Volume 104, Issue 10, 15 November 2008
- 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
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
Comparison of resonant-phonon-assisted terahertz quantum-cascade lasers with one-well injector and three-well module104(2008); http://dx.doi.org/10.1063/1.3021060View Description Hide Description
The authors compare resonant-phonon-assisted terahertz quantum-cascade lasers with different active region designs, i.e., one-well injector and three-well module, using an ensemble Monte Carlo method. Carrier transport under injection anticrossing, parasitic anticrossing, and collection anticrossing couplings are investigated for both structures. The results show that the main parasitic current channel is the resonance of the injector with the lower lasing level 3 and one-well injector design benefits a high injection selectivity because of a better ratio. Due to the diagonal radiative transition, the one-well injector design sacrifices the oscillator strength and modal gain to reduce the parasitic coupling. Temperature evaluation shows that longitudinal-optical phonon emission is the dominant scattering mechanism contributing to lasing and the calculated modal gain in the one-well design shows a slower decreasing rate with temperature.
104(2008); http://dx.doi.org/10.1063/1.3021088View Description Hide Description
To understand the modulation mechanisms of fluorescence emission induced by ultrasonic waves in turbid media, a mathematical model is proposed and compared with the recent experimental observations of Kobayashi et al. [Appl. Phys. Lett.89, 181102 (2006)]. Modulation of fluorophore concentration is considered as the source of the oscillation of fluorescence signals when fluorophore concentration is low enough so that quenching effects can be ignored. By solving the rate equation and photondiffusion equation, quantitative solutions are given to quantify the modulation strength. Our calculations predict that the modulation depth (the ratio of the modulated signal strength to the unmodulated signal strength) can reach when ultrasonicpressure with the order of magnitude of megapascals is applied in the ultrasound focal zone. Our model explains the relationship between the modulation strength and the average fluorophore concentration and also predicts a method to measure or imagefluorescence lifetime in the turbid medium. When fluorophore concentration is high enough so that fluorescence quenching occurs, the fluorescence modulation is attributed to the modulation of quenching efficiency. Quenching caused by fluorescence resonance energy transfer can lead to a nonlinear relationship between the modulation fluorescence strength and the applied ultrasound strength.
104(2008); http://dx.doi.org/10.1063/1.3000460View Description Hide Description
Laser annealing by pre-exposure to subdamage threshold laser pulses is a well-established method to increase the damage performance of bulk and nonlinear optical materials. The origin of laser-induced damage is believed to be localized absorption by a defect structure, either a light-absorbing foreign nanoparticle or a cluster of stoichiometric defects. It has been recently shown that there are at least three populations of such defect structures in these materials in which pre-exposure to laser pulses of specific fluence and wavelength results in a measurable reduction in their number density or decrease in their susceptibility to damage. In this work, we investigate the annealing characteristics of these three populations of defect structures in DKDP under variable irradiation conditions. The aim is to understand the similarities and differences between these populations in the way they interact with laser light. The results depict distinct behaviors that reveal information on the relationship between the defect populations and their modifications responsible for annealing.
104(2008); http://dx.doi.org/10.1063/1.3021142View Description Hide Description
Ferromagnetic and highly conductive copper doped ZnO (ZnO:Cu) films were prepared by filtered cathodic vacuum arc technique. By employing a biasing technique during growth, the electron concentration and resistivity of the ZnO:Cu films can be as high as and , respectively. The ferromagnetic behavior is observed in all the conductive films, but its magnetization is quenched with an increment in carrier concentration, suggesting that carrier induced exchange is not directly responsible for the ferromagnetism. Heterojunctionlight emitting diodes have been fabricated using the conductive ZnO:Cu layer as an electron injector and a -type GaN as hole injector. Electroluminescence can be detected from the devices.
