Volume 104, Issue 22, 02 June 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:
In this work, a propulsion system similar in motion to a sperm-cell is investigated. This system consists of a structure resembling a sperm-cell with a magnetic head and a flexible tail of 42 μm and 280 μm in length, respectively. The thickness, length, and width of this structure are 5.2 μm, 322 μm, and 42 μm, respectively. The magnetic head includes a 200 nm-thick cobalt-nickel layer. The cobalt-nickel layer provides a dipole moment and allows the flexible structure to align along oscillating weak (less than 5 mT) magnetic field lines, and hence generates a propulsion thrust force that overcomes the drag force. The frequency response of this system shows that the propulsion mechanism allows for swimming at an average speed of 158 ± 32 μm/s at alternating weak magnetic field of 45 Hz. In addition, we experimentally demonstrate controlled steering of the flexible structure towards reference positions.
- PHOTONICS AND OPTOELECTRONICS
Electroluminescence from the sidewall quantum wells in the V-shaped pits of InGaN light emitting diodes104(2014); http://dx.doi.org/10.1063/1.4880731View Description Hide Description
InGaN/GaN multi-quantum well (MQW) light emitting diodes with heavily Mg doped and unintentionally doped (UID) low-temperature Al0.2Ga0.8N electron blocking layer (EBL) were investigated. Broad short-wavelength electroluminescence peak, which has strong relative intensity to the main emission, was found in the UID-EBL sample at cryogenic temperatures. Study suggests that the broad peak is emitted by the sidewall MQWs. This result indicates that the electroluminescence of sidewall MQWs, in which the carrier density is high enough, can be detected at cryogenic temperatures. The lineshape variation with current density reveals detailed information on the process of carrier injection into the sidewall MQWs.
104(2014); http://dx.doi.org/10.1063/1.4880798View Description Hide Description
We report extremely strong optical activity and circular dichroism exhibited by subwavelength arrays of four-start-screw holes fabricated with single-pass focused ion beam milling of freely suspended silver films. Having the fourth order rotational symmetry, the structures exhibit the polarization rotation up to 90° and peaks of full circular dichroism and operate as circular polarizers within certain ranges of wavelengths in the visible. We discuss the observations on the basis of general principles (symmetry, reciprocity, and reversibility) and conclude that the extreme optical chirality is determined by the chiral localized plasmonic resonances.
Photo-guided sampling for rapid detection and imaging of traces of explosives by a compact Raman spectrometer104(2014); http://dx.doi.org/10.1063/1.4880940View Description Hide Description
Rapid acquisition of Raman spectra and Raman spectral maps of traces of compounds at proximal distances by a compact Raman system is demonstrated. The performance of the system was further improved by photographing the target, prior to data acquisition, allowing selective monitoring of spatially resolved particles and thus mapping at reduced sampling times, compared to raster-scanning. This spectrometer and its photo-guided sampling make it a potential candidate for detection of other partially covered substrates.
Carrier behavior in special multilayer device composed of different transition metal oxide-based intermediate connectors104(2014); http://dx.doi.org/10.1063/1.4881156View Description Hide Description
The impact of illumination on the connection part of the tandem organic light-emitting diodes was studied by using a special organic multilayer sample consisted of two organic active layers coupled with different transition metal oxide (TMO)-based intermediate connectors (ICs). Through measuring the current density-voltage characteristic, interfacial electronic structures, and capacitance-voltage characteristic, we observe an unsymmetrical phenomenon in current density-voltage and capacitance-voltage curves of Mg:Alq3/MoO3 and MoO3 composed devices, which was induced by the charge spouting zone near the ICs region and the recombination state in the MoO3 layer. Moreover, Mg:Alq3/MoO3 composed device displays a photovoltaic effect and the V oc shifts to forward bias under illumination. Our results demonstrate that the TMO-based IC structure coupled with photovoltaic effect can be a good approach for the study of photodetector, light sensor, and so on.
104(2014); http://dx.doi.org/10.1063/1.4881182View Description Hide Description
We demonstrate room temperature continuous wave THz sources based on intracavity difference-frequency generation from mid-infrared quantum cascade lasers. Buried ridge, buried composite distributed-feedback waveguide with Čerenkov phase-matching scheme is used to reduce the waveguide loss and enhance the heat dissipation for continuous wave operation. Continuous emission at 3.6 THz with a side-mode suppression ratio of 20 dB and output power up to 3 μW are achieved, respectively. THz peak power is further scaled up to 1.4 mW in pulsed mode by increasing the mid-infrared power through increasing the active region doping and device area.
104(2014); http://dx.doi.org/10.1063/1.4881183View Description Hide Description
2-μm micro-disks containing InGaN/GaN quantum wells supported on a tiny Si nanotip are fabricated via microsphere lithography followed by dry and wet etch processes. The micro-disks are studied by photoluminescence at both room-temperature and 10 K. Optically pumped blue lasing at room-temperature is observed via whispering-gallery modes (WGMs) with a lasing threshold as low as 8.43 mJ/cm2. Optical resonances in the micro-disks are studied through numerical computations and finite-difference time-domain simulations. The WGMs are further confirmed through the measured broadband transmission spectrum, whose transmission minima coincide well with predicted WGM frequencies.
