Volume 113, Issue 14, 14 April 2013
Index of content:
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4799062View Description Hide Description
We investigated time-resolved optical properties of the free carrier plasma excited by femtosecond laser pulses in silicon nanopillars. The optical response of the excited nanopillars was modelled by the 2D Maxwell-Garnett mixing rule modified to accommodate a Drude-like contribution of the free carriers. The experimental results of the plasma dynamics pump-probe study suggest that the main mechanism of the decay at high pumping is Auger recombination with a rate similar to that of the bulk silicon. We also show that the low reflective and almost fully transparent layer of the nanopillars becomes strongly absorptive when the pillars are pumped with the free carrier concentration in the range between and . The enhancement of the absorption is attributed to plasmon resonance at which the probe light and the excited plasma frequencies are matched.
113(2013); http://dx.doi.org/10.1063/1.4800546View Description Hide Description
We performed spectroscopy of the magnetic field effect (MFE) including magneto-photoinduced absorption (MPA) and magneto-photoluminescence (MPL) at steady state conditions in annealed and pristine fullerene C60 thin films, as well as magneto-conductance (MC) in organic diodes based on C60 interlayer. The hyperfine interaction has been shown to be the primary spin mixing mechanism for the MFE in the organics. In this respect, C60 is a unique material because 98.9% of the carbon atoms are 12C isotope, having spinless nucleus and thus lack hyperfine interaction. In spite of this, we obtained substantial MPA (up to ∼15%) and significant MC and MPL in C60 films and devices, and thus mechanisms other than the hyperfine interaction are responsible for the MFE in this material. Specifically, we found that the MFE(B) response is composed of narrow (∼10 mT) and broad (>100 mT) components. The narrow MFE(B) component is due to spin-dependent triplet exciton recombination in C60, which dominates the MPA(B) response at low pump intensities in films, or the MC response at small current densities in devices. In contrast, the broad MFE(B) component dominates the MPA(B) response at high pump intensities (or large current densities for MC( B)) and is attributed to spin mixing in the polaron pairs spin manifold due to g-factor mismatch between the electron- and hole-polarons in C60. Our results show that the organic MFE has a much broader scope than believed before.
113(2013); http://dx.doi.org/10.1063/1.4800826View Description Hide Description
We have developed a fluorescence enhancement system for biomolecular detection using a monolithic waveguide sensing plate. The plate consists of a thermally grown amorphous SiO2 layer, a Si single-crystal layer, and bulk amorphous SiO2. Waveguide-mode excitation in this plate produces an enhanced electric field on the plate surface, and therefore, signals from analyte-labeling fluorescent dyes are enhanced. In the present paper, we elucidate the impact of the system from viewpoints of the electric field enhancement factor calculated numerically and the analysis of the limit of detection estimated from experimental results. Optimal layer thicknesses of the plates to achieve the highest sensitivity are also discussed. Further, an interesting phenomenon that accompanied the waveguide-mode excitation, namely, photoluminescence from the amorphous SiO2 layer itself, is also reported.
113(2013); http://dx.doi.org/10.1063/1.4800896View Description Hide Description
SnO2 nanocrystalline films with different crystallite sizes were grown by direct current sputtering. All the films show radiative recombination of free exciton (FX) and surface exciton (SX) with emission peaks varied from 330 to 338 nm and from 364 to 375 nm, respectively. The emission intensities of FX and SX versus crystallite size, excitation intensity, and temperature were also investigated. It was found that the emission intensities of both FX and SX increase with the decrease of the crystallite size of the films. In addition, the crystallite size has significant influence on the emission intensity of FX than SX. A model was also established to describe the relationship between crystallite size and excitonic emission intensity. From the temperature-dependent photoluminescence spectra, the activation energies of FX and SX are deduced.
113(2013); http://dx.doi.org/10.1063/1.4800914View Description Hide Description
Using a real-space model Hamiltonian, we have theoretically studied the single-photon transmission in a waveguide side coupled with a Jaynes-Cummings chain (JCC). The JCC can induce the photon group advancement (GA) and group delay (GD) in different frequency ranges determined by JCC eigenmodes. For GA and GD, there exist different optimal JCC lengths. At certain frequency, the GA's maximum value as usual increases with decreasing the cavity dissipation, whereas the GD's eventually reaches saturation. For a 1.55 -μm photon, our calculation indicates that the GD's maximum value is about 400 ps simultaneously with a large transmission.
