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1 Dec 1998

Volume 84, Issue 11, pp. 5843-6471

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Influence of contacts on the conductivity of thin wires

H. Ruda and A. Shik

J. Appl. Phys. 84, 5867 (1998); http://dx.doi.org/10.1063/1.368901 (6 pages) | Cited 10 times

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The conductivity of thin wires was shown to depend noticeably on the properties and, particularly, dimensionality of contacts. Two- and one-dimensional contacts create a strongly nonuniform electric field in a wire which, in turn, causes the redistribution of electron density along a wire and nonlinear current–voltage characteristic. The particular shape of the latter has been calculated for both Ohmic and Schottky contacts of different dimensionality. The case of ac applied voltage is also considered. Spatial distribution of nonequilibrium carriers is found for different signal frequencies ω. The wire conductivity is shown to increase with ω for any type of low-dimensional contacts. © 1998 American Institute of Physics.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts

Influence of detection conditions on near-field optical imaging

B. Hecht, H. Bielefeldt, D. W. Pohl, L. Novotny, and H. Heinzelmann

J. Appl. Phys. 84, 5873 (1998); http://dx.doi.org/10.1063/1.368902 (10 pages) | Cited 19 times

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The process of image formation in transmission mode scanning near-field optical microscopy is analyzed both theoretically and experimentally. Changes in the dielectric and topographic properties of the sample influence not only the total transmitted intensity, but also its angular distribution in the far field. This opens up an additional source of optical information about the sample. Some of this additional information is retrieved by separate but simultaneous detection of the radiation emitted at angles smaller (allowed light) and larger (forbidden light) than the critical angle of total internal reflection, respectively. Different experimental setups and their respective advantages are discussed. High resolution, constant height mode optical images of test structures are compared with theoretical predictions. Forbidden-light optical images frequently provide enhanced resolution and/or contrast as compared to allowed light images. For small phase objects, in contrast to amplitude objects, a contrast reversal between forbidden and allowed light images is observed. © 1998 American Institute of Physics.
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07.79.Fc Near-field scanning optical microscopes

Threshold current of quantum-disk and quantum-wire gain-coupled distributed feedback lasers

Nobuhiko Susa

J. Appl. Phys. 84, 5883 (1998); http://dx.doi.org/10.1063/1.368903 (4 pages) | Cited 1 time

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This numerical study shows that an effective way to reduce the threshold current density (Jth) in the quantum well (QW) and bulk types of gain-coupled distributed feedback (DFB) lasers is to reduce the active volume. Although the threshold current per unit cavity width (Ith) is smaller for the quantum-disk type than it is for the quantum-wire or QW type of gain-coupled DFB laser, both the optimum number of vertically stacked quantum structures (Nst) for minimizing Ith when cavity length (L) is fixed and the optimum L for minimizing Ith when Nst is fixed are the largest for the quantum-disk type. When L=500 μm, for example, the optimum Nst is 7 for the quantum-disk type, 5 for the quantum-wire type and 3 for the QW type. Threshold current density (Jth) is almost independent of facet reflectivity (r), and this implies that all these lasers oscillate at the Bragg mode. The Jth of the bulk laser, however, is influenced most by r because this type has the smallest differential gain.© 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
78.66.Fd III-V semiconductors

Vacuum ultraviolet application of Li2B4O7 crystals: Generation of 100 fs pulses down to 170 nm

V. Petrov, F. Rotermund, F. Noack, R. Komatsu, T. Sugawara, and S. Uda

J. Appl. Phys. 84, 5887 (1998); http://dx.doi.org/10.1063/1.368904 (6 pages) | Cited 19 times

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We investigate and characterize the newly grown crystal Li2B4O7 which is transparent down to 160 nm for nonlinear optical conversion into the vacuum ultraviolet using sum frequency mixing with femtosecond pulses. This material exhibits excellent properties below 180 nm and makes possible the generation of wavelengths down to 170 nm with an all solid state laser system. The most important advantage of Li2B4O7 in this spectral range turns out to be the possibility of utilizing noncritical phase matching with maximized effective nonlinearity. We demonstrate generation of nearly transform limited 100 fs pulses between 170 and 185 nm with conversion efficiency of 4%. Their peak powers range from 200 kW at 170 nm to >2 MW at 185 nm. © 1998 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Nv Optical frequency converters
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Photorefractive polymer composite trapping properties and a link with chromophore structure

