Negative refraction at infrared wavelengths in a two-dimensional photonic crystal

We report on the first experimental evidence of negative refraction at telecommunication wavelengths by a two-dimensional photonic crystal field. Samples were fabricated by chemically assisted ion beam etching in the InP-based low-index constrast system. Experiments of beam imaging and light collection show light focusing by the photonic crystal field. Finite-difference time-domain simulations confirm that the observed focusing is due to negative refraction in the photonic crystal area.     

Wavelength dependence of focusing properties of two-dimensional photonic quasicrystal flat lens

We investigated the wavelength dependence of the focusing properties of a germanium-cylinder-based two-dimensional (2D) decagonal Penrose-type photonic quasicrystal (PQC) flat lens for the first time, to the best of our knowledge. We found that near the second bandgap and in the high-frequency side (between the bandgap boundary and the first light intensity peak) of the pass band, the flat lens can exhibit a focusing effect for a point light source and that the focusing wavelengths can directly be drawn from the photonic band structure. For all the focusing wavelengths, the summation of the object distance and the image distance is less than the thickness of the flat lens when the object distance is half the thickness of the flat lens. As the wavelength increases, the image distance, the image quality, and the effective refractive index of the flat lens increase, whereas the image power of the point light source decreases. The effective refractive index of the flat lens is less than -1.     

Focusing of light by negative refraction in a photonic crystal slab superlens on silicon-on-insulator substrate

We experimentally demonstrate the light focusing by negative refraction in a photonic crystal slab superlens at wavelengths lambda of 1.26-1.42 microm. The photonic crystal slab was fabricated on silicon-on-insulator substrate with an interface structure optimized for low reflection and diffraction losses. The light focusing in the photonic crystal slab was clearly observed through the intentional out-of-plane radiation or scattering of guided light in the slab. The minimum focused spot width was limited to 1.8 microm(1.4 lambda) owing to aberrations. The focusing characteristics were in good agreement with those obtained from photonic band and finite-difference time-domain analyses.     

Properties of the 3D photonic nanojet based on the refractive index of surroundings

We exhibit a three-dimensional （3D） photonic nanojet based on a dielectric microsphere irradiated by a plane wave with the finite-difference time-domain （FDTD） method. We investigate the influence of the refractive index of the surrounding on the properties of the nanojet by simulating the electric field distributions in it. The simulation results show that, by optimally choosing the size of the sphere and the ratio of the refractive indices of the sphere and the surrounding, the focus point can occur just on the surface of the sphere even if the refractive index of the surrounding is changed. Additionally, the peak amplitude of the nanojet increases with increasing the refractive index of the surrounding. However, the decay length and the jet width of the nanojet decrease simultaneously. These effects may have potential applications in observation or manipulation of nano-objects such as antibodies in biology. In nanojet-enabled optical data storage, the photonic nanojet may be also helpful for improving data-storage capacities and retrieval speed by controlling the field amplitude, the decay length, and jet width of the nanojet.     

Scatterer radius dependence of focusing properties in two-dimensional photonic quasicrystal flat lens

The focusing properties of a germanium decagonal photonic quasicrystal flat lens with different scatterer radii have been analyzed. For different wavelengths and polarization modes, the lenses have different scatterer radius thresholds resulting in different focusing properties. For a point source of light in the transverse-electric mode, we found dual-focusing occurs along the symmetry axis within a range of scattering radii. As scatterer radius increases, the dual-focusing image power and far-field-focusing image quality diminish, whereas near-field-focusing image quality, dual-focusing image distance, and the summed object-image distance all increase.     

Photonic Jet by a Near‐Unity‐Refractive‐Index Sphere on a Dielectric Substrate with High Index Contrast

Photonic jets are normally generated in transmission mode and are represented as a spatially localized high‐intensity region on the shadow side of a particle‐lens, with a background to medium refractive index contrast of 1.3–1.7 while illuminated by a plane wave. Here, a photonic jet is discovered in the opposing plane wave propagation direction and lies in the area in the upper boundary of a near‐unity refractive index sphere on a high refractive index dielectric substrate. The redistribution of the power flow is inhibited during the reflected wave passing the near‐unity refractive index sphere. This has led to a unique effect on the focus position and shape of the produced photonic jet in reflection mode which can be maximally maintained near the sphere regardless of the modulation of the refractive index for the dielectric substrate material.     

