Fabrication of autocloned photonic crystals using electron-beam guns with ion-assisted deposition

The autocloning technique is an attractive deposition method for the making of photonic crystals. With it various photonic crystals can be produced simply by changing the substrate periodicity and the structure of the stacking materials. We report on a method for the fabrication of autocloned photonic crystals. This method has better step-coverage, a higher deposition rate and a larger deposition area than can be achieved by the more traditional sputtering method and the periodic surface corrugation is preserved even after ion-assisted deposition (IAD) of multilayer stacks using E-beam gun evaporation. The shaping process can be freely controlled by controlling the IAD power and the ion source etching time. Ion source etching is a physical etching process which does not require any chemical reaction or dangerous reactive gas. The process parameters are described in this paper. The refractive index can be adjusted by changing the deposition rate and the substrate temperature during the deposition process. The deposition rate is about 0.7-1 nm/s for SiO₂ which is almost ten times faster than that of the sputtering method. This makes this method good for the mass production of photonic crystals.     

Effects of nano-structured photonic crystals on light extraction enhancement of nitride light-emitting diodes

The light extraction efficiency of an InGaN/GaN light-emitting diode (LED) can be enhanced by incorporating nano-structured photonic crystals inside the LED structure. We employed plane wave expansion (PWE) method and finite difference time domain (FDTD) method to reveal the optical confinement effects with the relevant parameters. The results showed that band-gap modulation could increase the efficiency for light extraction at the lattice constant of 200 nm and depth of 200 nm for the 468-nm LED. Focused ion beam (FIB) using Ga created the desired nano-structured patterns. The LED device micro-PL (photoluminescence) results have demonstrated that the triangular photonic crystal arrays could increase the peak illumination intensity by 58%. The peak wavelength remained unchanged. The integrated area under the illumination peak was increased by 75%. As the patterned area ratio was increased to 85%, the peak intensity enhancement was further improved to 91%, and the integrated area was achieved at 106%.     

Ripple suppression of third-order dispersion by using graded index one-dimensional photonic crystals

A novel one-dimensional photonic crystal design for ripple suppression of third-order dispersion in ultrafast laser systems has been introduced. We investigated and compared the compensation behaviour of a simple photonic crystal and the proposed graded index structure with the same total thicknesses and periodicity at the near infrared region by the transfer matrix method. The results reveal that implementing the proposed graded index structure removes the pulse wings, produces clean pulses without the use of additional optical elements and also results in a dramatic reduction in the fluctuations of the group delay and third-order dispersion curves.     

Preparation of one-dimensional photonic crystals by sol-gel process for magneto-optical materials

One-dimensional photonic crystal (PC) of periodically alternating low (SiO₂) and high (TiO₂) refractive index materials was prepared by sol-gel dip coating method that controls the thickness of each layer with nanometer level. The photonic band gap of high reflectivity was verified at wavelengths between 590 nm and 820 nm, which became significant with increasing the number of bilayer. The UV-vis spectra, SEM image and glow discharge optical emission spectroscopy indicate the periodic structure of SiO₂/TiO₂ multilayer. The magnetic layer of CoFe₂O₄ was also prepared by a sol-gel dip coating method. After the annealing at 700 °C, the single phase of CoFe₂O₄ film with spinel structure, without any preferred crystalline orientation, was obtained. In addition, the one-dimensional magnetophotonic crystal (MPC), in which the magnetic defect layer of CoFe₂O₄ is introduced into the periodic structure, was prepared. The light was localized at the magnetic defect due to the interference of the multilayer film, and the localized transmittance was observed around 530 nm. The Faraday rotation of MPC shows a peak at ∼ 570 nm which is close to the localized peak of transmittance. This may correspond to the enhancement of Faraday rotation due to the localization of light.     

Flexible photonic crystals for strain sensing

Three-dimensional (3-D) photonic crystals (PCs) have been studied as possible strain sensing materials, based on strain-induced stop band frequency shifting. Self-assembly of polystyrene microspheres, achieved by sedimentation over a flexible polyimide tape substrate whose surface hydrophilicity was optimized in order to achieve maximum wettability, led to an organized 3-D direct opal template. This was infiltrated with a silica sol-gel solution by dip-coating or by chemical vapour deposition and an inverse opal structure was ultimately obtained by chemical dissolution of the polymer template. The structural and optical properties of these PCs have been studied by scanning electron microscopy (FE-SEM) and UV/visible spectroscopy under variable degrees of strain. FE-SEM showed the presence of ordered domains up to ∼30 μm². A mechano-optical effect was evidenced by strain-induced shifting of the photonic stop band peak wavelength of the direct, infiltrated and inverse opals up to 50 nm in transmission mode, due to changes in interplanar spacing upon bending the flexible PCs. Optical response strain cycles were studied, suggesting the possible use of these structures in reversible photonic strain sensors integrated in sensor/actuator devices.     

