Double‐Inverse‐Opal Photonic Crystals: The Route to Photonic Bandgap Switching

Photonic crystals with a complete bandgap can stop the propagation of light of a certain frequency in all directions. We introduce double‐inverse‐opal photonic crystals (DIOPCs) as a new kind of optical switch. In the DIOPC, a movable, weakly scattering sphere is embedded within each pore of the inverse‐opal photonic crystal lattice. Switching between a diffusive reflector and a photonic crystal environment is experimentally demonstrated. Theory shows that a complete bandgap can be realized that can be opened or closed by moving the spheres. This functionality opens up new possibilities for the control of light emission and propagation. The close link and interaction between the chemical synthesis and the computational design and analysis underlines the interdisciplinary focus of this report.     

Engineered Light Scattering in Colloidal Photonic Heterocrystals

Photonic heterocrystals are prepared by sandwiching films of self‐assembled opal and force‐assembled Langmuir–Blodgett colloidal crystals. Anomalously strong light scattering in conjunction with low reflectivity is observed with increasing angle of incidence in the spectral range of photonic bandgaps. The occurrence of light scattering at the interface has been assigned to the optical mode mismatch between the two types of photonic crystals. Photonic bandgap‐related mechanisms of trapping the decaying photonic crystal modes at the interface are suggested.     

中红外完全光子带隙Ge反蛋白石三维光子晶体的制备

采用溶剂蒸发对流自组装法将单分散二氧化硅（SiO2）微球组装形成三维有序胶体晶体模板。以锗烷（GeH4）为先驱体气,用等离子增强化学气相沉积法向胶体晶体的空隙中填充高折射率材料Ge。酸洗去除二氧化硅微球,得到Ge反蛋白石三维光子晶体。通过扫描电镜、X射线衍射仪和傅立叶变换显微红外光谱仪对锗反蛋白石的形貌、成分和光学性能进行了表征。结果表明：Ge在SiO2微球空隙内填充致密均匀,得到的锗为多晶态,锗反蛋白石为三维有序多孔结构。锗反蛋白石的测试光谱图有明显的光学反射峰,表现出光子带隙效应。测试的完全光子带隙位于中红外3.4µm处,测试的光学性能与理论计算基本吻合。     

Fabrication of Photonic Microbricks via Crack Engineering of Colloidal Crystals

Evaporation‐induced self‐assembly of colloidal particles is one of the most versatile fabrication routes to obtain large‐area colloidal crystals; however, the formation of uncontrolled “drying cracks” due to gradual solvent evaporation represents a significant challenge of this process. While several methods are reported to minimize crack formation during evaporation‐induced colloidal assembly, here an approach is reported to take advantage of the crack formation as a patterning tool to fabricate microscopic photonic structures with controlled sizes and geometries. This is achieved through a mechanistic understanding of the fracture behavior of three different types of opal structures, namely, direct opals (colloidal crystals with no matrix material), compound opals (colloidal crystals with matrix material), and inverse opals (matrix material templated by a sacrificial colloidal crystal). This work explains why, while direct and inverse opals tend to fracture along the expected {111} planes, the compound opals exhibit a different cracking behavior along the nonclose‐packed {110} planes, which is facilitated by the formation of cleavage‐like fracture surfaces. The discovered principles are utilized to fabricate photonic microbricks by programming the crack initiation at specific locations and by guiding propagation along predefined orientations during the self‐assembly process, resulting in photonic microbricks with controlled sizes and geometries.     

Sacrificial‐Layer Atomic Layer Deposition for Fabrication of Non‐Close‐Packed Inverse‐Opal Photonic Crystals

A method is presented for predicting and precisely controlling the structure of photonic crystals fabricated using sacrificial‐layer atomic layer deposition. This technique provides a reliable method for fabrication of high‐quality non‐close‐packed inverse shell opals with large static tunability and precise structural control. By using a sacrificial layer during opal infiltration, the inverse‐opal pore size can be increased with sub‐nanometer resolution and without distorting the lattice to allow for a high degree of dielectric backfilling and increased optical tunability. For a 10 % sacrificial layer, static tunability of 80 % is predicted for the inverse opal. To illustrate this technique, SiO₂ opal templates were infiltrated using atomic layer deposition of ZnS, Al₂O₃, and TiO₂. Experimentally, a static tunability of over 600 nm, or 58 %, was achieved and is well described by both a geometrical model and a numerical‐simulation algorithm. When extended to materials of higher refractive index, this method will allow the facile fabrication of 3D photonic crystals with optimized photonic bandgaps.     

