A polarization insensitive dual-band tunable graphene absorber at the THz frequency

A dual-band and polarization insensitive tunable graphene absorber for THz frequency has been proposed and investigated. The absorber consists of a square graphene ring with a slit at the middle of each side separated from a metallic mirror by a dielectric spacer. Two distinct absorption peaks of 99.87% and 97.82% are observed at 3.92 THz and 6.96 THz, respectively. In addition, the distributions of electric field intensity are also presented for well-recognized the physical origin of such perfect absorption phenomenon. Furthermore, the influence of geometric parameters on the dual-band absorption properties is studied in detail to provide a useful guidance for practical fabrication. The perfect dual-band absorption properties can also be dynamically tuned through the change of the Fermi energy along with large angle insensitivity, both are interesting for real application. The proposed tunable graphene absorber should found potential applications in areas including sensors, modulators, and detectors.     

再论各向异性单轴晶体的布儒斯特角

从各向同性介质和各向异性单轴晶体界面间的菲涅耳公式出发,进一步研究了当自然光从各向同性介质射向各向异性且光轴任意取向的单轴晶体界面上时,其反射光成为垂直入射面的线偏振光时所对应的入射角,即布儒斯特角是否存在的问题,并对该问题进行了较全面的论述,计算了相应条件下的布儒斯特角和有关参量的数值,得到了正确的结论,同时也指出了某些文献在论述该问题时的不妥之处.     

布儒斯特角切割LiNbO3晶体的偏振态分析

基于琼斯矩阵和菲涅耳公式,建立了偏振光透过LiNbO3晶体的分析模型,研究了入射光偏振态、晶压等参数对输出光偏振态和p偏振态、s偏振态透射率的影响规律。搭建了分别含单块LiNbO3晶体、LiNbO3晶体和薄片、两块串联LiNbO3晶体的Ho,Tm,Cr:YAG激光电光调Q实验装置,并测得了输出激光的偏振态、输出能量等随晶压的变化关系曲线。     

Geometric visualization of the Brewster angle from dielectric-magnetic interface

A geometric visualization is presented for the Brewster angle for a plane wave reflecting from an interface. The surface is assumed to be isotropic but it is allowed to display both dielectric and magnetic susceptibility, and hence the Brewster (polarizing) angle can attain any value between 0 and 90^°, and can exist for both parallel and perpendicular polarizations. The geometric construction (a tetrahedron) is spanned by the basic material parameters of the surface. The Brewster angle appears in one of the faces of the tetrahedron.     

Tunable multi-band terahertz absorber based on graphene nano-ribbon metamaterial

The tunable multi-band selective absorption effect with graphene nano-ribbon metamaterial is investigated. It is achieved by depositing a set of graphene nano-ribbons on a dielectric spacer backed with a metallic mirror. A dual-band and tri-band absorbers are designed to illustrate such multi-band selective absorption behavior. The designed graphene absorbers exhibit near-unity absorption at multiple resonance frequencies, which is attributed to the plasmonic resonance of graphene nano-ribbons with different widths. Moreover, the multiple resonance wavelengths can be tuned in a wide frequency range by changing the Fermi energy and the absorbers possess large angle insensitivity. Last, a Fabry-Perot model is employed to give a physical understanding of such multi-band selective absorption effect. It is believed that the conclusions may be useful for designing of next-generation graphene-based optoelectronic devices.     

布儒斯特角附近的透射光栅传输特性

 减小光栅表面的反射率以得到更高的衍射效率是目前光栅设计与制造中需要解决的重要问题之一。提出利用偏振光沿布儒斯特角入射时具有的特性来实现降低光栅表面反射率的方案,介绍了这种方案的物理模型,并利用严格耦合波分析法进行了模拟计算。计算结果显示,对入射光为TM偏振,波长0.35 μm,当光栅周期较长为2.80 μm时,以布儒斯特角入射的光波,它的表面反射被大大抑制;当光栅周期较短为0.21 μm时也有类似的结论,并且透射光的一级衍射效率极大值出现在刻槽深度为3.50 μm处附近,衍射效率大于95%。     

A right cut method of Brewster angle to reduce the reflection of laser frequency doubler

The ordinary method to cut two light pass surfaces of an optical device in Brewster angle parallelly cannot reduce the harmonic's reflective losses of a laser frequency doubler. Right cut method with two unparallel light pass surfaces of Brewster angle of a laser frequency doubler, and preliminary experiment to demonstrate the usefulness of the technique are given.     

