Volume 104, Issue 12, 24 March 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:
We demonstrate a hole double quantum dot in an undoped GaAs/AlGaAs heterostructure. The interdot coupling can be tuned over a wide range, from formation of a large single dot to two well-isolated quantum dots. Using charge sensing, we show the ability to completely empty the dot of holes and control the charge occupation in the few-hole regime. The device should allow for control of individual hole spins in single and double quantum dots in GaAs.
- PHOTONICS AND OPTOELECTRONICS
104(2014); http://dx.doi.org/10.1063/1.4869297View Description Hide Description
We present a metal-free tunable anisotropic metamaterial where the iso-frequency surface is tuned from elliptical to hyperbolic dispersion by exploiting the metal-insulator phase transition in the correlated material vanadium dioxide (VO2). Using VO2-TiO2 heterostructures, we demonstrate the transition in the effective dielectric constant parallel to the layers to undergo a sign change from positive to negative as the VO2 undergoes the phase transition. The possibility to tune the iso-frequency surface in real time using external perturbations such as temperature, voltage, or optical pulses creates new avenues for controlling light-matter interaction.
Enhanced out-coupling efficiency of organic light-emitting diodes using an nanostructure imprinted by an alumina nanohole array104(2014); http://dx.doi.org/10.1063/1.4869468View Description Hide Description
We demonstrate organic light-emitting diodes (OLEDs) with enhanced out-coupling efficiency containing nanostructures imprinted by an alumina nanohole array template that can be applied to large-emitting-area and flexible devices using a roll-to-roll process. The nanostructures are imprinted on a glass substrate by an ultraviolet nanoimprint process using an alumina nanohole array mold and then an OLED is fabricated on the nanostructures. The enhancement of out-coupling efficiency is proportional to the root-mean-square roughness of the nanostructures, and a maximum improvement of external electroluminescence quantum efficiency of 17% is achieved. The electroluminescence spectra of the OLEDs indicate that this improvement is caused by enhancement of the out-coupling of surface plasmon polaritons.
Three-dimensional flow contrast imaging of deep tissue using noncontact diffuse correlation tomography104(2014); http://dx.doi.org/10.1063/1.4869469View Description Hide Description
This study extended our recently developed noncontact diffuse correlation spectroscopy flowmetry system into noncontact diffuse correlation tomography (ncDCT) for three-dimensional (3-D) flow imaging of deep tissue. A linear array of 15 photodetectors and two laser sources connected to a mobile lens-focusing system enabled automatic and noncontact scanning of flow in a region of interest. These boundary measurements were combined with a finite element framework for DCT image reconstruction implemented into an existing software package. This technique was tested in computer simulations and using a tissue-like phantom with anomaly flow contrast design. The cylindrical tube-shaped anomaly was clearly reconstructed in both simulation and phantom. Recovered and assigned flow contrast changes in anomaly were found to be highly correlated: regression slope = 1.00, R2 = 1.00, and p < 10−5 in simulation and regression slope ≥ 0.97, R2 ≥ 0.96, and p < 10−3 in phantom. These results exhibit promise of our ncDCT technique for 3-D imaging of deep tissue blood flow heterogeneities.
Carrier dynamics in inhomogeneously broadened InAs/AlGaInAs/InP quantum-dot semiconductor optical amplifiers104(2014); http://dx.doi.org/10.1063/1.4869489View Description Hide Description
We report on a characterization of fundamental gain dynamics in recently developed InAs/InP quantum-dot semiconductor optical amplifiers. Multi-wavelength pump-probe measurements were used to determine gain recovery rates, following a powerful optical pump pulse, at various wavelengths for different bias levels and pump excitation powers. The recovery was dominated by coupling between the electronic states in the quantum-dots and the high energy carrier reservoir via capture and escape mechanisms. These processes determine also the wavelength dependencies of gain saturation depth and the asymptotic gain recovery level. Unlike quantum-dash amplifiers, these quantum-dots exhibit no instantaneous gain response, confirming their quasi zero-dimensional nature.
