Volume 99, Issue 8, 15 April 2006
- 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
- proceedings of the 50th annual conference on magnetism and magnetic materials
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
99(2006); http://dx.doi.org/10.1063/1.2187196View Description Hide Description
Femtosecond lasers have proven to be effective tools for precise micromachining. Taking advantage of the reduced heat diffusion and the sharp ablation threshold at comparatively low energy densities, subdiffraction limit sized craters have been machined on silicon wafers by single near infrared Ti:sapphire laser pulses using a high numerical aperture objective lens. Two different ablation regimes have been identified by varying the laser fluence. While two-photon absorption dominates in the low fluence regime, electronic diffusion is a major energy transport mechanism at higher laser fluences. Time-resolved pump-and-probe side-view imaging has been performed to investigate the energy coupling to the target specimen over a wide range of fluences (up to around ) at lateral beam dimensions of the order of micrometers. The decrease of the ablation efficiency in the high fluence regime is attributed to the strong interaction of the laser pulse with the laser-induced plasma.
99(2006); http://dx.doi.org/10.1063/1.2189929View Description Hide Description
We use the theory of periodic structures, full-wave electromagnetic, and microwave circuit simulations to explain the resonant modes that propagate in metallic grids having rectangular unit cells constructed over a ground plane. We show that these metallic grids can support two types of resonant modes that have rectangular and hyperbolic isofrequency dispersion contours. By exploiting the spatial dispersion properties of these modes, a microwave harmonic splitter and a highly selective diplexer are designed and simulated. Furthermore, we provide experimental results for the diplexer and for the harmonic splitter, synthesized in microstrip technology. The proposed metallic grids utilize continuous unloaded transmission-line segments thus leading to spatial-filtering devices that are easy to fabricate and are scalable to terahertz frequencies and beyond.
99(2006); http://dx.doi.org/10.1063/1.2188050View Description Hide Description
Erbium-doped toroidal microcavity lasers are fabricated on a Si substrate using a combination of optical lithography, etching, Er ion implantation, and laser reflow. Erbium is either preimplanted in the base material or postimplanted into a fully fabricated microtoroid. Three-dimensional infrared confocal photoluminescence spectroscopy imaging is used to determine the spatial distribution of optically active Er ions in the two types of microtoroids, and distinct differences are found. Microprobe Rutherford backscattering spectrometry indicates that no macroscopic Er diffusion occurs during the laser reflow for preimplanted microtoroids. From the measured Er doping profiles and calculated optical mode distributions the overlap factor between the Er distribution and mode profile is calculated: and for postimplanted and preimplanted microtoroids, respectively. Single and multimode lasing around is observed for both types of microtoroids, with the lowest lasing threshold observed for the preimplanted microtoroids, which possess the smallest mode volume. When excited in the proper geometry, a clear mode spectrum is observed superimposed on the Er spontaneous emission spectrum. This result indicates the coupling of Er ions to cavity modes.
99(2006); http://dx.doi.org/10.1063/1.2188049View Description Hide Description
A metallic coating method is used to modify the optical properties of a dielectric photonic lattice and to achieve a near visible photonic band edge. It is experimentally shown that the linear scaling rule of a metallic band edge versus lattice constant holds only for perfect conducting metals. When a metal deviates from a perfect conducting behavior near the plasma wavelength, the metallic photonic band edge is pinned and is nearly independent of lattice constant. For our tungstenphotonic lattice, the pinning occurs at . By using a thin coppercoating to a dielectric photonic lattice, a photonic band edge at is observed. This achievement is made possible by the fact that copper is a good conductor at visible wavelengths and the linear scaling rule holds. Finally, this coating method allows for tailoring photonicproperties through material engineering at the nanometer scale.
99(2006); http://dx.doi.org/10.1063/1.2189973View Description Hide Description
The effects of doping on quantum dots-in-well infrared photodetectors have been studied by measuring the dark current,photocurrent, and spectral response. A significant reduction of dark current with decrease in doping concentration in the quantum dots has been observed. However, the photocurrent of the detectors increases with the doping. By measuring the background limited infrared photodetector temperature, we find that the optimum sheet doping concentration in these detectors is (corresponding to about one electron per dot).
