Volume 89, Issue 25, 18 December 2006
- 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
- interdisciplinary and general physics
Index of content:
The authors present low temperature transport measurements on double quantum dots in InAsnanowiresgrown by metal-organic vapor phase epitaxy. Two dots in series are created by lithographically defined top gates with a procedure involving no extra insulating layer. The authors demonstrate the full tunability from strong to weak coupling between the dots. The quantum mechanical nature of the coupling leads to the formation of a molecular state extending over both dots. The excitation spectra of the individual dots are observable by their signatures in the nonlinear transport.
- LASERS, OPTICS, AND OPTOELECTRONICS
Confined propagation and near single-mode laser oscillation in a gain-guided, index antiguided optical fiber89(2006); http://dx.doi.org/10.1063/1.2405860View Description Hide Description
The authors report laser oscillation in what appears to be a single transverse mode with very large mode area in optical fibers having heavily Nd-doped diameter cores with refractive index significantly lower in the core than in the surrounding cladding. Since fibers of this type cannot support conventional index-guided modes, their results appear to confirm a recent analysis which predicts gain-guided single-mode propagation in index antiguided fibers, provided the gain coefficient in the core exceeds a threshold value. Fibers of this type may be of significant interest for amplifiers and oscillators having large power outputs and/or small nonlinear pulse distortion.
89(2006); http://dx.doi.org/10.1063/1.2408652View Description Hide Description
The electrical conductivity of P-doped ZnO can be controlled by changing the P-doping concentration. With increasing P concentration, ZnO can be changed from type to type. At the same time, a redshift of the band gapenergy is observed by using the photoluminescence spectroscopy and UV-visible spectrophotometer. X-ray diffraction results show that lattice spacings of ZnO increase with P concentration, which indicates that P substitutes O, and this leads to a lattice spacing increase and an optical band gapenergy decrease.
89(2006); http://dx.doi.org/10.1063/1.2408658View Description Hide Description
The authors demonstrate electrically controlled negative refraction at the interface between an isotropic material and a uniaxial nematic liquid crystal, in which the optic axis is designed to make a large angle with the interface. Depending on the applied voltage, the refracted beam is either on the opposite side of the interface normal as compared to the incident beam (positive refraction) or on the same side (negative refraction).
89(2006); http://dx.doi.org/10.1063/1.2408659View Description Hide Description
Biophotonic sensors based on a polymer waveguide with Bragg reflection gratings are proposed in this work. For developing low-cost disposable sensors, UV curable polymer materials are utilized to demonstrate the waveguidesensors. The grating pattern is formed on the core layer of a polymer waveguide by exposing a laser interference pattern on a photoresist. To detect a small amount of analyte molecule through a specific binding, a calixarene crown molecule is immobilized on a sensing interface. Bragg reflection peak movement is observed depending on the potassium ion concentration dissolved in phosphate buffered saline solution. It has been confirmed experimentally that the sensor could detect the potassium ion concentration on the order of .
Dominant mode in closed photonic crystal microcavity filled with high scattering irregular gain medium89(2006); http://dx.doi.org/10.1063/1.2410218View Description Hide Description
The experimental results demonstrate a dominant vertical cavity mode oscillation in a closed microcavity filled with high scattering and irregular structure gain medium. The mechanisms are (A) ultralow loss in a closed microcavity, (B) strong feedback from the microcavity causes light to be dominant in the vertical cavity mode, and (C) the dominant light in the vertical cavity mode synchronizes light in random directions and localization state.
InGaN-light emitting diode with high density truncated hexagonal pyramid shaped hillocks on the emission surface89(2006); http://dx.doi.org/10.1063/1.2410229View Description Hide Description
To increase the light extraction efficiency, high density truncated hexagonal pyramid shaped submicron hillocks were formed on the emission surface of an multiple quantum well light emitting dicode (LED) using an in situ silicon carbon nitride self-masking layer. The self-assembled hillock density was raised up to a low using several nanometers of a self-masking layer. The self-assembled hillock LED resulted in the optical power improvement up to 80% with similar electrical properties as a normal LED. This device showed a higher electrostatic discharge pass yield at over reverse stress voltage.
