Volume 103, Issue 9, 01 May 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
103(2008); http://dx.doi.org/10.1063/1.2903582View Description Hide Description
doped and codoped and nanoparticles with a size distribution of 200–300 nm have been prepared by adopting a combustion-fluorization method. The luminescence spectra of and ions in the and nanoparticles have been investigated through comparison. It is found that the and ions in the two hosts show different luminescence properties although the sites occupied by the rare earth (RE)ions in the and hosts are of the same symmetry. The different luminescence properties may be ascribed to the difference in the RE-F bond nature in the and hosts.
103(2008); http://dx.doi.org/10.1063/1.2908212View Description Hide Description
We utilize photonic crystals to simulate enhanced light-trapping in thin filmsolar cells. A one dimensional photonic crystal or distributed Bragg reflector with alternating dielectric layers acts as low loss backreflector. A two dimensional photonic crystal between the absorber layer and the Bragg reflector diffracts light at oblique angles within the absorber. The photonic crystal geometry is optimized to obtain maximum absorption. The photonic crystal provides lossless diffraction of photons, increasing the photon path length within the absorber layer. The simulation predicts significantly enhanced photon harvesting between 600 and below the band edge, and an absorption increase by more than a factor of 10 near the band edge. The optical path length ratio can exceed the classical limit predicted for randomly roughened scattering surfaces at most wavelengths near the band edge. The optical modeling is performed with a rigorous scattering matrix approach where Maxwell’sequations are solved in Fourier space.
Optical properties of nitrogen-doped films: Effect of the electronegativity on refractive index and band gap103(2008); http://dx.doi.org/10.1063/1.2908197View Description Hide Description
The optical properties of nitrogen-doped films with different gas ratios grown by reactive sputtering were studied by spectroscopy ellipsometry. The optical dielectric functions of the films were simulated by the Tauc-Lorentz model. It was found that the refractive index and extinction coefficient of nitrogen-doped films increase and the band gap has a redshift with the increase of the ratios. The general influences of the electronegativity and bond ionicity on the band gap, the refractive index in the spectral region below the fundamental absorption edge of nitrogen-doped , and other doped semiconductors are demonstrated.
Temperature-dependent spectroscopic properties of in germanate, silica, and phosphate glasses: A comparative study103(2008); http://dx.doi.org/10.1063/1.2912952View Description Hide Description
Spectroscopic properties of thulium-doped germanate, silica, and phosphate glasses were measured and compared since such glasses are of interest as materials for fiber lasers in the eye-safe wavelength region. excited state fluorescence decay dynamics was investigated at temperatures from and the results revealed a strong dependence of the lifetime on the host matrix. The temperature-dependent stimulated emission cross section was obtained by using the Fuchtbauer–Ladenburg technique. In phosphate glass the fluorescent lifetime is short, making this material difficult to use for laser purposes. -doped germanate glass shows a longer lifetime than silica, a comparable value of stimulated emission cross section and some interesting temperature-independent properties.
103(2008); http://dx.doi.org/10.1063/1.2902952View Description Hide Description
Aluminumnanoparticles are formed into compacts, and electromagnetic wave reflectance and transmittance of the compacts are measured in a microwave frequency range. The dielectriclike behavior of the compact, which transmits an electromagnetic wave with quite a small loss, is observed. This dielectric behavior originates from electrically isolated nanoparticles by an oxide layer of a few nanometers formed at the surface.
103(2008); http://dx.doi.org/10.1063/1.2912983View Description Hide Description
Detailed polarized spectral properties of crystal have been investigated. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves were measured at room temperature. The fluorescence decay mechanisms of the and multiplets were discussed. Spectroscopic parameters related to the laser operation at around via the transition have been evaluated. Room-temperature quasi-cw laser emission from a Ti:sapphire laser pumped crystal has been demonstrated. The maximum output power of 0.5 W has been achieved with a slope efficiency of 50%.