Photochemically Controlled Photonic Crystals

We have developed photochemically controlled photonic crystals that may be useful in novel recordable and erasable memories and/or display devices. These materials can operate in the UV, visible, or near‐IR spectral regions. Information is recorded and erased by exciting the photonic crystal with ～ 360 nm UV light or ～ 480 nm visible light. The information recorded is read out by measuring the photonic crystal diffraction wavelength. The active element of the device is an azobenzene‐functionalized hydrogel, which contains an embedded crystalline colloidal array. UV excitation forms cis‐azobenzene while visible excitation forms trans‐azobenzene. The more favorable free energy of mixing of cis‐azobenzene causes the hydrogel to swell and to red‐shift the photonic crystal diffraction. We also observe fast nanosecond, microsecond, and millisecond transient dynamics associated with fast heating lattice constant changes, refractive index changes, and thermal relaxations.     

Quantitative analysis of lattice ordering in thin film opal‐based photonic crystals

This work is devoted to the quantitative evaluation of the lattice ordering of opal films. Assembling colloidal crystals in a moving meniscus under random noise agitation produced opal films with generically the same lattice but different disorders. The lattice ordering is quantified by the magnitudes of harmonics in the Fourier transforms of (i) the scanning electron microscopy images to address the in‐plane lattice ordering and (ii) rotation diagrams of the optical transmission to address the regularity of crystal planes. In prepared opals, the strong deviation of the lattice from the face‐centered cubic symmetry is demonstrated. We find uneven lattice responses to changing the growth conditions, e.g., the 30% improvement of the hexagonal lattice ordering in the (111) growth plane accompanied by a ten‐time better ordering of (220) planes as a result of noise agitation. The suggested approach to characterize crystalline quality of the lattice is a general methodology that can be applied to the analysis of other three‐dimensional photonic crystals.     

The Race for the Photonic Chip: Colloidal Crystal Assembly in Silicon Wafers

This paper surveys recent developments in engineering physics approaches and self‐assembly chemistry methodologies for creating 3D photonic crystals and how this has led to in‐wafer patterned colloidal crystals. These materials are comprised of single crystal micrometer scale features of silica colloidal crystals that have controlled thickness, area, and orientation and are embedded within a single crystal silicon wafer. Two processes for growing opal‐patterned chips are described. One is based upon microfluidic and the other spin coating driven self‐assembly of colloidal silica micro‐spheres within a lithographic patterned silicon wafer.     

Tunable Colors in Opals and Inverse Opal Photonic Crystals

Colloidal photonic crystals and materials derived from colloidal crystals can exhibit distinct structural colors that result from incomplete photonic band gaps. Through rational materials design, the colors of such photonic crystals can be tuned reversibly by external physical and chemical stimuli. Such stimuli include solvent and dye infiltration, applied electric or magnetic fields, mechanical deformation, light irradiation, temperature changes, changes in pH, and specific molecular interactions. Reversible color changes result from alterations in lattice spacings, filling fractions, and refractive index of system components. This review article highlights the different systems and mechanisms for achieving tunable color based on opaline materials with close‐packed or non‐close‐packed structural elements and inverse opal photonic crystals. Inorganic and polymeric systems, such as hydrogels, metallopolymers, and elastomers are discussed.     

Photonic bandgap formation and tunability in certainself-organizing systems

We describe the microfabrication and band structure of large scale three-dimensional (3D) photonic bandgap (PBG) materials based on self-organizing templates. The simplest of these templates is an fcc lattice of close-packed, weakly sintered spheres. Other templates include hcp and hexagonal AB₂ self-organizing photonic crystals. These photonic crystals may be converted into PEG materials by partially infiltrating the template with high refractive index semiconductors such as Si, Ge, or GaP and subsequently removing the template. The resulting “inverse opal” structure exhibits both a photonic pseudogap and a complete (3D) PBG in the near visible spectrum, spanning up to 15% of the gap center frequency. The local density of states (LDOS) for photons exhibits considerable variation from point to point in coordinate space and reveals large spectral gaps even in the absence of a PEG in the total density of states. These gaps in the LDOS may lead to novel effects in quantum and nonlinear optics when active atoms or molecules are placed within the PBG material. These effects include anomalous, low threshold nonlinear response, collective atomic switching, and low-threshold all-optical transistor action. When an optically birefringent nematic liquid crystal is infiltrated into the void regions of the “inverse” opal PBG material, the resulting composite material exhibits a completely tunable PBG. In particular, the 3D PBG can be completely opened or closed by applying an electric field which rotates the axis of the nematic molecules relative to the inverse opal backbone     

Air‐Pulse‐Drive Fabrication of Photonic Crystal Films of Colloids with High Spectral Quality

A fast and highly controllable method of fabricating large films of photonic crystals of colloids is reported. A charge‐stabilized colloidal suspension was run in a flat capillary driven by a pressure‐regulated air pulse. The colloidal crystal texture formed in the capillary was a sensitive function of air pressure. Above a critical pressure, the entire capillary was filled with a uniform single‐domain texture whose transmittance spectrum showed a high quality as a photonic crystal, i.e., excellent opacity at a photonic bandgap and high transparency at other wavelengths. The present method is easily applicable to industrial processes for mass production.     

