一种新型光子晶体波导定向耦合型超微偏振光分束器

将两个二维空气孔光子晶体波导平行放置,两波导之间由三排空气孔相隔,构成一个定向耦合器.数值分析了TE(磁场平行于空气孔)和TM(电场平行于空气孔)偏振态光波在该定向耦合器中的传播行为.结果表明,减小耦合区两波导间的一排介质柱的半径,TE模的耦合长度减小,而TM模的耦合长度不变.基于此结构,设计了超微偏振光分束器,整个器件的尺寸为10.1μm,与已报道的24.2μm的结果相比,该器件具有更小的器件尺寸和更高的输出效率.     

Optical interleaver based on directional coupler in a 2D photonic crystal slab with triangular lattice of air holes

A small-size optical interleaver based on directional coupler in a 2D photonic crystal slab with triangular lattice of air holes is designed and theoretically simulated using plane wave expansion and finite-difference time-domain method. The interleaver is formed by two parallel and identical photonic crystal slab waveguides which are separated by three rows of air holes. The coupling region is designed below the light line to avoid vertical radiation. The simulated results show that the coupling coefficient is increased and the final length of the interleaver is decreased by enlarging the radius of the middle row of air holes. The transmission properties are analyzed after the interleaver’s structure is optimized, and around 100 GHz channel spacing can be got when the length of the interleaver is chosen as 40.5 μm.     

光子晶体耦合波导实现光开关效应的研究

提出了一种基于光子晶体耦合波导实现光开关效应的方法:将线缺陷引入二维光子晶体以形成两平行的邻近波导,两邻近波导及其中间的两排介质柱构成了光开关模型耦合区;利用平面波展开法计算了不同介质填充率情况下的色散特性.结果发现:减小介质填充率可以实现波导耦合长度的减小;分段调整中间介质柱的填充率和选择不同的耦合搭配长度,定向耦合...     

Power splitter based on cascaded multimode photonic crystal waveguides with triangular lattice of air holes

We propose a power splitter based on two-dimensional photonic crystals with triangular lattice of air holes. The structure can be divided into three sections consisting of the input waveguide, area-defect regions, and output waveguides. The operational principle of the splitter is that an incident field in the frequency range of interest is split into a twofold mode pattern with small distance in the first area-defect and then one of which is shifted by the secondary area-defect regions to output port. We optimize the size of area-defects for the mode-splitting and the shifting from the numerical analysis of distribution of time-averaged Poynting vectors by a finite-difference time domain method. And the transmittance over 45% per each output with a bandwidth about 0.06a/λ is achieved.     

Photonic band structure in a two-dimensional hexagonal lattice of equilateral triangles

In this paper, the plane wave expansion method is used to calculate the photonic band structure (PBS) for the transverse electric (TE) polarization in a two-dimensional hexagonal lattice composed air holes with cross-sections in the shape of equilateral triangles embedded in a GaAs background. We consider that the dielectric constant of GaAs is dependent on both hydrostatic pressure and temperature. Further, an increase can be observed in the width of the photonic band gap (PBG) while the lengths of the sides of the equilateral triangles are increased. When the pressure increases in the presence of a constant temperature, the dielectric constant of GaAs decreases; further, the PBS exhibits a more noticeable shift toward regions of higher frequencies than that observed while the temperature is increased at a given pressure. We also observed that the PBG obtained by rotating the triangular holes by an angle θ increases when compared against the results obtained for the lattice of holes without performing any rotation. In addition, the results obtained using the supercell technique denote that the position and width of the PBG remain unchanged while removing a triangular hole from the structure, denoting the presence of a defective band within the PBG.     

Photonic Crystal Power-splitter Based on Mode Splitting of Directional Coupling Waveguides

In the paper, a novel power-splitting scheme based on two dimensional photonic crystal (2D PhC) is proposed. The structure can be divided into three sections, including the input waveguide, coupling region, and output region, and the latter two sections consist of two parallel waveguides placed in proximity. The operation principle of the splitter is that only one of the super-modes splitting from the directional coupler can propagate through coupling region in the frequency range of interest. The radius of air holes next to the guiding region in coupling region is increased to avoid the acute back reflection at the entrance to the input waveguide induced by the modes mismatch between the input waveguide and coupling region. While in output region, the radius of these corresponding air holes is also increased so that the two splitting super-modes have same propagation constants to avoid the coupling between the two output waveguides. Moreover, as the length of coupling region is varied, its influence on the splitting performance is discussed, and it is verified that the relationship between the splitter length and bandwidth has a trade-off.     

A polymer photonic crystal fiber with high and flattened birefringence

A novel high birefringence polymer photonic crystal fiber (PCF) is proposed in this work. This PCF is composed of a polymer core and a cladding with elliptical air holes and squeezed triangular lattice. The high birefringence is introduced on the combined effect of elliptical air holes and the squeezed lattice. Our numerical results based on the supercell lattice method indicate that the birefringence can reach as high as 0.0018 at 650 nm wavelength with a properly designed cladding structure. We also analyze the dependence of the birefringence on structure parameters. And we design a PCF that has high and flattened birefringence.     

