Volume 108, Issue 8, 15 October 2010
Index of content:
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This paper presents a systematic review of long period fiber gratings (LPFGs) written by the laser irradiation technique. First, various fabrication techniques based on laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the -laser-induced LPFGs are analyzed. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the -laser-induced LPFGs. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Fifthly, sensing applications of the -laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Finally, communication applications of the -laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers.
- Lasers, Optics, and Optoelectronics
Suppression of bimolecular recombination by UV-sensitive electron transport layers in organic solar cells108(2010); http://dx.doi.org/10.1063/1.3488609View Description Hide Description
Incorporating UV-sensitive electron transport layers (ETLs) into organic bulk heterojunction (BHJ) photovoltaic devices dramatically impacts short-circuit current and fill factor characteristics.Resistivity changes induced by UVillumination in the ETL of inverted BHJ devices suppress bimolecular recombination producing up to a two orders of magnitude change in . Electro-optical modeling and light intensity experiments effectively demonstrate that bimolecular recombination, in the form of diode current losses, controls the extracted photocurrent and is directly dependent on the ETL resistivity.
108(2010); http://dx.doi.org/10.1063/1.3498811View Description Hide Description
We have studied the energy splitting of bound photonic states in Kerr-nonlinear double photonicwaveguides. The structure is formed by embedding two Kerr-nonlinear photonic crystals in a linear photonic crystal. When an intense external laser field is applied to the system, two coupled waveguides are induced. These waveguides may also be induced by applying a stress field to the system. Due to the coupling between waveguides,bound states split into symmetric and antisymmetric pairs. Using the transfer matrix method we obtained expressions for these split bound states and their energy separation. We have shown that the energy splitting depends on the separation of the waveguides and the intensity of the applied laser. The energy splitting predicted by our expressions agrees well with the splitting of resonant states in simulated transmission spectra. Our findings agree qualitatively with existing experimental observations of coupled photonic wells fabricated from photonic crystals. We found that the bound state energy levels can be tuned using the laser and stress fields, and that the system can be switched between zero to one or more pairs of resonant states. The results described here can be used to develop all-optical switches, tunable filters and nonlinear coupled waveguides.
108(2010); http://dx.doi.org/10.1063/1.3493266View Description Hide Description
An ultrafast microwaveannealing method, different from conventional thermal annealing, is used to activate Mg-implants in GaN layer. The x-ray diffraction measurements indicated complete disappearance of the defect sublattice peak, introduced by the implantation process for single-energy Mg-implantation, when the annealing was performed at for 15 s. An increase in the intensity of Mg-acceptor related luminescence peak (at 3.26 eV) in the photoluminescencespectra confirms the Mg-acceptor activation in single-energy Mg-implanted GaN. In case of multiple-energy implantation, the implant generated defects persisted even after annealing, resulting in no net Mg-acceptor activation of the Mg-implant. The Mg-implant is relatively thermally stable and the sample surface roughness is 6 nm after annealing, using a 600 nm thick AlN cap. In situ Be-doped GaN films, after annealing have shown Be out-diffusion into the AlN layer and also in-diffusion toward the GaN/SiC interface. The in-diffusion and out-diffusion of the Be increased with increasing annealing temperature. In fact, after annealing, only a small fraction of in situdoped Be remained in the GaN layer, revealing the inadequateness of using Be-implantation for forming p-type doped layers in the GaN.
Diffraction and fringing field effects in small pixel liquid crystal devices with homeotropic alignment108(2010); http://dx.doi.org/10.1063/1.3499279View Description Hide Description
Reducing the pixel dimensions of liquid crystal microdisplays in search of high resolution has a fundamental impact on their electro-optic behavior. The liquid crystal director orientation becomes distorted due to fringing fields and diffractioneffects influence the optical characteristics of the device once the structure features approach the wavelength of the incident light. Three-dimensional finite element simulation of the liquid crystal dynamics with a variable order approach is combined with a full-vector beam propagation analysis to investigate how elasticity and diffraction limit the resolution as a function of the pixel size for transmissive and reflective architectures with vertical liquid crystal alignment. The key liquid crystalproperties are considered and the importance of materials with high birefringence is confirmed for small pixel devices as these improve the contrast for a fixed pixel size.
108(2010); http://dx.doi.org/10.1063/1.3490766View Description Hide Description
We define the confinement factor for a subwavelength resonator, present a simple approach to calculate it numerically, and demonstrate that by using a specific resonator based on a silver rod coated with active material. Unlike the common approach of using the profile of the resonant electromagnetic mode, our formulation relies on using the characteristic equation for the complex frequency with inclusion of damping due to Joule and radiative losses. This removes an arbitrariness related to the definition of the mode volume in subwavelength resonators. Our analysis of a coated silver rod suggests that there is an optimal value for the active region thickness. The confinement factors can be comparable to that in Fabry–Perot resonators, radiative losses are comparable or exceed Joule losses and the threshold gain is high.
