Volume 101, Issue 4, 23 July 2012
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
We developed a split-illumination electron holography that uses an electron biprism in the illuminating system and two biprisms (applicable to one biprism) in the imaging system, enabling holographic interference micrographs of regions far from the sample edge to be obtained. Using a condenser biprism, we split an electron wave into two coherent electron waves: one wave is to illuminate an observation area far from the sample edge in the sample plane and the other wave to pass through a vacuum space outside the sample. The split-illumination holography has the potential to greatly expand the breadth of applications of electron holography.
- PHOTONICS AND OPTOELECTRONICS
Quenched ultraviolet defect emission and excellent scintillation performance on a photodiode from heavily cerium doped yttrium aluminum garnet101(2012); http://dx.doi.org/10.1063/1.4738788View Description Hide Description
Single crystal ceriumdopedyttriumaluminum garnet boules (Ce:YAG, 0.25-4.0 at. % Ce) were grown under similar conditions by the Czochralski method. Scintillation, photoluminescence, and lifetime measurements demonstrate reduced UVdefect emission and increased visible emission as ceriumdoping increases, resulting from greater competitiveness of ceriumluminescence centers among defect centers. Photodiode energy resolutions improved significantly, where 2.0 at. % Ce:YAG displayed 8.9% (6.14% with noise reduction) at 662 keV without post-processing. Therefore large, highly doped, as grown Ce:YAG, can achieve resolutions on cost effective photodiodes that are generally observed only with photo multiplier tubes and avalanche photodiodes at low doping levels.
101(2012); http://dx.doi.org/10.1063/1.4738890View Description Hide Description
We demonstrate efficient optical directional beaming using an array of sub-wavelength patterns on a metallic surface. Specifically, a sub-wavelength sized slit placed next to a periodic grating is designed and optimized to realize maximum coupling efficiency and directional radiation into a leaky-wave plasmonic mode. Collective scattering from the corrugations forming the grating is synthesized to radiate towards the desired direction, and efficient beaming is achieved through tailoring the design parameters with a simple analytical model. We also prove that directivity can be further enhanced by improving the slit-grating coupling efficiency through efficient plasmon generation, showing improved angular response in far-field radiation.
101(2012); http://dx.doi.org/10.1063/1.4738993View Description Hide Description
Plasmonic light trapping can increase the absorption of light in thin semiconductor films. We investigate the effect of embedded metal nanoparticle (MNP) arrays on the electrical characteristics of ultra-thin PbS colloidal quantum dot (CQD) photoconductors. We demonstrate that direct contact with the metalnanoparticles can suppress or enhance the photocurrent depending on the work function of the metal, which dominates the optical effects of the particles for ultra-thin films. These results have implications for designing plasmonic CQD optoelectronic devices.
101(2012); http://dx.doi.org/10.1063/1.4738995View Description Hide Description
We report an infrared (IR) optical switch using a position-shifting glycerol droplet. The droplet is surrounded by density-matched oil. In the voltage-on state, the droplet shifts in one direction. Upon removing the voltage, the droplet returns to its original position with the aid of interfacial tensions. Due to the strong absorption of glycerol at 1.55 μm, our IR optical switch shows ∼95:1 contrast ratio and ∼200 ms response time. Such a device is promising for fiber optical switch and various IR optical attenuators.
Generation of a beam of fast electrons by tightly focusing a radially polarized ultrashort laser pulse101(2012); http://dx.doi.org/10.1063/1.4738998View Description Hide Description
The generation of an electron beam through longitudinal field acceleration from a tightly focused radially polarized (TM01) laser mode is reported. The longitudinal field is generated by focusing a TM01 few-cycle laser pulse (1.8 μm, 550 μJ, 15 fs) with a high numerical aperture parabola. The created longitudinal field in the focal region is intense enough to ionize atoms and accelerate electrons to 23 keV of energy from a low density oxygen gas. The characteristics of the electron beam are presented.
101(2012); http://dx.doi.org/10.1063/1.4737944View Description Hide Description
Intermediate band solar cells were realized using a GaAs (311)A p-i-n junction with Si as both the p- and n-type dopant, where the intermediate band was realized with a stack of InGaAsquantum wires. This quantum wirephotovoltaic device demonstrates a non-trivial increase in solar cell efficiency over a reference p-i-n GaAs (311)A junction resulting from a significant increase in short circuit current and an only slight decrease in open circuit voltage. Presented are optical and electrical characterizations of these devices.
