基于磁激元效应的石墨烯-金属纳米结构近红外吸收研究

石墨烯作为一种单层碳原子二维材料,在可见光和近红外波段吸收率只有2.3%左右,这限制了石墨烯在光电探测、光电调制等领域的应用.本文基于纳米超材料结构的磁激元共振效应,设计了一种金属-绝缘层-金属-石墨烯混合二维浅光栅结构,通过设计混合二维浅光栅结构尺寸来改变石墨烯化学势,实现了石墨烯在近红外波段的吸收增强和调制.利用有限元仿真和等效电路模型,系统地分析了非正入射、结构参数和石墨烯化学势对吸收特性的影响.研究结果表明,混合二维浅光栅结构的磁激元共振效应可以明显提升石墨烯在近红外波段的吸收率,并且对入射角度和极化方向不敏感.在特定结构参数下,混合二维浅光栅结构在1480nm处吸收率达到了85%,其中石墨烯的吸收率为55%,提升了24倍;通过调控石墨烯化学势从0.1eV增大到1.0eV,分别实现了不同结构尺寸下54.8%,50.3%,46.8%的反射率调制深度.     

基于金属光栅实现石墨烯三通道光吸收增强

为了增强单层石墨烯在可见光和近红外波段的吸收效率并实现多通道光吸收.本文利用石墨烯-金属光栅-介质层-金属衬底混合结构在λ_1=0.553μm、λ_2=0.769μm、λ_3=1.130μm三通道上提高了石墨烯吸收效率,石墨烯吸收效率最高可达41%.对3个光吸收增强通道的磁场分布分析可得它们分别源于表面等离子体激元共振、法布里-帕罗干涉腔共振、磁激元共振.经过模拟分析可知,通过调节金属光栅宽度、介质层厚度可以调谐混合结构的共振峰波长和吸收效率,而石墨烯化学势仅能对共振峰λ_3的吸收效率有影响.最后优化结构参数,在最优结构参数下混合结构在3个光吸收增强通道的光吸收效率可达0.97以上,这可以作为超材料吸收器.     

利用窄刻槽金属光栅实现石墨烯双通道吸收增强

构建基底/窄刻槽金属光栅/覆盖层/石墨烯结构,利用金属光栅激发的表面等离子体激元共振和窄光栅刻槽支持的法布里-珀罗共振,在可见光波段实现单层石墨烯的双通道吸收增强,并结合简化模型估算出双吸收通道所在位置.在波长462和768 nm处,石墨烯的光吸收效率分别为35.6%和40.1%,相比石墨烯本征光吸收率的增强均超过15.5倍.进一步研究发现由于短波处吸收增强源于金属光栅的表面等离子体激元共振,其吸收特性受覆盖层厚度、刻槽深度和宽度变化的影响较小;而由于长波处吸收增强源于窄刻槽中的法布里-珀罗共振,因此呈现出良好的角度不敏感吸收特性.     

基于石墨烯纳米带的中红外等离激元调控

石墨烯在中红外和太赫兹波段可以产生表面等离激元,并且通过合理设计还可以对其表面等离激元进行调控。基于此,本文设计了一种共振可调结构。通过在电介质基底上沉积不同宽度的单层石墨烯条带,引入纳米尺度上的不连续性,从而有效控制石墨烯与光的相互作用。使用时域有限差分法对该结构的光谱和电磁场分布进行了理论研究。结果表明:当所设计的结构与入射光耦合时,会出现多个共振增强的吸收峰;改变每个周期内石墨烯条带的数目、带宽和带间距,可以控制共振峰的个数、位置和强度;另外,施加不同的偏置电压可以改变石墨烯带的费米能级,从而实现共振峰位置和强度的动态调控;该结构可以在较宽光谱范围内调控石墨烯等离激元。本文研究为设计中红外波段基于石墨烯的传感、滤波、吸收等器件提供了理论基础。     

基于石墨烯-hBN薄膜多波段光吸收特性调控

在红外波段,石墨烯和六方氮化硼(Hexagonal Boron Nitride,hBN)作为能够激发等离子体激元(Plasmons Polariton)和声子极化激元(Phonon Polariton)很好的二维(Two-Dimensional,2D)材料,在传感、波导和探测领域等将有着极大的应用价值。本文设计一种石墨烯-hBN/银光栅混合异质结构,利用光栅沟槽内激发的磁极化(Magnetic Polariton,MP)效应诱导等离子体-声子极化激元增强和调控混合异质结构的吸收率.混合异质结构的吸收率和电磁场特性基于有限元的计算方法,并研究了结构参数变化对吸收特性的影响。所提出的混合结构可为光学探测等领域提供潜在的参考。     

非对称银纳米双环表面等离激元光谱特性

贵金属纳米材料在入射光激发下能够产生表面等离激元,即金属表面自由电子产生集体振荡。当其振荡频率与入射光频率相同时,发生表面等离激元共振,形成一种特殊的电磁场模式和光谱特性。利用该电磁场模式和光谱特性, 能够调节金属纳米材料的光谱学行为,例如通过改变金属纳米结构的大小、形状以及周围介质介电常数等参数, 在微纳尺度上实现光谱学信号的有效调控。目前,除了具有一定对称性的贵金属纳米材料被大量研究和应用外,非对称纳米结构的表面等离激元光谱特性也受到广泛关注。研究表明,在可见-近红外波段光谱范围内设计表面等离激元光电传感器件的关键问题在于,如何有效地调节其消光谱的共振波长、半峰宽以及峰值强度等主要特征参数。提出一种基于银纳米双环组成的非对称结构,利用时域有限差分方法,在可见-近红外波段内,通过分别改变银纳米双环的尺寸、间距及入射光偏振方向等参数,计算了该纳米结构在不同条件下的消光谱。结果表明,在0.4～3 μm的消光谱内,入射光能够激发产生两个独立的表面等离激元共振峰。通过研究峰值波长处的电场分布图发现,上述共振峰分别对应两种不同的电磁场模式。结果还表明,消光谱内两个独立的共振峰可以通过改变该双环结构的不同参数,被分别地进行调节。其中,可以通过改变该双环结构的半径来有效调节短波长峰的共振波长和半峰宽,同时保持长波长峰的共振波长和半峰宽基本不变。此外,通过改变两环间距或入射光偏振方向,可以分别以不同趋势来调节两个共振峰的峰值强度。在提出的非对称银纳米双环的消光谱中,获得了能够被分别调节的两个表面等离激元共振峰,研究结果能够为可见-近红外波段内基于银纳米材料光电传感器件的开发设计提供理论基础。     

