Spontaneous emission in micro- and nano-structures

Spontaneous emission of emitters governing the performance of optoelectronic devices is a fundamental phenomenon, and it has strong environment-dependent characteristics. In this article, we mainly review the experimental and theoretical progresses in the control of spontaneous emission by manipulating optical modes with photonic crystals, optical microcavities and metallic nanostructures. The spontaneous emission from emitters in photonic crystals can be modified by the local density of states, and by employing photonic crystals, the devices’ efficiency is enhanced, the angular radiation pattern can be engineered, and highly efficient optoelectronic devices are achieved through decreasing the radiative lifetime. In quantum optical devices, microcavities would alter the lifetime of an excited state through tuning the resonance in the frequency and positioning between the emitters and cavity field, and inducing the emitters to emit spontaneous photons in a desired direction. The emerging enhanced electromagnetic field near metallic nanostructures can help to control and manipulate the spontaneous emission of an emitter. The use of micro- and nano-structures to manipulate spontaneous emission will open unprecedented opportunities for realizing functional photonic devices.     

Spontaneous emission in micro- and nano-structures

Spontaneous emission of emitters governing the performance of optoelectronic devices is a fundamental phenomenon, and it has strong environment-dependent characteristics. In this article, we mainly review the experimental and theoretical progresses in the control of spontaneous emission by manipulating optical modes with photonic crystals, optical microcavities and metallic nanostructures. The spontaneous emission from emitters in photonic crystals can be modified by the local density of states, and by employing photonic crystals, the devices’ efficiency is enhanced, the angular radiation pattern can be engineered, and highly efficient optoelectronic devices are achieved through decreasing the radiative lifetime. In quantum optical devices, microcavities would alter the lifetime of an excited state through tuning the resonance in the frequency and positioning between the emitters and cavity field, and inducing the emitters to emit spontaneous photons in a desired direction. The emerging enhanced electromagnetic field near metallic nanostructures can help to control and manipulate the spontaneous emission of an emitter. The use of micro- and nano-structures to manipulate spontaneous emission will open unprecedented opportunities for realizing functional photonic devices.     

Mid-infrared luminescence from coupled quantum dots and wells

Coupled nanostructures have been developed in the InAs/InSb/GaSb materials system in order to extend the emission wavelength further into the infrared, beyond 2 μm. The samples studied consist of a single narrow InAs quantum well grown below a layer of InSb quantum dots in a GaSb matrix, in which the coupling has been altered by changing the thickness of a GaSb spacer layer. The overall transition energy of the combined dot–well system is generally reduced with respect to the dots and well only but the dependence on spacer thickness is more complex than that expected from a simple envelope function model.     

Embedded dots inside glass for optical film using UV laser of ultrafast laser pulse

Microstructures are usually fabricated on the surface of optical sheets to improve the optical characteristics. In this study, a new fabrication process with UV (ultraviolet) laser direct writing method is developed to embed microstructures inside the glass. Then the optical properties of glass such as reflection and refraction indexes can be modified. Single- and multi-layer microstructures are designed and embedded inside glass substrate to modify the optical characteristics. Both luminance and uniformity can be controlled with the embedded microstructures. Thus, the glass with inside pattern can be used as a light guide plate to increase optical performance. First, an optical commercial software, FRED, is applied to design the microstructure configuration. Then, UV laser direct writing with output power 2.5-2.6 W, repetition rate 30 kHz, wave length: 355 nm, and pulse duration 15 ns is used to fabricate the microstructures inside the glass. The effect of dot pattern in the glass such as the dot pitch, the layer gap, and the number of layer on the optical performance is discussed. Machining capacity of UV laser ranges from micron to submicrometer; hence with this ultrafast laser pulse, objectives of various dimensions such as dot, line width, and layers can be easily embedded in the glass by one simple process. In addition, the embedded microstructures can be made with less contamination. Finally, the optical performance of the glasses with various configurations is measured using a Spectra Colorometer (Photo Research PR650) and compared with the simulated results.     

