- 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
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Volume 107, Issue 21, 23 November 2015
We have investigated nanometric magnetic textures in thin (<150 nm) plates of Ru-doped bilayered manganites La1.2Sr1.8(Mn1−yRuy)2O7. Ru substitution for Mn site changes the magnetic anisotropy from in-plane to out-of-plane easy axis type without any significant change of global magnetic and crystal structures. The combination of conventional and Lorentztransmission electron microscopy observations confirms the emergence of magnetic bubbles and skyrmions in the absence of magnetic field. With the changing Ru concentration, systematic changes in the type of magnetic bubbles are observed. A tiny residual magnetic field also affects the generation and the type-change of magnetic bubbles.
- PHOTONICS AND OPTOELECTRONICS
107(2015); http://dx.doi.org/10.1063/1.4936330View Description Hide Description
We investigate the linear and dynamical nonlinearoptical properties of a superlattice composed of ultra-narrow n-doped GaN/AlN quantum wells. Owing to huge band offsets, the structures feature a broad inter-miniband transition in the telecom window at 1.55 μm. Resonant pump-probe experiments directly reveal ultrafast intersubband relaxation occurring within <100 fs. We estimate an associated third order nonlinear optical susceptibility of m2/V2. The polarization and angular dependences of the optical response confirm the nonlinearity as originating from inter-miniband transitions in the heterostructure.
107(2015); http://dx.doi.org/10.1063/1.4935202View Description Hide Description
Nonlinear processes can be exploited to gain access to more information than is possible in the linear regime. Nonlinearity modifies the spectra of the excitation signals through harmonic generation, frequency mixing, and spectral shifting, so that features originally outside the detector range can be detected. Here, we present an experimental study of resolution enhancement for photoacoustic imaging of thin metal layers immersed in water. In this case, there is a threshold in the excitation below which no acoustic signal is detected. Above threshold, the nonlinearity reduces the width of the active area of the excitation beam, resulting in a narrower absorption region and thus improved spatial resolution. This gain is limited only by noise, as the active area of the excitation can be arbitrarily reduced when the fluence becomes closer to the threshold. Here, we demonstrate a two-fold improvement in resolution and quantify the image quality as the excitation fluence goes through threshold.
107(2015); http://dx.doi.org/10.1063/1.4936381View Description Hide Description
Generation of near-infrared light within the first biological optical window via frequency upconversion in Tm3+-doped PbGeO3-PbF2-CdF2 glass excited within the second biological window at 1.319 μm is reported. The upconversion emission at 800 nm is the sole light signal observed in the entire ultraviolet-visible-near-infrared spectral region making it possible obtaining high contrast imaging. The dependence of the 800 nm signal upon the sample temperature was investigated and results showed an increase by a factor of ×2.5 in the 30–280 °C range. Generation of detectable 690 nm for temperatures above 100 °C in addition to the intense 800 nm main signal was also observed. The proposed excitation mechanism for the 800 nm thulium emitting level is assigned to a multiphonon-assisted excitation from the ground-state3H6 to the 3H5excited-state level, a rapid relaxation to the 3F4 level and followed by an excited-state absorption of the pump photons mediated by multiphonons connecting the 3F4 level to the 3H4 emitting level.
107(2015); http://dx.doi.org/10.1063/1.4936194View Description Hide Description
We report on the properties of radially self-accelerating intensity distributions consisting of two components in the angular frequency domain. We show how this subset of solutions, in literature also known as helicon beams, possesses peculiar characteristics that enable a better control over its properties. In this work, we present a step-by-step optimization procedure to achieve the best possible intensity contrast, a distinct rotation rate and long propagation lengths. All points are discussed on a theoretical basis and are experimentally verified.
