On dip broadening in transmission spectra of synthetic opals

This paper reports on a comprehensive immersion spectroscopy study of the spectral position and profile of the dip corresponding to the (111) photonic band in the transmission spectra of synthetic opals. The variation in the opal-filler permittivity contrast in the region of the immersion-mediated disappearance of the (111) band has revealed its anomalous abrupt behavior. The width of the (111) photonic band at the L point of the Brillouin zone of the fcc lattice has been calculated as a function of the opal-filler permittivity contrast in the plane wave approximation. The experimentally observed (111) dip width is found to be substantially larger than that obtained in the calculations, which can be due to deviations in the size and dielectric parameters of the a-SiO₂ particles making up synthetic opals.     

Uncoupled photonic band gaps

We theoretically investigated the symmetry properties of the modes in two-dimensional square lattice photonic crystals in order to study phenomena that would enable new frontiers in the applications of photonic crystals. Using group theory, symmetry analysis of the photonic crystals bands has been done. Particular attention was given to the search for the uncoupled B modes that cannot be excited by the external plane wave because they are symmetry forbidden. The existence of the uncoupled modes enabled to define new physics phenomena: uncoupled photonic band gaps. For the frequency ranges inside the uncoupled photonic band gaps, zero transmission is obtained. Therefore, there are two different types of photonic gaps in the photonic crystals: photonic band gaps and uncoupled photonic band gaps. The appearance of uncoupled photonic band gaps in photonic crystals could at least improve the application of the existing photonic materials and structures or even enable the usage of new ones for devices like waveguides, filters, and lasers.     

Acoustic properties of globular photonic crystals based on synthetic opals

Acoustic properties of globular photonic crystals based on synthetic opals and composed of closely packed SiO₂ globules about 200 nm in diameter are theoretically investigated. Dispersion characteristics of the investigated samples are numerically simulated, and the group velocity of acoustic waves and the effective mass of acoustic phonons are found. It is shown that phononic bandgaps in these photonic crystals are within the gigahertz frequency range. The effective mass of the acoustic phonons corresponding to the edges of the bandgaps is found, and a possibility that bound states of acoustic phonon pairs, biphonons, manifest themselves in the light scattering spectra is discussed.     

Effect of stacking faults on the optical properties of inverted opals

Stacking faults appear to be the most common type of defect in inverted opals which are good candidates for photonic crystals with absolute gaps in the visible range of light. In this Letter we present for the first time a systematic study of the effect of stacking faults on the optical properties of self-assembled photonic crystals, by means of large-scale transmittance calculations for macroscopic slabs of inverted opals with randomly distributed stacking faults. We show that frequency gaps, as seen in optical transmission experiments, will in general appear wider in the presence of stacking faults. We attribute the above to Anderson localization of light due to disorder.     

Analysis and optimization of a new photonic crystal filters in near ultraviolet band

Transmission characteristics of one-dimensional (1D) photonic crystals (PCs) heterostructures containing defective are studied using the transfer matrix method. The key is to search the best combination style for different 1D PCs to form heterostructures. It is shown that the non-transmission range over near ultraviolet and visible range can be substantially enlarged and the phenomenon of narrow band PC filter in near ultraviolet can be realized by adjusting the repeat cycle counts of various photonic crystals. The theoretical results on multiple heterostructures containing TiO₂/SiO₂ multilayer films are presented. With a perfect omni-directional and high peak transmission filters for TE modes, this structure opens a promising way to fabricate ultra-narrow band PC filters with wide non-transmission range in near ultraviolet and visible range.     

One-dimensional comparison of Tio2/SiO2 and Si/SiO2 photonic crystals filters for thermophotovoltaic applications in visible and infrared

In this paper, two photonic crystal structures composed of Si/SiO₂ and TiO₂/SiO₂are optimized and compared for potential use as thermophotovoltaic (TPV) optical filters. The performance of the proposed one-dimensional structures comprises eight periods each. The infrared radiation included is in the wavelength range of 300 nm to 900 nm for the visible and from 800 nm to 2000 nm for the infrared. The limiting temperature of the system is 2000 K. The transfer matrix method is used to analyze the behavior of the frequency for a structure comprising a layer of defects. This method permitted to observe the behavior of the peak of light transmission when the period and angle of incidence change. We have subsequently found that the band gaps of the Si/SiO₂structure were wider than those of the TiO₂/SiO₂structure.     

Phonon propagation through photonic crystals—media with spatially modulated acoustic properties

The thermal conductivity κ of photonic crystals differing in degree of optical homogeneity (single crystals of synthetic opals) was measured in the 4.2–300 K temperature range. The thermal conductivity revealed, in addition to the conventional decrease in comparison with solid amorphous SiO₂ characteristic of porous solids, a noticeable decrease for T<20 K, the range wherein the phonon wavelength in amorphous SiO₂ approaches the diameters of the contact areas between the opal spheres. This effect is enhanced in the case of phonon propagation along the SiO₂ sphere chains (six directions in the cubic opal lattice). The propagation of light waves (photons) through a medium with spatially modulated optical properties (photonic crystals) is presently well studied. The propagation of acoustic waves through a medium with spatially modulated acoustic properties (phononic crystals) may also reveal specific effects, which are discussed in this paper; among them are, e.g., the ballistic mode of phonon propagation and waveguide effects.     