Nonlinear ionization mechanism dependence of energy absorption in diamond under femtosecond laser irradiation113(2013); http://dx.doi.org/10.1063/1.4801802View Description Hide Description
We present first-principles calculations for nonlinear photoionization of diamond induced by the intense femtosecond laser field. A real-time and real-space time-dependent density functional theory with the adiabatic local-density approximation is applied to describe the laser-material interactions in the Kohn-Sham formalism with the self-interaction correction. For a certain laser wavelength, the intensity dependence of energy absorption on multiphoton and/or tunnel ionization mechanisms is investigated, where laser intensity regions vary from 1012 W/cm2 to 1016 W/cm2. In addition, the effect of laser wavelength on energy absorption at certain ionization mechanism is discussed when the Keldysh parameter is fixed. Theoretical results show that: (1) at the fixed laser wavelength, the relationship between the energy absorption and laser intensity shows a good fit of E = cMIN (N is the number of photons absorbed to free from the valence band) when multiphoton ionization dominates; (2) while when tunnel ionization becomes significant, the relationship coincides with the expression of E = cTIn (n < N).
113(2013); http://dx.doi.org/10.1063/1.4801805View Description Hide Description
Room temperature photoluminescence (PL) was observed along 50 μm long Ge strips on insulator on bulk Si substrates fabricated by rapid melt growth. The PL peaks evidently exhibited a redshift from the origin to the end of the Ge strip because of the shrinkage of the direct bandgap of Ge. Moreover, PL intensities increased along the direction of lateral epitaxial growth primarily because of the decrease in the energy difference between the direct and indirect gaps of Ge. The change in the Ge band structure, which facilitated changes in PL peaks and intensities, was found to have resulted from the variation of tensile strain ratios and Si fractions along Ge strips. Furthermore, the PL intensity at the end of the strip was one magnitude higher than that of bulk Ge, which indicates the high quality of Ge-on-insulator structures.
Threshold of terahertz population inversion and negative dynamic conductivity in graphene under pulse photoexcitation113(2013); http://dx.doi.org/10.1063/1.4801916View Description Hide Description
We present a theoretical study of population inversion and negative dynamic conductivity in intrinsic graphene in the terahertz (THz) frequency range upon pulse photoexcitation at near-/mid-infrared wavelengths. The threshold pulse fluence required for population inversion and negative dynamic conductivity can be orders of magnitude lower when the pulse photon energy is lower, because of the inverse proportionality of the photoexcited carrier concentration to the pulse photon energy and because of the weaker carrier heating. We also investigate the dependence of dynamic conductivity on momentum relaxation time. Negative dynamic conductivity takes place either in high- or low-quality graphene, where Drude absorption by carriers in the THz frequency is weak.
Systematic study of near-infrared intersubband absorption of polar and semipolar GaN/AlN quantum wells113(2013); http://dx.doi.org/10.1063/1.4801528View Description Hide Description
We report on the observation of intersubband absorption in GaN/AlN quantum well superlattices grown on -oriented GaN. The absorption is tuned in the wavelength range by adjusting the well thickness. The semipolar samples are compared with polar samples with identical well thickness grown during the same run. The intersubband absorption of semipolar samples shows a significant red shift with respect to the polar ones due to the reduction of the internal electric field in the quantum wells. The experimental results are compared with simulations and confirm the reduction of the polarization discontinuity along the growth axis in the semipolar case. The absorption spectral shape depends on the sample growth direction: for polar quantum wells the intersubband spectrum is a sum of Lorentzian resonances, whereas a Gaussian shape is observed in the semipolar case. This dissimilarity is explained by different carrier localization in these two cases.
