Advanced quantum dot and photonic crystal technologies for integrated nanophotonic circuits

Two types of nanophotonic technologies—two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)—were developed for key photonic device structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach–Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip–flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements.     

Analysis and Optimization of Compact Demultiplexer Monitor Based on Photonic-Crystal Waveguide

A wavelength selective lateral outcoupler based on multimode photonic-crystal waveguides (WGs) is presented. Its fundamental advantages are a small footprint, an inherently flexible and versatile design, and a natural WG integration. Devices can serve demultiplexing or monitoring purposes. We numerically discuss design and optimization issues and corresponding details of experimental investigations.     

A model for the performance analysis and design of waveguide p-i-n photodetectors

A model to investigate the frequency response of waveguide (WG) p-i-n photodetectors (PDs) is presented. The model is based on the 2-D position-dependent distribution of carriers in the device and the results show that the contribution to the total current mainly comes from a small length of the PD measured from its input end. The effect of carrier trapping at a heterointerface has also been considered to study the frequency dependence of the photocurrent at low-bias voltages. The frequency response and bandwidth obtained from the model are in good agreement with published experimental results. A simplified and approximate relation for the fiber-to-WG coupling efficiency has also been used to calculate the overall quantum-efficiency of WGPDs. Then, the effect of WG geometry on the bandwidth-quantum efficiency product has been analyzed and some results on the optimum design of a WG p-i-n PD for maximum bandwidth-quantum efficiency product have been tabulated.     

Second-harmonic generation from CuInSe2 thin films:influence of the substrate-epilayer lattice mismatch

We investigate the second-order nonlinear optical (NLO) properties of tetragonal chalcopyrite-structure CuInSe₂ thin films using optical second-harmonic generation at a fundamental wavelength of 790 nm. An approximate doubling of the second-order optical nonlinearity of the CuInSe₂ thin films is achieved through changing the substrate from GaAs(001) to In_0.29Ga_0.71As(001), thereby reducing the lattice mismatch from 2.2% to 0.2%. This observation suggests that minimizing the lattice mismatch is a key requirement for the growth of high-quality NLO active chalcopyrite structure thin films and is in contrast to some other NLO active thin-film systems     

光学高分子薄膜纳米结构与性能关系的探讨

应用AFM及力曲线的统计方法(forcecurvemethod)和Tapping/Phase功能,比较系统地研究了Glass/ITO基底上旋涂厚度为5μm的非线性光学功能高分子薄膜。结果发现,不同基底对于薄膜结构及分子构象没有明显影响。客体分子(molecule-2)在常温条件下能够以氢键的方式镶嵌在主体高分子(molecule-1)链上的—OH官能团周围和相邻的分子链间,形成特征的纳米级带状结构。ITO基底上非线性光学功能高分子薄膜在160～180℃条件下和经3000V强电场极化后,客体分子和主体高分子不同程度地进行了化学交联反应,生成新的非线性光学高分子;薄膜的微观结构由常温时的带状结构变为纳米环状结构,膜表面呈现出均匀平滑的表面形貌,并获得了理想的电光性能信号。分子键合类型的变化使得薄膜的组分发生了变化,并引起了薄膜的微观结构的改变。力曲线的统计方法和Phase的相关信息为此提供了重要证据。     

Direct Attach of Photonic Components on Substrates With Optical Interconnects

Direct attach of lasers and photodiodes on boards with optical interconnects can facilitate high-density packaging of high-speed optical components. For the technology demonstration, test chips are assembled by means of optical polymer pillars on substrates with embedded arrays of waveguides (WGs) and 45deg mirrors. The light couples vertically though the optical pillars enabling 1.5-dB reduction of the WG-to-chip coupling loss to 0.5dB. The insertion loss of the module tested is less than 2 dB and 10-Gb/s signal transmission through the module is demonstrated with bit-error rate <10^-12. Three-dimensional finite-element analysis provides results on the stress distribution in the displaced pillars of different shapes     

