High Efficiency One-Way Transmission by One-Dimensional Photonic Crystals with Gratings on One Side

We propose a scheme of optical one-way transmission by using one-dimensional photonic crystals （PhCs） with diffraction gratings on one side. The one-way transmission is realized by making the PhC opaque to the zeroth diffraction order and transparent to another propagating （in air） diffraction order. For such a structure with 10-period PhC, 93% of the incident energy passes through when an electromagnetic wave impinges from one side, and the transmittance decreases to the order of 0.001% as the electromagnetic wave illuminates from the other side.     

Transmission Properties of One-Dimensional Photonic Crystals Containing Anisotropic Metamaterials

The transmission properties of one-dimensional photonie crystals （1DPCs） containing anisotropic metamaterials are theoretically studied. It is shown that the 1DPCs can possess a similar zero average index （zero-n） gaps, the edges of zero-~ gap are weakly dependent on the incident angles, scale length and the polarization of the electromagnetic wave. When an impurity is introduced, a defect mode appears inside the zeron gap with a very weak dependence on incident angles and sealing. It is found that in such photonic crystals, a transmitted Gaussian pulse with its carrier frequency lying in the lower gap edge, in the defect mode and in the bandgap, can experience a positive or negative group delay and hence a subluminal, ultra.slow or superluminal propagation with small distortions. These properties of the photonic crystals have potential applications in the transfer of information.     

Tunable Pair Transmission in One-Dimensional Photonic Crystals Consisting of Single-Negative Materials

We study the transmission of one-dimensional photonic crystals consisting of single-negative permittivity and single-negative permeability media by using transfer matrix method. A pair of transmission modes is found in the gap. The transmission modes are dependent only on the ratio of the thicknesses of the two alternating layers. The separation of a pair of transmission modes can be tuned by varying the thickness of the defect layer or the ratio of thicknesses of the two alternating layers.     

Photonic Crystal Polarizing and Non-Polarizing Beam Splitters

A polarizing beam splitter （PBS） and a non-polarizing beam splitter （NPBS） based on a photonic crystal （PC） directional coupler are demonstrated. The photonic crystal directional coupler consists of a hexagonal lattice of dielectric pillars in air and has a complete photonic band gap. The photonic band structure and the band gap map are calculated using the plane wave expansion （PWE） method. The splitting properties of the splitter are investigated numerically using the finite difference time domain （FDTD） method.     

Photonic Crystal Waveguide Intersection Based on Self-Imaging of Multi-Mode Interference

A new mechanism of intersection formed by two line defect photonic crystal （PC） waveguides are numerically investigated using the finite-diFference time-domain method. The results show that the normalized crosstalk is smaller than 10^-4; the reflection is smaller than 10^-3, and the transmission is larger than 0.999. The authors analyse the physical origins and find that a modified self-imaging process in the intersected multi-mode region is the main reason of the excellent performance. This kind of multi-mode interference based intersection may find potential applications in PC optical circuits.     

Photonic crystal waveguide sampled gratings

Photonic crystal waveguide sampled gratings (PCWSG) are proposed to realize multi-channel optical filter in a photonic crystal (PC) waveguide. The reflection characteristics of the filter are analyzed by using the coupled-mode theory (CMT) together with transfer-matrix method (TMM). It is shown that the reflection spectrum of the filter exhibits a group of reflection peaks, which are distributed with equal frequency spacing within the photonic crystal bandgap (PBG). The theoretically calculated results are numerically verified by using two-dimensional (2D) finite-difference time-domain (FDTD) method.     

Roughness losses and volume-current methods in photonic-crystal waveguides

We present predicted relative scattering losses from sidewall roughness in a strip waveguide compared to an identical waveguide surrounded by a photonic crystal with a complete or incomplete gap in both 2d and 3d. To do so, we develop a new semi-analytical extension of the classic “volume-current method” (Green’s functions with a Born approximation), correcting a longstanding limitation of such methods to low-index contrast systems (the classic method may be off by an order of magnitude in high-contrast systems). The resulting loss predictions show that even incomplete gap structures such as photonic-crystal slabs should, with proper design, be able to reduce losses by a factor of two compared to an identical strip waveguide; however, incautious design can lead to increased losses in the photonic-crystal system, a phenomena that we explain in terms of the band structure of the unperturbed crystal.     