Bandgap properties of Kagomé photonic crystal fibers

Photonic crystal fibers (PCFs) can guide light by the photonic bandgap (PBG) effect created by the periodically arranged air holes in the cladding. In this paper, the bandgap properties of Kagomé photonic crystal fibers (KPCFs) are investigated in detail. First, the bandgap properties of PCFs based on the basic Kagomé lattice are analyzed and compared with the PBGs of PCFs based on honeycomb and triangular lattices. We highlight the similarities between KPCFs and honeycomb PCFs in their PBGs, both having air-guiding regions only at very large air filling fractions (AIFs), whereas the PBGs of triangular PCFs can have large air-guiding regions at smaller AIFs due to the difference in the gap structure. In the second half of this paper, we show how the PBGs of KPCFs can be modified by introducing an extra air hole into the vacant space of the original lattice. In particular, KPCFs with medium-sized air holes can be designed to guide air by introducing extra air holes of a larger size. The air-guiding regions of KPCFs with very large air holes can also be greatly extended by the extra air holes. These air-guiding regions occur at higher normalized frequencies, resulting in larger air hole pitches favorable for fabrication. PACS 42.70.Qs; 42.25.Bs; 42.81.Qb     

Broadband waveguide intersections with low cross talk in photonic crystal circuits

We propose a new mechanism for constructing waveguide intersections with broad bandwidth and low cross talk in photonic crystal (PC) circuits. The intersections are created by combination of coupled-cavity wave-guides (CCWs) with conventional line-defect waveguides. This mechanism utilizes the strong dependence of the defect coupling on the field pattern in the defects and the alignment of the defects (i.e., the coupling angle) in CCWs. By properly designing the defect mode, we demonstrate through numerical simulation the establishment of such a waveguide intersection in one of the most useful PCs, which is based on a two-dimensional triangular lattice of air holes made in a dielectric material. The transmission of a 500-fs pulse at ~1.3 microm is simulated by use of the finite-difference time-domain method, showing negligible distortion and low cross talk.     

Optimal tunability of waveguides based on silicon photonic crystals infiltrated with liquid crystals

In this work we study the optimization of the tunability range in waveguides based on two-dimensional silicon photonic crystal infiltrated with liquid crystal. The analyzed structure consists of a two-dimensional silicon photonic crystal with a triangular lattice of circular holes where a line of scatterers in the direction Γ–K has been replaced by a line of circular holes with different radius infiltrated by E7 liquid crystal. To this end, we use the plane-wave expansion method considering anisotropy and modelling supercells to account for the lattice defects that define the waveguide. Finally we study the field distributions of the guided modes in order to analyze their symmetries and confinement.     

Design of integrated optical circulator based on photonic crystals

Photonic crystals containing defects produce enhanced Faraday rotation. They have opened up the possibility of fabricating very compact magneto-optics structures. In this work, we have designed a two-dimensional photonic crystal waveguide for use in optical packaging and integrated optical circuits. For design purposes, a temporal coupled mode theory was utilized at the first step. It examined the coupling between cavity and optical ports. After acquiring a general solution, it would be applied to specific problem in hand. Then, optical characteristics of photonic crystal were investigated to design the practical parts such as cavity and waveguides which eventually a triangular crystal lattice of air holes in Bi:YIG (BIG) was considered to be the best candidate. Finally, the results of analytical investigations were evaluated using OptiFDTD software and then were confirmed.     

Channel drop filters using photonic crystal Fabry-Perot resonators

We have proposed channel drop filters based on a combination of Fabry-Perot (F-P) resonators in a two-dimensional photonic crystal with a triangular lattice of air holes. By tuning the separation, the radii of air holes between F-P resonators, and the length of two F-P resonators, we can selectively improve filter characteristics and obtain a forward-dropping efficiency of 95% and a backward-dropping efficiency of 90% at desired resonance frequencies. For the case where two F-P resonators are directly connected without separation, it forms a ring behavior and has a backward-dropping efficiency of 75% at resonance. Moreover, we investigate the dependence of optical bistability in one of the optimized resonators on initial frequency detunings. A switching speed of 2 ps is obtained. By decreasing the initial frequency detuning, overshoot on the switching-on power can be greatly suppressed.     

Efficient photonic crystal directional couplers

We demonstrate highly efficient and spectrally flat broadband coupling in photonic crystal directional couplers. The result is obtained by use of a novel design with smaller holes between coparallel photonic crystal waveguides for efficient channel-to-channel coupling. The system studied is based on a planar hexagonal photonic crystal lattice of holes made in silicon-on-insulator material. Results from three-dimensional finite-difference time domain modeling are shown to closely match results measured on fabricated samples.     