104(2014); http://dx.doi.org/10.1063/1.4869575View Description Hide Description
We demonstrate high-density, multi-level crystallization of a Ge2Sb2Te5 thin film using tightly focused femtosecond laser pulses. The submicron spots with 8 distinct data storage states are written on a 1.08 μm square grid. The significant change in reflectivity of every specific state of crystallized spot allows easy optical reading and identification. As a demonstration, two gray-scale images are written into the storage medium. Our results open up potential applications in ultra-fast two-dimensional parallel cognitive computing and holography.
104(2014); http://dx.doi.org/10.1063/1.4869578View Description Hide Description
A planewave incident on an active etalon with net roundtrip gain may be expected to diverge in field amplitude, yet applying the Fresnel formalism to Maxwell's equations admits a convergent solution. We describe this solution mathematically and provide additional insight by demonstrating the response of such a cavity to an incident beam of light. Cavities with net roundtrip gain have often been overlooked in the literature, and a clear understanding of their behavior yields insight to negative refraction in nonmagnetic media, a duality between loss and gain, amplified total internal reflection, and the negative-index lens.
104(2014); http://dx.doi.org/10.1063/1.4869751View Description Hide Description
We report on the design, fabrication, and electro-optical characterization of a light emitting device operating at 1.54 μm, whose active layer consists of silicon oxide containing Er-doped Si nanoclusters. A photonic crystal (PhC) is fabricated on the top-electrode to enhance the light extraction in the vertical direction, and thus the external efficiency of the device. This occurs if a photonic mode of the PhC slab is resonant with the Er emission energy, as confirmed by theoretical calculations and experimental analyses. We measure an increase of the extraction efficiency by a factor of 3 with a high directionality of light emission in a narrow vertical cone. External quantum efficiency and power efficiency are among the highest reported for this kind of material. These results are important for the realization of CMOS-compatible efficient light emitters at telecom wavelengths.
104(2014); http://dx.doi.org/10.1063/1.4869753View Description Hide Description
Photonic crystal (PC) nanolasers often consist of air-bridge PC slab, which enhances optical confinement while limiting its size to 30 × 30 μm2 due to the mechanical fragileness. This limit is broken by resin-mediated bonding of the PC slab on a host substrate. In this paper, we demonstrate a GaInAsP PC slab with a size of over 100 × 100 μm2 in which 1089, 2376, and 11 664 nanolasers showing high-yield laser operation are integrated.
Raman peak enhancement and shift of few-layer graphene induced by plasmonic coupling with silver nanoparticles104(2014); http://dx.doi.org/10.1063/1.4869577View Description Hide Description
Few-layer graphene was transferred directly on top of Ag nanoparticles, and the coupling between graphene and localized surface plasmons (LSPs) of Ag nanoparticles was investigated. We found that the surface enhanced Raman spectroscopy of graphene was increased approximately 7-fold by near-fields of plasmonic Ag nanoparticles and the enhancement factor of graphene G peak increased with the particle size. Meanwhile, the LSP resonances of Ag nanoparticles exhibit a 10 nm redshift and a 13 nm broadening by the presence of graphene, which can be attributed to the coupling between the Ag LSPs and the graphene.
104(2014); http://dx.doi.org/10.1063/1.4869754View Description Hide Description
Diamond/CeF3/SiO2 multilayered films electroluminescent (EL) devices were made, and we found that the EL spectrum at room temperature depends on the CeF3 layer thickness. The EL spectrum shows that the main peaks are located at 527 nm, 593 nm, and 742 nm when the CeF3 layer thickness is less than 0.5 μm, but when the CeF3 layer thickness is greater than 0.5 μm, the electroluminescence spectrum obviously exhibits three bands, which are centered at 310–380 nm (ultraviolet emission), 520–580 nm (green-yellow emission), and 700–735 nm (red emission). The white EL brightness of the device (for thicker CeF3 layer) reaches a maximum of 15 cd/m2 at a forward applied voltage of 225 V, which can be distinguished at the sunlight in the light by the naked eye.
