Broadband Peak Research Articles

Optical Absorption in SiGe/Si Quantum Well Structures Created by Subband Transitions

The absorption in Si1-xGex/Si multiple quantum-well structuresis measured. Several separated well absorption peaks corresponding toboth intersubband and intervalence band transitions in the samples areobserved. In the normal incidence, two broadband peaks are attributed tointervalence band transitions HH0-SO0(2.5 µm),HH0-LH0(~3 µm), respectively. Using 45° incidenceof unpolarized light, both the intervalence band andintersubband transitions are observed. The intervalence band transitions(HH0-LH0) are Ge composition dependent, but the intersubbandtransitions, HH0-HH1(5.9 µm) and HH0-HH2(4.3 µm), are not sensitive to the Ge composition.

Photoluminescence characteristics of Mg- and Si-doped GaN thin films grown by MOCVD technique

We have studied the optical, structural and surface morphology of doped and undoped GaN thin films. The p- and n-type thin films have been successfully prepared by low-pressure MOCVD technique by doping with Mg and Si, respectively. The different carrier concentrations were obtained in the GaN thin films by varying dopant concentrations. Photoluminescence (PL) studies were carried to find the defect levels in the doped and undoped GaN thin films at low temperature. In the undoped GaN thin films, a low intensity and broad yellow band peak was observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in both the Si or Mg lightly doped GaN thin films. The dominance of the blue and the yellow emissions increased in the PL spectra, as the carrier concentration was increased. The XRD and SEM analyses were employed to study the structural and surface morphology of the films, respectively. Both the doped and the undoped films exhibited hexagonal structure and polycrystalline nature. Mg-doped GaN thin films showed columnar structure whereas Si-doped films exhibited spherical shape grains.

Reduction of plasma-induced damage by electron beam excited plasma CVD

The electron beam excited plasma (EBEP-) CVD has succeeded in making nano-crystaline films. On the other hand, the existence of the plasma-induced damage by EBEP-CVD has been confirmed using the hydrogen plasma by measuring the photoluminescence (PL). After plasma exposure, broad band peak appears in the region of 1.0–0.78 eV (1.2–1.6 μm), and intensity of bound exciton peak with energy of 1.093 eV, which is measured and the non-irradiated silicon has been decreased. The same experiment was also performed with RF plasma and the peak appeared not only for EBEP but also for conventional RF plasma. The damage peak tends to disappear over 420°C of the substrate temperature. The damage recovery analysis has been done in relation to the annealing temperature of the substrate after the plasma exposure. Exciton peak has been increased by increasing the temperature especially at 350°C. Furthermore, plasma-induced peak intensity has been decreased at temperature higher than 500°C. Similar peak has been observed in the samples irradiated with high-energy protons. Therefore, positive ions in the plasma are thought to be the source of the damage of the silicon. The origin of the plasma-induced defect in Si is also considered. According to these results, the electric potential of substrate was controlled in order to avoid collision with positive ions in the plasma. When it was set to zero, plasma-induced peaks did not appear.

The effect of acoustic mode on time-resolved temperature measurements in a Rijke-tube pulse combustor

Radial profiles are reported of average and rms temperature in a propane flame for the first, second, and third acoustic modes at four different axial positions above the burner in a Rijke-tube combustor. Selected plots of the power spectral density (PSD) of temperature fluctuations are also reported. These radial profiles are then compared to similar ones made in the same flame, but in the absence of the acoustic field. Visual observations and photographs of the flame showed a remarkable change in flame height and structure with the onset of acoustic oscillations. This reduction in flame length, caused by the enhanced mixing due to the acoustic velocity fluctuations, gave rise to higher and lower average and rms temperatures near or well above the burner, respectively. In general, the PSD plots had a broad frequency content. The general trend was a decrease in magnitude with an increase in frequency. All cases presented broad-band peaks at around 5 Hz related to the flame flickering phenomenon. Preferred frequencies were observed in the oscillating PSD plots related to the fundamental frequency as well as subharmonics in the tube.

Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating

A novel and simple fiber-optic sensor based on a superstructure fiber Bragg grating (SFBG) for simultaneous strain and temperature measurement is proposed and demonstrated. The transmission spectrum of the sensor possesses several narrow-band loss peaks situated on the slope of a broad-band loss peak. By measuring the transmitted intensity and wavelength at one of the loss peaks, strain and temperature can be determined simultaneously. The accuracy of the sensor in measuring strain and temperature is estimated to be /spl plusmn/20 /spl mu//spl epsiv/ in a range from 0 to 1200 /spl mu//spl epsiv/ and /spl plusmn/1.2/spl deg/C from 20/spl deg/C to 120/spl deg/C, respectively.

Characteristics of the low- and high-frequency ambient sound from breaking waves in the surf zone

An experiment was conducted along the Scripps Institution of Oceanography Pier to study the characteristics of the ambient sound in the surf zone. The role of the single large bubble’s (order of a few centimeters) oscillations versus the collective oscillations of bubble clouds as possible mechanisms of generation of the low-frequency sound, especially the relatively broadband peak that appears around 20 Hz in the sound spectrum, is discussed. The frequency band at which ambient sound from breakers in the surf leaks out is identified. The acoustic attenuation which makes the surf zone extremely quiet above 2 kHz, and a possible method to use ambient sound to infer bubble size distribution in shallow water near the surf zone, are discussed. [Work supported by ONR.]

Characteristics of low-frequency ambient sound in the surf zone

An experiment was conducted along the Scripps Institution of Oceanography Pier to study the characteristics of the ambient sound in the surf zone. The role of the single large bubble oscillations versus the collective oscillations of bubble clouds as possible mechanisms of generation of the low-frequency sound, especially the relatively broadband peak that appears around 20 Hz in the sound spectrum, are discussed. The frequency band at which ambient sound from breakers in the surf leaks out is identified. The acoustic attenuation, which makes the surf zone extremely quiet above 2 kHz, and a possible method to use ambient sound to infer bubble size distribution in shallow water near the surf zone are discussed. [Work supported by ONR.]

Time–frequency distribution of heart rate variability below 0.05 Hz by Wigner–Ville spectral analysis in congestive heart failure patients

The Wigner–Ville spectral analysis was utilized to demonstrate a high-resolution time–frequency distribution of heart rate variability below 0.05 Hz. There are different time–frequency characteristics between the normal subject and the patient with severe congestive heart failure. The former consists of multiple and broad-band spectral peaks, while the latter presents unique spectral peaks. Based on Bayes theory, a classifier for the unique spectral peaks was developed. After the beneficial improvement with low-dose beta-blockers, the unique spectral peaks had disappeared or the time of occurrence was reduced in most patients.

Preparation and characterization of sol-gel derived Er3+: Al2O3-SiO2 planar waveguides

Er 3+ doped aluminosilicate thin films were prepared on silica and silica Si substrates by the sol-gel method. The Er ion concentration was varied from 0.25% to 1%. Fluorescences of Er ions in the visible and infrared regions were characterized. Green and red up-conversion fluorescences centered at 548 and 655 nm for, respectively, the (2H11/2+4S3/2)→4I15/2 and 2F9/2→4I15/2 were observed. A broadband peak was observed at 1531 nm corresponding to the 4I13/2 → 4I15/2 transition, with the full width at half-maximum of 47 nm comparable to those prepared by other methods. The fluorescence lifetime of the 4I13/2 excited state was found to be constant and equal to 3.5 ms until a high Er doping of 0.5 at. %.

CEMS study of Au/Fe multilayers in an annealing process

The magnetic and structural properties of Au/Fe multilayers (MLs) have been investigated in an annealing process by means of conversion electron Mössbauer spectroscopy (CEMS). In the [1.2 nm Au/0.3 nm Fe] and [1.2 nm Au/0.5 nm Fe] MLs, the CEM spectra show sextet magnetic splitting peaks, the broad band peaks and the doublet peaks with rising annealing temperature. These are suggested to be related to the change in the interface structure between Au and Fe layers and the formation of Fe islands having superparamagnetic nature in the annealing process.

