Volume 91, Issue 8, 20 August 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
The authors report on the fabrication of pentacene-based inverter with two -channel thin-film transistors(TFTs) on bilayer dielectric, which has been patterned by high energy ultraviolet (UV)illumination. After pentacene channel growth on the dielectric, the inverter showed a high voltage gain of under supply voltage but at a transition voltage of which is too marginal to guarantee a desirable inverter operation between 0 and . When low energy UV was applied onto one of the two TFTs, which plays as a load in the inverter circuit, the transition voltage shifted to an adequate value because the UV changes the threshold voltage of the load TFT to be lower. The UV-treated inverter demonstrated a high voltage gain of under a of .
- LASERS, OPTICS, AND OPTOELECTRONICS
91(2007); http://dx.doi.org/10.1063/1.2772756View Description Hide Description
An in-plane Fresnel zone plate (FZP) for focusing surface plasmon polariton (SPP) fields has been designed, fabricated, and tested. The fabricated device consists of tall by wide amorphous Si-based SPP FZP on an Al film integrated with a pair of two-dimensional nanohole arrays for excitation of the incident and detection of the diffracted SPP fields. Diffracted SPP fields from each Fresnel zone constructively interfere at the expected focal point to produce focusing with threefold intensity enhancement. Temporal and spatial characteristics of the focused SPP fields are studied with time-resolved spatial-heterodyne imaging technique. Good agreement with average power measurements is demonstrated. Diffractive focusing of SPP fields, based on Fourier plasmonics, represents an approach to SPP in-plane microscopy.
Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation91(2007); http://dx.doi.org/10.1063/1.2772766View Description Hide Description
The authors report on a spraydeposition method as a cost-efficient technique for the fabrication of organic solar cells (OSCs). Active layers of OSCs were fabricated using conventional handheld airbrushes. Although the spraydepositedfilm showed a relatively rougher surface than spin coated ones, pinhole-free and constant thickness films could be obtained. An optimized OSC showed 2.83% of power conversion efficiency and 52% of incident photon to current conversion efficiency even though the device was fabricated in air. The performance of sprayed OSCs was comparable to that of the spin coated devices fabricated in air.
Design of negative-refractive-index materials on the basis of rods with a gradient of the dielectric constant91(2007); http://dx.doi.org/10.1063/1.2770964View Description Hide Description
The authors propose an approach to the design of negative-refractive-index materials based on the use of dielectric rods with a gradient of the dielectric constant. A triangular-lattice photonic crystal assembled from multilayer dielectric rods with a refractive index approximating a fish-eye profile is shown to exhibit a negative refractive index in the wavelength range defined by the inequality , where is the lattice constant of the photonic crystal. A lens consisting of a plane-parallel slab of such a photonic crystal slab is shown to be able to form an image of a point source in this wavelength range. According to the calculations, particularly high-quality images can be obtained at the wavelength , where the fish-eye dielectric rods scatter the light like a medium with the refractive index equal to .
91(2007); http://dx.doi.org/10.1063/1.2759985View Description Hide Description
Surface plasmonpolaritons launched at concentric arcs can be focused into a subwavelength wide focal spot of high near-field light intensity. The focused plasmons give rise to enhanced Raman scattering from R6G molecules placed in the focal area. By exploiting the polarization dependence of the focusing the authors establish an enhancement of the Raman signal by a factor of . The results show that focusing of propagating surface plasmons on flat metal surfaces may be an alternative to localized plasmons on metal nanostructures for achieving enhanced Raman scattering. In particular, a flat metal substrate enables better control over the local electric fields and the placement of analyte molecules, and, therefore, ultimately better fidelity of Raman spectra.
Beam shaping and effective guiding in the bulk of photorefractive crystals through linear beam dynamics91(2007); http://dx.doi.org/10.1063/1.2772177View Description Hide Description
The authors demonstrate a technique to optically imprint through linear beam propagation an index pattern in the bulk of a photorefractive crystal capable of beam reshaping and waveguiding. The procedure is based on the separation into two distinct phases of the photosensitive and refractive response, so that light is in all cases undergoing only linear propagation. When saturation in the response becomes dominant, the scheme is able to achieve both one-dimensional and two-dimensional waveguiding. The result allows the straightforward writing of multiwaveguide circuits, where traditional schemes based on spatial solitons are in practice burdened by nonlinearity.
Narrow inhomogeneous broadening and its correlation with confinement potential of pyramidal quantum dot arrays91(2007); http://dx.doi.org/10.1063/1.2772178View Description Hide Description
The inhomogeneous broadening in the luminescence spectra of ordered arrays of pyramidal semiconductorquantum dots(QDs) was studied as a function of the dot size. Dot arrays with inhomogeneous broadening as small as and a corresponding ground state to first excited state transition separation of were obtained. By evaluating the QD energy levels using a multiband model, the authors estimated that the observed inhomogeneous broadening corresponds to dot height fluctuations of about 1–2 ML across the array.
