一维三元光子晶体缺陷模的特性研究

在考虑色散关系的基础上,利用传输矩阵法讨论了一维三元光子晶体缺陷模的特征.结果表明:缺陷层几何厚度增加时,缺陷模中心红移,透射率下降,品质因子变小.三介质的几何厚度各自独立递增时,缺陷模都向长波方向移动,缺陷模的品质因子先增后降.入射角增大时,缺陷模中心蓝移,透射率不变,半峰全宽度变窄,品质因子变小.若忽略缺陷介质的色散,当其折射率减小时,缺陷模蓝移,透射率变大,半峰全宽度保持不变,品质因子增大.     

含有吸收材料对称结构一维三元光子晶体的透射谱

利用传输矩阵法理论,研究含吸收材料对称结构一维三元光子晶体的光传输特性。结果表明:当各层介质无吸收时,在较宽的禁带范围内出现一条透射率为100%的透射峰;当介质的折射率为带有正虚部的复数,即介质存在吸收时,禁带中的透射峰出现明显的透射衰减,且随着复折射率虚实比β的增大透射率出现单调衰减规律;当单个介质存在吸收时,随着各介质折射率虚实比的增大,透射率的衰减速度存在各异,其中以C介质的折射率虚实比βc对透射峰透射率的影响较大,其次为B,A的影响相对较小;当所有介质同时存在吸收时,透射衰减最为明显。这些特性为光学衰减器的设计提供理论指导意义。     

由单负材料组成的对称型一维光子晶体的偏振特性

利用传输矩阵法,讨论了由单负材料组成的对称型一维光子晶体的偏振特性。结果表明:入射角较小时,TE、TM两波隧穿模的中心位置基本相同。入射角θ增加,两隧穿模均向短波方向移动,且其半峰全宽变窄。入射角θ>20°时,两隧穿模的蓝移量增加,且TM波的移动量大于TE的,入射角越大,这一变化越明显。周期数N增加时,两隧穿模的位置保持不变,半峰全宽变窄;TM波隧穿模的透射率保持为1,而TE波的有所下降。介质的几何厚度增加时,两隧穿模均向长波方向移动,隧穿模的透射率保持不变。两介质几何厚度的变化量相同,两隧穿模的移动量也分别相同。     

周期不对称度对光子晶体透射谱特性的影响研究

利用传输矩阵法,研究周期不对称度对光子晶体透射谱的影响,结果表明:当周期不对称度为零时,光子晶体透射峰随着周期数的增大而变得越精细,但其透射率均为100%不变;当周期不对称度为不等于零的恒定值时,光子晶体透射峰的透射率低于100%,但随着周期数的增大,透射峰仅变得精细而透射率不变;光子晶体透射峰的透射率随着周期不对称度增大而下降,而且不对称度越大,透射率下降越快,同时透射峰变窄的速度也越快。周期不对称度对光子晶体透射谱的影响规律,为光子晶体模型的构造和设计制备等提供方法依据。     

含左手材料的一维准周期光子晶体的透射特性研究

利用传输矩阵法,计算了含左手材料的一维准周期光子晶体的透射谱,并分析了其变化规律.结果表明:nR/ηL的比值对其透射谱影响较大,当比值为1时,透射率T恒为1,与N和△dR、△dL均无关;而当nR/ηL的比值在1.7～2.0之间、N=10时,透射谱的通带呈尖峰状,且通带中心在偶数倍频处有红移,通带峰值不变;而当nR/ηL的比值大于2.0时,在高频处的通带峰值变小,两介质的递增厚度△dR、△dL越大,nR越大,这种变化越明显,改变周期数N,对透射谱有较大的影响.     

