Volume 98, Issue 4, 15 August 2005
- applied physics reviews
- 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:
- APPLIED PHYSICS REVIEWS
98(2005); http://dx.doi.org/10.1063/1.1992666View Description Hide Description
The semiconductorZnO has gained substantial interest in the research community in part because of its large exciton binding energy which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of -type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev.142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys.6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett.16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett.18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors2, 624 (1968)], in which was used as the -type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys.13, 1475 (1974)], junctions (1975) by Tsurkan et al. [Semiconductors6, 1183 (1975)], and Ohmic contacts by Brillson [J. Vac. Sci. Technol.15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity -type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of -type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of -type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, -type doping, band-gap engineering, devices, and nanostructures.
- LASERS, OPTICS, AND OPTOELECTRONICS
Improvement of the atmospheric discharge laser-triggered ability using multiple pulses from a kilohertz KrF laser98(2005); http://dx.doi.org/10.1063/1.2009076View Description Hide Description
The potential ability of lasers to control lightning can be improved by using a train of pulses with submillisecond separations. Laser-triggered experiments in a small-scale ( gap) atmospheric discharge facility show that the triggering is dramatically enhanced when a five-pulse train of sub-Joule energy is used instead of a single pulse. This effect increases rapidly as the pulse interval is reduced. It appears that at a submillisecond pulse interval, sufficient positive and negative ions survive in subsequent pulses, thus enabling easy deionization. Hence, significant plasma buildup occurs from one pulse to the next. However, this persistence of ions would appear to imply that the rate of recombination (effectively a charge transfer between ions) is considerably lower than previously believed.
98(2005); http://dx.doi.org/10.1063/1.2001150View Description Hide Description
Theoretical investigations are carried out for close-to-lasing two-dimensional finite-sized photonic crystals with active (gain) lattice points. First, laser oscillations with lower thresholds are found to occur near the photonic band edges where optical gain is enormously intensified. For several modes isolated around the band edge, the field-intensity spectra in reciprocal space and the Poynting-vector distributions in real space are investigated in detail in close-to-lasing photonic crystals. By comparing the phenomena that occur in photonic crystals with a symmetric or an asymmetric outward form, this paper clarifies the differences in the feedback mechanisms of these crystals. In a symmetric photonic crystal, laser oscillation occurs through the waves propagating along the straight passages. This feedback is basically the same as that of ordinary one-dimensional lasers, although it exhibits a complicated behavior that light waves propagating in a variety of directions interfere with each other. In an asymmetric photonic crystal, laser oscillation occurs through the waves circulating within the crystal, which could be called recurrent-photon feedback. This feedback, however, can be construed as an extension of the feedback in ordinary one-dimensional distributed-feedback lasers.
Analytical stress modeling of high-energy laser windows: Application to fusion-cast calcium fluoride windows98(2005); http://dx.doi.org/10.1063/1.2007874View Description Hide Description
The performance of a laser-window material must be assessed not only in terms of its ability to transmit high-power beams without generating undue optical distortion but also in terms of the constraints imposed by stress-related failure modes. In operational use, the stress field images the superposition of stresses originating from the mechanical load created by the pressure differential and the thermal load created by the laser beam. Here, we provide the tools to carry out an analysis of both pressure- and beam-induced stresses, and illustrate the procedure in the context of assessing the performance of a “model” window made of fusion-cast . The analysis assumes (a) operation on a time scale such that lateral heat diffusion can be ignored, and (b) cylindrically symmetric Gaussian beam shapes, which permit straightforward calculations of stress distributions that should be representative of worst case situations. Pressure-induced stresses strongly depend on the window’s aspect ratio, which suggests increasing the thickness to minimize the stress, but considerations relating to the optical performance require minimum allowable thicknesses based on a Weibull statistical analysis of the fracture probability. Beam-induced stresses are best evaluated in terms of (a) thickness-averaged radial and azimuthal stresses, which increase linearly with exposure time and depend on radial distances through the truncation parameter, and (b) across-the-thickness stress deviations relative to the average stress, which are caused by surface absorption and reach steady-state configurations on a time scale much shorter than the characteristic time for lateral heat transport. The average stress is always compressive and equibiaxial in the central region of the window, but its azimuthal component turns tensile in the rim region, thus threatening the structural integrity through brittlefracture. In addition, the coating-induced stress results in on-axis surface compressions that may exceed the yield strength of the windowpane material. In this light we formulate a figure of merit for stress, which demonstrates that promising laser-window materials must combine a small stress factor (expansion coefficient times elastic modulus) with superior thermal properties in terms of the product of heat capacity and thermal conductivity; and are the only known candidates that exhibit outstanding optical features at chemical laser wavelengths together with acceptable thermomechanical properties.