Temperature dependence of middle infrared absorption lines in silver halide crystals doped with , , and ions108(2010); http://dx.doi.org/10.1063/1.3483954View Description Hide Description
The electron-phonon coupling was investigated for mid-infrared transitions in silver halide crystals doped with Pr, Dy, or Nd. The temperature dependence of the absorptionspectral line widths and the line shifts were measured in mid-infrared absorption lines in these crystals. It was shown that in each case the temperature dependence is consistent with the two-phonon Raman process model having a single characteristic Debye temperature. This Debye temperature was found to be in the range of 142–155.5 K. It was also found that below 50 K the line widths are caused by inhomogeneous crystal strains.
108(2010); http://dx.doi.org/10.1063/1.3504599View Description Hide Description
We studied concentration quenching of electroluminescence(EL) in organic light-emitting diodes with a neat fac-tris(2-phenylpyridinato-N, ) iridium (III) emitting layer of different thicknesses sandwiched between electron and hole blocking layers. The intensity of the green emission decreased rapidly with increasing thickness and was reversely correlated with the tail band emission. The overall light output power reached the minimum at 4 nm, and attained a saturated value for thicker than 6 nm. These results are interpreted as evidence that concentration quenching in originates from both short and long-range energy transfer between excited and ground states of molecules. The EL quenching magnitude was found to be independent of the injection current, indicating that biexcitonic annihilation plays a minor role.
108(2010); http://dx.doi.org/10.1063/1.3498805View Description Hide Description
In this paper, we discuss the optical characteristics and polarization anisotropy of a tensile strained polar -plane InGaN/AlInN quantum well. We found that if the quantum well is under the tensile strain, the -like state will be lifted up so that the emitted light will be TM mode. In addition, with a particular aluminum composition of the AlInN alloy as the barrier for the tensile strained InGaNquantum well, it is possible to reduce quantum-confined Stark effect. The self-consistent Poisson and Schrödinger solver has been used for studying light emitting characteristics. Our results show that the tensile strained InGaNquantum well on AlInN barrier has much larger optical gain and lower threshold carrier density compared to the conventional InGaN/GaN system, and it has a potential to be TM light source for edge emitting laser diodes with the photonic crystal cavity made by nanorod arrays.
108(2010); http://dx.doi.org/10.1063/1.3475369View Description Hide Description
The temporal and spatial evolution of electron temperature and electron density from Sn plasma produced by a laser has been investigated in vacuum using spectroscopic methods. The plasma parameters were inferred by the Boltzmann plot method from experimentally observed line profiles of singly ionized Sn and Stark broadened profiles. At a laser intensity of , electron temperature and density were measured to be within 1.13 eV to 0.53 eV and to , respectively, for delay times between 200 ns and 1100 ns, and at distances up to 5 mm along the target normal. The results show the electron temperature and density from Sn plasma produced by a laser with wavelength of to be lower than previously reported results using a 1064 nm laser in a similar parameter regime. The lower temperature in the region far away from the target surface confirms the smaller interaction region for laser as compared with that of neodymium-doped yttrium aluminum garnet laser.
108(2010); http://dx.doi.org/10.1063/1.3499638View Description Hide Description
The piezoelectric fields of localized states in trapezoidal InGaNquantum wells(QWs) were investigated using electric field dependent electroreflectance (ER) spectroscopy. From the energy shift in ER peak in a bias range from 0 to −24 V, the piezoelectric field of the dominant QWs was estimated to be −1.22 MV/cm. In contrast, the localized states exhibited a piezoelectric field that was about 30%–60% weaker than for the dominant QW states. The reduced piezoelectric field in the localized states can be attributed to the partial release of compressive strain in the local InGaN regions, along with a relative reduction in the effective reverse-bias. The relative decrease in the reverse-bias can be explained by the voltage drop in the localized states that is caused by an increase in the leakage current under a reverse-bias.
108(2010); http://dx.doi.org/10.1063/1.3500547View Description Hide Description
All optical techniques are used to inject and to study the relaxation dynamics of ballistic charge currents in clean germanium at room temperature without the application of external contacts or the use of externally applied fields. Ballistic currents are injected by the quantum interference between the transition amplitudes for direct one and two photon absorption of a pair of phase-locked and harmonically related ultrafast laser pulses. The transport of carriers following ballistic injection is temporally and spatially resolved using optical differential transmission techniques that are sensitive to the relative optical phase of the two injection pulses. The electron-hole dynamics are determined by the initial ballistic injection velocity, momentum relaxation, and space charge field effects. The injection process in Ge is similar to that in direct band gap materials but the indirect nature of Ge complicates the monitoring of the carrier dynamics, allowing the holes to play a more prominent role than in direct gap materials. The latter opens the possibility of following the hole (as opposed to the electron) dynamics.
