Negative Refraction In Photonic Crystals Research Articles

Subwavelength focusing by combining negative-refractive photonic crystal and silicon lens

We propose a new and dynamically focusable subwavelength focusing system, consisting of a negatively refracting photonic crystal and a single silicon lens. The system can adjust the focal length in the range of 370 nm to 4610 nm. We set directional light with a wavelength of 1612nm to incident on the whole system. Numerical simulation through RSOFT shows the smallest focus spot with the half-width of 0.116 λ at a distance of 0.44 λ and calculates focusing efficiency of 45%. In addition, a good result is found that the half-width of the spot is less than 1/2 wavelength at a distance of 2.85 λ from the surface of the structure.

Research on the resolution of NR-PC flat lenses array detecting small target

On the basis of the Finite-Difference-Time-Domain (FDTD) method, the resolution of two dimensional triangular negative-refractive-photonic-crystal (NR-PC) flat lenses array is studied by scanning small target with dynamic scanning scheme. The NR-PC flat lens array studied in this paper is constituted by some identical flat lenses with different arrangements. Numerical simulation shows that the lateral scanning resolutions of NR-PC flat lenses array, as well as the detection area in lateral and longitudinal direction is significantly improved. Because the point defects are introduced in the propagation direction of light field (Z-axis direction) between two flat lens in lateral direction after the single flat lens combined as flat lenses array, leading to the optical field localized in the vicinity of point defects, and thus the lateral detecting and canning performance of NR-PC flat lenses array is improved. Then metal silver defects is introduced to replace some air holes in 3×2(three rows in Z-axis and 2 rows in X-axis) NR-PC flat lenses array. The later detecting and scanning resolution is further improved by an appropriate combination, nearly double of single non-defective flat lens. Theoretical basis of the detecting to small target by NR-PC flat lenses array is provided in this paper.

Broadband polarization beam splitter based on a negative refractive lithium niobate photonic crystal slab

Using a lithium niobate (LN) material, we propose a broadband polarization beam splitter (PBS) with high efficiency by employing a negative refractive photonic crystal (PhC) wedge slab with an angle of 60°. It can split the incident light into two parts at about 90° with TE and TM polarizations. The transmissions of polarized light for an LN-based PBS are more than 80% with a broad angle and wavelength bandwidth of 8° and 70 nm at 1.55 μm, while with a Si-based PhC, no PBS with high efficiency can be realized for the relatively lower transmission of TM output light.

Refocusing resolution based on negative refractive-photonic crystal group with Ag defects for target detection and imaging

Negative refractive-photonic crystal (NR-PC) lenses that can exceed the diffraction limit of focus resolution for imaging and target detection in the near field have gotten more and more special attention in recent years. Three flat lens groups with Ag defects based on NR-PC are designed, and the focusing imaging in the NR-PC three flat lens groups is concluded with the extension of Snell’s law, and the influence on the resolution for a target detection dynamic scanning scheme is simulated by using the finite difference time domain method. An optimal-doped structure with Ag defects is achieved by different simulation combinations. The refocusing resolution 0.18834λ is achieved in the optimal structure and there is approximately a 0.06806λ improvement in the refocusing resolution compared to those undoped with Ag (0.2564λ); it also possesses distinct smaller side-lobes than a single flat lens doped with Ag. This means the optimal detecting ability for the three NR-PC flat lens groups with Ag defects is more improved than that for a single undoped and doped with Ag. This is significant for the perfect imaging being achieved for a particle structure.

