Volume 102, Issue 7, 18 February 2013
- 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
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
We demonstrate optically pumped polymer band-edge lasers based on a two-dimensional photonic crystal slab fabricated by nanoimprint lithography (NIL). Lasing was obtained at the photonic band-edge, where the light exhibits a low group velocity at the Γ point of the triangular lattice photonic crystal band structure. The active medium was composed of a dye chromophore-loaded polymer matrix directly patterned in a single step by nanoimprint lithography. Plane-wave and finite difference time domain algorithms were used to predict experimental lasing frequencies and the lasing thresholds obtained at different Γ points. A low laser threshold of 3 μJ/mm2 was achieved in a defect-free photonic crystal thus showing the suitability of nanoimprint lithography to produce cost-efficient optically pumped lasers.
- PHOTONICS AND OPTOELECTRONICS
Molecular beam epitaxial growth and optical properties of red-emitting (λ = 650 nm) InGaN/GaN disks-in-nanowires on silicon102(2013); http://dx.doi.org/10.1063/1.4793300View Description Hide Description
We have investigated the radiative properties of InGaN disks in GaN nanowires grown by plasma enhanced molecular beam epitaxy on (001) silicon substrates. The growth of the nanowire heterostructures has been optimized to maximize the radiative efficiency, or internal quantum efficiency (IQE), for photoluminescence emission at λ = 650 nm. It is found that the IQE increases significantly (by ∼10%) to 52%, when post-growth passivation of nanowire surface with silicon nitride or parylene is applied. The increase in efficiency is supported by radiative- and nonradiative lifetimes derived from data obtained from temperature dependent- and time-resolved photoluminescence measurements. Light emitting diodes with p-i-n disk-in-nanowire heterostructures passivated with parylene have been fabricated and characterized.
Numerical simulations of time-domain interferometric soft X-ray microscope with broadband high-order harmonic light sources102(2013); http://dx.doi.org/10.1063/1.4793301View Description Hide Description
A scheme for achieving high spatial resolution in soft X-ray microscopy with coherent broadband light sources is proposed, in which the chromatic aberration at a Fresnel zone plate lens can be canceled out by introducing time-delayed double pulses as an input light and by the Fourier transformation of recorded images with respect to the time delay. Numerical simulations of microscope images show that the spatial resolution of the proposed method is determined only by specifications of zone plates even when a broadband soft X-ray is used as a light source.
102(2013); http://dx.doi.org/10.1063/1.4793401View Description Hide Description
Chemically synthesized colloidal particles featuring large-scale surface asperities can be trapped and manipulated in fluid media through holographic optical trapping. Light scattering by these particles' surface features provides a mechanism for holographic optical traps also to exert torques on them, thereby setting them in steady rotation about arbitrary axes in three dimensions. When pairs of rotating particles are brought close enough that their surface features mesh, they form microscopic gear trains. These micro-opto-mechanical systems can be arranged in any desired three-dimensional configuration.
Improved Kerr constant and response time of polymer-stabilized blue phase liquid crystal with a reactive diluent102(2013); http://dx.doi.org/10.1063/1.4793416View Description Hide Description
A polymer-stabilized (PS) blue phase liquid crystal (BPLC) with fast response time and large Kerr constant is investigated by doping a low molecular weight monomer (N-vinylpyrrollidone) into a conventional PS-BPLC consisting of BPLC, RM257, and 1,1,1-trimethylolpropane triacrylate. With this polymer network system, Kerr constant and the response can be improved simultaneously. Compared to the conventional PS-BPLC, Kerr constant of the proposed PS-BPLC can increase by 54% and the response time can decrease by 23% at the same time. The contrast ratio can be kept at a high level, over 1000:1 at λ = 633 nm.
102(2013); http://dx.doi.org/10.1063/1.4792759View Description Hide Description
We demonstrate an experimental technique that allows to achieve a robust control on the emission spectrum of a micro random laser and to select individual modes with sub-nanometer resolution. The presented approach relies on an optimization protocol of the spatial profile of the pump beam. Here we demonstrate not only the possibility to increase the emission at a wavelength but also that we can “isolate” an individual peak suppressing unwanted contributions form other modes.
102(2013); http://dx.doi.org/10.1063/1.4793210View Description Hide Description
An ultraviolet heterojunction photodiode consisting of epitaxially grown p-GaN layers and polyvinyl alcohol coated ZnO colloidal nanoparticles exhibits a lowpass and bandpass alternative property depending on the illumination direction. At 0 V bias, a time response on the order of 10 s of milliseconds was demonstrated with a responsivity on the order of mA/W with about 100 nW of ultraviolet illumination. The rectification ratio at ±5 V was 1000 under dark environment. Deposition of colloidal ZnO nanoparticles on an independent p-GaN substrate introduces a technique to create a heterostructure pn junction photodiode with wavelength selection by back illumination.
