二维电子气等离激元太赫兹波器件

固态等离激元太赫兹波器件正成为微波毫米波电子器件技术和半导体激光器技术向太赫兹波段发展和融合的重要方向之一。本综述介绍AlGaN/GaN异质结高浓度和高迁移率二维电子气中的等离激元调控、激发及其在太赫兹波探测器、调制器和光源中应用的近期研究进展。通过光栅和太赫兹天线实现自由空间太赫兹波与二维电子气等离激元的耦合,通过太赫兹法布里-珀罗谐振腔进一步调制太赫兹波模式,增强太赫兹波与等离激元的耦合强度。在光栅-谐振腔耦合的二维电子气中验证了场效应栅控的等离激元色散关系,实现了等离激元模式与太赫兹波腔模强耦合产生的等离极化激元模式,演示了太赫兹波的调制和发射。在太赫兹天线耦合二维电子气中实现了等离激元共振与非共振的太赫兹波探测,建立了太赫兹场效应混频探测的物理模型,指导了室温高灵敏度自混频探测器的设计与优化。研究表明,基于非共振等离激元激发可发展形成室温高速高灵敏度的太赫兹探测器及其焦平面阵列技术。然而,固态等离激元的高损耗特性仍是制约基于等离激元共振的高效太赫兹光源和调制器的主要瓶颈。未来的研究重点将围绕高品质因子等离激元谐振腔的构筑,包括固态等离激元物理、等离激元谐振腔边界的调控、新型室温高迁移率二维电子材料的运用和高品质太赫兹谐振腔与等离激元器件的集成等。     

Commensurability oscillations in fast particle energy loss to a density modulated 2DEG in a magnetic field

We examine fast particle energy loss as a probe of the properties of a density modulated 2DEG in a normal magnetic field. In the high velocity limit, the local plasmon spectrum makes two separate contributions to energy loss for both the cases in which the particle velocity is parallel and perpendicular to the planar 2DEG. The inter-Landau band plasmon makes a distinct contribution to energy loss through excitation of the hybrid 2D magnetoplasmon. The other local plasmon, associated with the spread of a Landau level into a miniband by application of the periodic potential associated with the density modulation contributes to energy loss with features corresponding to Shubnikov-de Haas oscillations jointly with an amplitude envelope exhibiting oscillations associated with the commensurability of the modulation period and the cyclotron radius at the Fermi energy.     

Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas(2DEG)in AlGaN/GaN high electron mobility transistors(HEMTs).By introducing a drifted Fermi–Dirac distribution,we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation.Then,the nonequilibrium Fermi–Dirac function is obtained by applying the calculated electron drift velocity and electron temperature.Under random phase approximation(RPA),the electric field driven plasmon dispersion is investigated.The calculated results indicate that the plasmon frequency is dominated by both the electric field and the angle between wavevector and electric field.Importantly,the plasmon frequency could be tuned by the applied source–drain bias voltage besides the gate voltage(change of the electron density).     

Optimal structure for resonant THz detection of plasmons–polaritons in the 2D quantum wells

We investigate terahertz plasmon?Cpolariton (PP) resonances for hetero-structures (AlGaN/GaN, SiGe/Si/SiGe, AlGaAs/GaAs, and InAlN/GaN) with a grating coupler in order to find the overall optimal structure showing the strongest absorption for terahertz detection (THz). We show by a parametric study (influence of geometric dimensions, electron concentration, temperature, etc.) that refined and intense resonances can be obtained at specific frequency. GaN based heterostructures present the higher PP resonances at room temperature. The roles of the finite thicknesses of lossy metal grating and a two-dimensional gas (2DEG) layer on observed absorption are also investigated. Absorption spectra for three kinds of heterogeneous charge density profiles (piecewise, linear, and parabolic) of 2DEG was investigated and compared for an AlGaAs/GaAs structure because some physical parameters such as the Fermi level pinning at the interface semiconductor/air are well established only for this heterostructure. We show that the PP resonance (amplitude and frequency position) is modulated by the charge concentration but also by the metallization biasing.     

