太赫兹频段开口环谐振器的可调谐振模式转换

在硅衬底上设计了一种单开口环谐振器,对其太赫兹频段内的透射性质进行了研究。假定通过光注入方式改变衬底硅的电导率,实现了谐振环的双谐振透射率可调。将砷化镓材料生长于该谐振环的开口处,通过光注入方式改变砷化镓材料的电导率,可以实现谐振环的双频LC共振和偶极子共振模式与单频闭合环共振模式之间的转换。这种通过光注入改变半导体材料电导率的方法,可以在不破坏原来谐振器件物理结构的前提下,实现谐振环谐振模式的可逆转换。     

Origin of strain-induced resonances in flexible terahertz metamaterials

Two types of flexible terahertz metamaterials were fabricated on polyethylene naphthalate(PEN) substrates. The unit cell of one type consists of two identical split-ring resonators(SRRs) that are arranged face-to-face(i.e., Flex Meta F); the unit cell of the other type has nothing different but is arranged back-to-back(i.e., Flex Meta B). Flex Meta F and Flex Meta B illustrate the similar transmission dips under zero strain because the excitation of fundamental inductive–capacitive(LC)resonance is mainly dependent on the geometric structure of individual SRR. However, if a gradually variant strain is applied to bend Flex Meta F and Flex Meta B, the new resonant peaks appear: in the case of Flex Meta F, the peaks are located at the lower frequencies; in the case of Flex Meta B, the peaks appear at the frequencies adjacent to the LC resonance. The origin and evolution of strain-induced resonances are studied. The origin is ascribed to the detuning effect and the different responses to strain from Flex Meta F and Flex Meta B are associated with the coupling effect. These findings may improve the understanding on flexible terahertz metamaterials and benefit their applications in flexible or curved devices.     

异向介质谐振响应的Floquet模分析

将开口谐振环单元的场分布利用Floquet模展开,从电磁波传播角度揭示了凋落模引起异向介质谐振响应的事实.数值计算了任意平面电磁波照射下异向介质的传输特性.研究表明,对于来自主平面上的垂直或平行极化的入射波,具有相同极化方式的Floquet透射模将发生谐振,并且磁谐振和电谐振将随着入射角的不同而改变.而对于来自其他入射平面的电磁波,两种极化的Floquet透射模将共同发生谐振. 关键词： 开口谐振环 异向介质 Floquet模 谐振     

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.     

Excellent polarization-independent reflector based on guided mode resonance effect

A broad band polarization-independent reflector working in the telecommunication C+L band is proposed using the guided mode resonance effect of a periodic surface relief element deposited by a layer of silicon medium. It is shown that this structure can provide high reflection (R>99.5%) and wide angular bandwidth (θ≈20°, R>98%) for both TE and TM polarizations over a wide spectrum band 1.5 μm～1.6 μm. Furthermore, it is found by rigorous coupled wave analysis that the polarization-independent reflector proposed here is tolerant of a deviation of grating thickness, which makes it very easy to fabricate in experiments.     

基于平面超材料的Fano谐振可调谐研究

基于两段相同金属分裂环谐振器构成的单层结构, 从理论及实验方面研究了平面超材料的可调谐Fano谐振. 实验测量了平面超材料对TE和TM入射波的电磁响应, 利用电磁波的入射角度控制Fano谐振的强度, 实现了谐振的开关特性, 谐振频率红移可达到21%. 基于有限元法给出了平面超材料的场分布, 强的正常色散表明平面超材料的电磁响应可类比经典电磁诱导透明现象, 仿真与实验结果相符合. 对称结构超材料Fano谐振的开/关特性为超材料性能的可调谐控制提供了有效途径.     

Elastic waves propagation in thin plate metamaterials and evidence of low frequency pseudo and local resonance bandgaps

A comprehensive numerical simulation study on elastic wave propagation for a novel engineered thin plate metamaterial structural system is presented. To enhance the accuracy of model, a mesh convergence study is validated. The wave dispersion spectra with bandgaps for three types of structures are presented. Based on local resonance phenomena, the low frequency pseudo and local resonance bandgaps are reported. Highlights are also included for the Bragg scattering induced directional bandgaps. The effects of material and geometric parameters on bandgaps are discussed. The finite element based frequency response study further validates the reported bandgaps. The conclusions provide a new perspective for designs and applications of thin plate metamaterials for subwavelength wave manipulation including seismic shielding of civil infrastructures.     

