Volume 106, Issue 4, 26 January 2015
- 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
106(2015); http://dx.doi.org/10.1063/1.4906802View Description Hide Description
A method to improve the sensitivity and selectivity of a monolithically integrated mid-infrared sensor using a distributed feedback laser (DFB) is presented in this paper. The sensor is based on a quantum cascade laser/detector system built from the same epitaxial structure and with the same fabrication approach. The devices are connected via a dielectric-loaded surface plasmon polariton waveguide with a twofold function: it provides high light coupling efficiency and a strong interaction of the light with the environment (e.g., a surrounding fluid). The weakly coupled DFB quantum cascade laser emits narrow mode light with a FWHM of 2 cm−1 at 1586 cm−1. The room temperature laser threshold current density is 3 kA∕cm2 and a pulsed output power of around 200 mW was measured. With the superior laser noise performance, due to narrow mode emission and the compensation of thermal fluctuations, the lower limit of detection was expanded by one order of magnitude to the 10 ppm range.
Mobility enhancement in electric double layer gated n-ZnO ultraviolet photodetector by synergy of gate and illumination: A photo Hall study106(2015); http://dx.doi.org/10.1063/1.4906598View Description Hide Description
We report a large enhancement of the Hall mobility of a ZnO film (channel) by simultaneously application of an ultraviolet illumination along with a gate bias in an electric double layer field effect transistor configuration. The effect arises from a synergy between the illumination and the field effect (FE), leading to large enhancement of the channel conductivity and the photo response. We propose that large carrier density created by the simultaneous presence of the illumination and the FE leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to the enhanced mobility.
106(2015); http://dx.doi.org/10.1063/1.4906611View Description Hide Description
We use a picosecond acoustics technique to modulate the laser output of electrically pumped GaAs/AlAs micropillar lasers with InGaAs quantum dots. The modulation of the emission wavelength takes place on the frequencies of the nanomechanical extensional and breathing (radial) modes of the micropillars. The amplitude of the modulation for various nanomechanical modes is different for every micropillar which is explained by a various elastic contact between the micropillar walls and polymer environment.
106(2015); http://dx.doi.org/10.1063/1.4906596View Description Hide Description
Here, we investigate both numerically and experimentally, the polarization conversion capabilities of a rectangular array of holes with two unequal orthogonal periodicities. We show that it is possible to tune the periodicities in such a way that the transmitted light is circularly polarized for a nominated wavelength, λ CPL, when the structure is illuminated with appropriately oriented linearly polarized light at normal incidence. A device was fabricated and experiments confirmed that a degree of circular polarization of 0.89 could be achieved at the resonant wavelength.
106(2015); http://dx.doi.org/10.1063/1.4906860View Description Hide Description
There has been a considerable amount of interest in the development of various types of electromagnetic wave absorbers for use in different wavelength ranges. In particular, infrared (IR) absorbers with wavelength selectivity can be applied to advanced uncooled IR sensors, which would be capable of identifying objects through their radiation spectrum. In the present study, mushroom plasmonic metamaterial absorbers (MPMAs) for the IR wavelength region were designed and fabricated. The MPMAs consist of a periodic array of thin metal micropatches connected to a thin metal plate with narrow silicon (Si) posts. A Si post height of 200 nm was achieved by isotropic XeF2 etching of a thin Si layer sandwiched between metal plates. This fabrication procedure is relatively simple and is consistent with complementary metal oxide semiconductor technology. The absorption spectra of the fabricated MPMAs were experimentally measured. In addition, theoretical calculations of their absorption properties were conducted using rigorous coupled wave analysis. Both the calculated and measured absorbance results demonstrated that these MPMAs can realize strong selective absorption at wavelengths beyond the period of the array by varying the micropatch width. Absorbance values greater than 90% were achieved. Dual- or single-mode absorption can also be selected by varying the width of the Si posts. Pixel structures using such MPMAs could be used as high responsivity, high resolution and fast uncooled IR sensors.
106(2015); http://dx.doi.org/10.1063/1.4906923View Description Hide Description
We demonstrate one-dimensional photonic-crystal nanobeam cavities in amorphous silicon carbide. The fundamental mode exhibits intrinsic optical quality factor as high as 7.69 × 104 with mode volume at wavelength 1.5 μm. A corresponding Purcell factor value of ∼104 is the highest reported to date in silicon carbide optical cavities. The device exhibits great potential for integrated nonlinear photonics and cavity nano-optomechanics.