91(2007); http://dx.doi.org/10.1063/1.2772231View Description Hide Description
An InGaAs single photonavalanchedetector capable of sub-Geiger mode (Photomultiplier-tube-like) operation is reported. The device achieves a stable gain at around . The gain fluctuation is greatly suppressed through a self-quenching effect, thus an equivalent excess noise factor as low as 1.001 is achieved. In the photon counting experiment, the device is operated in the nongated mode under a dc bias. Because of its unique characteristics of self-quenching and self-recovery, no external quenching circuit is needed. The device shows a single photon response of around and a self-recovery time of about .
91(2007); http://dx.doi.org/10.1063/1.2761833View Description Hide Description
A study of the amplified spontaneous emission(ASE)properties of three bisfluorene-cored dendrimers in the solid state is reported. The results show that the dendron type has a strong impact on the photoluminescence quantum yield and affects the ASE threshold, the optical gain, and loss coefficients. Optically pumped distributed feedback lasers operating in the blue spectral region were fabricated by spin coating the dendrimer films on top of a two-dimensional corrugated fused silica substrate. A best lasing threshold of and a slope efficiency of 8.3% were obtained, which demonstrate the high potential of these materials for laser applications.
91(2007); http://dx.doi.org/10.1063/1.2772783View Description Hide Description
We present a nonresonant photoconductive terahertz detectionantenna suitable for detection of both focused and unfocused terahertz radiations. Our system consists of a scalable terahertz emitter based on an interdigitated electrode structure and a detectionantenna with similar electrode geometry. While the emitter is fabricated on semi-insulating GaAs we compare different ion-implanted GaAs-based detectionantennas. We studied the dependence of the measured terahertz signal on the power and spot size of the gating laser pulse. In addition we compare the performance of our antenna with that of electro-optical sampling.
91(2007); http://dx.doi.org/10.1063/1.2771531View Description Hide Description
The authors studied the photorefractive performance of bistable surface stabilized chiral smectic C cells doped with . Diffraction efficiency strongly depends on the combination of (a) the application time of the electric field used during the writing process and (b) light intensity. By choosing the conditions of the writing process, gratings with high efficiency can form only for certain levels of light intensity.
91(2007); http://dx.doi.org/10.1063/1.2772774View Description Hide Description
Dielectric thin film stripes on a metal surface can be applied as surface plasmon (SP) waveguides. Here, the authors demonstrate experimentally that such structures can be used to build SP waveguide bends and couplers. On one hand, they show that SP transmission through a waveguide bend can be maximized by assuming the trade-off of propagation and bend induced radiation losses. On the other hand, the authors combine two waveguides to form a directional SP coupler.
Effect of facet angle on effective facet reflectivity and operating characteristics of quantum dot edge emitting lasers and superluminescent light-emitting diodes91(2007); http://dx.doi.org/10.1063/1.2772845View Description Hide Description
The authors report the creation of low reflectivity angled facets by focused-ion-beam postfabrication etching. A method to directly measure the effective facet reflectivity of such facets, utilizing gain saturation effects in the quantum dots is described. The reflectivities of the angled facets are shown to decrease by increasing the facet angle from 0° to 15°. With a reflectivity of obtained for a facet with a 15° angle, allowing quantum dot superluminescent light-emitting diodes to be fabricated. The use of different angled facets to control the emission wavelength of both quantum dot lasers and superluminescent light-emitting diodes is outlined.
91(2007); http://dx.doi.org/10.1063/1.2773740View Description Hide Description
To investigate electroluminescence behaviors of a series of poly(-phenyl-2,7-carbazole)s having 2-ethylhexyloxy (PEHOC), triethylene oxide (PTEOC), or diphenylamino (PDPAC) group at the -phenyl portion, single-layer devices of organic light-emitting diode using the as-prepared polymer as the emitting layer material were fabricated. All the devices showed brightness above at with moderate luminous efficiencies. The PEHOC (, -disubstituted) and PTEOC (-substituted) devices exhibited a trivial but undesirable redshift due to electro-oxidation and aggregation of the polymer under high driving voltages, while the PDPAC (-substituted) device displayed a blue electroluminescence [ and CIE ] which had no difference compared with the photoluminescence spectrum.
