Volume 101, Issue 26, 24 December 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 have studied quantum transport in graphenenanoribbon tunnel field-effect transistors. Unlike other studies on similar structures, we have included dissipative processes induced by inelastic electron-phonon scattering and edge roughness in the nanoribbon self-consistently within a non-equilibrium transport simulation. Our results show that the dissipative scattering imposes a limit to the minimum OFF current and a minimum subthreshold swing that can be obtained even for long channel lengths where direct source-drain tunneling is inhibited. The edge roughness, in the presence of dissipative scattering, somewhat surprisingly, shows a classical behavior where it mostly reduces the maximum ON current achievable in this structure.
- PHOTONICS AND OPTOELECTRONICS
Electrically controlled absorption in a slab waveguide formed by the implantation of protons in a potassium lithium tantalate niobate substrate101(2012); http://dx.doi.org/10.1063/1.4772987View Description Hide Description
Electrically controlled absorption was observed in a slab waveguide, fabricated in a potassium lithium tantalate niobate substrate by protonimplantation, at an energy of and a fluence of . The implantation created an amorphous layer which acted as the cladding with an adjacent proton doped layer at its bottom. It is suggested that a n-i junction is formed at the interface between the proton layer and the substrate, which is the core of the waveguide. The electrically controlled absorption is attributed to changes in the width of the depletion area of the n-i junction induced by the applied field.
Enhanced photocurrent in crystalline silicon solar cells by hybrid plasmonic antireflection coatings101(2012); http://dx.doi.org/10.1063/1.4773038View Description Hide Description
Photocurrent enhancement induced by plasmonic light trapping is of great interest for photovoltaics. We design and demonstrate hybrid plasmonicantireflection coatings as an efficient light trapping strategy for broadband absorption and photocurrent enhancement in crystalline silicon solar cells. Goldnanoparticles of size ranging from 15 to 150 nm are embedded in standard SiNxantireflection coatings with a thickness of 90 nm. Through optimizing the location of tailored nanoparticles within the SiNx layer, both light scattering enhancement and near-field light concentration can be harnessed. A maximum increase of 6.3% in photocurrent is achieved for textured multi-crystalline Si solar cells with the optimum configuration.
101(2012); http://dx.doi.org/10.1063/1.4773246View Description Hide Description
A scheme for the generation of broadband and tunable mid-infrared pulses based on two-color filamentation in air is presented. The mid-infrared pulses, spanning the spectral region between 4 and 6.5 μm, result from a third-order nonlinear interaction between two short pulses, one at 800 nm and the other at 437 nm. The latter was created by frequency doubling a Raman-shifted pulse at 873 nm amplified in a chirped-pulse Raman amplifier. Tunability of this mid-infrared laser source was provided by introducing spatial chirp across the fundamental pulse profile which created a transversal array of chirped filaments.
101(2012); http://dx.doi.org/10.1063/1.4773236View Description Hide Description
It is known that one can determine the topological charge of a vortex beam based on the Fourier transform of its intensity. We demonstrate both theoretically and experimentally that this method will be invalid for determining the topological charge of a vortex beam with low coherence. Furthermore, we propose a method to determine the topological charge of a vortex beam with low coherence based on its complex degree of coherence.
101(2012); http://dx.doi.org/10.1063/1.4772476View Description Hide Description
In this work, we present the third order nonlinear optical investigation of two gold complexes, which differ by the nature of the counter cations. The impact of the different design in the architecture through a set of hydrogen bonds in the case of Au-Mel of the systems on the nonlinearity has been studied by means of the Z-scan setup under 532 nm, 30 ps laser excitation, allowing for the determination of the nonlinear absorption and refraction of the samples. Significant modification of the nonlinear optical response between the two metal complexes has been found suggesting a clear effect of the counter cation.
101(2012); http://dx.doi.org/10.1063/1.4773187View Description Hide Description
In this work, the advantages of the p-InGaN/AlGaN electron blocking layer (EBL) for InGaN/GaN light-emitting diodes(LEDs) were studied numerically and experimentally. The LEDs with p-InGaN/AlGaN EBL exhibited better optical performance over a wide range of carrier concentration due to the enhancement of holes' injection and electrons' confinement. The values of A, B, C, and D coefficients had been iteratively obtained by fitting quantum efficiency in the modified rate equation model. The analysis indicated that the improvement in the device properties could be attributed to the relatively small band gap and p-type doping of InGaN insertion layer.
