Compact WDM demultiplexer for seven channels in photonic crystal

We demonstrate a new device concept for wavelength division demultiplexing based on planar photonic crystal waveguides. The filtering of wavelength channels is realized by shifting the cutoff frequency of the fundamental photonic bandgap mode in consecutive sections of the waveguide. The shift is realized by modifying the size of the border holes.The proposed demultiplexer has an area equal to (16.5 μm × 6.5 μm) and thus it is verified that this structure is very small and can be integrated easily into optical integrated circuits with nanophotonic technologies. The output wavelengths of designed structure can be tuned for communication applications, around 1550 nm. The wavelengths of demultiplexer channels are λ₁ = 1.590 μm, λ₂ = 1.566 μm, λ₃ = 1.525 μm, λ₄ = 1.510 μm, λ₅ = 1.484 μm, λ₆ = 1.450 μm, λ₇ = 1.400 μm respectively. Designs offering improvement of number of the separate wavelengths (seven), miniaturization of the structure (107.25 μm²) is our aim in this work.In our structure, we consider that the 2D triangular lattice photonic crystal is composed of air holes surrounded by dielectric. Its parameters are: radius of holes (r = 0.130 μm), lattice constant (a = 0.380 μm), and index of membrane (n = 3.181:InP). The numerical model used to simulate the structure of the demultiplexer is based on the finite difference time domain (FDTD).     

Tunable characteristics of one dimensional magnetic photonic crystal composed with single-negative materials

In this paper, we present the study of dispersion and transmittance characteristics of one dimensional magnetic photonic crystal composed by single negative indexed materials. For this structure, we have considered magnetic negative (MNG) with ? = 1 and μ < 0 and electric negative (ENG) with ? < 0 and μ = 4. We used simple transfer matrix method and Bloch's theorem for its analytical explanations. Analyzing transmittance characteristics of the proposed structure, we obtain the tunneling of certain frequency range where as the dispersion characteristic shows total forbidden for the same range for TM mode. The tunable property is found inside the band structure due to zero-?, zero-μ and magnetic behavior of the material. To identify zero-? and zero-μ of the structure, we have calculated the dispersion and the transmittance of the magnetic structure of MNG–ENG on different angles of incidence and thickness of layers.     

High spatial resolution imaging in a clipping Kagomé lattice photonic crystal

In this paper, a two-dimensional Kagomé lattice photonic crystal (PC) made of GaAs (ɛ = 12.96) dielectric rods in air is considered. This Kagomé lattice PC has an effective refractive index n_eff = −1 at a low normalized frequency ω = 0.187 × 2πc/a. Imaging quality and the capability of the super-resolution of two point sources are studied for a superlens made of such PC structure. In order to achieve a high quality image and to improve the spatial super-resolution of two sources, a clipping Kagomé lattice PC is designed. Results simulated by finite-difference time-domain method show that imaging quality and super-resolution of two sources can be enhanced greatly as the perfect Kagomé lattice structure are superseded by the clipping Kagomé lattice structure. Coupled-mode theory analysis gives an explanation why the clipping structure is superior to the perfect one for both the imaging quality and the capability of the super-resolution of two sources. This clipping Kagomé lattice PC structure would be widely used in optical devices and integrated circuit.     

Large photonic band gap and strong attenuation of multiconnected Peano network

In this paper, by means of the network equation and generalized eigenfunction method we investigate the optical transmission spectra and attenuation behavior of electromagnetic (EM) waves in multiconnected Peano networks composed of one-dimensional (1D) waveguides. It is found that for some two-segment-connected networks a very large photonic band gap (PBG) can be created in the middle of a frequency period and the width of the large PBG can be controlled by adjusting the matching ratio of waveguide length, d₂ : d₁. When d₂ : d₁ = 2 : 1, the width of the large PBG is bigger than half of frequency period. The influence of generation on the width and attenuation of the large PBG are also studied and the numerical results demonstrate that the first-generation Peano network with d₂ : d₁ = 2 : 1 is a good selectable structure for the designing of optical devices with large PBG and strong attenuation.     

