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Applied Physics Letters

Issue 15 • Date Apr 2014

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Displaying Results 1 - 25 of 98
  • Selective generation of an intense single harmonic from a long gas cell with loosely focusing optics based on a three-color laser field

    Page(s): 151101 - 151101-4
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    The selective generation of an intense single harmonic has been experimentally achieved in argon from a long gas cell with loosely focusing optics using a three-color laser field. When compared with the single harmonic emission from a continuous gas jet, both the intensity and the purity of the selected single harmonic emission from the long gas cell show dramatic improvements; the peak intensity is more intense by as much as 1–2 orders of magnitude, while the contrast ratio (i.e., the spectral purity) is simultaneously increased by several times. The underlying physics of this enhancement can be explained using the strong field approximation model with the propagation effect. View full abstract»

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  • Modulating emission intensity of GaN-based green light emitting diodes on c-plane sapphire

    Page(s): 151102 - 151102-3
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    The asymmetric dual-wavelength (green/blue) coupled InGaN/GaN multiple quantum wells were proposed to modulate the green emission intensity. Electroluminescent measurements demonstrate the conspicuous increment of the green light intensity by decreasing the coupled barrier thickness. This was partly attributed to capture of more carriers when holes tunnel across the thinner barrier from the blue quantum wells, as a hole reservoir, to the green quantum wells. While lower effective barrier height of the blue quantum wells benefits improved hole transportation from p-GaN to the active region. Efficiency droop of the green quantum wells was partially alleviated due to the enhanced injection efficiency of holes. View full abstract»

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  • Ultralow-light-level all-optical transistor in rubidium vapor

    Page(s): 151103 - 151103-4
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    An all-optical transistor (AOT) is a device in which one light beam can efficiently manipulate another. It is the foundational component of an all-optical communication network. An AOT that can operate at ultralow light levels is especially attractive for its potential application in the quantum information field. Here, we demonstrate an AOT driven by a weak light beam with an energy density of 2.5 × 10−5 photons/(λ2/2π) (corresponding to 6  yJ/(λ2/2π) and about 800 total photons) using the double-Λ four-wave mixing process in hot rubidium vapor. This makes it a promising candidate for ultralow-light-level optical communication and quantum information science. View full abstract»

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  • Micro-electro-mechanically tunable metamaterial with enhanced electro-optic performance

    Page(s): 151104 - 151104-5
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    We experimentally demonstrate a micro-electro-mechanically tunable metamaterial with enhanced electro-optical performance by increasing the number of movable cantilevers in the symmetrical split ring resonator metamaterial unit cell. Simulations were carried out to understand the interaction of the incident terahertz radiation with out-of-plane deforming metamaterial resonator. In order to improve the overall device performance, the number of released cantilever in a unit cell was increased from one to two, and it was seen that the tunable range was doubled and the switching contrast improved by a factor of around five at 0.7 THz. This simple design approach can be adopted for a wide range of high performance electro-optical devices such as continuously tunable filters, modulators, and electro-optic switches to enable future photonic circuit applications. View full abstract»

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  • On-chip focusing in the mid-infrared: Demonstrated with ring quantum cascade lasers

    Page(s): 151105 - 151105-4
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    We report on collimated emission beams from substrate emitting ring quantum cascade lasers with an on-chip focusing element fabricated into the bottom side of the device. It is formed by a gradient index metamaterial layer, realized by etching subwavelength holes into the substrate. The generated optical path length difference for rays emitted under different angles from the ring waveguide flattens the wavefront and focuses the light. Our far field measurements show an increased peak intensity corresponding to 617% of the initial value without the focusing element. Far field calculations, based on a Fourier transformation of the metamaterial area, are in good agreement with our experimental data. View full abstract»

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  • Cascading metallic gratings for broadband absorption enhancement in ultrathin plasmonic solar cells

