Volume 104, Issue 6, 15 September 2008
- 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:
- LASERS, OPTICS, AND OPTOELECTRONICS
Testing asymmetry in plasma-ball growth seeded by a nanoscale absorbing defect embedded in a thin-film matrix subjected to UV pulsed-laser radiation104(2008); http://dx.doi.org/10.1063/1.2980054View Description Hide Description
Previous studies of ultraviolet, nanosecond-pulsed-laser damage in thin films revealed nanoscale absorbing defects as a major source of damage initiation. It was also demonstrated that damage (crater formation) is facilitated by plasma-ball formation around absorbing defects. In this work an attempt is made to verify the symmetry of the plasma ball by irradiatingthin film with embedded goldnanoparticles from the side of either the air/film or substrate/film interfaces. Crater-formation thresholds derived in each case support preferential plasma-ball growth in the direction of the laser-beam source. The strong impact of internal -field distribution is identified.
104(2008); http://dx.doi.org/10.1063/1.2977666View Description Hide Description
The reflection notch of cholesteric liquid crystals (CLCs) formed from highly photosenstive azobenzene nematic liquid crystalsdoped with light-insensitive, large helical twisting power chiraldopants is shown to be widely phototunable by green laser beams. The nonlinear transmission properties of these materials were studied. We have shown that the relative shift in Bragg wavelength is independent of the chiraldopant concentration and develop a predictive theory of such behavior. The theory describes the dynamics of phototuning as well. Reflection shifts greater than 150 nm were driven with low power, cw of 532 nm in these photosensitive CLCs, previously attainable only through UV pre-exposure. A nonlinear feedback mechanism was demonstrated for CLCs of left, right, and both handedness upon laser-induced blueshifting of the reflection notch from a red wavelength using a green cw laser.
Electrical conduction and electroluminescence in nanocrystalline silicon-based light emitting devices104(2008); http://dx.doi.org/10.1063/1.2977749View Description Hide Description
Electrical transport and light emission properties of plasma-enhanced chemical vapor deposition grown light emitting devices(LEDs) based on nanocrystalline silicon have been studied. Various active layer compositions have been used. Electroluminescence and current-voltage measurements have been performed on metal-oxide-semiconductor structures. We found that Poole–Frenkel emission and trap-assisted tunneling between traps located at the nanocrystalline silicon interfaces are consistent with the measurements. The interface trap density was estimated. Its dependence on the composition of the active layer is discussed. We propose an equivalent electrical circuit model for the LED based on complex impedance measurements. Nanocrystalline siliconelectroluminescence in the near infrared region is explained by hot-electron injection and impact ionization mechanism. It is concluded that the trap-assisted tunneling and charge trapping limit the external power efficiency of this kind of devices.
104(2008); http://dx.doi.org/10.1063/1.2981054View Description Hide Description
This paper reports the effects of GaN and AlN nucleation layers (NLs) on the characteristics of the subsequently grownAlGaN templates, which were grown by a two-step growth method on sapphire substrates using metal-organic chemical vapor deposition. The in situ monitored reflectance spectra reveal that the thickness variation in AlGaN templates grown on the GaN NL is much smaller than that on the AlN NL. X-ray diffraction patterns show that the AlGaN template with a GaN NL exhibits a better crystalline quality as compared to that with an AlN NL. Observed from the transmission electron microscopy, it is also shown that the dislocation density of the AlGaN template with a GaN NL can be substantially reduced. In addition, the fabricated light-emitting diodes from the AlGaN template with a GaN NL exhibit a lower forward voltage, a lower series resistance, a lower leakage current, and a narrower linewidth of electroluminescence peak than those with the AlN NL.
104(2008); http://dx.doi.org/10.1063/1.2978359View Description Hide Description
The emission wavelength of broad-area AlGaInP/InGaP quantum-well lasers is tuned by the application of high hydrostaticpressure and low temperature from 645 down to 575 nm, i.e., from the red through the orange to yellow spectral range. Emission powers up to 300 mW are obtained in the full tuning range. The pressure and temperature dependence of threshold currents indicates that leakage occurs into the and minima in the barriers.
