Photonic bands, gap maps, and intrinsic losses in three-component 2D photonic crystal slabs

We obtain the photonic bands and intrinsic losses for the triangular lattice three-component two- dimensional （2D） photonic crystal （PhC） slabs by expanding the electromagnetic field on the basis of waveguide modes of an effective homogeneous waveguide. The introduction of the third component into the 2D PhC slabs influences the photonic band structure and the intrinsic losses of the system. We examine the dependences of the band gap width and gap edge position on the interlayer dielectric constant and interlayer thickness. It is found that the gap edges shift to lower frequencies and the intrinsic losses of each band decrease with the increasing interlayer thickness or dielectric constant. During the design of the real PhC system, the effect of unintentional native oxide surface layer on the optical properties of 2D PhC slabs has to be taken into consideration. At the same time, intentional oxidization of macroporous PhC structure can be utilized to optimize the design.     

含负磁导率或双负材料的一维光子晶体的全方位带隙中波的反射相位特性

运用传输矩阵法研究了含负磁导率或双负材料的一维光子晶体结构在全方位光子禁带中波的反射相位谱.研究发现"负磁导率材料-正折射率材料"光子晶体和"双负材料-正折射率材料"光子晶体,在所述带隙中,对TE波而言,反射相位随入射角的增大而增大;对TM波而言,禁带频率范围内波的反射相位随入射角的增大而减小;TE波和TM波,两种结构的反射位相均随介质层的厚度缩放因子增大而增大,且都与周期数无关.     

Gap maps, diffraction losses, and exciton–polaritons in photonic crystal slabs

A theory of photonic crystal (PhC) slabs is described, which relies on an expansion in the basis of guided modes of an effective homogeneous waveguide and on treating the coupling to radiative modes and the resulting losses by perturbation theory. The following applications are discussed for the case of a high-index membrane: gap maps for photonic lattices in a waveguide; exciton–polariton states, when the PhC slab contains a quantum well with an excitonic resonance; propagation losses of line-defect modes in W1 waveguides, also in the presence of disorder; the quality factors of photonic nanocavities. In particular, we predict that disorder-induced losses below 0.2 dB/mm can be achieved in state-of-the-art samples by increasing the channel width of W1 waveguides.     

Extremely large birefringence and shifting of zero dispersion wavelength of photonic crystal fibers

Three different types of photonic crystal fibers have been investigated which promise very large birefringence. The first type fiber is band gap guiding, the second index guiding, while the third type is index guiding with high refractive index circular and elliptical regions in the innermost ring. The birefringence, group velocity dispersion, modal effective index and mode field area of these fibers have been numerically estimated by employing finite difference time domain method. When elliptical regions are introduced in the first and second rings with the combination of small circular regions, each of these proposed fibers exhibits large birefringence with shifted zero dispersion point. Among these three different types of fibers, the band gap guiding photonic crystal fiber promises the largest birefringence (～5.45×10^?2) reported so far. The value of the birefringence and group velocity dispersion of these fibers can be controlled by controlling the hole pitch. Largest birefringence is achieved with a specific value of hole pitch.     

Planar defects in three-dimensional chalcogenide glass photonic crystals

Here we report on the direct laser writing fabrication of Fabry-Perot-type planar microcavities in a three-dimensional (3D) photonic crystal (PhC) embedded within a high-refractive nonlinear chalcogenide glass (ChG) film. The fabricated planar microcavities in a nonlinear ChG 3D PhC facilitate the observation of resonant modes inside the stop gap. The experimental results show that the length of the planar cavity can be well controlled by the fabrication power and thus be used to tune the defect modes. The tunability of the observed defect modes is confirmed by the theoretical prediction.     

Demonstration of temperature resilient properties of 2D silicon carbide photonic crystal structures and cavity modes

In this paper, photonic crystal (PhC) based on two dimensional (2D) square and hexagonal lattice periodic arrays of Silicon Carbide (SiC) rods in air structure have been investigated using plane wave expansion (PWE) method. The PhC designs have been optimized for telecommunication wavelength (λ = 1.55 μm) by varying the radius of the rods and lattice constant. The result obtained shows that a photonic band gap (PBG) exists for TE-mode propagation. First, the effect of temperature on the width of the photonic band gap in the 2D SiC PhC structure has been investigated and compared with Silicon (Si) PhC. Further, a cavity has been created in the proposed SiC PhC and carried out temperature resiliency study of the defect modes. The dispersion relation for the TE mode of a point defect A1 cavity for both SiC and Si PhC has been plotted. Quality factor (Q) for both these structures have been calculated using finite difference time domain (FDTD) method and found a maximum Q value of 224 for SiC and 213 for Si PhC cavity structures. These analyses are important for fabricating novel PhC cavity designs that may find application in temperature resilient devices.     