Broadband non-polarizing beam splitter based on guided mode resonance effect

A broadband non-polarizing beam splitter (NPBS) operating in the telecommunication C+L band is designed by using the guided mode resonance effect of periodic silicon-on-insulator (SOI) elements. It is shown that this double layer SOI structure can provide ～50/50 beam ratio with the maximum divergences between reflection and transmission being less than 8% over the spectrum of 1.4 μm～1.7 μm and 1% in the telecommunication band for both TE and TM polarizations. The physical basis of this broadband non-polarizing property is on the simultaneous excitation of the TE and TM strong modulation waveguide modes near the designed spectrum band. Meanwhile, the electric field distributions for both TE and TM polarizations verify the resonant origin of spectrum in the periodic SOI structure. Furthermore, it is demonstrated with our calculations that the beam splitter proposed here is tolerant to the deviations of incident angle and structure parameters, which make it very easy to be fabricated with current IC technology.     

含奇异材料的掺杂一维光子晶体色散关系和空间局域度理论

给出了描述掺杂的一维光子晶体结构缺陷态的变化规律及其空间局域度特性的理论表达式.该式对由具有任意色散关系材料组成的、中间掺杂、两边对称的一维光子晶体结构均成立.通过研究含奇异材料的光子晶体组态发现，当缺陷层材料为右手材料时，光子带隙中缺陷态的频移方向随缺陷层厚度的增大而红移；当缺陷层材料是左手材料时，频移方向刚好相反，表现为蓝移.该缺陷态的空间局域度只与两边对称结构有关.两边对称结构含奇异材料的光子晶体的缺陷态比传统右手材料的结构具有更强的空间局域度.     

经典局域共振型水声超材料的吸隔声性能研究*

基于水声超材料吸声机理和多层平行介质平面波理论,建立局域共振型水声超材料结构,通过COMSOL进行建模计算,研究该结构的吸声性能机理,此外为了验证钢背衬的隔声性能,在该水声超材料结构基础上添加一层0.005m厚的钢背衬进行仿真对比。研究结果表明,在频段为200Hz-4000Hz时,水声超材料声学性能较好,吸声性能整体较优,且添加钢背衬的水声超材料隔声性能较优,甚至在某频率点达到15dB的隔声差值;此外通过位移场图进一步揭示水声超材料的吸声机理,发现水声超材料结构的位移场和钢背衬都对吸声性能会产生影响,钢背衬通过影响共振吸收来影响吸声性能,而位移场则通过位移幅度大小影响吸声性能。     

Dynamically controlled asymmetric transmission of linearly polarized waves in VO2-integrated Dirac semimetal metamaterials

We numerically demonstrated a novel chiral metamaterial to achieve broadband asymmetric transmission (AT) of linearly polarized electromagnetic waves in terahertz (THz) band. The proposed metamaterial unit cell exhibits no rotational symmetry with vanadium dioxide (VO$_{2}$) inclusion embedded between Dirac semimetals (DSMs) pattern. The resonant frequency of AT can be dynamically tunable by varying the Fermi energy ($E_{\rm F}$) of the DSMs. The insulator-to-metal phase transition of VO$_{2}$ enables the amplitude of the AT to be dynamically tailored. The transmission coefficient $|T_{yx}|$ can be adjusted from 0.756 to nearly 0 by modifying the conductivity of VO$_{2}$. Meanwhile, the AT parameter intensity of linearly polarized incidence can be actively controlled from 0.55 to almost 0, leading to a switch for AT. When VO$_{2}$ is in its insulator state, the proposed device achieves broadband AT parameter greater than 0.5 from 1.21 THz to 1.80 THz with a bandwidth of 0.59 THz. When the incident wave propagates along the backward ($-z$) direction, the cross-polarized transmission $|T_{yx}|$ reaches a peak value 0.756 at 1.32 THz, whereas the value of $|T_{xy}|$ well below 0.157 in the concerned frequency. On the other hand, the co-polarized transmission $|T_{xx}|$ and $|T_{yy}|$ remained equal in the whole frequency range. This work provides a novel approach in developing broadband, tunable, as well as switchable AT electromagnetic devices.     

