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1 Oct 2008

Volume 104, Issue 7, Articles (07xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 104, 071101 (2008); http://dx.doi.org/10.1063/1.2981642 (10 pages)

Ailish O’Halloran, Fergal O’Malley, and Peter McHugh
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The anatomy of the Gunn laser

S. Chung and N. Balkan

J. Appl. Phys. 104, 073101 (2008); http://dx.doi.org/10.1063/1.2977719 (9 pages)

Online Publication Date: 1 October 2008

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A monopolar GaAs Fabry–Pérot cavity laser based on the Gunn effect is studied both experimentally and theoretically. The light emission occurs via the band-to-band recombination of impact-ionized excess carriers in the propagating space-charge (Gunn) domains. Electroluminescence spectrum from the cleaved end-facet emission of devices with Ga1−xAlxAs (x = 0.32) waveguides shows clearly a preferential mode at a wavelength around 840 nm at T = 95 K. The threshold laser gain is assessed by using an impact ionization coefficient resulting from excess carriers inside the high-field domain.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.79.Gn Optical waveguides and couplers

Effect of molecular weight of poly(N-vinyl carbazole) on photorefractive performances

Naoto Tsutsumi and Hideto Kasaba

J. Appl. Phys. 104, 073102 (2008); http://dx.doi.org/10.1063/1.2978210 (5 pages) | Cited 6 times

Online Publication Date: 1 October 2008

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We present here the effect of molecular weight of poly(N-vinyl carbazole) (PVCz) on photorefractive performances. Photorefractive composites with higher molecular weight of PVCz led to higher diffraction efficiency and faster speed of grating buildup. The fastest response time of 18 ms (grating buildup speed of 55.6 s−1) was obtained at 45 V μm−1. Higher diffraction efficiency and faster buildup speed of photorefractive composite with higher molecular weight of PVCz was ascribed to the larger Pockels effect due to larger number density of traps. Dimer cation sites preferentially formed along the longer molecular chain in higher molecular weight of PVCz seem to work as effective traps for hole carriers.
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78.20.Jq Electro-optical effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
36.20.Cw Molecular weights, dispersity

Effect of intermixing on bulk and interface Raman modes in GaAs:AlAs superlattice waveguide structures

P. Scrutton, B. Fung, and A. S. Helmy

J. Appl. Phys. 104, 073103 (2008); http://dx.doi.org/10.1063/1.2986149 (9 pages) | Cited 2 times

Online Publication Date: 1 October 2008

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Spatially resolved Raman spectroscopy at room temperature is used to study quantum well intermixing in GaAs:AlAs superlattice structures. Phonon modes are probed from the side facet along the [110] direction. The intermixing leads to the appearance of interfacial alloy modes and degraded the intensity of the superlattice interface (IF) modes, which can be used as a sensitive indicator of superlattice quality. These changes in the Raman spectra, along with spatially resolved photoluminescence, are used to distinguish the degree of intermixing in samples intermixed by impurity free vacancy diffusion at 850–950 °C, and to investigate the bandgap modulation in a periodically intermixed bandgap grating fabricated using ion implantation induced disordering. The shift of the GaAs-transverse optic mode to the GaAs-like transverse optic alloy mode and the degradation of the GaAs-IF mode are shown to provide the best indication of the extent of intermixing. Due to the large contrast in the GaAs-IF mode intensity between as-grown and intermixed superlattice, using IF modes is found to be a promising route for characterizing the resolution of the superlattice bandgap grating.
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42.79.Gn Optical waveguides and couplers
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
78.67.De Quantum wells
78.30.Fs III-V and II-VI semiconductors
78.55.Cr III-V semiconductors
71.20.Nr Semiconductor compounds

Laser induced plane acoustic wave generation, propagation, and interaction with rigid structures in water

Seung H. Ko, Sang G. Ryu, Nipun Misra, Heng Pan, Costas P. Grigoropoulos, Nick Kladias, Elias Panides, and Gerald A. Domoto