107(2015); http://dx.doi.org/10.1063/1.4936235View Description Hide Description
A chip scale tunable laser in the visible spectral band is realized by generating a periodic droplet array inside a microfluidic channel. Combined with a gain medium within the droplets, the periodic structure provides the optical feedback of the laser. By controlling the pressure applied to two separate inlets we can change the period of the droplet array. As a result, the lasing frequency is tuned over a broad spectral range. Using this configuration, we demonstrate wavelength tunability of about 70 nm and lasing threshold of about .
107(2015); http://dx.doi.org/10.1063/1.4936236View Description Hide Description
Three-dimensional optical microscopy suffers from the well-known compromise between transverse resolution and depth-of-field. This is true for both structural imaging methods and their functional extensions. Interferometric synthetic aperture microscopy (ISAM) is a solution to the 3D coherent microscopy inverse problem that provides depth-independent transverse resolution. We demonstrate the extension of ISAM to polarization sensitive imaging, termed polarization-sensitive interferometric synthetic aperture microscopy (PS-ISAM). This technique is the first functionalization of the ISAM method and provides improved depth-of-field for polarization-sensitive imaging. The basic assumptions of polarization-sensitive imaging are explored, and refocusing of birefringent structures is experimentally demonstrated. PS-ISAM enables high-resolution volumetric imaging of birefringent materials and tissue.
Polarization properties of light scattered off solutions of chiral molecules in non-forward direction107(2015); http://dx.doi.org/10.1063/1.4936342View Description Hide Description
Measuring the optical activity from an ensemble of chiral molecules is a common tool to know their stereo-structure. These measurements are done in the same propagation direction of the probe beam of light, because that is the direction where most signal is emitted. We provide experimental and theoretical evidence that, even though other interesting information may be gathered when collecting light emitted in other directions, for most molecules, the phenomenon of optical activity is only present in the forward scattering direction. The fundamental reason behind this is that forward scattered light preserves the circular polarization states due to the cylindrical symmetry of the system, an essential requirement for optical activity. An important exemption happens in dual molecules, i.e., molecules which present the same response to electric and magnetic fields. We present a series of experiments measuring the optical activity and the scattering of chiral solutions in the forward and perpendicular directions. We experimentally show that these molecules present optical activity and preservation of circular polarization in the forward direction, while the polarization pattern in non-forward directions is much more complex and, in particular, does not preserve the circular polarization. Finally, we show that when probing the particle with different wavelengths, the scattering in non-forward directions presents some interesting structural features which are hidden in the forward measurements.
Enhanced quality factor of Fano resonance in optical metamaterials by manipulating configuration of unit cells107(2015); http://dx.doi.org/10.1063/1.4936385View Description Hide Description
By changing unit cell configurations, we demonstrated enhancement of quality factors (Q-factors) of Fano resonance in optical metamaterials composed of asymmetric double bars. The Q-factors of Fano resonance at wavelengths around 1500 nm were extracted from absorption spectra, and the dependence of the degree of asymmetry was studied. Observed enhancement is qualitatively interpreted by dipole-dipole interactions, and destructive interactions were essential for achieving high Q-factors. These results will be useful for improving performance of potential applications using metamaterialresonators such as light emitting devises and sensors.
Impact of carrier localization on radiative recombination times in semipolar plane InGaN/GaN quantum wells107(2015); http://dx.doi.org/10.1063/1.4936386View Description Hide Description
Semipolar plane InxGa1−xN quantum wells(QWs) of varying alloy composition were studied by time-resolved photoluminescence. A large difference in effective radiative lifetimes, from sub-ns for to ∼30 ns for was found. This effect is attributed to different properties of carrier localization. In low In content QWs, recombination at extended states with short recombination times is prevalent. In QWs with a high In content, the lifetimes are increased by localization of electrons and holes at separate sites. The zigzag shape of the QW interfaces and the resulting in-plane electric field are proposed as the cause for the separate electron and hole localization.