Intersubband absorption energy shifts in 3-level system for asymmetric quantum well terahertz emitters108(2010); http://dx.doi.org/10.1063/1.3487953View Description Hide Description
Intersubband absorption energy shifts in 3-level system stemming from depolarization and excitonlike effects are investigated. Analytically, the expressions we derive present good explanations to the conventional 2-level results and bare potential transition energy results; and numerical results show that they are more exact than the previous studies to describe the 3-level system depolarization and excitonlike shift (DES) character especially for higher carrier density (more than ). One interesting detail we find is that the “large blue” DES becomes “slight redshift” in the low doping limit (less than ), which may be neglected by the previous studies of intersubband transitions. Temperature character of DES in the step well structure is also numerically studied. Finally the above are applied to calculate asymmetric step quantum well structures. The two main functional aspects of terahertz (THz) emitters are discussed and several basic optimizing conditions are considered. By adjusting the well geometry parameters and material composition systematically, some optimized structures which satisfy all of the six conditions are recommended in tables. These optimizations may provide useful references to the design of 3-level-based optically pumping THz emitters.
108(2010); http://dx.doi.org/10.1063/1.3493736View Description Hide Description
We designed, implemented, and experimentally characterized electrically thin microwave absorbers by using the metamaterial concept. The absorbers consist of (i) a metal back plate and an artificial magnetic material layer; (ii) metamaterial back plate and a resistive sheet layer. We investigated absorber performance in terms of absorbance, fractional bandwidth, and electrical thickness, all of which depend on the dimensions of the metamaterial unit cell and the distance between the back plate and metamaterial layer. As a proof of concept, we demonstrated a thick absorber of type I, with a 99.8% absorption peak along with a 8% fractional bandwidth. We have shown that as the electrical size of the metamaterial unit cell decreases, the absorber electrical thickness can further be reduced. We investigated this concept by using two different magnetic metamaterial inclusions: the split-ring resonator (SRR) and multiple SSR (MSRR). We have also demonstrated experimentally a and a thick absorbers of type II, based on SRR and MSRR magnetic metamaterial back plates, respectively. The absorption peak of the SRR layout is 97.4%, while for the MSRR one the absorption peak is 98.4%. The 10 dB bandwidths were 9.9% and 9.6% for the SRR and MSRR cases, respectively.
- Plasmas and Electrical Discharges
108(2010); http://dx.doi.org/10.1063/1.3468176View Description Hide Description
When an intense beam encounters an aperture, the transmitted current depends on the properties of the beam and the transport channel, as well as those of the aperture itself. In some cases, an increase in the incident beam current will be exactly compensated by an increase in the incident beam area, so that the current density at the aperture remains unchanged. When this occurs, the transmitted beam current becomes independent of changes in the incident beam current, providing a passive means for suppressing current fluctuations in the beam. In this article, a key requirement for the existence of this condition is derived. This requirement is shown to be fulfilled in the case of an idealized uniform focusing channel in the small-signal limit, but to be violated when the current fluctuations are not small. Even in this case, the apertured transport system retains the ability to suppress—but not totally eliminate—fluctuations in the transmitted beam current for a wide range of incident beam currents.
Optical and electrical characterization of pulse-modulated argon atmospheric-pressure inductively coupled microplasma jets108(2010); http://dx.doi.org/10.1063/1.3499272View Description Hide Description
The critical parameters determining the generation of the pulse-modulated argon atmospheric-pressure inductively coupled plasma (AP-ICP) microjet were studied by varying the power, , pulse-modulation frequency, , and duty ratio, DR. The temporal changes in the net output power, , monitored between the very high frequency power supply and matching network by an rf sampler, and emission from the antenna were measured to elucidate the behavior of this plasma. The AP-ICP microjet, which produces high-density nonequilibrium plasma, consists of an alumina discharge tube with the inner diameter of 0.8 mm. The generation diagram of the pulse-modulated plasma was created by having as the horizontal axis and DR as the vertical axis while varying up to 50 W. At , the plasma was generated at above the linear lines of and DR, which indicated the existence of the critical power-off period of approximately . At , the pulse-modulated plasma was produced above constant DR and almost independent of . The time-averaged power, , which is the product of and DR, had to be more than 8–10 W to sustain the pulse-modulated plasma. From the measurement of the temporal changes in the net power and ArI emission, the dynamic behavior of the pulse-modulated plasma was revealed as follows. The prebreakdown period was present for after the power was turned on. Once the plasma was generated, the impedance was changed and the reflected power gradually decreased. A strong emission peak was observed immediately after the breakdown, followed by the gradual increase up to the steady state. Finally, the intense afterpeak was observed at after the power was turned off.