98(2005); http://dx.doi.org/10.1063/1.1991967View Description Hide Description
We report on various sexiphenyl derivatives as gain media in organic solid-state lasers. The molecules involved in this research are simple -sexiphenyl, the laser dye molecule 2,5,2””’,5””’-tetra--butyl--sexiphenyl (TBS) and the spirolinked sexiphenyl-derivative 2,7-bis(biphenyl-4-yl)-2’,7’-di-tert-butyl-9,9’-spirobifluorene. It appears that the morphology of vacuum-deposited thin films is highly dependent on the sterical dimensions of the respective molecules. Whereas thin films based on simple -sexiphenyl comprise large clusters which significantly deteriorate their waveguiding properties; films formed by TBS, and the spiroderivative show a dramatically improved morphology with reduced surface roughness. Therefore amplified spontaneous emission(ASE) and lasing are demonstrated in both of the last but not in films based on -sexiphenyl. Second-order distributed-feedback lasers with TBS as the active medium have been prepared with an emission between 390 and 435 nm depending on the grating period of the Bragg reflector. While the ASE characteristics are similar in films formed by TBS and the spiroderivative, TBS exhibits even superior laser threshold densities which are as low as at a wavelength of 396 nm.
98(2005); http://dx.doi.org/10.1063/1.2009820View Description Hide Description
A special technique for the modification of laser-induced breakdown spectroscopy(LIBS) has been developed to improve the spectral quality of hydrogen emission from a solid sample in helium gas at atmospheric pressure. In this technique, the plasma was generated by focusing a fundamental Nd-YAG (yttrium aluminum garnet) laser into a surrounding helium gas. The helium atoms excited to their metastable states would then serve to excite the atoms of the solid material vaporized by using another Nd-YAG laser. When properly synchronized, the resulting hydrogen emission line of H I 656.2 nm shows a dramatic improvement of the emission intensity and the spectral quality over what was obtained by conventional LIBS technique. This study further reveals that this improvement is mainly due to the role of the metastable excited state in a helium atom, which allows the delayed detection to be performed at a favorable moment when the charged particles responsible for the strong Stark broadening effect in the plasma have mostly disappeared.
98(2005); http://dx.doi.org/10.1063/1.2007853View Description Hide Description
We propose an isotropic three-dimensional (3D) negative-refractive-index medium using transmission lines loaded with reactive elements. The medium is referred to as the 3D dual transmission line (TL). Analytical expressions are derived which provide insight into the backward-wave propagation supported by the proposed medium. The dispersion characteristics of a physically realizable 3D dual TL are computed using a full-wave eigenmode solution of Maxwell’sequations based on the finite-element method. In addition, scattering parameters for a slab made of the 3D dual TL are presented. The described approach can be generalized for synthesizing a wide range of 3D metamaterials with tailored material parameters including both isotropic and anisotropic designs.
