Spatial Emission Characteristics Research Articles

Spatial characteristics of Kalpha x-ray emission from relativistic femtosecond laser plasmas.

The spatial structure of the Kalpha emission from Ti targets irradiated with a high intensity femtosecond laser has been studied using a two-dimensional monochromatic imaging technique. For laser intensities I<5 x 10(17) W/cm(2), the observed spatial structure of the Kalpha emission can be explained by the scattering of the hot electrons inside the solid with the help of a hybrid particle-in-cell/Monte Carlo model. By contrast, at the maximum laser intensity I=7 x 10(18) W/cm(2) the half-width of the Kalpha emission was 70 microm compared to a laser-focus half-width of 3 microm. Moreover, the main Kalpha peak was surrounded by a halo of weak Kalpha emission with a diameter of 400 microm and the Kalpha intensity at the source center did not increase with increasing laser intensity. These three features point to the existence of strong self-induced fields, which redirect the hot electrons over the target surface.

Improved coherent terahertz emission by modification of the dielectric environment

We present a generally applicable approach to enhance the conversion efficiency from optical input to usable coherent output power for THz emitters based on optically excited charge carriers. Guided by numerical simulations, the dielectric environment of the emitter is modified to improve radiation rate and spatial emission characteristics. The modifications comprise a sapphire layer applied on the semiconductor surface and a gold backside metallization. Comparison between a standard InGaAs semiconductor surface field emitter and an optimized version yields an increase of one order of magnitude in coherently emitted THz power. This result is in close agreement with the numerical simulations.

Ion acceleration with ultrafast lasers

Hot-electron confinement can build up fields capable of accelerating ions up to MeV energies when an ultrashort 35-fs laser pulse at ∼2×1018 W/cm2 interacts with a small spherical target. Singly charged ions with different masses have similar energies. A simple phenomenological model describes how ultrashort and less-energy-consumptive pulses drive ions to MeV energies. The energetic and spatial-emission characteristics of protons, deuterons and oxygen ions released from water and heavy-water droplets of ∼15 μm in size was determined for this interaction scenario.

Experimental and Theoretical Characterization of Sonochemical Cells. Part 1. Cylindrical Reactors and Their Use to Calculate the Speed of Sound in Aqueous Solutions

The sound field generated from a cylindrical sonochemical cell is discussed in depth. This sound field is considered with a variable operating frequency. An acoustic model is developed to predict the spatial distribution of acoustic pressure, which is compared with experimental measurements of the spatial characteristics of luminescent emission from a cylindrical cell. A sound speed (averaged in space and time) within the cavitation environment in the range of 868−1063 m s-1 was calculated. Under certain assumptions, this corresponds to a void fraction range of 2.9 × 10-3 to 4.2 × 10-3%. The implications for electrochemical and chemical activity within cylindrical cells are discussed.

Propagation of femtosecond light pulses through near-field optical aperture probes

The propagation of femtosecond light pulses through near-field optical fiber tips has been modelled numerically in three spatial dimensions by means of the finite integration technique. Ideally conducting as well as real metallic coatings of the tip have been considered, and the influence of surface plasmon polaritons (SPP) on shape, spectrum, and amplitude of the light pulse in the near and far fields of the tip have been investigated in this way. Special attention has been devoted to the superluminal tunneling of light through the fiber tip. The variation of phase and group velocities along the fiber axis has been characterized for a number of real metals and for different tip angles. A maximum of both velocities in the near field of the tip is characteristic for coatings of finite conductivity. For some tip angles negative values of the phase and/or group velocities are observed, which are caused by the propagation of SPP on the outer surface of the coating and their conversion into photons. It is shown, that the excitation of SPP on the metallic coating leads to strongly altered spatial emission characteristics of the tip.