非密堆积TiO2空心微球光子晶体的制备与能带分析

利用胶体自组装、高温烧结和HF酸刻蚀技术,制备了SiO₂微球非密堆积面心立方(FCC)结构的胶体晶体,并以此为模板,利用溶胶凝胶方法和NaOH湿法刻蚀技术制备了TiO₂空心微球非密堆积FCC结构光子晶体.利用电子显微镜分析了晶体的结构,用平面波展开法对该结构进行了能带计算与分析.计算结果表明,制备的TiO₂空心微球非密堆积FCC结构光子晶体在低能区的第二、第三能带之间除布里渊区的W点仍保持简并外,其余各点简并都已经消除. 关键词： 空心微球 非密堆积 光子晶体 光子带隙     

Investigation of luminescence properties in SiO2: Tb,Yb upconversion inverse opal

The SiO₂: Tb, Yb inverse opals with photonic band gap at 465 or 543 nm were prepared, and an effect of photonic band gap on upconversion spontaneous emission from Tb³⁺ was investigated. The results show that the photonic band gap has a significant influence on the upconversion emission of the SiO₂: Tb, Yb inverse opals. The upconversion luminescence of the Tb³⁺ ions is suppressed in the inverse opal compared with the luminescence of that of the reference sample.     

Fabrication of silica/zinc oxide core–shell colloidal photonic crystals

We have fabricated three-dimensional ZnO photonic crystals by the self-assembly of SiO₂/ZnO core–shell microspheres of diameter 180–220 nm by means of a vertical deposition method. The organized crystals adopt a uniform close-packed hexagonal structure with long-range order and a high filling fraction. They exhibit strong partial photonic band gaps at 565–688 nm in the transmission spectrum and a broadened band edge emission at 383 nm in the photoluminescence spectrum. For comparison with these ZnO photonic crystals, we have also prepared pure silica crystals of the same dimensions. The photonic band gaps of the former have been found to be shifted to a longer wavelength by about 200 nm compared to those of the latter as a result of the higher refractive index of ZnO. This approach provides an efficient, economical alternative means of obtaining other, more complicated photonic crystals. PACS 82.70.D; 61.46.w; 61.82.Rx; 68.37.Hk     

由介质球构成的三维光子晶体能带结构的平面波研究

采用平面波展开方法计算由介质球构成的面心立方三维光子晶体的能带结构及透射性质.选 用合适的平面波个数研究了SiO₂蛋白石结构光子晶体的能带及透射性质，并采 用转移矩阵 方法计算了电磁波沿［111］方向的传输特性，两种方法得到的结果相符合.还研究了反蛋白 石结构光子晶体的全带隙.最后，研究了壳层介质球构成的面心立方结构光子晶体的能带特 性，发现在高介质球外面包裹适当厚度的低介电常数介质壳层所构成的光子晶体，可以增大 L点相对带隙宽度50％，并证明了其优化内外半径比值约为0．69. 关键词： 光子晶体 光子能带 平面波展开方法 Core-Shell结构     

Multiple Bragg diffraction in low-contrast photonic crystals based on synthetic opals

The phenomenon of multiple Bragg diffraction in low-contrast photonic crystals based on synthetic opals has been studied both experimentally and theoretically. The transmission and reflection spectra of opal films near the K point of the Brillouin zone of the face-centered cubic lattice in s-polarization exhibit the effect of anticrossing of dispersion curves corresponding to the (111) and ([`1]11)(\bar 111) photonic stop bands. The effect of quasi-Brewster suppression of stop bands is clearly pronounced in p-polarization. The experimental data are analyzed using the calculation of the band structure of opal with the inclusion of the polarization of incident light.     

Secondary radiation of synthetic opals loaded by the sodium nitrite ferroelectric

This paper reports on a comparison of transmission, reflection, and secondary radiation spectra of unloaded synthetic opals and samples filled with the sodium nitrite ferroelectric (NaNO₂). The radiation is provided by semiconductor light-emitting diodes operating in the ultraviolet and visible spectral regions. Selective excitation of slow electromagnetic waves in a photonic crystal, an effect observed when the exciting radiation frequency approaches the stop-band edge, is studied.     

Photoluminescence from ZnO-SiO2 opals with different sphere diameters and thicknesses

We systematically investigated the photoluminescence (PL) and transmittance characteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325 nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stop-bands center of 410 nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570 nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals.     

Template-directed self-organized silica beads on square and Penrose-like patterns

Since a decade photonic band gap research is expanding quickly and especially with self-assembled silica beads, called opals. However, a lot of problems are arising in their preparation, in particular the difficulty to produce large frequencies band gaps in common (periodic) silica photonic materials. To solve this problem quasiperiodic opals are known to be good candidates. Actually, the high degree of isotropy and the high homogeneity of Penrose tilings make them a good way to generate complete photonic band gaps. The aim of our group is to elaborate quasiperiodic photonic crystals by directed-assembly (dip-coating) on Penrose-like patterned surfaces (through electron beam lithography). We will briefly present the different elaboration steps of our opals. Thus, the patterning methods and the dip-coating system will be discussed with our recent results in dip-coated photonic crystals.     

