Plasmonic nanostructure enhanced graphene-based photodetectors

Graphene exhibits electrical and optical properties promising for future applications in ultra-fast photonics[1]. High carrier mobility and Fermi velocity[2, 3] combined with its constant absorption over the visible wavelength range to the near-infrared[4] potentially allow its application for photodetection over a broad wavelength spectrum, operating at high frequencies. However, absorption being 2.3% per monolayer[4], responsiv-ity of these devices is rather low[5, 6]. Here we show that by combining graphene-based photodetectors with metal-nanostructures, plasmonic effects lead to an increased respon-sivity. © 2011 by the Author(s); licensee Accademia Peloritana dei Pericolanti, Messina, Italy.

Optical investigations of the effect of solvent and thermal annealing on the optoelectronic properties of Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) films

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most promising conducting polymers that can be used as transparent electrode or as buffer layer for organic electronic devices. However, when used as an electrode, its conductivity has to be optimized either by the addition of solvents or by post-deposition processing. In this work, we investigate the effect of the addition of the polar solvent dimethylsulfoxide (DMSO) to an aqueous PEDOT:PSS solution on its optical and electrical properties by the implementation of the Drude model for the analysis of the measured pseudo-dielectric function by Spectroscopic Ellipsometry from the near infrared to the visible-far ultraviolet spectral range. The results show that the addition of DMSO increases significantly the film conductivity, which reaches a maximum value at an optimum DMSO concentration as it has confirmed by experimentally measured conductivity values. The post-deposition thermal annealing has been found to have a smaller effect on the film conductivity. © 2013 Elsevier B.V.

Non-destructive optical characterization of phase separation in bulk heterojunction organic photovoltaic cells

The dithiophene donor-acceptor copolymers that are bridged either with carbon (C-PCPDTBT) or silicon atoms (Si-PCPDTBT) belong to a promising family of materials for use in photoactive layers for organic photovoltaic cells (OPVs). In this work, we implement the non-destructive Spectroscopic Ellipsometry technique in the near infrared to the far ultraviolet spectral region in combination with advanced theoretical modeling to investigate the vertical distribution of the C-PCPDTBT and Si-PCPDTBT polymer and fullerene ([6,6]-phenyl C71-butyric acid methyl ester - PC70BM) phases in the blend, as well as the effect of the polymer-to-fullerene ratio on the distribution mechanism. It was found that the C-PCPDTBT:PC70BM blends have donor-enriched top regions and acceptor-enriched bottom regions, whereas the donor and acceptor phases are more homogeneously intermixed in the Si-PCPDTBT:PC70BM blends. We suggest that the chemical incompatibility of the two phases as expressed by the difference in their surface energy, may be a key element in promoting the segregation of the lower surface phase to the top region of the photoactive layer. We found that the increase of the photoactive layer thickness reduces the polymer enrichment at the cathode, producing a more homogeneous phase distribution of donor and acceptor in the bulk that leads to the increase of the OPV efficiency. © 2014 Elsevier B.V.

Normal incidence p-i-n Ge heterojunction photodiodes on Si substrate grown by ultrahigh vacuum chemical vapor deposition

We report on normal incidence p-i-n heterojunction photodiodes operating in the near-infrared region and realized in pure germanium on planar silicon substrate. The diodes were fabricated by ultrahigh vacuum chemical vapor deposition at 600 degrees C without thermal annealing and allowing the integration with standard silicon processes. Due to the 0.14% residual tensile strain generated by the thermal expansion mismatch between Ge and Si, an efficiency enhancement of nearly 3-fold at 1.55 mu m and the absorption edge shifting to longer wavelength of about 40 nm are achieved in the epitaxial Ge films. The diode with a responsivity of 0.23 A/W at 1.55 mu m wavelength and a bulk dark current density of 10 mA/cm(2) is demonstrated. These diodes with high performances and full compatibility with the CMOS processes enable monolithically integrating microphotonics and microelectronics on the same chip.