- Plasmas and Electrical Discharges
113(2013); http://dx.doi.org/10.1063/1.4798596View Description Hide Description
The dielectric breakdown properties of SF6–N2 mixtures were investigated at different concentrations of N2, 0.01–1.6 MPa, and 300–3000 K. The equilibrium compositions of different SF6–N2 mixtures at several gas pressures and temperatures up to 3000 K were first calculated by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. The electron energy distribution function was then obtained using the composition data by Boltzmann equation analysis. It was found that adding N2 to SF6 gas can markedly reduce the kinetic energy of electrons at relatively high gas temperatures, which enhances the dielectric field strength. Finally, the critical reduced electric field (E/N)cr, defined as the value for which ionization is equal to attachment, of hot SF6–N2 mixtures was determined. The results indicate that in the gas temperature range around 2000–3000 K, increasing the concentration of N2 effectively enhances the (E/N)cr of SF6–N2 mixtures, and the (E/N)cr of SF6–N2 mixtures at a wide range of concentrations of N2 (5%, 50%, and 95%) are all higher than that of pure SF6 gas. Further, this trend exists at all the gas pressures considered (0.01–1.6 MPa).
Diode laser spectroscopic measurements of gas temperature in a pulsed dielectric barrier discharge using collisional broadening and shift of 1s3−2p2 and 1s5−2p7 argon transitions113(2013); http://dx.doi.org/10.1063/1.4800556View Description Hide Description
The average gas temperature rise within an argon dielectric barrier discharge operating from 50 to 500 Torr excited by 5 kHz repetition rate fast rise time high voltage pulse has been measured. For these measurements within high pressure discharges, we have employed two novel gas temperature measurement techniques based upon tunable diode laser absorption spectroscopy that take advantage of the nonresonant collision line broadening and collisional frequency shifts from relatively strong argon 1s 3-2p 2 and 1s 5-2p 7 transitions from the metastable states. An in situ estimate of collisional broadening coefficients for both transitions have been obtained from 5 to 30 Torr data using an independent estimate of gas temperature from Doppler line width measurements. Our measurements show that the effect of the isolated line assumption inherent within the van der Waals collisional impact approximation limit begins to fail for the 1s 3-2p 2 and 1s 5-2p 7 spectral lines when collisional line broadening exceeds ∼23 GHz line separation at gas pressure >300 Torr with gas temperature near ambient. A comparison of gas temperature estimates from both line broadening and peak frequency shift shows that the frequency shift provides a more reliable measurement of gas temperature, indicating that the isolated line assumption holds for collisional peak frequency shifts, even for partially overlapping lines.
113(2013); http://dx.doi.org/10.1063/1.4799052View Description Hide Description
Particle beam experiments were conducted in an ultra-high-vacuum vessel to mimic target poisoning during reactive magnetron sputtering of aluminum. Aluminum targets were exposed to quantified beams of argon ions, oxygen atoms and molecules, and aluminum vapour. The growth and etch rates were measured in situ by means of an Al-coated quartz crystal microbalance. The chemical state of the target surface was monitored in-situ by real-time Fourier transform infrared spectroscopy. The surface processes were modelled through a set of balance equations providing sputter yields and sticking coefficients. The results indicate that the oxygen uptake of the aluminum surface is enhanced by a factor 1 to 2 by knock-on implantation and that the deposition of aluminum is not affected by the oxidation state of the surface.
113(2013); http://dx.doi.org/10.1063/1.4797481View Description Hide Description
We consider the charging of a dust grain sitting on a surface exposed to plasma. The stochastic model of Sheridan and Hayes [Appl. Phys. Lett. 98, 091501 (2011)] is solved analytically for the charging time, which is found to be directly proportional to the square root of the electron temperature and inversely proportional to both the grain radius and plasma density.
