2D Photonic Crystal Structures Research Articles

Photonic band gaps of a two-dimensional square lattice composed by superconducting hollow rods

In this paper by means of the plane wave expansion method, we have calculated the photonic band structure of 2D photonic crystals consisting of high temperature superconducting hollow cylinders arranged in a square lattice. Band structures were obtained at low frequencies and assuming TM polarization of the incident wave, for different inner radii of the cylinders and for two different temperatures (5K and 15K), showing the tunability of photonic band gaps with respect to these parameters. Interesting features, such as the decreasing of cutoff frequency and separation of photonic modes were observed by increasing both the temperature and inner radius. Permittivity contrast and the difference between the inner and outer radius lead to the appearance of new band gaps when compared with the case of solid cylinders. These band gaps can be modulated by the width of the shell and temperature, which may be used for the development of novel optical devices.

Magnetic field role on the structure and optical response of photonic crystals based on ferrofluids containing Co0.25Zn0.75Fe2O4 nanoparticles

Ferrofluids based on magnetic Co0.25Zn0.75Fe2O4 ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions of Co (II), ZnSO4, and Fe (III) in an alkaline medium. Ferrofluids placed in an external magnetic field show properties that make them interesting as magneto-controllable soft photonic crystals. Morphological and structural characterizations of the samples were obtained from Scanning Electron Microscopy and Transmission Electron Microscopy studies. Magnetic properties were investigated with the aid of a vibrating sample magnetometer at room temperature. Herein, the Co0.25Zn0.75Fe2O4 samples showed superparamagnetic behavior, according to hysteresis loop results. Taking in mind that the Co-Zn ferrite hysteresis loop is very small, our magnetic nanoparticles can be considered soft magnetic material with interesting technological applications. In addition, by using the plane-wave expansion method, we studied the photonic band structure of 2D photonic crystals made of ferrofluids with the same nanoparticles. Previous experimental results show that a magnetic field applied perpendicular to the ferrofluid plane agglomerates the magnetic nanoparticles in parallel rods to form a hexagonal 2D photonic crystal. We calculated the photonic band structure of photonic crystals by means of the effective refractive index of the magnetic fluid, basing the study on the Maxwell-Garnett theory, finding that the photonic band structure does not present any band gaps under the action of applied magnetic field strengths used in our experimental conditions.

Measurement of glycerol concentration in B–H–G solution using 3D photonic crystal structure

A novel technique is used to measure the concentration of glycerol in blood-hemoglobin-glycerol (B–H–G) solution using 3D photonic crystal structure is presented in this paper. Glycerol concentration is estimated accurately by measuring the intensities of transmitted light. Here, reflection as well as absorption losses is considered to measure the transmitted intensity of light having wavelength 540nm. The principle of measurement is based on the linear variation of photonic band gap with respect to glycerol concentration. Simulations for photonic band gap are made using plane wave expansion (PWE) method. An experimental set up is theoretically designed to measure the concentration of glycerol at different hemoglobin concentration.

Ultrasensitive silicon photonic-crystal nanobeam electro-optical modulator: Design and simulation

Design and simulation results are presented for an ultralow switching energy, resonator based, silicon-on-insulator (SOI) electro-optical modulator. The nanowire waveguide and Q ~8500 resonator are seamlessly integrated via a high-transmission tapered 1D photonic crystal cavity waveguide structure. A lateral p-n junction of modulation length L(m) ~λ is used to alter the index of refraction and, therefore, shift the resonance wavelength via fast carrier depletion. Differential signaling of the device with ΔV ~0.6 Volts allows for a 6 dB extinction ratio at telecom wavelengths with an energy cost as low as 14 attojoules/bit.

Structural color mechanism in the Papilio blumei butterfly

The structural color found in biological systems has complicated nanostructure. It is very important to determine its color mechanism. In this study, the 2D photonic crystal structures of the Papilio blumei butterfly were constructed, and the corresponding reflectance spectra were simulated by the finite-difference time-domain method. The structural color of the butterfly depends on the incident angle of light, film thickness, film material (film refractive index), and the size of the air hole (effective refractive index). Analysis of simulations can help us understand the hue, brightness, and saturation of structural color on the butterfly wing. As a result, the analysis can help us fabricate expected structural color.