Role of Bi3+ ions for Er3+ ions efficient 1.54 mu m light emission in Er/Bi codoped SiO2 thin film prepared by sol-gel method

Er/Bi codoped SiO2 thin films were prepared by sol-gel method and spin-on technology with subsequent annealing process. The bismuth silicate crystal phase appeared at low annealing temperature while vanished as annealing temperature exceeded 1000 degrees C, characterized by X-ray diffraction, and Rutherford backscattering measurements well explained the structure change of the films, which was due to the decrease of bismuth concentration. Fine structures of the Er3+-related 1.54 mu m light emission (line width less than 7 nm) at room temperature was observed by photoluminescence (PL) measurement. The PL intensity at 1.54 gm reached maximum at 800 degrees C and decreased dramatically at 1000 degrees C. The PL dependent annealing temperature was studied and suggested a clear link with bismuth silicate phase. Excitation spectrum measurements further reveal the role of Bi3+ ions for Er3+ ions near infrared light emission. Through sol-gel method and thermal treatment, Bi3+ ions can provide a perfect environment for Er3+ ion light emission by forming Er-Bi-Si-O complex. Furthermore, energy transfer from Bi3+ ions to Er3+ ions is evidenced and found to be a more efficient way for Er3+ ions near infrared emission. This makes the Bi3+ ions doped material a promising application for future erbium-doped waveguide amplifier and infrared LED

Preferential orientation growth of AIN thin films on Si (111) substrates by LP-MOCVD

Aluminum nitride (AIN) thin films were deposited on Si (111) substrates by low pressure metalorganic chemical vapor deposition system. The effects of the V/III ratios on the film structure and surface morphology were systematically studied. The chemical states and vibration modes of AIN films were characterized by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer. The optical absorption property of the AIN films, characterized by ultraviolet-visible-near infrared spectrophotometer, exhibited a sharp absorption near the wavelength of 206 mm. The AIN (002) preferential orientation growth was obtained at the V/III ratio of 10,000 and the preferential growth mechanism is presented in this paper according to the thermodynamics and kinetics process of the AIN growth.

Anomalous-circular photogalvanic effect in a GaAs/AlGaAs two-dimensional electron gas

We have studied the circular photogalvanic effect (CPGE) in a GaAs/AlGaAs two-dimensional electron gas excited by near infrared light at room temperature. The anomalous CPGE observed under normal incidence indicates a swirling current which is realized by a radial spin current via the reciprocal spin-Hall effect. The anomalous CPGE exhibits a cubic cosine dependence on the incidence angle, which is discussed in line with the above interpretation.

Simulation of light coupling enhancement and localization of transmission field via subwavelength metallic gratings

Surface plasmons(SPs) generated in nano metallic gratings on medium layer can greatly enhance the transmission field through the metallic gratings. The enhancement effect is achieved from lambda = 500 nm to near-infrared domain. The enhancement rate is about 110 % at the wavelength of about 6 10 nm and about 180 % at lambda = 700 nm and 740 nm where most kinds of thin film solar cells have a high spectral response. These structures should provide a promising way to increase the coupling efficiency of thin film solar cells and optical detectors of different wavelength response.

Molecular beam epitaxy growth and photoluminescence study of room temperature 1.31 mu m (InyGa1-yAs/GaAs1-x Sb-x)/ GaAs bilayer quantum wells

Molecular beam epitaxy (MBE) growth of (InyGa1-yAs/GaAs1-xSbx)/GaAs bilayer quantum well (BQW) structures has been investigated. It is evidenced by photo luminescence (PL) that a strong blue shift of the PL peak energy of 47 meV with increasing PL excitation power from 0.63 to 20 mW was observed, indicating type II band alignment of the BQW. The emission wavelength at room temperature from (InyGa1-yAs/GaAs1-xSbx)/GaAs BQW is longer (above 1.2 μ m) than that from InGaAs/GaAs and GaAsSb/GaAs SQW structures (1.1 μ m range), while the emission efficiency from the BQW structures is comparable to that of the SQW. Through optimizing growth conditions, we have obtained room temperature 1.31 μ m wavelength emission from the (InyGa1-yAs/GaAs1-xSbx)/GaAs BQW. Our results have proved experimentally that the GaAs-based bilayer (InyGa1-yAs/GaAs1-xSbx)/GaAs quantum well is a useful structure for the fabrication of near-infrared wavelength optoelectronic devices. © 2005 Elsevier B.V. All rights reserved.

