Volume 100, Issue 7, 13 February 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 describe a straightforward technique for selective graphenegrowth and nanoribbon production onto 4H- and 6H-SiC. The technique presented is as easy as ion implanting regions where graphene layers are desired followed by annealing to 100 °C below the graphitization temperature (TG ) of SiC. We find that ion implantation of SiC lowers the TG , allowing selective graphenegrowth at temperatures below the TG of pristine SiC and above TG of implantedSiC. This results in an approach for patterning device structures ranging from a couple tens of nanometers to microns in size without using conventional lithography and chemical processing.
- PHOTONICS AND OPTOELECTRONICS
100(2012); http://dx.doi.org/10.1063/1.3684250View Description Hide Description
We report terahertz metamaterialwaveguides based on the concept of composite right/left-handed transmission-lines. The waveguides are implemented in a metal-insulator-metal geometry fabricated with spin-coated Benzocyclobutene and contact photolithography. Angle-resolved reflection spectroscopy shows strong resonant absorption features corresponding to both right-handed and left-handed (backward wave) propagating modes within the leaky-wave bandwidth. Tuning of the waveguidedispersion is achieved by varying the effective lumped element series capacitance. The experimental results are in good agreement with full-wave finite element method simulations as well as an intuitive transmission-line circuit model.
100(2012); http://dx.doi.org/10.1063/1.3680209View Description Hide Description
Multiplex detection of single beam coherent anti-Stokes Raman spectra is achieved by phase shaping of a narrow gate probe. The spectrally resolved signal is amplified via a local oscillator derived from the same single beam and is disentangled via double quadrature spectralinterferometry.Spectral resolution and spectral range are defined by the width and position of the phase gate, which can be easily set by a femtosecond pulse shaper. Such phase-only multiplexed single beam coherent anti-Stokes Raman spectroscopy is demonstrated for simple molecules and is combined with Raman line reconstruction methods to obtain dispersion free Raman resonances.
High quality factor two dimensional GaN photonic crystal cavity membranes grown on silicon substrate100(2012); http://dx.doi.org/10.1063/1.3684630View Description Hide Description
We report on the achievement of freestanding GaNphotonic crystal L7 nanocavities with embedded InGaN/GaN quantum wellsgrown by metal organic vapor phase epitaxy on Si (111). GaN was patterned by e-beam lithography, using a SiO2 layer as a hard mask, and usual dry etching techniques. The membrane was released by underetching the Si (111) substrate. Micro-photoluminescence measurements performed at low temperature exhibit a quality factor as high as 5200 at ∼420 nm, a value suitable to expand cavity quantum electrodynamics to the near UV and the visible range and to develop nanophotonic platforms for biofluorescence spectroscopy.
100(2012); http://dx.doi.org/10.1063/1.3685487View Description Hide Description
We use femtosecond optical pump-probe spectroscopy to study ultrafast carrier dynamics in single quasi-one-dimensional silicon nanowires. By isolating individual nanowires, we avoid complications resulting from the broad size and alignment distribution in nanowire ensembles, allowing us to directly probe ultrafast carrier dynamics. Spatially-resolved experiments demonstrate the influence of surface-mediated mechanisms on carrier dynamics in a single NW, while polarization-resolved experiments reveal a clear anisotropy in carrier lifetimes measured parallel and perpendicular to the long axis of the NW, due to density-dependent Auger recombination. These results suggest the possibility of tailoring carrier relaxation in a single nanowire for a desired application.
100(2012); http://dx.doi.org/10.1063/1.3685496View Description Hide Description
The electronic energy relaxation of polycrystalline BiFeO3films is studied using ultrafast pump-probe spectroscopy. After photo-excitation with femtosecond laser pulses, the relaxation of hot electrons is identified to decay with two different characteristic times. The fast process is attributed to scattering of electrons with lattice-vibration modes, and the slow one is corresponding to the spin-lattice thermalization. The electron-phonon coupling is characterized by the second moment of the Eliashberg function, λ〈ω2〉. Due to the structural strain and symmetry breaking, the electron-phonon interaction strength of tetragonal BiFeO3films is larger than that of rhombohedral counterparts.
Fiber delivered two-color picosecond source through nonlinear spectral transformation for coherent Raman scattering imaging100(2012); http://dx.doi.org/10.1063/1.3686149View Description Hide Description
We demonstrate a two-color, fiber-delivered picosecond source for coherent Raman scattering (CRS) imaging through nonlinear spectral transformation. The wavelength tunable picosecond pump is generated by nonlinear spectral compression of a prechirped femtosecond pulse in a fiber wavelength division multiplexer (WDM). The 1064-nm synchronized picosecond Stokes pulse is generated through pulse carving of a continuous wave laser, nonlinear spectral broadening in 100-m standard single-mode fiber, and subsequent dispersive compression with a fiber compressor. The pump and Stokes beams are combined and delivered by the fiber WDM. CRS imaging of mouse skin is performed to demonstrate the practicality of this source.
