Polarization Beam Splitter Based on Self-Collimation of a Hybrid Photonic Crystal

A photonic crystal polarization beam splitter based on photonic band gap and self-collimation effects is designed for optical communication wavelengths. The photonic crystal structure consists of a polarization-insensitive self-collimation region and a splitting region. TM- and TE-polarized waves propagate without diffraction in the self-collimation region, whereas they split by 90 degrees in the splitting region. Efficiency of more than 75% for TM- and TE-polarized light is obtained for a polarization beam splitter size of only 17 μm x 17 μm in a wavelength interval of 60 nm including 1.55 μm

Structural, morphological, and optical properties of AlGaN/GaN heterostructures with AlN buffer and interlayer

Cataloged from PDF version of article.AlxGa1-xN/GaN (x similar to 0.3) heterostructures with and without a high-temperature (HT) AlN interlayer (IL) have been grown on sapphire (Al2O3) substrates and AlN buffer/Al2O3 templates by metal organic chemical vapor deposition. The effects of an AlN buffer layer (BL) grown on an Al2O3 substrate and an AlN IL grown under the AlGaN ternary layer (TL) on structural, morphological, and optical properties of the heterostructures have been investigated by high-resolution x-ray diffraction, spectroscopic ellipsometry, atomic force microscopy, and photoluminescence measurements. The AlN BL improves the crystal quality of the AlGaN TL. Further improvement is achieved by inserting an AlN IL between GaN BL and AlGaN TL. However, experimental results also show that a HT AlN IL leads to relatively rough surfaces on AlGaN TLs, and an AlN IL changes the strain in the AlGaN TL from tensile to compressive type. In addition, an AlN BL improves the top surface quality of heterostructures. (c) 2007 American Institute of Physics

Design of a Polarization-Independent Dual-Band Electromagnetically Induced Transparency-Like Metamaterial

In this study, the classical analog of single and dual-band electromagnetically induced transparency is demonstrated with a four-fold symmetric metamaterial consisting of a Minkowski fractal ring resonator surrounded by a square ring resonator. The proposed metamaterials show high transmission ratios at the polarization independent resonances, as confirmed by the applied two different numerical methods. Delay-bandwidth products are found to be 0.34 and 0.61 at the resonances of the dual-band metamaterial. The peak frequencies and transmission ratios maintain also for oblique angle of incidences. These features of the proposed metamaterials are promising for single and multi-band filtering applications as well as for slow light and sensing devices

Consequences of Unit Cell Design in Metamaterial Perfect Absorbers

Metamaterials are a new class of composite materials with unusual properties that allow controlling of electromagnetic waves by properly engineering the response functions, which are not observed in constituent materials. However, since absorption of metamaterials is mainly based on electromagnetic resonances, the operating bandwidth is relatively narrow. Utilization of more than a single metallic structure with different geometrical parameters in each unit cell is a common way of accomplishing multiple band and/or broadband absorption. There are two usual approaches for this purpose: (a) multilayer unit cell design where metallic structures on dielectric substrate are stacked one on top of the other; (b) side by side unit cell design where metallic structures are distributed on a dielectric substrate. However, to the best of our knowledge, these two different approaches are not comparatively investigated. In this study, we propose metamaterial-based perfect absorbers with two different unit cell designs and simulate transmittances, reflectances and absorbances for each design by a commercial electromagnetic solver, CST Microwave Studio. It is found that each design has its own advantages in terms of device thickness, absorption bandwidth and angular dependence, which might be severely important for particular purposes

Enhanced Electro-Optic Modulation Performance in Optical Buffers by Slowing Light in Optimized Photonic Crystal Slab Structures

Slow light holds the key to advanced optical buffering and time-domain optical signal processing technologies. Photonic crystal based optical buffers are particularly attractive due to their nanoscale size, room temperature operation, and enhanced field dependent nonlinear response associated with the presence of slow light. In this study, the slow light and electro-optic modulation characteristics of a line-defect Si photonic crystal slab with triangular arrangement of holes filled with an electro-optic polymer (n = 1.6) are investigated by three-dimensional plane-wave expansion and finite-difference time-domain methods. The first rows adjacent to the line-defect are shifted gradually in the direction of light propagation and a slow light region with a high group index below the light-line is obtained for a shifting amount between 0.22a and 0.27a. For the photonic crystal configuration with 0.22a shifted rows, under modulated voltage change, the average group index is found to be decreasing with an increase in the bandwidth. The results show that the low group velocity supports a large delay time in a small modulated voltage variation. A linear change of group index with modulated voltage is obtained and the modulation sensitivity of central wavelength is obtained as 9.45 nm/V for a delay line length of 0.5 mm. Almost the same buffer capacity and bit length are found which provides the control of delay time flexibly while keeping the buffer capacity and the bit length almost unchanged