104(2008); http://dx.doi.org/10.1063/1.2976337View Description Hide Description
Diffraction behavior of cholesteric liquid crystal (CLC) grating with the surface plasmon effect was investigated. One indium tin oxide plate of the CLC grating cell was covered with silvernanoparticles. With the application of a proper voltage, a well formed phase grating was constructed in the CLC cell. The CLC grating was probed by a beam of the polarized-monochromatic light, and the wavelength range was from 450 to 700 nm. It was shown that an extra first-order diffraction band was observed around 505 nm. The physical reason of the extra diffraction band could be the surface plasma effect emerged from silvernanoparticles. The extra diffraction band due to the surface plasmon effect can offer potential applications in nano-optics, such as the optical switch function.
Laser induced gratings enhanced by surface-charge mediated electric field in doped nematic liquid crystals104(2008); http://dx.doi.org/10.1063/1.2980274View Description Hide Description
The optically induced grating enhanced by dcelectric field in nematic liquid crystal (LC)doped with copper porphyrin is studied. The reorientational effect of the LC was not observed for the writing beams with the intensity up to , but the threshold of the reorientation was reduced to with the assistance of an electric pulse applied to the LC. The low threshold for the induced grating can be retained even after switching off the dc pulse, indicating that an induced field inside the LC is present to sustain the low threshold operation for the induced grating. The low threshold operation does not persist by short circuiting of LC electrodes. The underlying mechanism is explained by an optical surface-charge mediated effect.
Electrotunable band gaps of one- and two-dimensional photonic crystal structures based on silicon and liquid crystals104(2008); http://dx.doi.org/10.1063/1.2975832View Description Hide Description
One- and two-dimensional photonic crystals based on silicon with infiltrated liquid crystals are investigated in this paper. We show that the photonic band gap can be continuously tuned changing the orientation of the director of the liquid crystal. For the one-dimensional case, we considered arbitrary direction of propagation of the electromagnetic waves, and we show that it is possible to tune the photonic band gap by an adequate orientation of the liquid crystal. For the two-dimensional case and propagation in the plane of periodicity, we show that there exists no complete photonic band gap in the system for both polarizations. We consider two different configurations, square array of solid Si cylinders in liquid crystal background and a triangular array of liquid crystal cylinders surrounded by Si. We show that for the triangular array it is possible to tune the photonic band gap only for the transversal electric modes. We used the plane wave expansion method to solve the Maxwell equations for anisotropic systems.
104(2008); http://dx.doi.org/10.1063/1.2980332View Description Hide Description
We theoretically and experimentally analyze the angular selectivity curves of nonuniform gratings recorded in a photopolymerizable silica glass. Due to the rigidity of this material, shrinkage would not cause fringe bending, so we propose that the coupling of the beams at the recording stage (two-wave mixing) produces the resulting nonuniformities. Nonuniform transmission holographic gratings can be analyzed in rigid media using the coupled-wave theory but introducing the results obtained from the two-wave mixing treatment and taking into account the absorption and the nonlinear effects produced by the mechanism of photopolymerization and diffusion. Good agreement between theory and experience is obtained.
104(2008); http://dx.doi.org/10.1063/1.2980340View Description Hide Description
A midinfrared quantum cascade laser with Mach–Zehnder cavity and split contacts is investigated with respect to interference effects. By increasing the temperature in one of the two coupled active waveguides, the value of the effective refractive index is varied and the modal phase is shifted. As a result, destructive interference is observed within the resonator, which manifests itself in a minimum of the modulated output power. The dissipated heat is controlled by locally adding a continuous current to the drive current pulses. In the first step, thermal properties, threshold values, and far fields are analyzed and compared to a Fabry–Pérot resonator to gain insight into the physical principles of the monolithic interferometer. Based on these findings, the temperature distribution is calculated in a two-dimensional heat transfer simulation, which leads to a match between the thermal change of the effective refractive index and the condition for destructive interference; a phase shift of between the two interfering beams is confirmed. By modulating the effective refractive index using evanescent fields instead of temperature variations, a monolithic midinfrared interferometric sensing device becomes feasible.