Characteristics of photonic nanojets from two-layer dielectric hemisphere

The properties of the photonic nanojet generated by a two-layer dielectric microsphere are studied. Simulation results indicate that this novel structure can generate a photonic nanojet outside its volume when the refractive index contrast relative to the background medium is higher than 2:1 in the condition of plane wave incidence. When the refractive index is smaller than 2, we show that an ultralong nanojet generated by the two-layer hemisphere has an extension of 28.2 wavelengths, and compared with the homogeneous dielectric hemisphere, it has superior performance in jet length and focal distance. Its dependence on the configuration and refractive index is investigated numerically. According to the simulation of the two-layer dielectric microsphere, a photonic nanojet with a full width at half maximum(FWHM) less than 1/2 wavelength is obtained and the tunable behaviors of the photonic nanojet are demonstrated by changing the reflective indices of the material or radius contrast ratio.     

二氧化硅胶体球自组装光子晶体的研究

光子晶体由于具有光子带隙和光子局域等一系列优异的光学特性而受到了人们广泛的关注。由于采用胶体颗粒自组装法制备光子晶体制备工艺简单,所需要的费用也较低,因此已成为制备可见光至红外波段三维光子晶体的一种简便有效的方法。采用垂直沉积法制得了三维光子晶体薄膜,并用扫描电子显微镜和紫外-可见光-近红外分光光度计对其显微结构和光学特性进行了详细的研究。结果表明,自组装薄膜在三维方向上都具有有序结构,其密排面平行于载波片的表面。制备的光子晶体薄膜具有明显的光子带隙特性,带隙中心波长为956nm。研究了带隙中心波长同入射线与密排面法线夹角之间的变化关系,其结果与理论值吻合得很好。     

On‐chip single photon sources using planar photonic crystals and single quantum dots

We review the basic light‐matter interactions and optical properties of chip‐based single photon sources, that are enabled by integrating single quantum dots with planar photonic crystals. A theoretical framework is presented that allows one to connect to a wide range of quantum light propagation effects in a physically intuitive and straightforward way. We focus on the important mechanisms of enhanced spontaneous emission, and efficient photon extraction, using all‐integrated photonic crystal components including waveguides, cavities, quantum dots and output couplers. The limitations, challenges, and exciting prospects of developing on‐chip quantum light sources using integrated photonic crystal structures are discussed.     

Graded photonic quasicrystals

We introduce graded photonic quasicrystals and investigate properties of such structures on the example of a Luneburg lens based on a dodecagonal photonic quasicrystal. It is shown that the graded photonic quasicrystal lens has better focusing properties as compared with the graded photonic crystal lens in a frequency range suitable for experimental realization. The proposed graded photonic quasicrystals can be used in optical systems where compact and powerful focusing elements are required.     

Characterization of internal order of colloidal crystals by optical diffraction

We report the results of the optical characterization of photonic crystals consisting of close-packed arrays of polymeric monodispersed microspheres. The excellent agreement of the diffraction peak wavelengths with Bragg's law enables a precise determination of the sphere diameter. Optical diffraction is used to assess the quality of the internal order of colloidal crystals and to identify optimum growth conditions. This work leads to an estimate of the average free propagation distance of light, which is limited by incoherent scattering due to lattice imperfections, of up to 30 lattice constants.     

Rainbow trapping using chirped all-dielectric periodic structures

We report a numerical investigation of rainbow trapping (light of different wavelengths) at different spatial locations in a newly designed two-dimensional photonic structure that is formed using chirping parameters in two-dimensional photonic crystals. Chirped parameters ensure trapping of certain light wavelengths inside these structures. To achieve broadband electromagnetic wave trapping, we properly adjust and chirp the position and dielectric filling factor of each unit cell within a photonic crystal structure. The low group velocity regions of the dielectric continuum bands at the Brillouin zone edge enable different wavelengths to be slowed and stopped along the propagation direction. The all-dielectric transparent material nature of the proposed structure realizes light trapping in different electromagnetic regions by spatially varying the effective refractive index of the structure.     

Design of leaky modes of two-dimensional photonic crystal slabs to enhance the luminescence from Er3N@C80 fullerenes

We develop an effective way to engineer a two-dimensional GaAs photonic crystal slab with its leaky eigenmodes at desired wavelengths by investigating its spectral dispersion, particularly in terms of transmission efficiency spectra at different launch angles of the light beam. Structural parameters for the photonic crystal slab with leaky eigenmode wavelengths at both 1492 nm and 1519 nm are obtained. This may lead to the enhanced luminescence from erbium-doped trinitride-template fullerenes (Er₃N@C₈₀) on the surface of the photonic crystal slabs.     