The research and progress of micro-fabrication technologies of two-dimensional photonic crystal

The novel material of photonic crystal makes it possible to control a photon, and the photonic integration will have breakthrough progress due to the application of photonic crystal. It is based on the photonic crystal device that the photonic crystal integration could be realized. Therefore, we should first investigate photonic crystal devices based on the active and the passive semiconductor materials, which may have great potential application in photonic integration. The most practical and important method to fabricate two-dimensional photonic crystal is the micro-manufacture method. In this paper, we summarize and evaluate the fabrication methods of two-dimensional photonic crystal in near-infrared region, including electron beam lithography, selection of mask, dry etching, and some works of ours. This will be beneficial to the study of the photonic crystal in China.

Optical properties of semiconductor quantum-well material using photonic crystal fabricated by micro-fabrication machine

Two-dimensional photonic crystals in near infrared region were fabricated by using the focused ion beam ( FIB) method and the method of electron-beam lithography (EBL) combined with dry etching. Both methods can fabricate perfect crystals, the method of FIB is simple,the other is more complicated. It is shown that the material with the photonic crystal fabricated by FIB has no fluorescence,on the other hand, the small-lattice photonic crystal made by EBL combined with dry etching can enhance the extraction efficiency two folds, though the photonic crystal has some disorder. The mechanisms of the enhanced-emission and the absence of emission are also discussed.

Fabrication and characterization of two-dimensional photonic crystal on silicon by efficient methods

Two-dimensional photonic crystals working in near infrared region are fabricated into silicon-on-insulator wafer by 248-nm deep UV lithography. We present an efficient way to measure the photonic crystal waveguide's light transmission spectra at given polarization states.

Polarization properties of elliptical core non-hexagonal symmetry polymer photonic crystal fibre

In this paper, polarization properties and propagation characteristics of polymer photonic crystal fibres with elliptical core and non-hexagonal symmetry structure are investigated by using the full vectorial plane wave method. The results how that the birefringence of the fibreis induced by asymmetries of both the cladding and the core. Moreover, by adjusting the non-symmetrical ratio factor of cladding eta from 0.4 to 1 in step 0.1, we find the optimized design parameters f the fibre with high birefringence and limited polarization mode dispersion, operating in a single mode regime at an appropriate wavelength range. The range of wavelength approaches the visible and near-infrared which is consistent with the communication windows of polymer optical fibres.

D−A型有机近红外材料的设计、合成及表征

近年来，随着近红外领域研究的深入，特别是光通讯工业的发展，迫切需要一大批具有良好发光性质的近红外材料。有机材料由于成本低、工艺简单和结构易调控等优点而备受关注。目前，对近红外有机发光材料的研究主要集中在两大类：一是稀土元素配合物；二是有机离子染料。但由于稀土元素的4f-4f跃迁是宇称禁阻的，其分子的激发需通过配体与中心离子的能量转移，发光效率低。同时有机离子染料由于静电相互作用，分子容易因聚集而导致发光萃灭，其器件结构主要采用主客体掺杂技术，存在着较强的主体材料发光和器件发光效率低等缺点。传统有机非离子型材料则不受上述因素的限制，可得到聚集态下高效的发光效率。本论文通过将电子给体与受体共轭连接，设计并合成了系列非离子型的窄带隙有机小分子，研究了它们在近红外电致发光等器件中的应用，主要工作内容和结果如下： (1)在分子内同时引入电子给体和受体，将它们共轭连接，设计并合成了系列D-π-A-π-D型的有机小分子，利用分子内电荷转移，实现近红外区域的吸收和发射。我们选择三苯胺和芴为电子给体，苯并双噻二唑及其衍生物为电子受体，苯、噻吩和吡咯为连接基团，得到的系列化合物的光谱及电化学性质能在很大的范围内进行调节。化合物的吸收光谱可从600nm至1400nm，发射光谱从900nm至1600nm范围内调节。化合物的带宽可从1.19eV减小到0.56eV。由于分子间的强相互作用，化合物在混合溶剂中可自组装成带状结构。同时该系列化合物在隔绝氧气的环境中有良好的光化学稳定性，可应用于电致发光或光伏器件中。 (2)多层器件结构是提高有机电致发光效率的有效方法，但前提是发光材料最好是可真空蒸镀。我们在前部分工作的基础上，合成了系列热稳定的可蒸镀型有机小分子。通过改变受体及给体结构可调节发光波长及效率。利用“掺杂剂/主体材料的思想”制备的电致发光器件，发光波长覆盖700nm到1500nm的范围，最长中心波长为1115nm。发光波长752nm时，器件最高外量子效率为1.12%。基于化合物IV-5制备的非掺杂器件，发光波长为1080nm，外量子效率为0.28%。该效率比基于有机离子染料的器件提高了近10倍。基于化合物IV-7的器件最大发射波长为1220nm，为迄今为止非掺杂有机电致发光器件中的最长发射波长。以上结果证明，该系列分子是良好的近红外电致发光材料。 (3)我们研究了含苯并双噻二唑分子与常见阴离子的相互作用关系，发现其只对CN-离子和F-离子有响应，而对其它阴离子如Cl , Br , I , AcO , H2PO4 , HSO4 , 和NO3 等则没有任何响应。并且通过将其中噻二唑环换成硝基，可实现对CN-离子和F-离子的区分。化合物V-2可高选择性的检测氰离子，其它离子的存在不会干扰检测信号，包括氟离子。检测信号为外观颜色的改变，可作为显色传感器。化合物V-3可定量检测CN-离子，检测极限为1μM。并且检测信号包括近红外发射的萃灭、可见发射的增强和吸收光谱的改变，多重检测信号可增加检测的可靠性。由于响应信号处于近红外区的生物波段，可应用于生物领域

