Effect of coupling between linear absorption and nonlinear absorption on Z-scan measurements

The contribution of multiphoton absorption to open-aperture Z-scan measurements is analysed by coordinate transformation. The approximate closed-form solution is obtained, which is suitable for application in higher-optical power regime. As an example, the coupling between linear absorption and two-photon absorption is discussed in detail.     

Thermodynamic study of advanced absorption heat transformers—II. Double absorption configurations

A thermodynamic analysis has been carried out to study the performance of double absorption heat transformers (DAHT) assuming water/lithium bromide as the working fluid. The performance of single (SSHT) and two stage heat transformers (TSHT) analyzed in Part I, was compared with the performance of double absorption heat transformers (DAHT) under the same operating conditions. The results showed that single stage heat transformers (SSHT) were the simplest and most efficient. Greater absorber temperatures were reached with two stage heat transformers (TSHT). However, these systems were in general less efficient than the others and technically the most complex. Double absorption heat transformers (DAHT) were technically simpler than two stage heat transformers (TSHT) and may reach absorber temperature as high as these systems.     

On the performance of advanced absorption heat transformers—II. The double absorption configuration

An innovative heat transformer can be derived from the TSHT discussed in Part I. Its configuration is essentially based on a serial arrangement of the two absorbers (“double absorption heat transformer”, DAHT) and allows a drastic reduction of the number of items necessary for the TSHT.The performance of a DAHT operated with the water-sulphuric acid system was computed by means of a lumped-parameter mathematical model and expressed in terms of the four indexes of performance previously used for the TSHT examined in Part I. The results show that the DAHT reaches its optimum performance at values of gross temperature lift intermediate between those optimal for the SSHT and the TSHT, respectively.     

Optical absorption properties of Ge-Sb-Te films

The optical properties of monolayer Ge2Sb2Te5 thin films with three different thicknesses prepared by dc magnetron sputtering method at the range of 400-800 nm were studied. The optical absorption coefficients and the optical energy gap (Eg) were calculated, The results gave values for the absorption coefficients in the range of (1.3-7.5)×105 cm-1 which were in the high absorption wavelength region of     

Wavelength-dependent nonlinear absorption of gold nanocages

The dipole resonances of gold nanocages were investigated theoretically using finite difference time domain method.The results show that field enhancement is obtained at the walls of the gold nanocages.It is believed that the effect can cause a strong optical nonlinear property.To test the hypothesis,nonlinear absorption was investigated using a broadband 5 ns Z scan.It was found that at low intensities the sample shows saturable absorption(SA),while at higher intensities a switch from SA to reverse SA occurs.Moreover,the nonlinear absorption of the sample is sensitively wavelength-dependent,and,in the resonant region,saturation intensity is the largest.     

Statistical absorption coefficient of microperforated absorbers

@Christian Nocke     

Practical absorption limits of MPP absorber

The construction and properties of microperforated panel (MPP) absorber are discussed. The absorption limit of the absorber had been shown that low values of the perforate constant k = d(f/10)1/2 and the orifice diameter d (in mm) are essential for MPP to have high absorption in wide frequency band. To find the exact limits, take 1 for k as a start, because both specific resistance and high absorption require k around one. And the orifice diameter d is chosen as 0.1 mm, so that the peak absorption coefficient (resonance absorption) is at 1000 Hz, and high sound frequency may be in the absorption region. Is it possible for a single layer of such an MPP to cover the whole absorption region required in practice? The half-absorption limit is not a good criterion, because low absorption comes in also in some cases. The 0.5 absorption coefficient limit is suggested for practical region, as a standard for comparison. Absorption curves were drawn for different load resistances, of absorption coefficients versus frequency. Ordinary MPP absorber absorbs in slightly over two octaves, and the new absorber with r = 1 (specific resistance equal to the characteristic impedance in air)is slightly better than these, 2.5 octaves. The new absorbers with r > 1, are much better than these, and some satisfies high absorption in broad frequency range. Realization of these will mean great progress of MPP absorbers.     

π-Meson absorption by three-particle nuclei

The reactions ⁺ +³He 3p and ^– +³H 3n are considered with both two- and three-nucleon pion-absorption taken into account. The pion-absorption with all nucleons of the nucleus involved and the interference of the two absorption mechanisms are shown to be of the same importance as a pure two-particle absorption in nuclei. The latter cannot be considered dominating as it used to be.     

Models of optical absorption in amorphous semiconductors at the absorption edge — A review and re-evaluation

Davis-Mott and Tauc models of optical absorption at the absorption edge in the high absorption coefficient region (10⁴cm^–1) are carefully reviewed with regard to their theoretical foundations, assumptions, mathematical derivations, and results. The full implications of these models are exploited, and it is found that the Davis-Mott model for negligible matrix elements between localised states could account for the cubic power law behaviour of with photon energy of some amorphous semiconductors such as a-Si. A fractional power law to find the optical band gapE _opt, of the form [ (–E _opt)^r; 2r3] based on Davis-Mott model is proposed in which the indexr can be a function of disorder. The Tauc model has further been extended to the case of negligible matrix elements between localised states, in which the same square power law for vs. with the same meaning of the optical gap as in the original Tauc model has resulted. A consideration of the case of unequal matrix elements for those transitions between localised states and those between extended states is also included. The meaning ofE _opt has been re-assessed and it is emphasized that it is an extrapolation of delocalised states to the zero of the density of states rather than a threshold energy for the onset of some kind of optical transitions.This paper is cordially dedicated to Professor C. A. Hogarth who taught us the Physics of amorphous materials.     