104(2008); http://dx.doi.org/10.1063/1.2976320View Description Hide Description
The properties of planar defectstructures, which are created by changing the dielectric distribution at the central layer of a three-dimensional (3D) layer-by-layer photonic crystal, are theoretically investigated by utilizing a parallel 3D finite-difference time-domain method and the plane-wave expansion method. Two different kinds of resonant modes, the defect mode and the band-edge resonant mode, have been clarified by spectrum analysis and calculated mode profiles. It is shown that the resonant modes can be controlled by changing the periodicity, the thickness, or the dielectric constant of materials at the defect layer. Besides, photonic band edges can be shifted by applying dislocation to a layer of dielectric rods.
104(2008); http://dx.doi.org/10.1063/1.2982093View Description Hide Description
A reflection technique to determine a contact angle by laser glancing incidence method, which is based on analyzing the reflection pattern from the up curved liquid surface (UCLS) around a smooth flat plate, is presented. In the experiment, a glass slide is vertically dipped into a tested liquid. Due to the wettingeffect, the UCLS is formed around the glass slide. When an expanded and collimated laser beam impinges on the UCLS at glancing incidence, the steady and visible strip-shape dark area reflection patterns are observed. The relation of the dark region width and the maximal height of the UCLS is derived theoretically. The contact angles of distilled water and kerosene on the glass slides are calculated directly by utilizing the dark area width of the reflection patterns. Results show that an effective and practical technique for measuring the contact angle of the Wilhelmy plate is found.
104(2008); http://dx.doi.org/10.1063/1.2982374View Description Hide Description
Emission characteristics, spectral properties, and quantum efficiencies of light-emitting diodes, with aluminum compositions between 0% and 8.75%, have been investigated as a function of temperature from 25 to 300 K, and a function of current from 1 to 100 mA. As both current and temperature are varied a change in the dominant recombination mechanism is observed as indicated by changes in the measured emission. An analysis of the light-current characteristics shows that Auger processes become important in all devices at temperatures above 100 K, implying an activation energy of approximately 7–13 meV depending on the aluminum composition.
Influence of applied electric fields on the absorption coefficient and subband energy distances of intersubband transitions in AlN/GaN coupled double quantum wells104(2008); http://dx.doi.org/10.1063/1.2980320View Description Hide Description
The influence of applied electric fields on the absorption coefficient and subband energy distances of intersubband transitions (ISBTs) in AlN/GaN coupled double quantum wells (CDQWs) has been investigated by solving the Schrödinger and Poisson equations self-consistently. It is found that the absorption coefficient of the ISBT between the ground state and the second excited state can be equal to zero when the electric fields are applied in AlN/GaN CDQWs, which is related to the applied electric field induced symmetry recovery of these states. Meanwhile, the energy distances between and subbands have different relationships from each other with the increase of applied electric fields due to the different polarization-induced potential drops between the left and right wells. The results indicate that an electrical-optical modulator operated within the optocommunication wavelength range can be realized in spite of the strong polarization-induced electric fields in AlN/GaN CDQWs.
104(2008); http://dx.doi.org/10.1063/1.2981196View Description Hide Description
Monomode waveguide was formed by ion implantation with 500 keV ions at a dose of and the following ion exchange in pure at for 45 min. Results indicate that it is an effective method to modulate the waveguide modes by combining ion exchange with ion implantation. Positive changes in both and refractive indices occur in the waveguide region. Continuous and homogenous field pattern of the propagation light in the waveguide was collected and studied using the end-coupling method. The irradiation damage as well as the Rb distribution was analyzed by means of the Rutherford backscattering spectrometry technique. The concentration profiles of Rb and K in were measured by the time of flight secondary ion mass spectrometry. Results show that the lattice damages formed by ion implantation act as a diffusion barrier, which stop the Rb ion and K ion exchange in the deeper depth.