    Page(s): 151106 - 151106-5
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    The incorporation of plasmonic nanostructures in the thin-film solar cells (TFSCs) is a promising route to harvest light into the nanoscale active layer. However, the light trapping scheme based on the plasmonic effects intrinsically presents narrow-band resonant enhancement of light absorption. Here we demonstrate that by cascading metal nanogratings with different sizes atop the TFSCs, broadband absorption enhancement can be realized by simultaneously exciting multiple localized surface plasmon resonances and inducing strong coupling between the plasmonic modes and photonic modes. As a proof of concept, we demonstrate of 66.5% in the photocurrent in an ultrathin amorphous silicon TFSC with two-dimensional cascaded gratings over the reference cell without gratings. View full abstract»

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  • Laser pulse amplitude changes induced by terahertz waves under linear electro-optic effect

    Page(s): 151107 - 151107-5
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    Changes in the amplitude of femtosecond laser pulses and in the energy of terahertz wave radiation induced during their co-propagation in ZnTe and GaP crystals are studied theoretically and experimentally. The results show that variation of the optical field amplitude leads to changes in the laser pulse energy and spectrum shift. We investigate the quantitative correlations between variations of the optical pulse energy, spectrum, phase and terahertz radiation energy. The values of laser pulse energy change and spectrum shift are proportional to the first time derivative of the magnitude of terahertz electric field, which enables coherent electro-optic detection. A simple and convenient calibration technique for terahertz energy detectors based on the correlation between laser and terahertz energy changes is proposed and tested. View full abstract»

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  • Nonradiative recombination mechanisms in InGaN/GaN-based light-emitting diodes investigated by temperature-dependent measurements

    Page(s): 151108 - 151108-4
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    Two kinds of InGaN-based light-emitting diodes (LEDs) are investigated to understand the nonradiative carrier recombination processes. Various temperature-dependent measurements such as external quantum efficiency, current-voltage, and electroluminescence spectra are utilized from 50 to 300 K. Based on these experimental results, we analyze the dominant nonradiative recombination mechanism for each LED device. We also analyze the effect of the dominant nonradiative recombination mechanism on the efficiency droop. On the basis of correlation between the efficiency droop and nonradiative recombination mechanisms, we discuss an approach to reducing the efficiency droop for each LED device. View full abstract»

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  • Polarization degenerate micropillars fabricated by designing elliptical oxide apertures

    Page(s): 151109 - 151109-3
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    A method for fabrication of polarization degenerate oxide apertured micropillar cavities is demonstrated. Micropillars are etched such that the size and shape of the oxide front is controlled. The polarization splitting in the circular micropillar cavities due to the native and strain induced birefringence can be compensated by elongating the oxide front in the [110] direction, thereby reducing stress in this direction. By using this technique, we fabricate a polarization degenerate cavity with a quality factor of 1.7 × 104 and a mode volume of 2.7 μm3, enabling a calculated maximum Purcell factor of 11. View full abstract»

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  • Determination of the polarization state of x rays with the help of anomalous transmission

    Page(s): 151110 - 151110-4
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    Besides intensity and direction, the polarization of an electromagnetic wave provides characteristic information on the crossed medium. Here, we present two methods for the determination of the polarization state of x rays by polarizers based on anomalous transmission (Borrmann effect). Using a polarizer-analyzer setup, we have measured a polarization purity of less than 1.5 × 10−5, three orders of magnitude better than obtained in earlier work. Using the analyzer crystal in multiple-beam case with slightly detuned azimuth, we show how the first three Stokes parameters can be determined with a single angular scan. Thus, polarization analyzers based on anomalous transmission make it possible to detect changes of the polarization in a range from degrees down to arcseconds. View full abstract»

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  • Microscopic analysis of non-equilibrium dynamics in the semiconductor-laser gain medium