Efficient transmission in cascaded photonic crystal waveguides via a strong coupling effect

A design of cascaded photonic crystal waveguide is proposed in this paper inspired by the work of Tang et al. [D. Tang, L. Chen, W. Ding, Appl. Phys. Lett. 89 (2006) 131120]. In contrast to a conventional waveguide source, a plane wave source is applied in the current design. We show that an efficient guide mode in the photonic band gap can be achieved. The same idea also works for a slight variation by defects introduction in the photonic crystal. Finally, the strong coupling effect present in the cascaded waveguides is demonstrated by an analogy with photonic quantum wells.     

Polarization-Independent Guided-Mode Resonance Filters under Oblique Incidence

We present the dispersion relation of guided-mode resonances in planar periodic waveguides, both for s-polarized （TF, mode） and p-polarized （TM mode） incident waves. For a fixed homogeneous planar waveguide, dispersion curves of the TE eigenmode cannot cross that of the TM eigenmode at all. That is to say, at a certain wavelength, TE and TM modes cannot be excited with the same propagation constant. Due to Bragg reflection in the planar periodic waveguide, dispersion curves of the TE leaky mode may intersect with that of the TM leaky mode in the first Brillouin zone. We employ these intersections to achieve polarization-independent guided-mode resonance filters.     

Photoinduced Birefringence and Broadband All-Optical Photonic Switch in a Bacteriorhodopsin/Polymer Composite Film

Photoinduced birefringence with large optical nonlinearity in a bacteriorhodopsin/polymer composite film is observed. A high refractive index change of 8.5×10^-5 photoinduced by 476nm pumping beam is reached at the low intensity of 6.5mW/cm². Based on it, a broadband all-optical photonicswitch is realized with an optical controlling switch system. Because of controlling beam's selectivity in switching, the transporting beams of different wavelengths with different intensities and shapes can be modulated by adjusting the wavelength and intensity of the controlling beam.     

High-Efficiency Bistable Switching Based on One-Dimensional Photonic Crystals with Single-Negative Materials

We study theoretically the nonlinear responses of one-dimensional photonic crystals （PCs） composed of alternating two kinds of single-negative （permittivity-negative and permeability-negative） materials embedded with a Kerrtype nonlinear defect layer. In conventional PCs, it is difficult to realize a bistable switching with both low threshold and quick response time. However, in PCs with single-negative materials, by changing the ratio of the thicknesses of the two types of layers, with the decreasing size of the structure, the switching response time is shortened and the threshold intensity decreases simultaneously.     

Negative Refraction and Near-Field Imaging of an Elliptical-Rod Photonic Crystal Slab in the Second Band

Negative refraction and imaging properties of the electromagnetic wave through a two-dimensional photonic crystal （PC） slab, which consists of a square lattice of elliptical dielectric rods immersed in the air background, is studied by the plane-wave expansion method and the finite-difference time-domain method. A point source placed in the vicinity of the PC slab can form a good-quality image spot through the PC slab for the incident frequencies within the second photonic band. The calculated result also shows that negative refraction occurs in this kind of PC slab.     

Photonic band gap properties of GaP opals with a new topology

In this paper, we propose that the “anomalous” optical response exhibited by GaP and InP infiltrated opals is due to the peculiar morphology shown by these materials when grown within the pores. In order to account for their optical response, we propose a new structural model consisting of a network of high dielectric spheres located in the pores of the bare opal, interconnected by cylinders of the same material. A fair agreement between the theoretical predictions using this model and the experimental measurements has been found. We also show that the inverse structure presents very interesting optical properties.     