The enlargement of high reflectance range in ultra-narrow bandpass filter with disordered one-dimensional photonic crystal

The conventional ultra-narrow bandpass filter structure has only a very limited width of high-reflectance range. This study, by introducing disorder into one-dimensional (1D) photonic crystal, attempts to enlarge the width of high-reflectance range while keeping the ultra-narrow bandpass. Enlargement by 46.8% was obtained after theoretical design. Since this structure contains some degree of disorder already, it has a strong tolerance of the variation of layer thicknesses. Unlike studies using conventional periodic structures, theoretical statistical results in this study demonstrate that high quality remains even after allowing for ±5% variation of layer thicknesses. This indicates that only a very low thickness control precision is required in the future and the production difficulty is immensely lowered. To put the construction to test, a structure has been developed and demonstrated by a magnetron reactive sputtering coating system, which agrees with the theoretical result very well. By introducing disorder into the periodic 1D photonic crystal structure, the high-reflectance range is significantly extended by 37%, with an ultra-narrow pass band of 0.8 nm and intensity of 82%.

Anomalous transport of light in photonic crystal

Photonic crystal (PC) offers a powerful means to mold the flow of light and manipulate lightmatter interaction at subwavelength scale. In this paper, we review some recent theoretical and experimental work in our group on design and fabrication of microwave and infrared PC structures with the capability to achieve various anomalous transport behaviors of light. We discuss several microwave 2D PC and quasi-crystal structures that exhibit nearly isotropic equi-frequency surface (EFS) contours with effective refractive index equal to -1. In these structures, we can observe negative refraction induced focusing of microwave against a flat slab lens in non-near field regions. In comparison, if PC structures have anisotropic EFS contours in the lowest photonic band, only near-field focusing is expected. We move forward to high frequency infrared band and exploreremarkable dispersion properties of silicon 2D PC slab to achieve broad-band negative refraction and self-collimation transport of infrared light beam. We also explore the possibility to realize negative refraction and flat-lens focusing of light in 3D PC made from inverse opal. These studies show that PCs can offer a powerful route to manipulate various anomalous transport of light via photonic band gap and band structure engineering, which can be harnessed to build a wide variety of integrated optical devices for large-scale optical integration.

Experimental System for the Micro-Nanofabrication of Three-Dimensional Structures by Femtosecond Laser Two-Photon Absorption

Fundamentals of two-photon photopolymerization have been introduced and a 3D femtosecond laser micro-nanofabrication system has been built. In this paper, 3D CAD data model based on femtosecond laser micro-nanofabrication system have been also discussed. The 3D various sphere-rod photonic crystal structure mimicking real atom structures in electronic crystals have been fabricated.

Study of Defect Modes in 1d Photonic Crystal Structure Containing High and Low T c Superconductor as a Defect Layer

The present paper describes the study of defect modes in a one-dimensional photonic crystal (1d-PC) containing a high and low temperature superconductor as a defect layer at different temperatures below the superconducting transition temperature (T c ). Since the refractive index of the superconducting material is dependent on the penetration depth, which depends on the temperature of the superconducting material, hence by changing the temperature of the superconducting material its refractive index can also be changed. Analysis of the transmission spectra of defect modes in the reflection band of 1d-PC structure shows a shift in the wavelength peak of the defect mode. The shift in peak is different for different superconducting materials and it increases with the increase in temperature whether the defect layer is high T c or low T c superconductors. We also study the presence two defect layers in a 1d-PC structure, one with high T c and other with a low T c superconductor. Further, the effect of variation in the thickness of the defect layer on the defect modes of the PC structure has also been studied In order to obtain the transmission (reflection) spectra of a 1d-PC structure with a defect, we employ the transfer matrix method (TMM). This property of the defective PC structure can be exploited in designing the temperature sensor and narrow optical filters. Further, this tunable feature of superconductor photonic crystal has technical use in the superconducting electronics and photonics.

