Index of content:
Volume 113, Issue 10, 14 March 2013
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4794017View Description Hide Description
The effect of laser-induced plasma confinement on lifetime and temperature is reported using cylindrical reflectors. This is determined experimentally in a gas cell, with hydrogen as test gas, and cylindrical shock reflectors of different diameters. The temperature evolution of confined and unconfined laser-induced plasma has been measured using plasma emission spectroscopy. Temperatures were determined through the plasma line-to-continuum thermometry technique in the hydrogen Balmer series using the and transitions at λ = 656 nm and λ = 486 nm, respectively. The experiments found that re-focusing the blast wave can significantly increase temperatures during the exponential decay of the plasma. The experimental results also show that confinement increases peak plasma temperatures, and that plasma lifetimes are only marginally affected by the confinement.
Study of sensitization process on mid-infrared uncooled PbSe photoconductive detectors leads to high detectivity113(2013); http://dx.doi.org/10.1063/1.4794492View Description Hide Description
For nearly a century, oxygen has been widely accepted as the key element that triggers photo-response in polycrystalline PbSe photoconductive detectors. Our photoluminescence and responsivity studies on PbSe samples, however, suggest that oxygen only serves as an effective sensitization improver and it is iodine rather than oxygen that plays the key role in triggering the photo-response. These studies shed light on the sensitization process for detector applications and ways to passivate defects in IV–VI semiconductors. As a result, high peak detectivity of 2.8 × 1010 cm·Hz1/2·W−1 was achieved at room temperature.
Holographic formation of compound photonic crystal and nano-antenna templates through laser interference113(2013); http://dx.doi.org/10.1063/1.4795119View Description Hide Description
This paper presents a holographic formation of compound photonic crystal and nano-antenna templates through a reflective optical element based laser interference. The reflective optical element consists of four Si facets where a circularly polarized single beam impinges at the Brewster angle and is reflected into four linearly s-polarized beams for the inference lithography. By tuning the phase delay in one of the interfering beams, dual-lattice photonic crystal, and nano-antenna templates are fabricated and compared with theoretic simulation. The design conditions for the nano-antenna formation are discussed.
Propagation of magnetically controllable lasers and magneto-optic dual switching using nitrogen-vacancy centers in diamond113(2013); http://dx.doi.org/10.1063/1.4795275View Description Hide Description
We study coherent laser-induced optical behaviors in weak twin-light propagation through strained diamond nitrogen-vacancy (NV) centers via switching on and off an external magnetic field. By numerically solving the coupled Bloch-Maxwell equations for NV center and field simultaneously in space and time, we address dynamic control of pulse propagation and magneto-optic dual switching in such a laser-driven NV center system. The proposed scheme may have applications in the design of magneto-optic switching and magneto-optic storage devices.
113(2013); http://dx.doi.org/10.1063/1.4794813View Description Hide Description
Direct frequency comb spectroscopy is currently one of the most precise techniques for studying the internal structure of atomic and molecular systems. In this technique, a train of ultrafast laser pulses excites states in the target system which then relax, emitting fluorescence. The measured fluorescence is then plotted as a function of the comb parameters. But according to recent theory, the ultrashort pulses from the comb laser can also significantly ionize the target. Here, we test this theory by measuring the ion signal from direct frequency comb spectroscopy. Furthermore, instead of actively controlling the frequency comb parameters, we allow them to drift passively, measuring them and the ion signal simultaneously. The experiments were found to be in satisfactory agreement with theory, and the passive comb approach was found to be functional, though not as convenient as the conventional actively locked comb.
A novel way to improve the quantum efficiency of silicon light-emitting diode in a standard silicon complementary metal–oxide–semiconductor technology113(2013); http://dx.doi.org/10.1063/1.4795170View Description Hide Description
Silicon diode at avalanche breakdown has visible light emission in the depletion region. It is believed that this optical radiation comes from the kinetic energy loss of carriers generated by impact ionization colliding with immobile charge centers in the avalanche region. A theoretical model is presented to show the correlation of the hot carrier effect with the related photonic emission in high field. Meanwhile, a PMOSFET-like silicon light source device fabricated completely in the standard silicon CMOS process technology is measured to demonstrate that avalanching current is linearly proportional to optical emission power whether this light source acts as a two-terminal device (i.e., diode, the “p+ Source/Drain to n-Substrate junction” with floating the gate) or acts as a three-terminal device (i.e., gate-diode, the “p+ Source/Drain to n-Substrate junction” in the course of varying the gate voltage). Such linearity implies that control of the increasing current is a significant way to enhance the quantum efficiency of this light source device no matter what the physical structure (i.e., two terminals or three terminals) of this device is. For the first time, it has been discovered that, at the same avalanching current, the optical output power in gate-diode structure is higher than the optical output power in diode structure. In other words, for this PMOSFET-like device, the three-terminal operating mode is more efficient than the two-terminal operating mode.