Si wafer bonded of distributed Bragg reflectors for -wavelength vertical cavity surface emitting lasers98(2005); http://dx.doi.org/10.1063/1.2009075View Description Hide Description
Amorphous silicon and amorphous silicon nitride layers deposited by magnetron sputtering have been analyzed in order to determine their optical and surface properties. A large value of of index difference is found between these materials.Distributed Bragg reflectors(DBRs) based on these dielectric material quarter wave layers have been studied by optical measurements and confronted to theoretical calculations based on the transfer matrix method. A good agreement has been obtained between the experimental and expected reflectivities. A maximum reflectivity of 99.5% at and a large spectral bandwidth of are reached with only four and a half periods of . No variation of the DBR reflectivity has been observed with the time nor when annealed above and stored during few months. This result allows us to use this DBR in a metallic bonding process to realize a vertical cavity surface emitting laser(VCSEL) with two dielectricDBRs. This bonding method using as the bonding medium and Si substrate can be performed at a low temperature of without damaging the optical properties of the microcavity. The active region used for this VCSEL is based on lattice-matched quantum wells and a laser emission has been obtained at room temperature on an optically pumped device.
98(2005); http://dx.doi.org/10.1063/1.2010627View Description Hide Description
We report a comparative study on the performance of three optically pumped, type-II quantum well lasers with differing quantum well(QW) confinement. One of the active regions emphasized hole confinement, another emphasized electron confinement, while the third incorporated both electron and hole confinements. In all cases the wells were inserted in a thick waveguide/absorber region. The lasing wavelengths at were 2.26, 3.44, and , respectively. The maximum peak output powers and differential quantum efficiencies at were similar for the hole well and W lasers , but significantly reduced in the electron-well-only laser . Waveguide loss measurements via the traditional quantum efficiency versus cavity length method and by a Hakki-Paoli method revealed that all three lasers had low waveguide loss that either increased slowly or not at all with increasing temperature. However, the laser’s internal efficiency, , showed a linear decline with increasing temperature, with the of the electron-well-only laser significantly less than the other two. The data suggest that for antimonide-based type-II designs, strong hole confinement is essential for improved performance. The data further suggest that it is hole leakage from the QW and/or hole dilution that is largely responsible for the degradation in laser performance.
98(2005); http://dx.doi.org/10.1063/1.2008385View Description Hide Description
Measurements of the attenuation and excitation efficiency of the long-range surface-plasmon-polariton mode supported by waveguides comprised of one or many thin metalfilms of finite width embedded in dielectric were made in the near infrared . Aufilms 31, 25, and 20 nm thick, and Agfilms 20 nm thick were used to implement the structures. The lowest attenuations measured among the Au and Ag waveguides are 0.42 and , respectively, corresponding to propagation lengths of 10 340, and , respectively. These propagation lengths are longer than those of the single-interface surface-plasmon polariton in the corresponding semi-infinite structures by factors of 93 and 138, respectively. These factors are the largest reported to date for long-range surface-plasmon-polariton waves. The largest excitation efficiency measured among the set of Au structures is 98%. Theoretical results were obtained for all of the structures characterized experimentally using an accurate electromagnetic-field model. Theory and experiment agree to within about 5% over the 31- and 25-nm-thick Au structures, but a thickness-dependant permittivity must be assumed in order to achieve agreement to within 12% for the 20-nm Au structures.
Nonlinear pulse evolution in seeded free-electron laser amplifiers and in free-electron laser cascades98(2005); http://dx.doi.org/10.1063/1.2010624View Description Hide Description
The advances in laser technology have made available very short and intense laser pulses which can be used to seed a high-gain single-pass free-electron laser(FEL)amplifier. With these seed pulses, a regime of the FEL interaction where the radiation evolution is simultaneously dominated by nonlinear effects (saturation) and time-dependent effects (slippage) can be explored. This regime is characterized by the propagation of a solitary wavelike pulse where the power of the optical wave grows quadratically with time, its pulse length decreases and the spectral bandwidth increases. We analyze the interplay between the field and particle dynamics of this propagation regime which was studied before and termed super-radiance. Furthermore we analyze the properties of the strong higher-order harmonic emission from this wave and its behavior when propagating in a cascade FEL. The super-radiant pulse is indeed capable of passing through the stages of a cascade FEL and to regenerate itself at the wavelength of the higher-order harmonic. The optical pulse obtained is shorter than a cooperation length and is strongly chirped in frequency, thus allowing further longitudinal compression down to the attosecond time scale.
