Volume 113, Issue 20, 28 May 2013
Index of content:
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4807638View Description Hide Description
Calculations of the optical properties of GeTe in the cubic NaCl and rhombohedral ferroelectric structures are reported. The rhombohedral ferroelectric distortion increases the band gap from 0.11 eV to 0.38 eV. Remarkably, substantial changes in optical properties are found even at high energies up to 5 eV. The results are discussed in relation to the bonding of GeTe and to phase change materials based on it.
113(2013); http://dx.doi.org/10.1063/1.4807407View Description Hide Description
We investigated the characteristics of terahertz pulses generated from antireflective GaAs surfaces with nanopillars under femtosecond laser excitation. Although the antireflective nanostructures contribute to the enhancement of free photocarrier excitation in GaAs, they could reduce the transient photocurrent density and advance the start time of the photocurrent decay. Thus, the relative amplitudes of the high-frequency spectral components of terahertz pulses increased, whereas the energies of the pulses decreased. However, we showed that thinly distributed nanopillar structures could generate a short terahertz pulse without a reduction in the pulse energy.
113(2013); http://dx.doi.org/10.1063/1.4807636View Description Hide Description
Focussed ion beam milling can be used to introduce aperiodic distributed feedback (ADFB) gratings into fully packaged, operational terahertz (THZ) quantum cascade lasers to achieve electronically controlled, discretely tunable laser emission. These aperiodic gratings—designed using computer-generated hologram techniques—consist of multiple slits in the surface plasmon waveguide, distributed along the length of the laser cavity. Tuning behaviour and output power in ADFB lasers operating around 2.9 THz are investigated with a variety of slit dimensions and grating scales. Mode selectivity and grating losses are found to be strongly dependent on milling depth into the upper waveguide layers, dramatically increasing as the metallic layers are penetrated, then rising more slowly with deeper milling into the laser active region. Grating scale and placement along the laser cavity length are also shown to influence mode selection.
First-principle prediction of single-carrier avalanche multiplication in chalcopyrite semiconductors113(2013); http://dx.doi.org/10.1063/1.4807650View Description Hide Description
A critical requirement for high gain and low noise avalanche photodiodes is the single-carrier avalanche multiplication. We propose that the single-carrier avalanche multiplication can be achieved in materials with a limited width of the valence band resulting in a restriction of kinetic energy for holes while allowing electrons to participate in the multiplication cascade. This feature of the electric structure is not common to the majority of technologically relevant semiconductors, but it can be anticipated in chalcopyrite Cu(AlGa)Se2 alloys based on the presented electric structure calculations.
Threshold improvement in uniformly lying helix cholesteric liquid crystal laser using auxiliary π-conjugated polymer active layer113(2013); http://dx.doi.org/10.1063/1.4807402View Description Hide Description
We propose a device structure to lower the lasing threshold of a uniformly lying helix cholesteric liquid crystal (ChLC) laser. We place a π-conjugated polymer active layer beneath the ChLC layer to provide auxiliary gain, and demonstrate an improvement in the lasing threshold by a factor of 2.3. We also perform finite difference time domain calculations coupled with rate equations for a four-level system, and clarify the effect of the additional active layer on both the photonic density of states and the inversion population density. Although the addition of an extra layer lowers the photonic density of states, the gain provided by the auxiliary layer is sufficient to overcome the losses and decrease the lasing threshold. Our concept is useful for obtaining high-performance ChLC lasers.
113(2013); http://dx.doi.org/10.1063/1.4807644View Description Hide Description
Extreme ultraviolet Mo/Si multilayers protected by capping layers of different materials were exposed to 13.5 nm plasma source radiation generated with a table-top laser to study the irradiation damage mechanism. Morphology of single-shot damaged areas has been analyzed by means of atomic force microscopy. Threshold fluences were evaluated for each type of sample in order to determine the capability of the capping layer to protect the structure underneath.
An indirectly pumped terahertz quantum cascade laser with low injection coupling strength operating above 150 K113(2013); http://dx.doi.org/10.1063/1.4807580View Description Hide Description
We designed and demonstrated a terahertz quantum cascade laser based on indirect pump injection to the upper lasing state and phonon scattering extraction from the lower lasing state. By employing a rate equation formalism and a genetic algorithm, an optimized active region design with four-well cascade module was obtained and epitaxially grown. A figure of merit which is defined as the ratio of modal gain versus injection current was maximized at 150 K. A fabricated device with a Au metal-metal waveguide and a top GaAs contact layer lased at 2.4 THz up to 128.5 K, while another one without the top GaAs lased up to 152.5 K ( ). The experimental results have been analyzed with rate equation and nonequilibrium Green's function models. A high population inversion is achieved at high temperature using a small oscillator strength of 0.28, while its combination with the low injection coupling strength of 0.85 meV results in a low current. The carefully engineered wavefunctions enhance the quantum efficiency of the device and therefore improve the output optical power even with an unusually low injection coupling strength.
Optical absorption of Mg-doped layers and InGaN quantum wells on c-plane and semipolar GaN structures113(2013); http://dx.doi.org/10.1063/1.4806997View Description Hide Description
We studied optical absorption of Mg-doped AlInGaN layers using excitation-position dependent and polarization resolved photoluminescence from the slab-waveguide edge of a laser structure. The major absorption in the Mg-doped layers was found only when p-doping is activated. It increases with the removal of residual hydrogen, which in case of Mg doping is a p-type passivation impurity, and reversibly disappears after passivation by hydrogen. This absorption is weakly wavelength and temperature dependent, and isotropic. This can be attributed to acceptor-bound hole absorption, because those holes concentration is nearly equal to that of activated acceptors and weakly temperature dependent (unlike the free hole concentration, which is much lower and is an exponential function of temperature due to high ionization energy). The cross section of photon absorption on such activated acceptor was quantified to be in the order of 10−17 cm−2. The absorption cross section of free electrons was found to be at least one order of magnitude lower and below detection limit. The same technique was used to experimentally quantify band structure polarization components along basis directions for green InGaN quantum wells (QWs) grown on c- and semipolar planes. The A1 and B1 valence subbands of c-plane QW were found to comprise mostly |X⟩ and |Y⟩ states. There was rather minor amount of |Z⟩ states with average square fraction of only 0.02. In (20-21) plane, due to small band anticrossing near gamma-point, we observed highly polarized absorption edges of A1- and B1-subbands consisting mainly of |Y⟩ and |X⟩ states, respectively, and found their energy splitting to be ∼40 meV. For (11-22) plane with smaller band splitting and polarization, we observed polarization switching with indium (In) concentration greater than 30% in the QW (or photon energy less than 2.3 eV). We confirmed our study of valence band structures by optical gain measurements.
Negative differential resistance induced by thermalization of two-dimensional electrons in terahertz quantum-well photodetectors113(2013); http://dx.doi.org/10.1063/1.4808343View Description Hide Description
Negative differential resistance (NDR) behavior existing in dark current-voltage (IV) curves of terahertz quantum-well photodetectors (QWPs) is theoretically investigated. Due to electron-electron scattering, the localized two-dimensional (2D) electrons in terahertz QWPs are thermalized. In steady state, the effective temperature of the 2D electrons is found to be higher than that of lattice. A self-consistent model is used to simulate the dark IV curves of terahertz QWPs, taking into account the thermalization effect of the 2D electrons. The NDR behavior is qualitatively reproduced. The periodic structures of electric-field domain and 2D electron occupation are formed in the NDR regime. The improved self-consistent model is useful for further understanding of the electron transport properties and improving the performance of terahertz QWPs.
113(2013); http://dx.doi.org/10.1063/1.4807416View Description Hide Description
In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.
- Plasmas and Electrical Discharges
113(2013); http://dx.doi.org/10.1063/1.4807303View Description Hide Description
The extrusion of the molten metal from a microcrater formed on a metal cathode during the operation of a vacuum arc is considered. The problem is thought to be similar to the classical hydrodynamic problem of a liquid drop impact on a solid surface. Based on this analogy, the conditions are analyzed under which the liquid will change its regular behavior (spreading over the cathode surface) into a singular behavior (formation of microjets and droplets). It is shown that the conditions realized in vacuum arc cathode spots at near-threshold currents are close to the threshold conditions for splashing of the molten metal. This points to a considerable contribution of hydrodynamic processes to the self-sustained operation of a vacuum arc and, in particular, gives grounds to relate the existence of a threshold arc current to the existence of a splashing threshold for liquid metal.
113(2013); http://dx.doi.org/10.1063/1.4807584View Description Hide Description
Breathing oscillations in the discharge of an enlarged cylindrical-anode-layer Hall plasma accelerator are investigated by three-dimensional particle-in-cell (PIC) simulation. Different from the traditional breathing mode in a circular Hall plasma accelerator, the bulk plasma oscillation here is trigged by the potential barrier generated by the concentrated ion beam and substantial enough to compete with the anode voltage. The electric field near the anode is suppressed by the potential barrier thereby decreasing the electron density by ∼36%. The discharge is restored to the normal level after the concentrated beam explodes and then it completes one cycle of electro-driven breathing oscillation. The breathing mode identified by the PIC simulation has a frequency range of ∼156 kHz–∼250 kHz and does not vary monotonically with the discharge voltage.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Effects of pressure, temperature, and hydrogen during graphene growth on SiC(0001) using propane-hydrogen chemical vapor deposition113(2013); http://dx.doi.org/10.1063/1.4806998View Description Hide Description
Graphene growth from a propane flow in a hydrogen environment (propane-hydrogen chemical vapor deposition (CVD)) on SiC differentiates from other growth methods in that it offers the possibility to obtain various graphene structures on the Si-face depending on growth conditions. The different structures include the (6√3 × 6√3)-R30° reconstruction of the graphene/SiC interface, which is commonly observed on the Si-face, but also the rotational disorder which is generally observed on the C-face. In this work, growth mechanisms leading to the formation of the different structures are studied and discussed. For that purpose, we have grown graphene on SiC(0001) (Si-face) using propane-hydrogen CVD at various pressure and temperature and studied these samples extensively by means of low energy electron diffraction and atomic force microscopy. Pressure and temperature conditions leading to the formation of the different structures are identified and plotted in a pressure-temperature diagram. This diagram, together with other characterizations (X-ray photoemission and scanning tunneling microscopy), is the basis of further discussions on the carbon supply mechanisms and on the kinetics effects. The entire work underlines the important role of hydrogen during growth and its effects on the final graphene structure.
113(2013); http://dx.doi.org/10.1063/1.4807312View Description Hide Description
The anisotropic optical and thermoelectric properties of In4Se3 and In4Te3 are studied by the first-principles calculation using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. The optical properties show highly anisotropic in the energy range between 0.0 and 12.0 eV for In4Se3 and between 0.0 and 10.0 eV for In4Te3 while it is isotropic in the higher energy range for In4Se3. In contrast to S, the anisotropies of the electrical conductivities and power factors are great affected by the change of the temperature. Their anisotropies become larger along three directions with the growth of the temperature. along the y direction is much higher than that along the x and z directions for In4Se3, which shows that the thermoelectric thin films with excellent performance can be obtained along the (010) surface. By studying the anisotropy of transport properties, we find that the transport properties of In4Se3 are better than that of In4Te3, which mainly comes from the small band gap of In4Se3. The anisotropy of for In4Se3 is larger than that for In4Te3, and the anisotropy of is mainly due to the anisotropy of .
Creation of freestanding wrinkled nano-films with desired deformation properties by controlling the surface morphology of a sacrificial layer113(2013); http://dx.doi.org/10.1063/1.4807579View Description Hide Description
Various wrinkle patterns can be formed due to the buckling of a stiff thin film on a compliant substrate. However, most wrinkled films previously reported were fixed on a large deformable substrate and thereby the potential deformability of the film was mechanically constrained by the substrate. In this study, we developed a technique for forming various wrinkled structures on the surface of a sacrificial resin layer. Since the sacrificial layer can be subsequently removed with a solvent, freestanding wrinkled films are created using the sacrificial layer. We found that a wrinkled structure is formed on the surface of the layer by applying a compressive strain to the resin layer at the appropriate moment during the hardening process. The wrinkle pattern depends on the curing time and the timing of the straining in two in-plane orthogonal directions. In addition to conventional stripe and labyrinth patterns by simple uniaxial and equi-biaxial strains, respectively, it was found that independent biaxial strains induce interesting structures, such as an orthogonally ordered wrinkle pattern and a nonsymmetrical buckling structure, in which the stripe array produced by the first straining remains and many finer wrinkles appear in each stripe by the second straining in the orthogonal direction. We conducted tensile experiments for 300-nm-thick freestanding Cu films having these wrinkled structures. The wrinkled nano-films have a variety of mechanical properties: the stripe structure has extremely high deformability (more than 10% strain) and reversibility, the labyrinth structure shows planar isotropic deformation, and the nonsymmetrical buckling structure has an anisotropic modulus and strength. Finite element analysis on the wrinkle structures revealed that the local stress concentration dominates the fracture limits.
Theoretical and experimental characterization of promising new scintillators: Eu2+ doped CsCaCl3 and CsCaI3113(2013); http://dx.doi.org/10.1063/1.4807401View Description Hide Description
An integrated approach was used to characterize Eu2+ doped CsCaCl3 and CsCaI3 crystals theoretically and experimentally. The temperature dependence of photoluminescence excitation, emission, and decay time was studied to better understand the energy transport and migration mechanism in these materials. The broadening and redshift of emission with increasing temperature was explained for both crystals by simultaneous quenching of emission and interaction of emission states with lattice vibration. The unusual increase of photoluminescence decay time with increasing temperature was ascribed to the presence of states with a lowered radiative rate slightly above the emitting states. The electronic and optical properties were also calculated theoretically with the help of Density functional theory in order to explain the Eu2+ emission properties in these crystals. The calculation explains the better scintillation light output and proportionality in CsCaI3. The promising cross-luminescent efficiency of these materials is also explained with the help of electronic band structure and dispersion of the partial density of the states of constituent atoms. Despite structural anisotropy, the calculated optical properties of CsCaI3 are nearly isotropic, and therefore the synthesis of optically transparent polycrystalline ceramics may be possible.
Effect of silane/hydrogen ratio on microcrystalline silicon thin films by remote inductively coupled plasma113(2013); http://dx.doi.org/10.1063/1.4807404View Description Hide Description
Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by remote low frequency inductively coupled plasma (ICP) chemical vapor deposition system, and the effect of silane/hydrogen ratio on the microstructure and electrical properties of μc-Si:H films was systematically investigated. As silane/hydrogen ratio increases, the crystalline volume fraction Fc decreases and the ratio of the intensity of (220) peak to that of (111) peak drops as silane flow rate is increased. The FTIR result indicates that the μc-Si:H films prepared by remote ICP have a high optical response with a low hydrogen content, which is in favor of reducing light-induced degradation effect. Furthermore, the processing window of the phase transition region for remote ICP is much wider than that for typical ICP. The photosensitivity of μc-Si:H films can exceed 100 at the transition region and this ensures the possibility of the fabrication of microcrystalline silicon thin film solar cells with a open-circuit voltage of about 700 mV.
Characteristics of ultrafast optical responses originating from non-equilibrium carrier transport in undoped GaAs/n-type GaAs epitaxial structures113(2013); http://dx.doi.org/10.1063/1.4807405View Description Hide Description
We have investigated the characteristics of ultrafast optical responses originating from a carrier transport process in undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structures with the use of a reflection-type pump-probe technique at room temperature. The built-in electric field in the i-GaAs top layer, whose strength is controlled by its thickness d, accelerates the transit of photogenerated carriers through the i-GaAs layer. We systematically observed that the decay time of a carrier-induced reflectivity change shortens with an increase in built-in electric field strength resulting from a decrease in d: 6.1, 12, and 28 kV/cm for d = 1200, 500, and 200 nm, respectively. In the i-GaAs/n-GaAs sample with d = 200 nm, which has the highest built-in electric field strength, the decay time is much shorter than the oscillation period of longitudinal optical (LO) phonon. From the spectrally resolved detection of the reflected light, it was found that the energy relaxation of the photogenerated carriers by the LO-phonon scattering hardly occurs in the i-GaAs layer, which indicates a quasiballistic transport. This finding demonstrates that the i-GaAs/n-GaAs structure with the non-equilibrium carrier transport process is useful for ultrafast optical applications.
Low temperature investigations and surface treatments of colloidal narrowband fluorescent nanodiamonds113(2013); http://dx.doi.org/10.1063/1.4807398View Description Hide Description
We report fluorescence investigations and Raman spectroscopy on colloidal nanodiamonds (NDs) obtained via bead assisted sonic disintegration (BASD) of a polycrystalline chemical vapor deposition film. The BASD NDs contain in situ created silicon vacancy (SiV) centers. Whereas many NDs exhibit emission from SiV ensembles, we also identify NDs featuring predominant emission from a single bright SiV center. We demonstrate oxidation of the NDs in air as a tool to optimize the crystalline quality of the NDs via removing damaged regions resulting in a reduced ensemble linewidth as well as single photon emission with increased purity. We furthermore investigate the temperature dependent zero-phonon-line fine-structure of a bright single SiV center as well as the polarization properties of its emission and absorption.
Judd-Ofelt analysis and emission quantum efficiency of Tb-fluoride single crystals: LiTbF4 and Tb0.81Ca0.19F2.81113(2013); http://dx.doi.org/10.1063/1.4807649View Description Hide Description
Terbium is the key element for highly efficient green phosphors and visible-near IR Faraday isolators. We have recently shown the potential of LiTbF4 and Tb0.81Ca0.19F2.81 as visible Faraday rotators. In this work, we present a detail spectroscopic analysis of Tb3+ (4f8) in these two compounds with different crystal structures. By means of the Judd-Ofelt theory, the emission branching ratios and lifetimes of the Tb3+ excited states have been estimated. These results are compared with experimental values obtained for the emitting 5D4 level, as well as with the absolute light yield measurements. Tb3+ in LiTbF4 exhibits a high quantum efficiency, and its radiative lifetime is confirmed to be 7 ms. Instead, the ionic conductor Tb0.81Ca0.19F2.81, which presents a high concentration of vacant sites, shows a lower quantum efficiency and a radiative lifetime about three times larger than estimated. Absorption and emission spectra of Tb0.81Ca0.19F2.81 are broad, so that any fine structure of energy levels can be resolved. In contrast, a detailed study of the splitting of Tb3+ multiplets in Stark energy levels is carried out for LiTbF4.