Effects of third-order aberrations on the irradiance of self-image

We examine the effects of third-order aberrations exerted on the irradiance of image that is observable in a coherent self-imaging system. Both spherical aberration and astigmatism degrade the visibility of the image of a sinusoidal-type grating as well as blur the outline of the image of a rectangular-type grating. Coma laterally shifts the image of a sinusoidal-type grating on the image plane as well as changes a rectangular-type pattern into an asymmetrically blurred pattern. According to our analysis, the self-image of a high-density grating with a period of two times the optical wavelength is not at all affected by spherical aberration. In general a self-imaging system can always be corrected for astigmatism by shifting the image plane in its normal direction. We show that the self-image with defect can be well explained by taking the third-order aberrations and the focus-shift aberration into consideration.     

AlGaN/GaN HEMT器件电流坍塌和膝点电压漂移机理的研究

考虑了势垒层、缓冲层体陷阱及表面电荷的浓度变化对电流坍塌和膝点电压的影响,发现表面电荷和势垒层体陷阱浓度的变化对沟道电子的浓度影响较小,表面电荷浓度变化下的膝点电压的偏移和坍塌强度的大小与势垒层势阱能量的变化有着主要的关系.缓冲层有着比势垒层更强的局域作用,势垒层和缓冲层的体陷阱浓度在一定范围变化时的膝点电压偏移主要是由沟道电子浓度的变化而引起的,但偏移量却比表面电荷浓度变化的情况下小很多.势阱能量的变化是造成膝点电压偏移的重要原因,坍塌强度主要取决于势阱能量和沟道的电子浓度. 关键词： AlGaN/GaN高电子迁移率晶体管 电流坍塌 膝点电压 陷阱俘获     

Photorefractive control of surface plasmon polaritons in a hybrid liquid crystal cell

We present a photorefractive hybrid liquid crystal system that allows strong photorefractive effects on surface plasmon polaritons. We demonstrate its capability to couple energy between two 1.03 eV surface plasmon polariton modes with an efficiency of 25.3±2.3%. We present the energy and grating pitch dependence of the diffraction and a model that can qualitatively explain them.     

Thermal emission properties of one-dimensional grating with different parameters

Thermal emission is often presented as a typical incoherent process. Incorporating periodic structures on the tungsten surface offers the possibility to obtain coherent thermal emission sources. Here we illustrate grating as an example to examine the influence of the geometric parameters on the thermal emission properties. It is found that for very shallow gratings, only surface plasmon polariton(SPP) modes can be excited and the emission efficiency is closely related with the filling factor. When the ratio of the depth to period of the grating is in the range from 1/20 to 1/2, the field between the adjacent corners can be coupled to each other across the air gap for the filling factor larger than 0.5 and produce a similar resonance as in an air rod. Further increase of the grating depth can cause the groove of the grating forming metal–insulator–metal(MIM) structures and induce surface plasmon standing wave modes. Our investigations will not only be helpful for manipulating thermal emission properties according to applications, but also help us understand the coupling mechanism between the incident electromagnetism waves and gratings with different parameters in various research fields.     

Formula for the absorption coefficient for multi-wall nanotubes

We present a formalism for calculating the absorption coefficient of a pair of coaxial tubules. A spatially nonlocal, dynamical self-consistent field theory is obtained by calculating the electrostatic potential produced by the charge density fluctuations as well as the external electric field. There are peaks in the absorption spectrum arising from plasma excitations corresponding either to plasmon or particle-hole modes. In this Letter, we numerically calculate the plasmon contribution to the absorption spectrum when an external electric field is applied. The number of peaks depends on the radius of the inner as well as outer tubule. The height of each peak is determined by the plasmon wavelength and energy. For a chosen wave number, the most energetic plasmon has the highest peak corresponding to the largest oscillator strength of the excited modes. Some of the low-frequency plasmon modes have such weak coupling to an external electric field that they are not seen on the same scale as the modes with larger energy of excitation. We plot the peak positions of the plasmon excitations for a pair of coaxial tubules. The coupled modes on the two tubules are split by the Coulomb interaction. The energies of the two highest plasmon branches increase with the radius of the outer tubule. On the contrary, the lowest modes decrease in energy as this radius is increased. No effects due to inter-tubule hopping are included in these calculations.     