Synthesis and characterization of nanocrystalline HAp powders prepared by using aloe vera plant extracted solution

Nanocrystalline hydroxyapatite (HAp) powders were synthesized by a simple method using aloe vera plant extracted solution. To obtain nanocrystalline HAp, the prepared precursor was calcined in air at 400–800 °C for 2 h. The phase composition of the calcined samples was studied by X-ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase. With increasing calcination temperature, the crystallite of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter, a, in a range of 0.9520–0.9536 nm and c of 0.6739–0.6928 nm. The particle sizes of the powder were obtained to be 43–171 nm. The optical properties of the calcined powders were characterized by Raman and FTIR spectroscopies. The Raman spectra showed a main peak of the phosphate vibration mode (ν₁(PO₄)) at ∼963 cm⁻¹ for all the calcined samples. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcined powders. The morphology tends to change from a spherical shape to a rod-like shape with increasing calcination temperature as revealed by TEM.     

A novel polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber

A kind of polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber has been proposed. The polarization splitter is based on the phenomenon of resonant tunneling. We use the finite element method and the full-vector beam propagating method to analyze the characteristics of three-core photonic crystal fiber. Compare with the silica glass three-core PCF, the ZnTe tellurite glass three-core PCF have higher extinction ratios and lower coupling loss, the extinction ratios ERA = ? 164.2681 dB and ERC = ? 37.1742 dB at the wavelength λ = 1.55 μm, and the coupling loss is lower than 0.02 dB. The 8.7983-mm-long splitter is proposed to achieve extinction ratio better than ? 20 dB and a bandwidthof 20 nm.     

Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V₂O₅/TiO₂ system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V₂O₅/TiO₂ system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO₂ calcined at 450 °C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 °C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO₂ phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 °C was predominantly V⁴⁺. However, the presence of V⁵⁺ atoms cannot be ruled out. A qualitative analysis of extended X-ray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 °C showed that the local structure around vanadium atoms is comparable to that of VO₂ crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 °C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a V_xTi_(1−x)O₂ solid solution formation. The conditions of sample preparation used here to prepare V₂O₅/TiO₂ samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V₂O₅/TiO₂ system.     

Broadband super-collimation with low-symmetric photonic crystal

We investigate dispersive properties of two dimensional photonic crystal (PC) called star-shaped PC (STAR-PC) in order to succeed super-collimation over a broad bandwidth. Both time- and frequency-domain numerical methods are conducted. Due to introduced low-symmetry in the primitive cell, flat contours are observed at the fifth band for transverse magnetic mode. The proposed structure supports a super-collimation effect over a broad wavelength range between 1443 nm and 1701 nm with a bandwidth of Δω = 16.42%. The intrinsic characteristic of STAR-PC provides in-plane beam propagation with a limited diffraction length of 120a, where a is the lattice constant. By means of STAR-PC, one may realize super-collimation based single-mode optical devices with a low insertion loss, reduced dispersion and wide bandwidth.     

Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Δλ = 2 nm around λ = 1565 nm with a group index of n_g = 60. We experimentally demonstrate a relatively low propagation loss, of 130 dB/cm, for waveguides up to 800 μm in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100 μm that exhibits stitching errors of typically 10–50 nm. We reduced the impact of these stitching errors by introducing “slow–fast–slow” mode conversion interfaces and show that these interfaces reduce the loss from 320 dB/cm to 130 dB/cm at n_g = 60. This significant improvement highlights the importance of the slow–fast–slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool.     

Design of a compact polarization splitter based on the dual-elliptical-core photonic crystal fiber

We propose a compact polarization splitter based on dual-elliptical-core photonic crystal fiber. Two elliptical cores are introduced to increase the difference of effective index between x-polarized and y-polarized mode and three elliptical modulation air holes are used to control the power transfer between the two cores. By optimizing the structure parameters, the length of the polarization splitter is distinctly shortened. Numerical results demonstrate that the compact splitter has the length of 775 μm and up to 50 dB extinction ratio at the central wavelength of 1.55 μm. The corresponding bandwidth of 32 nm could be achieved from the wavelength of 1.534–1.566 μm with the extinction ratio over 20 dB     

A new design of low-loss and ultra-flat zero dispersion photonic crystal fiber using hollow ring defect

New hollow ring defect structure is introduced in photonic crystal fiber design for ultra- flat zero dispersion with very low waveguide losses. The hollow ring defect consisted of a central hole surrounded by a doped silica ring provides highly flexible defect engineering capabilities in photonic crystal fibers to achieve precise control of dispersion value and dispersion slope while independently maintaining low waveguide losses, which was not attainable in previous designs. A nearly flat zero dispersion of D=0±0.51 ps/nm km was obtained in the wavelength range of 1.44–1.61 μm with the maximum slope of ?2.7×10^?2 ps/nm² km. The confinement loss was less than 5.75×10^?8 dB/m along with the bending loss of 2.8×10^?6 dB/m for the radius of 10 mm, and splice loss of less than 1.57 dB to conventional single mode fiber at 1.55 μm.     