108(2010); http://dx.doi.org/10.1063/1.3483300View Description Hide Description
We calculate the collision cross section of a charged finite cylinder (nanowire) with a beam of ions and electrons in collisionless plasma. We find that, while the shape and area of the cross section has complex dependence on the charge and orientation of the nanowire relative to the charged beam, its orientational average has a remarkably simple form: for attractive interactions, it is a linear function of the electrostatic ratio , where is the charge of the ions/electrons, is the charge on the cylinder, is the half-length of the nanowire, is the temperature of the charged species, and is the permittivity of free space. This linearity persists into the repulsive regime up until the cross sectional area is reduced to about 5% of its value for neutral collisions. We calculate the corresponding charging currents and show that the charging behavior of the nanowire in Maxwellianplasma is described by an equivalent sphere whose radius depends only on the aspect ratio of the nanowire. For small aspect ratios, the equivalent sphere has the same surface area as the nanowire.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Effect of nanoparticles on the thermal properties of decorated multiwall carbon nanotubes: A Raman investigation108(2010); http://dx.doi.org/10.1063/1.3496671View Description Hide Description
We have investigated multiwalled carbon nanotubes decorated with nanoparticles.Scanning electron microscopy and transmission electron microscopy measurements revealed that the incorporates on the nanotubes forming large cauliflowerlike aggregates and/or small crystalline particles attached to the nanotube wall, depending on the growth conditions. A detailed Raman study was performed in pristine and a series of decorated nanotubes, where we analyzed both the Raman signal from the nanotubes and from the nanoparticles. We demonstrate that the attached nanoparticles affect significantly the thermal properties of the resulting hybrid nanostructure, as revealed by differentiated reactions to laser heating. This is a crucial point for modeling and optimization of devices based on nanotubes, which properties are strongly temperature dependent.
108(2010); http://dx.doi.org/10.1063/1.3496516View Description Hide Description
Subband structure and depolarization shifts in an ultrahigh mobility quantum well are studied using magnetoinfrared spectroscopy via resonant subband Landau level coupling. Resonant couplings between the first and up to the fourth subbands are identified by well-separated antilevel-crossing split resonance, while the hy-lying subbands were identified by the cyclotron resonancelinewidth broadening in the literature. In addition, a forbidden intersubband transition (first to third) has been observed. With the precise determination of the subband structure, we find that the depolarization shift can be well described by the semiclassical slab plasma model and the possible origins for the forbidden transition are discussed.
108(2010); http://dx.doi.org/10.1063/1.3498804View Description Hide Description
In this work, the effect of confinement distance on the magnetorheological (MR)properties of a conventional MR fluid, constituted by of iron microparticles dispersed in a liquid carrier, is studied. With this aim a commercial magnetorheometer supplied with parallel-plate geometry was used. The distance between the upper and the lower plate (gap thickness) was tuned from 10 to . The steady-state and the dynamic regimes of the MR fluid in the presence of applied magnetic fields were studied as a function of the gap length. The experimental results show that in the preyield regime there is a strong increase in the magnitude of the viscoelastic moduli and the shear stress as the gap thickness is increased. The physical reason for this effect might be the influence of gap thickness on the particle structures induced by the field. This hypothesis is corroborated by microscopic observations in diluted systems. These experiments show that the aspect ratio (length/diameter) of the field-induced structures increases with the gap thickness. Theoretical analysis shows that the increase in the storage modulus with gap thickness can be explained by a decrease in the demagnetizing factor of these structures and, as a consequence, by an increase in the restoring torque, acting on them. The dissipation effects in the suspension under oscillating flow are rather produced by contact friction between the particles in the dense structures.
108(2010); http://dx.doi.org/10.1063/1.3499283View Description Hide Description
A new type of fluorophosphate glasses with high thulium doping concentration (up to ) is investigated. The intensive fluorescence is demonstrated with lower concentration quenching. On the basis of the measured Raman spectroscopy, it is revealed that the glass structure will be changed when adding ions into fluorophosphate glasses. Besides, the Judd–Ofelt parameters and radiative properties are calculated and discussed based on Judd–Ofelt theory. And the absorption and emission cross-sections of transition are also calculated by using McCumber and Beer–Lambert theories.