Volume 100, Issue 2, 09 January 2012
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- 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 have studied the effect of double-functionalization on goldelectrodes for improving nanopore-based DNAsequencing. The functionalizing molecular probes are, respectively, capable of temporarily forminghydrogen bonds with both the nucleobase part and the phosphate group of the target DNA, thus potentially minimizing the structural fluctuations of a single-stranded DNA molecule passing between the goldelectrodes. The results of our first-principles study indicate that the proposed setup yields current signals that differ by at least 1 order of magnitude for the four different nucleic acid bases, thus offering the possibility to electrically distinguish them.
- PHOTONICS AND OPTOELECTRONICS
Effect of strain and barrier composition on the polarization of light emission from AlGaN/AlN quantum wells100(2012); http://dx.doi.org/10.1063/1.3675451View Description Hide Description
For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes(LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well.
100(2012); http://dx.doi.org/10.1063/1.3675550View Description Hide Description
We report that a small two-dimensional array of gold nanoblock pairs separated by a nanometric gap significantly improves the performance of optical trapping compared to a single nanoblock pair. The array of 4 × 4 pairs suppresses the Brownian motion of a trapped 1 μm diameter particle by a factor of six compared to the single pair. In addition, the array enables particle trapping for a longer period of time. These results are essential for biological applications where intense optical irradiation is a concern.
100(2012); http://dx.doi.org/10.1063/1.3675618View Description Hide Description
We investigate the incorporation of nitrogen into (Ga,In)Sb grown on GaSb and report room temperature photoluminescence from GaInSb(N) quantum wells. X-ray diffraction and channeling nuclear reaction analysis, together with Rutherford backscattering, were employed to identify the optimal molecular beam epitaxial growth conditions that minimized the incorporation of non-substitutional nitrogen into GaNSb. Consistent with this hypothesis, GaInSb(N) quantum wells grown under the conditions that minimized non-substitutional nitrogen exhibited room temperature photoluminescence, indicative of significantly improved radiative efficiency. Further development of this material system could enable type-I laser diodes emitting throughout the (3-5 μm) wavelength range.
100(2012); http://dx.doi.org/10.1063/1.3675850View Description Hide Description
We demonstrate an electrically injected semipolar laser diode(LD) grown on an intentionally stress relaxed n-In0.09Ga0.91N waveguiding layer. Detrimental effects of misfit dislocations (MDs) in the proximity of the active region were effectively suppressed by utilizing a p/n-Al0.2Ga0.8N electron/hole blocking layer between the dislocated heterointerfaces and the active region. The threshold current density of the LD was ∼20.3 kA/cm2 with a lasing wavelength of 444.9 nm. This LD demonstrates an alternative approach in semipolar AlInGaN LD waveguide design where the thickness and composition of the waveguiding and/or cladding layers are not limited by the critical thickness for MD formation.
100(2012); http://dx.doi.org/10.1063/1.3675905View Description Hide Description
Design and characterization of a quantum dot quantum cascade detector for photovoltaic midwave infrared photodetection (λpeak = 5.5 μm) is demonstrated. The quantum cascade barrier region provides the internal electric field to transfer photoexcited electrons into quantum dots of the next stack, enabling zero bias operation. Increased carrier relaxation time for intersubband transitions in quantum dots provides a distinct advantage for the carrier transport. Responsivity of 10 mA/W and detectivity of 9 × 109 cm Hz1/2 W−1 at 77 K for f/2 optics has been obtained at zero bias. Dark current density is 6.5 × 10−7A cm−2, at 80 K at zero bias.
100(2012); http://dx.doi.org/10.1063/1.3675906View Description Hide Description
We investigate the photo-induced carrier dynamics and spin-lattice interaction in hexagonal YMnO3 film by the temperature-dependent femtosecond pump-probe spectroscopy. The spin-lattice interaction is identified from the slow component of the transient transmittance change with the excitation energies tuned to 1.7 eV and 2.0 eV, which are close to Mn3+ ions and transition, respectively. Temperature dependences of the spin-lattice relaxation parameters demonstrate that the spin-lattice interaction is strongly connected with the d-d transition within Mn3+ ions and enhanced by spin ordering.
100(2012); http://dx.doi.org/10.1063/1.3676085View Description Hide Description
We fabricated and tested a quantum well laser with asymmetric barrier layers. Such a laser has been proposed earlier to suppress bipolar carrier population in the optical confinement layer and thus to improve temperature-stability of the threshold current. As compared to the conventional reference laser structure, our laser with asymmetric barrier layers demonstrates reduced internal optical loss, lower threshold current density at elevated temperatures, and higher characteristic temperature (143 vs. 99 K at 20 °C).