107(2015); http://dx.doi.org/10.1063/1.4936599View Description Hide Description
We propose a concept for light polarization management: polarization-dependent diffraction in all-dielectric microstructures. Numerical simulations of light propagation show that with an appropriately configured array of twisted bands, such structures may exhibit zero birefringence and at the same time diffract two circular polarizations with different efficiencies. Non-birefringent structures as thin as 3 μm have a significant difference in diffraction efficiency for left- and right-hand circular polarizations. We identify the structural parameters of such twisted-band matrices for optimum performance as circular polarizers.
107(2015); http://dx.doi.org/10.1063/1.4936600View Description Hide Description
We present experimental evidence of plasmonic-enhanced optical tweezers, of polystyrene beads in deionized water in the vicinity of metal-coated nanostructures. The optical tweezers operate with a continuous wave near-infrared laser. We employ a Cu/Au bilayer that significantly improves dissipation of heat generated by the trapping laser beam and avoid de-trapping from heat convection currents. We investigate the improvement of the optical trapping force and the effective trapping quality factor, and observe an exponential distance dependence of the trapping force from the nanostructures, indicative of evanescent plasmonic enhancement.
107(2015); http://dx.doi.org/10.1063/1.4936612View Description Hide Description
In this study, we report the experimental demonstration of hyperbolic property in a conventional material—ferrite. The hyperbolic permeability dispersion originates from ferromagnetic resonance. Using 2D-Field mapping system, the negative refraction with a broad bandwidth of 1.5 GHz was observed in a bulk Ba-Ferrite with no magnetic field applied, therefore confirming the hyperbolic dispersionproperty. This work also provides an identification of several other similar hyperbolic materials based on permeability changes over a broad range of spectral frequencies.
107(2015); http://dx.doi.org/10.1063/1.4936618View Description Hide Description
Terahertz (THz) waves radiated from two noncollinear femtosecond plasma filaments with a crossing angle of 25° are investigated. The irradiated THz waves from the crossing filaments show a small THz pulse after the main THz pulse, which was not observed in those from single-filament scheme. Since the position of the small THz pulse changes with the time-delay of two filaments, this phenomenon can be explained by a model in which the small THz pulse is from the second filament. The denser plasma in the overlap region of the filaments changes the movement of space charges in the plasma, thereby changing the angular distribution of THz radiation. As a result, this schematic induces some THz wave from the second filament to propagate along the path of the THz wave from the first filament. Thus, this schematic alters the direction of the THz radiation from the filamentation, which can be used in THz wave remote sensing.
107(2015); http://dx.doi.org/10.1063/1.4936650View Description Hide Description
Antimony-based mid-infrared interband cascade (IC) photodetectors fabricated on (001) GaAs substrates are reported. By using a “buffer-free” interfacial misfit array growth method, an overall good crystalline quality is obtained on the largely lattice-mismatched GaAs substrate. The GaAs-based IC detectors show comparable optical performance, with similar electrical performance at temperatures higher than 140 K, as compared to the reference devices grown on GaSb substrate. The GaAs-based IC detectors demonstrate dark current density of 2.63 × 10−6 A/cm2 at 180 K, which is about twice as compared to that grown on GaSb substrate, with Johnson-limited D* of 1.06 × 1011 Jones at 180 K and 4.0 μm. The results indicate that IC detector design is robust and relatively insensitive to the material quality, and metamorphic IC detector is viable for large-format infrared focal plane array applications.
- SURFACES AND INTERFACES
107(2015); http://dx.doi.org/10.1063/1.4936240View Description Hide Description
The band alignment at the interface between GaAs and amorphous Al2O3 is studied through the use of hybrid functionals. For the oxide component, a disordered model is generated through density-functional molecular dynamics. The achieved structure shows good agreement with the experimental characterization. The potential line-up across the interface is obtained for two atomistic GaAs/Al2O3interfacemodels, which differ by the GaAs substrate termination. The calculated valence band offset amounts to 3.9 eV for an interface characterized by the occurrence of Ga–O bonds as dominant chemical bonding, favoring the high-energy side in the range of experimental values (2.6–3.8 eV). The effect of As antisite and As–As dimer defects on the band alignment is shown to be negligible.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
107(2015); http://dx.doi.org/10.1063/1.4936328View Description Hide Description
To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm2 V−1 s−1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no sub-bandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide electrode for tandem solar cells in modules.