104(2008); http://dx.doi.org/10.1063/1.3021150View Description Hide Description
An optical emission scheme was demonstrated, in which a high-refractive-index waveguide is excited by a traveling electron beam in a vacuum environment. The waveguide was made of layers. The velocity of light propagating in the waveguide was slowed down to 1/3 of that in free space due to the high refractive index of Si. The light penetrated partly into the vacuum in the form of a surface wave. The electron beam was emitted from an electron gun and propagated along the surface of the waveguide. When the velocity of the electron coincided with that of the light, optical emission was observed. This emission is a type of Cherenkov radiation and is not conventional cathode luminescence from the waveguide materials because Si and are transparent to light at the emitted wavelength. This type of emission was observed in an optical wavelength range from 1.2 to with an electron acceleration voltage of 32–42 kV. The characteristics of the emitted light, such as the polarization direction and the relation between the acceleration voltage of the electron beam and the optical wavelength, coincided well with the theoretical results. The coherent length of an electron wave in the vacuum was confirmed to be equal to the electron spacing, as found by measuring the spectral profile of the emitted light.
104(2008); http://dx.doi.org/10.1063/1.3021161View Description Hide Description
Titanium dioxide thin films have been deposited by reactive magnetron sputtering on glass and subsequently irradiated by UV radiation using a KrF excimer laser. The influence of the laser fluence on the constitution and microstructure of the depositedfilms is studied for . The diffraction data reveal that as depositedfilms are amorphous, while irradiated films present an anatase structure. Additional Raman spectroscopy study shows better crystal quality for the films irradiated with . The film morphology appears to be strongly modified after laser treatment. Atomic force microscopy and scanning electron microscopy measurements reveal fractally textured films presenting characteristics of high porosity and high specific surface area. Finally, contact angle analysis suggests hydrophobic or wetting behavior depending on . In order to explain the laser-induced structuration mechanisms, we have successfully applied a fractal as well as the nucleation theories. We propose that electronics effects could be responsible for the observed crystallization.
104(2008); http://dx.doi.org/10.1063/1.3021310View Description Hide Description
We present a numerical study of the electromagnetic properties of the three-dimensional metallic wire lattices operating at microwave frequencies with applications to advanced accelerating structures and microwave sources. The metallic lattices can be considered as “artificial plasmas” because they demonstrate the properties of plasmas with a negative dielectric constant. Bulk modes in a diamond lattice of metal wires and surface modes on its interface are calculated. It is shown that the lattice can be modeled as an anisotropic medium with spatial dispersion. In contrast to a simple cubic lattice, the diamond lattice allows the existence of three different interfaces—one isotropic and two anisotropic. The surface modes supported by these interfaces are affected by spatial dispersion, in sharp contrast with the surface mode on an isotropic vacuum/plasma interface. For particle accelerator applications, we identify the electromagnetic mode confined by a plasmonic waveguide formed as a defect in a diamond lattice. All deleterious higher order modes excited as wakefields from the accelerating particle are found to be leaky. The diamond lattice is also useful as a research tool for studying particle radiation in media with spatial dispersion.
104(2008); http://dx.doi.org/10.1063/1.3021466View Description Hide Description
We present optical emission studies of the laser ablationbrassplasma generated by the fundamental, second, and third harmonics of a neodymiumdoped yttrium aluminum garnet laser. The spectra predominantly reveal the spectral lines of the neutral and singly ionized copper and zinc. The excitation temperatures are determined by the Boltzmann plot method, whereas the electron number densities have been extracted from the Stark broadened line profiles. The spatial variations in the spectral line intensities and the plasma parameters at 1000, 500, and 100 mbar air pressures have been evaluated. Besides, the effect of the ambient gases (He, Ne, and Ar), the laser irradiance, and the laser wavelengths on the plasma parameters have been investigated.
104(2008); http://dx.doi.org/10.1063/1.3021056View Description Hide Description
A new principle of subwavelength imaging based on frequency scanning is considered. It is shown that it is possible to reconstruct the spatial profile of an external field exciting an array (or coupled arrays) of subwavelength-sized resonant particles with a frequency scan over the whole band of resonating array modes. During the scan it is enough to measure and store the values of the near field at one or at most two points. After the scan the distribution of the near field can be reconstructed with simple postprocessing. The proposed near-field microscope has no moving parts.
Observation of coherent acoustic and optical phonons in bismuth nanowires by a femtosecond pump-probe technique104(2008); http://dx.doi.org/10.1063/1.3021101View Description Hide Description
Coherent acoustic and optical phonon oscillations in Bi nanowire samples were studied with a femtosecond pump-probe technique. Laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the terahertz range. The transmission signal of nanowires on a glass substrate and the signal of light scattered from freestanding nanowires were measured. The acoustic velocity in nanowires was found to be close to that of bulk polycrystalline material. The changes in the optical phonon frequency at different laser fluences were simulated taking into account excitation inhomogeneity, lattice anharmonicity, diffusion, and recombination of the carriers and gave good agreement with experimental results.
Dependence on chirp rate and spectral resolution of the terahertz field pulse waveform measured by electro-optic detection using a chirped optical pulse and a spectrometer and its effect on terahertz spectroscopy104(2008); http://dx.doi.org/10.1063/1.3021151View Description Hide Description
Single-shot measurement of a terahertz field pulse waveform by electro-optic sampling using a chirped optical pulse and a spectrometer was demonstrated by and Jiang and Zhang [Appl. Phys. Lett.72, 1945 (1998)]. We have performed an experimental and theoretical investigation into the dependence of the waveform thus measured on the chirp rate and spectral resolution. It was found that the waveform exhibits multicyclic behavior at a chirp rate of , which corresponds to a chirped-pulse width of over 10 ps, for the monocyclic original terahertz field, while it approaches the monocyclic behavior with decreasing pulse width. Further, broadening of the spectral resolution of the spectrometer gives rise to a monocyclic waveform in the chirp rate range where the waveform is expected to be multicyclic. In addition, we have derived an analytical expression for the terahertz field pulse waveform thus measured without using the method of stationary phase. The theoretical results were found to be consistent with measured ones. Finally, we examined the spectral bandwidth and resolution of terahertz spectroscopy using this method.
Optical emission and energy disposal characterization of the laser ablation process of , , and NaCl at 1064 nm104(2008); http://dx.doi.org/10.1063/1.3021352View Description Hide Description
A method to simultaneously measure the fraction of incident energy spent as absorbed, scattered, and consumed energy by the plasma generated in the ablation process is presented. The results obtained allowed to make an energy balance in the ablation process of halogenated earth alkaline metal crystals (NaCl, , and ) and to relate the nature of the crystal structure to the absorbed energy. The latter could be determined by monitoring the magnitude of the breakdown induced by the discharge between the plates of a capacitor. Simultaneously, time resolved optical emission spectroscopy was used to analyze the ion emission intensity and as a function of the energy absorbed by the sample. This plot shows the usual sigmoid behavior for the ablation process with the three typical regions, corresponding to the vaporization, screening, and ablation regimes. A heuristic equation has been used to analyze this process in terms of the nature of the crystal structure. From this equation the critical absorbed energy for each of the studied samples could also be determined.
104(2008); http://dx.doi.org/10.1063/1.3028265View Description Hide Description
We report the measurement of efficient second harmonic generation at a metallic tip immersed in a molecular solution of highly nonlinear molecules. The dependence of the signal either with the tip-to-substrate applied voltage or with the tip-to-substrate distance is studied. We show that the signal originates from the orientation of a limited number of molecules following local centrosymmetry breaking under the tip. The results are discussed within the context of scanning probe microscopy.
104(2008); http://dx.doi.org/10.1063/1.3021292View Description Hide Description
Single crystal lithium niobate fibers with diameters ranging from have successfully been grown by the micro-pulling-down technique. The fibers are doped with various Er doping levels from and appear to be transparent and crack-free. The study includes x-ray structural investigation and complete spectroscopic characterization. The analysis shows a good homogeneity of composition, and the comparison with bulk crystals demonstrates the good optical quality of the samples. Particular attention has been devoted to the measurements of the fluorescence decay time of the Er level, which has been carried out with a particular technique in order to avoid radiation trapping effects. Results are in agreement with the literature both in the case of bulk crystals and of fibers. Reliable polarized emission cross section in the region are presented.
104(2008); http://dx.doi.org/10.1063/1.2975971View Description Hide Description
Using a planar cell consisting of a prism and a flat glass, we have carried out precise measurements of the angles at which quasi-in-plane leaky (QIPL) laser modes propagate within a thin layer of a cholesteric liquid crystal (CLC) doped with a laser dye. Both the prism and the glass have refraction indices higher than relevant CLC indices, therefore, the waveguide effect in CLC is excluded. For this type of almost thresholdless lasing neither mirrors nor distributed feedback is necessary. The modes are generated in the amplifying CLC layer due to strong Fresnel reflections from the glass boundaries at propagation angles very close to with respect to the cell normal. In the experiment, two modes polarized differently (- and -) have been found outgoing from the prism. Using equivalency between optical properties of a CLC and optically negative nematic liquid crystal at the propagation angles close to , the analytical approach known for a uniform nematic liquid crystal was used for calculations of the threshold gain of the QIPL modes in a helical CLC. The gain has been found for the particular eigenmodes whose propagation angles have been measured. The experimental and calculated data on the propagation angles, polarization and threshold gain are in quantitative agreement.
104(2008); http://dx.doi.org/10.1063/1.2985913View Description Hide Description
The evaluation of different surfacecoatings used in alkali metal atomic magnetometers is necessary for the improvement of sensitivity of these devices. A method to measure the polarization lifetime of alkali atoms in the region between substrates with different coatings was developed to determine the effectiveness of the coating at preserving alkali spin polarization as well as chemical compatibility and high-temperature stability. Multiple coatings can be compared under identical experimental conditions, using an experimental geometry that allows surface characterization before and after evaluation of the polarization lifetime. Multilayered, cross-linked octadecyltrichlorosilane films, alkyltrichlorosilane monolayers, and octadecylphosphonic acid monolayers were evaluated using this approach.
104(2008); http://dx.doi.org/10.1063/1.3031278View Description Hide Description
This paper reports a method for the efficient formation of surface relief grating (SRG) on azopolymer films in which goldnanoparticles are dispersed. The height and shape of the SRG, which were formed by a two-beam interference technique, were found to be dependent on the content of goldnanoparticles. The optimum concentration of goldnanoparticles for the efficient formation of SRG was confirmed by atomic force microscopy(AFM), and the height of SRG at the content of was about 2.2 times higher than that of pure azopolymer film without goldnanoparticles. This is due to the field enhancement effect derived from the localized plasmon excitations of goldnanoparticles dispersed in the azopolymer film.
Modification of emission of CdTe nanocrystals by the local field of Langmuir–Blodgett colloidal photonic crystals104(2008); http://dx.doi.org/10.1063/1.2981087View Description Hide Description
A light source on the surface of a slab of -dimensional photonic crystal has been prepared by the Langmuir–Blodgett deposition of a colloidal crystal on top of a thin film containing CdTe nanocrystals. The directional enhancement of the light emission intensity in the spectral range of the photonic bandgap has been revealed through the comparative examination of the angle-resolved transmission, diffraction, and photoluminescencespectra of the prepared structures. Changes in the emission spectrum have been tentatively explained in terms of the acceleration of the radiative recombination due to the increase in the local field strength at photonic bandgap resonance and changes in the emission diagram—as arising from the wavelength dependence of the topology of the local field pattern.
- PLASMAS AND ELECTRICAL DISCHARGES
104(2008); http://dx.doi.org/10.1063/1.2998907View Description Hide Description
Most previous three-dimensional modeling on gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) focuses on the weld pool dynamics and assumes the two-dimensional axisymmetric Gaussian distributions for plasma arc pressure and heat flux. In this article, a three-dimensional plasma arc model is developed, and the distributions of velocity, pressure, temperature, current density, and magnetic field of the plasma arc are calculated by solving the conservation equations of mass, momentum, and energy, as well as part of the Maxwell’s equations. This three-dimensional model can be used to study the nonaxisymmetric plasma arc caused by external perturbations such as an external magnetic field. It also provides more accurate boundary conditions when modeling the weld pool dynamics. The present work lays a foundation for true three-dimensional comprehensive modeling of GTAW and GMAW including the plasma arc, weld pool, and/or electrode.
104(2008); http://dx.doi.org/10.1063/1.2999343View Description Hide Description
Stable operation of a cylindrical Hall thruster has been achieved using a hot wire cathode, which functions as a controllable electron emissionsource. It is shown that as the electron emission from the cathode increases with wire heating, the discharge current increases, the plasma plume angle reduces, and the ion energy distribution function shifts toward higher energies. The observed effect of cathodeelectron emission on thruster parameters extends and clarifies performance improvements previously obtained for the overrun discharge current regime of the same type of thruster, but using a hollow cathode neutralizer. Once thruster discharge current saturates with wire heating, further filament heating does not affect other discharge parameters. The saturated values of thruster discharge parameters can be further enhanced by optimal placement of the cathode wire with respect to the magnetic field.