104(2014); http://dx.doi.org/10.1063/1.4881267View Description Hide Description
An ultra-narrow band absorber consisting of continuous silver and alumina films is investigated. Owing to Fabry–Pérot resonance and silver's inherent loss, an ultra-narrow spectral range of light can be entirely trapped in the structure. By varying thicknesses of metallic and dielectric films, absorption peak shifts in visible and near-infrared regions. When two such metal-insulator-metal stacks are cascaded, experimental results show that an ultra-narrow absorption bandwidth of 7 nm is achieved, though theoretical results give that of 2 nm. Features of high-efficiency and ultra-narrow band absorption have huge potential in optical filtering, thermal emitter design, etc.
104(2014); http://dx.doi.org/10.1063/1.4881839View Description Hide Description
We propose and experimentally show the mechanism of beam super-collimation by axisymmetric photonic crystals, specifically by periodic (in propagation direction) structure of layers of concentric rings. The physical mechanism behind the effect is an inverse scattering cascade of diffracted wave components back into on- and near-axis angular field components, resulting in substantial enhancement of intensity of these components. We explore the super-collimation by numerical calculations and prove it experimentally. We demonstrate experimentally the axial field enhancement up to 7 times in terms of field intensity.
104(2014); http://dx.doi.org/10.1063/1.4881887View Description Hide Description
We demonstrate the emission of indistinguishable photons along a semiconductor chip originating from carrier recombination in an InAs quantum dot. The emitter is integrated in the waveguiding region of a photonic crystal structure, allowing for on-chip light propagation. We perform a Hong-Ou-Mandel-type of experiment with photons collected from the exit of the waveguide, and we observe two-photon interference under continuous wave excitation. Our results pave the way for the integration of quantum emitters in advanced photonic quantum circuits.
104(2014); http://dx.doi.org/10.1063/1.4881935View Description Hide Description
Phase gradient metasurface (PGMs) are artificial surfaces that can provide pre-defined in-plane wave-vectors to manipulate the directions of refracted/reflected waves. In this Letter, we propose to achieve wideband radar cross section (RCS) reduction using two-dimensional (2D) PGMs. A 2D PGM was designed using a square combination of 49 split-ring sub-unit cells. The PGM can provide additional wave-vectors along the two in-plane directions simultaneously, leading to either surface wave conversion, deflected reflection, or diffuse reflection. Both the simulation and experiment results verified the wide-band, polarization-independent, high-efficiency RCS reduction induced by the 2D PGM.
104(2014); http://dx.doi.org/10.1063/1.4881936View Description Hide Description
We present an optically detected mechanical accelerometer that achieves a sensitivity of over a bandwidth of 10 kHz and is traceable. We have incorporated a Fabry-Pérot fiber-optic micro-cavity that is currently capable of measuring the test-mass displacement with sensitivities of , and whose length determination enables traceability to the International System of Units. The compact size and high mQ-product achieved combined with the high sensitivity and simplicity of the implemented optical detection scheme highlight our device and this category of accelerometers, outlining a path for high sensitivity reference acceleration measurements and observations in seismology and gravimetry.
104(2014); http://dx.doi.org/10.1063/1.4881976View Description Hide Description
We report on the fabrication and characterization of curved periodic microstructures formed through the controlled phase separation of a liquid crystal and a polymerizing matrix comprising self-aligned liquid crystal. Imaging through a “Fresnel like” structure imparts an intensity profile onto a photosensitive mixture which subsequently forms periodic alternating curved polymeric and liquid crystal slices. The phase separated concentric rings of nematic liquid crystal self-align in a radial alignment in between the polymer walls as indicated by polarizing optical microscopy analysis (Maltese cross). Electro-optical experiments confirm the possibility to control this alignment and the optical properties of the macroscopic structure by means of a quite low external voltage. The system exhibits high-quality and self-alignment of an ordered (liquid crystal) fluid without the need of surface chemistry or functionalization.
Frustrated total internal reflection and critical coupling in a thick plasmonic grating with narrow slits104(2014); http://dx.doi.org/10.1063/1.4882915View Description Hide Description
We demonstrate the possibility of critical coupling through frustrated total internal reflection in a thick plasmonic grating below the first diffraction order. Differently from conventional approaches relying on the excitation of surface plasmon-polaritons, here we exploit the light coupling with the leaky modes supported by the grating. This mechanism entails a wide-angle coupling and effectively access spectral bands that would otherwise be difficult to probe using conventional plasmonic critical coupling techniques, such as the Otto configuration. Our finding may pave the way to efficient plasmonic bio-sensor devices.
