Volume 102, Issue 6, 15 September 2007
- 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
102(2007); http://dx.doi.org/10.1063/1.2779265View Description Hide Description
We explore the use of quantum cascade structures as amplifiers for terahertz radiation. For this purpose, two section devices were fabricated, in which the output from a laser section is amplified in a subsequent stage. For relatively short devices we obtain an amplification of , in good agreement with the calculated level. Based on these results we predict that amplifications of up to 13 dB could be obtained for such structures using increased amplifier lengths of a few millimeters.
102(2007); http://dx.doi.org/10.1063/1.2779278View Description Hide Description
Precise ac electrical measurements as well as dc plots at forward bias have been used to characterize multi-quantum-well (MQW)laser diodes. Step offsets of apparent conductance, apparent capacitance,junction voltage, series resistance, and ideality factor at lasing threshold were observed. To compare the electrical characteristics of different diodes numerical simulations of the curve of the double-heterostructure (DH) laser were performed. We find that the conventional model used to explain this curve is not very satisfactory. Our simulations demonstrate that the junction voltage, series resistance, and ideality factor for both DH and MQW lasers exhibit the same sudden changes at threshold. Perhaps the electrical behavior near threshold for all laser diodes is very similar.
Noise characterization of light-emitting devices based on conjugated copolymers of fluorene and thiophene moieties102(2007); http://dx.doi.org/10.1063/1.2778739View Description Hide Description
Degradation induced changes in the structural and optical properties of polyfluorene-based light-emitting diodes are examined by using electroluminescence and low frequency noise (LFN) spectroscopic techniques. The materials studied are poly[2,7-(-dihexylfluorene)-alt-bithiophene] (P1) and poly[2,7-(-dihexylfluorene)-alt-thieno[3,2-]thiophene] (P2). Improved emission spectra for a light-emitting device based on polymer P2 in terms of current efficiency, spectra stability, and lifetime are observed. A polymer P2-based device also presents long lifetime predicted by the smaller slope in the initial LFN spectra. Correlation of device LFN spectra with polymer structure change and lifetime is established. The increase in noise level predicts the undergoing degradation in bulk material and the increase in the noise slope predicts the fluctuation of carrier number and change in polymer structure. The redshift in emission spectrum for P2 after long-time driving is also picked up by the LFN spectrum.
102(2007); http://dx.doi.org/10.1063/1.2779286View Description Hide Description
Detailed polarized spectral properties of crystal grown by the Czochralski method have been investigated. The Judd-Ofelt intense parameters , the absorption and emission cross sections, and the branching ratios were calculated for each polarization of the main transitions. The large absorption cross section of polarization) and broad absorption band ( of polarization) around indicate that this crystal can be pumped efficiently by diode lasers. The broad emission bands from the multiplet show that the crystal is a promising medium for tunable and short pulse lasers. Pumped by a Ti:sapphire laser, the maximum quasi-cw laser output around has been obtained with a 1.5% output couple, the threshold is , and the slope efficiency is 32%.
102(2007); http://dx.doi.org/10.1063/1.2779243View Description Hide Description
films are deposited by electron beam evaporation as a function of oxygen partial pressure. The packing density, refractive index, and extinction coefficient all decrease with the increase of pressure, which also induces the change of the film’s microstructure, such as the increase of voids and concentration in the film. The laser-induced damage threshold (LIDT) of the film increases monotonically with the rise of pressure in this experiment. The porous structure and low nonstoichiometric defects absorption contribute to the film’s high LIDT. The films prepared at the lowest and the highest pressure show nonstoichiometric and surface-defects-induced damage features, respectively.
102(2007); http://dx.doi.org/10.1063/1.2775310View Description Hide Description
Transparent -doped glass ceramics were prepared, and the optical properties of -doped glass ceramics were investigated. Broadband emission centered at was observed by excitation. The longer wavelength luminescence compared with -doped glass ceramics is ascribed to the low crystal field hold by in glass ceramics. The change in optical signals at the telecommunication bands with or without excitation was also measured when the seed beam passes through the bulk gain host.