113(2013); http://dx.doi.org/10.1063/1.4800925View Description Hide Description
We investigated the role of excitation laser wavelength on plasma generation and the expansion and confinement of ablation plumes at early times (0–500 ns) in the presence of atmospheric pressure. Fundamental, second, and fourth harmonic radiation from Nd:YAG laser was focused on Al target to produce plasma. Shadowgraphy, fast photography, and optical emission spectroscopy were employed to analyze the plasma plumes, and white light interferometry was used to characterize the laser ablation craters. Our results indicated that excitation wavelength plays a crucial role in laser-target and laser-plasma coupling, which in turn affects plasma plume morphology and radiation emission. Fast photography and shadowgraphy images showed that plasmas generated by 1064 nm are more cylindrical compared to plasmas generated by shorter wavelengths, indicating the role of inverse bremsstrahlung absorption at longer laser wavelength excitation. Electron density estimates using Stark broadening showed higher densities for shorter wavelength laser generated plasmas, demonstrating the significance of absorption caused by photoionization. Crater depth analysis showed that ablated mass is significantly higher for UV wavelengths compared to IR laser radiation. In this experimental study, the use of multiple diagnostic tools provided a comprehensive picture of the differing roles of laser absorption mechanisms during ablation.
Density of atoms in Ar*(3p54s) states and gas temperatures in an argon surfatron plasma measured by tunable laser spectroscopy113(2013); http://dx.doi.org/10.1063/1.4799152View Description Hide Description
This study presents the absolute argon 1 s (in Paschens’s notation) densities and the gas temperature, , obtained in a surfatron plasma in the pressure range . The absorption signals of 772.38, 772.42, 810.37, and 811.53 nm lines, absorbed by atoms in 1s3, 1s4, and 1s5 states, were recorded with two tunable diode lasers. is deduced from the absorption line shapes when scanning the laser wavelengths. The line profile, which is a Doppler broadening dominated Gaussian at gas pressures of , changes to a Voigt shape at , for which the pressure broadening can no more be neglected. is in the range of 480-750 K, increasing with pressure and decreasing with the distance from the microwave launcher. Taking into account the line of sight effects of the absorption measurements, a good agreement is found with our previous measurements by Rayleigh scattering of at the tube center. In the studied pressure range, the Ar(4 s) atom densities are in the order of , increasing towards the end of the plasma column, decreasing with the pressure. In the low pressure side, a broad minimum is found around and hence the Ar(4 s) atom densities increase slightly with rising pressure. For the studied pressure range and all axial positions, the density ratio: 1s5/1s4/1s3 is very close to a Boltzmann equilibrium by electron impact mixing at the local , which was previously measured by Thomson scattering. The Ar(4 s) densities are successfully compared to a detailed Collisional Radiative Model.
Charge distribution on the surface of a dielectric barrier discharge actuator for the fluid-dynamic control113(2013); http://dx.doi.org/10.1063/1.4799159View Description Hide Description
The electric potential distribution induced on the surface of an aerodynamic plasma actuator, operating by means of a surface dielectric barrier discharge (DBD), has been studied both numerically and experimentally. Three actuators made with three different dielectric materials (Teflon, Plexiglas, and glass) have been used. The geometric configuration of the three actuators is the same one. An electrode pair separated by a 2 mm thick dielectric sheet constitutes the DBD actuator. The exposed high voltage electrode has been fed by a 5 kHz a.c. electrical signal. Voltage values between 7.5 and 15 kVp have been used. Measurements of the distribution of the electrical potential in the dielectric surface, generated by the charge deposited on it, have been done. Numerical simulations allowed to evaluating the charge distribution on the dielectric surface. The discharge has been switched off after positive and negative plasma currents. The measurements have been carried out after both phases. The potential distribution is always positive. The charge build up takes place several centimeters downstream of the upper electrode for an extension broader than that of the plasma on the dielectric surface. The charge distribution strongly depends on the switching off phase and is heavily affected by the dielectric material. In order to evaluate the discharge structure and the extension of the plasma, images have been taken also.