Guided-mode resonant Brewster filter consisting of two homogenous layers and a single grating with an equal refractive index

In this paper,a new type of resonant Brewster filter(RBF) consisting of two homogenous layers and a single grating with an equal refractive index is presented.The properties are studied by using the plane waveguide method(PWM) and rigorous coupled-wave analysis(RCWA).It is found that the variation of the grating thickness does not effectively change the position of the resonant wavelength,however it has a remarkable effect on the line width,and the resonant peak can be adjusted back to its original position by slightly tuning the grating period.Moreover,by simultaneously tuning the thicknesses of the homogeneous layers above and beneath the grating structure,multiple channels can also be obtained when the RBF is illuminated at the Brewster angle calculated with the effective medium theory(EMT) of subwavelength grating.The adjacent optical thickness for acquiring the multiple channels is about three-quarters of the resonant wavelength.Furthermore,it is demonstrated that the line width at the operating resonant wavelength can be appreciably narrowed by tuning the thickness of the homogenous layer to its corresponding thickness without fine tuning the grating period or the thickness.Therefore,it is very useful for designing filters with different line widths at the desired wavelength.In addition,it is shown from our calculations that the symmetrical line feather can be obtained if the total optical thickness for the homogeneous layer meets the special condition.     

Guided-mode resonant Brewster filter consisting of two homogenous layers and a single grating with an equal refractive index

In this paper, a new type of resonant Brewster filter (RBF) consisting of two homogenous layers and a single grating with an equal refractive index is presented. The properties are studied by using the plane waveguide method (PWM) and rigorous coupled-wave analysis (RCWA). It is found that the variation of the grating thickness does not effectively change the position of the resonant wavelength, however it has a remarkable effect on the line width, and the resonant peak can be adjusted back to its original position by slightly tuning the grating period. Moreover, by simultaneously tuning the thicknesses of the homogeneous layers above and beneath the grating structure, multiple channels can also be obtained when the RBF is illuminated at the Brewster angle calculated with the effective medium theory (EMT) of subwavelength grating. The adjacent optical thickness for acquiring the multiple channels is about three-quarters of the resonant wavelength. Furthermore, it is demonstrated that the line width at the operating resonant wavelength can be appreciably narrowed by tuning the thickness of the homogenous layer to its corresponding thickness without fine tuning the grating period or the thickness. Therefore, it is very useful for designing filters with different line widths at the desired wavelength. In addition, it is shown from our calculations that the symmetrical line feather can be obtained if the total optical thickness for the homogeneous layer meets the special condition.     

Brewster’s angle method for absorption coefficient measurement of high-resistivity silicon based on CW THz laser

A simple method for measuring the absorption coefficient of low absorbing materials using CW THz laser was put forward. The method was based on transmittance measurements at the Brewster’s angle for p-polarized light, where the reflectance would be minimal, so interference caused by multiple reflections in the sample would be eliminated. Numerical simulations were carried out to evaluate errors in the proposed method. An experiment was also made to measure the absorption coefficient of high-resistivity Czochralski silicon at 118.83 μm by the method. Based on CW THz laser, the method offers a convenient way to measure the material absorption coefficient and has a low cost, so it shows promising application prospects.     

About the linewidth of cyclotron resonance in band-gap graphene

The critical amplitude of circularly polarized electromagnetic wave when the hysteresis of cyclotron absorption takes place, was found for band-gap graphene. The dependence of critical amplitude on the gap value and on the relaxation time was investigated. The conditions of applicability of linear theory describing the electromagnetic response of band-gap graphene in a non-zero magnetic field were found. The power of the circularly polarized electromagnetic radiation absorbed by band-gap graphene in the presence of a magnetic field was calculated. The linewidth of cyclotron absorption was shown to be not zero even for pure band-gap graphene.     

Measurement of surface absorption in optical materials by the method of laser photothermal radiometry

The present work is devoted to the development of the method of laser photothermal radiometry with a view to increasing its spatial resolution over the depth of the specimen when surface layers of the substance of thickness about 1 μm or less are to be investigated. As an example, results of an investigation of radiation absorption in the surface layer of a lithium niobate crystal are presented. The absorption index measured at a wavelength of 1.08 μm was ∼0.6 cm⁻¹. “Polyus” Research Institute, 3, Vernadskii Ave., Moscow, 117342, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 82–84, January–February, 1997.     