91(2007); http://dx.doi.org/10.1063/1.2773753View Description Hide Description
With the increasing use of siloxane polymers as optical waveguidematerial, there is a need to understand and thus to reduce the stress-induced birefringence in siloxane polymer due to thermal processing and material property mismatch. In this letter, stress-optical coefficients of a siloxane polymermaterial are experimentally determined by measuring the refractive indices of the uncured and cured siloxane polymermaterial in two orthogonal directions. Employing such coefficients and temperature- and direction-dependent material properties, a numerical modeling method is presented to determine the stress-induced birefringence in an optical waveguide system. In the case study that is presented in this work, it is seen that the coefficient of thermal expansion of the planarization layer has the maximum effect on the birefringence, and it is possible to reduce the stress-induced birefringence by reducing the property mismatch between the planarization layer and the core layer. The outlined methodology is generic in nature and can be applied to different waveguide geometries, planarization materials, and substrate/board materials to assess how stress-induced birefringence can be minimized for a given polymer core material.
Continuous-wave stimulated emission and optical amplification in europium (III)-aluminum nanocluster-doped polymeric waveguide91(2007); http://dx.doi.org/10.1063/1.2773933View Description Hide Description
The authors have demonstrated stimulated emission from polymer-based planar waveguide doped with Eu–Al nanoclusters under continuous-wave optical pumping. For the waveguide sample with a nanocluster concentration of , amplified spontaneous emission was observed in the spectra of the light decoupled from the waveguide edge under a pumping density of . The optical gain coefficient was when pumped at . The well-suppressed concentration quenching and multiphonon quenching may cause the high-gain and low-threshold optical amplification. These results show that the Eu–Al nanocluster is a promising material for polymer-based solid-state lasers and waveguide-type optical amplifiers.
Modified triphenylmethane dye for the efficient nonlinear generation of light in nanostructured materials91(2007); http://dx.doi.org/10.1063/1.2773934View Description Hide Description
Many nonlinear photonic materials exhibit large extinction coefficients as a result of the active molecule absorption band. The authors show that it is possible to redshift the absorption resonance of such material without a significant alteration of the nonlinear response. A drastic reduction in the absorption coefficient at the wavelength of interest is obtained by adding a triple bond unit to these active molecules. In such case, the nonlinear optical response of a photonic material would be much better than the one obtained with the original molecule or by following the conventional procedure of adding a double bond unit instead.
Effect of metal organic vapor phase epitaxy growth conditions on emission wavelength stability of quantum dot lasers91(2007); http://dx.doi.org/10.1063/1.2773971View Description Hide Description
InAs quantum dots(QDs) on InP were implemented as active layers in laser structures completely grown by metal organic vapor phase epitaxy (MOVPE). In laser structures due to growth of the upper InP cladding layers onto the QDs, a marked blueshift of the emission wavelength and a simultaneous degradation of photoluminescence intensity were observed. This behavior was systematically investigated using thermal treatment to simulate the growth of the upper cladding layer. Using an adequate growth procedure emission behavior was achieved that has proven to be fairly insensitive to over-/regrowth steps. Broad-area emitting laser devices were fabricated by incorporating seven QD layers in an all-MOVPE grown structure. Transparency current densities per dot layer of were achieved which is close to values of multiple quantum well lasers processed in the same way.
91(2007); http://dx.doi.org/10.1063/1.2775805View Description Hide Description
Surfacetexturing by solutiondeposition has been developed for antireflection in solar cells. The surfacetexture is formed by a monolayer of microscale silica particles partially immersed into a spin-on-glass film with a thickness less than the height of the particles. When the silica particles have a spherical shape, the low reflectance from this coating becomes omnidirectional, a desirable feature in fixed-orientation solar panels. It has been experimentally found that the coating improves the transmittance of a quartz wafer in the spectral range of and in the incident-angle range of surface normal to at least 30°. The surfacetexture can be applied to different types of solar cells as an add-on coating.
Direct real-time observation of catastrophic optical degradation in operating semiconductor lasers using scanning tunneling microscopy91(2007); http://dx.doi.org/10.1063/1.2775049View Description Hide Description
Cross-sectional scanning tunneling microscopy is performed on operating semiconductor quantum well laser devices to reveal real time changes in device structure. Low and nominally doped capping regions adjacent to the quantum well active region are found to heat under normal operating conditions. The increase in anion-vacancy defect formation and the generation of surface states pins the Fermi level at the surface and begins the process of catastrophic optical degradation which eventually destroys the device. The technique has implications for the study of defect generation and in-operation changes in all nanostructures.
91(2007); http://dx.doi.org/10.1063/1.2775087View Description Hide Description
The authors report a two-color, colocated quantum dot based imaging system used to take multicolor images using a single focal plane array (FPA). The dots-in-a-well (DWELL) detectors consist of an active region composed of InAs quantum dots embedded in quantum wells. DWELL samples were grown using molecular beam epitaxy and fabricated into focal plane arrays with indium bumps. The FPA was then hybridized to an Indigo ISC9705 readout circuit and tested. Calibrated blackbody measurements at a device temperature of yield midwave infrared and long wave infrared noise equivalent difference in temperature of and .