High-efficiency generation of both forward and backward optical phase conjugations in one-dimensional nonlinear photonic crystals99(2006); http://dx.doi.org/10.1063/1.2191575View Description Hide Description
By using only one pump beam, we show theoretically that both forward and backward phase conjugations can be generated simultaneously and efficiently in one-dimensional -nonlinear photonic crystals of only a few micrometers in size. The photonic crystal considered here is a superlattice. By using realistic optical parameters in our calculations, we find that the phase-conjugate generation efficiency can be enhanced by four to five orders of magnitude as compared with that in a homogeneous CdS medium of the same nonlinearity and length. The enhancement is caused by multiple reflections as well as strong field localization introduced by the shifted band-edge state, the gap localized state, or the shifted defect state.
99(2006); http://dx.doi.org/10.1063/1.2193164View Description Hide Description
A detailed study of surface laser damage performed on a nonlinear optical crystal, urea L-malic acid, using laser pulses at repetition rate from a Q-switched Nd:YAG laser at wavelengths of 532 and is reported. The single shot and multiple shot surface laser damage threshold values are determined to be and at and and at laser radiation, respectively. The laser damageanisotropy is consistent with the Vickers mechanical hardness measurement performed along three crystallographic directions. The Knoop polar plot also reflects the damage morphology. Our investigation reveals a direct correlation between the laser damage profile and hardnessanisotropy. Thermal breakdown of the crystal is identified as the possible mechanism of laser induced surface damage.
- PLASMAS AND ELECTRICAL DISCHARGES
99(2006); http://dx.doi.org/10.1063/1.2189932View Description Hide Description
High energy laser, pulse duration, irradiating heavy targets in vacuum produce intense plasma and generate emission of various energetic ion groups. The ion intensity is high along the normal to the irradiated target surface and high charge state and high velocity ions are produced. The characteristics of the ion streams were investigated by using an electrostatic ion energy analyzer and different ion collectors were placed at various angles with respect to the target normal. The ion energy distribution as a function of the ion charge state was measured and the comparison of the properties of different ion groups generated by laser beams at two different energies was carried out. Measurements point out that five ion groups or more can be generated by the laser interaction with the preformed plasma, with different “equivalent ion temperatures.” Slow, thermal, fast, and very fast ions follow a Boltzmann-like distribution; the equivalent temperatures of different ion groups were estimated to reach values up to about .
99(2006); http://dx.doi.org/10.1063/1.2189951View Description Hide Description
A laser-light-scattering (LLS) method for measuring the size and density of nanoparticles generated in reactive plasmas has been developed. The size and density of the nanoparticles are determined from their thermal coagulation that takes place after turning off the discharge. The measurable size and density range of the LLS method is , where , , , and are the density, size, and diffusion length of the nanoparticles, and the density of a background gas, respectively. The method has been demonstrated by measurement of the size and density of nanoparticles formed by the radio-frequency discharge of dimethyldimethoxysilane diluted with Ar. Using a simple optical setup for the LLS measurement,nanoparticles are detected down to in size when they are generated at a density of . The developed method is widely applicable to other systems in which thermal coagulation takes place.
99(2006); http://dx.doi.org/10.1063/1.2188084View Description Hide Description
Controlling the debris from a laser-generated tin plume is one of the prime issues in the development of an extreme ultraviolet lithographic light source. An ambient gas that is transparent to radiation can be used for controlling highly energetic particles from the tin plume. We employed a partial ambient argon pressure for decelerating various species in the tin plume. The kinetic energy distributions of tin species were analyzed at short and large distances using time and space resolved optical emission spectroscopy and a Faraday cup, respectively. A fast-gated intensified charged coupled device was used for understanding the hydrodynamics of the plume’s expansion into argon ambient. Our results indicate that the tin ions can be effectively mitigated with a partial argon pressure. Apart from thermalization and deceleration of plume species, the addition of ambient gas leads to other events such as double peak formation in the temporal distributions and ambient plasma formation.