ITO纳米棒结构红外波段消光特性模拟与分析

铟锡氧化物（ITO）作为一种高掺杂的半导体材料，其材料介电常数零点波长位于近红外波段，且其在近红外波段的吸收损耗较小，因此ITO可以成为近红外波段理想的局域表面等离激元共振效应（LSPR）材料。采用时域有限差分法模拟长方体状ITO纳米棒阵列的LSPR效应，通过调整ITO纳米棒的载流子浓度、尺寸、间距以及衬底折射率实现其红外波段LSPR共振峰的有效调节。这对于扩宽ITO纳米结构在红外波段LSPR效应的应用具有重要的研究意义。     

氮掺杂石墨烯纳米结构的等离激元激发（英文）

基于含时密度泛函理论,研究了氮掺杂石墨烯纳米结构的等离激元特性。吡啶型氮掺杂不影响石墨烯纳米结构的等离激元激发特性,而取代型氮掺杂主要基于石墨烯纳米结构对称性的改变和体系中电子密度的增加来影响石墨烯纳米结构的等离激元共振。相对于纯六角石墨烯纳米结构,在低能共振区,取代型氮掺杂六角石墨烯纳米结构的等离激元共振能量发生了红移。相对于纯矩形石墨烯纳米结构,在低能共振区,取代型氮掺杂矩形石墨烯纳米结构沿扶手椅型边界方向激发时,其等离激元共振能量发生了蓝移;沿Z字型边界激发时,其主要的等离激元共振模式受掺杂氮的影响较小。     

提高红外微测辐射热计光谱吸收率的金属光栅结构研究

为提高红外微测辐射热计的光谱吸收率,设计一种基于表面等离激元的金属光栅促吸收结构,研究光栅结构参数对光谱吸收率的影响规律。以金作为光栅材料,利用等离激元的谐振结构,克服金属材料的高反射特性,增强红外微测辐射热计的红外吸收能力。通过改变光栅的结构参数,对等离子共振波长进行调节,提高红外微测辐射计工作波段内的光谱吸收效率。利用有限差分法,分析光栅参数对光谱吸收率的影响机理,研究金属光栅的周期、占空比和高度对光谱吸收率的调控规律。随着光栅周期由2μm逐渐增加到5μm,吸收峰的峰值波长发生明显的红移现象,吸收峰高度呈现出较明显的下降趋势。随着光栅占空比从0.2逐渐增加到0.5,红外吸收峰的峰值波长向短波长移动,吸收峰高度也逐渐增高,但吸收峰宽度逐渐变窄。光栅厚度对吸收峰的峰值影响不大,当厚度达到一定程度后,峰值基本保持不变。峰值波长随厚度增加出现不同程度的减小,当厚度不足100 nm时,峰值波长减小的程度较大,随着厚度的继续增加,下降趋势逐渐变缓,基本维持在10.6μm附近。通过分析光栅结构参数对光谱吸收率的影响机理,对光栅结构参数的进一步优化,大幅提高氧化钒红外微测辐射热计的红外光谱吸收率, 8～14μm的平均吸收率达61.6%,峰值吸收率在99%以上。金属光栅的光谱吸收率促吸收结构研究,对高性能红外微测辐射热计的设计具有重要的指导意义。     

金微纳阵列表面等离激元中红外波段光谱特性

表面等离激元是金属表面自由电子在入射光激发下产生的电子集体振荡行为及其相应的电磁场分布。目前，金微纳颗粒表面等离激元除了在可见光波段内被大量研究和应用外，在中红外波段内也显示出独特的光谱特性，具备设计生产优良传感器的潜力，因而同样备受瞩目。研究表明，在中红外波段内设计表面等离激元传感器的关键问题在于如何有效地调节共振谱的共振波长、峰值吸收率以及半峰宽等主要特征参数。相比于单个微纳颗粒而言，阵列结构由于拥有良好的周期性，从而能够在上述参数的宽光谱调节方面具有独特的优势。基于此，提出一种基于金微纳颗粒组成的阵列结构，利用时域有限差分方法，在4～18 μm波段范围内，通过分别改变该阵列的结构参数，包括颗粒半径、高度、间距及颗粒形状等，系统地研究了该微纳阵列结构在中红外波段对入射光的反射光谱、透射光谱和吸收光谱等特性的影响。研究发现，在8～10 μm光谱内，入射光能够与其所激发的金微纳阵列表面等离激元产生共振效应，表现出明显的共振峰特性。可以通过分别改变上述结构参数来有效调节吸收率谱线共振峰的共振波长、峰值吸收率和半峰宽等主要特征参数。研究结果对中红外光谱内基于金微纳阵列结构传感器的科学研究和实际设计具有独特的理论应用价值。     

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.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.