J. Appl. Phys. 104, 073104 (2008); http://dx.doi.org/10.1063/1.2980014 (8 pages) | Cited 1 time

Online Publication Date: 2 October 2008

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Short pulsed laser induced single acoustic wave generation, propagation, interaction with rigid structures, and focusing in water are experimentally and numerically studied. A large area short duration single plane acoustic wave was generated by the thermoelastic interaction of a homogenized nanosecond pulsed laser beam with a liquid-solid interface and propagated at the speed of sound in water. Laser flash schlieren photography was used to visualize the transient interaction of the plane acoustic wave with various submerged rigid structures [(a) a single block, (b) double blocks, (c) 33° tilted single block, and (d) concave cylindrical acoustic lens configurations]. Excellent agreement between the experimental results and numerical simulation is observed. Our simulation results demonstrate that the laser induced planar acoustic wave can be focused down to several tens of micron size and several bars in pressure.
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43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
43.30.Ky Structures and materials for absorbing sound in water; propagation in fluid-filled permeable material
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
46.25.Hf Thermoelasticity and electromagnetic elasticity (electroelasticity, magnetoelasticity)
62.60.+v Acoustical properties of liquids
62.65.+k Acoustical properties of solids

Terahertz emission from a laser pulse with tilted front: Phase-matching versus Cherenkov effect

M. I. Bakunov, S. B. Bodrov, and M. V. Tsarev

J. Appl. Phys. 104, 073105 (2008); http://dx.doi.org/10.1063/1.2989999 (13 pages) | Cited 8 times

Online Publication Date: 3 October 2008

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A theory that describes the terahertz generation via optical rectification of a femtosecond laser pulse with tilted front in an electro-optic crystal is developed. The theory accounts for the transverse size of the laser pulse and allows us to explore the dependence of the terahertz yield on laser focusing. Two typical experimental situations—LiNbO3 excited with Ti:sapphire laser at room and cryogenic temperatures—are considered. It is shown that depending on temperature the dominant generation mechanism can be either phase-matching or Cherenkov effect. The parameters of the laser pulse (transverse size, tilt angle, and pulse duration) and crystal size maximizing the terahertz yield for LiNbO3 are calculated.
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78.70.Gq Microwave and radio-frequency interactions
78.47.-p Spectroscopy of solid state dynamics
78.20.Jq Electro-optical effects

Diffraction of cholesteric liquid crystal gratings probed by monochromatic light from 450 to 750 nm

Wen-Chi Hung, Wood-Hi Cheng, Tung-Kia Liu, I-Min Jiang, Ming-Shan Tsai, and Pochi Yeh

J. Appl. Phys. 104, 073106 (2008); http://dx.doi.org/10.1063/1.2990055 (4 pages) | Cited 1 time

Online Publication Date: 3 October 2008

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We investigated both theoretically and experimentally the diffraction property of cholesteric liquid crystals (CLCs) in the configuration of transmission grating. The CLCs are treated as birefringent index gratings whose diffraction property is strongly dependent on the polarization state of the incident beam. In our experimental studies, the CLC index gratings are constructed by applying an electric voltage to liquid crystal cell. The grating is probed by a monochromatic light in spectral range from 450 to 750 nm and the measured diffraction efficiency approximately agrees with the theoretical predictions. The dependence of polarization and wavelength of polarized monochromatic light may possibly lead to a light switch in optical communication system.
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42.79.Dj Gratings
42.70.Df Liquid crystals
42.79.Sz Optical communication systems, multiplexers, and demultiplexers
42.79.Kr Display devices, liquid-crystal devices

Saturable and reverse saturable absorption in silver nanodots at 532 nm using picosecond laser pulses

Ullas Gurudas, Elijah Brooks, Daniel M. Bubb, Sebastian Heiroth, Thomas Lippert, and Alexander Wokaun