99(2006); http://dx.doi.org/10.1063/1.2189215View Description Hide Description
We report the observation of the Kerr effect in a streamer head in experiments with electrical breakdown of water. The propagation of the streamer in water is associated with localized enhancement of the electric field surrounding the streamer head. Polarizational measurements show that the electric field around the streamer head is estimated to be , which is approximately six times higher than the maximum interelectrode electric field of . Electrostatic simulations support the experimental data.
99(2006); http://dx.doi.org/10.1063/1.2191567View Description Hide Description
Surface reactions during etching of organic low- film by and plasmas were studied through observations of the surface resident species using in situ infrared spectroscopy and in vacuo electron-spin-resonance techniques. We observed surface modifications by the formation of CN and NH bonds after exposure to plasmas generated from and . The number of carbon dangling bonds were greater in processes where was present. The passivation of carbon dangling bonds leads to , , and CN functionalities, which are the precursors for etching products that are desorbed, which includes volatile forms such as HCN and .
99(2006); http://dx.doi.org/10.1063/1.2191568View Description Hide Description
For a number of years it has been known that the alkali rf-discharge lamps used in atomic clocks can exhibit large amplitude intensity oscillations. These oscillations arise from ion-acoustic plasma waves and have typically been associated with erratic clock behavior. Though large amplitude ion-acoustic plasma waves are clearly deleterious for atomic clock operation, it does not follow that small amplitude oscillations have no utility. Here, we demonstrate two easily implemented methods for generating small amplitude ion-acoustic plasma waves in alkali rf-discharge lamps. Furthermore, we demonstrate that the frequency of these waves is proportional to the square root of the rf power driving the lamp and therefore that their examination can provide an easily accessible parameter for monitoring and controlling the lamp’s plasma conditions. This has important consequences for precise timekeeping, since the atomic ground-state hyperfine transition, which is the heart of the atomic clock signal, can be significantly perturbed by changes in the lamp’s output via the ac-Stark shift.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
Extended Mie-Grüneisen molecular model for time dependent dielectric breakdown in silica detailing the critical roles of tetragonal bonding, stretched bonds, hole capture, and hydrogen release99(2006); http://dx.doi.org/10.1063/1.2189930View Description Hide Description
An extended Mie-Grüneisen molecular model is presented, which describes a bond-breakage process for tetragonal molecules in silica and the trap-generation process that occurs during time dependent dielectric breakdown (TDDB) testing. This quantitative molecular model correctly describes important physics routinely reported for silica TDDB testing: the generation of centers, a breakdown strength of , an effective dipole-moment range of , and a zero-field activation energy range for bond breakage of . The bond-breakage/trap-generation mechanism is shown to occur when the Si ion transitions from its primary energy minimum (with fourfold coordination) to a secondary saddle point (with threefold coordination). The molecular model also shows clearly that current induced hole capture and hydrogen release can play critically important roles in the TDDB process.
99(2006); http://dx.doi.org/10.1063/1.2188079View Description Hide Description
The spectroscopic properties of single crystal for the transition near are presented. The absorption cross section, the emission cross section, and the potential laser gain near are investigated for both and polarizations. The broad absorption band at about and high value of the above cross section are promising features for obtaining a laser diode pumped laser. The maximum emission cross sections are and at for the and polarizations, respectively. The potential laser gain curves indicate that more than tunability range is expected for this material.
99(2006); http://dx.doi.org/10.1063/1.2189018View Description Hide Description
Variable angle spectroscopicellipsometry (VASE™) is used as a tool to characterize properties such as optical constant, thickness, refractive index depth profile, and pore volume fraction of single and bilayer porous low-films. The porous films were prepared using sacrificial pore generator (porogen) approach. Two sets of porous films with open- and closed-pore geometries were measured. Three models were used for data analysis: Cauchy, Bruggeman effective medium approximation (BEMA), and graded layer. Cauchy, a well-known model for transparent films, was used to obtain thickness and optical constant, whereas BEMA was utilized to calculate the pore volume fraction from the ellipsometric data. The Cauchy or BEMA models were then modified as graded layers, resulting in a better fit and a better understanding of the porous film. The depth profile of the porous film implied a more porous layer at the substrate-film interface. We found 3%–4% more porosity at the interface compared with the bulk for both films. This work shows that VASE™, a nondestructive measurement tool, can be used to characterize single- and multigraded layer porous films quickly and effectively.
