Experimental verification of negative refraction in a double cross metamaterial

We report on microwave experiments with a metamaterial composed of pairs of metallic crosses. The transmission properties of the structure show a left-handed transmission band at frequencies around 10.2 GHz. The validity of the negative effective index of refraction is verified by a Snell’s law refraction experiment performed on a wedge-shaped sample of the metamaterial. A second measurement of a similar wedge made from blank FR4 boards is done for reference. The results of the measurements show positive refraction over the whole measured frequency band for the FR4 wedge as well as the refraction of the incident radiation to negative angles within the designated left-handed frequency band for the metamaterial sample.     

Omnidirectional and independently tunable defect modes in fractal photonic crystals containing single-negative materials

The properties and applications of omnidirectional and independently tunable defect modes in fractal photonic crystals containing single-negative materials are demonstrated. The proposed fractal structures can produce as many defect modes as desired by adjusting its structural parameters. The interaction effect between the defect states of such fractal structures is avoided, so the frequency of each defect mode can be tuned independently. Furthermore, these defect modes inside the zero effective-phase gap are insensitive to the incident angle. With perfect transmission, mode controllability and omnidirectional compatibility, these structures open a promising way for designing omnidirectional multichannel filters with specific channels.     

Experiment study of tunneling phenomenon occurring in the photonic heterostructure containing single-negative metamaterials

In this paper, the tunneling phenomenon occurring in the photonic heterostructure consisting of single-negative (SNG) metamaterials is experimentally studied. First, the SNG metamaterials are fabricated using coplanar waveguide with lumped-element series capacitors and shunt inductors loading. Then, the tunneling phenomenon occurring in the photonic heterostructure containing SNG media is experimentally demonstrated. Moreover, the insensitivity of the tunneling frequency to the breakdown of periodicity of phonotic crystals was also illustrated.     

Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro-spectroscopy experiments and beam propagation calculations

The effect that femtosecond laser filamentation has on the refractive index of Nd:YAG ceramics, and which leads to the formation of waveguide lasers, has been studied by micro-spectroscopy imaging, beam propagation experiments and calculations. From the analysis of the Nd³⁺ luminescence and Raman images, two main types of laser induced modifications have been found to contribute to the refractive-index change: (i) a lattice defect contribution localized along the self-focusing volume of the laser pulses, in which lattice damage causes a refractive-index decrease, and (ii) a lattice strain-field contribution around and inside the filaments, in which the pressure-driven variation of the inter-atomic distances causes refractive-index variations. Scanning near-field optical-transmission and end-coupling experiments, in combination with beam propagation calculations, have been used to quantitatively determine the corresponding contribution of each effect to the refractive-index field of double-filament waveguides. Results indicate that the strain-field induced refractive-index increment is the main mechanism leading to waveguiding, whereas the damage-induced refractive-index reduction at filaments leads to a stronger mode confinement.     

Highly collimated emission from a left-handed photonic crystal with a quasi-cavity

We study the collimated emission characteristics from a dipole source inside a negative-effective-refractive-index photonic crystal with a quasi-cavity constructed by a concave photonic crystal reflector. The emissions along the ±X and −Y directions are forbidden by the quasi-cavity, so that most emissions propagate along the +Y direction. Simulation results show that a narrow collimated beam is achieved due to the near-zero negative effective refractive index. Moreover, the half-power beam width of such a collimated beam can be reduced to 3.48° by optimizing the size of the source area. Such a compact structure would have potential applications in micro-optical devices.     

Zero-averaged refractive-index gaps extension by using photonic heterostructures containing negative-index materials

We show theoretically that the frequency range of the zero-averaged refractive-index gap can be substantially extended in a photonic heterostructure containing negative-index materials. This photonic heterostructure consists of different one-dimensional (1D) photonic crystals. The constituent 1D photonic crystals have to be properly chosen in such a way that their zero-averaged refractive-index gap of the adjacent photonic crystals overlap each other.     

Photonic crystal slow light waveguides with large delay–bandwidth product

In this paper, we propose the use of two two-dimensional photonic crystal line defect waveguides for slow light with large delay–bandwidth product (DBP). One includes air rings localized at each side of the line defect and the other modifies the radius and distance of holes at each side of the waveguide. We show that we can achieve a very flat band corresponding to nearly constant group index over a broad frequency range by adjusting the parameters of the structure. We show further that the group velocity dispersion (GVD) can reach a relatively small amount and the DBP can be more than 0.6 for the first waveguide and 0.34 for the second waveguide. Numerical simulation by the finite-difference time-domain (FDTD) method demonstrates the propagation of the broadband pulse.     