有机近红外电致变色器件

光通讯近年来发展迅速，正在从元件向集成发展，迫切需要利于集成的元件。主要的一些光学元件如激光器、放大器等均采用无机材料，连接和集成存在困难。发展易于集成的有机光学元件是目前该领域研发的一个热点。电致变色是一个古老的题目，一般都在可见区，而光通讯波长主要是1300和1550nm，是近红外区。在近红外区的工作刚刚开始，．用于光衰减器的研究已有报道，但还没有达到实际要求，寻找新型的电致变色材料及其适合这些材料的新的器件构型是目前研究的重点。本文研究了三种新的有机近红外电致变色材料及其器件：PEDOT:PSS、Ru星状化合物和酞警噜。本文通过电化学、吸收光谱、X射线衍射、AFM等实验方法和手段表征了样品的本体性质、近红外电致变色，性质和器件性能。结果如下：本论文首先研究了PEDOT:PSS的近红外电致变色性质。实验结果首次表明通过掺杂如Liclo4的电解质可以提高近红外电致变色器件的性能。通过掺杂，褪色时间从30秒缩短到2.8秒，着色时间从9.2秒缩短到2.7秒，变色效率从105cm2／C提高到3llcm2／C。掺杂后薄膜离子导电率提高是器件性能改善的原因。另外还制备了基于不同厚度的PEDOT：PSS薄膜的器件，发现对于一定掺杂浓度有一个最佳的厚度，在这个厚度下器件有较大的调制能力和较小的透过损失。本论文第二部分研究Ru星状化合物。通过对基于Ru星状化合物溶液的动力学的研究，证明了调制能力与溶液浓度和外加电压的关系。之后研究开发了有新构型的近红外电致变色器件，以Ru（II，II）态化合物溶液代替其交联薄膜和电解质层来形成固液型器件，同时改善了氧化钨的制备方法。将器件对1550nm光的调制能力从文献报道的2dB提高到16dB。本论文还首次报道了酞警噜的近红外电致变色性质，发现它既可以作为阳极电致变色材料也可以作为阴极电致变色材料。酞普噜的溶液和薄膜均可用于制备电致变色器件，器件的响应速度分别为23秒和1秒。但因为中性态酞警噜对1300nm和1550nm的光吸收较小，器件的调制能力也较小（-0.3dB）。 论文的最后部分详细阐述了有机光电器件表征系统的搭建及测量方法，包括电调制光的电致变色器件测试系统、光转换为电的有机光电二极管或太阳能电池等光电器件测试系统和电转换为光的有机电致发光二极管的器件测试系统。本论文的结果证明采用新构型可以提高近红外电致变色器件的性能；通过掺杂可以缩短电致变色器件的响应时间和提高其变色效率；同时还首次报道了酞瞥噜的近红外电致变色性质。