J. Appl. Phys. 104, 073107 (2008); http://dx.doi.org/10.1063/1.2990056 (8 pages) | Cited 15 times

Online Publication Date: 3 October 2008

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Saturable absorption (SA) and reverse saturable absorption (RSA) at 532 nm were observed in Ag nanodots prepared by pulsed laser deposition. The real [Re χ(3)] and imaginary [Im χ(3)] parts of the third order nonlinearity of these films were measured using the Z-scan technique. At low input irradiances, the decrease in absorption near the focal point results from a negative Im χ(3) and yields SA. At higher input irradiance, RSA becomes dominant. The transition from SA to RSA suggests that there is a threshold irradiance at which additional nonlinear process(es) is (are) involved and become dominant. The recovery time of these nonlinear processes was measured by a degenerate pump-probe experiment. The effects are explained in terms of the electron dynamics in the excited states.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.47.J- Ultrafast spectroscopy (<1 psec)
78.40.Kc Metals, semimetals, and alloys
42.65.-k Nonlinear optics

Off-axis beaming from subwavelength apertures

Humeyra Caglayan, Irfan Bulu, and Ekmel Ozbay

J. Appl. Phys. 104, 073108 (2008); http://dx.doi.org/10.1063/1.2990063 (4 pages) | Cited 9 times

Online Publication Date: 3 October 2008

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Photonic crystal waveguides and metallic subwavelength apertures are promising tools for light manipulation. It is possible to obtain enhanced directional beams by using these structures via coupling to surface waves. In addition, these apertures can be designed to steer such directional beams by introducing asymmetrical gratings on the output surface. In the present paper, we report directional yet off-axis beaming from subwavelength apertures at microwave frequencies. The full width at half maximum of the beam is 10° while the beaming angle is 15°. Our results show that it is possible to steer the beam by the appropriate modification of the output surface.
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42.79.Gn Optical waveguides and couplers
42.79.Dj Gratings
42.82.Et Waveguides, couplers, and arrays
42.70.Qs Photonic bandgap materials

Realization and characterization of tetra(tert-butyl) zinc phtalocyanine poly(methyl methacrylate) films for optical limiting applications

M. C. Larciprete, S. Mangialardo, A. Belardini, C. Sibilia, and M. Bertolotti

J. Appl. Phys. 104, 073109 (2008); http://dx.doi.org/10.1063/1.2985914 (4 pages) | Cited 1 time

Online Publication Date: 6 October 2008

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We investigated optical limiting effects in films of tetra(tert-butyl) zinc phtalocyanine dissolved into poly(methyl methacrylate) within a 0.15% maximum weight ratio. Spectroscopic analysis was performed on the realized films in order to retrieve the corresponding linear optical absorption coefficient, whose interpolation according to a model including a set of three Lorentzian oscillators enabled the evaluation of the degrees of monomer and dimer concentrations. Thereafter, nonlinear transmission measurements were carried out at two different wavelengths, 532 and 634 nm. All the investigated films showed a nonlinear optical response, which was found to increase as the resonant condition was approached.
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42.70.Jk Polymers and organics
78.66.Qn Polymers; organic compounds
42.65.-k Nonlinear optics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Sulphide GaxGe25−xSb10S65(x = 0,5) sputtered films: Fabrication and optical characterizations of planar and rib optical waveguides

J. Charrier, M. L. Anne, H. Lhermite, V. Nazabal, J. P. Guin, F. Charpentier, T. Jouan, F. Henrio, D. Bosc, and J. L. Adam

