Volume 108, Issue 3, 01 August 2010
Index of content:
- Lasers, Optics, and Optoelectronics
108(2010); http://dx.doi.org/10.1063/1.3460810View Description Hide Description
Polarizers capable of producing linearly polarized beams with axial (radial and azimuthal) symmetry have been fabricated with the aid of a dichroic liquid crystal. Photoalignment was achieved using a printing technique to reduce the UV exposure time required for production of axially aligning substrates from 1 h, typical for direct writing techniques, to 10 min. The polarizing features of axial polarizers and their pairs are characterized and their differences outlined. We demonstrate that the transmission switching contrast of an axial polarizer/analyzer pair, comprised of an electrically controlled liquid crystal cell, is comparable to conventional systems with linear polarizers. The opportunities for using axial polarizers for polarizationimaging,sensor protection, and nonlinear optics are discussed. Particularly, we show that the technology could reduce the fluence of a laser beam on an optical sensor without affecting imaging.
108(2010); http://dx.doi.org/10.1063/1.3466981View Description Hide Description
Fiber Bragg grating(FBG) is side-polished to enable interaction with sensitive materials around FBG fiber core. Using TbFeCo magneto-optic thin filmdeposited onto FBG fiber core as transducer, a FBGrefractive index senor for magnetic field/current characterization is first proposed and demonstrated in this paper. Magnetic field sensing experiments show 19 pm of wavelength shift at a magnetic field intensity of 50 mT, the average linearity of magnetic field response is 0.9877.
108(2010); http://dx.doi.org/10.1063/1.3466996View Description Hide Description
Millisecond, nanosecond, and picosecond laser pulse induced damage thresholds on single-crystal are investigated in this study. The thresholds of laser-induced damage on silicon are calculated theoretically for three pulse widths based on the thermal damage model. An axisymmetric mathematical model is established for the transient temperature field of the silicon. Experiments are performed to test the damage thresholds of silicon at various pulse widths. The results indicate that the damage thresholds obviously increase with the increasing of laser pulse width. Additionally, the experimental results agree well with theoretical calculations and numerical simulation results.
108(2010); http://dx.doi.org/10.1063/1.3467004View Description Hide Description
The electroluminescence(EL) characteristics of a set of InAs/GaInNAs quantum dot(QD)light-emitting diodes with varying In and N contents are analyzed. Room-temperature EL around is obtained with 15% In and 2% N in the QD capping layer. It is shown that the addition of N results in a degradation of the external efficiency, , probably due to an increase in the nonradiative recombination in the QD heterostructure and an increase in the carrier escape from the QD to the capping layer, which yield a degradation of the current injection efficiency into the QD. Nevertheless, can be partially recovered if a postgrowth rapid thermal annealing is performed, although this also results in a blueshift in the EL peak wavelength. The different contributions to the EL spectra are also analyzed and identified by looking at their dependence on injected current and temperature. It is found that N-containing devices show two radiative transitions. The lowest energy transition has been ascribed to the QDground state recombination, whereas the higher energy transition has been attributed to recombination of carriers confined in the capping layer. Moreover, the carrier loss mechanisms responsible for the quenching of the EL in the dilute nitride-based devices are studied. It is found that the EL thermal quenching has an activation energy which can be ascribed to carrier escape from the QD to the capping layer.
108(2010); http://dx.doi.org/10.1063/1.3467529View Description Hide Description
A theoreticalmodel is developed to evaluate the roles of various mechanisms, including the molar polarization and molar volume, the spontaneous polarization and the photoelastic effect, for the modifications of refractive indices in ion-implanted waveguides. Based on the model, numerical calculations of refractive indices as a function of the lattice damage level in crystals of different crystalline orientations (X, Y, and Z), are performed, with results in a good agreement with experimental data. The analysis indicates that the spontaneous polarization, the molar polarization, and molar volume play important roles in determining the index profiles in ion implanted. In addition, the contribution of the strain-induced photoelastic effect has been identified for different damage levels.
108(2010); http://dx.doi.org/10.1063/1.3460280View Description Hide Description
In order to evaluate the energy transfer between Eu–Mn in solid solution,phosphors were prepared by flux method. The crystal structure and the morphology of the solid solution were demonstrated by x-ray dirrfactometer and scanning electron microscopy. The photoluminescence mechanisms were explained by the energy transfer of to and the Dexter theory. A redshift of green emission peak and a decrease in decay time with the increase in concentration were observed. These phenomena are attributed to the formation of paired centers after analysis by a method of Pade approximations.