Interaction of terahertz electromagnetic waves with periodic gratings of graphene micro- and nanoribbons

An original mathematical model of the interaction of terahertz (THz) electromagnetic waves with periodic gratings of graphene micro- and nanoribbons is based on the solution to the boundary-value problem of diffraction for the Maxwell equations with electrodynamic boundary conditions and material equations. The electrodynamic calculations of the transmission coefficients of the TEM wave versus frequency are performed for the 2D grating of graphene micro- and nanoribbons at several chemical potentials, grating periods, and geometrical sizes of ribbons. The results of the calculations show that the transmission spectrum exhibits a minimum in the THz range if the electric field of the wave is perpendicular to the graphene ribbons. The minimum is due to the plasmon resonance of the fundamental mode in graphene, and the absorption peaks at higher frequencies in the upper part of the THz range are related to the highorder plasmon modes.     

等离激元增强的石墨烯光吸收

石墨烯中等离激元具有特殊的光电性质,其和入射光的强烈耦合可以引起光吸收的增强.本文基于时域有限差分法和多体自洽场理论研究了等离激元对处于光学谐振腔中的石墨烯光吸收的影响.由于石墨烯中等离激元与入射光动量和能量不匹配而不能直接相互作用,因此石墨烯上施加了金属光栅结构.研究发现光栅结构能够对入射光进行动量补偿并且能够引起其下石墨烯中的电场强度产生很大程度增强,从而导致在该石墨烯结构中太赫兹等离激元和入射光发生强烈耦合而产生太赫兹等离极化激元,同时引起石墨烯光吸收的增强.希望本文能够加深对石墨烯光电特性的理解以及可以为基于石墨烯的太赫兹光电装置提供一定的理论依据.     

利用双光栅的多普勒频移测速度

衍射光的多普勒频移是与光栅运动速度有关的量.激光束依次通过动光栅、静光栅衍射,然后使1对光频不同、彼此平行又能部分重叠的衍射光束通过硅光电池进行光电转换,得到的信号电流的频率等于多普勒频移.信号电流经I/V转换、f/V转换和A/D转换后,由单片机测出电压,由此就可以确定动光栅的运动速度.最后对利用双光栅的多普勒频移测气垫导轨上滑块的速度进行分析和讨论.     

Energy-loss function of TTF-TCNQ

We investigate the energy-loss function for a previously developed model of quasi-one-dimensional metals with two one-dimensional electron bands per donor and acceptor chains and the three-dimensional long-range Coulomb electron-electron interaction within the random phase approximation. It is essentially influenced by two hybridized collective modes which result from the strong coupling of the intraband plasmon and the interband dipolar modes. Our calculations show that the spectral weights of the renormalized plasmon and the dipolar mode dominate within the long wavelength limit, while for large longitudinal wave vectors the intraband electron-hole quasi-continuumgains some experimentally observable spectral weight as the second mode approaches it. The function obtained is brought into correspondence with the data of the quasi-one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) obtained from electron energy-loss spectroscopy (EELS) measurements.     

表面等离子体-微腔激元对顶入射有机薄膜太阳能电池光吸收效率的增强

为了提高顶入射有机薄膜太阳能电池(TOSCs)的光吸收效率,我们将周期性矩形光栅结构引入到TOSCs中,分析了具有光栅结构的空气/Ag_1/有源层/Ag_2/空气(IMIMI)结构理想模型中复合表面等离子激元(SPPs)与微腔模式的耦合机制。通过调节光栅周期和有源层厚度,实现了复合SPPs、微腔模式以及有机材料本征吸收3个区域的重合。由于复合SPPs与微腔模式的反交叉耦合作用形成了表面等离子体-微腔激元,其局域场增强作用有效地提高了有源层的光吸收效率,提高了近19%。     

Rashba spin–orbit effect on the magnetocapacitance of a 2DEG in a diluted magnetic semiconductor

We investigate the magnetocapacitance of the two-dimensional electron gas (2DEG) embedded in diluted magnetic semiconductors in the presence of Rashba spin–orbit interaction (SOI). We present calculations on the energy spectrum and density of states and show that the tunable spin–orbit coupling and the enhanced Zeeman splitting have a strong effect on the magnetocapacitance of the structure. In the presence of Rashba SOI, a typical beating pattern with well defined node-positions in the oscillating capacitance is observed. A simple relation that predicts the positions of nodes in the beating patterns is obtained. The interplay between the total Zeeman splitting (including the s–d exchange interaction) and the Rashba SOI is discussed.     