102(2007); http://dx.doi.org/10.1063/1.2779271View Description Hide Description
We investigate a photon recycling scheme using two-dimensional metallic photonic crystals made of silver to improve the energy efficiency of an incandescent light source. A theoretical framework is presented to analyze the resultant photon-recycled lighting system. Calculation results show that the system can reach a maximum luminous efficiency of 125 lm/W, which is 8 times higher than that of a bare blackbody radiation at 2800 K. The color temperature of the system is calculated to be around 3500 K or below, while the color rendering index is between 68 and 90. These results suggest that a photon-recycled incandescent light source using metallic photonic crystals can be a viable alternative future lighting solution.
Optical method for measuring the azimuthal anchoring strength of liquid crystals using pitch values determined in imperfect samples102(2007); http://dx.doi.org/10.1063/1.2781320View Description Hide Description
An improved Grandjean–Cano wedge method for measuring the pitch of a chiralnematic liquid crystal is demonstrated. This method is easy to implement and can yield results of high accuracy when it is used for measuring the liquid crystal surface anchoring strength.
102(2007); http://dx.doi.org/10.1063/1.2778740View Description Hide Description
Femtosecond pulsed laser ablation of silicon substrates with thin thermally grown oxide films (20–1200 nm) was studied using pump-probe microscopy techniques. Images from both the front and side of the ablation event produced at a laser fluence of were obtained, and results from the two imaging geometries were compared yielding the optical properties of the ablatedmaterial.Ablation dynamics were studied over the time scale from 0 to 10.35 ns following the onset of ablation, and ablatedmaterialvelocities ranging from to were determined depending on the thermal oxide film thickness.
102(2007); http://dx.doi.org/10.1063/1.2781300View Description Hide Description
We present two functions for characterization of birefringent media with application to twisted nematicliquid crystal(LC) cells in the field off state. The first function is for reflective LC cells and the second is for transmissive LC cells. Based on the Stockes formalism, it is shown that these functions describe the ability of a layer of the twisted birefringent medium to change polarization of the output light and are invariant to the layer orientation. The characteristic functions are found in simple procedures and can be used for simultaneous determination of retardation, its wavelength dispersion, and twist angle, as well as for solving optimization problems.
Porous silicon omnidirectional mirrors and distributed Bragg reflectors for planar waveguide applications102(2007); http://dx.doi.org/10.1063/1.2784019View Description Hide Description
The use of omnidirectional mirrors (an special case of distributed Bragg reflectors) as cladding for planar waveguides is proposed and analyzed. The proposed structure is an all-porous siliconmultilayer consisting of a core layer inserted between two omnidirectional mirrors. The transfer matrix method is applied for the modal analysis. The influence of the parameters of the waveguide structure on the guided modes is analyzed. These parameters are the layer thickness and number of periods of the omnidirectional mirror, and the refractive index and thickness of the core layer. Finally, the confinement of the omnidirectional mirrorcladding is analyzed with respect to two other different distributed Bragg reflectorcladdings.
- PLASMAS AND ELECTRICAL DISCHARGES
102(2007); http://dx.doi.org/10.1063/1.2779287View Description Hide Description
The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to ) and reduced duty cycle (90%–50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at and 60% duty cycle, the total power density is , about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we predict values of power density much lower than those measured. The measured and calculated values are in close agreement for the results obtained in dc mode but diverge at high frequencies. It is believed that this is due to the Langmuir probemeasurements being unable to observe the presence of high-energy ions, created during the transient peaks in the electron temperature at the transitions from on off and off on [J. W. Bradley et al., Plasma Sources Sci. Technol.11, 165 (2002)] which subsequently bombard the substrate. This paper shows conclusively the benefit of pulsing the magnetron over and above dc operation for enhancing the ion power per depositing neutral in the ion assisted deposition process.