Fabrication of two-dimensional InGaN/GaN photonic crystal structure using a modified nanosphere lithography technique

By means of combining a very cost-effective lift-off process and a nanosphere lithography technique, we have fabricated two dimensional (2D) photonic crystal (PhC) structures on an InGaN/GaN multiple quantum well structure. Significant enhancement in photoluminescence (PL) intensity has been observed when the emission wavelength is within the photonic bandgap. Time-resolved PL measurements have shown that the spontaneous emission rate is strongly reduced by a factor of ∼4 due to the PhC effect. As a consequence, the emission intensity along 2D PhC slab-plane directions is effectively suppressed and redistributed to the direction normal to the 2D PhC slab-plane simultaneously. Temperature-dependent PL measurements have confirmed that the enhanced PL intensity is due to an increase in extraction efficiency as a result of the PhC effect.

A liquid refractive index detect method based on negative refraction phenomenon in 2D photonic crystal structure

This paper is mainly concerned with the investigation of negative refraction phenomenon dependent on the background refractive index in 2D photonic crystal, which consists of a hexagonal lattice of circular dielectric rods with Si. The paper presents the relationship between background refractive index and negative refraction. The relationship is investigated by using the finite-difference time-domain (FDTD) method on software RSoft. The results point out that the output power is added with the increase of the background refractive index. With the research, a new fluid refractive index detection method is proposed. Compared with other normal liquid refractive index analyzer reported, this detector has advantages of faster detection, less stray light interference and miniaturization.

Y-shaped design in two dimensional photonic crystal structure for applications in integrated photonic circuits

A new Y-shaped design in a 2D-photonic crystal structure composed of air holes in silicon substrate is proposed for realization of optical logic 1 and 0. A suitable operating principle for the realization of the logic operation is presented, which is then verified by FDTD simulations. The proposed structure is expected to be a better candidate for application in optical logic gates.

Position dependence of extraction efficiency in organic light-emitting diodes with photonic crystal structure

We demonstrate by finite-difference time-domain simulations that a one-dimensional (1D) photonic crystal (PC) structure between glass substrate and indium tin oxide layer can improve the light extraction efficiency of organic light-emitting diodes. The extraction efficiency depends on the emitters' positions varying laterally in a unit cell of PC. The highest efficiency is obtained when the emitters are under higher refractive index strips. Efficiency decreases when the emitters shift to lower refractive index strips. Simulations for both transverse magnetic and transverse electric modes indicate that when emitters are close to the middle of the higher refractive index strips, the guided wave transmits with less divergence and inhibited reflection because of the guiding effect of higher refractive index strips. A modified method that considers the position effects is proposed to calculate the extraction efficiency more precisely.

Four-branching waveguide in 2D photonic crystal structure for WDM system

Propagation and filtering characteristics of two dimensional pillar-type photonic crystal waveguide with triangular lattice were measured by a scale model in microwave frequency with lattice period P = 26.5 mm, diameter of pillar ϕ = 7.5 mm and its dielectric constant er = 36.0 around 4 GHz. First, symmetric Y-branch waveguide was set up by using line-defect in array of pillars. Two kinds of set of pillars which have length of 3.0P and 2.5P, were situated in each output waveguide as cavities to show and good resonances and extinction ratios. Next, cascaded Y-branch with four cavities with different lengths at each output port was observed, but unnecessary resonances appeared near main peaks of filtered frequency. To suppress them, stabs with length of λg/4 of unnecessary resonance were inserted after the cavities. Due to the stabs, peaks of the unnecessary resonances were suppressed over 20 dB. This structure is applicable as a filter component in wavelength division multiplexing (WDM) systems in photonic crystal optical or microwave circuits.

Stretchable Photonic Crystal Cavity with Wide Frequency Tunability

We report a new approach for realizing a flexible photonic crystal (PC) cavity that enables wide-range tuning of its resonance frequency. Our PC cavity consists of a regular array of silicon nanowires embedded in a polydimethylsiloxane (PDMS) matrix and exhibits a cavity resonance in the telecommunication band that can be reversibly tuned over 60 nm via mechanical stretching-a record for two-dimensional (2D) PC structures. These mechanically reconfigurable devices could find potential applications in integrated photonics, sensing in biological systems, and smart materials.