113(2013); http://dx.doi.org/10.1063/1.4800919View Description Hide Description
We report photoluminescence observed from ZnO nano particles/clusters formed in gaseous phase of two colliding plumes ZnO plasma using fourth harmonic 266 nm of Nd:YAG laser. The two expanding plumes generated using 1.06 μm of Nd:YAG laser interact with each other, resulting in an overlap region where the collision front in lateral direction of two plumes forms an overlapping region starting at ∼20 ns lasting ∼5 μs with respect to ablating pulse. The blue shift in peak position (∼30–5 meV) of photoluminescence profile of ZnO clusters to that observed in bulk ZnO shows the effect of quantum confinement. The dynamical growth of nanoparticles is ascertained using Rayleigh scattered second harmonic radiation at 532 nm of Nd:YAG laser. A combined photoluminescence and light scattering at longer time delays ∼1.5 ms where plasma emission is absent confirms the formation of the nano-particles/clusters of ZnO. Observed intensity variation in the Rayleigh scattered signal and blue shift in photoluminescence peak position at different temporal delays with respect to the ablation pulse corroborates the presence and size variation of nano-particles/clusters. The defect related emission band arising due to oxygen vacancy is not observed from the ZnO clusters.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
113(2013); http://dx.doi.org/10.1063/1.4800099View Description Hide Description
The transmittance and the optical constants of SrF2 thin films, a candidate material for multilayer coatings operating in the extreme ultraviolet and soft x-rays, have been determined in the spectral range of 25–780 eV, in most of which no experimental data were previously available. SrF2 films of various thicknesses were deposited by evaporation onto room-temperature, thin Al support films, and their transmittance was measured with synchrotron radiation. The transmittance as a function of film thickness was used to calculate the extinction coefficient k at each photon energy. A decrease in density with increasing SrF2 film thickness was observed. In the calculation of k, this effect was circumvented by fitting the transmittance versus the product of thickness and density. The real part of the refractive index of SrF2 films was calculated from k with Kramers-Krönig analysis, for which the measured spectral range was extended both to lower and to higher photon energies with data in the literature combined with interpolations and extrapolations. With the application of f- and inertial sum rules, the consistency of the compiled data was found to be excellent.
Enhancement mechanism of terahertz radiation from coherent longitudinal optical phonons in undoped GaAs/n-type GaAs epitaxial structures113(2013); http://dx.doi.org/10.1063/1.4799060View Description Hide Description
We have investigated the characteristics of monochromatic terahertz electromagnetic waves emitted from coherent longitudinal optical (LO) phonons in undoped GaAs/n-type GaAs epitaxial structures with various thicknesses, which range from 200 nm to 1200 nm, of the undoped GaAs layer. The terahertz waves were measured at room temperature using an optical gating method with a photoconductive dipole antenna. It was found that the intensity of the terahertz waves markedly depends on the thickness of the undoped GaAs layer. The analysis of Franz-Keldysh oscillations observed with photoreflectance spectroscopy demonstrates that the built-in electric field strength in the undoped GaAs layer, which results from the Fermi-level pinning at the surface, increases from 6.1 kV/cm to 28.4 kV/cm with a decrease in the thickness. It was revealed that the amplitude of the terahertz wave is proportional to the electric field strength in the undoped GaAs layer. This fact indicates that the efficiency of the terahertz radiation is dominated by the initial polarization of the LO phonons induced by the built-in electric field, which provides us a simple strategy for the enhancement of the terahertz radiation from the coherent LO phonons.
113(2013); http://dx.doi.org/10.1063/1.4799407View Description Hide Description
The comparison of the performances of SiO2 and SiOC layers as host matrices for optically active Eu ions is presented. A SiO2 matrix allows to observe light emission from both Eu2+ and Eu3+ ions, owing to a proper tuning of the thermal annealing process used for the optical activation of the rare earth. However, the photoluminescence efficiency of both ions remains relatively low and quite far from the requirements for technological applications, mainly due to the extensive formation of Eu-containing precipitates. A detailed study by transmission electron microscopy allowed us to analyze and elucidate the clustering process and to find suitable strategies for minimizing it. We found that the substitution of SiO2 matrix with a SiOC film allows to obtain a very bright light emission centered at about 440 nm from Eu2+ ions. In fact, SiOC is able to efficiently promote the Eu3+ → Eu2+ reduction; furthermore, Eu ions are characterized by an enhanced mobility and solubility in this matrix, and as a consequence, Eu precipitation is strongly reduced. Since SiOC is a material fully compatible with standard Si technology, Eu-doped SiOC layers can be considered a highly interesting candidate for photonic applications.