100(2012); http://dx.doi.org/10.1063/1.3675571View Description Hide Description
We experimentally studied the transmission spectrum of a coupled resonatorstructure in which a low-Q microdisk and a high-Q microtoroid indirectly interact with each other mediated by a fiber taper. Asymmetric Fano resonances were observed and could be controlled to change periodically by adjusting the distance between the two microresonators. It is revealed that the Fano resonance originates from the coupling of the two modes belonging to the two microresonators. The observed period of distance change is around 8 μm, which shows good agreement with the theoretical prediction by the beat of multiple propagating modes in the fiber taper.
Nonlinear optical properties of low temperature annealed silicon-rich oxide and silicon-rich nitride materials for silicon photonics100(2012); http://dx.doi.org/10.1063/1.3675882View Description Hide Description
We investigate the nonlinear optical properties of Si-rich silicon oxide (SRO) and Si-rich silicon nitride (SRN) samples as a function of silicon content, annealing temperature, and excitation wavelength. Using the Z-scan technique, we measure the non-linear refractive index n2 and the nonlinear absorption coefficient β for a large number of samples fabricated by reactive co-sputtering. Moreover, we characterize the nonlinear optical parameters of SRN in the broad spectral region 1100-1500 nm and show the strongest nonlinearity at 1500 nm. These results demonstrate the potential of the SRN matrix for the engineering of compact devices with enhanced Kerr nonlinearities for siliconphotonics applications.
High-reflectivity, high-Q micromechanical membranes via guided resonances for enhanced optomechanical coupling100(2012); http://dx.doi.org/10.1063/1.3658731View Description Hide Description
Using Fano-type guided resonances (GRs) in photonic crystal(PhC) slab structures, we numerically and experimentally demonstrate optical reflectivity enhancement of high-Q SiNx membrane-type resonators used in membrane-in-the-middle optomechanical (OM) systems. Normal-incidence transmission and mechanical ringdown measurements of 50-nm-thick PhC membranes demonstrate GRs near 1064 nm, leading to a ∼4 × increase in reflectivity while preserving high mechanical Q factors of up to ∼5 × 106. The results would allow improvement of membrane-in-the-middle OM systems by virtue of increased OM coupling, presenting a path towards ground state cooling of such a membrane and observations of related quantum effects.
100(2012); http://dx.doi.org/10.1063/1.3673856View Description Hide Description
A wavelength selective uncooled infrared (IR) sensor using two-dimensional plasmonic crystals (2D PLCs) has been developed. The numerical investigation of 2D PLCs demonstrates that the wavelength of absorption can be mainly controlled by the period of the surface structure. A microelectromechanical systems-based uncooled IR sensor with 2D PLCs as the IR absorber was fabricated through a complementary metal oxide semiconductor and a micromachining technique. The selective enhancement of responsivity was observed at the wavelength that coincided with the period of the 2D-PLC absorber.
- SURFACES AND INTERFACES
100(2012); http://dx.doi.org/10.1063/1.3675481View Description Hide Description
In contrast to the commonly employed high temperature chemical vapor depositiongrowth that leads to multilayer graphene formation by carbon segregation from the bulk, we demonstrate that below 600 °C graphene can be grown in a self-limiting monolayergrowth process. Optimum growth is achieved at ∼550 °C. Above this temperature, carbondiffusion into the bulk is limiting the surface growth rate, while at temperatures below ∼500 °C a competing surface carbide phase impedes graphene formation.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
100(2012); http://dx.doi.org/10.1063/1.3675572View Description Hide Description
The optical linear polarizationproperties of exciton complexes in asymmetric Stranski-Krastanov grown GaNquantum dots have been investigated experimentally and theoretically. It is demonstrated that the polarization angle and the polarization degree can be conveniently employed to associate emission lines in the recorded photoluminescencespectra to a specific dot. The experimental results are in agreement with configuration interaction computations, which predict similar polarization degrees for the exciton and the biexciton (within 10%) in typical GaNquantum dots. The theory further predicts that the polarization degree can provide information about the charge state of the dot.