Laser direct writing of GaN-based light-emitting diodes—The suitable laser source for mesa definition113(2013); http://dx.doi.org/10.1063/1.4794844View Description Hide Description
The development of a process chain allowing for rapid prototyping of GaN-based light-emitting diodes (LEDs) is presented, which does not rely on photolithography. Structuring of the epitaxial layers is realized by direct-writing laser ablation, allowing a flexible chip layout that can be changed rapidly and at low cost. Besides contact metallization and trench formation, mesa definition is the most critical processing step. For mesa formation and to expose the n-GaN contact layer, the epitaxial grown p-GaN layer together with the active region has to be removed completely without forming cracks or crystal defects in the n-GaN layer or the mesa sidewalls, which would cause sidewall leakage currents. In developing an appropriate laser ablation process that meets these requirements, three different laser systems have been employed in a comparative study. These are a frequency-tripled picosecond (ps) Nd:YVO4 laser emitting at a wavelength of 355 nm and a pulse length of 10 ps and two 20 nanosecond (ns) pulse length laser systems, operating at a wavelength of 248 nm (Excimer laser) and 355 nm (frequency-tripled Nd:YVO4 laser), respectively. First, the laser sources are compared regarding the morphological properties of the resulting laser trenches. Due to band filling effects resulting in optical bleaching of the GaN material when irradiating with ps-laser pulses at 355 nm, the resulting ablation process suffers from cracking. Laser ablation using ns-pulses at both 355 nm and 248 nm leads to crack-free material removal up to a well-defined depth. To keep reverse-bias leakage currents at a level comparable to that of conventional dry-etched mesa-LEDs, subsequent wet etching is essential to remove residues in the mesa-trenches irrespective of the laser source used. Besides wet etching, an additional annealing step has to be applied to mesa-trenches fabricated using ns- and ps-laser pulses at a wavelength of 355 nm. Due to the larger penetration depth at 355 nm, defects causing an increased leakage current are generated in the quantum well region during laser irradiation, which manifest themselves by a spectrally broad defect luminescence. To separate and quantify the contributions from the mesa sidewall leakage and from the areal leakage through the p-n-junction, a series of LEDs have been fabricated and analyzed for which the mesa perimeter length was varied while keeping the mesa area constant. In this way, it is shown that laser ablation with ns-pulses at 248 nm results in the lowest sidewall leakage current as well as forward bias voltage-current and output power-current characteristics very similar to those of conventionally dry etched reference LEDs.
Large electroluminescence excitation cross section and strong potential gain of erbium in ErYb silicate113(2013); http://dx.doi.org/10.1063/1.4795153View Description Hide Description
1.53 μm electroluminescence of erbium was observed in ErYb silicate metal-insulator-silicon light emitting device (MISLED). The erbium ions were excited by direct impact of hot carriers at electric field higher than 7 MV/cm. The conduction and carrier injection mechanisms were investigated. The impact excitation cross section of Er ions was measured to be 3 × 10−14 cm2, three times of Er-doped SiO2 MISLED. Rate equation modeling suggests that strong potential material's gain of more than 23 dB/cm from the ErYb silicate may be achieved by electrical pumping.
- Plasmas and Electrical Discharges
Characterization of the supersonic flowing microwave discharge using two dimensional plasma tomography113(2013); http://dx.doi.org/10.1063/1.4793494View Description Hide Description
A tomographic numerical method based on the two-dimensional Radon formula for a cylindrical cavity has been employed for obtaining spatial distributions of the argon excited levels. The spectroscopy measurements were taken at different positions and directions to observe populations of excited species in the plasmoid region and the corresponding excitation temperatures. Excited argon states are concentrated near the tube walls, thus, confirming the assumption that the post discharge plasma is dominantly sustained by travelling surface wave. An automated optical measurement system has been developed for reconstruction of local plasma parameters of the plasmoid structure formed in an argon supersonic flowing microwave discharge. The system carries out angle and distance measurements using a rotating, flat mirror, as well as two high precision stepper motors operated by a microcontroller-based system and several sensors for precise feedback control.