On the cylinder air holes of NR-PC double flats lens group with active impurities for lightwave target detection and imaging

In the paper, the influence of the cylinder air holes of negative-refraction photonic crystal (NR-PC) double flats lens group with active impurities on the performance of lightwave target detection and imaging are studied with finite-difference time-domain (FDTD) method. Firstly, we conclude the focusing imaging circuit diagram of NR-PC flat lens with the Snell extension law and geometrical optics principle. Then we use NR-PC flat lens to detect target with dynamic scanning system according to that. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained due to the use of the NR-PC flat lens. We further research the influence of active impurities on target detecting by using NR-PC double flats lens group with cylinder air holes. We use NR-PC flat with active impurities instead of the perfect ones. By using dynamic scanning scheme, we find that it could improve the lateral resolution of target scanning through introducing appropriate active impurities. In conclusion, our investigation optimized the performance of the small target detection and imaging system, and provided the basis for converting an idealized LHM lens into a physically realizable NR-PC double flats lens group.

The cylindrical air holes of the negative-refraction photonic crystal double flat lens group for lightwave target detection and imaging

The influence of the cylindrical air holes of the negative-refraction photonic crystal (NR-PC) double flat lens group on the performance of lightwave target detection and imaging is studied in this paper using the finite-difference time-domain (FDTD) method. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained by using a NR-PC flat lens; consequently, great improvement of the refocusing gain as well as the imaging resolution will be provided. We further research the effects of different positions for target detection by using a NR-PC double flat lens group with cylindrical air holes. Then we use defective air holes instead of perfect ones. By using a dynamic scanning scheme, we find that the distance between two flats could be changed flexibly. And it could improve the lateral resolution of target scanning and enlarge the distance between the target and flat greatly. In conclusion, our investigation optimized the performance of the detection and imaging system, and provided the basis for converting an idealized left-handed material lens into a physically realizable NR-PC double flat lens group.

On the defective NR-PC flat lens for lightwave target detection and imaging

Influence of the defective cylinder air holes of negative-refraction photonic crystal (NR-PC) lens on the performance of lightwave target detection and imaging are studied with finite-difference time-domain method in the paper. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained using the NR-PC flat lens; consequently, great improvement of the refocusing gain as well as the imaging resolution will be provided. We further study the effect on target detection and imaging using the defective NR-PC flat lens. Using dynamic scanning scheme, it is found that its focusing resolution is better than the non-defective lens. We can also get better resolution by appropriately decreasing the radius of the defective cylinder (R = 0.3a). In conclusion, appropriate radius of the defective cylinder (R = 0.3a) will provide better image resolution.

液晶光子晶体的负折射特性研究

Tunable negative refraction photonic crystals have attracted much attention for their potential application value.The photonic crystal with a square lattice of Si cylinders surrounded by optically controlled liquid crystals is investigated.The plane wave expansion method is used to calculate the equi-frequency surface(EFS).The General Snell’s Law and EFS analysis method is used to ensure the ability of tunable refraction.A new photonic crystal filled with liquid crystal structure is designed.Numerical simulation result shows that this kind of tunable negative refraction photonic crystal filled with optically controlled liquid crystals can be achieved.Compared with electronically controlled one,this way has a faster response speed and a more simple structure,which may be used in optically controlled router and all optical networks.

On the ellipse air holes of NR-PC flat lens for lightwave target detection and imaging

Influence of the cylinder air holes or ellipse air holes of negative-refraction photonic crystal (NR-PC) lens on the performance of lightwave target detection and imaging are studied with finite-difference time-domain (FDTD) method in the paper. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained due to the use of the NR-PC flat lens; consequently, great improvement of the refocusing gain as well as the imaging resolution will be provided. We furthermore research the effects of different targets for detecting using NR-PC lens with cylinder air holes. Then we use ellipse air holes instead of the cylinder ones. By using different scanning scheme (lens-fixed scanning scheme and lens-combined scanning scheme), we find that its focusing resolution is better than the cylinder ones. At the same time, the resolution using lens-combined scanning scheme is always better than that using lens-fixed scanning scheme. So our investigation optimizes the performance of the focus-scanning scheme.

