome all the aforementioned limitations through tilted subwavelength structures which provide anisotropy control of the resulting metamaterial. With a waveguide cross section of only 300 nm x 550 nm (height x width), the zero-birefringence point is obtained for an approximately 48?-tilt of the subwavelength structure. Birefringence of the nominal design deteriorates by only 9.10?3 even in the presence of size deviations of ± 10 nm. Moreover, birefringence is maintained under 6.10?3 in a 100-nm bandwidth around the central wavelength of 1550 nm. This innovative approach is readly adaptable to a wide range of waveguide sizes, while maintaining single-etch-step fabrication.
关键词： polarization independence,silicon-on-insulator (SOI),sub-wavelength grating (SWG),Zero-birefringence waveguides,anisotropy engineering
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Observation of Elastic Anisotropy in Strained Optical Nanofibers using Brillouin Spectroscopy
摘要： Observation of elastic anisotropy in strained optical nanofibers using Brillouin spectroscopy. Optical nanofibers (ONFs) are excellent nanophotonic platforms for many applications such as optical sensing, quantum photonics, and nonlinear optics, due to both tight optical confinement and their evanescent field. From an acoustic viewpoint, it has recently been reported the observation of a new class of Brillouin acoustic resonances in optical nanofibers, including hybrid shear/longitudinal acoustic waves (HAWs) and surface acoustic waves (SAWs) [1-2]. It has been later shown that, under axial tensile strain, the Brillouin frequency shifts (BFS) of these elastic resonances are fundamentally different from that of standard optical fibers [3]. This is principally due to the hybrid nature of acoustic waves and thus classical theory used for standard fibers can no longer be used [4]. Here, we develop a theoretical model based on third-order elasticity of silica to predict the strain dependence of acoustic waves in ONFs. We show in particular that the fundamental elastic properties of silica dramatically change due to elastic anisotropy and transverse hardening. The agreement with experimental results is excellent.
关键词： acoustic waves,Brillouin spectroscopy,optical nanofibers,tensile strain,elastic anisotropy
更新于2025-09-11 14:15:04