Enhancement of Refractive Index Sensitivity in Photonic Crystal Waveguide-Based Sensors by Selective Infiltration

We present a liquid refractive index sensor based on a photonic crystal waveguide slab structure. Sensing mechanism employed in this study is based on the shift in cut-off wavelength as the lattice holes are selectively infiltrated. Three-dimensional plane-wave expansion and finite-difference time-domain methods are used to determine the band structure and transmission spectra, respectively. First, the sensitivity of the device is analyzed for the structure where only the first rows of holes adjacent to the line-defect are infiltrated. In addition, this analysis is repeated for a range of hole diameters. Second, the effects of infiltrated holes which are placed in the line-defect are investigated. As these infiltrated central holes are introduced, the proposed device exhibits 5.3 times improved sensitivity

Thickness and optical constant distributions of PECVD a-SiCx : H thin films along electrode radial direction

Two sets of hydrogenated amorphous silicon carbide (a-SiCx:H) thin films were grown on glass and c-Si substrates by Plasma enhanced chemical vapor deposition (PECVD) technique at pressures of 0.5 and 0.1 Torr. The influence of the pressure on the distribution of thicknesses, refractive indices at 632.8 nm and optical gaps from the edge to the center of the bottom electrode of PECVD system is examined by transmission, single wavelength ellipsometry and Fourier transform infrared spectroscopy. A recently introduced optimization method considering some prior knowledge of common optical properties of amorphous semiconductor thin films have been applied to the optical transmission spectra for estimating thickness and optical constants of a-SiCx:H thin films at hand. In addition, spectral characteristics of these constants are analyzed by fitting experimental data through Forouhi-Bloomer, Tauc-Lorentz and single Lorentz oscillator models. The retrieved results show that while the thicknesses and optical band gaps are decreasing towards the center of the electrode, refractive index is slightly increasing in that direction. Chemical processes occurring during the film deposition are discussed. These distributions of the film constants have been interpreted as due to the carbon incorporation whose efficiency might increase towards the edge of the electrode corresponding to a longer residence time of carbon radicals

Correlation between optical path modulations and transmittance spectra of a-Si : H thin films

The optical constants of plasma-enhanced chemical-vapor-deposited amorphous silicon (a-Si:H) thin film upon a transparent substrate are determined within the UV-visible region by measurement of the transmittance spectrum. Apart from thickness irregularities, the effects of vertical film inhomogeneities (refractive-index distribution) on the spectrum are discussed. In this respect, although consideration of any possible variation in thickness of the film within the area illuminated by the probe beam is sufficient for correcting the modulation of the extrema of interference fringes, including in the model the thin transitional regions' at substrate-film and film-air interfaces might be an alternative method for understanding the overall optical behavior of the spectrum. (C) 2000 Optical Society of America

Carbon content influence on the optical constants of hydrogenated amorphous silicon carbon alloys

Hydrogenated amorphous silicon carbon alloys (a-Si1-xCx:H) with different carbon contents are deposited by a plasma enhanced chemical vapor deposition (PECVD) system with different hydrogen diluted ethylene (C2H4) concentrations at two power densities of 30 and 90 mW/cm(2). First, the carbon and hydrogen configurations and their relative concentrations in these films are investigated in details by IR spectroscopy. Then, the ultraviolet/visible transmittance spectroscopy and a relevant characterization software are used for finding out the effects of both the carbon content and power density on the refractive index, optical gap and Urbach parameter. This work shows that the refractive index and optical gap may be modulated on purpose by carbon content within the context of hydrogen dilution. However, beyond a critical rf power density, an inhomogencity may occur along the radial direction of the plasma electrode, rendering such a-Si1-xCx:H films useless in practical applications. Moreover, both the evaluated Urbach and slope parameters carry out an increased disorder for carbon rich a-Si1-xCx:H films at high power

Influences of carbon content and power density on the PECVD grown a-Si1-x : C-x : H thin films

Large area electronics require large size thin films whose eventual inhomogeneities arise as a problem. Hydrogenated amorphous silicon carbide thin films (a-Si1-xCx:H) for four different source gas mixtures at two power densities were deposited by plasma enhanced chemical vapor deposition (PECVD) technique. The degree of film homogeneity was investigated through measurements of deposition rate, refractive index and optical energy gap along the radial direction of bottom electrode. Both ellipsometer at various incident angles and optical transmittance at normal incidence were used in mutual control as diagnosing tools. It seems there is a critical power density beyond which inhomogeneities of the deposited films along the radial direction of the electrode are unavoidable