101(2012); http://dx.doi.org/10.1063/1.4773241View Description Hide Description
We demonstrate a proof-of-principle magnetometer that relies on the active oscillation of a cold atom Raman laser to continuously map a field-sensitive atomic phase onto the phase of the radiated light. We demonstrate wideband sensitivity during continuous active oscillation, as well as narrowband sensitivity in passive Ramsey-like mode with translation of the narrowband detection in frequency using spin-echo techniques. The sensor operates with a sensitivity of at 1 kHz and effective sensing volume of . Fundamental quantum limits on the magnetic field sensitivity of an ideal detector are also considered.
High sensitivity and fast response and recovery times in a ZnO nanorod array/p-Si self-powered ultraviolet detector101(2012); http://dx.doi.org/10.1063/1.4773245View Description Hide Description
High quality, vertically aligned ZnOnanorods were grown on a silicon substrate, using microwave-assisted chemical bath deposition with poly (vinyl alcohol)-Zn(OH)2nanocomposites as seed layer. The structure and surface morphology of the prepared ZnOnanorod arrays were characterized using X-ray diffraction and scanning electron microscopy. The optical properties were assessed using photoluminescence measurements; the results showed a high-intensity UV peak, and a lower intensity, broader visible peak. Upon exposure to 395 nm light at a zero-bias voltage, the UV detector showed a high sensitivity of 8000% and fast response and recovery times of 25 and 22 ms, respectively.
Germanium metal-semiconductor-metal photodetectors evanescently coupled with upper-level silicon oxynitride dielectric waveguides101(2012); http://dx.doi.org/10.1063/1.4773212View Description Hide Description
We demonstrate Ge-on-Si metal-semiconductor-metal (MSM) photodetectors monolithically integrated with silicon oxynitride (SiOxNy) waveguides. The waveguide is placed on top of the photodetector and between the metalelectrodes, evading the shading effect by metalelectrodes, which is typical in surface-illuminated MSM photodetectors. The devices showed responsivity of about 0.45 A/W for 80 μm long devices at 1550 nm. The photodetector with 1.5 μm electrode spacing showed 3 dB bandwidth of 2.0 GHz at −2 V and 2 μA dark current. Further studies suggest that with a modified design the structure is capable of achieving 1 A/W responsivity and greater bandwidth.
101(2012); http://dx.doi.org/10.1063/1.4773298View Description Hide Description
Scintillation materials based on polystyrene (PS) have been investigated. Para-terphenyl was employed as a fluorescent molecule (fluor) that functions as a wavelength shifter. A clear increase in photon yield of the scintillation materials relative to the pure PS was observed, which cannot be explained by the conventional theory of scintillation mechanism. Furthermore, the photon yield increased with flour concentration in accordance with a power-law. Here we reveal the emergence of a luminescence of PS-based scintillation materials and demonstrate that their photon yields can be controlled by the fluor concentration.
101(2012); http://dx.doi.org/10.1063/1.4773303View Description Hide Description
Motivated by the pursuit of a simple system to produce a continuous variable non-classical light source for long-distance quantum communication, we construct an all-fiber source of pulsed twin beams in the 1550 nm band by using a high gain fiber optical parametric amplifier. The intensity-difference noise of the twin beams is below the shot noise limit by 3.1 dB (10.4 dB after correction for losses). A detailed study reveals a number of limiting factors for noise reduction. Therefore, further noise reduction will be feasible once care is taken for these limiting factors.
101(2012); http://dx.doi.org/10.1063/1.4773307View Description Hide Description
This study investigates the electrical performance of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors(TFTs) with a Ta2O5 gate dielectric under monochromatic illumination. The relationship between the phototransistor performance and oxygen partial pressure is determined. The oxygen content of the a-IGZO channel significantly affects the electrical and optical characteristics of a-IGZO TFTs. At applied gate biases of 0, 0, and 0.25 V, oxygen partial pressures of 0%, 0.1%, and 0.2% yielded measured device responsivities of 0.23, 0.44, and 4.75 A/W, respectively. Oxygen content can be used to control the mobility of TFTs, which can amplify photocurrent and enhance the responsivity of a-IGZO TFTs with a Ta2O5 gate dielectric.