J. Appl. Phys. 104, 073110 (2008); http://dx.doi.org/10.1063/1.2968248 (8 pages) | Cited 3 times

Online Publication Date: 7 October 2008

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We report the fabrication and the physical and optical characterizations of sulphide GaxGe25−xSb10S65(x = 0,5) rib waveguides. High quality films fabricated on SiO2/Si wafer substrates were obtained using the sputtering magnetron rf deposition method. The slab waveguides obtained without annealing present propagation losses of about 0.6 dB/cm at 1550 nm. These optical losses are not important for implementation in optical devices based on silicon-on-insulator or polymer, for instance, atomic force microscopy measurements revealed low interface roughness between the different media (substrate/film and film/air). Reactive ion etching was used to pattern rib waveguides between 2 and 300 μm wide. The parameters were optimized to obtain a dry etching process that had low surface roughness, vertical sidewalls, etch depth of more than 1 μm, and reasonable etching rate. This technique was used to fabricate Y optical junctions for optical interconnections on chalcogenide amorphous films. Their optical transmission was demonstrated by optical near field of guided modes and optical losses were measured and discussed.
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42.82.Et Waveguides, couplers, and arrays
42.79.Gn Optical waveguides and couplers
42.70.-a Optical materials
78.66.-w Optical properties of specific thin films
81.15.Cd Deposition by sputtering
68.35.Ct Interface structure and roughness

Growth optimization for low residual carriers in undoped midinfrared InAs/GaSb superlattices

H. J. Haugan, S. Elhamri, G. J. Brown, and W. C. Mitchel

J. Appl. Phys. 104, 073111 (2008); http://dx.doi.org/10.1063/1.2993748 (4 pages) | Cited 9 times

Online Publication Date: 9 October 2008

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Reducing residual background carriers in InAs/GaSb superlattices (SLs) is an essential task to increase the operating temperature of photoconductive devices. This paper discusses how low-temperature Hall measurements were used to tune several SL growth parameters for the minimum residual carriers in a typical midinfrared 21 Å InAs/24 Å GaSb SLs designed for the 4 μm cutoff wavelength. Among the three growth parameters studied, neither growth temperature nor in situ postannealing significantly affected the intrinsic carrier type and doping concentration. The lowest carrier density of 1.8×1011 cm−2 was achieved at 400 °C. All SLs grown at 400 °C maintained the lowest density around 1.6×1011 cm−2 with or without postannealing. However, in-plane carrier mobility showed a slight improvement with annealing, especially at temperatures above 450 °C. The growth parameter most sensitive to the carrier density was interface control. With a minor variation in interface shutter sequence, the carrier density dramatically increased from ∼ 2×1011 to 5×1012 cm−2, and the corresponding mobility dropped from 6600 to 26 cm2/V s, indicating dramatic degradation of interfacial quality. All SLs investigated in this study were residually p-type.
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68.65.Cd Superlattices
81.40.Gh Other heat and thermomechanical treatments
61.72.Cc Kinetics of defect formation and annealing

One- and biphotonic reorientational nonlinearities in dye-doped nematic and cholesteric liquid crystals

H.-C. Yeh, H.-C. Lin, T.-S. Mo, Andy Y.-G. Fuh, S.-Y. Huang, and C.-R. Lee

J. Appl. Phys. 104, 073112 (2008); http://dx.doi.org/10.1063/1.2993751 (5 pages)

Online Publication Date: 9 October 2008

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This investigation elucidates one- and biphotonic reorientational nonlinearities in dye-doped nematic liquid crystals (DDNLCs) and dye-doped cholesteric liquid crystals (DDCLCs) using the Z-scan technique. Three cases—DDNLCs stimulated by one green beam polarized linearly parallel and perpendicular to the LC director and DDCLCs stimulated by one circularly polarized green beam—are considered without and with simultaneous stimulation by one linearly polarized red beam. Green-beam-induced dye reorientation through trans-cis isomerization and red-beam-induced suppression of dye reorientation by cis-trans back isomerization are responsible for the one- and biphotonic nonlinearities, respectively. The nonlinear refractive indices obtained herein are two orders of magnitude larger than those obtained elsewhere [ Fuh et al., Opt. Exp. 13, 10634 (2005) ]. The induced biphotonic nonlinearities can be modulated by controlling the intensity of the red beam in different ways due to the difference among the capabilities of the red beam to suppress the reorientation of the dyes and LCs in the three cases.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
82.30.Qt Isomerization and rearrangement
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Investigation of correlation between the microstructure and electrical properties of sol-gel derived ZnO based thin films

