Volume 101, Issue 11, 10 September 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 fabricated a refreshable organic light-emitting diode with a liquid emitting layer. This device has a mesh-structured cathode and a liquid reservoir in the back side of the cathode for easy convection of the liquid emitter. The small uniform gap between the electrodes was fabricated by means of micro-electro-mechanical systems (MEMS) processing. Although the device luminance decreased because of decomposition of the liquid emitters under the drive current, the decreased emission was quickly recovered to the initial state by convectional replacement of the decomposed emitters with fresh emitters through the holes of the mesh cathode.
- PHOTONICS AND OPTOELECTRONICS
Generation of high-frequency terahertz waves in periodically poled LiNbO3 based on backward parametric interaction101(2012); http://dx.doi.org/10.1063/1.4751843View Description Hide Description
Backward terahertz pulses at high frequencies are generated in multi-period periodically poled LiNbO3 using ultrafast pulses of a regenerative amplifier. The highest frequencies generated by us are centered at 4.8 THz at the poling period of 7.1 μm, corresponding to the output wavelength of 62.5 μm. Enhancement factors as large as 61 in the output powers are achieved and analyzed due to resonance-enhanced nonlinear optical coefficients.
101(2012); http://dx.doi.org/10.1063/1.4748974View Description Hide Description
Far field imaging of subwavelength magnetic objects in real time is a very challenging issue. We propose an original solution based on a planar array of closely spaced split ring resonators. Hybridization between the resonators of such metalens induces subwavelength modes with different frequencies. Thanks to these high Q resonating modes, Purcell like effect allows an evanescent source, close to the metalens, to emit waves that can be collected efficiently in the far field. We present the first microwave experimental demonstration of such metalens to image of a subwavelength magnetic pattern. Numerical simulation shows that this approach is still valid at THz frequencies.
101(2012); http://dx.doi.org/10.1063/1.4751840View Description Hide Description
We demonstrate enhanced nonlinear optical response from a one-dimensional metalnanocomposite based photonic crystal. A three-fold increase in the two photon absorption coefficient was observed for the photonic crystalstructure when compared to a single layer of the metalnanocomposite having comparable metal content. The photonic crystalstructure also shows a reduction in the optical limiting threshold by a factor of seven. The combination of metal nanoparticles with appropriately designed plasmon resonance in combination with photonic crystalstructure provides an attractive approach for developing practical nonlinear optical devices with low thresholds and wide spectral bandwidth.
101(2012); http://dx.doi.org/10.1063/1.4751847View Description Hide Description
We show how the cross-over effect of dipolar glasses can be used to observe diffraction cancellation in composite ferroelectric samples independently of composition. We are able to selectively frustrate the dielectric anomaly of different compositionally disordered photorefractive ferroelectrics to achieve scale-free optical propagation at one same temperature.
101(2012); http://dx.doi.org/10.1063/1.4746752View Description Hide Description
Doping of silicon-on-insulator layers with sulfur to concentrations far above equilibrium by ion implantation and pulsed laser melting can result in large concentration gradients. Photocarriers generated in and near the impurity gradient can separate into different coplanar transport layers, leading to enhanced photocarrier lifetimes in thin silicon-on-insulator films. The depth from which holes escape the heavily doped region places a lower limit on the minority carrier mobility-lifetime product of 10−8 cm2/V for heavily sulfur dopedsilicon. We conclude that the cross-section for recombination through S impurities at this concentration is significantly reduced relative to isolated impurities.
101(2012); http://dx.doi.org/10.1063/1.4752114View Description Hide Description
Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection, more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters.
101(2012); http://dx.doi.org/10.1063/1.4752158View Description Hide Description
We demonstrate time-resolved terahertz transmission ellipsometry of vertically aligned multi-walled carbon nanotubes. The angle-resolved transmission measurements reveal anisotropic characteristics of the terahertz electrodynamics in multi-walled carbon nanotubes. The anisotropy is, however, unexpectedly weak: the ratio of the tube-axis conductivity to the transverse conductivity, , is nearly constant over the broad spectral range of 0.4–1.6 THz. The relatively weak anisotropy and the strong transverse electrical conduction indicate that THz fields readily induce electron transport between adjacent shells within multi-walled carbon nanotubes.