102(2007); http://dx.doi.org/10.1063/1.2783848View Description Hide Description
In this paper, a one-dimensional bubble model of pulsed discharge in water is presented. With a total input energy of , the simulation results show that when the bubble collapses at the center of the bubble, the plasma pressureoscillates strongly. It oscillates between 800 and with an oscillation frequency of about , while at (: bubble radius), the gas velocity oscillates intensely at the same frequency. It oscillates between and when the bubble radius reaches its minimum. But it does not oscillate at because of the inertia of the surrounding water. The bubble collapses and reexpands with almost the same speed as that of the zero-dimensional (0D) model. This further confirms why the shock wavepressure from the 0D mode has a good agreement with the experimental results since the shock wavepressure is only determined by the bubble wall velocity .
Extracting ion emission lines from femtosecond-laser plasma x-ray spectra heavily contaminated by spikes102(2007); http://dx.doi.org/10.1063/1.2780872View Description Hide Description
Nowadays charged-coupled device(CCD) detectors are widely used for the registration of multicharged ions x-ray spectra. These spectra are generated in a plasma during interaction of ultrashort, ultraintense laser pulses with solid targets. Strong parasitic radiation from the plasma affects CCD detectors and contaminates resulting spectra, so that spectral features can be completely covered by noise even during measurements with a very short accumulation time. In this work we propose a “mean to median” (M2M) algorithm for noise suppression in femtosecond laserplasmax-ray spectra. Series of spectra is necessary for the identification of corrupted data points by the developed method. The algorithm was tested with model spectra which reflect main features of experimental data. In practice we used it for extracting information about spectral lines of Ne-like Fe ions and He-like Al ions which allowed us to calculate plasma parameters. It is demonstrated that M2M method is able to clean spectra with more than 10% of corrupted pixels. Fluctuations in intensity of spectral lines induced by laser instability do not affect validity of the proposed method.
102(2007); http://dx.doi.org/10.1063/1.2776374View Description Hide Description
The dispersion and radiation loss of propagating modes in a plasma-filled Bragg waveguide are investigated. The Bragg waveguide at a center frequency of 10 GHz is modeled with the transfer-matrix method, which has been used to analyze optical Bragg fibers. We calculate the dispersion and radiation loss of the , , and modes and show how they vary as a function of plasma density. As the plasma density inside the waveguide increases, the cutoff frequency of each mode increases. An increase in plasma density increases the radiation loss in the mode while it decreases the radiation loss in the mode; the effect on the mode is to increase the radiation loss for frequencies above 10.2 GHz and decrease the radiation loss for frequencies below 10.2 GHz.
102(2007); http://dx.doi.org/10.1063/1.2779268View Description Hide Description
This paper presents a systematic characterization of capacitively coupled radio-frequency hydrogen discharges, produced within a parallel plate cylindrical setup at different rf applied voltages , frequencies , and pressures . A two-dimensional, time-dependent fluid model for charged particle transport is self-consistently solved coupled to a homogeneous kinetic model for hydrogen, including vibrationally excited molecular species and electronically excited atomic species. Numerical simulations are compared with experimental measurements of various plasma parameters. A good quantitative agreement is found between simulations and experiment for the coupled electrical power and the plasma potential. The model underestimates the values of the electron density, the self-bias potential, and the atom density with respect to measurements, but agrees with experiment when predicting that all these parameters increase with either , , or . The dissociation degree is about for the work conditions considered. Simulations adopt a wall recombination probability for H atoms that was experimentally measured, thus accounting for surface modification with discharge operating conditions. Results show the key role played by the atomic wall recombination mechanism in plasma description.