Ultrafast response of tunnel injected quantum dot based semiconductor optical amplifiers in the 1300 nm range100(2012); http://dx.doi.org/10.1063/1.3686901View Description Hide Description
The ultrafast gain and refractive index dynamics of tunnel injected quantum dot based semiconductor optical amplifiers in the 1300 nm range are investigated using a heterodyne pump probe technique. In the gain regime, ground state wavelengths exhibit full gain recovery in less than 10 ps up to 3 times transparency, attributed to enhanced carrier refilling via the injector layer. The effect of the injector can also been seen in unusual phase dynamics at excited state wavelengths at this injection level.
100(2012); http://dx.doi.org/10.1063/1.3687170View Description Hide Description
The observation of four-wave mixing(FWM) in single-walled carbon nanotubes(SWCNTs) deposited around a tilted fiber Bragg grating (TFBG) has been demonstrated. A thin, floating SWCNTfilm is manually wrapped around the outer cladding of the fiber and FWM occurs between two core-guided laser signals by TFBG-induced interaction of the core mode and cladding modes. The effective nonlinear coefficient is calculated to be 1.8 × 103 W−1 Km−1. The wavelength of generated idlers is tunable with a range of 7.8 nm.
100(2012); http://dx.doi.org/10.1063/1.3687186View Description Hide Description
We have developed a modified theory of the spin Hall effect of reflected light from a planar interface composed of two dielectric media and obtain the analytical expression valid for any incident angle including the Brewster’s angle. We improved the experimental method and measured the spin-dependent transverse displacement of reflected light from a planar air-glass interface around the Brewster’s angle. The experimental results are in agreement with the theoretical prediction.
Electrically switchable random to photonic band-edge laser emission in chiral nematic liquid crystals100(2012); http://dx.doi.org/10.1063/1.3684804View Description Hide Description
Using a chiral nematic liquid crystal with a negative dielectricanisotropy, it is possible to switch between band-edge laser emission and random laser emission with an electric field. At low frequencies (<1 kHz), random laser emission is observed as a result of scattering due to electro-hydrodynamic instabilities. However, band-edge laser emission is found to occur at higher frequencies (>5 kHz), where the helix is stabilized due to dielectric coupling. These results demonstrate a method by which the linewidth of the laser source can be readily controlled externally (from 4 nm to 0.5 nm) using electric fields.
A statistical exploration of multiple exciton generation in silicon quantum dots and optoelectronic application100(2012); http://dx.doi.org/10.1063/1.3687184View Description Hide Description
We have carried out an investigation of multiple exciton generation(MEG) in Si quantum dots(QDs) and its application in optoelectronic devices. A simple yet effective statistical model has been proposed based on Fermi statistical theory and impact ionization mechanism. It is demonstrated that the MEG efficiency depends on both the radius of Si QDs and the energy of incident photons, with the MEG threshold energy in the range of ∼2.2–3.1 E g depending on the dot radius. While limited improvement has been observed in power conversion efficiency of single stage solar cells,MEG in Si QDs exhibits prospective for application in ultraviolet detectors due to the high internal quantum efficiency under short incident light.
- SURFACES AND INTERFACES
100(2012); http://dx.doi.org/10.1063/1.3681594View Description Hide Description
High heat dissipation rates are enabled by multi-phase cooling schemes owing to latent heat uptake. We demonstrate enhanced flow boiling from a carbon nanotube (CNT)-coated coppersurface exposed to low-intensity ultraviolet (UV)-visible excitation. Compared to non-illuminated results, the average boiling incipience temperature decreased by 4.6 °C and heat transfer coefficients improved by 41.5% with light exposure. These improved results are attributed to augmented hydrophilicity upon exposure to UV light and possible nanoscale opto-thermal effects, and suggest opportunities for active temperature control of temperature-sensitive devices.
100(2012); http://dx.doi.org/10.1063/1.3685508View Description Hide Description
We propose using the Zintl-Klemm (Z-K) bonding to engineer transition layers that provide wetting between ionic oxides and covalent semiconductors to ensure two-dimensional epitaxialgrowth. Using density functional theory to test this concept, we consider the thermodynamics of wetting at the GaAs/SrTiO3interface, and identify Sr aluminide SrAl2 as the Z-K wetting layer. We discuss the atomic structure and bonding at the interface, and estimate the conduction band discontinuity to be 0.6 eV, in good agreement with recent experiment.