Polymer‐Based Devices for Optical Communications

Polymers are emerging as new alternative materials for optical communication devices. We developed two types of polymer‐based devices for optical communications. One type is for ultra high‐speed signal processing that uses nonlinear optical (NLO) polymers in such devices as electro ‐optic (EO) Mach‐Zëhnder (MZ) modulators and EO 2×2 switches. The other is for WDM optical communications that use low‐loss optical polymers in such devices as 1×2, 2×2, 4‐arrayed 2×2 digital optical switches (DOSs) and 16×16 arrayed waveguide grating (AWG) routers. For these devices, we synthesized a polyetherimide‐disperse red 1 (PEI‐DR1) side chain NLO polymer and a low‐loss optical polymer known as fluorinated polyaryleneethers (FPAE). This paper presents the details of our development of these polymeric photonic devices considering all aspects from materials to packaging.     

Lattice-Matched GaN–InAlN Waveguides at λ = 1.55 μm Grown by Metal–Organic Vapor Phase Epitaxy

We report on the demonstration of low-loss, single-mode GaN-InAlN ridge waveguides (WGs) at fiber-optics telecommunication wavelengths. The structure grown by metal-organic vapor phase epitaxy contains AlInN cladding layers lattice-matched to GaN. For slab-like WGs propagation losses are below 3 dB/mm and independent of light polarization. For 2.6-mum-wide WGs the propagation losses in the 1.5- to 1.58-mum spectral region are as low as 1.8 and 4.9 dB/mm for transverse-electric- and transverse-magnetic-polarization, respectively. The losses are attributed to the sidewall roughness and can be further reduced by the optimization of the etching process.     

Incoherent Two-Dimensional Array Modulation Transceiver for Photonic CDMA

This paper proposes a novel incoherent optical code-division multiple-access (OCDMA) transceiver design, as a promising network access employing a newly introduced prime code family named as double-padded modified prime code as spreading sequences, based on the 2-D optical modulation scheme deploying frequency and polarization shift keying. Hereafter, it is referred to as F-PolSK. The novel F-PolSK-OCDMA system has been accurately analyzed taking into account the presence of 1) optical amplified spontaneous emission noise; 2) electronic receiver noise; 3) photo-diode shot-noise; and 4) multiple-access interference (MAI). The application of the optical tapped-delay lines at the receiver as a CDMA-decoder has also been investigated. The evaluation of 2-D array modulation in conjunction with OCDMA indicates that the binary combination of this hybrid scheme enhances the overall system performance. The results reveal that the proposed architecture can easily accommodate greater number of users and consume less power as compared with previous schemes. On the other hand, in polarization modulation since the optical beam power is constant, the system has immunity against the self- and cross-phase modulations. Besides, the system security is much enhanced due to 2-D advanced modulation in optical domain.     

Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

After a brief review on electro‐optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single‐ and multiple‐dendron‐modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three‐fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site‐isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r₃₃ = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double‐end functionalization of a three‐dimensional phenyl‐tetracyanobutadienyl (Ph‐TCBD)‐containing NLO chromophore with thermally crosslinkable trifluorovinylether‐containing dendrons.     

1D Van der Waals Polymers with Nonlinear Optical Performance Approaching Theoretical Upper Limit

Nonlinear optical (NLO) materials are of great importance for applications in lasers, atomic clocks, free-space communication, etc. Herein, inspired by the recent prediction of excellent second harmonic generation (SHG) performance in van der Waals (vdW) materials with 1D building blocks, 14 new NLO materials are found from 244 bulk crystals constructed with 1D polymers using high-throughput first-principles calculations. Nearly half of the new NLO materials exhibit superior NLO performance with SHG susceptibilities approaching the theoretical upper limit. The 2D form of 11 candidates inherits the NLO property covering UV, visible, and infrared regions. Bader charge analysis reveals that the SHG susceptibility is determined by the charge difference of ions on the chains. Finally, it is proposed that superior NLO materials can be found in materials with proper bandgaps and large charge differences on the chains. This work not only screens out candidates with outstanding NLO performance in vdW materials with 1D building blocks but also provides a guideline for the search and design of NLO vdW 1D polymer patterns with excellent NLO properties.     