Mode tuning in dispersive and non-dispersive photonic crystals by defects

Transmission spectrum of two-dimensional photonic crystal for dispersive and non-dispersive photonic crystals (PhC) is calculated. Calculations show that by considering defect electromagnetic waves can propagate in PhC band gap. Transmission spectrum for different types of defects is compared together. The number and position of transmission modes in PhC waveguide depend on host PhC and type of defects. By selecting suitable PhC material and defect type, the number and position of transmission modes can be controlled.     

Polarization-dependent angle filters based on one-dimensional photonic crystals using mu-negative materials

We study transmission properties of one-dimensional photonic crystals consisting of mu-negative and positive index materials by using transfer matrix methods. The results show that transmission properties of the transverse electric waves depend on permittivity (?), while transmission properties of the transverse magnetic waves depend on permeability (μ); there exists a transmission band inside the single-negative gap in this periodic structure without defects, and the transmission band is insensitive to the incident angle for the transverse electric waves but sensitive for the transverse magnetic waves. This property can be used to make polarization-dependent angle filters.     

Chiral photonic materials self-assembled by cellulose nanocrystals

Cellulose nanocrystals are natural nanomaterials with a high aspect ratio, high specific area, excellent stability, and favorable optical performances. Cellulose nanocrystals can form cholesteric liquid crystals through a left-handed spiral arrangement. The suspension liquid of cellulose nanocrystals can retain the chiral cholesteric structure in the solid film after being completely dried, leading to the appearance of Bragg reflection and bright structural color in the visible spectrum. By changing the conditions or mixing with polymers, the cellulose nanocrystals film will show different structural colors due to the change of pitch. The film can cover almost the entire visible spectrum, which can be applied to various aspects such as sensing, anti-counterfeiting, detection, and so on. In this review, we elaborated on the synthesis and properties of cellulose nanocrystals materials and introduced the mechanism of structural color formation, as well as the current research progress and applications. Cellulose nanocrystals have become a hot spot in the field of structural color, and provide more research value for providing a cheap, easy-to-obtain, green-friendly, and high-biocompatibility natural photonic material.     

Optical manipulation of photonic defect-modes in cholesteric liquid crystals induced by direct laser-lithography

Manipulation of photonic defect-modes in cholesteric liquid crystals (ChLCs), which are one-dimensional pseudo photonic band-gap materials have been demonstrated by an external optical field. A structural defect in which the pitch length of the ChLC in the bulk and the defect are different was introduced by inducing local polymerization in a photo-polymerizable ChLC material by a direct laser-lithography process, and infiltrating a different ChLC material as the defect medium. When an azobenzene dye-doped ChLC was infiltrated in the defect, the trans-cis isomerization of the dye upon ultraviolet (UV) exposure caused the pitch to shorten, changing the contrast in the pitch lengths at the bulk and the defect, leading to a consequent shifting of the defect-mode. The all-optical manipulation was reversible and had high reproducibility.     

Slow-light Mach–Zehnder modulators based on Si photonic crystals

Mach–Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p–n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p–n diode. This length is shorter than commercial LiNbO₃ modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index n_g. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.     

Fabrication of three-dimensional photonic crystals by interference lithography with low light absorption

We report on the fabrication of polymer templates of photonic crystals by means of holographic (or interference) lithography. The grating is written in a SU-8 photoresist using a He-Cd laser of wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous structure corresponding to an orthorhombic lattice are determined.     

椭圆柱透镜组准直半导体激光束的优化设计

采用光线矢量法设计了两个相互垂直的椭圆截面柱透镜组来准直半导体激光束。利用多目标优化中的加权和法进行优化计算,并提出自适应加权系数优化法。此方法能根据计算结果自动调整加权系数,从而达到最佳的优化结果。计算结果表明,该准直系统可达到45μrad左右的发散角,远远优于其它截面柱透镜组（毫弧度数量级）的准直效果。同时两柱透镜间距取适当值,还可在远场形成圆形光宽。     

Study of band structure of one-dimensional photonic crystal composed of dispersive and nonlinear dielectric materials

The analysis of band structure of one-dimensional (1-D) photonic crystal containing dispersive and non-linear dispersive materials has been presented. The band spectra for the different combination of photonic crystals have been calculated by the well-known plane wave expansion method. The effect of the dispersive and non-linear materials on the band structures has been determined. The third-order nonlinearity has been considered in the non-linear material, and Lorentz–Drude model has been taken for dispersive material. The band gaps of considered photonic crystal are affected by the nonlinearity in the presence of dispersive material like gold. We have observed that the normalized frequency difference between photonic bands decreases on increasing intensity of input beam. This work may be useful in optical switching devices.     