104(2014); http://dx.doi.org/10.1063/1.4869752View Description Hide Description
n-CdS/p-PbSe heterojunction is investigated. A thin CdS film is deposited by chemical bath deposition on top of epitaxial PbSe film by molecular beam epitaxy on Silicon. Current-voltage measurements demonstrate very good junction characteristics with rectifying ratio of ∼178 and ideality factor of 1.79 at 300 K. Detectors made with such structure exhibit mid-infrared spectral photoresponse at room temperature. The peak responsivity Rλ and specific detectivity D* are 0.055 A/W and 5.482 × 108 cm·Hz1/2/W at λ = 4.7 μm under zero-bias photovoltaic mode. Temperature-dependent photoresponse measurements show abnormal intensity variation below ∼200 K. Possible reasons for this phenomenon are also discussed.
104(2014); http://dx.doi.org/10.1063/1.4869762View Description Hide Description
This study theoretically demonstrates a broadband circular asymmetric transmission effect on the basis of two dimensional planar spiral metamaterials. We found that by increasing the number of turns in a single spiral unit cell, the bandwidth of the asymmetric transmission effect will be broaden dramatically and reach to about 940 nm in the near-infrared spectral range.
104(2014); http://dx.doi.org/10.1063/1.4869959View Description Hide Description
We report an experimental realization of lensless ghost imaging for a phase-only object with pseudo-thermal light, which was proposed by W. Gong and S. Han [Phys. Rev. A 82, 023828 (2010)]. In contrast with conventional ghost imaging, the scheme involves the interference of two correlated fields and the phase information of the object can be retrieved. This imaging technique completes the nonlocally lensless spatial reconstruction of both amplitude and phase distributions in ghost imaging with thermal light.
Experimental verification of effects of barrier dopings on the internal electric fields and the band structure in InGaN/GaN light emitting diodes104(2014); http://dx.doi.org/10.1063/1.4870256View Description Hide Description
We experimentally clarify the effects of barrier dopings on the polarization induced electric fields and the band structure in InGaN/GaN blue light emitting diodes. Both effects were independently verified by using electric field modulated reflectance and capacitance-voltage measurement. It is shown that the Si barrier doping does reduce the polarization induced electric field in the quantum wells. But the benefit of Si-doping is nullified by modification of the band structure and depletion process. With increased number of doped barriers, smaller number of quantum wells remains in the depletion region at the onset of the diffusion process, which can reduce the effective active volume and enhance the electron overflow.
Absorber and gain chip optimization to improve performance from a passively modelocked electrically pumped vertical external cavity surface emitting laser104(2014); http://dx.doi.org/10.1063/1.4870048View Description Hide Description
We present an electrically pumped vertical-external-cavity surface-emitting laser (EP-VECSEL) modelocked with a semiconductor saturable absorber mirror (SESAM) with significantly improved performance. In different cavity configurations, we present the shortest pulses (2.5 ps), highest average output power (53.2 mW), highest repetition rate (18.2 GHz), and highest peak power (4.7 W) to date. The simple and low-cost concept of EP-VECSELs is very attractive for mass-market applications such as optical communication and clocking. The improvements result from an optimized gain chip from Philips Technologie GmbH and a SESAM, specifically designed for EP-VECSELs. For the gain chip, we found a better trade-off between electrical and optical losses with an optimized doping scheme in the substrate to increase the average output power. Furthermore, the device's bottom contact diameter (60 μm) is smaller than the oxide aperture diameter (100 μm), which favors electro-optical conversion into a TEM00 mode. Compared to optically pumped VECSELs we have to increase the field enhancement in the active region of an EP-VECSEL which requires a SESAM with lower saturation fluence and higher modulation depth for modelocking. We therefore used a resonant quantum well SESAM with a 3.5-pair dielectric top-coating (SiNx and SiO2) to enhance the field in the absorber at the lasing wavelength of 980 nm. The absorption bandedge at room temperature is detuned (965 nm) compared to the resonance (980 nm), which enables temperature-tuning of the modulation depth and saturation fluence from approximately 2.5% up to 15% and from 20 μJ/cm2 to 1.1 μJ/cm2, respectively.