Metal-induced efficient enhancement of nonlinear optical response in conjugated azo-based iminopyridine complexes

The nonlinear optical (NLO) properties of conjugated azo-based iminopyridine complexes with zinc and silver metal cations were studied. The processes of second and third harmonic generations in guest-host polymeric films were investigated and NLO parameters were extracted. Obtained second and third order NLO susceptibilities of zinc containing complex exceeds the latter of silver containing one. Using the Z-scan technique the NLO refractive index, NLO absorption coefficient, second order hyperpolarizability and NLO absorption cross section for the azo-based iminopyridine zinc (II) and silver (I) complexes were obtained and analyzed. Estimated nonlinearity/loss figure of merit of these complexes show promise for optical device applications.     

Coupling properties in a 2-D photonic crystal slab directional coupler with a triangular lattice of air holes

Two-dimensional (2-D) photonic crystal (PC) directional couplers (DCs) that have a triangular lattice pattern of air holes in a planar dielectric slab are theoretically and experimentally analyzed. Unlike the 2-D PC DC structure with a dielectric rod in air, which is frequently used in theoretical studies, more practical PC DCs tend to be multimode in nature and exhibit a large group velocity dispersion, thus creating decoupling points in the dispersion relation without any additional modifications to the structure. The multimode nature and large dispersion lead to interference which degrades the coupling properties. By inserting three rows of air holes between neighboring line-defect waveguides in order to separate them, we have successfully reduced the multimode region and obtained a single-mode region. In this case, the large dispersion allows the creation of a PC DC with wavelength selectivity and a coupling length as short as 30 a,/spl sim/10 /spl mu/m for a=345 nm, where a is the lattice constant. The transmission spectra obtained experimentally showed good agreement with the theory whereas their transmission ranges were restricted to those of bent waveguides. These results are encouraging for practical application to optical communications.     

金属团簇化合物的结构及光限幅特性

简单回顾了非线性光学材料研究的发展历程。详细地介绍了金属团簇化合物的基本结构、非线性光学特性及光限幅特性。     

二维有机-无机杂化钙钛矿非线性光学研究进展（特邀）

自从钙钛矿材料问世以来,有机-无机杂化钙钛矿材料在近数十年的时间得到了蓬勃发展。二维（2D）有机-无机杂化钙钛矿是由典型的无机八面体框架以及不同的有机阳离子构成,其具有本征的量子阱结构和有趣的光电特性,也因此在发光、传感器、调谐、光伏体系以及通讯设备等领域引起了人们的密切关注。低成本、可溶液法制备、以及可替换的有机阳离子等特性使二维杂化钙钛矿在光学和光子应用中具有灵活的层间距,层数和可变的晶格扭曲,从而实现可调节的框架以及在光学和光子学应用中的高调节度。尤其地,它们也表现出突出的二阶、三阶和高阶非线性光学特性,例如在激光脉冲激发下的二次谐波产生（SHG）、太赫兹 、双光子吸收（2PA）和饱和吸收（SA）和三光子吸收（3PA）等。讨论了具有不同结构特点的二维杂化钙钛矿的构建,并重点介绍了这些二维杂化钙钛矿在线性和非线性光学领域地特性和应用。最后,对二维杂化钙钛矿的研究现状进行了评估,并对其未来发展进行了展望。     

A New Family of 2-D Codes for Fiber-Optic CDMA Systems With and Without the Chip-Synchronous Assumption

In this paper, we propose a new family of wavelength-time codes, which are based on one-dimensional optical orthogonal codes (1-D OOCs) of cross-correlation functions of at most two. By relaxing the maximum cross-correlation values to two, the new 2-D codes provide larger code cardinality for accommodating more subscribers and support heavier code weight for better code performance. It is known that the traditional chip-synchronous assumption used in the analyses of optical codes gives a pessimistic performance upper bound, while the newer chip-asynchronous assumption offers a more accurate analysis. The performance of the new 2-D codes is here analyzed under both assumptions for comparison. Under certain conditions, our results show that the new wavelength-time codes outperform our recently reported multiple-wavelength OOCs and 2-D codes, which were based on 1-D OOCs of cross-correlation functions of at most one and two, respectively.      