103(2008); http://dx.doi.org/10.1063/1.2913326View Description Hide Description
The thermal and electrical tunability of a cholesteric liquid crystal containing a negative dielectricanisotropyliquid crystal in a planar alignment was studied. The physical, optical, and electro-optical characteristics of mixtures containing different ratios of chiraldopant S811 and the negative dielectricanisotropyliquid crystal ZLI-2806 were examined. A smectic A phase was seen at room temperature for S811 loadings . Below 20%, a room temperature cholesteric phase was observed. Upon heating mixtures with composition S811 , the selective reflection notch of the cholesteric phase appeared and blueshifted with temperature. Thermal tuning from was observed over the temperature range of . Polarized optical microscopy, differential scanning calorimetry, and x-ray studies were utilized to confirm the temperature-dependent phase behavior. Tuning of by the application of a direct current electric field was also observed with no onset of electrohydrodynamic instabilities for voltages up to . Bandwidth broadening but not tuning was obtained with the application of alternating current fields. Electrical tuning is likely due to pitch contraction brought about through the annealing of defects.
103(2008); http://dx.doi.org/10.1063/1.2908862View Description Hide Description
The evolution of polariton features with increasing thickness in -polarized (TM) reflectance spectra of indium tin oxide (ITO) thin filmsdeposited on BK7 glass reveals the nature of plasmons in conducting thin films without interference from band-to-band transitions or the tendency of very thin films to form islands, both of which are complicating factors with the noble metalsAu and Ag. Although the dependence on energy, film thickness, and angle of incidence is complex, these features are accurately described by the three-phase (substrate/overlayer/ambient) Fresnel model using only the Drude free-electron representation for the dielectric function of the ITO film. For film thicknesses less than the relevant excitation is a one-dimensional screened-bulk plasmon (SBP) that corresponds to charge transfer across the entire film. The associated SBP polariton (SBPP) occurs at the energy of the SBP and is relatively independent of the angle of incidence. For film thicknesses greater than , the relevant excitation is the surface plasmons (SP). The associated surface plasmonpolariton (SPP) exhibits the usual strong dependence of energy on the angle of incidence. For larger thicknesses this structure gradually weakens, in agreement with theory. No other collective excitations are observed. The optimum thicknesses for the SPP in ITO is , whereas the SBPP is observed only when the film thickness is less than . The SBPP exhibits many of the features that make the SPP attractive for both science and technology, but has not been observed previously. Our results show that ITO films, in particular, and conducting-metal-oxide films in general provide new opportunities for investigating plasmons in conductors and obtaining new insights into plasmons,plasmonpolaritons, and related optical phenomena.
103(2008); http://dx.doi.org/10.1063/1.2919779View Description Hide Description
Charge-sensitive infrared phototransistors (CSIPs) with a active area, which are fabricated in a GaAs/AlGaAs double-quantum-well structure, are studied with an all-cryogenic spectrometer operated at 4.2 K. Extremely low level of background radiation makes reliable determination of detector characteristics at 4.2 K possible: The detection band is found to be centered at the wavelength with a bandwidth (full width at maximum) . The quantum efficiency , the current responsivity , the noise equivalent power (NEP), and the specific detectivity are derived to be , , , and . The dynamic range of detection is demonstrated to exceed (approximately attowatts to picowatts), but the upper limit of the radiation power is limited by the radiation source intensity. The intrinsic dynamic range of the detector is suggested to reach (approximately attowatts to microwatts). The detection speed is suggested to be around 3 ns (300 MHz). The sensitivity of CSIPS is so high that single-photon signals are discerned in the photocurrent as stepwise increases in given amplitude. The value of is by a few orders of magnitude higher than that of the state-of-the-art multi-quantum-well infrared photodetectors. The extremely high sensitivity will open up the possibility of developing ultrahigh-speed imaging and/or ultrahigh-resolution passive microscopy system in the long wavelength infrared region.