Experimental energy difference between heavy- or light-hole valence band and crystal-field split-off-hole valence band in89(2006); http://dx.doi.org/10.1063/1.2410242View Description Hide Description
The energy difference between heavy- or light-hole (HH or LH) and crystal-field split-off-hole (CH) valence bands of - and -plane was experimentally investigated as the peak-energy difference between - and -polarized emissions. The energy difference was zero at . However, this value was significantly larger than , at which the spontaneous surface-emission intensities of and polarizations from the were equal. These results suggest a shorter carrier lifetime (larger optical gain) in the CH than in the HH and/or LH.
89(2006); http://dx.doi.org/10.1063/1.2405420View Description Hide Description
Using the rare-earth special feature of a sharp emission spectrum, voltage-controlled continuous color tuning of organic light-emitting diodes is achieved. is used as the strategic starting point close to the red corner of the Commission International de I’Eclairage chromaticity diagram for a wide color tuning. The end point and path of the color tuning can be engineered by doping the hole-transport emitting layer with dyes. The mechanisms of color tuning have been investigated and explained by the efficiency reduction of the europium complex and the extension of carrier recombination zone with driving voltage. The effect of exciplex on the color tuning is also studied.
89(2006); http://dx.doi.org/10.1063/1.2405880View Description Hide Description
The crystal quality of -plane GaNfilms was improved by using epitaxial lateral overgrowth on trenched -plane GaN buffer layers. Not only the threading dislocation density but also the difference of anisotropic in-plane strain between orthogonal crystal axes can be mitigated by using trenched epitaxial lateral overgrowth (TELOG). The low threading dislocation density investigated by the cross-sectional transmission electron microscopy was estimated to be on the N-face GaN wing. On the other hand, the Ga-face GaN wing with a faster lateral overgrowth rate could be influenced by the thin GaN layer grown on the bottom of the trenches, resulting in higher dislocation density generated. As a result, the authors concluded that a narrower stripped GaN seeds and deeper stripped trenches etched into the surface of sapphire could derive a better quality -plane GaNfilm. Finally, they demonstrated the fast coalescence process of TELOG GaNfilms below thick.
Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide89(2006); http://dx.doi.org/10.1063/1.2405884View Description Hide Description
Waveguide terahertz time-domain spectroscopy is used to demonstrate the narrowing of vibrational lines for a thin polycrystalline film of 1,2-dicyanobenzene in a parallel plate metal waveguide. When compared to corresponding linewidths for 1,2-dicyanobenzene in a pellet sample at room temperature, the linewidths for the waveguide film are found to be significantly sharper. For measurements near , a dramatic line narrowing is observed for the waveguide film, yielding linewidths as much as five times sharper than found in the pellet. These effects are attributed to much smaller inhomogeneous broadening in the waveguide film and result in a more informative spectrum.
89(2006); http://dx.doi.org/10.1063/1.2408651View Description Hide Description
Optical interferenceholography has been proven to be a useful technique in fabricating periodic photonic crystals in which electromagnetic waves are forbidden in certain frequency band gaps. Compared to periodic crystals, quasicrystals, having higher point group symmetry, are more favorable in achieving complete band gaps. In this letter, we propose two seven-beam optical interference configurations based on the reciprocal vector space representation to fabricate icosahedral quasicrystals. Interference simulations for the quasicrystals exhibit the full symmetry of an icosahedron. The result paves the way for the fabrication of photonic quasicrystals using holographiclithography.
Femtosecond coherent anti-Stokes Raman scattering measurement of gas temperatures from frequency-spread dephasing of the Raman coherence89(2006); http://dx.doi.org/10.1063/1.2410237View Description Hide Description
Gas-phase temperatures and concentrations are measured from the magnitude and decay of the initial Raman coherence in femtosecond coherent anti-Stokes Raman scattering (CARS). A time-delayed probe beam is scattered from the Raman polarization induced by pump and Stokes beams to generate CARS signal; the dephasing rate of this initial coherence is determined by the temperature-sensitive frequency spread of the Raman transitions. Temperature is measured from the CARS signal decrease with increasing probe delay. Concentration is found from the ratio of the CARS and nonresonant background signals. Collision rates do not affect the determination of these quantities.
89(2006); http://dx.doi.org/10.1063/1.2402241View Description Hide Description
The authors present a polarization-resolved photoluminescence study of single semiconductorquantum dots(QDs) interconnected to quantum wires, measured both in a top geometry, and in a less conventional cleaved-edge geometry. Strong polarizationanisotropy is revealed for all observed transitions, and it is deduced that closely spaced QD hole states exhibit nearly pure heavy-or light-hole character. These effects are attributed to the large aspect ratio of the dot shape.