Room-temperature transverse-electric polarized intersubband electroluminescence from InAs/AlInAs quantum dashes101(2012); http://dx.doi.org/10.1063/1.4773360View Description Hide Description
We report the observation of transverse electric polarized electroluminescence from InAs/AlInAs quantum dash quantum cascade structures up to room temperature. The emission is attributed to the electric field confined along the shortest lateral dimension of the dashes, as confirmed by its dependence on crystallographic orientation both in absorption measurements on a dedicated sample and from electroluminescence itself. From the absorption, we estimate a dipole moment for the observed transition of 〈x〉 = 1.7 nm. The electroluminescence is peaked at around 110 meV and increases with applied bias. Its temperature dependence shows a decrease at higher temperatures limited by optical phonon emission.
101(2012); http://dx.doi.org/10.1063/1.4773373View Description Hide Description
An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) photoresponse is reported. Using a combined quaternary In0.21Al0.21Ga0.58As and GaAs capping that relieves strain and maintains strong carrier confinement, we demonstrate a four color infrared response with peaks in the midwave- (5.7 μm), longwave- (9.0 and 14.5 μm), and far- (17 μm) infrared regions. Narrow spectral widths (7% to 9%) are noted at each of these wavelengths including responsivity value ∼95.3 mA/W at 14.5 μm. Using strain field and multi-band theory, we map specific bound-to-bound and bound-to-quasibound transitions to the longwave and midwave responses, respectively.
101(2012); http://dx.doi.org/10.1063/1.4773374View Description Hide Description
We propose and discuss terahertz (THz) electro-absorption modulators based on grapheneplasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.
101(2012); http://dx.doi.org/10.1063/1.4773376View Description Hide Description
We present a time-resolved study of the logical operation of a polariton condensate transistor switch. Creating a polariton condensate (source) in a GaAs ridge-shaped microcavity with a non-resonant pulsed laser beam, the polariton propagation towards a collector, at the ridge edge, is controlled by a second weak pulse (gate), located between the source and the collector. The experimental results are interpreted in the light of simulations based on the generalized Gross-Pitaevskii equation, including incoherent pumping, decay, and energy relaxation within the condensate.
101(2012); http://dx.doi.org/10.1063/1.4773377View Description Hide Description
We report on the demonstration of an array of master-oscillator power-amplifier quantum cascade lasers (QCLs) operating in single-mode at different wavelengths between 9.2 and 9.8 μm. In each device, the output of a distributed feedback QCL is injected into a tapered QCL section which acts as an amplifier while maintaining a high beam quality due to adiabatic mode spreading. All array elements feature longitudinal as well as transverse single-mode emission at peak powers between 0.8 and 3.9 W at room temperature. The high output power and excellent beam quality render the array highly suitable for stand-off spectroscopy applications.
101(2012); http://dx.doi.org/10.1063/1.4773484View Description Hide Description
We have investigated the optical and electrical crosstalk in HgCdTe photovoltaic pixel arrays employing a photon trapping (PT) structure realized with a periodic array of pillars. We have used a finite-difference time-domain approach to compute the carrier generation rate in the device and evaluate the optical crosstalk. Subsequently, solving the drift-diffusion equations, we have computed the quantum efficiency and the electrical crosstalk. We have found that, compared to a conventional pixel array, the one employing the photon trapping structure has a slightly higher optical crosstalk. However, the presence of the photon trapping region drastically reduces the electrical (total) crosstalk.
Subterahertz electromagnetic wave generation in a randomly networked single-walled carbon nanotubes photoconductive switch101(2012); http://dx.doi.org/10.1063/1.4773487View Description Hide Description
We demonstrated an electromagnetic (EM) wave generation that reaches up to 250 GHz in the photoconductive switch based on randomly networked single-walled carbon nanotubes(SWNTs). Furthermore, we investigated the bias dependence of the electromagnetic wave amplitudes. This subterahertz radiation is generated by the acceleration of photogenerated carriers through fluctuation-induced tunneling in single-walled carbon nanotube bundles. Below the bias field of 20 kV/cm, the signal was enhanced with an increase in the bias field. However, the signal amplitudes decreased above 20 kV/cm due to emerging space-charge accumulation and scattering effect occurring at the defects and contact points.
101(2012); http://dx.doi.org/10.1063/1.4773513View Description Hide Description
We observe a 104 times enhancement of the second harmonic generation of 1.535 μm laser light in a two-dimensional AlGaAs photonic crystal at normal incidence. The linear properties of the optical modes that are resonant with the fundamental frequency are well described by a Fano model and have a typical quality factor of 100. We present an extended version of the Fano interpretation to quantitatively analyze the second harmonic signal using the measured linear properties as input and find good agreement with the data.