Real-time observation of dynamic process of oxygen vacancy migration in cerium oxides under electric field107(2015); http://dx.doi.org/10.1063/1.4936333View Description Hide Description
The dynamic process of oxygen vacancy migration driven by the external electric field is directly observed at atomic scale in the cerium oxides (CeO2) thin film by in-situtransmission electron microscopy method. When a bias voltage of a proper value is applied across the CeO2 film, the oxygen vacancies are formed near the interface of CeO2/anode, followed by their migration along the direction of the external electric field. The structural modulation occurs in the  zone axis due to the ordering of oxygen vacancies. The migration of oxygen vacancies results in the reversible structural transformation, i.e., releasing and storing oxygen processes in CeO2, which is of great significance for the ionic and electronic applications of the cerium oxides materials, such as oxygen pump, gas sensor, resistive random access memory, etc.
Inorganic/organic nanocomposites: Reaching a high filler content without increasing viscosity using core-shell structured nanoparticles107(2015); http://dx.doi.org/10.1063/1.4936339View Description Hide Description
Extensive research is being conducted on the development of inorganic/organic nanocomposites for a wide variety of applications in microelectronics, biotechnologies, photonics, adhesives, or optical coatings. High filler contents are usually required to fully optimize the nanocompositesproperties. However, numerous studies demonstrated that traditional composite viscosity increases with increasing the filler concentration reducing therefore significantly the material processability. In this work, we synthesized inorganic/organic core-shell nanocomposites with different shell thicknesses. By reducing the shell thickness while maintaining a constant core size, the nanoparticle molecular mass decreases but the nanocomposite filler fraction is correlatively increased. We performed viscosity measurements, which clearly highlighted that intrinsic viscosity of hybridnanoparticles decreases as the molecular mass decreases, and thus, as the filler fraction increases, as opposed to Einstein predictions about the viscosity of traditional inorganic/polymer two-phase mixtures. This exceptional behavior, modeled by Mark-Houwink-Sakurada equation, proves to be a significant breakthrough for the development of industrializable nanocomposites with high filler contents.
107(2015); http://dx.doi.org/10.1063/1.4936327View Description Hide Description
We numerically investigate the propagation of small-amplitude elastic waves in random fiber networks. Our analysis reveals that the dynamic response of the system is not only controlled by its overall elasticity, but also by the local microstructure. In fact, we find that the longest fiber-segment plays a key role in dynamics when the network is excited with waves of short wavelength. In this case, the Bloch modes are highly non-affine as the longest segments oscillate close to their resonances. Based on this observation, we predict the low frequency dispersion curves of random fiber networks.
107(2015); http://dx.doi.org/10.1063/1.4936332View Description Hide Description
Temperature dependent optical property of diamond has been considered as a very important factor for realizing high performance diamond-based optoelectronic devices. The photoluminescence feature of the zero phonon line of silicon-vacancy (Si-V) centers in Si-doped chemical vapor deposited single crystal diamond (SCD) with localized surface plasmon resonance (LSPR) induced by gold nanoparticles has been studied at temperatures ranging from liquid nitrogen temperature to 473 K, as compared with that of the SCD counterpart in absence of the LSPR. It is found that with LSPR the emission intensities of Si-V centers are significantly enhanced by factors of tens and the magnitudes of the redshift (width) of the emissions become smaller (narrower), in comparison with those of normal emissions without plasmon resonance. More interestingly, these strong Si-V emissions appear remarkably at temperatures up to 473 K, while the spectral feature was not reported in previous studies on the intrinsic Si-doped diamonds when temperatures are higher than room temperature. These findings would lead to reaching high performance diamond-based devices, such as single photon emitter, quantum cryptography, biomarker, and so forth, working under high temperature conditions.