Polarization characteristics of chiral photonic crystal fibers with an elliptical hollow core

The theoretical study of dielectric-chiral photonic crystal fiber(PCF)with an elliptical hollow core is presented.The band structure of chiral photonic crystal(PhC)is calculated by using a modified plane-wave expansion(PWE)method.By examining the out-of-plane photonic bandgaps(PBGs)of chiral PhC,a kind of chiral PCF with a hollow core is designed and their eigenstates are calculated.The distributions of mode field and polarization state are demonstrated,and how the structural asymmetry of the core together with the chirality in the background affects the modal polarization is discussed.The dependences of birefringence on chirality for different ellipticities of core are investigated.     

Coupling into the slow light mode in slab-type photonic crystal waveguides

Coupling external light signals into a photonic crystal (PhC) waveguide becomes increasingly inefficient as the group velocity of the waveguiding mode slows down. We have systematically studied the efficiency of coupling in the slow light regime for samples with different truncations of the photonic lattice at the coupling interface between a strip waveguide and a PhC waveguide. An inverse power law dependence is found to best fit the experimental scaling of the coupling loss on the group index. Coupling efficiency is significantly improved up to group indices of 100 for a truncation of the lattice that favors the appearance of photonic surface states at the coupling interface in resonance with the slow light mode.     

Photonic crystal nanocavity laser in an optically very thick slab

A photonic crystal (PhC) nanocavity formed in an optically very thick slab can support reasonably high-Q modes for lasing. Experimentally, we demonstrate room-temperature pulsed lasing operation from the PhC dipole mode emitting at 1324 nm, which is fabricated in an InGaAsP slab with thickness (T) of 606 nm. Numerical simulation reveals that when T≥800 nm, over 90% of the laser output power couples to the PhC slab modes, suggesting a new route toward an efficient in-plane laser for photonic integrated circuits.     

Analysis of multiple reflections in hybrid photonic crystal multimode interference coupler

In this paper the analysis of multiple reflections in photonic crystal (PhC) multimode interference (MMI) couplers using eigen-mode expansion method is presented. The analysis is conducted on a hybrid PhC structure which consisted of 1-D PhC multimode waveguide sandwiched between 2-D PhC input/output waveguides. In PhC multimode waveguide, where the mechanism of wave confinement is not due to total internal reflection but due to photonic bandgap properties, the reflectivity at 2-D PhC facet wall would be very large for all the guided modes in the waveguide when ever the image formed due to MMI effect does not coincides with the output access waveguide.     

有限开敞介质光子晶体的模式及其带结构分析

针对光子晶体在行波管中的应用,对有限开敞介质光子晶体的模式和带结构进行了分析和计算. 分析表明,有限开敞介质光子晶体中所能存在的模式包括EH和HE混合模式,如果是二维光子晶体,还存在E模式,E₁模式即为无限光子晶体中的E极化. 计算表明,被用作行波管慢波电路的光子晶体可以不必拥有完全带隙,而只需要具有带间隙即可. 关键词： 光子晶体 本征值方程 行波管 带结构     

Enhancement of photoluminescence from germanium by utilizing air-bridge-type photonic crystal slab

We fabricated germanium-based photonic crystal (PhC) slabs and characterized them by photoluminescence (PL) measurements at room temperature. Air-bridge-type Ge PhC slabs showed stronger PL than non-processed Ge layers on SiO₂ and than Ge PhC slabs on SiO₂. This enhancement is attributed to improved extraction efficiency due to the PhC patterns and to suppressed light leakage into the substrate by utilizing the air-suspended structure. In particular, when flat photonic band-edge modes around the Γ point are tuned to the Ge emission range, larger enhancement of integrated PL intensity was observed. A maximum enhancement ratio of integrated intensity up to 22 was demonstrated in an air-suspended Ge PhC slab with appropriate structural parameters. This is the largest enhancement factor of Ge PL using photonic nanostructures reported so far.     

Dispersionless slow light by photonic crystal slab waveguide with innermost elliptical air holes

We report a low-loss photonic crystal slab waveguide formed by deforming the innermost circle air holes in the conventional photonic crystal slab waveguide into elliptical ones. We obtain the photonic bands and group index of guided modes in this photonic crystal waveguide by guided-mode expansion method and investigate the dependence of photonic bands and group index of guided modes on the parameters of the innermost elliptical air holes. The group velocity and group velocity dispersion of this waveguide strongly depend on the innermost elliptical air holes. Photonic crystal slab waveguide with the optimum innermost elliptical air holes possesses a wider single mode region below the light line, in which light can easily propagate without intrinsic loss. At the same time, the guided mode supported by this waveguide has nearly constant group velocity and vanishing group velocity dispersion in a 3-5 nm bandwidth.     

