用一维光子带隙结构增强硫化镉双光子吸收研究

用真空镀膜方法制备了含有单个CdS缺陷层的具有不同周期和结构参量的TiO2／SiO2一维光子晶体。用抽运一探测技术研究了CdS缺陷层的双光子吸收（TPA）现象。实验结果表明：一维光子晶体中CdS缺陷层的双光子吸收显著增强。不同周期和结构参量的一维光子晶体中CdS缺陷层的双光子吸收系数不同。双光子吸收的增强来源于由光局域化导致的缺陷层的电场强度的增加。缺陷层电场强度与一维光子晶体的结构有关,如周期,光子带隙的位置与宽度及缺陷模式等因素都会影响缺陷层电场强度。采用四分之一波长的高低折射率介质层和与入射波长匹配的缺陷模可以得到最大的缺陷层电场强度。     

Magnetorefractive effect in magnetic nanocomposites

The magnetorefractive effect in ferromagnetic metal-insulator granular nanostructures (CoFeZr)-SiO_n, Co-Al-O, FeSiO_n, and (CoFe)-(Mg-F) is investigated in the infrared spectral region in a wavelength range from 5 to 20 μm. The magnitude of the effect varies from 0.1 to 1.5% for different nanocomposites and strongly depends on the frequency of light and magnetoresistance. It is shown that the reflection coefficient changes in a magnetic field not only due to the magnetorefractive effect, but also due to the even magnetooptical effect. Simple relations describing this effect are given for the case when the reflection from the substrate is insignificant and in the case of a three-layer (insulator-film-substrate) system. The expression for the frequency dependence of the magnetorefractive effect in nanocomposites is derived and its features in the case of high-frequency spin-dependent tunneling are analyzed.     

Giant magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films

Complex experimental investigations of the structural, optical, and magneto-optical properties (magnetotransmission, magnetoreflection, and transversal Kerr effect, as well as the magnetoresistance, of La_0.7Ca_0.3MnO₃ epitaxial films indicate that magnetoreflection and magnetotransmission in manganite films can reach giant values and depend strongly on the magnetic and charge homogeneity of the films, their thickness, and spectral range under investigation. It has been shown that the optical enhancement of the magnetorefractive effect occurs in thin films as compared to manganite crystals. In the region of the minimum of the reflectance near the first phonon band, the resonance-like magnetorefractive effect has been observed, which is accompanied by change of the sign of the magnetoreflection. A model based on the theory of the magnetorefractive effect has been proposed to qualitatively explain this behavior.     

Magnetorefractive effect and giant magnetoresistance in Fe(<Emphasis Type="Italic">t</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)/Cr superlattices

The magnetorefractive effect in Fe(t _x , Å)/Cr(10 Å) samples grown by molecular-beam epitaxy with a variable thickness of the iron layer (superlattices, cluster-layered nanostructures) has been studied in the IR region (λ = 2–13 µm) in s and p polarizations of light. The magnetoresistive effect in a dc magnetic field, H ≤ 32 kOe, has been measured on the same samples. The iron layer thickness required for the magnetorefractive response to appear has been found to be t _Fe ≥ 3 Å. The correlation between the magnitude of the magnetorefractive effect in the mid-IR region and magnetoresistance has been discussed.     

Magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La_0.7Ca_0.3MnO₃ crystals have been complexly investigated. The measurements have been performed in wide temperature and spectral ranges in magnetic fields up to 3.5 kOe. It has been found that magnetic reflection is a high-frequency response in the infrared spectral range to the colossal magnetore-sistance near the Curie temperature. Correlation between the field and temperature dependences of the magnetic reflection and colossal magnetoresistance has been revealed. The previously developed theory of the magnetorefractive effect for metallic systems makes it possible to explain the experimental data at the qualitative level. Both demerits of the theory of the magnetorefractive effect in application to the magnets and possible additional mechanisms responsible for the magnetic reflection are discussed.     

Magnetoreflection and Magnetostriction in Ferrimagnetic Spinels CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>

It is shown that magnetoreflectance of natural light up to +4% exists in magnetostrictive ferrimagnetic spinel CoFe₂O₄ single crystal; this effect is associated with a change of the fundamental absorption edge, the impurity absorption band, and the phonon spectrum under the action of a magnetic field. The correlation between the field dependences of magnetoreflectance and magnetostriction has been established. The physical mechanisms responsible for the spectral and field peculiarities of magnetoreflection have been explained. It is shown that the magnetorefractive effect in CoFe₂O₄, which is associated with magnetoelastic properties of the spinel, amounts to +1.5 × 10^–3 in magnetic fields exceeding the saturation field. Analysis of magnetooptical and magnetoelastic data has made it possible to estimate deformation potential as Ξ _u = 20 eV for the valence band of the spinel.     