Chemical solution processing and properties of Sr2FeMoO6 thin films

Crack-free and oriented Sr₂FeMoO₆ (SFMO) thin film with double perovskite structure has been fabricated by the chemical solution deposition (CSD) method. A homogeneous and stable SFMO precursor solution was successfully prepared by controlling the reaction of starting metal-organic compounds in a mixture solvent of 1-propanol and 2-methoxyethanol. SFMO thin films with c-axis preferred orientation could successfully be synthesized on MgO (0 0 1) and SrTiO₃ (0 0 1) substrates by optimizing the several processing conditions. SFMO thin film prepared on SrTiO₃ (0 0 1) showed a magnetoresistance effect at a low magnetic field.     

Development of perovskite and phase transition in lead cobalt niobate modified lead zirconate titanate system

Ferroelectric lead zirconate titanate–lead cobalt niobate ceramics with the formula (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃ where x = 0.0–0.5 were fabricated using a high temperature solid-state reaction method. The formation process, the structure and homogeneity of the obtained powders have been investigated by X-ray diffraction method as well as the simultaneous thermal analysis of both differential thermal analysis (DTA) and thermogravimetry analysis (TGA). It was observed that for the binary system (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃, the change in the calcination temperature is approximately linear with respect to the PCoN content in the range x = 0.0–0.5. In addition, X-ray diffraction indicated a phase transformation from a tetragonal to a pseudo-cubic phase when the fraction of PCoN was increased. The dielectric permittivity is remarkably increased by increasing PCoN concentration. The maximum value of remnant polarization P_r (25.3 μC/cm²) was obtained for the 0.5PZT–0.5PCoN ceramic.     

Theoretical analysis on the optical characteristics of pure and N-doped Ta2O5 2D photonic crystals with honeycomb lattice

Density functional theory (DFT) is performed on the structural and optical properties of undoped and N-doped Ta₂O₅. The optimized lattice constants of β-Ta₂O₅ are in good agreement with the experimental values. When O is replaced by N in Ta₂O₅, the substitutional doping of N in Ta₂O₅ clearly increases the refractive indices. The propagation of acoustic wave in two-dimensional (2D) photonic crystal of a honeycomb structure of air cylinder is investigated by the plane wave expansion method (PWEM). Our numerical results show that Ta₂O₅ has incomplete band gaps, indicating that only the TE mode appears. Ta₂O_4.5N_0.5 is more suitable for background material. When a = 338 nm, r/a = 0.46, and Δ = 0.022 (ωa/2πc), a complete band gap appears in the red light range.     

Comparative study of large-solid-core photonic crystal fibers: Dispersion and effective mode area

In this study, large-solid-core photonic crystal fibers with fixed air-hole diameter d = 0.84 μm and with fixed pitch length Λ = 4.2 μm are investigated for different d/Λ ratios. The dispersions and the effective mode-areas are obtained and compared for both the structures. It is seen that the dispersion management is easier by using the fixed d structures, but for working around the same zero dispersion points in a large interval of d/Λ the fixed Λ structures are more available. The A_eff values larger than 100 μm² are obtained with d/Λ smaller than 0.2 for both the two structures. A_eff increases rapidly with decreasing d/Λ to 0.1 and then reaches to A_eff value of 500 μm² at the d/Λ = 0.1 for the fixed d structures. The single-mode regime for the two structures is also discussed.     

Design of tapering one-dimensional photonic crystal ultrahigh-Q microcavities

One-dimensional (1D) photonic crystal (PC) microcavities can be readily embedded into silicon-on-insulator waveguides for photonic integration. Such structures are investigated by 2D Finite-Difference Time-Domain method to identify designs with high transmission which is essential for device integration. On-resonance transmission is found to decrease with the increasing mirror pairs, however, the quality factor (Q) increases to a saturated value. The addition to the Bragg mirrors of tapered periods optimized to produce a cavity mode with a near Gaussian shaped envelope results in a major reduction in vertical loss. Saturated Q up to 2.4 × 10⁶ is feasible if the internal tapers are properly designed. The effect of increasing transmission is also demonstrated in a structure with the external tapers.     