Brewster Anomalies in Opaque Random Amorphous Multilayers a-Si:H/a-Si3N4+x:H

Reflectance anomalies of random amorphous multilayers are shown to be different dependence on the incident angle for s- and p-polarized light by simulation. Disappearance of these anomalies has especially been observed for p-polarized light propagation at a certain incident angle. It is shown that this incident angle is the extended Brewster angle defined for opaque materials. This phenomenon, i.e., the disappearance of reflectance anomalies, is the Brewster anomaly in random amorphous multilayers made with absorbent materials. Preliminary results of experiments are also presented.     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.     

Polarization‐independent absorption enhancement in a graphene square array with a cascaded grating structure

The polarization‐independent enhanced absorption effect of graphene in the near‐infrared range is investigated. This is achieved by placing a graphene square array on top of a dielectric square array backed by a two‐dimensional multilayer grating. Total optical absorption in graphene can be attributed to critical coupling, which is achieved through the combined effect of guided‐mode resonance with the dielectric square array and the photonic band gap with the two‐dimensional multilayer grating. To reveal the physical origin of such a phenomenon, the electromagnetic field distributions for both polarizations are illustrated. The designed graphene absorber exhibits near‐unity polarization‐independent absorption at resonance with an ultra‐narrow spectrum. Moreover, the polarization‐independent absorption can be tuned simply by changing the geometric parameters. The results may have promising potential for the design of graphene‐based optoelectronic devices.     

Rashba coupling induced spin susceptibility and magnetic phase transition of conduction electrons in monolayer graphene

Isolated graphene cannot be obtained by the known synthesis processes and it should be placed on a substrate. This substrate introduces a new type of spin–orbit interaction known as Rashba coupling. Using the Kubo formalism, the magnetic properties of the system in the linear regime have been investigated. Mainly the effect of non-magnetic substrate on the spin susceptibility is calculated. Results show that the Rashba coupling has a central role in the magnetic response function of the system and it is really remarkable since this type of spin orbit coupling can be effectively controlled by an external gate voltage. Most importantly, it was shown that, in the presence of the Rashba interaction a magnetic phase transition could be observed. This magnetic phase corresponds to a magnetic order of conduction electrons that takes place at some special frequencies of external magnetic field.     

Anisotropic resistivity of the monolayer graphene in the trigonal warping and connected Fermi curve regimes

In the present study, the anisotropic resistivity of the monolayer graphene has been obtained in semiclassical regime beyond the Dirac point approximation. In particular, detailed investigations were made on the dependence of conductivity on the Fermi energy. At low energies, in the vicinity of the Dirac points, band energy of the monolayer graphene is isotropic at the Fermi level. Meanwhile, at the intermediate Fermi energies anisotropic effects such as trigonal warping is expected to be the origin of the anisotropic resistivity. However, besides the band anisotropy there also exists an other source of anisotropic resistivity which was introduced by scattering matrix. At high energies it was shown that the band anisotropy is less effective than the anisotropy generated by the scattering matrix. It was also shown that there exist two distinct regimes of anisotropic resistivity corresponding the trigonal warping and connected Fermi curve at intermediate and high energies respectively.     

MATLAB在光学界面发生反射时的分析与应用

基于MATLAB的计算功能及可视化功能计算并分析了光学中的菲涅耳公式,更加直观的揭示了光波在两介质界面发生反射时的变化规律。使学生更加深入、直观地掌握光波在界面发生变化的条件、实质及布儒斯特角与全反射临界角的物理意义,同时也学会了分析研究方法,且提高了学习的主动性和创造性。     

介质光栅导模共振耦合波分析

导模共振是由于光栅介质内高级次子波耦合进光栅所支持的泄漏模中,导致传播波能量重新分布的结果.采用严格的耦合波方法,通过分析波导的导波模式,正确估计出弱调制介质光栅导模共振的位置.并对导模共振与光栅厚度、基底厚度以及入射角的关系作了讨论.通过采用抗反射设计,获得了具有对称、低旁带特点的窄带共振峰 关键词： 导模共振 耦合波