Negative index materials with gain media for fast modulation

Negative index materials (NIM) enable subwavelength resolution and are promising for applications in integrated optical systems, since the mode volume is small. Most promising NIM systems essentially use noble metals (Ag, Au) with material losses much lower than in other metals, but still rather hefty, like in metal–dielectric–metal “fishnets”. Therefore, we perform extensive finite-difference time-domain modeling of NIM “fishnets” in combination with gain medium, InGaAsP multiple quantum wells in the present work. The signal recovery is weak, which is related to weak overlap between the radiation field and the gain medium. The signal modulation speed may be very large, in a picosecond range.     

Transmission characteristics of wave modes in a rectangular waveguide filled with anisotropic metamaterial

In this paper, transmission characteristics of wave modes in a rectangular waveguide filled with lossless anisotropic metamaterial are theoretically investigated. The wave equation and dispersion relations for TE and TM modes in the waveguide are obtained and analyzed. It is shown that the negative constitutive parameters of the filling anisotropic metamaterial can be used to control the wave magnitude and the phase velocity direction in the waveguide over the entire frequency domain, both below and above the cutoff frequency. Particularly, not only backward waves, but also forward waves can propagate below the cutoff frequency in the waveguide. Furthermore, a typical example is calculated to demonstrate transmission characteristics of waves inside the waveguide. Numerical results are obtained in the paper and compared with theoretical predictions: a good agreement is found.     

Binary metal and semiconductor quantum dot metamaterials with negative optical dielectric constant and compensated loss for small volume waveguides,modulators and switches

We study numerically and analytically a binary mixture of quantum dots exhibiting gain and loss. For a mixture of gain quantum dots and silver nanoparticles, we find conditions when the composite shows negative dielectric constant operation and lossless operation. The composites of this kind may be used for dense integration of photonic components as well as modulation and switching in optical interconnect systems L. Thylen is also at Dept of Microelectronics and Applied Physics, Royal Institute of Technology (KTH), 164 40 Kista, Sweden.     

Fabrication of three-dimensional (3D) woodpile structure photonic crystal with layer by layer e-beam lithography

Photonic crystal based superprism offers a way to design new optical components for beam steering and DWDM (Dense Wavelength Division Multiplexing) application. Three-dimensional (3D) photonic crystals are especially attractive as they could offer more control of the light beam. A FCT (Face-Centered-Tetragonal) woodpile structure has been fabricated using layer by layer stacking techniques with E-Beam lithography. Special planarizations and processes have been introduced to ensure the survivability and good alignment of the fabricated nanostructures. Scanning electron microscopy results proved the structure uniformity. With the proper design, the structure exhibits superprism effects around 1550 nm, and such effects have been observed in the experiments.     

Polarization-independent low-pass spatial filters based on one-dimensional photonic crystals containing negative-index materials

A new application of one-dimensional photonic crystals containing negative-index materials is proposed as low-pass spatial filters. Through optimizing the parameters of defect layer, a series of polarization-independent defect modes in the zero-average-index gap of the photonic crystals are obtained with the increase of the incident angle. Based on these defect modes, polarization-independent low-pass spatial filters are designed. The spatial-frequency bandwidth of the spatial filters can be adjusted by changing the period number of the defective photonic crystal structures. In addition, the effect of the losses of negative-index materials on the spatial filters is considered.     

Effects of disorder on the frequency and field of photonic-crystal cavity resonators

In recent years the application of 2-Dimensional (2D) metallic Photonic-Crystal (PC) structures to high-power microwave devices, such as particle accelerators and gyrotrons, has gained increased interest. In this paper we focus on the effect disorder has on the resonant frequency and peak electric field in the defect site of a 2D PC structure. For disorders up to a maximum of 15% variation in position and radius, we found that disorder applied to the innermost rods surrounding the defect site dominates in determining the peak field and resonant frequency of the structure. We also show that small disorder (∼1%) can lead to an increase in peak field in certain cases due to structure optimization. We find that increasing levels of disorder lead to a decreasing average peak field for all structures. Whereas the mean resonant frequency remains constant for increasing disorder while the standard deviation increases. We then develop an understanding for this behaviour in terms of frequency detuning and mode confinement.     