    Page(s): 151111 - 151111-4
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    Fully microscopic many-body calculations are used to analyze the carrier dynamics in situations where a strong sub-picosecond pulse interacts with an inverted semiconductor quantum well. Electron-electron and electron-phonon scatterings are calculated on a second Born-Markov level. Intra-subband scatterings on a scale of tens of femtoseconds are shown to quickly re-fill the kinetic holes created in the carrier distributions during the pulse amplification. Even for sub-100 fs pulses, this significantly influences the pulse amplification as well as its spectral dependence. Interband scatterings on a few picosecond timescale limit the possibly achievable repetition rate in pulsed semiconductor lasers. View full abstract»

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  • A broadband transformation-optics metasurface lens

    Page(s): 151601 - 151601-4
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    We present a transformational metasurface Luneburg lens based on the quasi-conformal mapping method, which has weakly anisotropic constitutive parameters. We design the metasurface lens using inhomogeneous artificial structures to realize the required surface refractive indexes. The transformational metasurface Luneburg lens is fabricated and the measurement results demonstrate very good performance in controlling the radiated surface waves. View full abstract»

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  • Enhanced Pt performance with H2O plasma modified carbon nanofiber support

    Page(s): 151602 - 151602-4
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    The insufficient durability and catalytic activity in low loading of platinum (Pt) are main obstacles to the development of low-temperature fuel cells. Our study demonstrated an efficient way to simultaneously improve the durability and electro-catalytic activity of Pt catalysts on carbon supports by water vapor (H2O) plasma functionalization. We report the finding that H2O plasma modification can introduce hydroxyl groups on carbon nanofiber (CNF) surface, and at the same time, highly preserve the microstructure of carbon support. Pt/CNF-H2O electrode possesses ultra-low Pt loading and superior electro-catalytic activity, poisoning-resistance ability and stability, suggesting a good candidate for fuel cell applications. View full abstract»

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  • Resistive switching of a TaOx/TaON double layer via ionic control of carrier tunneling

    Page(s): 151603 - 151603-5
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    Resistance random access memory (RRAM) is an attractive candidate for future non-volatile memory due to its superior features. As the oxide thickness is scaled down, the charge transport mechanism is also subject to the transition from hopping to tunneling dominant process, which is critically related to the interfacial electronic band structure. A TaOx/TaON double layer-based RRAM is fabricated and characterized in this work. Upon TaON insertion at the lower interface, the improved switching behavior is observed. The TaON at the bottom electrode interface blocks oxygen vacancy percolation due to strong N-O bonds and also modifies interfacial band alignment to lower the injected electron energy from bottom electrode due to higher tunneling barrier height than that of TaOx/Pt. This study suggested that a defect-minimized insertion layer like TaON with a proper interfacial band alignment is pivotal in RRAM for the effective ionic control of carrier tunneling resulting in non-linear I-V behavior with improved properties. View full abstract»

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  • Quantitative analysis of interfacial reactions at a graphene/SiO2 interface using the discharge current analysis method

    Page(s): 151604 - 151604-4
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    Using the discharge current analysis method, the contribution of charge generation through an interfacial reaction at a graphene /substrate interface is assessed to be on the order of 1014/cm2, which is ∼20% of the total charging sites. The validity of this method, which separately extracts the density of the charging sites related to the initial defect density of the graphene from the contribution of interfacial reactions is examined by measuring the discharge current of graphene field-effect-transistors at different ambient and temperatures. This method will be crucially instrumental in finding an optimal substrate material for graphene devices. View full abstract»

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  • Stacking faults and interface roughening in semipolar (202¯1¯) single InGaN quantum wells for long wavelength emission

    Page(s): 151901 - 151901-5
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    The microstructure of InGaN single quantum wells (QWs) grown in semipolar (202¯1¯) orientation on GaN substrates was studied by transmission electron microscopy. Stress relaxation in the lattice mismatch InxGa1−xN layer was realized by forming partial misfit dislocations associated with basal plane stacking faults (BPSFs). For given composition x = 0.24, BPSFs formation was observed when the QW thickness exceeded 4 nm. The high density of partial threading dislocations that bound the BPSFs is detrimental to light-emitting device performance. Interface roughening (faceting) was observed for both upper and lower QW interfaces (more pronounced for upper interface) and was found to increase with the thickness of the QW. BPSFs had a tendency to nucleate at roughened interface valleys. View full abstract»

