Effects of Parameter Modulation on Near-Field Imaging in Photonic Crystal Consisting of Alternately Left-Handed Material and Right-Handed Material

By means of the transfer-matrix method, the effects of parameter modulation on the quality of near-field imaging in one-dimensional photonic crystal consisting of alternately left-handed material and right-handed material are investigated. Based on analyses of the recovery rate and phase shift, the results show that the imaging quality is not obviously affected by the minor changes of layer thickness. In addition, by modulating the material parameters of the left-handed material, it is found that for both the real part and the imaginary part, the system is more sensitive to the permeability than the permittivity for the TE wave. For the TM wave, it is reverse. These properties are very useful to fabricate left-handed material photonic crystals in practice.     

Imaging Properties of a Rectangular-Lattice Metallic Photonic-Crystal Slab

We report the imaging properties of a two-dimensional rectangular-lattice photonic crystal （PC） slab consisting of rectangular metallic rods immersed in a dielectric background. By simulating the electromagnetic wave propagation through such a PC slab with the finite-difference time-domain method, we find that a point source placed in the vicinity of the PC slab can form a good-quality image through the slab. The frequency region where a good-quality image is formed can be controlled by choosing the direction along which the PC slab surface normal is placed.     

Influence of Dielectric Loss on Quality Factors of Photonic Crystal Microcavity Modes

We numerically investigate the quality factors of two-dimensional （2D） photonic crystal （PC） microcavities using an auxiliary differential equations （ADE） technique in the context of finite-difference time-domain （FDTD） method. The microcavities are formed by point defects in the air hole lattice hexagonally patterned in ZnO （zinc oxide） matrix. The quality factors of these microcavities are limited primarily by the absorption of the background dielectric. We show that the ratio between the quality factors of microcavities in lossy and lossless background dielectric depends on the overlap between the field of cavity modes and the absorbing background dielectric in addition to the magnitude of absorption. These results will be helpful when designing and optimizing photonic crystal microcavities formed in lossy medium.     

Optical Tamm States in Dielectric Photonic Crystal Heterostructure

We investigate one-dimensional dielectric photonic crystal and optical Tamm modes formed by superposition of two band gaps and find that this kind of mode can be explained by the single negative materials tunnelling effect. A finite-size dielectric photonic band gap can mimic one kind of effective single negative material and this property sensitively depends on the frequency location in stop-band regions and surface termination and so on. The effective impedance match and effective phase match give the precise position of the optical Tamm mode. Complete transparency via tunnelling is achieved by two opaque media and demonstrates the validity of our approach.     

Focusing Properties in Photonic Crystal Structure Formed by Air Holes in Dielectric Background with Concave Interface

We investigate the focusing properties of photonic crystal structures with a concavo-concave as the photonic crystal boundary. The photonic crystal is constituted by air holes parallelly distributed in a uniform dielectric. A good-quality focus of a plane wave can appear out of the photonic crystal structure, and a strong far field focus is also formed from the photonic crystal interface. A negative-refractive beam at the frequency 0.195 （2πc/α） for the excited Bloch wave mode near the Brillouin zone edges under all incident angles are obtained by simulation.     

A Defect Effect to Light Transmission through Acute Bending Coupled Cavity Waveguide in a Two-Dimensional Photonic Crystal

Light propagation through a coupled-defect waveguide with a 63.5° bend in a two-dimensional （2D） photonic crystal is investigated. The waveguide modes are non-degenerate monopole state and dipole defect state of a square lattice for two different branches. To increaze the transmission in the bending waveguide, we propose a method to rotate the localized state by introducing a cavity. The higher coupling efficiency and transmission shift. new type defect with a sheared square rod into coupled in the bending waveguide are obtained with proper shear     

Zero-Dispersion Slow Light with Wide Bandwidth in Photonic Crystal Coupled Waveguides

By introducing an adjustment waveguide besides the incident waveguide, zero-dispersion slow light with wide bandwidth can be realized due to anticrossing of the incident waveguide mode and the adjustment waveguide mode. The width of the adjustment waveguide （W2） and the hole radii of the coupling region （r＇） will change the dispersion of incident waveguide mode. Theoretical investigation reveals that zero dispersion at various low group velocity vg in incident waveguide can be achieved. In particular, proper W2 and r＇ can lead to the lowest vg of 0.0085c at 1550 nm with wide bandwidth of 202 GHz for zero dispersion.     

Reducing Insertion Loss of Photonic Crystal Couplers by Suppressing the Remained Power

We find that the increasing coupling strength can lead to a decreasing density of power inside the waveguide of a photonic crystal waveguide （PC-WG） directional coupler, which is called the mode power remaining phenomenon. This phenomenon is detrimental to achieving low insertion loss of the coupler. An improved structure of the PC- WG directional coupler is proposed by simply increasing the radii of air holes in the post coupling region. The simulation results demonstrate that the insertion loss can be reduced dramatically by suppressing the remained power, and therefore both the short coupling length （8μm） and low insertion loss （lower than 0.5dB） can be obtained.     