Optimization of slow light one-dimensional Bragg structures forphotocurrent enhancement in solar cells

In 1D photonic crystal Bragg structures, strong localization of the light occurs in the high refractive index layers at wavelengths on the red edge of the photonic bandgap. We exploit this slow light effect for thin film solar cells in order to increase the absorption of light in silicon, as the latter has a high refractive index. Amorphous silicon and a transparent conductive oxide are chosen as high-index and low-index materials, respectively. Reference thin film cells have equivalent total thickness of amorphous silicon, plus antireflection coating and optional metallic back mirror. Through transfer-matrix calculations, we demonstrate that the spectrally integrated photon flux absorbed in active layers, hence the photocurrent, is enhanced by at least 10% with respect to reference using only a few periods. The enhancement is robust with respect to the light incidence angle. The key of such an enhancement is the tuning of the red edge to both the solar irradiance spectrum and the intrinsic material absorption spectrum, which is achieved by suitably selecting the layer thicknesses.

The optical responses of one-dimensional photonic crystals based on the transparent Ag‐anodic aluminum oxide composites with super low-refractive index

The super low refractive index of less than 1 is obtained through adjusting the deposition time of nano-metal Ag in Ag-anodic aluminum oxide (Ag-AAO) composite films. It is very interesting that the Ag-AAO composite films with super low refractive index are still transparent. Based on the results, the Ag-AAO composite films and titanium dioxide (TiO2) films were used to assemble 1D TiO2/Ag-AAO photonic crystal (PC) structure and its optical responses were simulated in the 500–2500nm range by using the finite difference time domain method. It is found that the increase of Ag-AAO layer thickness makes the photonic band gap (PBG) become broad greatly, but has few effects on the blue edge of the PBG, which is different from the effect of TiO2 layers. It is more important that a broad omnidirectional PBG can be easily obtained from the simple 1D TiO2/Ag-AAO photonic crystal structure through the modulation of both the TiO2 and Ag-AAO layer thicknesses. Furthermore, the absorption of 1D TiO2/Ag-AAO PC structure at the PBG edges is very strong due to the photon localization in Ag-AAO layer. The different roles of Ag-AAO layer in the above mentioned 1D PC structure are mainly attributed to the different transmission process of optical pulse in Ag-AAO layer.

Continuous wave mirrorless lasing in cholesteric liquid crystals with a pitch gradient across the cell gap

Despite numerous efforts, continuous wave (CW) lasing in dye doped, one-dimensional (1D) photonic bandgap cholesteric liquid crystal (CLC) structures has not been previously reported, to our knowledge. Here we report on the observation of lasing in such structures under both coherent (laser) and incoherent (LED) CW light excitation. To achieve this effect, we used a 1D-photonic bandgap structure made of a polymer stabilized CLC with a pitch gradient across the cell thickness. A spectral reflectivity profile of such a CLC structure reveals local minima in the area within a photonic stopband and close to it. The realization of lasing pumped by low power CW light sources opens the possibility of all-organic, compact, tunable CW lasers for display and medical applications.

Implementation of optical logic gates using closed packed 2D-photonic crystal structure

A proposal for implementation of optical logic gates using a two dimensional closed packed photonic crystal structure is presented here. The logic operations are realized by a control signal and the input signal(s) applied across two adjacent faces, while the output is obtained along one of the remaining faces. Finite Difference Time Domain simulations show that, this structure can be used to realize AND, OR, NOT, XNOR, NAND operations, if phase difference of π/2 is maintained between the control and the input signal. Further it is found that the same structure can be used as a switch, if Kerr nonlinearity is introduced to the dielectric rods constituting the photonic crystal structure.

Efficient Transparent Thin Dye Solar Cells Based on Highly Porous 1D Photonic Crystals

AbstractA working electrode design based on a highly porous 1D photonic crystal structure that opens the path towards high photocurrents in thin, transparent, dye‐sensitized solar cells is presented. By enlarging the average pore size with respect to previous photonic crystal designs, the new working electrode not only increases the device photocurrent, as predicted by theoretical models, but also allows the observation of an unprecedented boost of the cell photovoltage, which can be attributed to structural modifications caused during the integration of the photonic crystal. These synergic effects yield conversion efficiencies of around 3.5% by using just 2 μm thick electrodes, with enhancements between 100% and 150% with respect to reference cells of the same thickness.