Noise-Induced Coherence in Neural Networks.

We investigate numerically the dynamics of large networks of $N$ globally pulse-coupled integrate and fire neurons in a noise-induced synchronized state. The powerspectrum of an individual element within the network is shown to exhibit in the thermodynamic limit ($N\to \infty$) a broadband peak and an additional delta-function peak that is absent from the powerspectrum of an isolated element. The powerspectrum of the mean output signal only exhibits the delta-function peak. These results are explained analytically in an exactly soluble oscillator model with global phase coupling.

The optical characterization of high molecular mass carbonaceous structures produced in premixed laminar flames across the soot threshold limit

The consistence of the dark material in interstellar space is still under discussion. Reasonable candidates are structures containing silicon or carbon. In that light it is interesting to know the physical and chemical properties of carbonaceous structures produced in flames and to compare them with the spectroscopical properties of interstellar dust. Light scattering, fluorescence induced by UV laser and light absorption have been employed to analyse phenomenological aspects of formation; destruction and chemical transformation of carbonaceous structures formed in rich premixed Laminar methane-oxygen and ethylene-oxygen flames at atmospheric pressure with different C/O ratio, at and above the soot threshold limit. Light scattering measurements also show that in non-sooting conditions, i.e. in the non-sooting zone of sooting flames or in flames below the soot threshold limit, high molecular mass structures are formed. Absorption spectra measured in non-sooting conditions show a continuous decay from 200 to 300 Dill, while in fully sooting conditions an additional peak at around 235 nm appears. Further information is obtained by laser-induced fluorescence investigations. Here. a broad-band peak with a maximum at around 320 nm is always measurable. Approaching the soot nucleation zone a further broad-band fluorescence emission in the visible. in addition to that in the UV, is observed. All the experimental results produce evidence that the early formation of high molecular mass carbonaceous molecules containing aromatic functionalities with not more than two rings occurs. Their role as soot precursors seems to be connected to their change into higher polycondensed aromatic structures. Our results give a hint that these soot precursors and the interstellar dust are members of the same family of carbonaceous structures.

World Patent Information : HEКOTOPЫE BOзMOзHOCTИ БAзЫ ДAHHЫЧ “ПATEHTЫ” ПPИ OПPEДEЛEHИИ ПOЛИTИКИ ФИPMЫ Beнчиcлaв Cтeфaнoв Vol. 17(3), pp. 201–204

Na2O–PbO–Al2O3–B2O3 (NPAB) glasses mixed with different concentrations of WO3 (ranging from 0 to 2.5 mol%) are synthesized by conventional melt quenching method. The samples are characterized by X-ray diffraction (XRD), optical absorption, Electron paramagnetic resonance (EPR) and Fourier transform infrared (FT-IR) spectroscopic techniques. Glass formation is confirmed by X-ray diffraction spectra. The optical absorption spectra of these glasses exhibited a predominant broad band peak at about 850–870 nm is identified due to dxy–dx2−y2 transition of W5+ ions. From the optical absorption spectral data, optical band gap (Eopt) and Urbach energy (ΔE) are evaluated. From EPR spectra the strength of the signal is increased and hyperfine splitting is resolved with increasing concentration of WO3 in the glass matrix. The FT-IR spectral studies have pointed out the existence of conventional BO3, BO4, B–O–B, PbO4, WO4 and WO6 structural units of these glasses. Various physical properties and optical basicity are also evaluated with respect to the concentration of WO3 ions.