108(2010); http://dx.doi.org/10.1063/1.3467766View Description Hide Description
This paper presents the conception, modeling, and simulation of a silicon-based group-IV semiconductor injection laser diode in which the GeSn-alloy active region has a direct band gap wavelength in the 1.8 to midwave infrared for 6%–12% . The strain-free monolithic P-type semiconductor/Intrinsic semiconductor/N-type semiconductor (PIN) bulk heterostructure, grown lattice matched upon a relaxed GeSn-buffer on silicon-on-insulator, is believed to be manufacturable in a complementary metal-oxide semiconductor fab. Detailed modeling is given for the type-I band offsets, carrier lifetimes, infrared gain profile and laser threshold current density in a Fabry–Perot cavity having loss. The laser’s temperature of operation is determined by a combination of the radiative lifetime and the nonradiative lifetime due to unwanted Auger electron-hole recombination. If we keep below , then we find that this laser requires cooling in the 100–200 K range, whereas at 300 K appears to be too high for a practical device. However, the GeSn quantum-well laser diode does offer a pathway to room-temperature operation.
Highly doped layers as efficient electron–hole recombination contacts for tandem organic solar cells108(2010); http://dx.doi.org/10.1063/1.3467786View Description Hide Description
A key feature of stacked organic solar cells is an efficient recombination contact at the interface between the solar cells in the stack. Here, an electron current has to be converted into a hole current without loss of energy. Furthermore, the recombination contact has to be highly transparent. We present a new approach for small molecule organic solar cells using highly doped organic layers. Our approach adapts the use of tunnel diodes known from inorganic tandem solar cells. We compare a metal cluster based recombination contact reported in literature to the new approach using different organic tandem solar cell structures. For this purpose, current-voltage characteristics of adequate solar cells are measured. The experiments show that highly doped layers as recombination contacts in tandem organic solar cells are superior to the metal cluster based approach. The proposed concept allows an addition of the open circuit voltages of the subcells of a tandem solar cell, without absorption or reflection at the recombination contact. The results further show that our concept does not depend on the specific choice of materials as it is seen for metal cluster based recombination contacts. It therefore represents a general approach which is compatible to mass manufacturing.
108(2010); http://dx.doi.org/10.1063/1.3457787View Description Hide Description
This work proposes a new optoelectronic measurement of quantum well(QW) thickness and applies it to doped and undoped multiple-QW structures. Near-infrared spectroscopic identification of the interband optical transition at 100–300 K gave the eigenenergies of the conduction band in the QW. Evaluation of the QW thickness involved analysis of the effective mass at the corresponding eigenenergy. QW thicknesses in the range of 5.45–20.8 nm were determined in six different wafers. These thicknesses agreed well with the QW thicknesses estimated by double-crystal x-ray diffraction within almost two monolayers. This measurement was used to determine the distance of potential boundaries confining the electron wave functions.
Mechanism of femtosecond-laser induced refractive index change in phosphate glass under a low repetition-rate regime108(2010); http://dx.doi.org/10.1063/1.3468490View Description Hide Description
Raman microscopy and refractive near-field profilometry were used to analyze waveguides written in Yb-doped Kigre QX glass under the low repetition-rate (noncumulative-heating) regime. It was found that femtosecond-laser induced refractive index change was due to an increase in the proportion of P-tetrahedra and the associated increase in the polarizability of the glass. The role of color centerformation and removal in this process is clearly defined, phosphorous–oxygen hole centers (POHCs) and ions form as a result of P–O bonds being broken during the modification process, and the subsequent removal of POHCs give rise to the increased proportion of P-tetrahedra. This result, when compared to other studies undertaken in the cumulative-heating regime, show conclusively that the mechanism of refractive index change in a particular type of glass can be very different, depending on the irradiation conditions.
108(2010); http://dx.doi.org/10.1063/1.3466746View Description Hide Description
In this study we describe a simple method to fabricate microarrays of polystyrene microbeads (PS-) on Thermanox coverslip surfaces using laser induced forward transfer (LIFT). A triazene polymer layer which acts as a dynamic release layer and propels the closely packed microspheres on the receiving substrate was used for this approach. The deposited features were characterized by optical microscopy, scanning electron microscopy,atomic force microscopy, and Raman spectroscopy. Ultrasonication was used to test the adherence of the transferred beads. In addition, the laser ejection of the PS- pixels was investigated by time resolved shadowgraphy. It was found that stable PS- micropatterns without any specific immobilization process could be realized by LIFT. These results highlight the increasing role of LIFT in the development of biomaterials, drug delivery, and tissue engineering.