104(2014); http://dx.doi.org/10.1063/1.4882916View Description Hide Description
Nanoscale multichannel filter is realized in plasmonic circuits directly, which consists of four plasmonic nanocavities coupled via a plasmonic waveguide etched in a gold film. The feature device size is only 1.35 μm, which is reduced by five orders of magnitude compared with previous reports. The optical channels are formed by transparency windows of plasmon-induced transparencies. A shift of 45 nm in the central wavelengths of optical channels is obtained when the plasmonic coupled-nanocavities are covered with a 100-nm-thick poly(methyl methacrylate) layer. This work opens up the possibility for the realization of solid quantum chips based on plasmonic circuits.
Multi-phonon-assisted absorption and emission in semiconductors and its potential for laser refrigeration104(2014); http://dx.doi.org/10.1063/1.4880799View Description Hide Description
Laser cooling of semiconductors has been an elusive goal for many years, and while attempts to cool the narrow gap semiconductors such as GaAs are yet to succeed, recently, net cooling has been attained in a wider gap CdS. This raises the question of whether wider gap semiconductors with higher phonon energies and stronger electron-phonon coupling are better suitable for laser cooling. In this work, we develop a straightforward theory of phonon-assisted absorption and photoluminescence of semiconductors that involves more than one phonon and use to examine wide gap materials, such as GaN and CdS and compare them with GaAs. The results indicate that while strong electron-phonon coupling in both GaN and CdS definitely improves the prospects of laser cooling, large phonon energy in GaN may be a limitation, which makes CdS a better prospect for laser cooling.
- SURFACES AND INTERFACES
104(2014); http://dx.doi.org/10.1063/1.4881334View Description Hide Description
The structural phase transition from the 1 × 1 to the √31 × √31 ± R9° structure of two types of α-Al2O3(0001) surfaces has been investigated using reflection high-energy electron diffraction. One of the α-Al2O3(0001) surfaces was composed of regularly arranged monolayer steps (M-surface), and the other surface was composed of multiples of bilayer high steps (B-surface). The surface transition from the 1 × 1 to the √31 × √31 ± R9° structure starts at 1200 °C on the M-surface, but the transition occurs at 1600 °C on the B-surface. We discuss the difference in the stability of these surfaces at high temperature from the viewpoint of the stoichiometry near the step edge and conclude that the B-surface is more stable than the M-surface.
104(2014); http://dx.doi.org/10.1063/1.4881612View Description Hide Description
Combined x-ray diffraction and first-principles studies of various epitaxial rutile-type metal dioxide films on Al2O3(0001) substrates reveal an unexpected rectangle-on-parallelogram heteroepitaxy. Unique matching of particular lattice spacings and crystal angles between the oxygen sublattices of Al2O3(0001) and the film(100) result in coexisted crystal rotation and lattice twinning inside the film. We demonstrate that, besides symmetry and lattice mismatch, angular mismatch along a specific crystal direction is also an important factor determining epitaxy. A generalized theorem has been proposed to explain epitaxial behaviors for tetragonal metal dioxides on Al2O3(0001).
104(2014); http://dx.doi.org/10.1063/1.4881718View Description Hide Description
An attempt has been made to utilize uniquely high electron mobility of graphene on hexagonal boron nitride (h-BN) to electron emitter. The field emission property of graphene/h-BN/Si structure has shown enhanced threshold voltage and emission current, both of which are key to develop novel vacuum nanoelectronics devices. The field emission property was discussed along with the electronic structure of graphene investigated by Fowler-Nordheim plot and ultraviolet photoelectron spectroscopy. The result suggested that transferring graphene on h-BN modified its work function, which changed field emission mechanism. Our report opens up a possibility of graphene-based vacuum nanoelectronics devices with tuned work function.
Enhancing controllability and stability of bottom-gated graphene thin-film transistors by passivation with methylamine104(2014); http://dx.doi.org/10.1063/1.4881841View Description Hide Description
This paper is intended to aid to bridge the gap between chemistry and electronic engineering. In this work, the fabrication of chemical vapour deposited graphene field-effect transistors employing silicon-nitride (Si3N4) gate dielectric is presented, showing originally p-type channel conduction due to ambient impurities yielding uncontrollable behaviour. Vacuum annealing has been performed to balance off hole and electron conduction in the channel, leading to the observation of the Dirac point and therefore improving controllability. Non-covalent functionalisation by methylamine has been performed for passivation and stability reasons yielding electron mobility of 4800 cm2/V s and hole mobility of 3800 cm2/V s as well as stabilised controllable behaviour of a bottom-gated transistor. The introduction of interface charge following the non-covalent functionalisation as well as the charge balance have been discussed and analysed.
104(2014); http://dx.doi.org/10.1063/1.4881927View Description Hide Description
The crystallization of amorphous GeTe and Ge 2Sb2Te5 phase change material films, with thickness between 10 and 100 nm, sandwiched between either Ta or SiO2 layers, was investigated by optical reflectivity. Ta cladding layers were found to increase the crystallization temperature, even for films as thick as 100 nm. X-Ray diffraction investigations of crystallized GeTe films showed a very weak texture in Ta cladded films, in contrast with the strong texture observed for SiO2 cladding layers. This study shows that crystallization mechanism of phase change materials can be highly impacted by interface effects, even for relatively thick films.