106(2015); http://dx.doi.org/10.1063/1.4906924View Description Hide Description
An approach to the optimization of the spectral power distribution of solid-state light sources with the tunable non-image forming photobiological effect on the human circadian rhythm is proposed. For tetrachromatic clusters of model narrow-band (direct-emission) light-emitting diodes (LEDs), the limiting tunability of the circadian action factor (CAF), which is the ratio of the circadian efficacy to luminous efficacy of radiation, was established as a function of constraining color fidelity and luminous efficacy of radiation. For constant correlated color temperatures (CCTs), the CAF of the LED clusters can be tuned above and below that of the corresponding blackbody radiators, whereas for variable CCT, the clusters can have circadian tunability covering that of a temperature-tunable blackbody radiator.
InAs-based interband-cascade-lasers emitting around 7 μm with threshold current densities below 1 kA/cm2 at room temperature106(2015); http://dx.doi.org/10.1063/1.4907002View Description Hide Description
Interband cascade lasers (ICLs) grown on InAs substrates with threshold current densities below 1 kA/cm2 are presented. Two cascade designs with different lengths of the electron injector were investigated. Using a cascade design with 3 InAs quantum wells (QWs) in the electron injector, a device incorporating 22 stages in the active region exhibited a threshold current density of 940 A/cm2 at a record wavelength of 7 μm for ICLs operating in pulsed mode at room temperature. By investigating the influence of the number of stages on the device performance for a cascade design with 2 QWs in the electron injector, a further reduction of the threshold current density to 800 A/cm2 was achieved for a 30 stage device.
106(2015); http://dx.doi.org/10.1063/1.4907004View Description Hide Description
Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses. These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current.
106(2015); http://dx.doi.org/10.1063/1.4906460View Description Hide Description
Optical characterisation methodologies are employed to validate a nanorod self-alignment technique for use in luminescent solar concentrators (LSCs). The nanorods utilised in this work were CdSe/CdS core/shell nanorods, and the self-alignment technique relied on the evaporation of a highly concentrated nanorod/xylene solution onto a glass substrate. Position and angular dependent light absorptivity measurements revealed evidence of vertical nanorod alignment over a limited region at the centre of the LSC sample. Vertical nanorod alignment is beneficial for absorbing diffuse/scattered sunlight and provides for a high light trapping efficiency in the LSC.
Texturing the cathode of white organic light-emitting diodes with a lattice of nanoscale scatterers for enhanced light out-coupling106(2015); http://dx.doi.org/10.1063/1.4907253View Description Hide Description
The external quantum efficiency of white organic light-emitting diodes is often limited by light out-coupling losses due to surface plasmons. We demonstrate how texturing of the metal-cathode surface using a two-dimensionally periodic lattice of nanoscale scatterers with limited disorder can be used to reduce plasmonic losses while simultaneously enhancing both the light out-coupling and the spontaneous-emission rate of the excitons. We use electrodynamic simulations and statistical modeling to explore the relationship between the topology of the surface texture and its corresponding scattering efficiency. From this, we outline attributes of textures that can most enhance device performance.
- SURFACES AND INTERFACES
106(2015); http://dx.doi.org/10.1063/1.4906417View Description Hide Description
This letter reports observations of the breakdown characteristics of helium/nitrogen parallel jets when AC voltage was applied across them. Below a certain voltage level, the plasma emission was fully confined to the helium stream as in a traditional dielectric barrier discharge, where the role of the dielectric was played by an insulating gas barrier (nitrogen as compared to helium). The discharge had characteristics of self-organized patterns. Some differences to traditional solid dielectrics are discussed.
106(2015); http://dx.doi.org/10.1063/1.4906867View Description Hide Description
Electronic properties of fullerene derivatives containing oligothiophene pendant chain (1–3 thiophene moieties) were investigated using the Kelvin probe technique and quantum chemistry methods. For electrochemical examination of these systems, Langmuir–Blodgett (LB) layers were prepared by the deposition on a gold substrate. The analysis of the experimental data shows that the value of the work function depends strongly on the length of oligothiophene chain. Similar dependence was also found for the surface photovoltage measurements conducted for the layers consisting of multiple LB films of the examined compounds deposited on gold surfaces. The assumption has been made that these changes are associated with the influence of oligothiophene chain on the electrostatic potential distribution near the surface of the sample. The hypothesis was confirmed by the results of DFT calculations, which revealed that the value of Fermi level energy shifts in the opposite direction to the determined work function. The key highlights of this study are as follows: electronic structure tuning by oligothiophene side chain; DFT calculation on fullerene-thiophene system; work function measurements of thin molecular layers.
Direct observation of electron emission and recombination processes by time domain measurements of charge pumping current106(2015); http://dx.doi.org/10.1063/1.4906997View Description Hide Description
To analyze the charge pumping (CP) sequence in detail, the source/drain electron current and the substrate hole current under the CP mode of transistors are simultaneously monitored in the time domain. Peaks are observed in both the electron and hole currents, which are, respectively, attributed to the electron emission from the interface defects and to the recombination with holes. The peak caused by the electron emission is found to consist of two components, strongly suggesting that the present time-domain measurement can enable us to resolve different kinds of interface defects. Investigating the correlation between the number of emitted and recombined electrons reveals that only one of the two components contributes to the CP current for the gate-pulse fall time from 6.25 × 10−4 to 1.25 × 10−2 s.