98(2005); http://dx.doi.org/10.1063/1.2008389View Description Hide Description
This work presents an investigation of the photoemission properties of niobium. The quantum efficiency (QE) of niobiumcathodes was measured for a variety of surface preparations relevant to the operation of a superconducting injector. The dependence of the QE on wavelength, applied field, and laser cleaning energy was determined. The three-step model of photoemission was adapted to fit the observed emission behavior. QE values of for 266 nm, for 248 nm, and for 193 nm were observed with a bias field of .
Polarization-independent and fast-response phase modulation using a normal-mode polymer-stabilized cholesteric texture98(2005); http://dx.doi.org/10.1063/1.2037191View Description Hide Description
Fast-response, polarization-independent, and hysteresis-free phase-only modulation using a normal-mode polymer-stabilized cholesteric texture (PSCT) is demonstrated. Although the remaining phase change in the high-voltage regime is small, it is still useful for making microdevices. Polarization-independent tunable-focus microlens arrays using such a PSCT are demonstrated.
98(2005); http://dx.doi.org/10.1063/1.2030414View Description Hide Description
Responsivity measurements are reported for electro-optic field sensors operated under applied electric fields at ac frequencies in the vicinity of the acoustic resonance values of the crystals. At these frequencies, piezoelectric effects dominate the sensor output. These resonanceeffects are well known and commonly considered parasitic. However, we propose their use as a sensitivity-enhancing mechanism for electric-field detection. We have found that our field sensors operated within these resonances responded linearly with the applied field strength and exhibited increases in their intrinsic sensitivities as high as 350 times larger than their normal, electro-optic values. Our modeling of the data suggests that the sensitivity enhancements are produced by the interplay between photoelastic shifts in the refractive indices and the physical vibration modes of the crystals. Aside from narrowband applications, these resonant enhancements can be exploited with fields at frequencies well beyond the narrow bandwidth of the acoustic resonance, if the fields are properly modulated.
98(2005); http://dx.doi.org/10.1063/1.2030416View Description Hide Description
We use an analytic Bloch-mode approach to investigate one-dimensional planar Bragg waveguides (PBWGs) with an air- or a glass-core layer sandwiched within two symmetric Bragg gratings formed by alternate arrays of air and glass layers. The calculation results show that there exist simultaneously two kinds of guided modes as the gap-guided modes resulting from the photonic band-gap effect and the index-guided modes formed by the total internal reflection effect. We calculate the electromagnetic (EM)-field distributions of these guided modes with different parallel wave vectors and find remarkably different EM-field profiles between the index-guided modes and the gap-guided modes. In order to explore the propagation properties of the PBWGs, we analyze the group velocity and the group-velocity dispersion of different types of guided modes. The results show that the guided mode properties are significantly influenced by the core material as well as the cladding structure. The analytic Bloch-mode approach can help to investigate and design the PBWGs in a simple, efficient, and accurate manner.
- PLASMAS AND ELECTRICAL DISCHARGES
Time evolution of electron energy distribution function and plasma parameters in pulsed and unbalanced magnetron argon discharge98(2005); http://dx.doi.org/10.1063/1.1990264View Description Hide Description
The temporal behavior of the electron energy distribution function and plasma parameters in the vicinity of the substrate have been investigated in detail by performing time-resolved probe measurements in a pulsed and unbalanced magnetron argon discharge. A midfrequency unipolar dc pulse with an on-time average power of and a duty cycle of 50% was applied to the metallic cathode target. It was found that the high-energy electrons with energies higher than the sheath potential energy are generated within a few microseconds after the dc pulse is turned on and the electron energy distribution functions during the pulse-on period show a bi-Maxwellian distribution with the high-energy electron group. In the afterglow after the dc pulse is turned off, the initial fast decay of the high-energy electrons and the subsequent diffusive slower decay of the low-energy electrons were observed. This temporal behavior of the electron energy distribution function reflected two-fold decay characteristics of electron density and electron temperature when approximated by a biexponential function with two characteristic decay times and an initial fast decay time of less than a few microseconds and a subsequent slower decay time of few tens of microseconds were observed. The temporal behavior of the other plasma parameters were presented and these results were explained in view of electron heating by deeply penetrating the high-voltage cathodesheath and electron transport.