101(2012); http://dx.doi.org/10.1063/1.4737945View Description Hide Description
We demonstrate twisted nematicliquid crystal(LC)polarizationgratings (TNPGs) with a high first-order diffraction efficiency (>90%) that have the twist angle as constant, whereas the azimuthal angle of the mid-plane liquid-crystal director varies linearly in space. The high diffraction-efficiency design scheme is derived by means of the Jones matrix method. The designed first-order diffraction efficiencytheoretically reaches 100% for the thick LC layer in which the retardation equals (n + 1/2)λ (n is an integer and λ is the wavelength). We also present a simple model for the grating period- and twist angle-dependent threshold voltage of the Freedericksz transition in TNPGs.
Continuous-wave operation and differential gain of InGaN/GaN quantum dot ridge waveguide lasers (λ = 420 nm) on c-plane GaN substrate101(2012); http://dx.doi.org/10.1063/1.4738499View Description Hide Description
The differential gain and coherent output characteristics of blue-emitting In0.18Ga0.82N/GaN quantum dot ridge waveguide lasers have been measured. The laser heterostructures were grown by molecular beam epitaxy. Injected carrier lifetimes in the quantum dots have been measured by temperature dependent and time resolved photoluminescence measurements. The radiative lifetime at 280 K is 480 ps. The threshold current densities at room temperature are 930 and 970 A/cm2 for pulsed and continuous wave bias operation, respectively. The measured differential gain is 2 × 10−16 cm2. The output slope and wall plug efficiency at 1050 A/cm2 under continuous wave operation are 0.4 W/A and 0.4%, respectively. The measured blue shift in the emission wavelength due to screening of the piezoelectric field with injection is as small as 4.4 nm.
101(2012); http://dx.doi.org/10.1063/1.4738982View Description Hide Description
Coupling of focused terahertz radiation into tapered metal parallel plate waveguides with sub-wavelength gap widths is presented. A line-focus terahertz time-domain spectroscopy setup with flexible foci is used to investigate coupling of broadband terahertz radiation from free-space into waveguides for different gap widths. Various waveguide lengths are compared in experiment and calculation in terms of occurring loss and divergence phenomena. Amplitude coupling ratios of 80% from free-space into the waveguides are obtained.
Ultraviolet electroluminescence from horizontal ZnO microrods/GaN heterojunction light-emitting diode array101(2012); http://dx.doi.org/10.1063/1.4739002View Description Hide Description
ZnO microrods were assembled on p-GaN substrate to form a heterostructural light-emitting diode(LED) array. Ultraviolet (UV) emission was obtained under a low forward bias of 3.5 V. The investigation on the electroluminescence,photoluminescence demonstrated three distinct electron-hole recombination processes. The relative intensity of these three emission bands changed with increase of the forward bias, and resulted in blue shift and spectral narrowing of electroluminescence. The present work provides a facile technique for micro-/nano-devices fabrication besides obtaining UVLED arrays.
101(2012); http://dx.doi.org/10.1063/1.4737646View Description Hide Description
We introduce a system of light driven microscopic autonomous moving particles that move on a flat surface. The design is simple, yet effective: Micrometer sized objects with wedge shape are produced by photopolymerization, and they are covered with a reflective surface. When the area of motion is illuminated perpendicularly from above, the light is deflected to the side by the wedge shaped objects, in the direction determined by the position and orientation of the particles. The momentum change during reflection provides the driving force for an effectively autonomous motion. The system is an efficient tool to study self propelled microscopic robots.
Ultrafast third order nonlinear optical response of donor and acceptor codoped and compensated silicon quantum dots101(2012); http://dx.doi.org/10.1063/1.4739237View Description Hide Description
Nonlinear optical responses of phosphorus and boroncodoped and compensated siliconquantum dots (Si-QDs) embedded in borophosphosilicate glass were studied by a z-scan and an optical Kerr gate methods under femtosecond excitation at 780 nm. The nonlinear refractive index (n2) and the two photon absorption coefficients (β) of compensated Si-QDs were found to be enhanced several times compared to those of intrinsic Si-QDs. The response time was shorter than our time resolution of 100 fs at room temperature.
101(2012); http://dx.doi.org/10.1063/1.4739004View Description Hide Description
We demonstrate high thermal dissipation of quantum cascade lasers(QCLs) using multi-stripes array technology. Buried QCL arrays offer both lateral dissipation enhancements while keeping beam quality control for large active region lasers. Experimental thermal resistances down to 2 K/W are reported. InP:Fe regrowth morphology has been optimized to limit current leakage. Thermal resistance decreasing with both number and width of emitters is demonstrated. Comparison with simulation shows excellent agreement, with a reduction factor of 3 when comparing to standard ridges QCL. These low thermal resistances project up to 40 W in continuous wave operation using state-of-the-art QCL design.
