PBG properties of three-component 2D photonic crystals

In this paper we analyze theoretically how the introduction of the third component into the two-dimensional photonic crystal influences the photonic band structure and the density-of-states of the system. We consider the periodic array of cylindrical air rods in a dielectric, and the third medium is introduced as a ring-shaped intermediate layer of thickness d and dielectric constant _i between the air pores and the dielectric background. Using the plane wave method, we have obtained the band structures for the 2D triangular lattice photonic crystals. The dependencies of TE and TM band gaps’ widths and gaps’ edges position on the interlayer dielectric constant and interlayer thickness were analyzed. In the framework of this approach, we have estimated the influence of the surface oxide layer on the band structure of macroporous silicon. We observed the shift of the gaps’ edges to the higher or lower frequencies, depending on the interlayer thickness and dielectric constant. We have shown that the existence of a native oxide surface layer should be taken into consideration to understand the optical properties of 2D photonic crystals, particularly in macroporous silicon structures.     

Elastic wave localization in two-dimensional phononic crystals with one-dimensional random disorder and aperiodicity

The band structures of in-plane elastic waves propagating in two-dimensional phononic crystals with one-dimensional random disorder and aperiodicity are analyzed in this paper. The localization of wave propagation is discussed by introducing the concept of the localization factor, which is calculated by the plane-wave-based transfer-matrix method. By treating the random disorder and aperiodicity as the deviation from the periodicity in a special way, three kinds of aperiodic phononic crystals that have normally distributed random disorder, Thue-Morse and Rudin-Shapiro sequence in one direction and translational symmetry in the other direction are considered and the band structures are characterized using localization factors. Besides, as a special case, we analyze the band gap properties of a periodic planar layered composite containing a periodic array of square inclusions. The transmission coefficients based on eigen-mode matching theory are also calculated and the results show the same behaviors as the localization factor does. In the case of random disorders, the localization degree of the normally distributed random disorder is larger than that of the uniformly distributed random disorder although the eigenstates are both localized no matter what types of random disorders, whereas, for the case of Thue-Morse and Rudin-Shapiro structures, the band structures of Thue-Morse sequence exhibit similarities with the quasi-periodic (Fibonacci) sequence not present in the results of the Rudin-Shapiro sequence.     

Enhanced light trapping for the silver nanoparticles embedded in the silica layer atop the silicon substrate

The optical properties of the structures with silver nanoparticles embedded in the silica layer atop the silicon substrate are simulated by the finite-difference time-domain method. The effects of nanoparticle size, period, silica layer thickness, and the angle of incidence of the illuminated light on optical transmissions are studied. It is found that there is the red-shift for the maximum of the total light transmitting into the silicon substrate as the silica layer thickness increases. The electric field intensity distributions and the average power densities for the structure with largest optical transmission is studied, and the strong electric field intensities are found in the silica regions surrounding to the silver nanoparticles, which can help the light energy going into the silicon substrate. By controlling the structure parameters, the optical transmissions of the structures with the silica layer can have higher optical transmissions than the cases without the silica layer. The silica layer plays the role as the graded refractive index layer between the air and the silicon substrate, and the light power from the incident wave can transmit into the silicon substrate with less optical reflections for choosing a suitable silica layer thickness. A guideline to design the structures with high optical transmissions for the solar spectra is given. This study cannot only be useful for the solar cells applications, but also other antireflection applications.     

Confinement in a planar waveguide with porous silicon omnidirectional mirrors as confining walls

We present the design and study of waveguide structures based on porous silicon where the light confinement is not due to the usual total reflection effect but to the use of photonic crystals (PCs) as confining walls. These PC are omnidirectional mirrors (OMs), consisting of the periodic repetition of two porous silicon layers with different refractive indices and thicknesses. They reflect the radiation for all angles of incidence within a frequency range called the omnidirectional band gap (OBG). We have followed the PC formalism to investigate the properties of the OM as a multimode waveguide: the number of modes within the band gap, their field spatial distribution and their confinement as a function of the frequency and the core thickness.     

Light propagation in polytype Thue–Morse structures made of porous silicon

For the first time, two different sets of polytype Thue–Morse multilayered porous silicon structures are studied to investigate the reflection of light in aperiodic dielectrics. The optical response of the samples was studied before and after oxidation. The results were compared with the classical periodic structure, and an enhancement in the number of photonic bandgaps with a significant blue shift in reflectance peaks, in some of the structures, were observed. Numerical simulation along the lines of the transfer matrix approach is also presented.     