通过光致还原调制氧化石墨烯寿命并用于微纳图形制备

氧化石墨烯因其宽带可调谐的荧光发射特性已被广泛应用于荧光成像、金属离子高灵敏检测和光电器件的制备.相比于荧光强度,氧化石墨烯荧光寿命不受材料厚度和激发功率的影响,具有更为稳定和均一的特性.本文研究了在激光还原过程中氧化石墨烯荧光寿命逐渐减小的变化行为,发现了长寿命sp~3杂化结构向短寿命sp~2杂化结构的转变.通过精确控制还原时间,结合激光直写技术,在单层氧化石墨烯薄膜上实现了二维码、条形码、图形和数字等微纳图形的制备,还在多层氧化石墨烯薄膜结构上获得了多寿命多层微纳图形.这种微纳图形的制备具有灵活无掩膜、高对比和多模式的特点,可用于高密度光学存储、信息显示和光电器件制备等诸多领域.     

Light propagation for time-domain fluorescence diffuse optical tomography by convolution using lifetime function

Time-domain light propagation in biological tissue is studied by solving the forward problem for fluorescence diffuse optical tomography using a convolution of the zero-lifetime emission light and the exponential function for a finite lifetime. We firstly formulate the fundamental equations in a time-domain assuming that the fluorescence lifetime is equal to zero, and then the solution including the lifetime is obtained by convolving the emission light and the lifetime function. The model is a two-dimensional (2-D) 10 mm-radius circle with the optical properties simulating biological tissue for the near infrared light, and contains some inclusions with fluorophores. Temporal and spatial profiles of excitation and emission light are calculated and discussed for several models with different inclusions. The results are physically reasonable and will be used for the inverse problem of fluorescence diffuse optical tomography.     

Modulation of light absorption by optical spacer in perovskite solar cells

Lead halide perovskite solar cells with planar heterojunction configuration have recently attracted tremendous attention because of their excellent power conversion efficiencies. The modulation of optical absorption by using an optical spacer layer is a unique method to enhance the device efficiency. Here, we demonstrate the application of thin ZnO layer that act as an optical spacer that enhance the power conversion efficiency perovskite devices from 8.92% to 10.7%, which is mainly due to increment in short‐circuit current density by 16% compared to the reference solar cell. The simulation data revealed that ZnO acts as an optical spacer layer that shifts length (average) of electric field |E|² distribution from 500 nm to 750 nm wavelength is 25 nm in the perovskite layer. Which represents that exciton generation region is moved to near the hole transport layer that enhances the exciton dissociation efficiency and device efficiency.     

Broadband light emitting from multilayer-stacked InAs/GaAs quantum dots

We report the effect of the GaAs spacer layer thickness on the photoluminescence(PL) spectral bandwidth of InAs/GaAs self-assembled quantum dots(QDs).A PL spectral bandwidth of 158 nm is achieved with a five-layer stack of InAs QDs which has a 11-nm thick GaAs spacer layer.We investigate the optical and the structural properties of the multilayer-stacked InAs/GaAs QDs with different GaAs spacer layer thicknesses.The results show that the spacer thickness is a key parameter affecting the multi-stacked InAs/GaAs QDs for wide-spectrum emission.     

Aqueous chemical growth of free standing vertical ZnO nanoprisms,nanorods and nanodiskettes with improved texture co-efficient and tunable size uniformity

Tuning the morphology, size and aspect ratio of free standing ZnO nanostructured arrays by a simple hydrothermal method is reported. Pre-coated ZnO seed layers of two different thicknesses (≈350 nm or 550 nm) were used as substrates to grow ZnO nanostructures for the study. Various parameters such as chemical ambience, pH of the solution, strength of the Zn²⁺ atoms and thickness of seed bed are varied to analyze their effects on the resultant ZnO nanostructures. Vertically oriented hexagonal nanorods, multi-angular nanorods, hexagonal diskette and popcorn-like nanostructures are obtained by altering the experimental parameters. All the produced nanostructures were analysed by X-ray powder diffraction analysis and found to be grown in the (002) orientation of wurtzite ZnO. The texture co-efficient of ZnO layer was improved by combining a thick seed layer with higher cationic strength. Surface morphological studies reveal various nanostructures such as nanorods, diskettes and popcorn-like structures based on various preparation conditions. The optical property of the closest packed nanorods array was recorded by UV-VIS spectrometry, and the band gap value simulated from the results reflect the near characteristic band gap of ZnO. The surface roughness profile taken from the Atomic Force Microscopy reveals a roughness of less than 320 nm.     