89(2006); http://dx.doi.org/10.1063/1.2420777View Description Hide Description
This letter describes the generation of femtosecond cylindrical vector beams by passing the optical pulse through a microfabricated spiral phase plate and an azimuth-type polarization analyzer. The resulting beam resembles a doughnut and has a polarization that is axially symmetric. Portions of the wave front were sampled via the GRENOUILLE frequency resolved optical grating device; it was found that the original pulse of was stretched slightly by . Compared to an interferometric beam combination technique, this experiment geometry is simple, minimizes material dispersion, and has negligible spatial chirp.
89(2006); http://dx.doi.org/10.1063/1.2422885View Description Hide Description
The authors consider propagation of a plasmon polariton on the interface between a metal and a magneto-optical material and show that in presence of transverse magnetic field a strong nonreciprocal phase shift results. This phenomenon can be used to achieve optical isolation action in short nanoplasmonic guides with low insertion loss.
Noncritical phase-matched lithium triborate optical parametric oscillator for high resolution coherent anti-Stokes Raman scattering spectroscopy and microscopy89(2006); http://dx.doi.org/10.1063/1.2420773View Description Hide Description
An efficient, widely tunable, narrow-bandwidth, green-pumped, noncritical phase-matched lithium triborate based optical parametric oscillator (OPO) is applied to coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy. The tunable signal beam of the OPO is combined with the fundamental of a pump laser ( , ) to obtain high resolution vibrational spectra of molecules around the CH vibrational stretch . The straightforward and convenient tunability of the OPO is demonstrated by CARS microscopy for the identification of different polymer microparticles on the same substrate.
89(2006); http://dx.doi.org/10.1063/1.2422908View Description Hide Description
A configurable charge-coupled-device (CCD)detector is used in a scanning transmission x-ray microscope to record the transmitted intensity distribution for every pixel in a raster scan of the sample. Real-time processing of the CCD frames gives simultaneous absorption and phase contrast image signals from a single scan. The CCD combines fast frame-transfer readout with very high sensitivity and makes use of x-ray to visible-light coupling to allow operation over a wide range of photon energies, from the oxygen edge upwards. Tests on the Twinmic end station at the Elettra synchrotron are reported.
89(2006); http://dx.doi.org/10.1063/1.2422911View Description Hide Description
Small-molecule organic solar cells with a structure of indium tin oxide (ITO)\tris-8-hydroxy-quinolinato aluminum \fullerene \copper phthalocyanine (CuPc) were fabricated. The shelf lifetime of unencapsulated devices was over , and the power conversion efficiency reached 0.76% under AM1.5G (air mass 1.5 global) . The long lifetime was attributed to the inverted structure compared to the conventional structure since the former could effectively protect from the diffusion of oxygen and modify interfacial electrical properties. The introduction of a layer into the cells enhanced the power conversion efficiency by more than 20 times. The presence of the thin film on the ITO substrate lowered the substrate work function and hence increased the electric field in the organic layers, which was beneficial to the collection of free carriers. The reasons for the degradation of such kind of organic solar cells are analyzed in detail.
89(2006); http://dx.doi.org/10.1063/1.2408639View Description Hide Description
The authors present high-power continuous-wave (cw) operation of distributed-feedback quantum-cascade lasers. Continuous-wave output powers of at and at are obtained. Single-mode emission near with a side-mode suppression ratio of and a tuning range of was obtained between 15 and . The device exhibits no beam steering with a full width at half maximum of 27.4° at in cw mode.
Control of self-collimated Bloch waves by partially flat equifrequency contours in photonic crystals89(2006); http://dx.doi.org/10.1063/1.2423237View Description Hide Description
Self-collimation effects in photonic crystals are generally investigated by employing flat equifrequency contours. Here we report, based on a partially flat equifrequency contour inducing two different group velocity vectors, the simultaneous excitation of dual self-collimated beams and the selective excitation of either of them by varying the incident angle or the width of an input Gaussian beam. With combination of the finite-difference time-domain simulation and the Fourier analysis as well as the wave vector diagram, we analyze the refractive behaviors of these self-collimated Bloch waves.