Electrochromic effect in photoluminescent porous silicon–polyaniline hybrid structures

Changes in the intensity of light transmitted through polyaniline films caused by an external voltage are used to control the photoluminescence spectrum of nanoporous silicon. A study of the optical-luminescence properties of hybrid porous silicon-polyaniline structures under the influence of an applied external potential shows that polyaniline films deposited on a porous silicon surface can serve as an optical filter for the photoluminescence of porous silicon with an electrically controlled transmission band.     

Optical filter based on omnidirectional reflectors

Optical filters based on dielectric omnidirectional reflectors are theoretically analyzed both in the frequency domain and in time domain. It is shown that an optical filter can be made by drilling periodic air holes in a dielectric omnidirectional reflector. The filter’s optical properties can be controlled by varying the lattice constant and the radius of air holes without changing the reflector’s thickness. Thus different filters can be easily integrated in one reflector. This kind of filter is expected to be used in optical communication devices and vertical cavity surface emitting lasers. PACS 42.70.Qs; 42.79.Ci; 78.20.Bh     

Luminescence and resonance fluorescence of an adatom near a surface of a superlattice film periodic of quasi-periodic structure

The surface-dressed optical Bloch equations derived from the reservoir theory and Dekker's quantization procedure and the Maxwell equations are used to obtain the spontaneous decay time and frequency shift, the resonance fluorescence spectrum, and squeezing effects of an adatom adsorbed near a surface of a periodic or quasi-periodic superlattice film. The effects of the thickness and dielectric properties of the layers in the film, number of layers, and different geometry structures are discussed, and the results for the two different cases of periodic structure and the quasi-periodic one are compared.     

镶嵌于多孔硅微腔中8-羟基喹啉铝的窄峰发射

利用浸渍法将8羟基喹啉铝(Alq₃)镶嵌到多孔硅微腔中,制备了多孔硅微腔—Alq₃镶嵌膜,研究了多孔硅微腔对镶嵌其中的Alq₃自发发射的微腔效应,观察到了光谱窄化、发光强度增强等现象。镶嵌于多孔硅微腔中的Alq₃荧光光谱的半峰全宽只有15nm,而非微腔样品,即镶嵌于普通的单层多孔硅中Alq₃荧光谱半峰全宽在85nm以上。并且有微腔时Alq₃发光强度比没有微腔时Alq₃发光强度增强一个数量级。随机改变微腔中Bragg反射镜高折射率层的几何厚度可使高反射区展宽,从而更加有效地抑制了多孔硅本身的发光模,使发光色度更纯,但由于峰值透射率减小,导致共振峰强度有所减小。多孔硅微腔有机镶嵌膜有可能成为进一步发展Alq₃在电致发光器件方面应用的一条新途径。     

Filtering properties of defect-bearing periodic and triadic cantor multilayers

Defect-bearing periodic as well as pre-fractal dielectric multilayers can be designed to operate as narrow-band filters. For the same dielectric constituents and the same (or closest to same) number of layers, pre-fractal structures are remarkably better than the periodic ones, both in terms of filter bandwidth and transmittance peak, and are less sensitive to variations in the defect thickness. For structures having about 30 layers, optical losses do not significantly affect the filtering features of both morphologies if the loss tangent of the constituent materials is less than 10⁻⁵.     

Surface wave photonic device based on porous silicon multilayers

Porous silicon is widely studied in the field of photonics due to its interesting optical properties. In this work, we present theoretical and first experimental studies of a new kind of porous silicon photonic device based on optical surface wave. A theoretical analysis of the device is presented using plane-wave approximation. The porous silicon multilayered structures are realized using electrochemical etching of p⁺-type silicon. Morphological and optical characterizations of the realized structures are reported.     

Ellipsometric studies of porous silicon

Multiple-angle-of-incidence (MAI) ellipsometry at 632.8 nm is used to characterize P and P⁺ porous silicon of high porosity. Complex dielectric constants are obtained, from which the porosity can be estimated qualitatively. The properties of the imaginary part of the dielectric constants are studied and the possible causes are discussed. Two dielectric constants, perpendicular and parallel to the interface, respectively, are measured based on a semi-infinite anisotropic model. Ellipsometric studies demonstrate a larger difference between the two dielectric constants in P⁺ porous silicon, but both P and P⁺ samples only show weak anisotropy, i.e., a network-like structure, which tends towards isotropy, is more suitable for porous silicon than a column-like one, which shows strong anisotropy.     