Spatially Inhomogeneous Gain Modification in Photonic Crystals

The electric and magnetic energy distributions in photonic crystals （PC） are calculated by using the plane wave expansion （PWE） method. Even though the total electric and magnetic energy in each unit cell of photonic crystals are equal to each other, the ratio of electric and magnetic energy densities varies depending on the local position. Based on Fermi＇s golden rule, the optical gain is analysed in the full quantum framework that takes the nonuniform energy density ratio into account. This nonuniform energy density ratio in photonic crystals, defined in an equal form as gain modification factor, leads to spatially inhomogeneous modification of optical gain. Results reported in the paper provide a new perspective for analysing gain characteristics, as well as the lasing properties, in photonic crystals.     

Nonlinear Localization due to a Double Negative Defect Layer in a One-Dimensional Photonic Crystal Containing Single Negative Material Layers

We investigate the effects of introducing a defect layer negative material layers on the transmission properties in a one-dimensional photonic crystal containing single The width of the defect layer is taken to be the same or smaller than the period of the structure. Different cases of the defect layer being linear or nonlinear and double positive or double negative are discussed. It is found that only a nonlinear double negative layer gives rises to a localized mode within the zero-Φeff gap in this kind of structure. It is also shown that the important characteristics of the nonlinear defect mode such as its frequency, its FWHM and the threshold of the associated bistability can be controlled by changing the widths of the defect layer and the host layers.     

TM-like and TE-like Modes Coupling in a Two-Dimensional Photonic Crystal Slab Composed of Truncated Cone Silicon Rods

We numerically investigate the coupling of TE-like modes and TM-like modes in a two-dimensional （2D） photonic crystal （PC） slab composed of truncated cone silicon rods. In such structures, the classification of TE-like modes and TM-like modes is generally impossible and the coupling occurs due to vertical structural asymmetries. The frequency and wavevector dependences of the mode coupling are discussed by investigating the photonic band structures, and the coupling efficiency is studied by examining the transmittance. The results show that the efficiency of mode conversion is strengthened by the vertical asymmetry and weakened by the clear sinall gap. These structures could be used as polarization conversion devices in integrated optics.     

Effective dielectric constant of two-dimensional photonic crystals in optical bands

By using the Fourier expansion method, we have developed an approach to calculate the effective dielectric index of a two-dimensional photonic crystal. The approach is very general: it can take into account various Bravais lattice structure as well as arbitrary spatial variation of the dielectric index. It has been found that near a nondegenerate frequency ω_n(Γ) at Γ point, the transverse magnetic (TM) mode is ordinary, as it is independent of the propagation direction, whereas in general the transverse electric (TE) mode depends on the propagation direction, it is extraordinary. Therefore, a two-dimensional photonic crystal can always be described by an effective dielectric index for TM mode near the nondegenerate frequency ω_n(Γ). However, the TE mode is much more complicated unless the lattice structure is highly symmetric. Moreover, a two-dimensional square photonic crystal has been identified as an effective birefringent crystal having two negative refractive indexes from the perspective of Snell's law.     

Emissions of Photonic Crystal Waveguides with Discretely Modulated Surfaces

Transmission properties of photonic crystal （PC） waveguides with discretely modulated exit surfaces are investigated numerically using the finite-difference time-domain （FDTD） method. Unlike the case of periodically modulated surfaces, where the transmission beam tends to be a single and directional beam, when the exit surfaces are modulated only at several discrete points, the emission power tends to split into multiple and directional beams. We explain this phenomenon using a multiple point source interference model. Based on these results, we propose a 1-to-N beam splitter, and numerically realized high efficiency coupling between a PC sub-wavelength waveguide and three traditional dielectric waveguides with a total efficiency larger than 92%. This simple, easy fabrication, and controllable mechanism may find more potential applications in integrated optical circuits.     

Sub-terahertz on-off switch based on a two-dimensional photonic crystal infiltrated by liquid crystals

A sub-terahertz switch is realized by infiltration of a two-dimensional photonic crystal (PC) with the liquid crystal 5CB. On-off switching is based on a shift of the bandgap of the PC by applying an external electric field which rotates the 5CB molecules. We confirm theoretically and experimentally that rotating the optical axis of the 5CB molecules considerably affects the transmission of the electromagnetic waves of TM polarization in the stop band. The effect can be used for on-off switching of the electromagnetic waves in the sub-terahertz range. Experimentally we demonstrate an extinction ratio of 13.3 dB at 91 GHz.