Hot electron effects on efficiency degradation in InGaN light emitting diodes and designs to mitigate them108(2010); http://dx.doi.org/10.1063/1.3460271View Description Hide Description
Hot electrons and the associated ballistic and quasiballistic transport, heretofore neglected endemically, across the active regions of InGaN light emitting diodes(LEDs) have been incorporated into a first order simple model which explains the experimental observations of electron spillover and the efficiency degradation at high injection levels. The model is in good agreement with experiments wherein an adjustable barrier hot electron stopper, commonly called the electron blocking layer (EBL), is incorporated. The model is also in agreement with experiments wherein the electrons are cooled, eliminating hot electrons, inside a staircase electron injector (SEI) prior to their injection into the active region. Thermionic emission from the active region, even if one uses an uncharacteristically high junction temperature of 1000 K, fails to account for the carrier spillover and the experimental observations in our laboratory in samples with varying EBL barrier heights. The model has been successfully applied to both -plane (lacking polarization induced electric field) and -plane (with polarization induced field) InGaN double heterostructure (DH) LEDs with a 6 nm active region featuring a variable barrier hot electron stopper, and a SEI, and the various combinations thereof. The choice of DH LEDs stems from our desire to keep the sample structure simple as well as the model calculations. In this paper, the theoretical and experimental data along with their comparison followed by an insightful discussion are given. The model and the approaches to eliminate carrier spillover proposed here for InGaN LEDs are also applicable to GaN-based laser diodes.
Long-lived laser-induced microwave plasma guides in the atmosphere: Self-consistent plasma-dynamic analysis and numerical simulations108(2010); http://dx.doi.org/10.1063/1.3457150View Description Hide Description
A detailed model of plasma dynamics, which self-consistently integrates plasma-kinetic, Navier–Stokes, electron heat conduction, and electron-vibration energy transferequations, is used to quantify the limitations on the lifetime of microwaveplasma waveguides induced in the atmosphere through filamentation with high-intensity ultrashort laser pulses further sustained by long laser pulses. We demonstrate that a near-infrared or midinfrared laser pulse can tailor plasma decay in the wake of a filament, efficiently suppressing, through electron temperature increase, the attachment of electrons to neutral species and dissociative recombination, thus substantially increasing the plasma-guide lifetime and facilitating long-distance transmission of microwaves.
108(2010); http://dx.doi.org/10.1063/1.3457147View Description Hide Description
We report on the enhancement of photoluminescence (PL) from polymer thin films by nanotexturing their surfaces using nanoporous anodic alumina oxide templates. Chromophore-embedded polystyrene films with nanostructured surfaces are prepared by imprinting 200 nm diameter nanopillars with various heights, and their PL output and angular emission are observed. The PL output increases and the angular distributions broaden as the height of the nanopillars increases. For tall nanopillars, the PL output is enhanced by a factor of 2.5 relative to the smooth surface. An effective refractive index model provides a qualitative description of the angular emission and PL output of nanotextured surface but underestimates the degree of PL enhancement. Comparison of the nanopillared films with surfaces randomly roughened using sandpaper shows that the details of the texturing have a significant impact on the PL output characteristics. These results show that imprinted nanopillars provide a simple and controlled way to increase the PL output of luminescent polymer films.
- Plasmas and Electrical Discharges
108(2010); http://dx.doi.org/10.1063/1.3383056View Description Hide Description
The asymmetrically mounted flat plasma actuator is investigated using a self-consistent two-dimensional fluid model at atmospheric pressure. The computational model assumes the drift-diffusion approximation and uses a simple plasma kinetic model. It investigated the electrical and kinetic properties of the plasma, calculated the charged species concentrations, surface charge density, electrohydrodynamic forces, and gas speed. The present computational model contributes to understand the main physical mechanisms, and suggests ways to improve its performance.
Atmospheric pressure plasma jets beyond ground electrode as charge overflow in a dielectric barrier discharge setup108(2010); http://dx.doi.org/10.1063/1.3466993View Description Hide Description
With a proper combination of applied voltage and the width of ground electrode, atmospheric pressure plasma jets extending beyond the ground electrode, whether it sits on the downstream or the upstream side, can be equally obtained with a dielectric barrier discharge setup, which can be ascribed to the overflow of depositedcharges [J. Appl. Phys.106, 013308 (2009)]. Here, we show that, by using narrower ground electrodes, such an overflow jet can be successfully launched at a much reduced voltage (down to below 10 kV). Moreover, by using transparent and triadic ground electrodes, development of chargeoverflow beneath the ground electrode was temporally and spatially resolved. Temporal evolution of discharge currents measured on the severed ground electrode helps establish the propagation dynamics of discharges along the dielectric surface beneath ground electrode, and also reinforces the conception that the streamer’s head is in connection to the active electrode via a conducting channel. A small propagation velocity of was measured for the “overflow” jet inside the dielectric tube. The availability of such overflowjets is enormously advantageous concerning their application to living bodies, to which the high voltage at active electrode is lethally dangerous.
