Volume 91, Issue 7, 13 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
- interdisciplinary and general physics
Index of content:
Hypervelocity impact deformation mechanisms of -alumina are studied using atom molecular dynamics simulation on massively parallel computers. The projectile impact on the (0001) surface of -alumina at exhibits a fundamentally different symmetry of the deformationpatterns from those under lower strain rates. The simulation reveals a sequence of atomistic deformation mechanisms following localized melting and amorphization. These include pyramidal slips, basal slips and twins, rhombohedral twins, and twins along the . Some of these deformationpatterns are not observed under lower impact velocities.
- LASERS, OPTICS, AND OPTOELECTRONICS
91(2007); http://dx.doi.org/10.1063/1.2770768View Description Hide Description
The authors report on the development of strain-balanced InP-based quantum cascade lasers with high wall plug efficiency and room temperature continuous-wave operation. The use of narrow-ridge buried heterostructure waveguides and thermally optimized packaging is presented. Over 9.3% wall plug efficiency is reported at room temperature from a single device producing over of continuous-wave output power. Wall plug efficiencies greater than 18% are also reported for devices at a temperature of , with continuous-wave output powers of more than .
Pump-power-controlled luminescence switching in codoped water-free low silica calcium aluminosilicate glasses91(2007); http://dx.doi.org/10.1063/1.2771051View Description Hide Description
Intense infrared-to-visible upconversion emissions in codoped water-free low silica calcium aluminosilicate glasses have been obtained under excitation at . The results showed that as the pump power/intensity is increased, a reduction of up to one order of magnitude at the emitted intensity ratio is observed; characterizing what can be denominated as luminescent switching. The physical origin of this switching is discussed and explained in terms of the tailoring of luminescent switchers to operate in a large range of pump powers, what could be used in the development of sensors and networks for optical processing and optical communications.
91(2007); http://dx.doi.org/10.1063/1.2771052View Description Hide Description
Low threshold cw laser emission from a semiconductor disk laser emitting at was demonstrated. Using high quality metal-organic vapor phase epitaxy (GaIn) (NAs) material, nonradiative processes could be minimized. As a consequence, threshold pump power densities have been reduced down to . The capture efficiency of carriers into the quantum wells was found to be uncritical.
Edge emitting quantum cascade microlasers on InP with deeply etched one-dimensional photonic crystals91(2007); http://dx.doi.org/10.1063/1.2771054View Description Hide Description
Edge emitting quantum cascade microlasers based on InP have been fabricated and investigated. Deeply etchedBragg mirrors have been manufactured at the end of the resonators, to reduce the mirror losses of short cavity devices. In order to achieve the required high aspect ratio, an optimized plasma dry etching process has been developed, which allows monolithic device fabrication in a single dry etching step. The short resonator length results in a large Fabry-Pérot mode spacing, which leads to a single mode operation. A side mode suppression ratio of about at has been observed.
Compact and polarization controlled vertical-cavity surface-emitting laser using single-layer photonic crystal mirror91(2007); http://dx.doi.org/10.1063/1.2771085View Description Hide Description
Vertical-cavity surface-emitting lasers(VCSELs) consist of a gain medium in which feedback is generally accomplished by two distributed Bragg reflectors(DBRs), for which sufficient reflectivity has to be achieved in order to obtain laser emission. As a consequence, DBRs generally consist of very thick layer stacks. In this letter, the authors report the realization of a VCSEL, of which the top mirror consists of a single-layer one-dimensional photonic crystalmirror (PCM). This PCM enables significantly improved device compactness and total polarization control, while providing a directive and single lateral mode emission.
91(2007); http://dx.doi.org/10.1063/1.2770965View Description Hide Description
The authors report an experiment in which optical pulses are effectively stored via maximal atomic coherence in a three-level -type atomic system, and then selectively released at one of the two different wavelengths by turning on the retrieve control pulse at 794.9842 or . The storage time up to was demonstrated.