102(2013); http://dx.doi.org/10.1063/1.4793432View Description Hide Description
The photoresponses of gallium nitride (GaN) doped with manganese (Mn) grown by metalorganic vapor-phase epitaxy were investigated. The transmission spectroscopy obtained from the Mn-doped GaN exhibited three distinct absorption thresholds at approximately 365, 650, and 830 nm, respectively. The below-band-gap absorption peaks were attributed to the fact that the deep Mn-related states mediate the electronic transition between the valence and conduction bands. A below-band-gap spectral response ranging from 400 nm to 1000 nm was also observed from a typical GaN p-i-n photodetector with Mn-doped absorption layer. The significant below-band-gap spectral responses showed that the Mn-doped GaN-based materials have promising applications in intermediate band solar cells.
- SURFACES AND INTERFACES
102(2013); http://dx.doi.org/10.1063/1.4792509View Description Hide Description
The structural and electrical properties of conducting NdGaO3/SrTiO3 (NGO/STO) heterostructures grown at various deposition temperatures were investigated. X-ray diffraction and X-ray photoelectron spectroscopy reveal a strong impact of the growth temperature on both crystallinity and cation stoichiometry of the NGO thin films. This stoichiometry variation significantly affects the electrical properties of the NGO/STO interface. High temperature conductance measurements under oxygen equilibrium conditions show a distinct conductance contribution of the NGO/STO interface up to 1000 K and exclude a conduction effect caused by a mere reduction of the STO substrate. Above 1000 K, the interface conduction is degrading due to a thermal instability. Both stoichiometry variation in as-grown films and thermal instability are attributed to the preferential evaporation of gallium from the NGO thin films at elevated temperatures.
102(2013); http://dx.doi.org/10.1063/1.4793197View Description Hide Description
In this work, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of single layer graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure.
Unveiling the mechanisms of dressed-photon–phonon etching based on hierarchical surface roughness measure102(2013); http://dx.doi.org/10.1063/1.4793233View Description Hide Description
Dressed-photon–phonon (DPP) etching is a disruptive technology in planarizing material surfaces because it completely eliminates mechanical contact processes. However, adequate metrics for evaluating the surface roughness and the underlying physical mechanisms are still not well understood. Here, we propose a two-dimensional hierarchical surface roughness measure, inspired by the Allan variance, that represents the effectiveness of DPP etching while conserving the original two-dimensional surface topology. Also, we build a simple physical model of DPP etching that agrees well with the experimental observations, which clearly shows the involvement of the intrinsic hierarchical properties of dressed photons, or optical near-fields, in the surface processing.
Comparison of CsBr and KBr covered Cu photocathodes: Effects of laser irradiation and work function changes102(2013); http://dx.doi.org/10.1063/1.4793214View Description Hide Description
Thin films of CsBr and KBr were deposited on Cu(100) to investigate photoemission properties of these potential photocathode materials. After prolonged laser ultraviolet irradiation photoemission, quantum efficiency increases by factors of 26 and 77 for KBr/Cu(100) and CsBr/Cu(100) photocathodes, respectively. Immediately following thin film deposition, a decrease in work function is observed, compared to bare Cu, in both cases. Quantum efficiency enhancements are attributed to the decrease in photocathode work function, and photo-induced processes that introduce defect states into the alkali halide bandgap. Our results suggest that KBr, a relatively stable alkali-halide, also has potential for photocathode applications.
102(2013); http://dx.doi.org/10.1063/1.4793576View Description Hide Description
We present a systematical study on the electronic transport properties of the insulating LaAlO3 (3 unit cells)/SrTiO3 interfaces capping with thin layers of La1−xSrxMnO3, whose formal polarization is continually tuned by Sr doping. When the Sr doping is lower than 2/3, the LaAlO3/SrTiO3 interfaces show metallic behaviors. The carrier mobility is almost independent on the Sr doping for metallic interface, indicating that the capping layer does not change the density of the oxygen vacancies and the interface intermixing. However, the sheet carrier densities monotonically decrease as increasing Sr doping, which is ascribed to the decrease of the La1−xSrxMnO3 formal polarization. These results strongly support the intrinsic mechanism of the polar catastrophe model and provide a new approach to tailor the interface states of complex oxide heterostructures.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
Microcrystalline diamond micromechanical resonators with quality factor limited by thermoelastic damping102(2013); http://dx.doi.org/10.1063/1.4793234View Description Hide Description
Thin-film microcrystalline diamond micromechanical resonators with mechanical quality factor limited by thermoelastic dissipation in the diamond film are demonstrated. Surface micromachined double ended tuning fork resonators were fabricated from in-situ boron doped microcrystalline diamond films deposited using hot filament chemical vapor deposition. Time-domain thermoreflectance measurements show thermal conductivity of 110 W m−1 K−1 for heat transport through the thickness of the diamond film. Measurement of the quality factor of resonators spanning a frequency range 0.5–10 MHz shows a maximum Q = 81 646 and demonstrates good agreement with quality factor limited by thermoelastic dissipation using 100 W m−1 K−1 for the in-plane thermal conductivity of the diamond film.