Research on light-wave target detection and imaging by using NR-PC flat lens with active impurities

The authors mainly use the two-dimensional finite-difference time-domain method to study the effection on image resolution of target detection and imaging when active impurities are introduced to negative refraction photonic crystals (NR-PC) flat lens. It is demonstrated that because of the influence of the mini-forbidden band and resonance excitation effect, high transmissivity will appear at the resonance frequency of 0.3068(a/λ) when the light wave goes through the NP-PC lens. Also, the results show that the refocused wave backscattered from a target is significantly stronger than the directly backscattered wave when no NR-PC flat lens is applied. Further studies demonstrated that the introduction of active impurities into NR-PC lens provides improved refocusing and image resolution. In conclusion, the authors' investigation optimises the performance of the focus-scanning scheme, and provides the basis for converting an idealised left-handed materials (LHM) lens into a physically realisable NR-PC flat lens.

Two-square target detection and imaging using NR-PC flat lens

The characteristics of two-target detection and imaging from negative-refraction photonic crystal (NR-PC) flat lens are studied by using the two-dimensional finite-difference time-domain method. It is demonstrated that due to the influence of the mini-forbidden band and resonance excitation effect, high transmissivity will appear at the normalized resonance frequency of 0.3068 when the lightwave goes through the NR-PC lens. Meanwhile, the use of the NR-PC lens may introduce at least fourfold improvement of the refocusing resolution, if compared to the directly backscattered lightwave without using the NR-PC lens. In addition, by giving a defination for two-target minimum distinguishable distance, we further investigate the performance of two-target detection and imaging system using the proposed focus-scanning scheme. The results show that the smaller the size of the target pair is, the weeker the interference between the two targets will be, which results in a better performance in the minimum distinguishable distance.

The application of dynamic scanning in target detection and imaging based on an NR-PC flat lens

In this paper a finite-difference time-domain method is used to model and analyze the application of dynamic scanning in target detection and imaging by using an effective negative-refraction photonic crystal (NR-PC) flat lens. The results show that there is a transmission peak, with a value far greater than unity, resulting from the influence of mini-forbidden bands and resonance excitation effect at a resonance frequency of 0.3068(a/λ). Thus, the lightwave emitted from the point source will provide strong backscattered waves after being focused on the target by the NR-PC lens that greatly improves the refocusing resolution and imaging resolution of the backscattered wave. Furthermore, a comparison with a non-dynamic scanning scheme clearly demonstrates that the dynamic scanning scheme provides improved refocusing resolution. In conclusion, our investigation optimized the performance of a detection and imaging system, and provided the basis for converting an idealized LHM lens into a physically realizable NR-PC flat lens.

Numerical research on properties of optical-wave target detection and imaging from NR-PC flat lens

The characteristics of optical-wave target detection and imaging from a negative-refraction photonic crystal (NR-PC) flat lens are studied with a two-dimensional (2D) finite-difference time-domain (FDTD) method in the paper. The results show that there exists a transmission peak, with a value far greater than unit, resulting from the influence of the mini-forbidden bands and resonance excitation effect at resonance frequency. Also, the refocusing of backscattered optical wave from the target leads to its image with a sub-wavelength lateral resolution and great amplitude as long as the target is placed at the focus of the NR-PC flat lens. Therefore, the NR-PC flat lens will find many implications in optical area, especially in the detection and imaging of small targets.

Image resolution and the properties of optical-wave target detection and imaging by using the NR-PC flat lens

We used the method of two-dimensional (2D) finite-difference time-domain (FDTD) to study the characteristics of optical-wave target detection and imaging from negative-refraction photonic crystal (NR-PC) flat lens in the paper. The theory of image resolution was introduced. The results show that there exists a transmission peak, with a value far greater than unit, resulting from the influence of the mini-forbidden bands and resonance excitation effect at resonance frequency. And, the refocusing of backscattered optical wave from the target leads to its image with a sub-wavelength lateral resolution and great amplitude as long as the target is placed at the focus of the NR-PC flat lens. And the application of NR-PC flat lens can be extended in optical area, especially in the detection and imaging of small target.