Multi-layer cladding leaky planar waveguide for high-power applications

We design a multi-layer cladding large-core planar waveguide that supports a single guided mode. The waveguide works on the principle of higher-order mode discrimination. The cladding of the waveguide is formed by alternate low- and high- index regions, which helps leaking out of higher-order modes while retaining the fundamental mode over the entire length of the waveguide. The structure is analyzed by the transfer-matrix method and the leakage losses of the modes have been calculated. We show that a waveguide formed in silica with numerical aperture 0.24 and core width 10 μm can be designed to exhibit single-mode operation at 1550-nm wavelength. Such a structure should find applications in high-power planar waveguide lasers and amplifiers.     

Improved amplitude–frequency characteristics for T-splitter photonic crystal waveguides in terahertz regime

Based on the plane-wave expansion method, we calculate TE/TM gaps of 2-D photonic crystals (PCs) with typical square lattices composed of the silicon rods in air. Using the finite-difference time-domain method, we simulate the electromagnetic field distribution of THz waves in photonic crystals T-splitters. By the improved T-splitter with a rod in the junction, we achieved the amplitude–frequency characteristics of a pass band of 84% from 1.12 to 1.22 THz and surpassed by 76% the amplitude consistency of common T-splitters. And using the finite-difference time-domain method, we demonstrated that the improved T-splitter excels a common T-splitter in the degree of separation between the two output ports. These results provide a useful guide and a theoretical basis for the developments of THz functional components.     

Photorefractive properties of optical waveguides in Fe:LiNbO<Subscript>3</Subscript> crystals produced by O<Superscript>3+</Superscript> ion implantation

The photorefractive properties of optical planar waveguides in Fe:LiNbO₃ crystals fabricated by O³⁺ ion implantation are investigated. Two-wave mixing experiments are carried out for both the waveguide and the bulk. The results show that the measured gain coefficients are almost identical for the waveguiding layer and the substrate. In the waveguide, the response time could be reduced by one order of magnitude, with respect to the bulk, at the same power level of the incident light.     

The directional emission sensitivity of photonic crystal waveguides to air hole removal

To obtain highly directional light output from photonic crystal waveguides (PCWs), the emission characteristics of the narrow-width waveguide structures are investigated by tailoring the geometry of the exit sides. The local structural deformations in the form of air hole removal from the triangular-lattice photonic crystal (PC) show the effectiveness of the previously proposed approach that was implemented by us for another type of PC. The spatial broadening of the beam is greatly suppressed. With the modified waveguide exits, highly directional emissions with small side lobes are achieved. The frequency dependency of the directional emissions is evaluated. We show that the divergence angles of the beams depend linearly on the wavelength for a regular type of PCW but the modified PCW exits have local minima with respect to wavelength in terms of the divergence angle. The present work may prove to be helpful in the design of couplers and edge-emitting lasers and in the implementation of free-space optical communications.     

Transparence and electrical properties of ZnO-based multilayer electrode

Three-layered ZnO/Ag–Ti/ZnO structures were prepared using both the sol-gel technique and DC magnetron sputtering. This study focuses on the electrical and optical properties of the ZnO/Ag–Ti/ZnO multilayers with various thicknesses of the Ag–Ti layer. The ZnO thin film prepared by the sol–gel method was dried at 300°C for 3 minutes, and a fixed thickness of 20 nm was obtained. The thickness of the Ag–Ti thin film was controlled by varying the sputtering time. The Ag–Ti layer substantially reduced the electrical resistivity of the sol–gel-sprayed ZnO thin films. The sheet resistance of the Ag–Ti layer decreased dramatically and then became steady beyond a sputtering time of 60 s. The sputtering time of Ag–Ti thin film deposition was determined to be 60 s, taking into account the optical transmittance. Consequently, the transmittance of the ZnO/Ag–Ti/ZnO multilayer films was 71% at 550 nm and 60% at 350 nm. The sheet resistance was 4.2 Ω/sq.     

Design of a polymer directional coupler electro-optic switch using two-section reversed electrodes

By using the coupled mode theory, electro-optic modulation theory, conformal transforming method and image method, the structure is designed, the parameters are optimized, and the characteristics are analyzed for a polymer directional coupler electro-optic switch with two-section reversed electrodes. Simulation shows that the designed device exhibits excellent switching functions. Under the operation wavelength of 1550 nm, the electro-optic coupling region length is 4751 μm, the cross-state and bar-state voltages are about 1.22 and 2.65 V, and the insertion loss and crosstalk are less than 2.21 and −30 dB, respectively. By slightly adjusting the state voltages, the blight of the fabrication errors on the switching characteristics can be easily eliminated. The calculation results of the presented technique are in good agreement with those of the beam propagation method (BPM).