二维点缺陷正方光子晶体的微腔结构

通过平面波展开法对由Al₂O₃介质棒在空气背景介质中构成含有点缺陷的二维正方光子晶体微腔结构进行研究,计算得出缺陷态能带以及缺陷态模场分布。缺陷模对应的电磁波波长为470~476nm。对该微腔结构的品质因数的求解,得出缺陷态光谱曲线。在光谱曲线中,随着传输波长的增大,将产生几个峰值,并且在475nm处的波动最为明显,反映出在475nm附近的电磁波段在缺陷处的光强较大。进一步利用全矢量等效折射率法研究该结构缺陷模频率的稳定性,得出等效折射率的变化曲线。从等效折射率变化曲线可以看出,当传输波长达到475nm时,该结构已经达到稳定传输的区域。含缺陷模的二维光子晶体微腔结构在光子晶体发光二极管以及高阈值半导体激光器等方面有着重要的应用价值。     

一维新型阶梯函数光子晶体透射特性

研究了阶梯型折射率n₂₂、n₁₁(阶梯分布高度)的大小、对应的分布厚度、不同入射角以及缺陷模对阶梯函数型光子晶体透射特性的影响.由费马原理给出光在函数光子晶体中的运动方程,再由电磁传播理论给出函数光子晶体的传输矩阵,进一步推导出函数光子晶体的透射率以及电场分布的表达式.研究表明,1)随n₂₂,n₁₁大小或者厚度改变,其禁带变宽;2)随光的入射角增加,其禁带变窄;3)当加入缺陷层时,随着缺陷层介质折射率增加,缺陷模强度减小且位置发生红移;4)在函数光子晶体中,缺陷层前电场分布保持不变,而在缺陷层处以及之后的电场强度都明显增强,这不同于常规光子晶体的电场分布仅在缺陷层处局域增强.     

Optics of globular photonic crystals

Recent experimental and theoretical results on the optical properties of globular photonic crystals coauthored by the author are presented. The dispersion relation for electromagnetic waves in a 1D photonic crystal that simulates the properties of a selected direction in the globular photonic crystal is calculated. The spectral ranges that are characterized by the anomalous slowing of electromagnetic waves in the photonic crystal and correspond to the stop-band edges are determined. A method for the measurement of the transmission and reflection spectra of the broadband radiation in photonic crystals is proposed. The method enables one to find the characteristics of the stop bands. The features of the secondary emission that emerges in opals due to the UV and visible excitation are reported. The conditions for the low-threshold lasing in opals filled with rareearth elements are presented. The experimental results on the induced-globular light scattering are demonstrated. Such a scattering implies the coherent excitation of vibrational states of the globules in a globular photonic crystal. A new phenomenon (slow light scattering) which involves the excitation of slow photons (slowtons) that correspond to the stop-band edges of the photonic crystal is observed. The conditions for the measurement of the slow light scattering in opals excited using the ruby and nitrogen lasers are experimentally determined. The experimental and theoretical results open up the prospects for low-threshold nonlinear optical processes in material media.     

Dispersion properties of two-dimensional plasma photonic crystals with periodically external magnetic field

Dispersion properties of two types of two-dimensional periodically magnetized plasma array structures with square lattices have been investigated by using plane wave expansion method. It is found that two different regions of flatbands and photonic band gaps occur in the TE polarization due to the external magnetic field. The two types of system can be seen as a kind of unusual plasma photonic crystals. The results show that not only the location of flatbands but also the position and bandwidth of photonic band gaps can be tuned by external magnetic field. The cutoff frequency decreases as external magnetic field increases. The edge of two different of flatbands regions and cutoff frequency shift downward to lower frequencies obviously with increasing plasma collision frequency, while they shift upward to higher frequencies notably with increasing plasma frequency. The filling factor has little effect on the location of flatbands regions. The width of flatbands regions and photonic band gaps are almost unchanged by increasing filling factor, but the number of ominidirectional photonic band gap for type-1 structure plasma photonic crystals can be effectively controlled by adjusting filling factor. It is worth to be noted that the first band gaps above the cutoff frequencies in Г–X and Г–M direction for two types of PPCs can be modulated by the parameters as mentioned above, and the relative bandwidth of band gap in Г–X direction is wider than the one in Г–M direction. The results may provide theoretical instructions to design new tunable photonic crystals devices.     

溶胶凝胶法制备圆柱形大孔二氧化硅反蛋白石结构晶体

利用溶胶凝胶法在不同直径的毛细管表面制备了圆柱形二氧化硅反蛋白石多孔结构.利用扫描电子显微镜表征了其形貌,发现胶体晶体的类(111)面结构整体为圆柱形,平行于毛细管表面.透射光谱结果表明,二氧化硅多孔结构在[111]方向具有典型的光子带隙效应,与布拉格理论相符合,有望应用到光通讯和传感领域.