Terahertz mirage: Deflecting terahertz beams in an inhomogeneous artificial dielectric based on a parallel-plate waveguide101(2012); http://dx.doi.org/10.1063/1.4752241View Description Hide Description
The field of metamaterials and the formalism of transformation optics have provided a prescription for constructing artificial dielectrics with unique properties such as light trapping and cloaking. Here, we describe a different approach to creating an inhomogeneous artificial medium, based on waveguide techniques, which does not rely on engineered subwavelength-scale components. We demonstrate a mirage effect in which an object several times larger than the selected wavelength is rendered invisible by bending a beam around it.
High-power high-efficiency optically pumped semiconductor disk lasers in the green spectral region with a broad tuning range101(2012); http://dx.doi.org/10.1063/1.4751352View Description Hide Description
Optically pumped semiconductor disk lasers with an infra-red fundamental emission at around 1050 nm are presented. Design and characteristics of the devices are discussed and evaluated. A maximum output power of infra-red radiation of 18 W close to room temperature and rather high slope and differential quantum efficiencies are demonstrated. Utilizing intra-cavity second-harmonic generation in a folded resonator, green optical output powers exceeding 8 W are generated. A maximum total conversion efficiency of 22% and a relatively broad tuning range of 22 nm in the second-harmonic regime are achieved.
101(2012); http://dx.doi.org/10.1063/1.4752435View Description Hide Description
To fully utilize graphene's remarkable optical properties for optoelectronic applications, it needs to be integrated in planar photonic systems. Here, we demonstrate integration of graphene on siliconphotonic circuits and precise measurement of the optical absorption coefficient in a graphene/waveguide hybrid structure. A method based on Mach-Zehnder interferometry is employed to achieve high measurement precision and consistency, yielding a maximal value of absorption coefficient of 0.2 dB/μm when graphene is located directly on top of the waveguide. The averaged results obtained from a large number of samples agree with theoretical model utilizing the universal ac conductivity in graphene. Our work provides an important guide for the design and optimization of integrated grapheneoptoelectronic devices.
Propagation of light in serially coupled plasmonic nanowire dimer: Geometry dependence and polarization control101(2012); http://dx.doi.org/10.1063/1.4752718View Description Hide Description
We experimentally studied plasmon-polariton-assisted light propagation in serially coupled silvernanowire (Ag-NW) dimers and probed their dependence on bending-angle between the nanowires and polarization of incident light. From the angle-dependence study, we observed that obtuse angles between the nanowires resulted in better transmission than acute angles. From the polarization studies, we inferred that light emission from junction and distal ends of Ag-NW dimers can be systematically controlled. Further, we applied this property to show light routing and polarizationbeam splitting in obtuse-angled Ag-NW dimer. The studied geometry can be an excellent test-bed for plasmonic circuitry.
101(2012); http://dx.doi.org/10.1063/1.4752453View Description Hide Description
We report on the optical properties of single quantum dots in nanowires probed along orthogonal directions. We address the same quantum dot from either the nanowire side or along the nanowire axis via reflection on a micro-prism. The collected photoluminescence intensity from nanowires lying on a substrate is improved 3-fold using the prism as compared to usual collection from the top. More importantly, we circumvent the polarizing effect of the nanowire and access the intrinsic polarization properties of the quantum emitter. Our technique is compatible with the design of complex nanowire devices for the development of quantum opto-electronics.
- SURFACES AND INTERFACES
Growth of continuous and ultrathin platinum films on tungsten adhesion layers using atomic layer deposition techniques101(2012); http://dx.doi.org/10.1063/1.4749819View Description Hide Description
Continuous and ultrathin platinum (Pt) films were deposited on tungsten (W) adhesion layers using atomic layer deposition(ALD) techniques. Pt ALDfilms were deposited at 120 °C using MeCpPtMe3 and H2plasma as the reactants. X-ray reflectivity studies observed the rapid nucleation of the Pt film. X-ray photoelectron results were consistent with layer-by-layer growth suggesting a continuous Pt film at thicknesses ≥1.5 nm. The high surface energy of the W ALDadhesion layer enables the growth of continuous and ultrathin films of lower surface energy metals and should facilitate the use of precious metals for a variety of applications.