Coupling between opposite-parity modes in parallel photonic crystal waveguides and its application in unidirectional light transmission

Directional coupling between the even- and odd-parity modes of two parallel dissimilar linear defect waveguides in a square photonic crystal of cylindrical air holes in dielectric background is numerically demonstrated. Projected band-structure computations through the plane-wave expansion method reveal that high-efficiency coupling can be achieved in a frequency range of approximately 9 % extent around the central frequency. Coupling occurs if one row of spacing is maintained between the waveguides supporting even and odd modes, which are composed of annular air holes with outer radii equal to the photonic crystal’s scatterer radii and inner radii of 0.19 and 0.44 periods, respectively. Extinction ratio for coupling from the even to odd mode at the central frequency is 4.0 dB. Coupling length calculated through finite-difference time-domain simulations is approximately 25 periods at the central frequency, in agreement with the estimation through band diagram. Unidirectional light transmission is also demonstrated through finite-difference time-domain simulations, provided that waveguide and coupling lengths are equal. Forward and reverse transmittances of 71 and 0.3 %, respectively, are achieved at the central operation frequency in a 25-period system.     

Design of a novel wideband single-mode waveguide in a photonic crystal slab structure

We propose a novel photonic crystal slab waveguide that provides a single-mode band with large bandwidth. The proposed waveguide is obtained by introducing a line defect in a triangular lattice of air holes in a dielectric slab. This line defect consists of holes which are not located in the original lattice points. The plane wave expansion (PWE) method is used to extract the band diagram of guiding modes and based on the results, photonic crystal holes in the defect row and its adjacent rows are modified to maximize the waveguide bandwidth. We show that using the proposed structure, a single-mode bandwidth of 17% can be achieved.     

Electrical and optoelectronic properties of graphene Schottky contact on Si-nanowire arrays with and without H2O2 treatment

We show a methodology for how to construct Dirac points that occur at the corners of Brillouin zone as the Photonic counterparts of graphene. We use a triangular lattice with circular holes on a silicon substrate to create a Coupled Photonic Crystal Resonator Array (CPCRA) which its cavity resonators play the role of carbon atoms in graphene. At first we draw the band structure of our CPCRA using the tight-binding method. For this purpose we first designed a cavity which its resonant frequency is approximately at the middle of the first H-polarization band gap of the basis triangular lattice. Then we obtained dipole modes and magnetic field distribution of this cavity using the Finite Element Method (FEM). Finally we drew the two bands that construct the Dirac points together with the frequency contour plots for both bands and compared with the Plane Wave Expansion (PWE) and FEM results to prove the existence of Dirac point in the H-polarization band structure of lattices with air holes.     

光子晶体定向耦合三波长功分器

以二维三角晶格光子晶体为研究对象,在该光子晶体中引入两行以一行耦合介质柱为间距的平行单模线缺陷波导.通过分析和研究光子晶体波导耦合结构的耦合和解耦合特性,发现在不同频率下耦合波导的耦合长度不同.利用平面波展开法和定向耦合原理计算了在不同入射光频率下,缺陷波导间耦合波导的耦合长度,设计了一种新型超微光子晶体波导耦合型三波长功分器,实现了归一化频率分别为0.369、0.394、0.435的光波的分束效果.采用时域有限差分法验证了该功分器具有很好的功率分配效果.本文结果有助于光子晶体新型滤波器、定向耦合器、波分复用器、偏振光分束器以及光开关等光子器件的研究.     

Numerical test of the theory of pseudo-diffusive transmission at the Dirac point of a photonic band structure

It has recently been predicted that a conical singularity (=Dirac point) in the band structure of a photonic crystal produces an unusual 1/L scaling of the photon flux transmitted through a slab of thickness L. This inverse-linear scaling is unusual, because it is characteristic of radiative transport via diffusion modes through a disordered medium - while here it appears for propagation of Bloch modes in an ideal crystal without any disorder. We present a quantitative numerical test of the predicted scaling, by calculating the scattering of transverse-electric (TE) modes by a two-dimensional triangular lattice of dielectric rods in air. We verify the 1/L scaling and show that the slope differs by less than 10% from the value predicted for maximal coupling of the Bloch modes in the photonic crystal to the plane waves in free space.     

Design of photonic crystal superlens with improved image resolution

In this paper, we theoretically investigate the spatial resolution of a triangular two-dimensional photonic crystal superlens. We prove that this resolution can be improved by optimizing the air hole radius as well as the air hole shape. First, it is found that by decreasing the radius of the air holes, the spatial resolution is improved significantly. Next, we demonstrate that using elliptic air holes instead of circular air holes in a triangular two-dimensional photonic crystal can leads to good-quality images and focusing, with effective refractive index n_eff=−1 and enhanced image resolution. It is also shown that for the case of two sources, the resolution of such a photonic crystal lens can be made indeed better than the radiation wavelength.     

Two-dimensional absolute photonic band gaps in the visible

The most promising two-dimensional photonic crystals are the graphite lattice of dielectric rods in air background and the triangular lattice of air holes in dielectric background. In this paper, we compare their convenience in achieving structures which inhibit the propagation of visible electromagnetic waves. For visible waves, etching is difficult because the structure period must be smaller than the light wavelength. Furthermore, the semiconductor materials whose electronic band gap does not absorb any optical waves have little dielectric constant, which reduces photonic band gap widths. We show, using the Plane Wave Method and the Transfer Matrix Method, that the triangular structure is not appropriate because its gap is too narrow and its dimensions are too small for fabrication. On the other hand, wider gaps and larger dimensions that should be etched easily in wide gap semiconductors make graphite a much more suitable structure.