99(2006); http://dx.doi.org/10.1063/1.2187417View Description Hide Description
Grain-boundary relaxation in nanocrystalline Fe was studied by high-resolution transmission electron microscopy (HREM), and its effect on plasticity was characterized by nanoindentation. Samples with grain size of were synthesized by mechanical attrition and subsequently annealed at low temperatures (80 and ) without affecting the grain size. While the hardness is not significantly affected by annealing, the strain-rate sensitivity peaks as a function of annealing time, suggesting two competing processes. HREM images show grain-boundary relaxation during annealing. Initially disconnected lattice planes were observed to evolve into a more continuous and ordered structure with regularly spaced grain-boundary dislocations.
Energy transfer and frequency upconversion involving triads of ions in (, ) doped fluoroindate glass99(2006); http://dx.doi.org/10.1063/1.2189207View Description Hide Description
Blue and ultraviolet luminescence in (, ) doped fluoroindate glass is studied for excitation in the red region . Frequency upconversion (UC) is observed due to energy transfer(ET) among three ions initially excited to the state corresponding to the ET process . Additionally, UC luminescence from states and of is observed for an excitation wavelength resonant with transitions of the ions. The characterization of the luminescence signals allowed to determine ET rate among the ions and provides evidence of interconfigurational ET between and ions.
High quality films with controllable in-plane orientations grown on yttria-stabilized zirconia substrates99(2006); http://dx.doi.org/10.1063/1.2194228View Description Hide Description
The pulsed-laser deposition (PLD) technique was used to grow high superconducting(YBCO)films on both virgin and ion-bombarded yttria-stabilized zirconia (YSZ) substrates. To pattern high films for device applications, the ion milling technique is often used to turn virgin YSZ substrates into ion-bombarded substrates. Multilayered processes require the growth of high films on these ion-bombarded substrates. The purpose of this work was to investigate the growing conditions for these two kinds of substrate surfaces. We found that high quality 0° in-plane orientation films can be grown on either substrate when the growthtemperature is about . The thin filmgrown at this temperature has of about and of about at . On virgin substrates, the in-plane orientations of YBCOfilmsgrown within the temperature range of exhibit a mixture of 0° and 45° domains. As the growthtemperature decreases, the dominant orientation shifts gradually from 0° to 45°. On the other hand, on ion-bombarded YSZ substrates, the in-plane orientation of YBCOfilmsgrown within the same temperature range shows that the 45° domain is more prominent. Furthermore, 9° subpeaks appear around the 0° peak on ion-bombarded YSZ substrates. At a lower growthtemperature of around , only the 45° domain exists on the virgin substrate, while a small amount of 0° domain is present with the majority of 45° domain on the ion-bombarded substrate. The and of the filmsgrown at around on virgin substrates are as good as filmsgrown at high temperatures, despite the difference in the in-plane orientations.
Influence of postdeposition annealing on the structural and optical properties of cosputtered Mn doped ZnO thin films99(2006); http://dx.doi.org/10.1063/1.2188083View Description Hide Description
The influence of postgrowth annealing on the structural and optical properties of rf cosputtered Mn dopedZnOthin filmsdeposited on glass substrate at room temperature has been investigated. All as depositedfilms are highly textured, with the axis of the wurtzite structure along the growth direction. The as grownfilms are in a state of compressive stress and a reduction in stress with postgrowth annealing treatment are observed. The band gap of Mn dopedZnOfilms is slightly larger than the pure ZnOfilm and is found to decrease with an increase in annealing temperature for all the samples. The optical dispersion of refractive index with photon energy in films with varying and different annealing temperature is studied in the light of single oscillator and Pikhtin-Yas’kov [Sov. Phys. Semicond.15, 81 (1981)] model, respectively.