异质双周期结构光子晶体光学特性

利用传输矩阵法研究了异质双周期结构一维光子晶体的带隙结构和光学传输特性。此光子晶体能形成512~960 nm的含5个极窄透射峰的光子带隙。重点研究了一种材料为复折射率时的光学传输特性,以及当结构发生轴向应变时的"介观压光效应"。结果表明:当复折射率虚部为负时,透射峰随虚部绝对值的增大先增大后减小,甚至出现衰减;虚部为正时,发生吸收现象。当光子晶体发生轴向应变时,透射峰随应变量的增加呈现近似L形变化,应变量很小时透射率几乎线性减小,波长越大透射峰变化越缓慢。     

AIN/Si3N4 纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN,Si3N4薄膜和不同调制周期的AlN/S3N4纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si3N4层的晶体结构和多层膜的硬度依赖于Si3N4层的厚度.当AlN层厚度为4.0 nm、Si3N4层厚度为0.4nm时,AlN和Si3N4层共格外延生长,多层膜形成穿过若干个调制周期的柱状晶结构,产生硬度升高的超硬效应.随着Si3N4层厚的增加,Si3N4层逐步形成非晶并阻断了多层膜的共格外延生长,多层膜的硬度迅速降低,超硬效应消失.采用材料热力学和弹性力学计算了Si3N4层由晶态向非晶转变的临界厚度.探讨了AlN/Si3N4纳米多层膜出现超硬效应的机理.     

一维增透亚波长光栅的研究

将具有高透射性的亚波长光栅置于微机械波长可调谐垂直腔面发射激光器(VCSEL)的内腔当中可以提高波长的调谐范围,为了使波长调谐范围达到最优则必须优化高透射性的亚波长光栅使其透射率达到最大。利用严格耦合波法分析了亚波长光栅的占空比、周期、厚度和入射角对其透射率的影响并找出最优的光栅参数。通过计算分析可得,对于TE和TM偏振存在最佳的占空比使其透射率达到99.5%。在文中条件下,它们对应的占空比分别为0.23和0.80。而光栅厚度对于TE和TM偏振透射率的影响是周期性的,在一个周期内存在一个最佳值使其透射率达到最高。在文中条件下,TE偏振的厚度周期是150 nm,TM偏振的厚度周期是300 nm。当光栅参数不变时,无论是TE还是TM偏振光,它们的透射率只有在垂直入射光栅时(入射角为0°)才能达到最大。而通过等效介质原理可以得出,周期对透射率没有影响。最后计算了透射率在光栅厚度和占空比同时变化时的变化趋势,并从中得出最优的光栅参数。     

一维光子晶体带隙结构研究

在考虑介质色散的基础上,研究了介质层厚度对光子晶体带隙结构的影响.利用传输矩阵法,计算了以LiF和Si两种材料组成的一维光子晶体带隙结构.结果表明,介质层厚度的增加会引起禁带的红移,厚度减小会引起蓝移.分析了含空气缺陷层、金属缺陷层的光子晶体结构,发现空气缺陷层对带隙结构的高反射区域变化不大,而在低反射区域,反射系数为零的波带之间出现了两边反射系数增加,中间反射系数减小的情况.在金属缺陷层的带隙结构中,金属对整个波长范围光的吸收作用不同,金属对低反射区1.6 μm、1.85 μm处透射率较大的透射光吸收作用明显,而在1.28~1.38 μm处透射率波长区间,几乎无吸收.     

激活材料对镜像对称一维光子晶体透射谱的影响

利用传输矩阵法理论,研究了激活杂质材料对镜像对称结构一维光子晶体（ABCB）m（BCBA）m的透射谱的影响。结果表明：当介质无掺入激活杂质,即介质的介电常量为一实数时,禁带中心出现一条透射率为100%的透射峰;当在镜像对称结构的任一介质中掺入激活杂质,即介质的介电常数为一带有负虚部的复数时,透射峰的透射率均出现增益现象,且随着各介质复介电虚部大小的增大,透射增益出现一极大值后又减小;各介质复介电虚部大小的变化对光子晶体透射谱的影响存在各异,其中介质A掺入激活杂质时增益幅度最大,其次为C,B相对较小,但透射率的增益对B介质的复介电虚部响应最为灵敏,其次为C,A相对较为迟钝。这些特性对新型光学器件的设计具有指导意义。     