91(2007); http://dx.doi.org/10.1063/1.2770995View Description Hide Description
A planar electroabsorption modulator suitable for spatial light modulation has been constructed. The device operates near the band edge of zinc oxide at and is based on broadening and shifting of the unconfined exciton with an externally applied electric field. The ZnOactive layer was deposited on an aluminum/titanium oxide dielectric on an indium tin oxide conducting layer on glass. A transparent conductive InGaZnO layer on a spin on glass insulator served as the top contact, allowing high electric fields to be applied transverse to the ZnO layer. The modulator operates at room temperature in transmission mode with modulation at and modulation at at applied bias, corresponding to electric field across the ZnOactive layer.
91(2007); http://dx.doi.org/10.1063/1.2771524View Description Hide Description
Bipolar transport in blends of a copolymer of fluorene, thiophene and electron accepting groups, and the substituted fullerene [6,6]-phenyl-C61-butyric acid methylester have been studied through charge extraction by linearly increasing voltage on solar cells and with field effect transistors. Between 10% and 90% polymer has been used and the results show a clear correlation to solar cell performance. Optimal solar cells comprise 20% polymer and have a power conversion efficiency of 3.5%. The electron mobility is increasing strongly with fullerene content, but is always lower than the hole mobility, thus explaining the low amount of polymer in optimized devices.
91(2007); http://dx.doi.org/10.1063/1.2769955View Description Hide Description
The authors report on the multistage optical memory of a liquid crystal(LC)diffraction grating fabricated using a dye-dispersed LC/polymer composite medium. Under a single pump beam, the reorientation of dye molecules, confined within spatially periodic interpolymer networks produced by a patterned exposure of ultraviolet light, allows for a pure optic-axis modulation which gives high diffraction efficiency in a planar LC configuration. The resultant multistage optical memory of the LCgrating is controlled solely by the polarization state of a single rewriting beam without the use of an erasing step prior to every rewriting process.
91(2007); http://dx.doi.org/10.1063/1.2770657View Description Hide Description
Polymeroptical fiber (POF) with a core of azopolymer has been fabricated. Based on the photoinduced birefringence of azopolymer materials, a mechanism for writing POF gratings is presented. After writing in azopolymer optical fiber (APOF) by two beams of mutually orthogonal polarizations, a birefringencegrating was formed, and then the gratings could be erased almost totally for a short time by the circularly polarized beam at the same wavelength. By manipulating the APOF gratings, the transmittance through APOF at can be adjustable as revealed by the transmittance change during write-erase-write procedure.
91(2007); http://dx.doi.org/10.1063/1.2771374View Description Hide Description
Single-color and two-color pump-probe measurements are used to analyze carrier dynamics in quantum dotamplifiers. The study reveals that hole recovery and intradot electron relaxation occur on a picosecond time scale, while the electron capture time is on the order of . A longer time scale of hundreds of picoseconds is associated with carrier recovery in the wetting layer, similar to that observed in quantum well semiconductor amplifiers.
91(2007); http://dx.doi.org/10.1063/1.2771528View Description Hide Description
The authors present apparently the observation of subterahertz electromagnetic wave emission from GaN based large aperture photoconductive switches (LA-PCSWs) excited by ultraviolet femtosecond laser pulses. The photoconductive layer of the GaN LA-PCSW is doped with carbon giving rise to a resistivity as high as . The absolute energy of the emitted radiation pulses is measured to be /pulse under bias voltage. The Fourier spectrum of the measured pulse by the time-domain-spectroscopy shows the main components lie in regime.
91(2007); http://dx.doi.org/10.1063/1.2768632View Description Hide Description
The authors characterize experimentally the anisotropy of two-photon absorption and the Kerr nonlinearity in silicon over a broad spectral region in the near infrared using the -scan technique. The results show that both of these parameters decrease by about 12% along the [0 1 0] direction compared with the direction, and this change occurs for wavelengths in the range of .