Highly birefringent ZBLAN photonic quasi-crystal fiber with four circular air holes in the core

A highly birefringent ZBLAN photonic quasi-crystal fiber with a rectangular array of four relatively small circular air holes in the core region is proposed. Through optimizing fiber structure parameters using a full-vector finite-element method combined with perfectly matched layers boundary condition, its birefringence is up to 2.88 × 10⁻² and the confinement losses of both polarized modes are less than 4.95 × 10⁻⁴ dB/m at 2 μm. To our knowledge, this is the first simulation study showing that a birefringence can be achieved with the order of 10⁻² by all-circular-hole PQFs around 2 μm.     

Oxygen permeation characteristics of zirconia with surface modification

Yttria-stabilized zirconia (YSZ) can be used as an oxygen-permeating membrane at elevated temperature (> 1400 °C) due to its chemical and mechanical stability. It was previously shown that the oxygen transport through YSZ membrane in reducing oxygen partial pressure (P_O2) was highly influenced by the surface-exchange kinetics that can be improved by porous surface coating layers such as YSZ, GDC (Gd-doped ceria) or YSZ–GDC mixture [H.J. Park, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2577]. However, the increased oxygen flux was still lower than that estimated assuming bulk-diffusion limit and rapidly decreased with time due to the sintering of coating layers and the reaction between bulk YSZ and coating layers. In this study, the oxygen fluxes through YSZ with LaCrO₃, GDC + LaCrO₃ (bilayer), LaCrO₃ + 5 wt.% GDC (mixture), or LaCr_0.7Co_0.3O₃ coatings were measured under controlled P_O2 gradient (permeate-side P_O2: ∼ 3 × 10^− 12 atm, feed-side P_O2: 2 × 10^− 10–2 × 10^− 8 atm) at 1600 °C. The oxygen flux drastically increased with these coatings. The highest increase in oxygen flux was shown with GDC + LaCrO₃ (bilayer) coating and was maintained for a long time. The presence of highly catalytic Ce ions while maintaining porous structure in the coating layer may explain the observation. The prevention of formation of resistive layer due to ceria coating may also be partly responsible for the observation.     

Synthesis and electrical properties of nanocrystalline Ca1 − xEuxMnO3 ± δ (0.1 ≤ x ≤ 0.4) powders prepared at low temperature using citrate gel method

Nanopowders of Ca_1 − xEu_xMnO₃ (0.1 ≤ x ≤ 0.4) manganites were synthesized as a single phase using the auto gel-combustion method. The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation or contamination. The Ca_1 − xEu_xMnO₃ system has been synthesized at 800 °C during 18 h, against the conventional method of mixing oxides used to obtain these materials at higher temperatures of synthesis. The formation reaction was monitored by X-ray diffraction (XRD) analysis and an infrared absorption technique (FTIR). The polycrystalline powders are characterised by nanometric particle size, ∼ 48 nm as determined from X-ray line broadening analysis using the Scherrer equation. Morphological analysis of the powders, using the scanning electron microscope (SEM), revealed that all phases are homogeneous and the europium-substituted samples exhibit a significant decrease in the grain size when compared with the undoped samples. The structure refinement by using the Rietveld method indicates that the partial calcium substitution by europium (for x ≥ 0.3) modifies the orthorhombic structure of the CaMnO₃ perovskite towards a monoclinic phase. All manganites show two active IR vibrational modes around 400 and 600 cm^− 1. The high temperature dependence of electrical resistivity (between 25 and 600 °C) allows us to conclude that all the samples exhibit a semiconductor behaviour and the europium causes a decrease in the electrical resistivity by more than one order of magnitude. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio.     

Tunable slow light and buffer capability in photonic crystal coupled-cavity waveguides based on electro-optic effect

An enhanced photonic crystal coupled-cavity waveguide (PC-CCW) is proposed to realize compact, ultra-fast modulated and high-performance buffering application. The slow light and buffer performances of PC-CCW are optimized by adjusting the cavity spacing and the hole size around the cavity. In the optimized structure, the group velocity is below 2.713 × 10^? 4c. The corresponding delay time, buffer capacity and Q factor can reach as high as 12.286 ns, 68.5 bit and 3525, respectively. Then the dynamic modulation of slow light transmission and buffer capacity in PC-CCW are systemically studied. The simulation shows that the buffer capacity and the physical size of each bit are unchanged as the applied voltage increases. While the wavelength shift and delay time decrease almost linearly as the applied voltage increases. And the modulation sensitivities are about 3.726 nm/mV and 0.875 ns/mV, respectively. The Q factor also decreases accordingly.