Self-organization of five species in a cyclic competition game

Cyclic competition game models, particularly the “rock–paper–scissors” model, play important roles in exploring the problem of multi-species coexistence in spatially ecological systems. We propose an extended “rock–paper–scissors” game to model cyclic interactions among five species, and find that two of the five can coexistent when biodiversity disappears, which is different from the “rock–paper–scissors” game. As the number of fingers is five, we named the new model the “fingers” game, where the thumb, forefinger, middle finger, ring finger, and little finger cyclically dominate their subsequent species and are dominated by their former species. We investigate the “fingers” model in two ways: direct simulations and nonlinear partial differential equations. An important finding is that the number of species in a cyclic competition game has an influence on the emergence of biodiversity. To be specific, the “rock–paper–scissors” model is in favor of maintaining biodiversity in comparison with the “fingers” model when the variables (population size, reproduction rate, selection rate, and migration rate) are the same. It is also shown that the mobility and reproduction rate can promote or jeopardize biodiversity.     

Contrastive analysis on tone between Putonghua and Taiwan Mandarin

The differences of the pitch and duration of Chinese syllables between Putonghua （PTH） and Taiwan Mandarin （TM） were studied. The speech materials to be used are not only isolated syllables, but also sentences. The results reveal that： For the isolated syllables, T1 and T2 in TM are influenced by Minnan dialect, therefore their pitch are lower than those in PTH. T3 is fall-rise in PTH, while it is fall in TM. Moreover, the syllable duration sequence for different tone is T3〉T2〉T1〉T4 in PTH, while it is T1〉T2〉T3〉T4 in TM. For the syllables in sentences, T2 is mid-rise in PTH, while it is mid-level in TM. And the T3 is longer than T4 but shorter than T1 or T2 in PTH, while it is the shortest in TM. Furthermore the effects of prosodic phrase boundary on duration for different tones are almost the same in PTH, but the lengthening part of T1 or T2 is longer than that of T3 or T4 in TM.     

Experimental study on downstep in Chinese intonation

Downstep in pitch contour of Chinese Putonghua is examined using subtly designed sentences by controlling tone combinations. The results show both automatic and nonautomatic downstep phenomena exist in Chinese. In non-automatic downstep, low tones compress downwards the pitch range of the following syllables. and the main influence of downstep is on topline. Low tone not only lower the topline behind it, but also raise the high tones before it, the effects are compatible with each other. In automatic downstep, the topline of pitch contour in intonational phrase is presented as a linear downtrend, but it differs among speakers due to the effect of personal stress practice. In comparison with downstep phenomenon in other tone or non-tone languages, the downstep ratio in Chinese is not constant, and the domain of downstep is not limited within the adjacent tones.     

Abelian-Higgs phase of SU(2) QCD and glueball energy

It is shown that SU(2) QCD admits an dual Abelian-Higgs phase, with a Higgs vacuum of a type-Ⅱ superconductor. This is done by using a connection decomposition for the gluon field and the random-direction approximation. Using a bag picture with soft wall, we presented a calculational procedure for the glueball energy based on the recent proof for wall-vortices [Nucl. Phys. B 741(2006)1].     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Signal generation in an isotropic medium in scanning electron acoustic microscope

Based on the research in Ref. [5][Materials Science and Engineering, 1989; A122： 57 63], an improved model of heat source is set up, the different modes of Lamb wave in an isotropic sample generated by a chopped electron beam at frequency f are obtained with integral transform and normal function expansion method, and the output signal of PZT coupled at the back surface of the sample is found out. The generation mechanism of SEAM （Scanning Electron Acoustic Microscopy） signal is discussed. It shows that the SEAM is a near field imaging technique with high spatial resolution and its best lateral spatial resolution is about 2√2α （α is the radius of the focused electron beam）. Some of experimental results of SEAM images are presented in the paper and it shows that the spatial resolution of SEAM is better than 0.5 μm and smaller than the thermal diffusion length of the sample. Therefore the character of near field imaging in SEAM is also proved experimentally.     

High efficiency filtering using two-dimensional photonic crystal coupled cavity structures

Transmission spectra of coupled cavity structures (CCSs) in two-dimensional (2D) photonic crystals (PCs) are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain (FDTD) method, and the results show that within a very short coupling distance of about 3λ, where λ is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.