Measurement of the junction temperature in high-power light-emitting diodes from the high-energy wing of the electroluminescence band103(2008); http://dx.doi.org/10.1063/1.2908176View Description Hide Description
By using pulsed driving currents with a small duty cycle, the high-energy wing of the electroluminescence band in AlGaInP and InGaN high-power light-emitting diodes(LEDs) was calibrated to measure the junction temperature in the range of . In a red AlGaInPLED with a thick active layer, an accuracy of 2% was achieved for the junction temperature derived from the high-energy slope in the spectral range free from parasitic absorption by taking into account the three-dimensional density of band states. Meanwhile, the far high-energy region of the slope distorted by parasitic absorption can be used for the extraction of the junction temperature by using only an appropriate linear correction procedure ( accuracy). In a blue InGaNLED with multiple-quantum-well active layers, the junction temperature can be determined with an accuracy of 2% from the inverse derivative of the spectra in a narrow spectral region above the peak energy by using a linear correction.
Femtosecond pulsed laser ablation dynamics and ablation morphology of nickel based superalloy CMSX-4103(2008); http://dx.doi.org/10.1063/1.2917418View Description Hide Description
Pump-probe shadowgraphic imaging of single pulse femtosecond laserablation was performed to investigate the dynamics of material removal during femtosecond lasermachining of the intermetallic superalloy CMSX-4. Time-resolved shadowgraphic images were collected, which showed the presence of an expanding shock wave in the air in front of the target, following the onset of laser ablation. The dimensions of the shock wave were measured as a function of time , following the onset of ablation. The energy release associated with the observed shock wave and the pressure at the shock wave front versus time as a function of incident laser fluence were inferred from the shock dynamics. The measured shock wavedynamics and inferred shock energy release are discussed in light of the evolving ablation morphology and ablated crater depth as a function of incident laser fluence.
103(2008); http://dx.doi.org/10.1063/1.2919765View Description Hide Description
Optical absorption and emission intensities are investigated for doped into single-crystal . Room temperature absorption spectra of , representing transitions from to excited multiplet manifolds observed between 400 and 2100 nm, have been analyzed using the Judd–Ofelt (JO) model to obtain the phenomenological intensity parameters , , and . The JO intensity parameters are used to calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios of the transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds of . The room temperature fluorescencespectrum in the near infrared region is reported between 850 and 1550 nm. The emission cross section for the intermanifold transition, has been determined. The 8 K absorptionspectrum was examined as well. The detailed structure observed in the absorption bands , , , and , was analyzed in terms of the crystal-field splitting of each manifold using a charge-compensation model first developed for doped into . The optical and spectroscopic characteristics of demonstrate that this material has potential as a laser host system at various wavelengths.
103(2008); http://dx.doi.org/10.1063/1.2907960View Description Hide Description
The recently increased efficiency of organic light emitting devices(OLED) brings lighting applications within reach. If the area of the OLED is of the order a , voltage and brightness losses related to the square resistance of the transparent electrode become important. The homogeneity of the voltage and brightness can be improved by contacting the transparent electrode from all edges and by adding a metallic grid to the transparent electrode. This grid should have narrow lines to minimize transmission losses and improve the total light emission from the OLED. The voltage losses in grids with different shapes (triangular, square, and hexagonal) are evaluated and the grid parameters are optimized to maximize the total light emission. It turns out that a hexagonal grid has lower loss than a square grid with the same line width.
103(2008); http://dx.doi.org/10.1063/1.2919151View Description Hide Description
Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR)spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.
103(2008); http://dx.doi.org/10.1063/1.2919710View Description Hide Description
We have determined a refractive index of an organic single crystal of a thiophene/phenylene co-oligomer in its laser oscillation wavenumbers. We estimated the refractive index to be 4.0 on the basis of the mode intervals of the well-resolved longitudinal multimodes that are highly polarized along the normal to the (001) plane of the single crystal. This large number is in good agreement with that obtained by the quantum chemical calculations at semiempirical levels. This is also consistent with the high factor obtained from the line shape analysis.