World Patent Information : КOMMEHTAPИЙ К ПATEHTHOMУ ДEКPETУ 1994 ГOДA (# 13 oт 1994 г.), ДOПOЛHЯЮЩEMУ ИHДИЙCКИЙ ПATEHTHЫЙ зAКOH 1970 г. qP.Гaнгyли Vol. 17(3), pp. 193–194

Na2O–PbO–Al2O3–B2O3 (NPAB) glasses mixed with different concentrations of WO3 (ranging from 0 to 2.5 mol%) are synthesized by conventional melt quenching method. The samples are characterized by X-ray diffraction (XRD), optical absorption, Electron paramagnetic resonance (EPR) and Fourier transform infrared (FT-IR) spectroscopic techniques. Glass formation is confirmed by X-ray diffraction spectra. The optical absorption spectra of these glasses exhibited a predominant broad band peak at about 850–870 nm is identified due to dxy–dx2−y2 transition of W5+ ions. From the optical absorption spectral data, optical band gap (Eopt) and Urbach energy (ΔE) are evaluated. From EPR spectra the strength of the signal is increased and hyperfine splitting is resolved with increasing concentration of WO3 in the glass matrix. The FT-IR spectral studies have pointed out the existence of conventional BO3, BO4, B–O–B, PbO4, WO4 and WO6 structural units of these glasses. Various physical properties and optical basicity are also evaluated with respect to the concentration of WO3 ions.

Fabrication of highly concentrated Er3+ doped aluminosilicate films via sol-gel processing

Optically active highly concentrated Er3+–Al3+ codoped silica thin films were fabricated on fused silica substrates by sol-gel method. The synthesis of the Er3+–Al3+ double alkoxide was intended to prevent the optically harmful Er clustering by the selective coordination of Al3+ around Er3+ ions. The molar Er ion concentration was varied from 3811 to 23719 ppm. Photoluminescence (PL) measurements indicated that Er ions were optically active in all films of different Er concentrations. A broadband peak was observed at 1531 nm with the full width at half-maximum (FWHM) of 42 nm, which corresponds to the 4I13/2→4I15/2 transition. The shape, peak position, and FWHM of the PL signals from the sol-gel derived Er3+ doped silica were quite comparable to those prepared by other methods. The PL peak showed maximum intensity at an Er concentration of 8535 ppm. This implies the concentration quenching at higher Er concentrations.

Quasiparticle dispersion of the t-J and Hubbard models.

The spectral weight ${\rm A({\bf p},\omega)}$ of the two dimensional ${\rm t-J}$ and Hubbard models has been calculated using exact diagonalization and quantum Monte Carlo techniques, at several densities ${\rm 1.0 \leq \langle n \rangle \leq 0.5}$. The photoemission $(\omega < 0)$ region contains two dominant distinct features, namely a low-energy quasiparticle peak with bandwidth of order J, and a broad valence band peak at energies of order t. This behavior $persists$ away from half-filling, as long as the antiferromagnetic (AF) correlations are robust. The results give support to theories of the copper oxide materials based on the behavior of holes in antiferromagnets, and it also provides theoretical guidance for the interpretation of experimental photoemission data for the cuprates.

Bubble production by capillary-gravity waves

In the absence of whitecapping, other physical mechanisms may contribute to the generation of high-frequency ambient noise. It has been suggested [Longuet-Higgins, in NATO Advanced Research Workshop on Sound Generation Mechanisms at the Open Surface (NATO, Geneva, 1987)] that capillary waves, with surface profiles that are peaked downward in the troughs and are relatively flat at the crests, can inject acoustically active bubbles into the ocean, and thus contribute to the ambient noise background. It has been demonstrated in the laboratory that bubble injection can be generated at the trough of capillary-gravity, short-fetched waves by blowing air over water contained in a long, narrow tank. Simultaneous in situ acoustic and high-speed video monitoring of the capillary-gravity waves demonstrate that these waves can produce acoustically active bubbles. The generation of capillary waves depends principally upon the surface tension, which can be changed by adding surface-active agents to the water. The bubble production rate per unit area of these capillary-gravity waves was measured, as well as the dependence of this rate on wind speed, laboratory wind fetch, and surface tension. It was determined that an increase in water salinity and a reduction in surface tension increases the bubble production rate. The spectra of radiated frequencies ranges from 1 kHz to over 100 kHz with a broadband peak located around 4 kHz. The measured spectral densities were weakly related to wind speed. The wind-speed threshold value for bubble production was determined to be approximately 8.6 m/s (14.6 m/s at 10-m level) in fresh water and salt water, which decreased to 8.1 m/s (13.8 m/s at 10-m level) with a surface tension of 40.5 dyn/cm.