Effect of the polarity of carbon-fluorine bonds on the work function of plasma-fluorinated epitaxial graphene101(2012); http://dx.doi.org/10.1063/1.4752443View Description Hide Description
Work function engineering of graphene facilitates its application as a transparent electrodematerial in organic electronic devices. Toward this end, we demonstrate the dependence of the work function of plasma-fluorinated epitaxial graphene on the polarity of carbon-fluorine bonds which is controlled by the nature of chemical bonding (ionic, semi-ionic, or covalent) between fluorine and carbon atoms. The work function of fluorinated graphene was measured using ultraviolet photoelectron spectroscopy and the polarity of carbon-fluorine bonds was established using x-ray photoelectron spectroscopy.
101(2012); http://dx.doi.org/10.1063/1.4752436View Description Hide Description
Understanding the mechanism of iceadhesion on surfaces is crucial for anti-icing surfaces, and it is not clear if superhydrophobicsurfaces could reduce iceadhesion. Here, we investigate iceadhesion on model surfaces with different wettabilities. The results show that the superhydrophobicsurface cannot reduce the iceadhesion, and the iceadhesion strength on the superhydrophilicsurface and the superhydrophobic one is almost the same. This can be rationalized by the mechanical interlocking between the ice and the surface texture. Moreover, we find that the iceadhesion strength increases linearly with the area fraction of air in contact with liquid.
101(2012); http://dx.doi.org/10.1063/1.4752439View Description Hide Description
A modulation-doping approach to control the carrier density of the high-density electron gas at a prototype polar/non-polar oxide interface is presented. It is shown that the carrier density of the electron gas at a GdTiO3/SrTiO3interface can be reduced by up to 20% from its maximum value (∼3 × 1014 cm−2) by alloying the GdTiO3 layer with Sr. The Seebeck coefficient of the two-dimensional electron gas increases concurrently with the decrease in its carrier density. The experimental results provide insight into the origin of charge carriers at oxide interfaces exhibiting a polar discontinuity.
101(2012); http://dx.doi.org/10.1063/1.4752470View Description Hide Description
A hybrid surface consisting of an array of hydrophobic and hydrophilic sites was designed and fabricated in an effort to better understand the effects of microscale surface features and chemistry on wettability. A model based on energy minimization was developed to design and predict the wettability of hybrid surfaces. Measured advancing, receding, and equilibrium contact angles fit the proposed model well. Experiments show that a higher degree of hydrophobicity results in higher contact angles and that contact angle hysteresis increases with decreasing micropillar spacing (b/a). Moreover, measured roll-off angle as an indicator of droplet shedding, decreases with b/a.
Experimental surface-enhanced Raman scattering response of two-dimensional finite arrays of gold nanopatches101(2012); http://dx.doi.org/10.1063/1.4752719View Description Hide Description
We experimentally investigate the nonlinear response of two-dimensional periodic arrays composed of gold nanopatches on silicon substrate, functionalized by means of a conjugated rigid thiol. The surface-enhanced Raman scattering(SERS) response is empirically evaluated using a laser source operating in the visible spectral range at λ = 633 nm. Nonlinear results are then correlated to optical and structural properties of the samples under investigation. SERS mapping and estimation of the SERS enhancement factor are examined to determine stability and reproducibility of the results, highlighting also the contribution of the plasmonic resonance excited in the two-dimensional periodic array, and the dependence on the numerical aperture of the microscope objective used in the micro-Raman system.
A photoelectron spectroscopy study of the electronic structure evolution in CuInSe2-related compounds at changing copper content101(2012); http://dx.doi.org/10.1063/1.4752723View Description Hide Description
Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy(XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS.
Scanning internal photoemission microscopy for the identification of hot carrier transport mechanisms101(2012); http://dx.doi.org/10.1063/1.4752734View Description Hide Description
Linear and nonlinear internal photoemission in a thin-film metal-insulator-metal heterosystem, i.e., a Ta-TaOx-Ag junction, together with surfacereflectivity are mapped with a lateral resolution of better than 5 μm. The spatial correlation of the different signals and time-resolved internal photoemission spectroscopy reveal excitation mechanisms and ballistichot carrier injection. The internal photoemission yield variation with Ag layer thickness is quantitatively explained by above-barrier injection. The hot-spot-like behavior of the two-photon induced internal photoemission observed for short pulse excitation is attributed to local field enhancements because of Ag-film thickness reduction and plasmonic effects at structural defects.