A pulse-train laser driven XUV source for picosecond pump–probe experiments in the water window

We describe the development of a table top soft X-ray source based on a laser produced plasma. The plasma is produced by a Nd:YLF pulse-train laser which delivers pulse trains of up to 400 pulses each of about 25 ps duration. Number and energy of the micropulses are fully computer controlled adjustable for an optimum interaction with a rotating solid target. Spectral, spatial and temporal characteristics of the X-ray emission from Mylar and copper targets have been studied. A calibrated back-illuminated CCD camera together with a slit grating spectrograph were utilized for the measurement of the absolute soft X-ray photon fluxes. The temporal measurements were performed using a streak camera while the plasma size was measured with a zone plate. An application of the system in a X-ray absorption experiment is shown. The advantages of the system in view of possible optical pump and soft X-ray probe experiments will be discussed.

Interaction of atomic wave packets with four-wave mixing:?detection of rubidium and potassium wave packets by coherent ultraviolet emission

The observation of an atomic wave packet by use of a coherent, nonlinear-optical process is reported. Wave packets formed in K or Rb vapor by two-photon excitation of ns and (n-2)dstates (n=8 for K; n=11 , 12 for Rb) with red (~620-nm) , 80-100-fs pulses were detected by four-wave mixing in pump-probe experiments. The temporal behavior of the wave packet is observed by monitoring the coherent UV radiation generated near the alkali mp(2)P? (2)S(1/2) (7</=m</=12 for Rb; 5</=m</=7 for K) resonance transitions when a probe pulse is scattered by the wave packet established by the earlier (identical) pump pulse. The spatial and spectral characteristics of the UV emission are well described by axially phase-matched four-wave mixing, and all the prominent frequency components of the wave packets are associated with energy differences between pairs of excited states for which Diota=0 or Diota=2 . These results demonstrate that the wave packet modulates chi((3))of the medium, thus rendering the wave packet detectable.

Modeling Light‐Extraction Characteristics of Packaged Light‐Emitting Diodes

We employ a Monte Carlo ray‐tracing technique to model light‐extraction characteristics of light‐emitting diodes. By relaxing restrictive assumptions on photon traversal history, our method improves upon available analytical models for estimating light‐extraction efficiencies from bare LED chips, and enhances modeling capabilities by realistically treating the various processes which photons can encounter in a packaged LED. Our method is not only capable of calculating extraction efficiencies, but can also provide extensive statistical information on photon extraction processes, and predict LED spatial emission characteristics.

Nonlinear reflectivity of an inhomogeneous plasma in the strongly damped regime

The nonlinear reflectivity of an inhomogeneous plasma slab in the strongly damped regime is investigated, taking into account the spatial and temporal characteristics of the thermal noise emission of waves. In the linear approximation, the spectral width corresponding to the frequencies that are effectively amplified is always found to be less than the spectral width of the unstable frequency domain. By conjecturing that this frequency filtering process remains valid in the nonlinear regime, the effective noise term appearing in Tang's formula [J. Appl. Phys. 37, 2945 (1966)] can be obtained analytically. The validity of this conjecture is numerically checked for different values of the inhomogeneity parameter. Conditions are given that must be satisfied for the validity of one-dimensional modeling of three-dimensional scattering.

Cathodoluminescence Studies of InGaN Quantum Wells

AbstractLow temperature cathodoluminescence has been used to investigate the spatial characteristics of light emission in InxGa1−xN single quantum wells. High spatial resolution, narrow band pass imaging shows the luminescence to be strongly inhomogeneous in wavelength as well as in intensity on a sub-micron scale. Cathodoluminescence spectra correlate favorably with photoluminescence spectra. However, when spectra are recorded from different areas in spot mode, the quantum emission varies significantly in wavelength. The observed variations are consistent with composition inhomogeneities in the quantum well.

Spatial and Temporal Characteristics of an Excimer Laser-Induced Lead Plasma Emission

Abstract A laser-induced plasma was generated from a lead target using an ArF excimer laser (λ = 193 nm) and characterized by time-resolved and time-integrated spatially resolved spectrometry. The ambient atmosphere (gas composition and pressure) influenced the emission intensity for both atomic and ionic lines. The emission of laser-induced lead plasma varied with time as well as the location in the plasma. Lead ion emission decayed more rapidly than lead atomic line emission. High excitation temperatures and nonlinear optical phenomena were observed in the laser-induced lead plasma. Gas breakdown and the subsequent shielding effect of the incident laser beam in different ambient gas compositions and pressures were discussed to explain the different efficiency of metal ion for matioa in the plasma. The experimental results show that spatial discrimination of the laser-induced plasma emission is desirable for direct spectrochemical analysis.

The crystal structure of the light-harvesting complex II (B800–850) from Rhodospirillum molischianum

Background: The light-harvesting complexes II (LH-2s) are integral membrane proteins that form ring-like structures, oligomers of α β-heterodimers, in the photosynthetic membranes of purple bacteria. They contain a large number of chromophores organized optimally for light absorption and rapid light energy migration. Recently, the structure of the nonameric LH-2 of Rhodopseudomonas acidophila has been determined; we report here the crystal structure of the octameric LH-2 from Rhodospirillum molischianum. The unveiling of similarities and differences in the architecture of these proteins may provide valuable insight into the efficient energy transfer mechanisms of bacterial photosynthesis. Results The crystal structure of LH-2 from Rs. molischianum has been determined by molecular replacement at 2.4 å resolution using X-ray diffraction. The crystal structure displays two concentric cylinders of sixteen membrane-spanning helical subunits, containing two rings of bacteriochlorophyll-a (BChl-a) molecules. One ring comprises sixteen B850 BChl-as perpendicular to the membrane plane and the other eight B800 BChl-as that are nearly parallel to the membrane plane; eight membrane-spanning lycopenes (the major carotenoid in this complex) stretch out between the B800 and B850 BChl-as. The B800 BChl-as to exhibit a different ligation from that of Rps. acidophila (aspartate is the Mg ligand as opposed to formyl-methionine in Rps. acidophila). Conclusion The light-harvesting complexes from different bacteria assume various ring sizes. In LH-2 of Rs. molischianum, the Q y transition dipole moments of neighboring B850 and B800 BChl-as are nearly parallel to each other, that is, they are optimally aligned for Förster exciton transfer. Dexter energy transfer between these chlorophylls is also possible through interactions mediated by lycopenes and B850 BChl-a phytyl tails; the B800 BChl-a and one of the two B850 BChl-as associated with each heterodimeric unit are in van der Waals distance to a lycopene, such that singlet and triplet energy transfer between lycopene and the BChl-as can occur by the Dexter mechanism. The ring structure of the B850 BChl-as is optimal for light energy transfer in that it samples all spatial absorption and emission characteristics and places all oscillator strength into energetically low lying, thermally accessible exciton states.

Spatial and spectral characteristics of spontaneous emission from semiconductor quantum wells in microscopic cylindrical cavities

The spontaneous emission rate from an In0.2Ga0.8As quantum well in a cylindrical dielectric cavity with submicron radii and without mirrors in the emission plane is calculated based on a rigorous description of the cavity modes and their interaction with the 2D electronic system. The rate is suppressed for radii much less than the peak optical wavelength (rcav≪λ/n), enhanced by up to a factor of ≊8 for rcav≊λ/n, and remains similar to its value in the absence of a cavity for rcav≫λ/n. The emitted light produces a highly collimated vertical beam in the intermediate region with the majority of photons emitted into the guided cavity modes, and its spatial distribution broadens dramatically in the smallest structures. The global spectral width of spontaneous emission in cavities that can presently be fabricated is largely unaffected by the introduction of lateral dielectric boundaries.

Monte Carlo simulation of light-emitting diode light-extraction characteristics

We employ a Monte Carlo ray-tracing technique to model light-extraction characteristics of light-emitting diodes. By keeping track of the various processes that photons can encounter in a packaged LED, consisting of a semiconductor chip, a reflector dish, and an epoxy magnifying lens, our model can provide valuable information for LED design. The simulations can be performed very effectively on modern workstations. Preliminary results from a sample parametric study indicate that, with some refinement, our method could be used as a computer-aided design tool for optimizing light-extraction efficiencies and for designing LED spatial emission characteristics.

Krypton excimer emission from discharge excited rare gas clusters

We report the time-resolved observation of krypton second continuum excimer emission near 146 nm from a pulsed-discharge excited gas jet. The excimer emission reaches a peak intensity several millimeters from the nozzle and lasts up to several microseconds after the termination of the discharge. We report the temporal behavior of the excimer emission as a function of gas pressure, discharge energy, and position within the cathode-anode gap. The temporal and spatial characteristics of the excimer emission during the discharge are due to cluster excitation in the jet expansion region and is from a combination of the A 3∑+u and the B 1Σ+u states. A kinetic mechanism involving metastable atoms and clusters is described to explain the observed afterglow emission.

X-ray emission in a small plasma focus operating with H2-Ar mixtures

The temporal and spatial characteristics of the X-ray emission in a 3 kJ plasma focus are investigated when operating in H2 with an Ar admixture. The experiments are performed in a UNU/ICTP PFF device. The X-ray emission is detected with a multichannel system of PIN-Si diodes with different filtering looking axially and radially into the focusing region. Two main periods are observed in the X-ray emission, corresponding to two successive compressions in the focus. X-ray emission arising from electron beam activity and dense plasmas is identified. On the basis of the X-ray emission characteristics several features of the plasma focus dynamics are discussed.

A computer‐controlled data acquisition and analysis system for use in thecharacterization of plasma atomic emission sources

The characterization of new spectroscopic plasma sources for use in atomic emission spectroscopy often presents the need for information about the spatial emission characteristics of the source. In addition, information about the variation of the emission over time is often beneficial. It is also useful to be able to graphically see, in real-time, the effects of varying a particular experimental parameter. This paper describes a flexible data acquisition system in which an echelle spectrometer is interfaced to an IBM-PC compatible microcomputer. The FORTH-based ASYST software, which proves real-time display of acquired data, allows any point in the emission source to be imaged on the entrance slit of the spectrometer by using the cursor keys of the PC and a computer-controlled mirror. The different acquisition modes of the system are illustrated with examples of two-dimensional spatial profiles of the plasma and monitoring of plasma stability over time. In addition, an illustration of the optimization of the echelle's aperture plate and photomultiplier tube positions in the focal plane is presented.

Evaluation of a 13.56 MHz capacitively coupled plasma as a detector for gas chromatographic determination of organotin compounds

A demountable, parallel plate, capacitively coupled plasma source operating at 13.56 MHz has been developed as a gas-chromatographic detector for the determination of organotin compounds. The effect of operating power and gas flow-rate on analyte emission intensity has been measured and spatial emission characteristics of the plasma have been evaluated. The detection limits for Me4Sn, Me3SnCl and Pr4Sn were 0.079, 0.190 and 0.168 ng s–1, respectively.

LINE-FOCUSED LASER PRODUCED PLASMA STUDIES USING A PTGS

The spectral and spatial characteristics of X-ray emission (5-200?) along or off the axis of the plasma column, produced by 1.06 μm laser beam irradiating luminum flat target in line focused configurations with an average power density 2×1012W/cm2, are studied by using a pinhole transmission grating spectrograph (PTGS). Problems for the plasma as an X-ray-laser gain medium are also discussed.

Introduction of analyte-loaded poly(dithiocarbamate) into inductively coupled argon plasmas by electrothermal vaporization. Spatial emission characteristics of the resulting dry plasmas

Analyte loaded poly(dithiocarbamate) resin, used in sample enrichment procedures, was introduced into inductively coupled argon plasmas by means of electrothermal vaporization. Net line intensities, radial excitation temperatures, and signal-to-noise ratios were determined as a function of observation height, inner gas flow rate, and forward power. The properties of plasmas fed with dry aerosols were compared with those of plasmas fed with aqueous aerosols. It was observed that excitation condition in dry plasmas notably differ from those in aqueous plasmas. In routine operation, limits of detection for the technique developed are at the pg ml −1 level.