A study of broad band photoluminescence from Si 1-xGe x⧸Si superlattices

Using photoluminescence (PL) obtained with ultraviolet (UV) laser excitation, we have examined Si 1- x Ge x ⧸Si long-period superlattices (SLs) grown by molecular beam epitaxy at 500°C and annealed post-growth at 550 to 750°C. Our as-grown samples show broad PL bands, 120 meV below the band gap when the effects of quantum confinement of holes are added in. The broad PL band peak positions shift to higher energies as the anneal temperature increases. This coincides with a reduction in the activation energies of the broad PL bands. The broad bands from both the as-grown and annealed samples shift to lower energies as the sample temperature increases.

SiGe/Si Quantum Wells by MBE : A Photoluminesence Study

AbstractStrained Sil-xGex quantum wells and multi-quantum wells, synthesized by solid source ebeam evaporated MBE on Si(100) substrates have been studied by low temperature photoluminescence (PL) spectroscopy. Phonon resolved transitions originating from excitons bound to shallow impurities in Sil-xGex layers were observed over the temperature range 2K to 100K and used to characterize Sil-xGex/Si heterostructures. Thin Sil-xGex quantum wells exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain, quantum well width and Ge fraction. In single quantum wells confinement shifts up to ∼200 meV were observed (1.2 nm wells with x = 0.38) and NP linewidths down to 1.37 meV were obtained. The confinement shifts were modeled by hole confinement in Sil-xGex wells. An annealing study was performed to investigate the role of Si-Ge interdiffusion on luminescence. Both the geometrical shape and optical emission of the quantum well were found to significantly change through intermixing. In addition to near edge luminescence a broad band of intense luminescence was obtained from several Sil-xGex/Si heterostructures. Some layers exhibited both types of PL spectra. However, the broad PL band (peak energy ∼120meV below the strained bandgap) was predominant when the alloy layer thickness was greater than 2 - 10nm, depending on x, growth temperature, and substrate surface preparation. The strength of the broad PL band was correlated with the areal density of strain perturbations (∼109cm−2 per quantum well corresponding to a spacing of 300nm in the plane of the well; local lattice dilation ∼ 1.5 nm in diameter) observed in plan-view TEM. The first few wells of MQW exhibited only band edge luminescence as was revealed by etching off the upper MQW periods. In addition, post growth anneals at temperatures in the range 700°C to 1100°C were found to enhance band edge luminescence, while the broad luminescence band decayed to zero intensity. Interdiffusion at these these temperatures has been shown to dramatically change the QW shape and consequently interfacial asperities would be expected to disappear, consequently only shallow phonon resolved luminescence is observed in PL after annealing. The influence of PL measurement parameters such as excitation power density and PL sample temperature on the relative strengths of band edge versus broad band luminescence were also consistent with the presence of exciton traps at sites of reduced bandgap.

Spectroscopic analysis of proton-induced fluorescence from yttrium and gadolinium oxysulfide phosphors

The effect of proton-induced damage on the relative scintillation efficiency of various inorganic phosphors including Y/sub 2/O/sub 2/S:Eu, Gd/sub 2/O/sub 2/S:Pr, and Gd/sub 2/O/sub 2/S:Tb. Materials were selected for high efficiency, fast prompt fluorescence response, and minimal delayed fluorescence. Phosphors were exposed to a 3-MeV proton beam at ambient temperatures to control heating damage. Real-time, in-situ measurements of the fluorescence spectra permitted observation of the special characteristics for the deterioration of scintillation yield due to particle-induced damage. The light spectra emitted were nearly identical to those excited by electrons, indicating a common excitation process. Rare earth oxysulfides give intense emissions but the corresponding light intensity drops rapidly with dose. Light intensity decreases with dose, following the Birks and Black model (1951) where damage sites compete for excitons with light-producing sites. Decay of broadband and spectral peak intensity follows similar patterns. The Birks and Black deterioration relation is fitted to each spectral line with empirical decay parameters. >