Characterization of nanosecond pulse driven dielectric barrier discharge plasma actuators for aerodynamic flow control113(2013); http://dx.doi.org/10.1063/1.4794507View Description Hide Description
Positive polarity nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators are studied experimentally in quiescent atmosphere. Pulse energy and instantaneous pulse power (hereafter referred to as energy and power) are calculated using simultaneous voltage and current measurements. Electrical characteristics are evaluated as a function of peak voltage, pulse frequency, discharge length, and dielectric thickness. Schlieren imaging is used to provide a relative estimate of discharge energy that is coupled to the near surface gas as heat for the same parameters. Characteristics of the DBD load have a substantial effect on the individual voltage and current traces which are reflected in the energy and power values. Power is mainly dependent on actuator length which is inconsistent with schlieren data as expected. Higher per unit length energy indicates a stronger compression wave for a given actuator geometry, but this is not universally true across different actuators suggesting some constructions more efficiently couple energy to the gas. Energy and compression wave strength are linearly related. Higher pulse frequency produces higher energy but is primarily attributed to heating of the actuator and power supply components and not to an optimal discharge frequency. Both energy and wave strength increase as peak voltage to the power of approximately 3.5 over a substantial range similar to ac-DBD plasma actuators.
113(2013); http://dx.doi.org/10.1063/1.4795148View Description Hide Description
Supersonic vehicles are surrounded by a plasma layer which produces a cutoff layer for electromagnetic waves. Methods to remove the layer by gas releases, dc magnetic fields, and flows have been proposed earlier. The present work suggests a new approach which is based on laboratory observations. Ions are expelled by a time varying magnetic field which creates a Hall electric field. The ion expulsion opens up a window of transparency for wave communications.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Temperature and pressure dependent geometry optimization and elastic constant calculations for arbitrary symmetry crystals: Applications to MgSiO3 perovskites113(2013); http://dx.doi.org/10.1063/1.4794360View Description Hide Description
To optimize lattice parameters for arbitrary symmetry crystals under high temperature and high pressure conditions, a new “geometry optimization method for arbitrary symmetry crystals” has been proposed in this work. By minimizing non-equilibrium Gibbs energy functions for a series of deformed crystal configurations, the components of deformation tensors have been obtained, and allowing the optimized crystal lattice parameters to be determined. Based on our method and Zhao's method, a new method for calculating high temperature and high pressure elastic constants in arbitrary symmetry crystals has been deduced. To verify the effectiveness of the new method, the high temperature and high pressure lattice parameters and elastic constants of orthorhombic symmetry MgSiO3 pv have been studied, and a good agreement between calculated and experimental results has been obtained.
Internal friction associated with the premartensitic transformation and twin boundary motion of Ni50+ x Mn25− x Ga25 (x = 0−2) alloys113(2013); http://dx.doi.org/10.1063/1.4794509View Description Hide Description
The internal friction (IF) during the martensitic transformation (MT) has been extensively studied in NiMnGa alloys. In this paper, temperature dependence of the IF associated with the premartensitic transformation (PMT) and twin boundary motion (TBM) was investigated in Ni 50+ x Mn25− x Ga25 (x = 0−2) alloys. Both the composition and frequency had no obvious effect on the IF peak position of the PMT and TBM. With increasing frequency from 0.1 Hz to 5 Hz, the IF peak height corresponding to the TBM was significantly decreased, but was kept constant during the PMT. The observed phenomena were discussed in terms of the different microscopic mechanisms of the TBM and PMT.
Shape memory metamaterials with tunable thermo-mechanical response via hetero-epitaxial integration: A molecular dynamics study113(2013); http://dx.doi.org/10.1063/1.4794819View Description Hide Description
We show that nanoscale epitaxial superlattices (SLs) can be used to engineer the energy landscape that governs the martensitic transformation in shape memory alloys and tune their thermo-mechanical response. We demonstrate the approach using large-scale molecular dynamics simulations of a SL material consisting of alternate layers of a shape memory Ni-rich NiAl alloy and NiAl B2 alloy. The non-martensitic NiAl alloy was chosen to reduce the energy barrier that separates the martensite and austenite phases of the SL and its incorporation leads to a reduction in the thermal hysteresis of the transition. This is a desirable feature in applications involving actuation and our approach represents a generally applicable and powerful avenue to engineer desired behavior in mechanically active materials.
113(2013); http://dx.doi.org/10.1063/1.4794094View Description Hide Description
Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm−3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence.
Effects of oxidation on reaction front instabilities and average propagation speed in Ni/Ti multilayer foils113(2013); http://dx.doi.org/10.1063/1.4794183View Description Hide Description
Vapor-deposited, equiatomic Ni/Ti multilayer foils exhibit low-speed, self-propagating formation reactions that are characterized by a spin-like reaction front instability. In addition to the intermetallic reaction between Ni and Ti, reactions performed in air can also exhibit a discrete combustion wave associated with the oxidation of Ti. In general, the oxidation wave trails the complex intermetallic reaction front. Multilayers that have a large reactant layer periodicity (≥200 nm) exhibit a decrease in net reaction speed as air pressure is reduced. Oxidation has a much smaller effect on the net propagation speed of multilayers with small layer periodicity (<100 nm). The net propagation speed of the multilayers is increased when air is present, due to the added energy release of Ti oxidation. High-speed optical microscopy shows that the increased front speed is associated with an increased nucleation rate of the reaction bands that typify the spinning reaction instability of the Ni/Ti system.
113(2013); http://dx.doi.org/10.1063/1.4794002View Description Hide Description
The influence of shock-wave-loading profile on the failure processes in a brittle material has been investigated. Tungsten heavy alloy (WHA) specimens have been subjected to two shock-wave loading profiles with a similar peak stress of 15.4 GPa but different pulse durations. Contrary to the strong dependence of strength on wave profile observed in ductile metals, for WHA, specimens subjected to different loading profiles exhibited similar spall strength and damage evolution morphology. Post-mortem examination of recovered samples revealed that dynamic failure for both loading profiles is dominated by brittle cleavage fracture, with additional energy dissipation through crack branching in the more brittle tungsten particles. Overall, in this brittle material, all relevant damage kinetics and the spall strength are shown to be dominated by the shock peak stress, independent of pulse duration.
113(2013); http://dx.doi.org/10.1063/1.4793282View Description Hide Description
A series of shock compression experiments on hexagonal α-Ce3 Al have been carried out using a two-stage light gas gun. No phase transition was observed in the recovered sample shock compressed at 23.5 GPa. However, as the shock pressure was increased to 27.3 GPa, a face-centered cubic Ce3 Al phase was detected in the samples recovered at ambient conditions. Furthermore, a Ce2 Al phase was found in the 37.1 GPa shocked sample with a space group Fd-3m and lattice parameter a = 8.26(1) Å. These Ce-based alloys may have potential industrial applications due to the heavy-fermion related properties.
Origin of ultrafast Ag radiotracer diffusion in shear bands of deformed bulk metallic glass Pd40Ni40P20113(2013); http://dx.doi.org/10.1063/1.4795260View Description Hide Description
Measurements of Ag radiotracer diffusion in shear bands of deformed bulk metallic glass, Pd40 Ni 40P20 [Bokeloh et al., Phys. Rev. Lett. 107, 235503 (2011)], have found a colossal enhancement of diffusion coefficient by more than eight orders of magnitude than in undeformed Pd40 Ni 40P20. Suggestion was made by Bokeloh et al. that enhanced diffusion occurs in high-mobility pathways originating from some excess free volume distribution inside the shear bands. Although plausible, this qualitative suggestion does not allow quantitative calculation of the enhancement. The impasse is avoided by using the coupling model to calculate the maximum of the enhancement of diffusivity possible in high-mobility pathways of the shear bands. Within the range of eight to ten orders of magnitude, the calculated maximum enhancement is capable to account for the experimental observation.
113(2013); http://dx.doi.org/10.1063/1.4795261View Description Hide Description
Comprehensive characterization of defects formed in bulk ZnO single crystals co-implanted with N and Zn as well as N and O atoms is performed by means of optically detected magnetic resonance (ODMR) complemented by Raman and photoluminescence (PL) spectroscopies. It is shown that in addition to intrinsic defects such as Zn vacancies and Zn interstitials, several N-related defects are formed in the implanted ZnO. The prevailed configuration of the defects is found to depend on the choices of the co-implants and also the chosen annealing ambient. Specifically, co-implantation with O leads to the formation of (i) defects responsible for local vibrational modes at 277, 511, and 581 cm−1; (ii) a N-related acceptor with the binding energy of 160 ± 40 meV that is involved in the donor-acceptor pair emission at 3.23 eV; and (iii) a deep donor and a deep NO acceptor revealed from ODMR. Activation of the latter defects is found to require post-implantation annealing in nitrogen ambient. None of these defects are detected when N is co-implanted with Zn. Under these conditions, the dominant N-induced defects include a deep center responsible for the 3.3128 eV PL line, as well as an acceptor center of unknown origin revealed by ODMR. Formation mechanisms of the studied defects and their role in carrier recombination are discussed.