由同时具有磁谐振和电谐振结构组成的左手材料

为了在同一结构中同时实现负介电常数和负磁导率(双负),采用一种“巨”字形结构,利用丝网印刷技术在氧化铝基板上进行加工制作,并通过仿真和实验测试证实了这种结构的“双负”特性. 与利用磁谐振器和电谐振器组阵实现左手材料的方法相比,这种单一结构可以同时实现负介电常数和负磁导率,制作方便,等效电路简单,双负频段较宽. 利用丝网印刷技术进行样品的制作,工艺简单,成本低廉. 关键词： 左手材料 磁谐振 电谐振     

Transient optical modulation properties in the terahertz metamaterial of split ring resonators

The ultrafast optical modulation properties of split ring resonators are characterized by utilizing optical pump-terahertz probe spectroscopy.The experimental results show that when the terahertz electric vector is perpendicular to the gap of the split ring resonator,resonant absorption can be quenched significantly under high pump excitation.However,when the terahertz electric vector is parallel to the gap,the resonant absorption is less sensitive to pump excitation due to the structural properties of the metamaterial.Our numerical simulations also demonstrate that the pump pulse significantly influences the split ring resonator current by generating carriers in the substrate.     

腔镜倾斜对平面环形腔激光振荡模式的影响

从Fox-Li迭代法出发,用光学仿真软件GLAD实现了对平面环形腔的建模。利用此光学模型进行了腔镜倾斜对其本征模式影响的研究,给出了腔镜在两个方向不同倾斜角度下的谐振腔模式光强分布、光强峰值点位置及谐振腔损耗的变化。研究结果表明：腔镜倾斜使平面环形腔模式发生畸变,随着腔面倾斜加大,腔损耗增大,光强峰值点向镜边沿偏移,且腔镜在垂直于腔面方向上的倾斜对振荡模式的影响比腔镜在腔面内的倾斜对振荡模式的影响大。     

垂直入射条件下金属环的谐振特性

本文研究垂直入射条件下水平极化和垂直极化时金属开口谐振环(split ring resonator, SRR)的电磁响应行为. 通过分析这两种情况下的透射系数、介电常数和磁导率, 发现垂直极化时, SRR可以产生电谐振实现负介电常数, 其频段远高于磁谐振频段; 水平极化时, SRR只能产生磁谐振实现负磁导率, 其频段与水平入射时的SRR的磁谐振频段相对应. 通过仿真对此进行了证明, 并对产生电谐振和磁谐振的原因进行了分析.     

一种基于导模共振的电光调制器研究

实验研究了基于导模共振的反射型电光调制器采用的棱镜耦合方式,在棱镜上面镀有4层膜,依次为银膜下电极-极化聚合物-缓冲层-金膜上电极。对这种衰减全反射（ATR）型的聚合物波导电光调制器进行了视频调制实验,采用静态偏置电压稳定模式同步角方法,结合对入射激光束进行监测,解决了视频调制中激光器噪声对模拟信号调制影响的问题,提高了工作稳定性。实验结果表明：该电光调制器具有良好的线性和较高的灵敏度。     

复合腔CuBr激光器中的模式研究

本文报道了在两种结构的多镜复合腔中CuBr激光器的纵模淬灭现象,并得到了重复频率几倍于基频纵模间隔的多重锁模脉冲串,最高重复频率可达440MHz.实验结果表明,通过调整复合腔的结构参数,可以在不缩短增益介质长度的条件下,实现CuBr激光多重锁模脉冲高重复频率的大范围调节.     

Influence of spherical aberrations on fundamental mode beam quality under different laser resonators

Spherical aberrations of the thermal lens of the active media are severe when solid state lasers are strongly pumped. The fundamental mode profile deteriorates due to the aberrations. Self-consistent modes of a resonator with aberrations are calculated by using the Fox--Li diffraction iterative algorithm. Calculation results show that the aberration induced fundamental mode beam quality deterioration depends greatly on the resonator design. The tolerance of a flat--flat resonator to the aberration coefficient is about 30λ in the middle of stability, where λ is the wavelength of laser beam. But for a dynamically stable resonator, 2λ of spherical aberration will create diffraction loss of more than 40%, if inappropriate design criteria are used. A birefringence compensated laser resonator with two Nd:YAG rods is experimentally studied. The experimental data are in quite good agreement with simulation results.     

Design and characterization of guided mode resonance filters with the improved structure for facile fabrication

In this study, a guided mode resonance filter with the improved structure for facile fabrication is designed; the properties of the designed filters are simulated by rigorous coupled wave analysis. It is found that the resonance wavelength and spectral linewidth are slightly increased with the increasing of the grating thickness, and seldom changed with the thickness of the grating filling factor, as the parameter error of the grating thickness and the grating filling factor are deviated from the designed value by ±10%, respectively, which are very favorable for simplifying preparation process of the GMR filter.