106(2015); http://dx.doi.org/10.1063/1.4906920View Description Hide Description
An oxygen vacancy driven structural response at the epitaxial interface between La0.7Sr0.3MnO3 films and SrTiO3 substrates is reported. A combined scanning transmission electron microscopy and electron energy loss spectroscopy study reveal the presence of an elongated out-of-plane lattice parameter, coupled to oxygen vacancies and reduced manganese oxidation state at the La0.7Sr0.3MnO3 side of the interface. Density functional theory calculations support that the measured interface structure is a disordered oxygen deficient brownmillerite structure. The effect of oxygen vacancy mobility is assessed, revealing an ordering of the vacancies with time.
Patterning of self-assembled monolayers by phase-shifting mask and its applications in large-scale assembly of nanowires106(2015); http://dx.doi.org/10.1063/1.4907042View Description Hide Description
A nonselective micropatterning method of self-assembled monolayers (SAMs) based on laser and phase-shifting mask (PSM) is demonstrated. Laser beam is spatially modulated by a PSM, and periodic SAM patterns are generated sequentially through thermal desorption. Patterned wettability is achieved with alternating hydrophilic/hydrophobic stripes on octadecyltrichlorosilane monolayers. The substrate is then used to assemble CdS semiconductor nanowires (NWs) from a solution, obtaining well-aligned NWs in one step. Our results show valuably the application potential of this technique in engineering SAMs for integration of functional devices.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
106(2015); http://dx.doi.org/10.1063/1.4906593View Description Hide Description
The overprints produced in inkjet technology with graphene oxide dispersion are presented. The graphene oxide ink is developed to be fully compatible with standard industrial printers and polyester substrates. Post-printing chemical reduction procedure is proposed, which leads to the restoration of electrical conductivity without destroying the substrate. The presented results show the outstanding potential of graphene oxide for rapid and cost efficient commercial implementation to production of flexible electronics. Properties of graphene-based electrodes are characterized on the macro- and nano-scale. The observed nano-scale inhomogeneity of overprints' conductivity is found to be essential in the field of future industrial applications.
106(2015); http://dx.doi.org/10.1063/1.4906612View Description Hide Description
We demonstrate large-area (∼600 μm), (111)-oriented, and high-crystallinity, i.e., pseudo-single-crystalline, germanium (Ge) films at 275 °C, where the temperature is lower than the softening temperature of a flexible substrate. A modulated gold-induced layer exchange crystallization method with an atomic-layer deposited Al2O3 barrier and amorphous-Ge/Au multilayers is established. From the Raman measurements, we can judge that the crystallinity of the obtained Ge films is higher than those grown by aluminum-induced-crystallization methods. Even on a flexible substrate, the pseudo-single-crystalline Ge films for the circuit with thin-film transistor arrays can be achieved, leading to high-performance flexible electronics based on an inorganic-semiconductor channel.
Raman spectroscopic characterization of the core-rim structure in reaction bonded boron carbide ceramics106(2015); http://dx.doi.org/10.1063/1.4906969View Description Hide Description
Raman spectroscopy was used to characterize the microstructure of reaction bonded boron carbide ceramics. Compositional and structural gradation in the silicon-doped boron carbide phase (rim), which develops around the parent boron carbide region (core) due to the reaction between silicon and boron carbide, was evaluated using changes in Raman peak position and intensity. Peak shifting and intensity variation from the core to the rim region was attributed to changes in the boron carbide crystal structure based on experimental Raman observations and ab initio calculations reported in literature. The results were consistent with compositional analysis determined by energy dispersive spectroscopy. The Raman analysis revealed the substitution of silicon atoms first into the linear 3-atom chain, and then into icosahedral units of the boron carbide structure. Thus, micro-Raman spectroscopy provided a non-destructive means of identifying the preferential positions of Si atoms in the boron carbide lattice.
106(2015); http://dx.doi.org/10.1063/1.4906579View Description Hide Description
We observe distinct interlayer shear mode Raman spectra for trilayer graphene with ABA and ABC stacking order. There are two rigid-plane shear-mode phonon branches in trilayer graphene. We find that ABA trilayers exhibit pronounced Raman response from the high-frequency shear branch, without any noticeable response from the low-frequency branch. In contrast, ABC trilayers exhibit no response from the high-frequency shear branch, but significant Raman response from the low-frequency branch. Such complementary behaviors of Raman shear modes can be explained by the distinct symmetry of the two trilayer allotropes. The strong stacking-order dependence is not found in the layer-breathing modes, and thus represents a unique characteristic of the shear modes.