Volume 102, Issue 12, 15 December 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
102(2007); http://dx.doi.org/10.1063/1.2817591View Description Hide Description
One knows the imaging system's properties are central to the correct interpretation of any image. In a scanning electron microscope regions of different composition generally interact in a highly nonlinear way during signal generation. Using Monte Carlo simulations we found that in resin-embedded, heavy metal-stained biological specimens staining is sufficiently dilute to allow an approximately linear treatment. We then mapped point-spread functions for backscattered-electron contrast, for primary energies of 3 and 7 keV and for different detector specifications. The point-spread functions are surprisingly well confined (both laterally and in depth) compared even to the distribution of only those scatteredelectrons that leave the sample again.
Time-resolved photoluminescence and optically stimulated luminescence measurements of picosecond-excited SrS:Ce,Sm phosphor102(2007); http://dx.doi.org/10.1063/1.2822474View Description Hide Description
Doubly activated alkaline-earth phosphors are known to present luminescent and charge-storage properties. In this work, we investigate the photoluminescence(PL) and the optically stimulated luminescence(OSL) of the SrS:Ce,Sm phosphor by means of time-resolved spectroscopy with excitation laser pulses of duration. By comparing the PLmeasurements obtained with direct UV excitation and the OSL experiments performed using infrared wavelength, it was possible to measure a charge conversion lifetime of about . A luminescence lifetime of has also been found. These measurements are presented and discussed in connection with applications where the speed of the SrS:Ce,Sm luminescence is a crucial parameter.
102(2007); http://dx.doi.org/10.1063/1.2825041View Description Hide Description
We present a detailed model for the evolution of resonance enhanced multiphoton ionization (REMPI) produced plasma during and after the ionizing laser pulse in inert gas (argon, as an example) at arbitrary pressures. Our theory includes the complete process of the REMPI plasma generation and losses, together with the changing gas thermodynamic parameters. The model shows that the plasma expansion follows a classical ambipolar diffusion and that gas heating results in a weak shock or acoustic wave. The gas becomes involved in the motion not only from the pressure gradient due to the heating, but also from the momentum transfer from the charged particles to gas atoms. The time dependence of the total number of electrons computed in theory matches closely with the results of coherent microwavescattering experiments.
102(2007); http://dx.doi.org/10.1063/1.2821151View Description Hide Description
The damaged region of a semiconductor laser diode that failed in a vacuum environment was analyzed using focused ion beam(FIB) serial sectioning, time-of-flight secondary ion mass spectrometry (ToF-SIMS), high resolution transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), energy dispersive x-ray spectroscopy (EDS), and nanodiffraction. The FIB nanotomography models and the TEM cross sections show a damage structure extending deep into the core and originating at the diode/antireflective (AR) coatinginterface. Nanocrystalline gold was detected at this interface using both TEM diffraction and EDS, and the localization of gold along the core at the diode/AR interface was corroborated using 3D ToF-SIMS. A thinning of the AR coating above the failure site was observed by TEM with a corresponding increase in carbon content on the AR surface detected with EELS. It is suggested that failure proceeded by pyrolysis of adsorbed hydrocarbons on the AR coating, which, in the presence of a high optical flux, contributed to carbothermal reduction of the AR coating. As the optical flux increased, thermal gradients facilitate metal migration, leading to larger gold clusters. These clusters are sites for deep level traps and may promote catalytic reactions.
102(2007); http://dx.doi.org/10.1063/1.2825403View Description Hide Description
The effect of laser beam operation mode on transition of grain morphology was studied during surface modification of alumina ceramic. A 4 kW HL 4006D continuous wave Nd:yttrium-aluminum-garnet(YAG) laser and a JK 701 pulsed Nd:YAG laser were employed to conduct surface modification of alumina. The laser beam operated in continuous wave mode generated faceted grain structure on the surface of the ceramic while the beam operated in pulsed mode produced nonfaceted grain structure. Such transition in grain morphology is mainly attributed to different levels of undercooling which is a function of cooling rates encountered in two different modes of laser operation.
