Zhifang Feng

Channel-drop filters in three-dimensional woodpile photonic crystals

Optical waveguides are characterized by high-efficiency transmission of electromagnetic (EM) waves and optical cavities have frequency selective property. The combination of optical waveguides and cavities can result in a large amount of devices in optical integrated circuit, among which a channel-drop filter is an important member. A three-dimensional (3D) woodpile photonic crystal (PC) can possess a complete band gap that allows perfect confinement of EM waves. We theoretically and experimentally study the property of the x-type waveguide and acceptor-type defect cavity that are created in the 3D woodpile PC working in the microwave regime. On the basis of resonant coupling between the x-type wave guide and acceptor-type cavity, a series of three-port in-plane-type channel-drop filters are built: single-cavity, double-cavity, three-cavity, and four-cavity channel-drop filters. All of them exhibit good frequency responding property by changing the size of cavity. The multicavity channel-drop filter demonstrates the possibility to build multichannel wavelength division multiplexers in the 3D PC platform

Focusing properties of a rectangular-rod photonic-crystal slab

The focusing properties of a photonic-crystal (PC) slab consisting of a square lattice of rectangular dielectric rods in the air background are studied theoretically. We employ the finite-difference time-domain method to investigate the field patterns of a point source placed in the vicinity of the PC slab and find that an image can form in the opposite side of the slab in a frequency window located slightly below the fundamental band gap. We change the orientation of the rectangular rods and find that when the rods are arranged asymmetrically with respect to the surface normal of the slab, the image spot can show a vertical shift relative to the point source. The influence of the PC slab thickness on the quality of the image is also analyzed. From these simulation results and the equifrequency-surface contour analysis, we find that the dominant physical mechanism that shapes the focusing behavior of these rectangular-rod PC slabs in the ground photonic band is the self-collimation effect instead of the negative refraction effect.

Engineering the imaging properties of a metallic photonic-crystal slab lens

Focusing properties of a two-dimensional metallic photonic crystal (PC) slab lens are studied through the finite-difference time-domain technique. The PC consists of a square lattice of metallic cylinders immersed in a dielectric background. A good-quality image can form in the opposite side of the slab in both the lowest and second higher transverse magnetic-polarized photonic bands, consistent with the analysis of equifrequency-surface contours. The all-angle negative refraction can be realized and controlled by simply adjusting the background material permittivity,.

Effect of rotational randomness on focusing in a two-dimensional photonic-crystal flat lens

Rotating the basic unit randomly is an important method for the transformation from periodical structures to disordered systems. In this work, we investigate in detail the influence of rotational randomness and surface modification on focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens by the finite-difference time-domain method and experimental measurements. We have found that focusing can appear within a large range of random rotation angles although the quality degrades with the increase of the rotation angle randomness. When the degree of randomness goes beyond a certain value (30°), the focusing effect disappears. However, it is interesting that focusing can reappear with the modification of the surface of the random lens.

Influence of surface termination morphologies on the imaging properties of a composite two-dimensional photonic crystal lens

By the numerical simulation based on the finite-difference time-domain method, we investigate the adjustability of image distance for the same object distance in two-dimensional photonic crystals (PCs). When we add a fraction of a metallic component to the center of each dielectric bar, the PC slab lens can form a non-near-field image and the image distance changes for different surface terminations formed by introducing cylinders at the surface layers whose geometric and physical parameters are different from those of the PC bulk. Furthermore, the image distance can be further tuned by combining the two kinds of cylinders at the surface layers with different ratios of slab length. These simulation results clearly show that the imaging properties can be controlled effectively by changing the surface termination of PC slab lenses.

Photonic band-gap engineering of quasiperiodic photonic crystals

Band-gap engineering of quasiperiodic photonic crystals (QPC) is studied. Owing to the specific structural properties of the QPC, the distribution—especially its maximum position—of the electric field at different upper gap edge frequencies is quite varied. Therefore, by inserting metallic cylinders at different positions, the width of the gaps can be separately adjusted. This could hardly be realized in a periodic photonic crystal.

Experimental Investigation of Relationship between the object- and Image Distance

We experimentally investigate the focusing properties of a triangular two-dimensional photonic crystals of which all the cylinders are coated. The relationship between the location of object and image point is analyzed. Its demonstrated that at a certain frequency with rela- tive refractive index of i1, the position of image point obey the geometric-optics analysis and non-near-fleld imaging can be achieved. DOI: 10.2529/PIERS061211014939

Negative refraction and localized states of a classical wave in high-symmetry quasicrystals

Recently, negative refraction of electromagnetic waves in periodic photonic crystals has been demonstrated experimentally and sub-wavelength images observed. A theoretical and experimental investigation is reported of the electromagnetic wave transport in high-symmetry photonic quasicrystals (QCs). It is shown that negative refraction can appear in these transparent quasicrystalline photonic structures. It is interesting that highly symmetric two-dimensional photonic QCs possess a universal feature for non-near-field focus of two kinds of polarized waves (S wave and P wave). That is, the non-near-field focus for two kinds of polarized waves can be realized by using flat lenses, which consist of some high-symmetric two-dimensional photonic QCs with the same structures and parameters. In addition, some two-dimensional and three-dimensional localized states in defect-free photonic QCs have been found. It is evident that these unusual localized states can be explored by means of electron energy loss spectroscopy.

Influence of unit cell rotated on the focusing in a two-dimensional photonic-crystal-based flat lens

Focusing in photonic crystal (PC) slab has been studied theoretically and experimentally. PCs are composed of a unit cell repeated periodically. In this article, we investigate in detail the influence of the continuous rotation of the unit cell on the focus in a two-dimensional photonic-crystal-based flat lens by the finite-difference time-domain method and experimental measurement. It is found that the focusing can appear within a large range of rotated angle of the unit cell. When the rotation exceeds a certain angle, the focusing disappears. However, it is interesting that the focusing does not depart from the symmetric center because of rotation of the unit cell.

Channel drop filters in 3D photonic crystal

In this paper, we experimentally investigate a channel drop filter in a three-dimensional layer-by-layer photonic crystal (PC) based on the resonant coupling between waveguides and cavities. The working frequency range of the PC device is in the microwave regime. We gradually increase the cavity size and measure the transmission spectra, and found that the channel drop filter is tunable in a wide range of frequency bands. Furthermore, it has excellent frequency selectivity.