Investigation of the roles of gas-phase molecules and F atoms during fluorocarbon plasma processing of Si and substrates108(2010); http://dx.doi.org/10.1063/1.3467776View Description Hide Description
The molecular-level chemistry involved in the processing of silicon and zirconia substrates by inductively coupled fluorocarbon (FC) plasmas produced from and precursors has been explored. The roles of gas-phase excited, neutral, and ionic species, especially and F, were examined as they contribute to FC film formation and substrate etching. The surface reactivity of radicals in plasmas has a dependence on substrate material and plasma system, as measured by our imaging of radicals interacting with surfaces (IRIS) technique. Relative concentrations of excited state species are also dependent upon substrate type. Moreover, differences in the nature and concentrations of gas-phase species in and plasmas contribute to markedly different surface compositions for FC filmsdeposited on substrates as revealed from x-ray photoelectron spectroscopic analysis. These data have led to the development of a scheme that illustrates the mechanisms of film formation and destruction in these FC/substrate systems with respect to and F gas-phase species and also Si and substrates.
Ion focusing in enhanced glow discharge plasma immersion ion implantation of hydrogen and nitrogen into silicon108(2010); http://dx.doi.org/10.1063/1.3467967View Description Hide Description
Ion focusing in enhanced glow dischargeplasma immersion ion implantation (EGD-PIII) of hydrogen into silicon affects the lateral ion fluence uniformity. The phenomenon and its effects are investigated experimentally and theoretically under different conditions and compared to those in nitrogen EGD-PIII. Consistent results are obtained from experiments and numerical simulation disclosing that the lower the plasma density, the more severe is the ion focusing effect. The influence of the negative high voltage on the ion focusing effect is small compared to that of the plasma density.
108(2010); http://dx.doi.org/10.1063/1.3468603View Description Hide Description
Experiments with a Langmuir probe and optical emission spectroscopy are carried out in inductively coupled rf (13.56 MHz) discharges at three total pressures of 30, 60, and 110 mTorr, varying the Ar fractions from 50% to 95%. The electron energy probability functions (EEPFs) measured at all Ar fractions can be approximated using different exponentials in the bulk and high-energy regions, resulting in two temperature approximation of the measured EEPFs. The electron density slightly increases with increasing the Ar fraction at the Ar fractions below 70%, beyond which it relatively abruptly increases. On the other hand, the electron temperatures gradually decrease with the increase in the Ar fraction. The vibrational temperature does not strongly depend on the Ar fraction, whereas the rotational temperature gradually increases with the increase in the Ar fraction. The density of nitrogen atoms remains constant at the Ar fractions below 80%, beyond which it monotonically decreases with increasing the Ar fraction. A global model for electropositive plasma is used in order to investigate the plasma chemistry in discharges assuming the Maxwellian electron energy distribution. The model results are compared with the experimental results, obtaining reasonably good agreement.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
108(2010); http://dx.doi.org/10.1063/1.3456005View Description Hide Description
The redistribution of boron during the formation of the Ni silicides was investigated using atom probe tomography and transmission electron microscopy. A 7 nm amorphous intermixed region was found after deposition of a 30 nm thick Nifilm at room temperature. The formation of this Ni–Si layer was found to have almost no influence on the boron implantation profile. After heating at for 1 h, three types of silicides (, NiSi, and ) were identified below a thin remaining film of Ni (8 nm). The unexpected presence of the silicon-rich phase at this temperature may be caused by the presence of a thin silicon oxide observed at the interface that may act as a diffusion barrier. The average boron profile in and NiSi silicides is similar to the profile in the silicon substrate before reaction. A segregation of boron at several interfaces was detected. Small boron clusters (1.5 at. %) were found in NiSi, , and Si phases but not in . After a 1 min heat treatment at , the NiSi phase is the only silicide present. Boron clusters with a platelet shape and a concentration of 3 to 5 at. % of boron were found in both NiSi and Si. The presence of boron in the Ni silicide and its precipitation in the form of tiny clusters is likely to affect the electrical properties of the contacts.