Angular tuning of defect modes spectrum in the one-dimensional photonic crystal with liquid-crystal layer

A one-dimensional ZrO₂/SiO₂ photonic crystal with a 4-n -pentyl-4' -cyanobiphenyl (5CB) nematic defect layer was used to investigate the transmission spectra of light polarized parallel and perpendicular to the liquid-crystal director at different angles of incidence. The spectra of the photonic crystal were shown to split into four polarized components T_ij at oblique incidence. When the incident angle increased, the bandgap edges and the defect modes shifted towards short wavelengths, while the amplitudes of the defect modes increased for the transverse magnetic polarization and decreased for the transverse electric polarization. The observed discrepancy between the defect mode amplitudes in the center and near the edges of the photonic bandgap was found to be related to the radiation losses inside the defect layer of a non-ideal photonic crystal. The simulated transmission spectra obtained using recurrence relations and taking into account the decay of defect modes are in good agreement with the experimental data.     

Properties of the defect mode in one-dimensional ferroelectric Si/C60 photonic crystals

In the present paper, a novel photonic crystal (PC) defect mode is designed by inserting a ferroelectric material layer (LiNbO₃) into Si/C₆₀ one-dimensional PCs. The band structure of the ferroelectric PCs is numerically analyzed by the transfer matrix method (TMM). The width of the photonic band gap increases by 80 nm and a defect mode appears at a central wavelength of 680 nm when a 150 nm LiNbO₃ layer is inserted into the Si/C₆₀ PC structure. The defect mode in the band gap shifts linearly with the change in electric field. The defect mode shifts by 11.2 nm toward shorter wavelengths when the thin film is subjected to a DC voltage of 1 KV.     

Field distribution of a light wave near a magnetic defect in one-dimensional photonic crystals

The field distribution of a light wave near a magnetic defect in a one-dimensional photonic crystal is analyzed. It is shown that, by properly varying the magnetic defect thickness or the parameters of the photonic crystal surrounding the defect, one can create a situation where the electric field of a light wave will be localized predominantly inside the magnetic layer or, conversely, in the immediate vicinity of the layer surface. This opens up possibilities for optimizing the Q factor of a magnetic microcavity in the presence of dissipation in the magnetic layer and, hence, for enhancing the linear and nonlinear magneto-optical effects. The possibility of separating the contributions from the surface and volume of the magnetic material to the nonlinear magnetooptical properties by properly varying the field distribution over the defect thickness is discussed.     

Magnetorefractive effect in manganites with a colossal magnetoresistance in the visible spectral region

The magnetotransmission, magnetoreflection, and magnetoresistance of the La_0.7Ca_0.3MnO₃ and La_0.9Ag_0.1MnO₃ epitaxial films have been investigated. It has been found that the films exhibit a significant magnetorefractive effect in the case of reflection and transmission of light in the fundamental absorption region both in the vicinity of the Curie temperature and at low temperatures. It has been shown that the magnetorefractive effect in the infrared spectral region of the manganites is determined by a high-frequency response to magnetoresistance, whereas the magnetorefractive effect in the visible spectral region of these materials is associated with a change in the electronic structure in response to a magnetic field, which, in turn, leads to a change in the electron density of states, the probability of interband optical transitions, and the shift of light absorption bands. The obtained values of the magnetotransmittance and magnetoreflectance in the visible spectral region are less than those observed in the infrared region of the spectrum, but they are several times greater than the linear magneto-optical effects. As a result, the magnetorefractive effect, which is a nongyrotropic phenomenon, makes it possible to avoid the use of light analyzers and polarizers in optical circuits.     

一维缺陷光子晶体温度的测量

采用Si和SiO₂两种介质材料构造一维缺陷光子晶体,缺陷层介质为Si,利用传输矩阵法对带有缺陷的一维光子晶体的传光特性进行了理论分析,并得到其带隙特性.由于缺陷的存在,使得光子晶体的透射谱中产生缺陷峰.当被测温度变化时,根据两种介质的热光效应和热膨胀效应,光子晶体介质和缺陷层的光学厚度和折射率发生变化,透射谱缺陷峰产生漂移,由缺陷峰的中心波长漂移量得到被测温度的大小.构建了一维缺陷光子晶体测量温度的实验系统,实验结果表明缺陷峰中心波长与光子晶体所受的温度呈线性关系,测量灵敏度为0—2 关键词： 温度测量 一维光子晶体 传输矩阵法 缺陷峰     

Giant magnetorefractive effect in magnetic granular CoFe-MgF alloys

This paper reports on an IR study (5–20 µm) of the dispersion and field dependences of the magnetorefractive effect in granular CoFe-MgF (metal-insulator) systems, which exhibit, in compositions close to the percolation threshold, a tunneling magnetoresistance of 7.5% in a field of 1700 Oe. The change in the reflectivity of p-polarized light under magnetization in the IR spectral region 5–7 µm is of the order of 0.1%, while in the 10-to 12-µm region, this change, reaches record-high values of 1.2–1.5%, which exceed the usual odd and even magnetooptic reflectivities in the IR range by two orders of magnitude. The magnetorefractive effect in magnetic systems with tunneling conduction originates from spin-dependent high-frequency tunneling.     