102(2013); http://dx.doi.org/10.1063/1.4793088View Description Hide Description
We show that, using molecular dynamic simulations, nanocrystalline (NC) graphene fails by brittle fracture along grain boundaries under uniaxial tension at room temperature. Initiated from either a grain-boundary triple junction or an array of vacancies on a preferential grain boundary, fracture occurs by unzipping atomic bonds along a preferential grain boundary. In sharp contrast to NC metals, no mobile dislocations are generated throughout the entire loading process, and the deformation remains fully elastic (albeit nonlinear) until the breaking of the first atomic bond due to high local stress near the initiation defect sites. Breaking of the first atomic bond triggers a cascade of bond breaking events along a preferential grain boundary that leads to the final brittle fracture failure. For the NC graphene monolayer sheet with an average grain size of ∼25 nm considered here, the predicted uniaxial tensile strength is 96.2 ± 4.2 GPa, which is one of the highest among all polycrystalline materials.
Impact of substrate temperature on the structural and optical properties of strain-balanced InAs/InAsSb type-II superlattices grown by molecular beam epitaxy102(2013); http://dx.doi.org/10.1063/1.4793231View Description Hide Description
Molecular beam epitaxial growth of strain-balanced InAs/InAs1− x Sb x type-II superlattices on GaSb substrates has been investigated for substrate temperatures from 400 °C to 450 °C. The Sb composition is found to vary linearly with substrate temperature at constant V/III ratios. For samples grown at the optimized substrate temperature (410 °C), superlattice zero-order peak full-width at half-maximums are routinely less than 25 arc sec using high-resolution X-ray diffraction. Cross-sectional transmission electron microscopy images show the absence of any visible defects. Strong photoluminescence covers a wavelength range from 5.5 to 13 μm at 12 K. Photoluminescence linewidth simulations show satisfactory agreement with experiments.
102(2013); http://dx.doi.org/10.1063/1.4793412View Description Hide Description
We study the martensitic transformation (MT) of metamagnetic shape memory alloy Ni 50Mn34.5In15.5 in the magnetic fields up to 12 T. The observed dependence of the MT temperature, T m, on the field is highly nonlinear. As far as magnetization change, ΔM, remains field-independent, a depart from linearity of T m(H) function is attributed to a decrease of the transformation entropy, ΔS. This decrease correlates with the parameter (T C-T m), controlled by magnetic field, where T C is the Curie temperature of austenite, and with the dependence of ΔS on the width of the MT temperature interval deduced from a ferroelastic model of MT.
102(2013); http://dx.doi.org/10.1063/1.4793413View Description Hide Description
The Stokes shift of colloidal 4.7 nm PbS quantum dots was measured between 5 and 300 K at incrementally increasing continuous laser intensities. The results demonstrate Stokes shift tuning by optical means only at stable given temperatures due to optically enforced electronic state alteration in the quantum dots. The tuning phenomenon is perfectly fit by a semi-empirical model, which provides a design tool for the chromaticity of quantum dots at different optical pump intensities.
102(2013); http://dx.doi.org/10.1063/1.4793420View Description Hide Description
Non-viscoelastic castable elastomers are replacing the polyacrylate VHB films in the new generations of dielectric elastomer actuators (DEAs) to achieve fast and reliable actuation. We introduce the optimum prestretch conditions to enhance the electromechanical behavior of the castable DEAs resulting in large actuation strain. For castable actuator in which the thickness is selected independent of the prestretch, uniaxial prestretch mode offers the highest actuation strain in the transverse direction compared to biaxial and pure shear. We experimentally demonstrate that miniaturization hinders the loss of tension and up to 85% linear actuation strain is generated with a 300 × 300 μm2 polydimethylsiloxanes-based DEA.
Why does the second peak of pair correlation functions split in quasi-two-dimensional disordered films?102(2013); http://dx.doi.org/10.1063/1.4793187View Description Hide Description
Molecular dynamics simulation has been performed to study the splitting of the second peak in pair correlation functions of quasi-two-dimensional disordered film. A quasi-two-dimensional inhomogeneous structural model, which contains both crystal-like and disordered regions, supports the hypothesis that the splitting of the second peak is result of a statistical average of crystal-like and disordered structural regions in the system, not just the amorphous structure. The second-peak splitting can be viewed as a prototype of the crystal-like peak exhibiting distorted and vestigial features.
102(2013); http://dx.doi.org/10.1063/1.4793191View Description Hide Description
We studied the nanoindentation of monolayer graphene by molecular dynamics simulations. It is found that the response of graphene to indentation is deflection dependent. In small deflection range, the response obeys point load model, while large-deflection indentation follows the sphere load model. Hence, we proposed to make sectional fittings and use different response models in different deflection ranges. In this way, a consistent Young's modulus is obtained that is almost independent of the size ratio of intender to graphene and the pretensions of graphene. The calculated Young's modulus is about 1.00 TPa, in good agreement with the experiments.