Improved fluorescent assay sensitivity using silver island films: Fluorescein isothiocyanate-labeled albumin as an example

We have studied enhancement of the fluorescence of fluorescein isothiocyanate (FITC), bound to albumin and near an annealed silver island film, as a function of the distance between the protein molecules and the metal. As the intermediate spacer layer between the albumin and the silver substrate, we used multilayer films based on polyelectrolytes. The maximum nine-fold enhancement coefficient for the fluorescence of FITC corresponds to a thickness of the intermediate layer of ≈4 nm, or three layers of the polyelectrolyte. In this case, we observe a significant decrease in the average photoluminescence decay time for the label near the silver film compared with a dielectric medium. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 797–800, November–December, 2006.     

周期和准周期超晶格固体薄膜表面吸附原子的共振荧光

本文使用表面修饰的光学Bloch方程,求得周期和准周期超晶格固体薄膜表面吸附原子的共振荧光谱,并研究了压缩效应。对该薄膜各层厚度和介电性质变化所带来的影响进行了讨论。同时,也对整个薄膜不同几何结构的影响进行了比较和分析。 关键词：     

倾斜L形手性结构的制备以及圆二色性

在单层聚苯乙烯小球模板上,制备了大面积倾斜L形手性结构.通过倾斜角沉积技术,在小球的一侧生长二氧化硅,在二氧化硅上面沉积金属层;在垂直方向沉积另一金属层,使两个金属层具有不同高度从而形成倾斜L形手性结构.研究发现,通过控制二氧化硅的厚度,可以实现倾斜L形手性结构的圆二色性的调控.数值模拟结果表明,倾斜L形手性结构的圆二色性机制符合Born-Kuhn模型理论.     

Metal-Enhanced Fluorescence from Plastic Substrates

We report the first findings of Metal-Enhanced Fluorescence (MEF) from modified plastic substrates. In the past several years our laboratories have reported the favorable effects of fluorophores in close proximity to silver nanoparticles. These effects include, enhanced fluorescence intensities, (increased detectability), and reduced lifetimes, (enhanced fluorophore photostability). All of these reports have featured silver nanostructures and fluorophores which have been immobilized onto clean glass or quartz surfaces. In this report we show how plastic surfaces can be modified to obtain surface functionality, which in turn allows for silver deposition and therefore metal-enhanced fluorescence of fluorophores positioned above the silver using a protein spacer. Our findings show that plastic substrates are ideal surfaces for metal-enhanced phenomena, producing similar enhancements as compared to clean glass surfaces. Subsequently, we speculate that plastic substrates for MEF will find common place, as compared to the more expensive and less versatile traditional silica based supports.     

Gearing up for optical microrobotics: micromanipulation and actuation of synthetic microstructures by optical forces

Optics is usually integrated into robotics as part of intelligent vision systems. At the microscale, however, optical forces can cause significant acceleration and so optical trapping and optical manipulation can enable the noncontact actuation of microcomponents. Microbeads are ubiquitous optically actuated structures, from Ashkin's pioneering experiments with polystyrene beads to contemporary functionalized beads for biophotonics. However, micro‐ and nanofabrication technologies are yielding a host of novel synthetic structures that promise alternative functionalities and new exciting applications. Recent works on the actuation of synthetic microstructures using optical trapping and optical manipulation are examined in this review. Extending the optical actuation down to the nanoscale is also presented, which can involve either direct manipulation of nanostructures or structure‐mediated approaches where the nanostructures form part of larger structures that are suitable for interfacing with diffraction‐limited optical fields.     

Confinement of acoustical vibrations in a semiconductor planar phonon cavity

Extending the idea of optical microcavities to sound waves, we propose a phonon cavity consisting of two semiconductor superlattices enclosing a spacer layer. We show that acoustical phonons can be confined in such layered structures when the spacer thickness is an integer multiple of the acoustic half-wavelength at the center of one of the superlattice folded minigaps. We report Raman scattering experiments that, taking profit of an optical microcavity geometry, demonstrate unambiguously the observation of a phonon-cavity confined acoustical vibration in a GaAs/AlAs based structure. The experimental results compare precisely with photoelastic model calculations of the Raman spectra.     

