Volume 104, Issue 15, 14 April 2014
- 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:
- PHOTONICS AND OPTOELECTRONICS
Selective generation of an intense single harmonic from a long gas cell with loosely focusing optics based on a three-color laser field104(2014); http://dx.doi.org/10.1063/1.4871513View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871090View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871384View Description Hide Description
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 (corresponding to 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.
104(2014); http://dx.doi.org/10.1063/1.4871517View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871520View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871584View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871700View Description Hide Description
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.
Nonradiative recombination mechanisms in InGaN/GaN-based light-emitting diodes investigated by temperature-dependent measurements104(2014); http://dx.doi.org/10.1063/1.4871870View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4872003View Description Hide Description
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  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.
104(2014); http://dx.doi.org/10.1063/1.4872180View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4872316View Description Hide Description
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.
- SURFACES AND INTERFACES
104(2014); http://dx.doi.org/10.1063/1.4870809View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871505View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871692View Description Hide Description
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.
Quantitative analysis of interfacial reactions at a graphene/SiO2 interface using the discharge current analysis method104(2014); http://dx.doi.org/10.1063/1.4871866View Description Hide Description
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.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
Stacking faults and interface roughening in semipolar single InGaN quantum wells for long wavelength emission104(2014); http://dx.doi.org/10.1063/1.4871512View Description Hide Description
The microstructure of InGaN single quantum wells (QWs) grown in semipolar 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.
Temperature dependent photoexcited carrier dynamics in multiferroic BiFeO3 film: A hidden phase transition104(2014); http://dx.doi.org/10.1063/1.4871689View Description Hide Description
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.
High quality factor nanocrystalline diamond micromechanical resonators limited by thermoelastic damping104(2014); http://dx.doi.org/10.1063/1.4871803View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871804View Description Hide Description
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.
104(2014); http://dx.doi.org/10.1063/1.4871868View Description Hide Description
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.