A model for Si, SiCH, , SiOCH, and porous SiOCH etch rate calculation in inductively coupled fluorocarbon plasma with a pulsed bias: Importance of the fluorocarbon layer102(2007); http://dx.doi.org/10.1063/1.2784016View Description Hide Description
In a previous paper we showed that selective etching of porous SiOCH with respect to and SiCH is clearly enhanced when using a pulsed bias in inductively coupled fluorocarbon plasma. To understand this pulsed process, a model for etch rate calculation is developed in the present paper. This model explains the etching/deposition threshold shift toward higher bias voltage in pulsed conditions. Rather good confidence is obtained with experimental , Si, SiOCH and SiCH etch rates. Porous SiOCH etching is found to behave slightly differently compared to or SiCH; its chemical etching is assumed to occur even during the beginning of off period. This point could explain why good selectivities between porous SiOCH and and SiCH are obtained.
Angular distribution and dose measurements of hard x-ray emission from intense laser-plasma interaction102(2007); http://dx.doi.org/10.1063/1.2781383View Description Hide Description
An experimental study on hard x-ray Bremsstrahlung radiation due to fast electron generation from a planar solid copper target irradiated by 150 mJ, 45 fs Ti:sapphire laser pulses at is reported. Angular distribution of the hard x-raydose rate is observed to be strongly forward peaked in the direction of target normal with a measured peak value of , at a distance of 500 mm from the target. Two sources of this radiation, one at the laser irradiated target and the other at the chamber wall facing the target, are inferred from the measurements of angular distribution and distance dependence of the x-raydose rate.
102(2007); http://dx.doi.org/10.1063/1.2783980View Description Hide Description
This paper investigates the spatial and temporal variation in plasma electron density over a region between 5 and above the race-track region of a pulsed magnetron sputtering target. The pulse operation is performed using an asymmetric bipolar pulsed dc power supply, which provides a sequence of large negative “on-phase” voltage and a small positive “reverse-phase” voltage for 55% of the pulse duration . The electron density is measured using a floating microwave hairpin resonance probe. The results show electron expulsion from the target in the initial on phase, which propagates with a characteristic speed exceeding the ion thermal speed. In the steady state on phase, a consistent higher density is observed. A quantitative model has been developed to explain the resultant density drops in the initial on phase. While in the reverse phase, we observed an anomalous growth in density at a specific location from the target . The mechanism behind the increase in electron density has been attributed to the modulation in spatial plasma potential, which was measured earlier in the same apparatus using a floating emissive probe [J. W. Bradley et al., Plasma Sources Sci. Technol.13, 189 (2004)].
Measurement and modeling of arc jet discharge chemical vapor deposition reactors. I. Intercomparison of derived spatial variations of H atom, , and CH radical densities102(2007); http://dx.doi.org/10.1063/1.2783890View Description Hide Description
Comparisons are drawn between spatially resolved absorption spectroscopy data obtained for a dc arc jet reactor, operating with gas mixtures, used for deposition of thin, polycrystalline diamond films, and the results of a two-dimensional computer model incorporating gas activation, expansion into the low pressure reactor, and the chemistry of the neutral and charged species. The experimental measurements, using either cavity ring-down spectroscopy or diode laser absorption spectroscopy, determined absolute number densities of atoms, and column densities of , , and radicals, with vibrational and rotational quantum state resolutions, and their variation with height through the horizontally propagating arc jet plume. Spectra were also analyzed to obtain temperatures and local electron densities [from Stark broadening of absorption lines]. The experimental data are directly compared with the output data of the model that returns spatially inhomogeneous temperature, flow velocities, and number densities of 25 neutral and 14 charged species. Under the base operating conditions of the reactor [ (standard liters per minute) of Ar and of entering the primary torch, with addition of (SCCM denotes cubic centimeter per minute at STP) of downstream; input power; reactor pressure of ], the calculated and measured column and number densities agree to within factors of 2–3, the model reproduces the spatial dependence of column densities, and the mean temperatures of and radicals derived from spectra and model results are in good agreement. The model also captures the variation of these parameters with changes to operating conditions of the reactor such as flows of and , and input power. Further details of the model and the insights it provides are the subject of the accompanying paper [Mankelevich et al., J. Appl. Phys.102, 063310 (2007) ].