An Integrative Biosensor Based on Contra-Directional Coupling Two-dimensional Photonic Crystal Waveguides

We propose an integrative biochemical sensor utilizing the dip in the transmission spectrum of a normal singleline defect photonic crystal （PC） waveguide, which has a eontra-directional coupling with another PC waveguide. When the air holes in the PC slab are filled with a liquid analyte with different refractive indices, the dip has a wavelength shift. By detecting the output power variation at a certain fixed wavelength, a sensitivity of 1.2 × 10^-4 is feasible. This structure is easy for integration due to its plane waveguide structure and omissible pump source. In addition, high signal to noise ratio can be expected because signal transmits via a normal single-line defect PC waveguide instead of the PC hole area or analyte.     

Independent Modulation of Omnidirectional Defect Modes in Single-Negative Materials Photonic Crystal with Multiple Defects

Single-negative materials based on photonic crystal with multiple defect layers are designed and the free modulation of defect modes is studied. The results show that the multi-defect structure can avoid the interference between the defect states. Therefore, the designed double defect modes in the zero effective-phase gap can be adjusted independently by changing the thickness of different defect layers. In addition, the two tunable defect modes have the omnidirectional characteristics. This multi-defect structure with above-mentioned two advantages has potential applications in modern optical devices such as tunable omnidirectional filters.     

Negative Refraction in One-Dimensional Photonic Crystals with Tilted Interface

We investigate the wave propagation through the tilted interface of one-dimensional photonic crystals. Negative refraction can be realized by excitation of the Bloch states in the extended Brillouin zone with suppressed reflection. Equi-frequency surface analysis shows that the positive refraction, negative refraction or birefringence in this configuration can be achieved under a proper incident angle, which is confirmed by finite-difference timedomain simulations. The results may be useful in applications in the new devices based on one-dimensional photonic or optical waveguide arrays.     

Negative refraction in a honeycomb-lattice photonic crystal

In this paper we theoretically investigate a photonic crystal with dielectric rods in a honeycomb lattice. Two left-handed frequency regions are found in the second and third photonic band by using the plane wave expansion method to analyze the photonic band structure and equifrequency contours. Subwavelength imaging by the photonic crystal flat lens are systematically studied by numerical simulations using the multiple scattering method. Different from the photonic crystals with noncircular dielectric rods in air, this structure is almost isotropic at the optimal frequency for superlensing. As a comparison, flat slab focusing is also demonstrated at other frequencies in the two left-handed regions.     

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.     

Band structure of comb-like photonic crystals containing meta-materials

We study the transmission properties and band structure of comb-like photonic crystals (PC) with backbones constructed of meta-materials (negative-index materials) within the frame of the interface response theory. The result shows the existence of a special band gap at low frequency. This gap differs from the Bragg gaps in that it is insensitive to the geometrical scaling and disorder. In comparison with the zero-average-index gap in one-dimensional PC made of alternating positive- and negative-index materials, the gap is obviously deeper and broader, given the same system parameters. In addition, the behavior of its gap-edges is also different. One gap-edge is decided by the average permittivity whereas the other is only subject to the changing of the permeability of the backbone. Due to this asymmetry of the two gap-edges, the broadening of the gap could be realized with much freedom and facility.     

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.     

Properties of defect modes in one-dimensional photonic crystals containing a graded defect layer

The properties of defect modes in one-dimensional photonic crystals (PCs) containing a graded defect layer are studied theoretically. The relative permittivity and magnetic permeability of the graded defect layer vary continuously along the direction perpendicular to the surface of the layer. The effect of the linear gradation profiles of the relative permittivity and permeability are studied in detail. It is shown that the defect modes appear inside the forbidden band gaps in its transmission spectra and the gradation profiles of the relative permittivity and permeability affect the defect modes significantly. By changing the gradation parameters, the intensity and position of the defect modes can be tuned. Therefore, introducing a graded defect layer in one-dimensional PCs provides possible mechanism for tuning the defect modes. This may be useful in the design of channeled filters.     

Optical properties of one-dimensional photonic crystals containing graded materials

The optical properties of an one-dimensional photonic crystal containing graded materials are studied theoretically. The graded layers have space dispersive permittivity and magnetic permeability which vary along the direction perpendicular to the surface of the layer. The gradation profiles of permittivity are studied in detail. We show that the structure possesses forbidden band gaps in its transmission spectra and the gradation profiles of permittivity affect the band gaps significantly. For the exponential gradation profile ε₁(x) = α e^βx, the number of the band gaps increases and the total frequency region corresponding to the gaps becomes large with increasing parameter β. On the other hand, the position of band gaps can be changed by the adjustment of the gradation profiles even if possessing same volume-average permittivity in the graded layers. Therefore, we can achieve suitable photonic band gaps by choosing gradation profiles of permittivity.     

Ultra-Compact, Subwavelength and Single-Mode Cavity Resonator

We propose an ultra-compact subwavelength cavity resonator that is composed of a bilayer of metamaterials, one with only negative dielectric constant and the other one with only negative permeability. It is shown that the total thickness of the resonator can be much smaller than the resonant wavelength. In addition, this resonator is always single-mode.     

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.     

Design, Fabrication, and Measurement of Two-Dimensional Photonic Crystal Slab Waveguides

Two-dimensional photonic crystal slab waveguides on SOI wafer are designed and fabricated. Photonic band gap, band gap guided mode, and index guided mode are observed by measuring the transmission spectra. The experimental results are in good agreement with the theoretical ones.