Efficient tunable negative refraction photonic crystal achieved by an elliptic rod lattice with a nematic liquid crystal

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated the electromagnetic wave propagation inside an elliptic rod PC slab by means of finite-difference time-domain simulations. The band structure of the PC composed of elliptic rod in the square and triangular lattices is studied by solving Maxwell's equations using the plane wave expansion method. Numerical simulations show that the refractive angle can be tuned greatly by rotating the directors of elliptic rod in the PC slab. Furthermore, an optical switch based on elliptic rod PC structures with nematic liquid crystals was proposed. In the on/off switching system, the partial band gap can be controlled when the normalized operation frequency is 0.28. The modulation induced by liquid crystals created a sharp switching in the photonic devices. Such a mechanism of negative refraction PCs should open up a new application for designing components in photonic integrated circuits.

Optical imaging superresolution by a negative refraction anisotropic photonic crystal

Abstract Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated the focusing of electromagnetic wave propagation inside an anisotropic PC slab by means of finite-difference time-domain simulations. The PC structure composed of air cylinders in the anisotropic–dielectric medium is studied by solving Maxwell's equations using the plane wave expansion method. Numerical simulations show that the optical imaging resolution can be improved greatly in the square lattices consisting of anisotropic–dielectric background. Such a mechanism of negative refraction PCs should open up a new application for designing components in optical imaging systems.

Improvement of optical imaging resolution by a negative refraction photonic crystal with a solid immersion lens

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated the solid immersion lens (SIL) technology in imaging system based on negative refraction PCs and analyzed the influence of refractive index and geometric parameters of SIL on imaging resolution. In the finite element method calculation, the resolution of our optical system has improved greatly. The high performance of imaging resolution was achieved with shorter radius and larger refractive index of SIL. Furthermore, the effects of the three kinds of SILs at the same radius were analyzed. Such a mechanism of negative refraction PCs and SILs should open up a new application for designing components in optical imaging systems.

Uncoupled modes and all-angle negative refraction in walled honeycomb photonic crystals

Left-handed materials have superlensing effects that enable them to surmount the optical diffraction limit. A photonic crystal is able to mimic some properties of all-angle left-landed materials. In this study, the all-angle negative refraction criteria of photonic crystals are evaluated. The MIT Photonic-Bands program is employed to calculate the band structure of walled honeycomb photonic crystals, and the finite-difference time-domain method is used to provide a snapshot of the electric field distribution inside and outside the honeycomb photonic crystals. The results indicate that the all-angle negative refraction phenomena of the honeycomb photonic crystals are correlated with the orientation of the photonic crystals. Furthermore, the role of the uncoupled modes varies based on their orientation to the all-angle negative refraction photonic crystals, in one case assisting negative refraction and in the other case preventing it. The results suggest that symmetric properties should not be ignored when considering the negative refraction of photonic crystals.

Achieving Subdiffraction Imaging through Bound Surface States in Negative Refraction Photonic Crystals in the Near-Infrared Range

We report the observation of imaging beyond the diffraction limit due to bound surface states in negative refraction photonic crystals. We achieve an effective negative index figure of merit [-Re(n)/Im(n)] of at least 380, approximately 125x improvement over recent efforts in the near-infrared range, with a 0.4 THz bandwidth. Supported by numerical and theoretical analyses, the observed near-field resolution is 0.47lambda, clearly smaller than the diffraction limit of 0.61lambda. Importantly, we show this subdiffraction imaging is due to the resonant excitation of surface slab modes.

Calculation of effective permittivity, permeability, and surface impedance of negative-refraction photonic crystals

We consider the eigen-fields of a two-dimensional negative-refraction photonic crystal and obtain negative effective permittivity and negative effective permeability. Effective permittivity, permeability, and surface impedance are calculated by averaging the eigen-fields. The value of the surface impedance is shown to be location-dependent and is validated by finite-difference time-domain simulations. The unique power propagation mechanism in the photonic crystal is demonstrated through time-evolution of eigen-fields.