- PLASMAS AND ELECTRICAL DISCHARGES
Impact of magnetron configuration on plasma and film properties of sputtered aluminum nitride thin films104(2008); http://dx.doi.org/10.1063/1.2978226View Description Hide Description
We have investigated the growth of the -axis oriented aluminum nitride(AlN)thin films on (100) silicon by reactive dc magnetron sputtering at low temperature, considering the effect of the magnet configuration on plasma and filmproperties. It appears that a magnet modification can significantly modify both the plasmacharacteristics and the filmproperties. Electrical and optical characterizations of the plasma phase highlight that depending on the magnet configuration, two very different types of deposition process can be involved in the same deposition chamber. On the one hand, with a balanced magnetron (type 1), the deposition process enhances the production of AlN dimers in the plasma phase and enables to synthesize AlN films with different preferential orientations (100, 002, and even 101). On the other hand, a strongly unbalanced magnetron (type 2) provides a limited production of AlN species in the plasma phase and a strong increase in the ratio of ions to metal atom flux on the growingfilms. In the latter case, the ion energy provided by the ion flux to the growingfilm is typically in the 20–30 eV range. Thus, dense (002) oriented films with high crystalline quality are obtained without any substrate heating.
104(2008); http://dx.doi.org/10.1063/1.2979715View Description Hide Description
An inductive plasma source driven with phase shifted antenna coils at 2 MHz has been developed to accelerate ions for semiconductor etching process. The experiment was carried out in gas mixtures in the pressure range between 0.3 and 0.9 mTorr and rf power between 0.6 and 1.5 kW. Measurement of the ion energy spectra behind the wafer has shown high energy ions (up to 70 eV). An anisotropicetching (without rf biasing) of a polysilicon film has been demonstrated in this experiment. The acceleration of the electrons was numerically studied based on the fluid theory. The numerical results show that electrons affected by Lorentz force and thermal pressure gradient make axial electron currents, which contribute to form axial electric fields and ion acceleration.
104(2008); http://dx.doi.org/10.1063/1.2980336View Description Hide Description
The physical properties of Cherenkov radiation (CR) are theoretically investigated for a charged particle traveling along the axis of a cylindrical waveguide filled with anisotropic double-negative metamaterials (DNMs). The reversed CR and CR conditions are obtained using analytical method. The influence of the particle velocity, the waveguide radius, and the constitutive parameters of the anisotropic DNMs are discussed. A numerical example illustrates that the total radiated energy increases with increasing particle velocity, the radiated energy spectral density has different poles at the different frequencies for different anisotropic DNMs when the loss of the anisotropic DNM is smaller, and when the radius has the same order as the operating wavelength, the influence of the waveguide radius on the total radiated energy is smaller on the whole. Since most of the metamaterials realized so far are anisotropic, our theoretical work based on anisotropic DNMs will be helpful for future experimental realizations.
Experiment and simulation of the compositional evolution of Ti–B thin films deposited by sputtering of a compound target104(2008); http://dx.doi.org/10.1063/1.2978211View Description Hide Description
The evolution of the coating stoichiometry with pressure, target-substrate distance, and angle was analyzed for dc sputtering of compound targets by elastic recoil detection analysis. For an investigation of the underlying fundamental processes primarily Ar was used as sputter gas. Additionally, the effect of a reactive gas as well as bias voltage (floating up to ) was briefly cross-checked. For deposition along the target normal a pronounced Ti-deficiency of up to 20% is detected. Increasing the pressure or distance from 0.5 to 2 Pa and from 5 to 20 cm, respectively, leads to an almost equivalent linear increase in Ti/B ratio surpassing even the target composition. Off-axis depositions at lower angles ( and ) on the other hand result in a higher Ti/B ratio. This is consistent with results obtained from Monte Carlo simulations combining the respective emission characteristics from the sputter process as well as the gas-phase transport. Hence, the pressure, distance, and sample position induced changes in chemical film composition can be understood by considering gas scattering and the angular distribution of the sputtered flux. The theoretically determined transition from a directional flux to thermal diffusion was experimentally verified by mass-energy analysis of the film-forming atoms.
104(2008); http://dx.doi.org/10.1063/1.2980278View Description Hide Description
Detailed characterization of a microwave cavity discharge in the supersonic flow of mixtures at static pressures of 1–10 Torr and Mach number 2 is mostly based on emission spectroscopy techniques. In the conditions close to real combustion environments, effects of hydrogen and air admixture to plasma parameters and population of excited species in the discharge are demonstrated. The effects resulting in ionization loss are discussed from the aspects of dominant mechanisms and consequences for the plasma assisted hydrogen oxidation.