101(2012); http://dx.doi.org/10.1063/1.4739245View Description Hide Description
Zinc-blende semiconductor heterostructures grown in the  direction with a small lattice mismatch accommodate stress by developing a cross-hatch dislocation pattern. In GaAs based planar microcavitiesgrown by molecular beam epitaxy, this pattern creates a potential landscape for exciton-polaritons, causing scattering and localization. We report here on suppressing the cross-hatch by introducing strain-compensating AlP layers into the center of the low index AlAs layers of the distributed Bragg reflectors. We observe a reduction of the cross-hatch dislocation density by at least one order of magnitude for 1.1 nm thick AlP layers, which correspond to an effective AlAs0.985P0.015 low index layer. These compensated structures show a remaining polariton disorder potential in the 10 μeV range.
Improved quantum efficiency in InGaN light emitting diodes with multi-double-heterostructure active regions101(2012); http://dx.doi.org/10.1063/1.4739419View Description Hide Description
InGaN light emitting diodes(LEDs) with multiple thin double-heterostrucutre (DH) active regions separated by thin and low energy barriers were investigated to shed light on processes affecting the quantum efficiency and means to improve it. With increasing number of 3 nm-thick DH active layers up to four, the electroluminescence efficiency scaled nearly linearly with the active region thickness owing to reduced carrier overflow with increasing total thickness, showing almost no discernible efficiency degradation at high injection levels up to the measured current density of 500 A/cm2. Comparison of the resonant excitation dependent photoluminescence measurements at 10 K and room temperature also confirmed that further increasing the number of DH layers beyond six results in degradation of the material quality, and therefore, increasing nonradiative recombination. Using multiple DH active regions is shown to be a superior approach for quantum efficiency enhancement compared with simply increasing the single DH thickness or the number of quantum wells in LED structures due to better material quality and larger number of states available.
101(2012); http://dx.doi.org/10.1063/1.4739445View Description Hide Description
GaN-based light emitting diodes(LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QWstructure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.
Demonstration of low-loss on-chip integrated plasmonic waveguide based on simple fabrication steps on silicon-on-insulator platform101(2012); http://dx.doi.org/10.1063/1.4739523View Description Hide Description
We report the experimental realization of a robust silicon-based plasmonicwaveguidestructure which can theoretically provide sub-wavelength confinement for E x- and E y-polarized surface plasmon polariton modes. Our waveguides exhibit propagation loss as low as 0.2 dB/μm with ∼50% coupling efficiency.
101(2012); http://dx.doi.org/10.1063/1.4739512View Description Hide Description
All-fiber mode-locked lasers with fundamental repetition rates of several gigahertz are sought after for applications in optical communications and metrology. In this paper, we propose a fiber Fabry-Pérot laser mode-locked by a graphene-based saturable absorber that operates at a fundamental repetition rate of 9.67 GHz. We use this laser as the seed for the generation of supercontinuum with 0.08 nm mode spacing.
- SURFACES AND INTERFACES
Towards secondary ion mass spectrometry on the helium ion microscope: An experimental and simulation based feasibility study with He+ and Ne+ bombardment101(2012); http://dx.doi.org/10.1063/1.4739240View Description Hide Description
The combination of the high-brightness He+/Ne+ atomic level ion source with secondary ion mass spectrometry detection capabilities opens up the prospect of obtaining chemical information with high lateral resolution and high sensitivity on the Zeiss ORION helium ion microscope. The analytical performance in terms of sputtering yield, useful yield, and detection limit is studied and subsequently optimized by oxygen and cesium flooding. Detection limits down to 10−6 and 10−5 can be obtained for silicon using Ne+ and He+, respectively. A simulation based study reveals furthermore that a lateral resolution <10 nm can be obtained.
Surface modification by nonthermal plasma induced by using magnetic-field-assisted gliding arc discharge101(2012); http://dx.doi.org/10.1063/1.4738766View Description Hide Description
The authors report on the introduction of a magnetic field to gliding arc discharge (GD) in order to enhance surface modification by nonthermal plasma at atmospheric-pressure. The GD is induced between two wire electrodes by using a pulse high-voltage power supply with peak-to-peak voltage of 5 kV. When a magnetic field of 0.25 T is applied, the GD enlarged and a 19-cm-long stretch of plasma is excited. The surface treatment of polyethylene terephthalate and polytetrafluoroethylene films is performed. The adhesion improved by up to ∼30 times due to the enhanced chemical activity in the films.