Tunable,Gap‐State Lasing in Switchable Directions for Opal Photonic Crystals

A photonic crystal laser that is tunable throughout the visible in three‐dimensionally switchable directions is demonstrated. This photo‐pumped laser utilizes a dye‐infiltrated, single‐crystal SiO₂ opal having incomplete bandgaps. Our results support a gap‐state‐enhanced distributed feedback mechanism for lasing. Three different types of wavelength tunability are demonstrated, each applicable over a different frequency range and involving either single or multiple bandgaps. The many independent laser cavities that exist in one photonic crystal are demonstrated by simultaneously obtaining lasing in various colors and directions from an opal crystal. The observation of characteristic laser emission lines provides a new spectroscopy for characterizing intra‐gap photonic states, which may be useful for developing the photonic crystal analogues of electronic circuitry.     

混合型Fe3O4@SiO2胶体光子晶体光传输特性的仿真分析

除了通过改变胶体粒子半径及晶格常数来实现对光子带隙的调控方式之外,能否利用不同尺寸粒子的混合精确控制胶体光子晶体的显色行为是人们关注的一个重要科学问题。在分析Fe₃O₄@SiO₂胶体光子晶体的带隙范围与介电常数、电磁波入射角度、晶格常数、颗粒尺寸、SiO₂包覆层厚度的依赖关系的基础上,利用数值仿真手段对不同尺寸粒子混合得到的光子晶体的光传输特性进行了研究。结果发现,两种粒径的磁性胶体粒子按不同质量比混合后,其光子带隙位置始终落在两种粒径胶体粒子各自形成光子晶体的带隙位置之间,且随着大粒径颗粒掺杂比的增加,反射光谱逐渐红移。这一结果证明混色原理对胶体光子晶体仍然是适用的。该结果对研究胶体光子晶体结构色的新型调控方式具有重要的参考价值。     

Superoleophilic and Superhydrophobic Inverse Opals for Oil Sensors

An inverse opal with both superoleophilic (oil contact angle (CA), 5.1° ± 1.2°) and superhydrophobic (water CA, 153.8° ± 1.2°) properties is fabricated using a phenolic resin (PR) as precursor and poly(styrene‐methyl methacrylate‐acrylic acid) (poly(St‐MMA‐AA)) colloidal crystals as templates. The stopband of the inverse opal can shift reversibly upon sorption of oils, whereby the peak position is a linear function of the refractive index of the adsorbed oil, e.g., a variation in refractive index of 0.02 will result in a stopband shift of 26 nm. Therefore, the inverse opals show a high sensitivity and selectivity for different petroleum oils. Moreover, as‐prepared PR inverse opals show excellent oil‐sensing stability in cyclic sorption experiments, which suggests a promising and economical alternative to traditional oil‐sensing materials, and will provide a new approach to in situ petroleum monitoring and detection.     

Mechanochromic and Thermochromic Sensors Based on Graphene Infused Polymer Opals

High quality opal‐like photonic crystals containing graphene are fabricated using evaporation‐driven self‐assembly of soft polymer colloids. A miniscule amount of pristine graphene within a colloidal crystal lattice results in the formation of colloidal crystals with a strong angle‐dependent structural color and a stop band that can be reversibly shifted across the visible spectrum. The crystals can be mechanically deformed or can reversibly change color as a function of their temperature, hence their sensitive mechanochromic and thermochromic response make them attractive candidates for a wide range of visual sensing applications. In particular, it is shown that the crystals are excellent candidates for visual strain sensors or integrated time‐temperature indicators which act over large temperature windows. Given the versatility of these crystals, this method represents a simple, inexpensive, and scalable approach to produce multifunctional graphene infused synthetic opals and opens up exciting applications for novel solution‐processable nanomaterial based photonics.     