Optimizing non-radiative energy transfer in hybrid colloidal-nanocrystal/silicon structures by controlled nanopillar architectures for future photovoltaic cells100(2012); http://dx.doi.org/10.1063/1.3675634View Description Hide Description
To optimize colloidal nanocrystals/Si hybrid structures,nanopillars are prepared and organized via microparticle patterning and Si etching. A monolayer of CdSe nanocrystals is then grafted on the passivated oxide-free nanopillarsurfaces, functionalized with carboxy-alkyl chain linkers. This process results to a negligible number of non-radiative surface state defects with a tightly controlled separation between the nanocrystals and Si. Steady-state and time-resolved photoluminescence measurements confirm the close-packing nanocrystal arrangement and the dominance of non-radiative energy transfer from nanocrystals to Si. We suggest that radially doped p-n junction devices based on energy transfer offer a viable approach for thin film photovoltaic devices.
Formation mechanism of cellular structures during unidirectional growth of binary semiconductor Si-rich SiGe materials100(2012); http://dx.doi.org/10.1063/1.3675860View Description Hide Description
The formation mechanism of a cellular structure during the growth of Si-rich SiGe crystals was studied by in situ observation. We directly observed the morphological transformation of the crystal-melt interface during the unidirectional growth of Si-rich SiGe. It was found that the morphology of the interface transformed from a planar to a zigzag facets to a faceted cellular interface with increasing growth rate. It is clarified that Ge segregation at valleys of zigzag facets leads to the formation of a cellular structure in Si-rich SiGe crystals.
100(2012); http://dx.doi.org/10.1063/1.3675881View Description Hide Description
Transparent Lu1−xScxBO3:Ce (x = 0.3, 0.5, 0.7, 0.8) scintillation crystals were grown by Czochralski method. Vacuum ultraviolet excitation spectra and thermoluminescence glow curves revealed the narrowing of bandgap and the redshift of the first electric dipole-allowed fd transition of Ce3+ with the increase of Sc content. The ionizationenergy evaluated from the photoluminescence decays as a function of temperature indicated that, with increasing Sc content, the thermal ionization probability occurring from 5d 1 relaxed state of Ce3+ was enhanced, which degraded the scintillation efficiency of Lu1−xScxBO3:Ce crystals.
Theoretical investigation of high velocity, temperature compensated Rayleigh waves along AlN/SiC substrates for high sensitivity mass sensors100(2012); http://dx.doi.org/10.1063/1.3675619View Description Hide Description
The operation of electroacoustic devices based on surface acoustic waves(SAW) propagation along β-SiC/AlN and amorphous-SiC/AlN substrates is theoretically studied with respect to the AlN film thickness, the SAW propagation direction, temperature and electric boundary conditions. GHz-range, enhanced electroacoustic coupling coefficient, temperature compensated around 20 °C electroacoustic devices are the advantages of SiC/AlN composite structures. These structures are also suitable for the implementation of sensors with improved performances with respect to SAW devices based on bulk single crystal piezoelectric substrates. The structures feasibility was confirmed by structural investigation and quantitative analysis of sputtered amorphous-SiC and AlN films on Si substrates.
Correlating the supercooled liquid region width with the fragility parameter in bulk metallic glasses100(2012); http://dx.doi.org/10.1063/1.3675910View Description Hide Description
A linear correlation of fragility parameter D * with supercooled liquid region width ΔT x for Ca-based bulk metallic glasses(BMGs) was revealed. This relationship is found in La- and Zr-based BMGs as well and extended to several glass-forming systems. The origin of this phenomenon lies in the close relation between crystallization and temperature dependence of viscosity. This relationship can be formulated by , indicating that the unique variation of the viscosity with the temperature correlates with the location and width of the supercooled liquid region. Moreover, an approximation of fragility parameter D * for BMGs can be evaluated by the formula.
100(2012); http://dx.doi.org/10.1063/1.3675912View Description Hide Description
In this letter, both entanglement length and pore size of carbon nanotube(CNT) buckypaper are studied numerically and found to scale with a characteristic length , where and denote the bending stiffness and binding energy of a CNT, respectively. For nm, the CNTs in buckypaper are “interwound” with a short entanglement length and a small pore size. However, when nm, CNT “ring”/“racket” structures dominate the buckypaper, exhibiting longer entanglement length and larger pore size. The acquired understanding of microscopic structures allows us to propose that CNT buckypaper with different mechanical properties and pore size can be designed through the choice of values.
100(2012); http://dx.doi.org/10.1063/1.3675913View Description Hide Description
Complex hydride YMn2H6 with a space group has two sorts of Mn, in which one presents as a divalent cation Mn2+ on the 8c site and the other forms a complex anion [MnH6]5− on the 4a site. We investigate the stabilities of Fe substitutions for a part of Mn atoms on either the site using first-principles calculations. Our results reveal that an Fe atom prefers the 4a occupation, and the origin of this site preference can be understood by the balance of two competing energy gains brought by the 3d bands for either the 4a or 8c site substituted Fe.