几种单晶半导体材料在压痕下的变形与断裂行为比较

采用显微压痕方法研究了Si、Ge、GaAs和InP四种半导体单晶的变形与断裂行为.通过测量[100]取向单晶体面内的显微硬度,裂纹开裂的临界压痕尺寸以及断裂韧性,分析了这四种材料力学性能的面内各向异性行为.结果表明:在压痕载荷的作用下,Si和Ge的塑性变形以剪切断层为主,而GaAs和InP则通过滑移系的开动协调变形.[100]取向的Si、Ge、GaAs和InP四种单晶的面内显微硬度、弹性模量和断裂韧性表现出不同程度的各向异性.裂纹长度与压痕尺寸间的关系表明,与GaAs和InP相比,Si、Ge具有较小的临界压痕尺寸和拟合直线斜率,这一临界压痕尺寸和拟合直线斜率的变化规律分别与材料的硬度和断裂韧性的变化规律一致.     

Beam splitter and beam bends based on self-collimation effect in two-dimensional photonic crystals

Basing on the self-collimation effect of photonic crystals, one-to-two beam splitter, beam bend and one-to-three beam splitter are, respectively, designed by introducing a different line defect along the same direction. From the equal-frequency contour plot which is calculated by the plane wave expansion method, we obtain the frequency and the propagate direction of the self-collimated beam. The self-collimated beam propagation in photonic crystals with different line defects is simulated by the two-dimensional finite-difference time-domain method with perfectly matched layer absorbing boundary conditions. The simulation results show that one-to-two beam splitter, beam bend and one-to-three beam splitter can be realized by appropriately arranging the line defect along the proper direction. Such devices can greatly enhance photonic crystals for use in high-density optical integrated circuits.     

Label-free detection of glycated haemoglobin in human blood using silicon-based photonic crystal nanocavity biosensor

Abstract

In this paper, we describe a two-dimensional photonic crystal-based biosensor that consists of a waveguide and a nanocavity with high sensitivity. A new method is employed for increasing sensitivity of the biosensor. The simulation results show that biosensor is highly sensitive to the refractive index (RI) variations due to injected biomaterials, like glycated haemoglobin, into the sensing surface. The proposed biosensor is designed for the wavelength range of 1514.4–1896.3 nm. The sensitivity and the quality factor are calculated to be 3000 and 272.43 nm/RIU, respectively. The designed structure can detect a 0.002 change in the RI via resonant wavelength shift of 0.9 nm. The band diagram and transmission spectra are computed using plane wave expansion and finite difference time domain methods.     

Polarization beam splitter based on honeycomb-lattice photonic crystal ring resonators

A new polarization beam splitter is proposed based on a photonic crystal ring resonator (PCRR) composed of honeycomb-lattice cylindrical silicon rods in air. By shrinking the width of the bus waveguide and adjusting the radii of two nearest-neighbor center rods of the PCRR, an unpolarized beam can be separated well into TE and TM polarization states, respectively, at the backward and forward output ports. Simulation results obtained by the two-dimensional finite-difference time-domain technique show that the insertion losses are 3.58 dB and 3.08 dB, and the polarization extinction ratios are 21.42 dB and 28.53 dB for TE and TM polarization, respectively, at a 1566.7 nm center wavelength. The excess loss is less than 0.34 dB and its dimensions are roughly 43.2 μm × 27.52 μm. These findings offer potential practical applications in high-density photonic integrated circuits.     

Computing equifrequency contours of two-dimensional photonic crystals by Dirichlet-to-Neumann maps

Equifrequency contours provide important information for designing special photonic crystal devices. In this paper, we present an efficient method to compute equifrequency contours for two-dimensional photonic crystals with triangular and honeycomb lattices. Our method is based on the Dirichlet-to-Neumann (DtN) operator of a unit cell in the photonic crystal. The DtN operator maps the wave field on the boundary of the unit cell to its normal derivatives. For photonic crystals with a triangular or honeycomb lattice, a small linear eigenvalue problem is formulated to calculate the dispersion relation. The formulation is based on the DtN map of the unit cell for a given angular frequency, and the eigenvalue is related to the wave vector. Our method is especially suitable for calculating the equifrequency contours, if a relatively small number of frequencies are involved.     

光子晶体微腔半导体激光器的研究进展

概述了光子晶体微腔半导体激光器的研究进展，从物理机理、数值模拟、以及工艺实现方法等方面作了详细的叙述，并对其在光子集成中的应用前景进行了展望。