Organic thin film transistors with ink-jet printed metal nanoparticle electrodes of a reduced channel length by laser ablation91(2007); http://dx.doi.org/10.1063/1.2771059View Description Hide Description
The authors have demonstrated organic thin film transistors (OTFTs) based on the ink-jet printed electrodes in which a reduced channel length is accomplished by laser ablation.Laser ablation on the dried silvernanoparticleelectrode formed by ink-jet printing effectively shortened the channel length down to , which is difficult to achieve by ink-jet printing alone. Reducing the channel length using this hybrid technique also allows them to observe the contact resistance effect in the OTFTs, which involves the printed silvernanoparticleelectrode of a lower work function with respect to the ionization energy of the organic semiconductor.
91(2007); http://dx.doi.org/10.1063/1.2771373View Description Hide Description
In this letter, the authors observe high quality ( up to 20 000) whispering gallery modes(WGMs) with small modal volumes in micropillars by employing an experimental geometry in which both excitation and collection of emission are in a direction normal to the sidewalls of the pillars. They show that WGMs provide at least two times larger values of the figure of merit for strong coupling applications compared to “photonic dot” states in pillars with comparable size.
Effect of nitric oxide on gain and output power of a non-self-sustained electric discharge pumped oxygen-iodine laser91(2007); http://dx.doi.org/10.1063/1.2771535View Description Hide Description
This letter discusses the effect of nitric oxide on gain and output power of a pulser-sustainer discharge excited oxygen iodine laser. Adding small amounts of NO to the laser mixture (a few hundreds of ppm) considerably increases gain and output power due to (i) O atom titration and resultant slower atom quenching and (ii) improved stability of the dc sustainer discharge, which allows stable operation at significantly higher discharge powers. Gain on the iodine atom transition and laser power in the transverse laser cavity are and , at a flow temperature of .
91(2007); http://dx.doi.org/10.1063/1.2771538View Description Hide Description
High efficient phosphors were prepared in a solid-state reaction method with various fluxes. The results show that addition of proper flux can decrease the phase-forming temperature of the TAG crystals and improve the morphology as well as enhance the photoluminescence of the phosphors greatly. High efficient and low color-temperature white light-emitting diodes (WLEDs) were fabricated with the phosphor. Good performances of the WLEDs indicate that a mixture of and LiF is an excellent flux for synthesis of phosphor and is beneficial to the fabrication of high efficient WLEDs.
91(2007); http://dx.doi.org/10.1063/1.2772198View Description Hide Description
The authors report the dynamic properties of bulk heterojunctionphotodiodes based on a polymer blend system consisting of poly(3-hexylthiophene-2,5-diyl) and the fullerene derivative [6,6]-phenyl -butyric acid methyl ester. Devices with a high-frequency contact layout were analyzed under continuous wave and pulsed laser illumination. The organic photodiodes exhibit a pulse response with a full width at half maximum of to the applied -long laser pulses. Rise times as small as and fall times were measured under applied reverse bias.
91(2007); http://dx.doi.org/10.1063/1.2772670View Description Hide Description
The so-called symmetric state has been reported to have submillisecond response times, which results from the symmetric profile of the liquid crystal director; however, no direct evidence has been obtained to show the profile symmetry. The difficulty in proving this symmetric structure by direct observation results from the short lifetime of state (typically around a few tens of milliseconds). In the work reported here, the authors utilize a burst driving method along with stroboscopic illumination from blue and red light emitting diodes to capture conoscopic images for the director profile; these showed good agreement with their modeling.
Use of ZnO thin films as sacrificial templates for metal organic vapor phase epitaxy and chemical lift-off of GaN91(2007); http://dx.doi.org/10.1063/1.2770655View Description Hide Description
Continued development of GaN-based light emitting diodes is being hampered by constraints imposed by current non-native substrates. ZnO is a promising alternative substrate but it decomposes under the conditions used in conventional GaNmetal organic vapor phase epitaxy(MOVPE). In this work, GaN was grown on using low temperature/pressure MOVPE with as a carrier and dimethylhydrazine as a source. Characterization confirmed the epitaxialgrowth of GaN. The GaN was lifted-off the by chemically etching away the ZnO underlayer. This approach opens up the way for bonding of the GaN onto a support of choice.