K. S. West, D. P. West, M. D. Rahn, J. D. Shakos, F. A. Wade, K. Khand, and T. A. King

J. Appl. Phys. 84, 5893 (1998); http://dx.doi.org/10.1063/1.368905 (7 pages) | Cited 14 times

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The photorefractive properties and the phase stability of polymer composites are dependent on the detail of the alkyl chain substituent attached to the electro-optic dye within the composite. Photorefractive composites based on poly (N-vinylcarbazole) (PVK), sensitized with trinitrofluorenone (TNF) and mixed with a concentration of 47.5 wt. % of electro-optic dye have been tested for photorefractive performance. Two alternative azo dyes of identical molecular weight have been used to produce alternative composites; both dyes were modified to suppress spatial isomerism and incorporated an eight carbon alkyl chain at the electropositive end of the chromophore: either a straight octyl chain or a branched ethylhexyl chain was substituted. The reorientational enhancement of photorefractive performance is similar in the composites resulting from these dyes. The dye with a straight octyl chain led to a composite with improved holographic performance. The dye with a branched ethylhexyl chain led to a composite exhibiting lower diffraction efficiency, but with superior phase stability. A tentative explanation is offered for these differences based on the shape of the alkyl substituent and its effect on a trapping mechanism involving the dye molecules and the sensitisor anions in PVK:TNF-based photorefractive composites. © 1998 American Institute of Physics.
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42.70.Ln Holographic recording materials; optical storage media
42.70.Jk Polymers and organics
61.41.+e Polymers, elastomers, and plastics
42.40.Lx Diffraction efficiency, resolution, and other hologram characteristics
78.20.Jq Electro-optical effects

Diode-pumped continuous-wave, quasi-continuous-wave, and Q-switched laser operation of Yb3+,Tm3+: YLiF4 at 1.5 and 2.3 μm

A. Diening, P. E.-A. Möbert, and G. Huber

J. Appl. Phys. 84, 5900 (1998); http://dx.doi.org/10.1063/1.368876 (5 pages) | Cited 15 times

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We report continuous-wave (cw), quasi-cw and Q-switched laser oscillation of Yb3+,Tm3+-doped YLiF4 at 1.5 and 2.3 μm. Maximum cw output powers of 300 mW at 1.5 μm and 450 mW at 2.3 μm have been obtained. Quasi-cw excitation revealed 20 mJ pulses at 1.5 μm. The cw pumped 1.5 μm laser was passively Q-switched using a U:CaF2 crystal as saturable absorber with a repetition rate of 65 kHz and a pulsewidth of 800 ns. An average output power of 19 mW was achieved. The dynamics of the system was simulated using a rate equation model. © 1998 American Institute of Physics.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water

Martin Frenz, Flurin Könz, Hans Pratisto, Heinz P. Weber, Alexander S. Silenok, and Vitaly I. Konov

J. Appl. Phys. 84, 5905 (1998); http://dx.doi.org/10.1063/1.368906 (8 pages) | Cited 12 times

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The starting mechanisms and dynamics of laser-induced bubble formation at a submerged fiber tip in distilled water were experimentally investigated using pressure measurements and fast flash videography. A fiber guided Ho:YAG laser operating in the free running (τ=200 μs) and Q-switched (τ=45 ns) mode at a wavelength of λ=2.12 μm was used as a light source. It is shown that the beam profile at the distal fiber tip (multimode fiber d = 300 μm) exhibits hot spots that result in an inhomogeneous temperature distribution in the heated water volume. Depending on the laser irradiance, three different bubble formation processes are distinguished: bubble formation by heating, by rarefraction (cavitation), and by a combination of these two processes. For laser irradiances of less than 0.5 MW/ cm2 bubble formation takes place at temperatures near the critical point of water (T = 280 °C). A rapid decrease in the threshold temperature for bubble formation was found for laser irradiances between 0.5 and 1 MW/cm 2. At laser irradiances higher than 3 MW/cm2, microbubbles with radii of up to 20 μm were formed at the front of the laser pulse even though the average water temperature was far below 100 °C. The water temperature distribution during the laser pulse was determined by numerical simulation. Simultaneous pressure measurements revealed that each subablative laser spike induces a bipolar pressure transient. The onset of the bubble expansion was found to be correlated with a characteristic pressure increase that can be used for on-line monitoring of the ablation process. The distortion of the temporal profile of the pressure wave is shown to be an effect of diffraction. The reduction of pressure by the negative part of the bipolar pressure transients leads to a lowering of the evaporation pressure and therefore to the initiation of bubbles by cavitation. With increasing irradiance this mechanism becomes more efficient. © 1998 American Institute of Physics.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
65.90.+i Other topics in thermal properties of condensed matter (restricted to new topics in section 65)
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects

Second-harmonic generation in nonlinear grating coupler

Mitsuhiro Yokota, Sin’ichi Takaishi, and Jun’ichi Yamane

J. Appl. Phys. 84, 5913 (1998); http://dx.doi.org/10.1063/1.368907 (9 pages) | Cited 1 time

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Second-harmonic (SH) wave in the grating coupler is investigated by using the singular perturbation technique with multiple space scales. In order to apply the perturbation technique, we first introduce the perturbation parameter concerned with the nonlinear coefficient and the depth of the grating, and the multiple space scales in the propagation direction. Substituting the expansion of the field and the multiple space scales into the nonlinear wave equation, we can get the governing equations to each order of the perturbation. From the solvability conditions to have nontrivial solutions for each perturbation solution, that coupled equations for the guided wave, incident wave, and radiated wave can be shown. Also, the radiated angle of fundamental and SH waves and the radiated SH wave for input/output couplers are examined numerically. © 1998 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.65.Wi Nonlinear waveguides
42.82.Et Waveguides, couplers, and arrays
42.79.Dj Gratings
42.79.Gn Optical waveguides and couplers
02.30.-f Function theory, analysis

Optical second-harmonic generation in sputter-deposited AlN films

G. T. Kiehne, G. K. L. Wong, and J. B. Ketterson

J. Appl. Phys. 84, 5922 (1998); http://dx.doi.org/10.1063/1.368908 (6 pages) | Cited 13 times

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Results of a study of optical second-harmonic (SH) generation in AlN films deposited on borosilicate glass by reactive direct current magnetron sputtering are reported. As determined by x-ray diffraction (XRD), the films were highly c-axis oriented. The transmitted SH signal for film thickness ranging from 340 to 7800 Å was measured. The signal was consistent with the existence of a dead layer (in which no SH was generated) on the film surface with an effective thickness of ∼300 Å. A set of 7800-Å-thick films were deposited at substrate temperatures from ambient to Ts = 400 °C. For films deposited at 100 °C or higher, minimal variation in the SH signal with Ts was found. Values of ∣d33∣ = 4 pm/V and ∣d31∣ = 0.04 pm/V, with d33 and d31 having the same sign, were obtained for Ts = 400 °C. The approximate tilt angle of the optic axis of the films with respect to the substrate surface normal was determined. XRD results showed that the tilt was due to a tilt of the crystallographic texture axis. © 1998 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
68.55.-a Thin film structure and morphology
78.66.Fd III-V semiconductors
42.79.Wc Optical coatings
42.70.Mp Nonlinear optical crystals

Effects of frequency on the two-dimensional structure of capacitively coupled plasma in Ar

T. Kitajima, Y. Takeo, N. Nakano, and T. Makabe

J. Appl. Phys. 84, 5928 (1998); http://dx.doi.org/10.1063/1.368909 (9 pages) | Cited 31 times

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A time-modulated and time-averaged two-dimensional (2D) profile of the net excitation rate of Ar(3p5) and Ar+(4p4D7/2) was observed in capacitively coupled plasma (CCP) in Ar as a function of the driving frequency (1–100 MHz) through the use of 2D-t optical emission spectroscopy (OES). A significant improvement in the detection efficiency was achieved for the time-resolved OES. Results are presented for the basic characteristics of 2D and 2D-t profiles of the net excitation rate in Ar from 1 to 100 MHz at 1.0 Torr. For comparison, 2D profiles in CCP between high frequency (13.56 MHz) and very high frequency (100 MHz) at 0.1 Torr and 25 mTorr under the same power dissipation are discussed. The advantages of VHF operation are described and discussed based on the radial uniformity and high density in CCP at low pressure. The appearance of a local emission peak at the edge of the rf electrode is observed in CCP, which is caused by changes in the sustaining mechanism due to increases in the driving frequency. © 1998 American Institute of Physics.
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52.25.-b Plasma properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Sheath overlap during very large scale plasma source ion implantation