大带隙2维正方晶格光子晶体的优化设计

为了研究2维正方晶格光子晶体的完全带隙特性,采用平面波展开方法模拟了两种结构2维光子晶体,在固定光子晶体周期常数a的前提下,研究了2维正方晶格光子晶体的完全禁带随柱半径和折射率的变化规律。结果表明,以空气为背景的锗介质柱组成的光子晶体,随着半径的增大,完全带隙宽度先增大后减小最后消失,填充比为38.3%时,同时增大介质柱的介电常数,在介质柱折射率为4.2处,完全带隙最大,带宽是0.02754(ωa/(2πc));以锗为背景的空气柱组成的光子晶体,光子禁带对应的无量纲频率随半径的增大而增大,填充比为48.3%时,同时增大背景介质的介电常数,出现多个完全带隙,在背景折射率为6.2处,完全禁带最大,带宽为0.02922(ωa/(2πc))。光子晶体带隙的频谱响应也表明了完全带隙的范围。这为大带隙2维正方晶格光子晶体的设计和制备提供了依据。     

Well‐Defined Pillararene‐Based Azobenzene Liquid Crystalline Photoresponsive Materials and Their Thin Films with Photomodulated Surfaces

Photoresponsive materials (PRMs) have long been a hot topic and photo‐modulated smart surface is very appealing. Particularly, liquid crystalline PRMs are able to amplify and stabilize photoinduced orientation thanks to their self‐assembling and ordering characteristics. Herein, the first pillararene‐based azobenzene liquid crystalline PRM with well‐defined structure is presented, which can avoid the usually ill‐defined composition drawback of polymer PRMs and prevent the severe H‐aggregation from suppressing or even completely blocking photoresponse in simple azobenzene derivatives. The pillar[5]arene‐based macrocyclic azobenzenes with variant length spacers show wide temperature range smectic liquid crystalline mesophases and excellent film‐formation property. The tubular pillar[5]arene macrocyclic framework provides sufficient free volume for azobenzene moieties to achieve reversible photoisomerization and photoalignment; thus, their thin films demonstrate excellent light‐triggered modulation of surface free energy, wettability, and even photoalignment‐mediated orientation of an upper layer discotic liquid crystal columnar mesophase. Such pillararene‐based azobenzene liquid crystals represent novel and promising PRMs with extensive fascinating applications.     

ZnO光子晶体的制备及其光子带隙特性研究

利用重力场下的自组装,将单分散的ZnO胶体颗粒悬浮液自组装为三维光子晶体.通过扫描电镜图谱、透射光谱对制得的ZnO光子晶体进行了表征.结果表明,这种方法可得到排列有序的光子晶体,并且改变反应条件可以控制ZnO胶体球的尺寸.制得的样品有较宽的光子禁带,且禁带波长位置随胶体球粒径的减小和前处理温度的降低而蓝移.当入射光角度逐渐增大时,禁带中心位置有规律地向短波长方向移动.     

New Encryption Strategy of Photonic Crystals with Bilayer Inverse Heterostructure Guided from Transparency Response

Combining functional response materials into colloidal photonic crystals is an accepted encryption strategy for information security. Here, bilayer inverse heterostructure photonic crystals that enable instantaneously transparentizing of the top layer and simultaneously releasing the reflected light of the bottom layer when exposed to ethanol are reported. The transition can quickly return to its original state after the evaporation of ethanol. In addition, the bilayer film is responsive to water, which shows redshift of the bandgap position. The mechanism of the design involves optical scattering and diffraction in the fabricated periodic nanostructures and uses the infiltration and capillary evaporation of fluids with low surface tension to realize the spectral diversity of reflectance. The effects of scattering and color superposition of the upper layer can be obliterated and re‐established for the fact of the infiltration and capillary evaporation of fluids with low surface tension; meanwhile, it provisionally displays the pattern of the bottom layer. Multiple reversible ways to hide and display information could be easily realized by these characteristics. Reconfigurable bilayer inverse heterostructure photonic crystals simultaneously provide a simple and sensitive optical technique for investigating the intriguing encryption effects at the nanoscale.     

垂直沉积法制备高质量胶体光子晶体

利用乳液聚合方法制备了粒径约为262 nm的单分散聚苯乙烯(PS)微球。通过控制溶剂蒸发温度和液体表面下降的速度,用垂直沉积法较快速地制备出了在较大范围呈现很好有序性的密排结构聚苯乙烯胶体光子晶体,其在626 nm波长处存在光子带隙。在扫描电子显微镜(SEM)下,观察到该胶体光子晶体是面心立方(fcc)密排结构。实验结果表明,对于粒径为262 nm的聚苯乙烯微球,在温度为55℃,质量分数为0.3%的情况下,当液体表面下降的速度约为每天3 mm时,可以得到高质量的胶体光子晶体。这种高质量的胶体光子晶体可以为利用模板技术制备具有完全带隙的有序孔结构提供较理想的模板。