102(2007); http://dx.doi.org/10.1063/1.2825636View Description Hide Description
Three dimensionally ordered photonic crystals are fabricated with rhodamine B dyed polystyrene colloidal spheres using the inward growing self-assembling method in less than 3 h. This avoids the difficulties due to infiltration of active materials into passive photonic crystals. The superior optical quality of the photonic crystalsfabricated using this method results in high reflectance values even at large angles of incidence. The study of emission characteristics on these functionalized photonic crystals shows a clear trend dependent on the angle of emission, wavelength, and the angle-resolved transmittance. The dip observed in the emission spectrum clearly matches the photonic stop band position at different angles of observation. The emission spectrum measured at different angles was found to follow a simple functional form related to the intrinsic emission of the dye and the stop band effect due to the photonic crystal environment.
102(2007); http://dx.doi.org/10.1063/1.2821789View Description Hide Description
The spectroscopic properties of dopedborosilicate glasses were investigated to assess their potential as gain media in the band. Analysis of the concentration and temperature dependences of the optical transitions from the level of was carried out. The emission lifetime of the level at was found to be and almost invariant for concentrations less than . Judd-Ofelt analysis was used to calculate the emission transition probabilities from the level, and the emission cross section at for the transition was at . Temperature dependent nonradiative relaxation of the level was observed in samples with 5 and . The quenching process of the level in the temperature range of might have been caused by a two-photon one-site resonant process, whereas it was dominated by a two-site nonresonant process at temperatures below . The results obtained indicate that dopedborosilicate glass is a promising candidate for optical gain media in the green band.
102(2007); http://dx.doi.org/10.1063/1.2825649View Description Hide Description
The mechanism by which is excited in the chemical oxygen-iodine laser was studied by means of emission spectroscopy. Using the intensity of the band as a reference, relative number densities were assessed by measuring the emission intensities. Vibrationally excited singlet oxygen molecules were detected using infrared emission spectroscopy. The measured relative density of for the conditions of a typical oxygen-iodine laser medium amounted to of the total content. Mechanisms for formation were proposed for both the dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine as an intermediate. It is proposed that in the dissociation zone the molecular iodine and states are populated in collisions with vibrationally excited singlet oxygen molecules , whereas in the downstream region of the dissociation zone those intermediate states are populated by the atomic iodine recombination process. is subsequently formed in collisions of with singlet oxygen. We also demonstrated that does not participate measurably in the dissociation process, and that energy transfer from does not excite to a significant degree.
Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence102(2007); http://dx.doi.org/10.1063/1.2825651View Description Hide Description
We have carried out detailed optical simulations of tandem solar cells based on the following organic semiconductors: poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-;3,4-]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), 1-(3- methoxycarbonyl) propyl-1-phenyl[6,6] , and 1-(3-methoxycarbonyl) propyl-1- phenyl[6,6] . We demonstrate that out of the many possible combinations of the component materials, one specific combination emerges as the best to reduce the spectral overlap of the two bulk heterojunction blends and thereby to ensure an optimized short-circuit current density. Furthermore, the calculations allow us to predict the maximum achievable in tandem cells based on P3HT and PCPDTBT. Finally, we show that the efficient tandem cell realized and described recently by Kim et al. [Science317, 222 (2007)] ensures balanced absorption in the top and bottoms cells.
Characterization of long-range surface plasmon-polariton in stripe waveguides using scanning near-field optical microscopy102(2007); http://dx.doi.org/10.1063/1.2826910View Description Hide Description
Propagation characteristics of long-range surface plasmon-polariton (LRSPP) guiding along thin gold stripes embedded in polymer cover layers are investigated by scanning near-field optical microscopy (SNOM). It is shown that the characterization of samples with cover layers up to is feasible in the optical communication wavelength range. We found that the spatial dimension of the optical signal is directly related to the geometrical dimension of the guiding layer, and the light collected by the SNOM is scattered light from the surface and not the evanescent field. We also discuss the limitations of the SNOM technique for the characterization of LRSPP modes.
102(2007); http://dx.doi.org/10.1063/1.2828153View Description Hide Description
In this article, we report an optical method for the nanogap measurements beyond optical diffraction limit of a half of wavelength. This method is derived from Fresnel’s equations of the traditional Kretschmann-Raether configuration based on surface plasmon resonance. Numerical simulation shows that a decrease of air gap width to be smaller than a half of wavelength of incident light will reveal a larger resonance angle shift. The resonance angle monotonically shifts from 45° to 50° by reducing the air gap width around from . The measured smallest air gap width is about by fitting the simulated curves to the experimental data. This gap measurement method provides a simple and real-time measurement system beyond the physical diffraction limit of a half of wavelength.
- PLASMAS AND ELECTRICAL DISCHARGES
102(2007); http://dx.doi.org/10.1063/1.2822467View Description Hide Description
A large amplitude surfaceplasma wave (SPW), propagating over a semiconductor-free space interface, is susceptible to filamentation instability. A small perturbation in the amplitude of the SPW across the direction of propagation exerts a ponderomotive force on free electrons and holes, causing spatial modulation in free carrier density and hence the effective permittivity of the semiconductor. The regions with higher attract more power from the nieghborhood, leading to the growth of the perturbation. The growth rate increases with the intensity of the surface wave. It decreases with the frequency of the SPW.
102(2007); http://dx.doi.org/10.1063/1.2822338View Description Hide Description
Gas temperature and convective velocity distributions are presented for an unconfined glow discharge in air at atmospheric pressure, with electric currents ranging between 30 and 92 mA. The vertically oriented discharge was formed between a pin anode (top) and an extended cathode. The temperature and velocity profiles were measured using laser-induced Rayleigh scattering and laser Doppler anemometry techniques, respectively. The temperature field exhibited a conical shape with the radius of hot temperature zone increasing toward the anode. A maximum temperature of 2470 K was observed on the discharge axis with the discharge current of 92 mA. Air velocity measurements around the discharge demonstrated that the shape and magnitude of the temperature field are strongly affected by natural convection. Estimates indicate that convective losses may account for more than 50% of the power input into the positive column of the discharge. The measuredtemperature fields and convective velocity profiles provide a set of data that is important for the evaluation of dc atmospheric glow discharges in various applications such as sound manipulation and acoustic noise mitigation.
Effective hard x-ray spectrum of a tabletop Mather-type plasma focus optimized for flash radiography of metallic objects102(2007); http://dx.doi.org/10.1063/1.2822449View Description Hide Description
The effective spectrum of the hard x-ray output of a Mather-type tabletop plasma focusdevice was determined from attenuation data on metallic samples using commercial radiographic film coupled to a phosphor intensifier screen. It was found that the radiation has relevant spectral components in the 40–150 keV range, with a single maximum around . The radiation output allows for 50 ns resolution, good contrast, and introspective imaging of metallic objects even through metallic walls. A numerical estimation of the induced voltage on the focus during the compressional stage is briefly discussed.
Effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster102(2007); http://dx.doi.org/10.1063/1.2822456View Description Hide Description
The effects of magnetic field configuration on thrust performance in a miniature microwavedischargeion thruster were investigated in order to improve thrust performance. First, the extracted ion beam current was measured for various levels of strength of the magnetic field. It was found that there is an optimum magnitude of the magnetic field. That this is due to the tradeoff between magnetic mirror confinement and microwave-plasma coupling was confirmed by measurement of the ion saturation current into the antenna of the ion thruster. The ion saturation current was found to decrease with an increase in magnetic field strength, due to the improvement in magnetic mirror confinement. The estimated electron temperature also decreases with an increase in magnetic field strength. This result shows that the increase in magnetic field strength leads to a decrease in microwave-plasma coupling. Next, the ion beam current for three magnetic field shapes was measured by changing the length of the central yoke. The results show that the optimum magnetic field shape depends on the mass flow rate because of the tradeoff between magnetic confinement and ionization probability. For the configurations tested, the 3 mm length central yoke is optimal for low mass flow, whereas 7 mm is the best for high mass flow. Overall, the extracted ion beam current is 21.4 mA, at a xenon mass flow rate of 0.036 mg/s, beam voltage of 1500 V, and incident microwave power of 16 W.
102(2007); http://dx.doi.org/10.1063/1.2825625View Description Hide Description
The effects of the magnetic gradient on the ion beam current in an end Hall-type ion source with a magnetic mirror field are investigated. In a cylindrical Hall ion source in which a cylindrical magnetic ring other than a regular magnetic pole is shortened and centrally inserted, a mirror magnetic field profile can be formed around the annular anode. A positive-negative variable magnetic gradient is shown experimentally to enhance ionization; the ionization efficiency is substantially affected by the different magnetic gradient. The high ionization results in efficiency in the conversion of discharge current to ion beam current. The experimental results and interpretation of the effects are presented in this paper.
102(2007); http://dx.doi.org/10.1063/1.2822450View Description Hide Description
The plume dynamics in the presence of an ambient gas is very intriguing physics. The expansion of a laser-produced plasma in the presence of an ambient gas leads to internal plume structures, plume splitting, sharpening, confinement, etc. We investigated propagation dynamics of an expanding tin plume for various spot sizes using a fast visible plume imaging and Faraday cup diagnostic tools. Our results indicate that the sharpening of the plume depends strongly on the spot size. With a smaller spot size, the lateral expansion is found to be higher and the plume expansion is spherical while with a larger spot size the plume expansion is more cylindrical. Analysis of time resolved imaging also showed internal structures inside the plume.
102(2007); http://dx.doi.org/10.1063/1.2822471View Description Hide Description
This paper studies the space-charge-limited current in an infinite wedge geometry in two dimensions. This geometry permits a reduction of the problem to a set of easily solved ordinary differential equations. The system, though very simplified, exhibits features similar to those expected to occur in many realistic systems with inhomogeneous electric fields. We obtain, in particular, a universal form for the particle trajectories and a nonmonotone charge distribution with accumulation at both the cathode and the anode. The explicit solution of the model can be useful for testing numerical schemes. The case of a very low density current is also considered. Relaxation of the geometrical limitations of the model are studied using conformal mapping techniques. Possible applications to realistic systems, which can be tested by simple experiments, are presented.
Effects of plasma loss on the luminous efficacy in a full high definition alternating current plasma display panel102(2007); http://dx.doi.org/10.1063/1.2825402View Description Hide Description
We have investigated the effect of cell resolution change on the luminous efficacy of an alternating current plasma display panel with the three-dimensional plasma simulation. The luminous efficacy decreased and the minimum sustain voltage increased as the resolution increased from VGA to full high definition (HD) resolution, due to the increased particle loss to the enclosing surfaces. A simple increase of the Xe content to realize the high luminous efficacy resulted in a much enhanced luminous efficacy decremental ratio as the cell resolution increases because of the increased plasma loss and decreased electron heating efficacy. We are proposing some effective ways of realizing high luminous efficacy in the high resolution 50-in. full HD cell, such as the increase of the sustain electrode gap, the adoption of a segmented electrode in delta color arrayed, enclosed subpixel, type cell with 4:3 aspect ratio, and the optimization of the barrier rib height. Because the adoption of those geometrical parameters causes the reduced plasma loss, the transport efficacy of visible light from phosphor through the front plate in a full HD cell resulted in much increased improvement with the increase of Xe content.
102(2007); http://dx.doi.org/10.1063/1.2822457View Description Hide Description
Investigations on the dynamics of virtual cathode oscillation are performed with a reflex triode virtual cathode oscillator powered by a 450 kV, 400 ns voltage pulse. Based on the diode impedance changes, the dynamics of virtual cathode oscillation are discussed. Four ensuing processes, corresponding to different stages of microwave emission, exist in the temporal behavior of diode impedance changes. When the turning point of diode impedance forms, the beam current develops into the parapotential flow. The process of microwave emission can be described by diode impedance changes. To confirm this mechanism, self-pinching of the beam current and current-voltage characteristics are introduced. This mechanism is independent of the cathode material and structure. When the experiments were carried out with a carbon fiber cathode, the 300 MW, 180 ns microwave emission was obtained, the beam current density reached level, and the plasma expansion rate was . These results show that the carbon fiber cathode is a potential candidate for high-power microwave sources.