Nanophotonics of isolated spherical particles

The problem of extreme focusing of an optical beam into the spatial region with wavelength dimensions is considered with the use of the special features of radiation interaction with isolated spherical particles. Results of numerical computations of the optical field intensity at the surface of silver particles of different radii upon exposure to laser radiation with different wavelengths are presented. It is demonstrated that the relative intensity of the plasmon optical field on the nanoparticle surface increases and the field focusing region decreases with increasing particle radius. Results of numerical computations illustrating the influence of the shell of composite nanoparticles comprising a dielectric core and a metal shell on the optical field intensity in the vicinity of the particle are presented. The problem of local optical foci of a transparent microparticle (photonic nanojets) is investigated. It is established that variation of the micron particle size, its optical properties, and laser radiation parameters allows the amplitude and spatial characteristics of the photonic nanojet region to be controlled efficiently.     

Nonlinear surface plasmon polaritonic crystals

We present an overview of the optical properties of nonlinear surface plasmon polaritonic crystals and their applications to control light with light. Surface plasmon polaritonic crystals are periodically nanostructured metal surfaces or thin metal films that act as two‐dimensional photonic crystals for surface polaritons. Hybritization of such nanostructures with dielectrics exhibiting an optical nonlinear response allows utilization of the electromagnetic field enhancement effects to observe nonlinear effects and bistable behaviour at low light intensities. By changing the geometry of the nanostructured film, the dispersion of the crystal is modified and, thus, electromagnetic mode structure and associated density of states can be controllably tuned in the desired spectral range. This provides enhanced flexibility in engineering the nonlinear optical response of plasmonic crystals in a chosen spectral range for both control and signal wavelengths.     

Selective stop-band switching in two-dimensional multicomponent photonic crystals

A comprehensive theoretical investigation of the stop-band switching in two-dimensional multicomponent photonic crystals is carried out. The calculations are performed within the analytical model based on analysis of the scattering form-factor. Earlier, this approach was used to describe experimental data obtained in a study of synthetic opals representing three-dimensional photonic crystals. We report here on the development of a new version of this model applicable to the class of two-dimensional photonic crystals. The existence of resonant (non-switchable) stop-bands and the possibility of selectively controlling light flows propagating at different wavelengths is predicted. These effects draw up new guidelines for the design of two-dimensional photonic crystals possessing desired optical qualities.     

双零色散光子晶体光纤结构参数的变化对其性能的影响

应用多极法,研究了正六边形结构光子晶体光纤的结构参数改变时,波长范围在0.8—1.8μm之间的双零色散光子晶体光纤的色散特性和非线系数随波长的变化规律.对具有相同结构参数的正六边形结构和正八边形结构进行比较,得到正六边形结构的双零色散光子晶体光纤的色散更加平坦,非线性系数有明显增大的结果.因此,正六边形结构更容易获得色散平坦的高非线性双零色散光子晶体光纤.最终设计了在800nm附近具有平坦色散和高非线性的正六边形双零色散光子晶体光纤.     

近红外波段硅基金属光子晶体平板的亚波长成像特性

利用时域有限差分方法,理论上研究了由正方形金属嵌入背景材料硅中组成的二维正方晶格和三角晶格光子晶体的亚波长成像特性。采用Drude模型描述金属银的色散特性,在近红外波段该模型可以很好地描述金属的实际介电常数。通过结构参数的设计,在上述两种结构中实现了波长在1550nm附近的亚波长成像,并且发现金属对入射光的吸收使得成像位置处的光强稍有降低,但是对于光子晶体亚波长成像的质量并无影响。相对于通常的硅基空气孔型光子晶体亚波长成像器件,该种硅基金属型全固态光子晶体结构更加稳定,因而可以更好地在复杂全光集成回路中加以实际应用。     

光子晶体波导耦合的波分复用研究

两平行光子晶体单模波导的相互耦合组成一个耦合结构。两本征模的色散曲线相交并出现简并,简并模之间的耦合作用使模式的分布发生了改变。由于耦合的作用,各个波长的光波会在不同的波导中传输。在简并点处两本征模发生解耦合,光波会沿着原来的方向传输。将两个不同耦合长度的光子晶体波导耦合结构集成在一起,就可以组成一个三波长的光子晶体波分复用器。     

Sensing properties of fiber Bragg gratings in small-core Ge-doped photonic crystal fibers

We report about fiber Bragg gratings (FBGs) inscribed in two different types of small-core Ge-doped photonic crystal fibers with a UV laser. Sensing properties of the FBGs were systematically investigated by means of demonstrating the responses of Bragg wavelengths to temperature, strain, bending, and transverse-loading. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. Moreover, the bending and transverse-loading properties of the FBGs are sensitive to the fiber orientations. The reasonable analyses for these sensing properties also are presented.