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  • Temperature dependent photoexcited carrier dynamics in multiferroic BiFeO3 film: A hidden phase transition

    Page(s): 151902 - 151902-5
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    The ultrafast carrier dynamics of the multiferroic BiFeO3 film in a broad temperature range is investigated using optical pump-probe spectroscopy. The photoexcited electrons release their energy with optical phonons emission through electron-phonon coupling in about 1 ps. The following intermediate process is identified as dynamical spin-lattice coupling in several picoseconds. Furthermore, the peak values of the optical reflectivity and the time constants of carrier relaxation channels show significant changes while the temperature varies from 137.5 K to around 195 K, this aligns with the previously reported hidden phase transition. Our study demonstrates that ultrafast spectroscopy is a sensitive method to look into the dynamical interactions among the on-site high-energy electrons accumulated in the p conduction band of Bi, coherent optical phonon, as well as the spin degree of freedom. These features play crucial roles in the characterization of phase transitions. View full abstract»

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  • High quality factor nanocrystalline diamond micromechanical resonators limited by thermoelastic damping

    Page(s): 151903 - 151903-5
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    We demonstrate high quality factor thin-film nanocrystalline diamond micromechanical resonators with quality factors limited by thermoelastic damping. Cantilevers, single-anchored and double-anchored double-ended tuning forks, were fabricated from 2.5 μm thick in-situ boron doped nanocrystalline diamond films deposited using hot filament chemical vapor deposition. Thermal conductivity measured by time-domain thermoreflectance resulted in 24 ± 3 W m−1 K−1 for heat transport through the thickness of the diamond film. The resonant frequencies of the fabricated resonators were 46 kHz–8 MHz and showed a maximum measured Q ≈ 86 000 at fn = 46.849 kHz. The measured Q-factors are shown to be in good agreement with the limit imposed by thermoelastic dissipation calculated using the measured thermal conductivity. The mechanical properties extracted from resonant frequency measurements indicate a Young's elastic modulus of ≈788 GPa, close to that of microcrystalline diamond. View full abstract»

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  • Enhanced acoustic wave localization effect using coupled sonic crystal resonators

    Page(s): 151904 - 151904-4
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    A coupled resonance structure of two sonic crystal resonators with different sizes is proposed to enhance the acoustic wave localization effect. Due to acoustic resonance coupling between sonic crystal resonators, the enhanced acoustic wave localization is observed in the coupled resonance structure, and the cavity pressure is much larger than that in each individual sonic crystal resonator. The experimental results show that the proposed coupled structure exhibits 2.1–3.3 times larger maximum pressure magnification than each individual sonic crystal resonator. This proposed structure can be further used to improve acoustic energy harvesting, acoustic sensing, and sound concentration. View full abstract»

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  • Conditions for the coexistence of liquid-like and solid-like behaviors in viscoelastic liquids

    Page(s): 151905 - 151905-4
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    Viscoelastic liquids at small scales and in the presence of strong gradients are known to exhibit anomalous behaviors. Despite recent advances, our understanding of the phenomena is far from complete. For example, it is not clear what causes the molecules in molecular liquids to act in a collective manner and why similar dynamic heterogeneity takes place in gels and polymers? Furthermore, we would like to know why particles in suspensions experience clustering? The “ordered” liquid is a liquid, and yet it exhibits some properties of a viscoelastic solid-like material. We conjecture that the liquid-like and solid-like behaviors can coexist but only in the presence of the dynamics heterogeneity. In liquids, the heterogeneity is an internal constraint. In amorphous viscoelastic solids, it destroys the solid-like microstructural organization. Thus, the two behaviors may converge and become indistinguishable. The transitional behavior occurs in the absence of an abrupt configurational change. For this reason, these transitions cannot be viewed as the first order phase transformations. View full abstract»

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  • Stress distribution in GaN nanopillars using confocal Raman mapping technique