M. W. Zhu, J. Gong, C. Sun, J. H. Xia, and X. Jiang

J. Appl. Phys. 104, 073113 (2008); http://dx.doi.org/10.1063/1.2993978 (7 pages) | Cited 12 times

Online Publication Date: 9 October 2008

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Pure ZnO and aluminum doped ZnO films (ZAO) were prepared by sol-gel method and the effect of Al doping on the microstructure and electrical properties of the films was investigated. The results showed that the transformation from granular to columnar structure could be observed in pure ZnO films with the increase in heating time while in aluminum doped films little structural changes occurred even after a prolonged heating time. Additionally, measurements of electrical properties showed that both microstructural evolution and doping could significantly improve the conductivity of the films, which could be assigned to an increase both in Hall mobility and carrier concentration. The relationship between microstructure and the electrical properties of the films was discussed, and various scattering mechanisms were proposed for sol-gel derived ZnO and ZAO films as a function of the carrier concentration.
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68.55.ag Semiconductors
73.61.Ga II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
72.20.My Galvanomagnetic and other magnetotransport effects
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Intracavity harmonic generation in figure-8 undulator free electron lasers

G. Dattoli, P. L. Ottaviani, and S. Pagnutti

J. Appl. Phys. 104, 073114 (2008); http://dx.doi.org/10.1063/1.2968552 (4 pages)

Online Publication Date: 10 October 2008

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We consider a free electron laser oscillator employing an exotic undulator provided by a figure-8, a device having the orthogonal field component oscillating with a period one-half of the radial component. The harmonic spectrum of such an oscillator device is fairly reach. We have indeed emission on odd and even harmonics with orthogonal polarizations. The intracavity nonlinear coherent harmonic generation follows the harmonic undulator spectrum and in addition allows the possibility of obtaining robust and very short pulses at different wavelengths.
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41.60.Cr Free-electron lasers
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Determination of liquid crystal orientation in holographic polymer dispersed liquid crystals by linear and nonlinear optics

M. Yemtsova, A. Kirilyuk, A. F. van Etteger, and Th. Rasing

J. Appl. Phys. 104, 073115 (2008); http://dx.doi.org/10.1063/1.2969656 (4 pages) | Cited 1 time

Online Publication Date: 13 October 2008

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The liquid crystal alignment imposed during the fabrication process of one-dimensional transmission gratings, holographically formed in a polymer dispersed liquid crystal, is determined by linear optical birefringence as well as rotational second harmonic generation. For a mixture consisting of nematic liquid crystal E7, a pentafunctional acrylate monomer, and a photoinitiator, the director orientation inside the liquid-crystal-rich layer is found to be approximately along the grating direction. We suggest that this molecular orientation is the consequence of the strong flow during the grating formation.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
42.70.Df Liquid crystals
42.79.Dj Gratings
42.25.Lc Birefringence
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.40.-i Holography

Mechanism of the linear electro-optic effect in potassium dihydrogen phosphate crystals

He Huang, Z. S. Lin, and C. T. Chen

J. Appl. Phys. 104, 073116 (2008); http://dx.doi.org/10.1063/1.2990769 (3 pages)

Online Publication Date: 13 October 2008

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The linear electro-optic effect in KH2PO4 (KDP) is studied using first-principles calculations combined with semiempirical methods previously proposed [Chem. Phys. Lett. 397, 222 (2004)] . The protons in the hydrogen bonds are much more flexible and better able to respond an external field than the other atoms in KDP, implying that displacement of hydrogen atoms makes the main contribution to the linear electro-optic effect in KDP. Optical property calculations show that the changes in the refractive indices induced by the displacement of hydrogen atoms are pertinent to the flexible degree of the lattice in our semiempirical model.
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78.20.Jq Electro-optical effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.50.Lt Crystal binding; cohesive energy
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