Fiber Fault Monitoring for Passive Optical Networks Using Hybrid 1-D/2-D Coding

We propose a novel, simple, and cost-effective technique to decrease beat noise in standard one-dimensional (1-D) optical coding passive optical networking (PON) monitoring. This scheme places low-cost 1-D encoders at the termination of the network branches and two-dimensional (2-D) decoders at the central office mixing the 1-D and 2-D schemes in a complementary way. Simulation shows that our proposed scheme allows the monitoring of a 64 customer PON with a signal-to-noise ratio of 9.1 dB.      

Optical scanning holography

This paper provides a tutorial on the principles of holography, followed by a review of a newly developed 3-D imaging technique in which 3-D optical information of an object cam be extracted by a 2-D optical scan of the object. The technique is called optical scanning holography (OSH). In the context of optical scanning holography, we discuss some of its potential applications such as holographic information transmission and television (TV) system, 3-D image coding and decoding, 3-D microscopy, and 3-D optical remote sensing. In most cases, feasibility has been proven but awaits broader application     

Designing Non‐Centrosymmetric Molecular Crystals: Optimal Packing May Be Just One Carbon Away

Molecular nonlinear optical (NLO) crystals feature important advantages compared to inorganic counterparts, such as low dielectric constants, ultrafast response times, and large electro‐optic coefficients. Conjugated push–pull chromophores connecting electron‐donating with accepting groups are often employed in the design of these crystals. However, associated large molecular dipole moments induce antiparallel or centrosymmetric conformations in the solid‐state, which leads to NLO inactivity. The cation–anion hydrogen bond interactions of a hydroxy‐piperidino electron donor group are combined with increased van der Waals volume effects induced by an ethyl modification of the electron‐accepting moiety. This produces non‐centrosymmetric packing in the organic salt EHPSI‐4NBS ((E)‐1‐ethyl‐2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐3,3‐dimethyl‐3H‐indol‐1‐ium 4‐nitrobenzenesulfonate). Converting a methyl group into ethyl changes the packing symmetry in the molecular crystal to switch on NLO activity. This behavior is attributed to the increased size of the ethyl group, which pushes apart the van der Waals contacts of the cation that lead to centrosymmetric packing in the methyl derivative. To test the NLO properties of EHPSI‐4NBS, THz generation experiments are performed at 1200 nm pump wavelength. Spectral amplitude similar to DAST ((E)‐4‐(4‐(dimethylamino)styryl)‐1‐methylpyridin‐1‐ium tosylate) crystal is observed with generation profile from 0 to 3.8 THz.     

Sequential Operations of Quantum Dot/Photonic Crystal All-Optical Switch With High Repetitive Frequency Pumping

A photonic crystal (PC)-based symmetric Mach-Zehnder type all-optical switch (PC-SMZ), previously operated by single pump pulse alone, has been operated newly by a multiple-pump pulse train corresponding to a repetition frequency of 40 GHz at pulse energy as low as 10 fJ. The device involves quantum dots (QDs) in two parallel PC arms as optical nonlinear media and functions as a time-differential phase modulator caused by the pump pulse inducing carriers in the QD. Prior to the switch operation, sequential time response of the phase shift for a probe pulse was investigated in detail by changing the power and repetition rate of the pump pulse in the straight PC waveguide configuration. Besides, PC and QD parameters were explored for possibility of 100% on-off switching ratio. As a result, five QD layers, 40-ps QD relaxation-time, 500-mum PC-length and use of as low as 0.05 c PC group-velocity (c; light velocity in vacuum) were found to implement the 100% switching ratio. Since each of these parameters has ever been achieved experimentally, the result will pave a promising way to an ultra-small and ultra-fast integrated all-optical switch.