98(2005); http://dx.doi.org/10.1063/1.2007851View Description Hide Description
Despite the high pressure employed in plasma display panels, the energy balance of low-energy electrons is found to be dominated by inelastic collisions, and the resulting nonlocal electron kinetics plays a key role in the striation formation. Surface charge accumulation on the anodedielectric, however, is also needed for striations to form. It is the combined effect of surface charges and nonlocal electron kinetics that results in the striation formation in plasma display panel cells. Two-dimensional fluid simulations, which assume local electron kinetics, and two-dimensional particle-in-cell Monte Carlo collision simulations with a bare conducting anode show that striations do not form if either the nonlocal electron kinetics or the surface charge accumulation is not considered.
98(2005); http://dx.doi.org/10.1063/1.2009821View Description Hide Description
A three-dimensional model of atom and ion dynamics in the accelerating channel of a stationary plasma thruster (SPT) is based on the model kinetic equations. Electron dynamics is described based on available experimental data. The expression for the electric field was obtained from the equation of electron impulse conservation law with the use of “thermalized potential” assumption. The simulation was performed for a particular SPT with a thrust vector deviation. A quantitative agreement with the experimental results was obtained on thrust and thrust vector deviation angles for two different SPT operation modes. Calculated distributions of macroparameters demonstrate azimuthally asymmetrical ionization and acceleration processes in this particular SPT.
Influence of addition on the microstructure and discharge properties of Mg–Zr–O protective layers in alternating current plasma display panels98(2005); http://dx.doi.org/10.1063/1.2009077View Description Hide Description
Mg–Zr–O protective layers for alternating current plasma display panels were deposited by -beam evaporation. The effect of the addition on both the discharge properties [firing voltage , minimum sustaining voltage , and memory coefficient (MC)] and the microstructure of deposited Mg–Zr–O films were investigated. The results show that the filmmicrostructure changes and the electron emission enhancement due to the addition are the main reasons for the improvements of the discharge properties of Mg–Zr–O films. A small amount of Zr solution in MgO under its solidsolubility can effectively increase the outer-shell valence electron emission yield so as to decrease and compared with using a pure MgO protective layer. The ratio has a great effect on the filmsurface conditions. Proper surface morphologies make a good contribution to obtain large MC in accordance with lower firing voltage.
98(2005); http://dx.doi.org/10.1063/1.2009819View Description Hide Description
This paper is aimed to investigating how the electrical power consumed in generating silent discharges is influenced by the width of the annular discharge gap in the reactor. The silent discharges have been generated in conventional tubular reactors forming annular discharge gaps of -, -, and width, with one dielectric barrier pasting on the ground electrodes of the reactors. The reactors were fed by dry air flowing with a constant rate at atmospheric pressure and room temperature, and stressed by an ac voltage. The discharge power consumed in generating silent discharges has been measured as a function of the voltage applied to the reactors up to a of 20 000 V, and calculated by Manley’s equation under the same discharge conditions. The experimental and calculated results have shown that when the reactor has a discharge gap of - or width, Manley’s equation agrees only with the experimental results in range of the voltage of . In the higher range of the applied voltage of , Manley’s equation has been modified to fit the experimental results. With the reactor that has a discharge gap of width, Manley’s equation agrees with the experimental results over the whole range of the voltage applied to the reactor. The results have been explained by recorded oscillograms showing the wave form of the ac voltage applied to the reactors and the corresponding discharge current.