100(2012); http://dx.doi.org/10.1063/1.3685469View Description Hide Description
This paper presents a numerical model which describes the underlying physical processes during laser induced forward transfer. The laser induced forward transfer uses a pulsed laser to transfer thin layers from a transparent support to a substrate. The model predicts the threshold energies as well as the blow-off time , thus allowing a profound physical understanding of the transfer process. The good agreement of simulated with measured and of thin nickel layers demonstrates the accuracy of the model. The model shows that gasification of the soda-lime glass support is the main driving force of the transfer process.
Altering the sulfur content in the propanethiol plasma polymers using the capacitive-to-inductive mode transition in inductively coupled plasma discharge100(2012); http://dx.doi.org/10.1063/1.3686902View Description Hide Description
The effect of the transition from capacitive (E) to inductive (H) mode on propanethiol plasmapolymer filmsproperties was investigated by optical emission as well as by x-ray photoelectron spectroscopy. The E mode is characterized by low deposition rate and by high sulfur content in the films (∼40% vs ∼20% in H mode). After aging, a strong decrease of sulfur to carbon content (from ∼0.75 to 0.13), attributed to desorption of unbounded sulfur-based molecules (e.g., H2S), is detected at low power in E mode. The importance of the E-H transition for altering the filmproperties is highlighted.
Vibrational properties of alkyl monolayers on Si(111) surfaces: Predictions from ab-initio calculations100(2012); http://dx.doi.org/10.1063/1.3685489View Description Hide Description
Vibrational properties of Si(111) surfaces terminated by different functional groups have been investigated using density functional theory(DFT). The variations in methyl-related frequencies in different chemical environments, e.g., in methane, methylsilane and ethylsilane, and the methyl- and ethyl-terminated Si(111) surfaces are well predicted by DFT within the local density approximation. In particular, DFT calculations provide useful information on trends and mode assignments in cases where the surface coverage and morphology are not well established experimentally, e.g., in the case of the ethyl-terminated Si(111) surface. Influences of DFTexchange-correlation functionals and anharmonic effects on computed vibrational frequencies are discussed.
High-k GaAs metal insulator semiconductor capacitors passivated by ex-situ plasma-enhanced atomic layer deposited AlN for Fermi-level unpinning100(2012); http://dx.doi.org/10.1063/1.3687199View Description Hide Description
This paper examines the utilization of plasma-enhanced atomic layer deposition grown AlN in the fabrication of a high-kinsulator layer on GaAs. It is shown that high-kGaAsMIS capacitors with an unpinned Fermi level can be fabricated utilizing a thin ex-situ deposited AlNpassivation layer. The illumination and temperature induced changes in the inversion side capacitance, and the maximum band bending of 1.2 eV indicates that the MIS capacitor reaches inversion. Removal of surface oxide is not required in contrast to many common ex-situ approaches.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
100(2012); http://dx.doi.org/10.1063/1.3685463View Description Hide Description
Strain-relaxation in LaAlO3/SrTiO3heterostructures was systematically investigated with LaAlO3film thickness in the range 4.9-84 nm. Heterostructures were characterized using reciprocal lattice mapping (RLM), high resolution rocking curve, and x-rayreflectivity. RLM enables the measurement of lattice constant with accuracy of 10−6 nm. Lattice constant, mismatch, and strain are independently determined in both out-of-plane and in-plane directions. Heterostructures are tetragonally distorted over the entire range of film thickness, even in the film with thickness of 84 nm, in which plastic deformation occurred. This strain-relaxation analysis of LaAlO3/SrTiO3heterostructure contributes, on the experimental basis, to the knowledge of the strained heterostructureinterfaces from thin film growth point of view.
100(2012); http://dx.doi.org/10.1063/1.3684612View Description Hide Description
We show reduction in the emission from nitrogen‐vacancy (NV) centers in single crystaldiamond due to exposure to hydrogen plasmas ranging from 700 °C to 1000 °C. Significant fluorescence reduction was observed beneath the exposed surface to 80 μm depth after ∼10 min and did not recover after post‐annealing in vacuum for 7 h at 1100 °C. We attribute the fluorescence reduction to the formation of nitrogen‐vacancy‐hydrogen centers by the plasma‐induced diffusion of hydrogen. These results have important implications for the formation of NV centers for quantum applications, whilst also providing experimental evidence of long range hydrogen diffusion through intrinsic high‐purity diamond.
100(2012); http://dx.doi.org/10.1063/1.3684616View Description Hide Description
Coalescence of droplets during reactive wetting is investigated for the liquid Ga/GaAs(001) system. In situmirror electron microscopy reveals that coalescence predominantly involves the motion of one reactive droplet relative to the other. This behaviour differs significantly from coalescence in non-reactive systems and is associated with contact line pinning at a ridge/etch pit edge which is identified using atomic force microscopy and selective etching. A simple geometrical model is presented to describe the pinning.