基于DMD微光刻的导光板模板的制作方法

提出了一种基于数字微反射镜(DMD)微光刻的导光模板的制作方法.导光板的网点单元图形由DMD输入,经过缩微光学成像系统缩微后,在光刻胶干板上逐单元网点曝光,再经过显影、微电铸得到导光板模板,在PC薄板材上用微纳米压印制成导光板,厚度仅为0.381 mm.采用自行研制的SVGwriting-DMD激光直写系统,图形的最小分辨率为2 μm,DMD微光刻法无需掩膜版,可实现不同形状、大小、微结构的单元网点图形及网点的排布,便于大幅面的导光板模板的制作.     

Spacer layer thickness fluctuation scattering in a modulation-doped Al_xGa_1-xAs/GaAs/Al_xGa_1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation(SLTF) on the mobility of a twodimensional electron gas(2DEG) in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Study of electrical field distribution of gold-capped nanoparticle for excitation of localized surface plasmon resonance

The specific optical characteristics which can be observed from noble metal nanostructured materials such as nanoparticles and nanoislands have wide variety of applications such as biosensors, solar cells, and optical circuit. Because, these noble metal nanostructures induce the increment of light absorption efficiency by the enhancing effect of electrical field from localized surface plasmon resonance (LSPR) excitation. However, the enhancing effects of electrical field from LSPR using simple structured noble metal nanostructures for several applications are not satisfactory. To realize the more effective light absorption efficiency by the enhancing effect of electrical field, quite different noble metal nanostructures have been desired for applying to several applications using LSPR. In this study, to obtain the more effective enhancing effect of electrical field, conditions for LSPR excitation using a gold-capped nanoparticle layer substrate are computationally analyzed using finite-difference time-domain (FDTD) method. From the previous research, LSPR excitation using such gold-capped nanoparticle layer substrates has a great potential for application to high-sensitive label-free monitoring of biomolecular interactions. For understanding of detailed LSPR excitation mechanism, LSPR excitation conditions were investigated by analyzing the electrical field distribution using simulation software and comparing the results obtained with experimental results. As a result of computational analysis, LSPR excitation was found to depend on the particle alignment, interparticle distance, and excitation wavelength. Furthermore, the LSPR optical characteristics obtained from the simulation analysis were consistent with experimentally approximated LSPR optical characteristics. Using this gold-capped nanoparticle layer substrate, LSPR can be excited easily more than conventional noble metal nanoparticle-based LSPR excitation without noble metal nanoparticle synthesis. Hence, this structure is detectable a small change of refractive index such as biomolecular interactions for biosensing applications.     

弹流油膜彩色光干涉系统的特性研究

彩色光干涉技术是润滑油膜厚度测量的最有效的方法之一,但对其测量光学理论的研究并不十分透彻。以球-盘弹性流体动压润滑油膜测量装置为对象,建立了彩色光干涉的色度学计算模型,对该系统的干涉条纹和色彩进行了数值模拟与分析。分析结果与实验数据的一致性表明了所建立的模型和分析方法的正确。分析表明:铬膜厚度对图像的色彩和对比度有明显的影响,当其厚度为5～10nm时,能够获得对比度和色彩理想的图像;垫层厚度影响各颜色分量强度分布的初相变;LED光源可产生对比度较好的干涉条纹。     

Influence of manganese concentration and annealing temperatures on the physical properties of CdO films grown by the SILAR method

Undoped and doped Cadmium oxide (CdO) films with different Mn concentrations were prepared on glass substrates by the successive ionic layer adsorption and reaction method and annealed at two different temperatures. Structural, morphological and optical conditions were performed by XRD, scanning electron microscope (SEM) and UV–vis spectrum measurements. The deposited films were dense and they adhere well to the glass substrates. From the SEM photographs, it was seen that Mn doping concentration and annealing process affect the morphologies of the nanostructures. It was found that the growth process modifies the microstructures of the CdO films. XRD results and UV–vis analysis reveal that the crystal structure and optical band gap of the films could be adjusted by varying the Mn concentration and annealing temperature. The results also indicate that there is a direct dependence of the physical and optical properties of the metal oxide films on doping concentration and annealing process.