Magnetic-field control of the transmission of a photonic crystal with a liquid-crystal defect

The method of modulation of transmittance of a multilayer photonic crystal with a nematic liquid-crystal defect upon the orientational transition from the homeotropic to the planar state is investigated. The orientation of the director of the nematic is controlled by a magnetic field in the B-effect mode. The method of recurrent relations is used for numerical simulation of transmission spectra of the photonic crystal structure under investigation.     

Mossbauer effect measurements in the dysprosium-silicon and dysprosium-gallium equiatomic compounds

Mössbauer effect measurements have been performed in dysprosium-silicon and dysprosium-gallium equiatomic compounds. The experimental results were compared with crystal field calculations based on a point charge model. The calculated second order crystal field parameters were found to be predominant. The magnetic moment of the ground state is highly anisotropic and its high value (9.8 μ_B) is in good agreement with the observed magnetic hyperfine field which is similar for both the pure and the diluted compounds.The same magnitude of the quadrupolar interactions in these compounds as well as in the Dy metal may be attributed to the predominant contribution of the 4f electrons.     

Magneto-optical activity of a one-dimensional photonic crystal with a magnetic defect

The influence of a magnetic defect on the field distribution and magneto-optical properties of a one-dimensional photonic crystal has been investigated. It has been shown that the maximum localization of the wave field in the defect layer is achieved in an asymmetric photonic crystal structure. A greater Faraday rotation, which significantly exceeds the angle of rotation of the polarization plane in an isolated magnetized layer, and a higher degree of localization of the wave field can be achieved when the magnetic layer is surrounded by layers of photonic crystal mirrors with a lower refractive index. An increase in the Faraday rotation angle is determined not only by an increase in the thickness of the magnetic defect but also by a symmetric increase in the number of periods in the photonic crystal mirrors.     

Waveguide modes of 1D photonic crystals in a transverse magnetic field

We analyze waveguide modes in 1D photonic crystals containing layers magnetized in the plane. It is shown that the magnetooptical nonreciprocity effect emerges in such structures during the propagation of waveguide modes along the layers and perpendicularly to the magnetization. This effect involves a change in the phase velocity of the mode upon reversal of the direction of magnetization. Comparison of the effects in a nonmagnetic photonic crystal with an additional magnetic layer and in a photonic crystal with magnetic layers shows that the magnitude of this effect is several times larger in the former case in spite of the fact that the electromagnetic field of the modes in the latter case is localized in magnetic regions more strongly. This is associated with asymmetry of the dielectric layers contacting with the magnetic layer in the former case. This effect is important for controlling waveguide structure modes with the help of an external magnetic field.     

Influence of crystal defects on the irreversibility line of YBa2Cu3O7-x single crystals

High quality flux grown and defect enriched peritectically grown YBa₂Cu₃O_7–x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsH _ac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y₂BaCuO₅(211)-precipitates and microcracks act as strong pinning centers. ForH _ac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations.     

基于磁光子晶体的低损耗窄带THz滤波器

本文提出一种采用石榴石型铁氧体磁性材料的太赫兹滤波器,利用波导线缺陷和腔内点缺陷的耦合特性,通过改变腔内介质柱半径及分布,实现对某个波长的耦合,达到了高效率滤波的功能;改变外磁场的大小,影响铁氧体材料的磁导率变化,使谐振频率发生改变,从而对THz波进行滤波.应用平面波展开法(PWM)和时域差分有限法(FDTD)进行仿真分析,研究结果表明,该滤波器其插入损耗为0.0997 d B,3 d B带宽为8.22 GHz,实现了低损耗窄带滤波.     

Magnetoreflection and magnetotransmission of light in manganites with CMR

The influence of a magnetic field on the reflection and transmission spectra of natural light is studied in the visible and infrared ranges of the spectrum in films and single crystals of manganite La_0.7Ca_0.3MnO₃ with colossal magnetoresistance. The magnetoreflection and magnetotransmission effects are analyzed using the magnetorefractive approach developed for manganites.     

光子晶体微腔中高偏振单偶极模的研究

三角晶格排列的光子晶体微腔中的偶极模式是简并的,通过改变其晶格的对称性可以消除模式简并.晶格的整体形变破坏了晶格对称性从而影响光场的分布,同时还改变了电磁场的偏振分布.晶格整体形变使得简并的偶极模式分离成x极和y极偶极模式.通过计算分析发现分离后的模式具有良好的偏振特性,从而为实现单偏振光子晶体激光器提供了一种很好的途径.文中针对光子晶体薄板结构的微腔,主要计算了偶极模中x极偶极模式在不同拉伸时以及不同填充因子情况下的Q值,并分别计算了x关键词： 光子晶体 偶极模 品质因子 偏振度