平面偏振光激励的多孔硅的光致发光

研究了以掠入射的平面偏振光激励的多孔硅的光致发光。实验结果显示，光的入射角对多孔硅的发光行为影响不大，然而，以ｚ方向偏振光激励的发光强度明显高于以ｘ方向偏振光激励的发光强度。激励光电场相对于样品表面的不同取向引起光致发光的差异，这反映多孔硅的光学性质是各向异性的，也排除了纯粹的硅量子点的集合作为多孔结构的可能性。     

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：     

Specific features of light propagation in a periodic structure with regular arrangement of defect layers

Specific features of light propagation in a periodic structure with a large number of identical regularly arranged defect layers are considered. Rigorous analytical formulas for the transmittance and reflectance of the structures are derived. It is shown that the presence of periodically arranged inclusions may give rise to new photonic band gaps, with their positions controlled by the dielectric properties and thickness of the defect layers.     

Laser removal of mold growth from paper

Femtosecond laser nano-processing by enhanced light scattered from nanospheres has received much attention. Enhanced scattered near field enables us to ablate nanoholes at nanometer scales below the diffraction limit. In addition, the interference between the scattered far field and the irradiated laser enables us to fabricate spatially controlled periodic surface structures. In this paper, we simulated the time evolution of scattered near field and far field during the free electron excitation in silicon (Si) by femtosecond laser irradiation. The optical property of Si changes from dielectric to metal-like Si due to the increase of the free electron number density excited by femtosecond laser pulse. It is elucidated that the scattered field of Si shifts from Mie scattering to plasmonic scattering during laser irradiation. We achieved the optimal free electron density and laser intensity for precisely controlled periodic surface structures fabrication. We explained the temporal behavior of the scattering near field and far field from the standpoint of dielectric function of the materials.     

溶胶-凝胶法制备SiO2-TiO2平板光波导工艺研究

采用溶胶-凝胶法制备了SiO₂-TiO₂平板光波导,计算了平板光波导通光条件,分析了硅/钛溶胶-凝胶材料的热性能,观测了平板光波导的结构形貌,并测试了其通光损耗。结果表明：经过200℃,30 min干燥处理的凝胶薄膜呈疏松多孔状态,对于非对称平板波导,存在芯层通光截止厚度,而且当SiO₂-TiO₂芯层厚度为0.5 μm时,SiO₂下包层厚度至少有6 μm才能防止1550 nm波长光泄露入单晶硅衬底中。制备的光波导对于1550 nm波长光传输损耗最小值为0.34 dB/cm。     

周期多孔板的面内振动衰减域及其优化

针对Bragg散射型周期多孔板难以实现较低起始频率并维持较宽衰减域的问题,优化设计了一种含菱形孔的周期多孔板。采用有限元法结合周期边界条件,并运用COMSOL对周期多孔板的面内弹性波频散关系进行计算,通过ANSYS模拟有限尺寸周期多孔板的频率响应,将周期多孔板悬吊进行了正弦波激励的振动试验。研究结果表明,含菱形孔的周期多孔板相比于含圆形和六边形孔的周期多孔板具有更宽的衰减域;材料属性对衰减域影响较大,丁晴橡胶和硅橡胶易于获得低频衰减域;孔隙率的增大有利于获得低频宽带的衰减域;增大菱形孔水平夹角能获得较宽的衰减域。对衰减域的形成机理分析发现,含菱形孔的周期多孔板同时具有Bragg散射型和局域共振型声子晶体的特性,表明两种衰减域机理具有内在的联系。优化设计的周期多孔板存在一条5281.76 Hz至8824.30 Hz的完全衰减域,经过至少2个周期,振动即得到较明显衰减。数值和试验得到的衰减区具有较高的一致性。该研究为减振降噪板的开发提供了新的思路,且由于制作过程便捷,在改善建筑声环境中具有潜在的应用前景。      

Light emission from porous silicon single and multiple cavities

Experimental and theoretical techniques are used to examine the effects of microstructuring on the optical properties of multilayer, single and multiple microcavity structures fabricated from porous silicon. Measurements of the reflectivity and photoluminescence spectra of three multilayer samples are presented. The results are modelled using a transfer matrix technique including a negative absorption term to represent the effect of spontaneous emission which gives luminescence. The emitted light is strongly controlled by the optical modes of the structures and very good agreement is observed between theory and experiment.     

Porous silicon layers prepared by electrochemical etching for application in silicon thin film solar cells

In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores.