Index of content:
Volume 116, Issue 6, 14 August 2014
- Lasers, Optics, and Optoelectronics
Fine tunable red-green upconversion luminescence from glass ceramic containing 5%Er3+:NaYF4 nanocrystals under excitation of two near infrared femtosecond lasers116(2014); http://dx.doi.org/10.1063/1.4892594View Description Hide Description
In this paper, we report fine tunable red-green upconversion luminescence of glass ceramic containing 5%Er3+: NaYF4 nanocrystals excited simultaneously by two near infrared femtosecond lasers. When the glass ceramic was irradiated by 800 nm femtosecond laser, weak red emission centered at 670 nm was detected. Bright red light was observed when the fs laser wavelength was tuned to 1490 nm. However, when excited by the two fs lasers simultaneously, the sample emitted bright green light centered at 550 nm, while the red light kept the same intensity. The dependences of the red and the green light intensities on the two pump lasers are much different, which enables us to manipulate the color emission by adjusting the two pump laser intensities, respectively. We present a theoretical model of Er3+ ions interacting with two fs laser fields, and explain well the experimental results.
Interface modified thermally stable hole transporting layer for efficient organic light emitting diodes116(2014); http://dx.doi.org/10.1063/1.4892396View Description Hide Description
Electrical transport in thermally stable 2, 7-bis [N, N-bis (4-methoxy-phenyl) amino]-9, 9-spirobifluorene (MeO-Spiro-TPD) thin films has been investigated as a function of temperature and organic layer thickness. ITO/MeO-Spiro-TPD interface was found to be injection limited and has been studied in detail to find barrier height for hole injection. The thickness of tetra-fluoro-tetracyano-quinodimethane thin films were optimized to be used as hole injection buffer layer which resulted in switching of charge transport mechanism from injection limited to space charge limited conduction above a critical thickness of 3 nm. Hole mobility has been measured using transient space charge limited conduction (SCLC), field dependent SCLC, and top contact transistor characteristics. The charge carrier transport in interface modified hole only devices was analysed using Gaussian disorder model. The thermal stability of MeO-Spiro-TPD has been investigated by atomic force microscopy and X-ray diffraction studies. The study indicates a thermally stable and highly efficient hole transport material for application in organic semiconductor based devices.
Experimental investigation of factors limiting slow axis beam quality in 9xx nm high power broad area diode lasers116(2014); http://dx.doi.org/10.1063/1.4892567View Description Hide Description
GaAs-based broad-area diode lasers are needed with improved lateral beam parameter product (BPP lat) at high power. An experimental study of the factors limiting BPP lat is therefore presented, using extreme double-asymmetric (EDAS) vertical structures emitting at 910 nm. Continuous wave, pulsed and polarization-resolved measurements are presented and compared to thermal simulation. The importance of thermal and packaging-induced effects is determined by comparing junction -up and -down devices. Process factors are clarified by comparing diodes with and without index-guiding trenches. We show that in all cases studied, BPP lat is limited by a non-thermal BPP ground-level and a thermal BPP, which depends linearly on self-heating. Measurements as a function of pulse width confirm that self-heating rather than bias-level dominates. Diodes without trenches show low BPP ground-level, and a thermal BPP which depends strongly on mounting, due to changes in the temperature profile. The additional lateral guiding in diodes with trenches strongly increases the BPP ground-level, but optically isolates the stripe from the device edges, suppressing the influence of the thermal profile, leading to a BPP-slope that is low and independent of mounting. Trenches are also shown to initiate strain fields that cause parasitic TM-polarized emission with large BPP lat, whose influence on total BPP lat remains small, provided the overall polarization purity is >95%.
Optical energy storage and reemission based weak localization of light and accompanying random lasing action in disordered Nd3+ doped (Pb, La)(Zr, Ti)O3 ceramics116(2014); http://dx.doi.org/10.1063/1.4892873View Description Hide Description
Multi-mode random lasing action and weak localization of light were evidenced and studied in normally transparent but disordered Nd3+ doped (Pb,La)(Zr,Ti)O3 ceramics. Noticeable localized zone and multi-photon process were observed under strong pumping power. A tentative phenomenological physical picture was proposed by taking account of diffusive process, photo-induced scattering, and optical energy storage process as dominant factors in elucidating the weak localization of light observed. Both the decreased transmittance (increased reflectivity) of light and the observed long lasting fading-off phenomenon supported the physical picture proposed by us.
116(2014); http://dx.doi.org/10.1063/1.4892158View Description Hide Description
We investigate the ultrafast absorption dynamics of fused silica irradiated by a single laser pulse in the context of micromachining applications. A -fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction.
116(2014); http://dx.doi.org/10.1063/1.4893176View Description Hide Description
Free carrier absorption in heavily doped silicon can have a significant impact on devices operating in the infrared. In the near infrared, the free carrier absorption process can compete with band to band absorption processes, thereby reducing the number of available photons to optoelectronic devices such as solar cells. In this work, we fabricate 18 heavily doped regions by phosphorus and boron diffusion into planar polished silicon wafers; the simple sample structure facilitates accurate and precise measurement of the free carrier absorptance. We measure and model reflectance and transmittance dispersion to arrive at a parameterisation for the free carrier absorption coefficient that applies in the wavelength range between 1000 and 1500 nm, and the range of dopant densities between ∼1018 and 3 × 1020 cm−3. Our measurements indicate that previously published parameterisations underestimate the free carrier absorptance in phosphorus diffusions. On the other hand, published parameterisations are generally consistent with our measurements and model for boron diffusions. Our new model is the first to be assigned uncertainty and is well-suited to routine device analysis.
Laser beam induced nanoscale spot through nonlinear “thick” samples: A multi-layer thin lens self-focusing model116(2014); http://dx.doi.org/10.1063/1.4892871View Description Hide Description
Self-focusing is a well-researched phenomenon. Nanoscale spots can be achieved through self-focusing, which is an alternative method for achieving high-density data storage, high-resolution light imaging, and maskless nanolithography. Several research groups have observed that self-focusing spots can be reduced to nanoscale levels via incident laser power manipulation. Self-focusing spots can be analyzed by solving the nonlinear Schrödinger equation and the finite difference time domain method. However, both procedures are complex and time-consuming. In the present work, a multi-layer thin-lens self-focusing model that considers diffraction effects and changes of refractive index along the radial and film thickness directions is proposed to analyze the self-focusing behavior and traveling process of light beams intuitively. The self-focusing behaviors of As2S3 are simulated, and results show that a nanoscale self-focusing spot with a radius of about can be formed at the bottom of nonlinear sample when the incident laser power exceeds . Our findings are basically consistent with experimental reports and provide a good method for analyzing and understanding the self-focusing process. An appropriate application schematic design is also provided.
The surface-plasmon-resonance and band bending effects on the photoluminescence enhancement of Ag-decorated ZnO nanorods116(2014); http://dx.doi.org/10.1063/1.4892874View Description Hide Description
Doping with noble metal nanoparticles (NPs) is an effective method to tune the photoluminescence (PL) properties of semiconductor materials. The mechanism is widely attributed to the localized surface-plasmon-resonance (SPR) effect of the metal NPs, while the contribution of potential barrier at the interface between metal and semiconductor is less discussed. Taking ZnO nanorods on fused silica substrates as a model system, we present a facile low-temperature fabrication method to derive Ag metal NPs coated ZnO nanorods. Great enhancement of the near-band-edge (NBE) ultraviolet emission of the ZnO nanorods was achieved by the surface decoration with Ag NPs, accompanied with a decrement of deep-level (DL) emission intensity. Since, the Ag NPs are active even at ambient conditions, spontaneous changes in PL properties have been observed with aging time. The influence of oxidation state of Ag on the contacting potential and subsequently on the PL properties of ZnO nanorods were studied by employing a subsequent thermal annealing process. The role of contacting behavior and the varied potential barrier are comprehensively discussed. It was demonstrated that both the SPR effect and band bending effect can enhance the NBE emission and suppress the DL emission. In the Ag-decorated ZnO nanorods, the SPR effect is the main reason for PL enhancement of the metallic silver, while the potential barrier enhancement is the main reason for the oxidized silver.
- Plasmas and Electrical Discharges
116(2014); http://dx.doi.org/10.1063/1.4892675View Description Hide Description
Tungsten plasmas induced by unipolar arcs were investigated using optical emission spectroscopy and imaging, and compared with laser-induced tungsten plasmas. The unipolar arcs were initiated in the linear-plasma simulator PISCES-A at UCSD under fusion relevant conditions. The electron temperature and density of the unipolar arc plasmas were in the range 0.5–0.7 eV and 0.7–2.0 × 1020 m−3, respectively, and increased with increasing negative bias voltage, but did not correlate with the surface temperature. In comparison, the electron temperature and density of the laser-induced plasmas were in the range 0.6–1.4 eV and 7 × 1019–1 × 1022 m−3, respectively.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Effects of rapid thermal annealing on properties of Ga-doped MgxZn1−xO films and Ga-doped MgxZn1−xO/AlGaN heterojunction diodes116(2014); http://dx.doi.org/10.1063/1.4892591View Description Hide Description
This study investigated the thermal annealing effects of Ga-doped Mg x Zn 1−xO (GMZO) films and GMZO/AlGaN heterojunction diodes. GMZO films were deposited using a radio-frequency magnetron sputtering system with a 4-in. ZnO/MgO/Ga2O3 target. In addition, the Hall results, X-ray diffraction, transparent performance, and X-ray photoelectron spectroscopy (XPS) spectra were measured. The as-grown GMZO film deposited in this study exhibited a high transparency with transmittances over 95% in the visible region (360–700 nm) and a sharp absorption edge in the UV region (275–350 nm). The phenomenon of phase separation in the GMZO films was investigated based on the XPS spectra, revealing that an increase in the O-Zn signal accompanied a decline in the O-Ga signal after the thermal annealing. Moreover, the current-voltage (I-V) characteristics of the GMZO/AlGaN n-p junction diodes were examined at different annealing temperatures. The light emission derived from the forward-biased junction and near-ultraviolet (near-UV) light emission was evident at all p-n junctions. The n-GMZO/p-AlGaN diode annealed at 800 °C exhibited a brighter near-UV emission compared with the other diodes. In addition, the spectrum of diode annealed at 800 °C exhibited a broad peak at 474 nm (2.62 eV) and a tail of the emission spectrum extending to 850 nm. Based on these findings, the GMZO films are suitable for forming transparent contact layers in optoelectronic devices, and the n-GMZO/p-AlGaN junction diode is a feasible alternative in near-UV light emission devices.
Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties116(2014); http://dx.doi.org/10.1063/1.4892618View Description Hide Description
The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.
Recovery of Biot's transition frequency of air-saturated poroelastic media using vibroacoustic spectroscopy116(2014); http://dx.doi.org/10.1063/1.4892627View Description Hide Description
The transition frequency marks the passage from low-frequency viscosity dominated flow to high-frequency inertia dominated one in porous media. It was one of the principal characteristics predicted by Biot's theory. The transition frequency has been a theoretical concept for which only theoretical expressions have been developed in recent years. A vibroacoustic spectroscopy experimental method to recover the characteristic frequency (fC) and for gaining insight into the frequency response of fluid-saturated porous materials has been developed. Long thin air-saturated porous rods solicited mechanically are employed for the experiment. Changes in the fluid flow profile with excitation frequency results in relative motion between the skeleton and the saturating-fluid. This enhances the frictional viscous forces, which, in turn, increases damping of the skeletal motion. These transitions are signaled by observable cues in the acquired laser-vibrometry spectrum of the rods' longitudinal vibration mode patterns. The resonance peaks exhibit sudden attenuation (increase in damping) and are interrupted at the transition frequencies evoking a change of propagation medium. These patterns are compared with those of two plains, single phase material (viscoelastic) rods whose modes stand out as regularly spaced moderately damped peaks.
Experimental observations on the links between surface perturbation parameters and shock-induced mass ejection116(2014); http://dx.doi.org/10.1063/1.4891449View Description Hide Description
We have assembled together our ejecta measurements from explosively shocked tin acquired over a period of about ten years. The tin was cast at 0.99995 purity, and all of the tin targets or samples were shocked to loading pressures of about 27 GPa, allowing meaningful comparisons. The collected data are markedly consistent, and because the total ejected mass scales linearly with the perturbations amplitudes they can be used to estimate how much total Sn mass will be ejected from explosively shocked Sn, at similar loading pressures, based on the surface perturbation parameters of wavelength and amplitude. Most of the data were collected from periodic isosceles shapes that approximate sinusoidal perturbations. Importantly, however, we find that not all periodic perturbations behave similarly. For example, we observed that sawtooth (right triangular) perturbations eject more mass than an isosceles perturbation of similar depth and wavelength, demonstrating that masses ejected from irregular shaped perturbations cannot be normalized to the cross-sectional areas of the perturbations.
Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices116(2014); http://dx.doi.org/10.1063/1.4892407View Description Hide Description
We investigate point defects in the buffer layers CdS and ZnS that may arise from intermixing with Cu(In,Ga)(S,Se)2 (CIGS) or Cu 2ZnSn(S,Se)4 (CZTS) absorber layers in thin-film photovoltaics. Using hybrid functional calculations, we characterize the electrical and optical behavior of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities in the buffer. We find that In and Ga substituted on the cation site act as shallow donors in CdS and tend to enhance the prevailing n-type conductivity at the interface facilitated by Cd incorporation in CIGS, whereas they are deep donors in ZnS and will be less effective dopants. Substitutional In and Ga can favorably form complexes with cation vacancies (A-centers) which may contribute to the “red kink” effect observed in some CIGS-based devices. For CZTS absorbers, we find that Zn and Sn defects substituting on the buffer cation site are electrically inactive in n-type buffers and will not supplement the donor doping at the interface as in CIGS/CdS or ZnS devices. Sn may also preferentially incorporate on the S site as a deep acceptor in n-type ZnS, which suggests possible concerns with absorber-related interfacial compensation in CZTS devices with ZnS-derived buffers. Cu, Na, and K impurities are found to all have the same qualitative behavior, most favorably acting as compensating acceptors when substituting on the cation site. Our results suggest one beneficial role of K and Na incorporation in CIGS or CZTS devices is the partial passivation of vacancy-related centers in CdS and ZnS buffers, rendering them less effective interfacial hole traps and recombination centers.
116(2014); http://dx.doi.org/10.1063/1.4892624View Description Hide Description
Periodic composite materials have many promising applications due to their unique ability to control the propagation of waves. Here, we report the existence and frequency tunability of complete elastic wave band gaps in bio-inspired periodic composites with nacre-like, brick-and-mortar microstructure. Numerical results show that complete band gaps in these periodic composites derive from local resonances or Bragg scattering, depending on the lattice angle and the volume fraction of each phase in the composites. The investigation of elastic wave propagation in finite periodic composites validates the simulated complete band gaps and further reveals the mechanisms leading to complete band gaps. Moreover, our results indicate that the topological arrangement of the mineral platelets and changes of material properties can be utilized to tune the evolution of complete band gaps. Our finding provides new opportunities to design mechanically robust periodic composite materials for wave absorption under hostile environments, such as for deep water applications.
Generalized analytical solutions and breather for nonlinear acoustic wave propagation in molecular magnets116(2014); http://dx.doi.org/10.1063/1.4892631View Description Hide Description
In this paper, we show that, in the presence of two strong ac magnetic fields, a weak acoustic wave interacts with a crystal of molecular magnets. We obtain the analytical expression for absorption and group velocity in the linear case and discuss the effect of two coupling fields intensity on absorption and group velocity. Then, we find and analyze a series of generalized analytical solutions for nonlinear acoustic wave propagation in molecular magnets with arbitrary linear and nonlinear coefficients.
116(2014); http://dx.doi.org/10.1063/1.4892162View Description Hide Description
The efficacy of strained layer threading dislocation filter structures in single crystal epitaxial layers is evaluated using numerical modeling for (001) face-centred cubic materials, such as GaAs or Si1− x Ge x , and (0001) hexagonal materials such as GaN. We find that threading dislocation densities decay exponentially as a function of the strain relieved, irrespective of the fraction of threading dislocations that are mobile. Reactions between threading dislocations tend to produce a population that is a balanced mixture of mobile and sessile in (001) cubic materials. In contrast, mobile threading dislocations tend to be lost very rapidly in (0001) GaN, often with little or no reduction in the immobile dislocation density. The capture radius for threading dislocation interactions is estimated to be approximately 40 nm using cross section transmission electron microscopy of dislocation filtering structures in GaAs monolithically grown on Si. We find that the minimum threading dislocation density that can be obtained in any given structure is likely to be limited by kinetic effects to approximately 104–105 cm−2.
116(2014); http://dx.doi.org/10.1063/1.4893021View Description Hide Description
GaAs nanowires were grown on (111)B GaAs substrates using the vapour-liquid-solid mechanism. The Au/Pt nanodots used to catalyse wire growth were defined lithographically and had varying diameter and separation. An in-depth statistical analysis of the resulting nanowires, which had a cone-like shape, was carried out. This revealed that there were two categories of nanowire present, with differing height and tapering angle. The bimodal nature of wire shape was found to depend critically on the diameter of the Au-Ga droplet atop the nanowire. Transmission electron microscopy analysis also revealed that the density of stacking faults in the wires varied considerably between the two categories of wire. It is believed that the cause of the distinction in terms of shape and crystal structure is related to the contact angle between the droplet and the solid-liquid interface. The dependency of droplet diameter on contact angle is likely related to line-tension, which is a correction to Young's equation for the contact angle of a droplet upon a surface. The fact that contact angle may influence resulting wire structure and shape has important implications for the planning of growth conditions and the preparation of wires for use in proposed devices.
Suppression of thermal carrier escape and efficient photo-carrier generation by two-step photon absorption in InAs quantum dot intermediate-band solar cells using a dot-in-well structure116(2014); http://dx.doi.org/10.1063/1.4892826View Description Hide Description
We investigated the effects of an increase in the barrier height on the enhancement of the efficiency of two-step photo-excitation in InAs quantum dot (QD) solar cells with a dot-in-well structure. Thermal carrier escape of electrons pumped in QD states was drastically reduced by sandwiching InAs/GaAs QDs with a high potential barrier of Al0.3Ga0.7As. The thermal activation energy increased with the introduction of the barrier. The high potential barrier caused suppression of thermal carrier escape and helped realize a high electron density in the QD states. We observed efficient two-step photon absorption as a result of the high occupancy of the QD states at room temperature.
Optically and elastically assembled plasmonic nanoantennae for spatially resolved characterization of chemical composition in soft matter systems using surface enhanced spontaneous and stimulated Raman scattering116(2014); http://dx.doi.org/10.1063/1.4892932View Description Hide Description
We present a method to locally probe spatially varying chemical composition of soft matter systems by use of optically controlled and elastically self-assembled plasmonic nanoantennae. Disc-shaped metal particles with sharp irregular edges are optically trapped, manipulated, and assembled into small clusters to provide a strong enhancement of the Raman scattering signal coming from the sample regions around and in-between these particles. As the particles are reassembled and spatially translated by computer-controlled laser tweezers, we probe chemical composition as a function of spatial coordinates. This allows us to reliably detect tiny quantities of organic molecules, such as capping ligands present on various nanoparticles, as well as to probe chemical composition of the interior of liquid crystal defect cores that can be filled with, for example, polymer chains. The strong electromagnetic field enhancement of optically manipulated nanoparticles' rough surfaces is demonstrated in different forms of spectroscopy and microscopy, including enhanced spontaneous Raman scattering, coherent anti-Stokes Raman scattering, and stimulated Raman scattering imaging modes.