Hybrid waveguide-surface plasmon polariton modes in a guided-mode resonance grating

We analyze the coupling between surface plasmon polaritons in a metal grating and the guided modes of a dielectric waveguide. Our model structure is a gold wire grating on a slab waveguide made of silicon nitride on silica wafer. The excitation of guided-mode resonances, surface plasmon polariton modes and hybrid waveguide-plasmon modes are observed in numerical simulations. Our experiments verify the existence of the predicted modes. These hybrid modes add significant degrees of freedom in designing structures for plasmonic applications.     

Influence of laser mode on splitting beam illumination effect of Dammann grating

The influences of various laser modes on the splitting beam effect of Dammann grating are studied in theory and by numerical simulation. The results show that fundamental mode laser resembles plane wave while high order mode laser differs from plane wave in the splitting beam effect by Dammann grating. Therefore, the fundamental mode laser is more suitable to be the light source to improve the energy efficiency in far-distance image detecting systems, such as laser image ladar, which use Dammann grating in the ilhlmination svstem.     

复合金属光栅模式分离与高性能气体传感器应用

为实现近红外波段表面等离子体共振(SPR)模式的分裂和移动, 同时提高光栅基SPR传感器的品质因数, 提出了一种由双金属光栅构成的新型复合结构光栅, 并研究了其气体传感特性. 运用有限时域差分算法对该结构进行了数值模拟, 发现由复合金属光栅激发的SPR出现模式分裂的现象. 通过增大双金属光栅阵列间的相对位移改变原结构的对称性, 导致复合金属光栅分裂的SPR模式朝相反方向移动. 当相对位移量进一步增大到双光栅合并成新的单一光栅时, 随光栅结构对称性的恢复, 分裂的两共振模式最后又重新合并为一个模式. 如果待测物的折射率为1.01≤n_a≤1.05, 当相对位移量为0时, 基于复合光栅结构气体传感器的折射率灵敏度为1207.5 nm/RIU, 且品质因数达到1290.7; 当相对位移量为100 nm时, 与双共振模式对应的折射率灵敏度分别为1205.0 nm/RIU和1210.0 nm/RIU, 品质因数分别为1295.4和762.3. 因此, 复合光栅SPR传感器具有超高品质因数的性能, 使得它在生物化学传感领域中有巨大的应用潜力.     

CT-6B托卡马克等离子体角向转动的光谱测量

利用高分辨光谱测量系统，根据杂质离子谱线ＯⅡ４６４.２ｎｍ，ＣⅢ４６４.７ｎｍ和氢Ｈα谱线的多普勒位移，测量了ＣＴ-６Ｂ托卡马克等离子体角向转动速度的径向分布．结果表明：杂质离子的角向转动速度的方向在等离子体内部为电子逆磁漂移方向，其线速度在小半径约９ｃｍ处达到极大值３.５ｋｍ／ｓ；在接近孔阑ｒ＝１０ｃｍ处，反转为离子逆磁漂移方向．由此导出等离子体内部的径向电场向里，其最大值为１８Ｖ／ｃｍ．中性氢原子只存在其方向为电子逆磁漂移方向的角向转动分量，其数值较杂质离子谱线所得结果要低．最后对所得的结果作了初步的讨论 关键词：     

Enhanced optical transmission through double-layer gold slit arrays

We investigate the relationship between the transmission and the layer distance of double-layer gold slit arrays by using the finite-difference time-domain method.The results show that the transmission properties can be influenced strongly by layer distance.We attribute the two types of resonant modes to surface plasmon resonance and the localised waveguide resonance.We find that the localised waveguide transmission peak redshifts and becomes broader with increasing layer distance D.We also describe and explain the splitting,shift,and degeneration of the surface plasmon resonant transmission peak theoretically.In addition,to clarify the physical mechanism of the transmission behaviours,we analyse the distributions of electric field and total energy for the three transmission peaks with distance D=45 nm for the double-layer system.Light transporting behaviours are mostly concentrated in the region of the slits as well as the interspaces of the two layers,and for different resonant wavelengths the electric field and energy distributions are different.It is expected that the results obtained here will be helpful for designing subwavelength metallic grating devices.