    Page(s): 151906 - 151906-5
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    In this Letter, high-resolution confocal Raman mapping of stress distribution in etched and re-grown GaN nanopillar structures is investigated. Results of the E2(high) phonon line mapping of the top surfaces of individual nanopillars reveal differences in stress between both the center and edge of the nanopillar top surfaces and between the etched and re-grown GaN nanopillar structures. In-plane biaxial compressive stress with the values of 0.36–0.42 GPa and 0.49–0.54 GPa is observed at the center of etched and re-grown GaN nanopillars, respectively. The in-plane biaxial compressive stress decreases from center to edge in re-grown GaN nanopillar due to the tilted facets. Also, the A1(LO) phonon frequency increases from center to edges, or tilted facets, due to the tilt of the c-axis of re-grown GaN nanopillar. View full abstract»

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  • Spatial density profile of electrons near the LaAlO3/SrTiO3 heterointerface revealed by time-resolved photoluminescence spectroscopy

    Page(s): 151907 - 151907-4
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    The depth profile of the electron density near the LaAlO3/SrTiO3 heterointerface has been studied by means of time-resolved photoluminescence (PL) spectroscopy. A broad blue PL band is observed at 2.9 eV, originating from the two-carrier radiative recombination of interface-induced electrons and photoexcited holes. The PL lifetime of LaAlO3/SrTiO3 heterointerface is dominated by the three-carrier Auger recombination of electrons and holes and is sensitive to electron density. We tuned the probing depth by changing the excitation photon energy and evaluated the carrier-density profile using the relation between the carrier density and the PL lifetime. Our non-contact probe method based on PL spectroscopy indicates that the carriers are confined within several nanometers in depth near the LaAlO3/SrTiO3 heterostructures. View full abstract»

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  • Experimental investigation of the latent heat of vaporization in aqueous nanofluids

    Page(s): 151908 - 151908-4
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    This paper reports an experimental investigation of the latent heat of vaporization (hfg) in nanofluids. Two different types of nanoparticles, graphite and silver, suspended in deionized water were exposed to a continuous laser beam (130 mW, 532 nm) to generate boiling. The latent heat of vaporization in the nanofluids was determined by the measured vapor mass generation and the heat input. To ensure that the measured hfg values are independent of heating method, the experiments were repeated with an electrically heated hot wire as a primary heat input. These experiments show considerable variation in the hfg of nanofluids. That is, graphite nanofluid exhibits an increased hfg and silver nanofluid shows a decrease in hfg compared to the value for pure water. As such, these results indicate that relatively low mass fractions of nanoparticles can apparently create large changes in hfg. View full abstract»

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  • Effects of sodium on electrical properties in Cu2ZnSnS4 single crystal

    Page(s): 152101 - 152101-4
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    We have studied the effect of sodium on the electrical properties of Cu2ZnSnS4 (CZTS) single crystal by using temperature dependence of Hall effect measurement. The sodium substitution on the cation site in CZTS is observed from the increasing of unit-cell size by powder X-ray diffraction. Sodium increases the effective hole concentration and makes the thermal activation energy smaller. The degree of compensation decreases with sodium incorporation, thus the hole mobility is enhanced. We revealed that sodium is important dopant in CZTS to control the electrical properties. View full abstract»

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  • On the reliable analysis of indium mole fraction within InxGa1−xN quantum wells using atom probe tomography

    Page(s): 152102 - 152102-5
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    Surface crystallography and polarity are shown to influence the detection probability of In, Ga, and N ions during atom probe tomography analysis of InxGa1−xN m-plane, c-plane, and (202¯1¯) quantum wells. A N deficit is observed in regions of the reconstruction generated from Ga-polar surfaces, and the probability of detecting group-III atoms is lower in InxGa1−xN quantum wells than in GaN barrier layers. Despite these artifacts, the detected In mole fraction is consistent throughout a given quantum well regardless of the crystal orientation of the quantum well or the evaporation surface from which the reconstruction was generated. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory