Volume 101, Issue 5, 01 March 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
101(2007); http://dx.doi.org/10.1063/1.2696436View Description Hide Description
Based on an effective propagation constant and a frequency-dependent dielectric constant, the plane wave method is extended to calculate the complex photonic band diagram and the density of states for electromagnetic waves propagating in a two-dimensional finite structure with nonoverlapping dielectric cylinders of arbitrary radii, which may be randomly embedded in a dielectric matrix. The effective propagation constant is obtained by using the Twersky formula [J. Math. Phys.3, 700 (1962)] and the scattering matrix method. A case study shows that an embedded defect with optimal radius in a finite photonic crystal can increase the first photonic band gap as compared to the similar structure without the defect. This work is expected to provide a useful tool in determining the photonic properties of a disorder dielectric medium with arbitrary embedded nanoparticles and nanowires.
101(2007); http://dx.doi.org/10.1063/1.2696204View Description Hide Description
We report ultrafast pulse radiolysis transient absorption (TA) spectroscopy measurements from the Terawatt Ultrafast High Field Facility (TUHFF) at Argonne National Laboratory. TUHFF houses a 20 TW Ti:sapphire laser system that generates 2.5 nC subpicosecond pulses of multi-mega-electron-volt electrons at 10 Hz using laser wakefield acceleration. The system has been specifically optimized for kinetic TA measurements in a pump-probe fashion. This requires averaging over many shots which necessitates stable, reliable generation of electron pulses. The latter were used to generate excess electrons in pulse radiolysis of liquid water and concentrated solutions of perchloric acid. The hydronium ions in the acidic solutions react with the hydrated electrons resulting in the rapid decay of the transient absorbance at 800 nm on the picosecond time scale. Normalization of the TA signal leads to an improvement in the signal to noise ratio by a factor of 5 to 6. Due the pointing instability of the laser this improvement was limited to a 5 to 10 min acquisition period, requiring periodic recalibration and realignment. Time resolution, defined by the rise time of TA signal from hydrated electron in pulse radiolysis of liquid water, of a few picoseconds, has been demonstrated. The current time resolution is determined primarily by the physical dimensions of the sample and the detection sensitivity. Subpicosecond time resolution can be achieved by using thinner samples, more sensitive detection techniques, and improved electron beam quality.
101(2007); http://dx.doi.org/10.1063/1.2464185View Description Hide Description
Nominally undoped ZnOfilms were deposited by reactive sputtering on the lightly boron-doped and heavily boron-doped silicon substrates. The sputteredZnOfilms were identified to be highly ⟨002⟩ oriented in crystallinity and type in electrical conductivity. The current-voltage characteristics revealed that the heterojunction exhibited well-defined rectifying behavior while the heterojunction did not possess rectifying function. As for the heterojunction, it was electroluminescent to a certain extent in the visible region under sufficient forward bias with the positive voltage on the silicon substrate, while it emitted ultraviolet light characteristics of near-band-edge emission of ZnO under the reverse bias, which significantly dominated the visible emission. In contrast to the heterojunction, the heterojunction did not exhibit detectable electroluminescence(EL) under either forward or reverse bias. The characteristics and EL mechanism of the above-mentioned heterojunctions have been tentatively explained in terms of the energy-band structures of the heterojunctions.
101(2007); http://dx.doi.org/10.1063/1.2437104View Description Hide Description
Measurements of the angular dependency of luminescence and lasing spectra were carried out for a planar layer of a dye-doped cholesteric liquid crystal (CLC). In addition to the known Bragg emission mode propagating along the cell normal, we observed that leaky modes also depend on the pump intensity and on the dimension and shape of the transverse cross section of the excitation beam. The leaky modes are valuable as a specific lasing regime but impose serious limitation on the energy of the Bragg lasing modes. This has to be taken into consideration when designing high energy lasing devices based on CLC.
Efficient terahertz generation using trap-enhanced fields in semi-insulating photoconductors by spatially broadened excitation101(2007); http://dx.doi.org/10.1063/1.2437113View Description Hide Description
The combined effects of large trap-enhanced fields (TEFs) near the anode in semi-insulating photoconductors and enhanced terahertz emission by spatially nonuniform optical illumination are investigated using a self-consistent Poisson–Monte Carlo solver for the carrier transport. In addition, we distinguish self-consistently the screening contributions to the terahertz radiation associated with the Coulomb and radiation fields by incorporating the three-dimensional Maxwell equations into the solver. Efficient terahertz generation is obtained by reducing the impact of these screening effects, which are the main factors underlying the saturation of the terahertz radiation power, by exploiting an elliptical excitation aperture as eccentric as possible with the major axis parallel to the anode and lying to the maximum extent within the large TEF region that occurs within of the anode.
101(2007); http://dx.doi.org/10.1063/1.2437122View Description Hide Description
ZnOthin films were epitaxialdeposited on sapphire substrates at various temperatures by using the pulsed laser deposition(PLD) technique. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin film. It was found that all of the thin films were oriented and the intensity of (0002) peak increased with the increasing growth temperature. The -scans for the thin films indicated that the thin filmgrown at a temperature higher than had an epitaxial relation with the substrate. An atomic force microscope (AFM) was used to investigate the surface morphologies of the thin films. The surface roughness and grain size of the thin films increased with increasing growth temperature. A double-beam spectrophotometer was used to measure the transmittances of the thin films. The band gap energies of the thin films were calculated by linear fitting the absorption edges for high-quality thin films. A spectrometer was used to investigate the photoluminescent (PL) properties of the thin films. It was discovered that all of the thin films showed two emissions. One was the near band edge (NBE) emission; the other was the broad deep-level (DL) emission. After checking the PL of the thin films on a different date, the aging effect of the ZnOthin film on the sapphire substrate deposited by PLD was observed. It was revealed that, the NBE emissions were enhanced and the DL emissions were decreased with time. To find the reason why the DL emission decreased with time, the as-deposited thin films were annealed at in a and atmosphere for 30 min, respectively. The surface morphologies and the transmittances of the annealedthin films were investigated by the AFM and spectrophotometer. The surface roughness and the transmittance decreased much after annealing. The PL measurements for the annealedthin films indicated that, the DL emission of the thin filmannealed in was enhanced and that annealed in was quenched. It was suggested that the oxygen vacancies, instead of zinc interstitials, played the most important role for DL emissions in ZnOthin filmsdeposited by PLD.
Nonlinear study of photoelectrothermal modulated optical reflectance for active dopant profile extraction101(2007); http://dx.doi.org/10.1063/1.2464183View Description Hide Description
Carrier illumination (CI) is a photoelectrothermal modulated optical reflectance (PMOR) technique for the one dimensional active doping profile characterization of ultrashallow junctions. The specificity of CI as a PMOR technique is to exploit the probe differential reflectance nonlinearity as a function of the pump laser irradiance . The probe differential reflectance as function of the pump power is called a power curve, and its interpretation provides information on the underlying active doping profile. In a previous work [F. Dortu et al., J. Vac. Sci. Technol.24, 375 (2006)], the independent extraction of the active doping concentration and the metallurgical junction of a chemical vapor deposited boxlike profile was based on two features of the power curve, namely, the inflexion power and the signal at end of range power. However, this method suffers from the difficulty to extract accurately the second derivative and has a limited extraction range (, ). In the present work, we present a method making use of the power curve’s first derivative at low and high illumination powers. This method, in principle, allows a much broader extraction range (, ) provided that the signal time dependence due to the native silicon oxide charging under intense illumination is taken into account properly. The present work is supported by a two-layer diffusionless nonlinear analytical model, which provides the basic insights of the method, and three dimensional axisymmetric numerical simulations in the framework of the drift-diffusion equations. A procedure to remove the time dependent charging effect is also presented.
101(2007); http://dx.doi.org/10.1063/1.2435822View Description Hide Description
Defects induced by are crucial for the improvement of persistent luminescenceproperties of . Those defects were investigated with thermoluminescence and positron annihilation techniques. The results indicate that the trap levels in are much deeper than that in , and the structure of those traps was not normally expected vacancies but complex clusters consisting of a vacancy and two ions. A possible mechanism based on the structure of those defects is presented and discussed.
101(2007); http://dx.doi.org/10.1063/1.2435917View Description Hide Description
Making use of the effective medium theory, we reduce the two-dimensional photonic crystals to an effective one-dimensional model. This one-dimensional model not only predicts correctly the dispersion in the long-wavelength limit (where the wavelength is much larger than the period of material) but also gives a good approximation for shorter wavelength. We further use the perturbation theory to improve the calculation of dispersion up to medium wavelength (which is of the order of space period of the material). Our work gives an insight into the modes of electromagnetic field propagating in photonic crystals.
101(2007); http://dx.doi.org/10.1063/1.2437583View Description Hide Description
In this paper, we report the fabrication of photoenhanced polarization mode separated fiber Bragg gratings(FBGs) in polarization maintaining (PM) fibers using IR femtosecond laserillumination. The separation of the Bragg resonant wavelengths between the two polarization modes is as large as due to the photoenhanced birefringenceeffect generated by the strong ultrashort laser pulses. This large polarization mode separation solves one of the major problems of the conventional PM Bragg gratings (i.e., the narrow spacing or even the partial spectral overlap between spectra of the two polarization eigenmodes) and substantially enhances the multiparameter sensing capability of FBGs by offering a wider sensing range and higher discrimination. Furthermore, the high thermal stability of FBGs (up to for silica fibers) inscribed by IR femtosecond lasers provides for multiparameter sensing in harsh, high temperature environments.
Effect of concentration quenching in highly doped lead lanthanum zirconate titanate transparent ferroelectric ceramics101(2007); http://dx.doi.org/10.1063/1.2435074View Description Hide Description
The concentration dependence of the fluorescencequantum efficiency in dopedleadlanthanum zirconate titanate (PLZT), transparent ceramics, is presented. The total emission decay of the emitting level is close to exponential, even for high concentration , due to the very low probability of the cross relaxation energy transfer processes among ions. Owing to this low probability, it was inferred that Nd:PLZT presents lower concentration quenching than other laser materials as Nd:YAG. The figure of merit , where is the fluorescencequantum efficiency, presents a maximum around , indicating the good prospects of concentrated samples for miniaturization of the laser medium (microchip laser).
101(2007); http://dx.doi.org/10.1063/1.2710293View Description Hide Description
We report on the fabrication and characterization of low-loss planar and stripe waveguides in a -doped glass by oxygen-ion implantation at a dose of . The dark mode spectroscopy of the planar waveguide was measured using a prism coupling arrangement. The refractive index profile of the planar waveguide was reconstructed from a code based on the reflectivity calculation method. The results indicate that a refractive index enhanced region as well as an optical barrier have been created after the ion beam processing. The near-field mode profiles of the stripe waveguide were obtained by an end-fire coupling arrangement, by which three quasitransverse electric modes were observed. After annealing, the propagation losses of the planar and stripe waveguides were reduced to be and , respectively.
101(2007); http://dx.doi.org/10.1063/1.2710780View Description Hide Description
The evolution of temperature and velocity fields during direct metal deposition with coaxial powder injection was simulated using a self-consistent three-dimensional model based on the solution of the equations of mass, momentum, energy conservation, and solute transport in the liquid pool. The basic physical phenomena, including heat transfer, phase changes, mass addition, fluid flow, and interactions between the laser beam and the coaxial powder flow, were considered in the model. The level-set method was implemented to track the evolution of the liquid/gas interface. The temperature and velocity fields, liquid/gas interface, and energy distribution at liquid/gas interface at different times were simulated. For verification purposes, the cladding depth and height were compared with experimental results.
101(2007); http://dx.doi.org/10.1063/1.2710354View Description Hide Description
A low pass polarizer that suppresses higher-order diffraction light from vacuum ultraviolet and soft x-ray monochromators is presented in this paper. This vacuum ultraviolet and soft x-raypolarizer is based on a concept of sandwiched metal-dielectric-metal triple reflection configuration. By appropriate optimization of material and angle of incidence, the proposed polarizer demonstrates the capability of matching to desired cutoff edge of photon energy. Furthermore, the optimized soft x-raypolarizer shows the possibility to tune cutoff photon energy in a broadband spectral region ranging from 80 down to down to .
Microcrystalline, nanocrystalline, and ultrananocrystalline diamond chemical vapor deposition: Experiment and modeling of the factors controlling growth rate, nucleation, and crystal size101(2007); http://dx.doi.org/10.1063/1.2696363View Description Hide Description
gas mixtures have been used to deposit microcrystalline diamond,nanocrystallinediamond, and ultrananocrystalline diamondfilms using hot filament chemical vapor deposition. A three-dimensional computer model was used to calculate the gas phase composition for the experimental conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology, growth rate, and across-sample uniformity can be rationalized using a model based on competition between H atoms, radicals, and other radical species reacting with dangling bonds on the surface. Proposed formulas for growth rate and average crystal size are tested on both our own and published experimental data for and conventional mixtures, respectively.
Scattering loss and effective index step of asymmetric cladding surface-etched distributed Bragg reflector lasers at 850 nm101(2007); http://dx.doi.org/10.1063/1.2696253View Description Hide Description
The reflectivity of surface-etched distributed Bragg reflector (DBR) laser gratings, suitable for incorporation in photonic integrated circuits, is determined from a series of samples having varied gratingetch depths. The effective index step and DBR scattering loss are used to characterize the relationship between the physical structure and associated optical characteristics of asymmetric cladding surface-etched lasers at 850 nm. These results enable the fabrication of frequency-selective DBR gratings with a predetermined grating index step and reflectivity.
101(2007); http://dx.doi.org/10.1063/1.2437575View Description Hide Description
We demonstrate that the phase and amplitude of the reflection coefficient for Bloch waves can be useful tools in understanding and predicting the spectral characteristics of distributed-feedback lasers. We further apply the Bloch wave formalism for engineering the asymmetry of the spectral response with respect to the gap through a judicious termination of the laser facets.
- PLASMAS AND ELECTRICAL DISCHARGES
101(2007); http://dx.doi.org/10.1063/1.2709578View Description Hide Description
Polyimide samples modified by aluminum(Al) ions produced by filtered cathodic vacuum arc (FCVA) with plasma immersion ion implantation(PIII) technique, under ambient argon and oxygen gases (flow rate ) were investigated by x-ray photoelectron spectroscopy(XPS). The working pressure was about and the plasma density was estimated to be . The applied bias voltages were varied from 5 to 12.5 kV but with fixed frequency at 900 Hz and duty time of . For 1 min process time, C and O spectra for modified samples clearly indicated that the carbonyl group (C=O) was largely destroyed by incident Al ions while Alspectra suggested Al atoms remain inside polyimide matrices in the form of C-O-Al complexes. For a 5 min process time, when the ion fluence became large, both C and O spectra suggested a structure of “aluminum oxide-mixed layer-polyimide” and Alspectra confirmed that most Al atoms were bonded to oxygen atoms on the top surface. These XPS results revealed the chemical bonds between implanted and deposited Al ions and polyimide matrix by using the PIII technique. The structural information can also be suggested. Furthermore in this paper, some discussions with the theoretical [the stopping and range of ions in matter (SRIM)] simulation were also mentioned in order to explore the effectiveness of Al ions irradiation on polyimide.
101(2007); http://dx.doi.org/10.1063/1.2437675View Description Hide Description
In this paper we describe experimental observations connected with the propagation of primary and secondary streamers in water. Using a Mach-Zehnder interferometer we determined the pressure field surrounding the streamer channel at a given instant in time with high temporal and spatial resolution. This pressure field contains information on the time evolution of the pressure pulse inside the discharge channel. The pressure history in the channel has been reconstructed by comparing the experimentally obtained fringe shifts in the interferograms with those derived from one-dimensional hydrodynamic calculations in cylindrical geometry. Assuming different trial pressure pulses, it has been possible to establish the channel pressure iteratively. A reproduction of the experimental data from secondary streamers requires short pressure pulses with amplitudes of . These findings are inconsistent with the assumption of bubble-initiated propagation of secondary streamers. It has also been inferred from estimates of the channel diameter that self-propagation of secondary streamers occurs at field strengths at the streamer tip of more than . We can therefore conclude that field induced dissociation and ionization of molecules in the bulk liquid are the most likely mechanism for secondary streamer propagation. Rather high electrical conductivity is achieved at fields of and an ionizationwave is launched from the streamer tip into the liquid. To advance the streamer the electric field must be expelled from the newly generated section. This occurs with the Maxwellian relaxation time of a few nanoseconds. During this time the region of high conductivity is transformed into a plasma channel of lower density and a pressure wave is launched into the liquid. A different mechanism is suggested for primary streamer formation. Because of the low conductivity in the channels it is more likely that gas bubbles or phase instabilities are involved in this case.
101(2007); http://dx.doi.org/10.1063/1.2435914View Description Hide Description
As a result of observations made by Shaw (M.S. thesis, Cornell University, 1964) in the mid-1960s, alkali rf discharges are known to operate in two spectralmodes, the so-called ring mode and the red mode. Experience has shown that the ring mode is best for discharge lamps used in quantum-electronic devices such as atomic clocks and optically pumped magnetometers and that the performance of these devices seriously degrades when the lamp operates in the red mode. Understanding the origin of these modes therefore has application to understanding and improving various quantum-electronic devices. Here we show that Shaw’s model for these modes is inconsistent with observation, and we propose an alternate model based on the role of radiation trapping in multistep ionization.