Sensitive detection of weak absorption signals in photoacoustic spectroscopy by using derivative spectroscopy and wavelet transform103(2008); http://dx.doi.org/10.1063/1.2924438View Description Hide Description
This report presents a practical analytical method of photoacoustic (PA) spectroscopy that is based on wavelet transform (WT) and the first-derivative PA spectrum. An experimental setup is specially designed to obtain the first-derivative spectrum, which aims to identify some unnoticeable absorption peaks in the normal PA spectrum. To enhance the detectability of overlapping spectral bands, the WT is used to decompose the PA spectrum signals into a series of localized contributions (details and approximation) on the basis of the frequency. For the decomposed contributions do not change the absorption peak position of PA spectrum, one can retrieve the weak absorption signals by the decomposed result of WT. Because of the use of derivative spectroscopy and WT, three unnoticeable absorption peaks that are hidden in the PA spectrum of carbonabsorption are precisely retrieved, the wavelengths of which are 699.7, 752.7, and , respectively. This analytical method, which has the virtue of using a physical method and using a computer software method, can achieve great sensitivity and accuracy for PA spectralanalysis.
103(2008); http://dx.doi.org/10.1063/1.2917224View Description Hide Description
The forward and backward light scattering that accompanied the almost ballistic light propagation in opal-based three-dimensional photonic crystal(PhC) has been experimentally studied. The light path memory and the high anisotropy of scattering have been associated with low-order scattering at large-scale crystal defects. The strong backscattered light intensity has been linked to the uncoupled light at a PhC surface. The exponent of power-law approximation of angle diagrams of the scattered light intensity has been suggested as a measure of the scatteringanisotropy. The scatteringanisotropyspectra have demonstrated that in a vicinity to the first photonic bandgap, the lower directionality of the forward scattering is complemented by the higher directionality of the backward scattering.
103(2008); http://dx.doi.org/10.1063/1.2919164View Description Hide Description
A simple optical method based on absorption of monochromatic light to investigate the dynamics of single walled carbon nanotube(SWCNT)suspensions is described. The well dispersed suspensions display a complex behavior, exhibiting peaks due to resonant scattering from SWCNT bundles with increasing diameters as a function of time. The results indicate that the bundling of SWCNTs initiates almost immediately after termination of sonication (after ) and continues to increase up to a critical time , above which precipitation according to the Stokes relationship occurs. The absorbance behavior can be explained by the depletion of the effective medium as well as the Mie scattering from growth of bundles. A semiquantitative analysis of the experimental data based on the Mie theory of light scattering from cylindrical particles allows the extraction of diameters at the nucleation and growth of SWCNT bundles. The bundling dynamics have been correlated with the electrical properties such as the sheet resistance and transistorcharacteristics of the SWCNTthin films. Our work is a useful step toward reproducible solution processed electronics because it provides a simple method to monitor the quality of SWCNTsuspensions in real time and correlate it to device characteristics.
103(2008); http://dx.doi.org/10.1063/1.2919574View Description Hide Description
An antireflection coating improves the trapping of high-index particles in optical tweezers by reducing the scattering force. This can allow the strong trapping of high-index particles that cannot normally be trapped, and the use of lower numerical aperture objectives while still obtaining strong trapping. The improvement is not overly sensitive to the refractive index or thickness of the coating.
- PLASMAS AND ELECTRICAL DISCHARGES
Quadrupole mass spectrometry and time-of-flight analysis of ions resulting from pulsed laser ablation of Ni, Al, and ZnO targets103(2008); http://dx.doi.org/10.1063/1.2903604View Description Hide Description
This work describes the design and validation of an instrument to measure the kinetic energies of ions ejected by the pulsed laser ablation (PLA) of a solid target. Mass spectra show that the PLA of Ni,Al, and ZnO targets, in vacuum, using the second harmonic of a Nd:YAG laser (, pulse duration ) generates abundant ions ( for Ni, for Al, and for Zn and O respectively from ZnO). Ions are selected by their mass∕charge ratio prior to the determination of their times of flight. PLA of Ni has been studied in most detail. The mean velocities of ablated ions are shown to follow the trend . Data from and are fitted to shifted Maxwellian functions and agree well with a model which assumes both thermal and Coulombic contributions to ion velocities. The dependence of ion velocities on laser pulse energy (and fluence) is investigated, and the high energy data are shown to be consistent with an effective accelerating voltage of within the plume. The distribution of velocities associated with indicates a population at cooler temperature than .