- SURFACES AND INTERFACES
104(2014); http://dx.doi.org/10.1063/1.4870085View Description Hide Description
Using in situ transmission electron microscopy, we show that the oxidation of the Cu-Au(100) results in the formation of Cu 2O islands that deeply embed into the Cu-Au substrate while the oxidation of the Cu-Pt(100) leads to the formation of Cu 2O islands that highly protrude above the Cu-Pt substrate. Their difference is attributed to the different mobilities of Pt and Au in the Cu base alloys for which the sluggish mobility of Pt in Cu results in trapped Pt atoms at the oxide/alloy interface while the faster mobility of Au in Cu leads to enhanced rehomogenization of the alloy composition.
104(2014); http://dx.doi.org/10.1063/1.4870093View Description Hide Description
A facile manganese ion Mn(II)-assisted assembly has been designed to fabricate microbowls by using graphene oxide nanosheets as basic building blocks, which were exfoliated ultrasonically from the oxidized soot powders in deionized water. From the morphology evolution observations of transmission electron microscope and scanning electron microscope, a coordinating-tiling-collapsing manner is proposed to interpret the assembly mechanism based on attractive Van der Waals forces, π-π stacking, and capillary action. It is interesting to note that the as-prepared microbowls present a room temperature superparamagnetic behavior.
Large enhanced dielectric permittivity in polyaniline passivated core-shell nano magnetic iron oxide by plasma polymerization104(2014); http://dx.doi.org/10.1063/1.4870098View Description Hide Description
Commercial samples of Magnetite with size ranging from 25–30 nm were coated with polyaniline by using radio frequency plasma polymerization to achieve a core shell structure of magnetic nanoparticle (core)–Polyaniline (shell). High resolution transmission electron microscopy images confirm the core shell architecture of polyaniline coated iron oxide. The dielectric properties of the material were studied before and after plasma treatment. The polymer coated magnetite particles exhibited a large dielectric permittivity with respect to uncoated samples. The dielectric behavior was modeled using a Maxwell–Wagner capacitor model. A plausible mechanism for the enhancement of dielectric permittivity is proposed.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
104(2014); http://dx.doi.org/10.1063/1.4869022View Description Hide Description
Although symmetry breaking is widely realized as one of the most powerful tools in modern scientific researches, it is unclear how symmetry breaking plays its role in nanocosm. Here, we show a correlation between spontaneous symmetry breaking and the formation of nanocrystals. In our experiments, some ZnO nanocrystals, including ZnO tetrapods, rod-based tetrapods, and aeroplane-like crystals, presented with specific structures and symmetries leading to an unexpected process of spontaneous symmetry breaking. According to the rule of spontaneous symmetry breaking, a hypothesis was proposed that the aeroplane-like nanocrystals might be resulted from the unequal development of the crystal twinnings. Subsequent work supported this hypothesis and proved the dramatic effect of spontaneous symmetry breaking. This work applies the rule of spontaneous symmetry breaking to the formation mechanisms for nanocrystals and highlights the causal contribution of spontaneous symmetry breaking to the intricate behaviors of the particles at nanoscale.
104(2014); http://dx.doi.org/10.1063/1.4869462View Description Hide Description
We investigate tunable acoustic absorption enabled by the coherent control of input waves. It relies on coherent perfect absorption originally proposed in optics. By designing appropriate acoustic metamaterial structures with resonating effective bulk modulus or density, we show that complete absorption of incident waves impinging on the metamaterial can be achieved for either symmetrical or anti-symmetrical inputs in the forward and backward directions. By adjusting the relative phase between the two incident beams, absorption can be tuned effectively from unity to zero, making coherent control useful in applications like acoustic modulators, noise controllers, transducers, and switches.