All-optical switch based on photonic crystal microcavity with multi-resonant modes

We present a design of all-optical switches based on one-dimensional photonic crystals (1D PhC) doped with nonlinear optical materials. The 1D PhC switch structure is composed of a PhC cavity sandwiched by two accessional PhC microcavities. The center PhC cavity has two resonant frequencies with nearly the same quality factors (Q), while the accessional PhC cavities have the same resonant frequency, which is equal to one of the resonant frequencies of the center cavity. The two accessional PhC cavities cause reduction of Q value in this resonant frequency and result in different Q values of two modes. We realize all-optical switch effect by selecting pump light wavelength at the low Q mode and probe light wavelength at the other mode. The theoretical simulations by using the finite difference time domain method show that the pump light intensity required to realize optical switch effect in the designed switch is 50 times smaller than that in one-dimensional photonic crystals cavity with only one resonant mode.     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

Modelling and design of complete photonic band gaps in two-dimensional photonic crystals

In this paper, we investigate the existence and variation of complete photonic band gap size with the introduction of asymmetry in the constituent dielectric rods with honeycomb lattices in two-dimensional photonic crystals (PhC) using the plane-wave expansion (PWE) method. Two examples, one consisting of elliptical rods and the other comprising of rectangular rods in honeycomb lattices are considered with a view to estimate the design parameters for maximizing the complete photonic band gap. Further, it has been shown that complete photonic band gap size changes with the variation in the orientation angle of the constituent dielectric rods.      

Tunable highly birefringent photonic bandgap fibers

A novel tunable highly birefringent photonic bandgap fiber (PBGF) is designed theoretically by filling its air holes with high-index material. The transmission band can be continuously tuned by changing the refractive index of the filling material. Accordingly, the tunable modal birefringence and polarization mode dispersion of the PBGFs are investigated by adjusting the refractive index of the filling material. Furthermore, we have also analyzed the effect of surface modes in the photonic bandgap on the characteristics of the tunable PBGFs. The simulation results show the feasibility of constructing birefringence-tunable photonic crystal fibers and related fiber devices in practical applications.     

Analysis of single-mode condition and high-order modes discrimination in photonic crystal VCSEL

A new technique for analyzing the single-mode condition and mode discrimination of high-order modes in photonic crystal VCSEL is proposed and reported in this paper. The technique proposed is a semi-empirical approach that uses analytical analysis based on experimental data. For that purpose, single- and multimode photonic crystal VCSEL devices are fabricated and characterized for comparison of device performance. The mode width for first- and second-order mode of PhC VCSEL is computed for analyzing the mode behavior. In order to verify the findings, the fabricated single- and multimode devices are analyzed using finite difference frequency domain technique for the purpose of showing the loss as a function of mode order for the PhC VCSEL. The results show close agreement between the computed and experiment findings, justifying the use of the proposed semi-empirical technique for analyzing single-mode condition and of high-order modes discrimination in photonic crystal VCSEL.     

Optics design and fabrication of 3D electrically switchable hexagonal photonic crystal

We demonstrate a holographic approach for easy and large-area fabrication of 3D photonic crystal (PhC) microstructures in a polymer-dispersed liquid crystal (PDLC) by applying a single top-cut hexagonal prism and a seven-beam interference system. Based on the expected photonic band gap of the photonic crystal, the prism parameters are designed and optimized. For example, a prism with a height h=3.73 cm and a cross angle α=66° between the side face and the base is used to fabricate 3D PhC hexagonal symmetrical structures in PDLCs. A good agreement is obtained between the theoretical calculation of the interference intensity and morphological structures. On the other hand, far-field diffraction patterns and electrical switching characteristics are also investigated. PACS 42.15.Eq; 42.40.Eq     

Symmetrically glass-clad photonic crystal nanocavities with ultrahigh quality factors

We design and fabricate ultra-high-quality (Q) photonic nanocavities in a symmetrically glass-clad silicon (Si) two-dimensional (2D) photonic crystal (PhC) structure. We theoretically investigate the dependence of the refractive index of the glass on the Q factors for asymmetric and symmetric structures. We show that the index-symmetric distribution of the glass is a critical factor to realize ultrahigh Q factors for glass-clad 2D PhC structures. We fabricate symmetrically glass-clad Si PhC nanocavities and achieve a record Q factor of 1×10(6), comparable with the highest Q factors of nanocavities in air-bridge structures.