B. P. Cluggish and C. P. Munson

J. Appl. Phys. 84, 5937 (1998); http://dx.doi.org/10.1063/1.368910 (8 pages) | Cited 5 times

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Measurements of plasma source ion implantation have been performed on a large target of complex geometry. The target consists of 1000 aluminum, automotive piston surrogates mounted on four racks; total surface area is over 16 m2. The four racks are positioned parallel to each other, 0.25 m apart, in an 8 m3 vacuum chamber. The racks of pistons are immersed in a capacitive radio frequency plasma, with an argon gas pressure of 20–65 mPa. Langmuir probe measurements indicate that the plasma density profile is highly nonuniform, due to particle losses to the racks of pistons. The plasma ions are implanted into the pistons by pulse biasing the workpiece to negative voltages as low as −18 kV for up to 20 μs. During the voltage pulse, the high-voltage sheaths from adjacent racks of pistons converge towards each other. At plasma densities less than 109 cm−3 the sheaths are observed to overlap. Measurements of the sheath overlap time are compared with standard analytic theory and with simulations run with a two-dimensional particle-in-cell code.
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61.72.up Other materials
61.80.Jh Ion radiation effects
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Dg Plasma sources
52.25.-b Plasma properties
52.70.Ds Electric and magnetic measurements

Secondary electron enhanced discharges in plasma source ion implantation

B. P. Cluggish and C. P. Munson

J. Appl. Phys. 84, 5945 (1998); http://dx.doi.org/10.1063/1.368889 (11 pages) | Cited 8 times

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The first published measurements of a plasma discharge enhanced by secondary electrons during plasma source ion implantation are presented. The ion implantation target consists of 1000 aluminum, automotive piston surrogates mounted on four racks; total surface area is over 16 m2. The four racks are positioned parallel to each other, 0.25 m apart, in an 8 m3 vacuum chamber. The racks of pistons are immersed in a capacitive radio frequency plasma, with an argon gas pressure of 20–65 mPa. Each plasma ion implanted into the target results in the emission of multiple secondary electrons. Langmuir probe measurements indicate that at high enough gas pressures, the energy of the secondary electrons can couple to the plasma, increasing the rate of ionization. Two different coupling mechanisms are observed. In the first, the secondary electrons directly ionize the background gas. In the second, the secondary electrons couple to the thermal electrons through a beam-plasma instability. Measurements of the instabilities are in agreement with two-dimensional particle-in-cell simulations.
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61.72.up Other materials
52.50.Dg Plasma sources
61.80.Jh Ion radiation effects
52.80.Pi High-frequency and RF discharges
79.20.Hx Electron impact: secondary emission
52.70.Ds Electric and magnetic measurements
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Multiply charged ion transport in free boundary vacuum arc plasma jet

M. Keidar, I. I. Beilis, and I. G. Brown

J. Appl. Phys. 84, 5956 (1998); http://dx.doi.org/10.1063/1.368890 (5 pages) | Cited 8 times

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The free boundary plasma arc jet expansion was analyzed based on a two-dimensional hydrodynamic model. The plasma jet expansion was modeled using the sourceless steady-state hydrodynamic equations, where the plasma free boundary was determined self-consistently. Due to the existence of an electric field in the quasineutral plasma the different charged ion species can be spatially separated. It was found that the mean charge state distribution is strongly nonuniform with a tendency for the highly charged species to appear near the plasma jet boundary region. Along the center line the density of singly charged ions falls off by about four times while the density of fourfold charged ions drops by more than 100 times. The radial charge state distribution becomes more nonuniform with increasing magnetic field. Good qualitative agreement between calculated and experimental radial distributions of different charged species was obtained. © 1998 American Institute of Physics.
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52.25.Fi Transport properties
52.30.-q Plasma dynamics and flow
52.50.Dg Plasma sources
52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.40.Hf Plasma-material interactions; boundary layer effects
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