Kun Ren

High resolution three-port filter in two dimensional photonic crystal slabs

An in-plane, three-port filter consisting of input/output waveguides and two point-defect cavities in a 2D PC slab is designed and fabricated, where a new feedback method is introduced, and its transmission properties are measured. The measured minimum output wavelength spacing between two channels is 1.5 nm, which is realized by slightly adjusting the size of the resonant cavities. The measured resonant wavelengths of two cavities agree well with the calculated ones and the quality factors of the cavities are almost the same. It is believed that this kind of filter may be useful in optical integrated circuits with high density.

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.

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.

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

Permanent tuning of the opal stop-band with the application of uniaxial pressure

Three-dimensional photonic crystals have been prepared by self-assembly from sub-micrometre polystyrene spheres. Such crystals are known as a synthetic opal and possess a partial stop-band in the direction. The stop-band is tuned towards the shorter wavelengths by applying uniaxial pressure along the direction of the face-centred cubic opal structure. The tuning is found to be irreversible and it was also observed that the normalized stop-bandwidth increased after applying uniaxial pressure. The experimental results are compared with the simulation results and found to be in very close agreement.

Designed Negative Refraction in Photonic Crystals

Summary form only given. We report our recent progress on the negative refraction and imaging properties of 2D and 3D photonic-crystal (PC) slab lens. We have employed several theoretical and numerical tools, including plane-wave expansion method, multiple-scattering method, and finite-difference time-domain technique, to analyze the negative refraction and imaging behavior of the PC slab lens. We have found that negative refraction and superlensing effect can occur in a large variety of 2D PC structures with appropriately designed geometric and physical parameters. The crystal can be made from circular, square, rectangular, and elliptical rods. The lattice can be either square or triangular lattices, or even 12-fold quasiperiodic. The composite material can be dielectric, metallic, or metallo-dielectirc. Negative refraction can happen at the lowest or higher photonic bands. Different mechanisms can contribute to the superlensing phenomenon, such as the negative refraction and self-collimation effects. We have also carried out experimental measurement on the negative refraction and imaging properties of these PC slab lens and find good agreement with the theoretical analysis.

A Photonic Crystal Slab Lens for Three-dimensional Negative Refraction

In this paper we study negative refraction of EM waves in three-dimensional (3D) inverted-opal photonic crystal that consists of periodic air-spheres embedded in a dielectric background. We investigate the focusing properties of flat lens made of this photonic crystal structure. We find that wave focusing can be achieved in such a 3D photonic crystal structure at the negative-refraction frequency window.