Partial absorption and “virtual” traps

The spatial probability distribution associated with diffusion and attenuation in partially absorbing media is studied. An equivalence is established between a system with free diffusion for x>0 and partial absorption for x<0, and a semi-infinite system (x>0) with a radiation boundary condition at x=0. By exploiting this equivalence, it is shown that the effect of a partially absorbing medium in the long-time limit is equivalent to that of a perfect, virtual trap whose size is smaller than the original absorbing medium. For short times, however, there is substantial penetration of diffusing particles into the absorber. The virtual trap approach is readily generalized to higher dimensions. This allows one to obtain the density profile of diffusing particles around a partially absorbing spherical trap. An unusual crossover between short-time penetration and long-time trapping occurs in two dimensions; the size of the virtual trap is exponentially small in the case of weak absorption, corresponding to an absorption time which is exponentially large.     

M-sequence three-point method to measure absorption coeffcient

1IntroductionTheconventionalmethodtomeasureabsorptioncoefficientobtainsresultsfromstanding-wave-ratioinanimpedancetube.Thismethodperformspreciselybutratherlaboriously.Ex-citedbybandrandomsignal,twomicrophonetransferfunctionmethodproposedbySybertandRossobtainsabsorptioncoefficientbythetransferfunctionbetweentheacousticpressureattwolocationinthet.b.[1].ThemethodisratherfasterthantheconventionaI.ta.ding-wavejratiomethod.However,thetransferfunctionmethodhastwofatalshortcomings-phasedismatchingan…     

Theoretical analysis on sound absorption by pseudostochastic diffusors

I.IntroductionIn1979,M.R.S.h.o.der[1]pro-posedanewdesignofsounddiffusorwhoseperiodcomprisesNelements(slotsorwells,referedtoaschannelsinfol1owing)ofequalwidths.ThedepthsofchanneIsvaryaccordingtoapseu-dostochasticsequencewithinonepe-riod.Atypicalstructureofq1ladraticresiduediffusor(N=11)isi1IustratedinFig.1.BychoosingthedePtl1ofchannels,thescatteringcharacteristicsofthestructurecanbeoptirnizeds11chastodistributethescattereden(}rgyequalIyoveralloweddirections.In1992,K.Fuiwaraandothers['repo…     

Pion absorption by16O using realistic interactions—II: Free pion absorption followed by emission of proton

Matrix element of the Gallilean invariant nonrelativistic reduction of the pseudoscalar-pseudovector interaction has been calculated for free pion absorption by a single nucleon inside the nucleus of¹⁶O. The Hartree-Fock wavefunctions obtained with the unitary-model-operator approach starting with the hard-core nucleon-nucleon interaction have been used for the π-capturing nucleon in the initial state. The initial pion distortion in the presence of nuclear field of the absorbing nucleus prior to its absorption together with the Coulomb interaction with the finite nuclear size has been taken into account. The distortion of the emitted proton in the field of the residual nucleus has also been considered. The differential cross-sections have been obtained and calculated results are compared with the previous experimental and theoretical work.     

K− absorption reactions from rest in deuterium

Quantitative agreement with the data on K⁻ d → NΛπ and NΣπ relative rates and spectra is found by solving a system of twelve Faddeev equations. The data imply that there is no stable bound state (second sheet pole above ΔN threshold) in the (ΣN, ΛN) system but do require a virtual state (fourth sheet pole) near the ΣN threshold.     

Impurity absorption in indium-doped Pb1−xSnxTe

Optical absorption spectra of indium doped Pb_1−xSn_xTe (x = 0–0.22) have been measured at 90 and 300 K. It was found that doping gives rise to a broad absorption band with its maximum at photon energies much greater than the thermal ionization energy of the indium impurity. The energetic position and the shape of impurity absorption band change little with alloy composition, but the band broadens substantially with temperature. The results are explained by a strong lattice relaxation around the indium impurity.     

X-ray absorption spectroscopy in biology – present and future

From May 10 to 12, the GISAXS Workshop 2005 was hosted at HASYLAB/DESY in Hamburg, Germany. GISAXS stands for Grazing Incidence Small Angle X-ray Scattering, a surface-sensitive technique to observe structures on large length scales up to several hundred nanometers. Organized by R. Gehrke and S. V. Roth from HASYLAB and P. Müller-Buschbaum from University of Technology Munich, the workshop attracted more than 50 participants from all over the world, originating a very stimulating atmosphere with invited talks, a contributed poster session and practical training, including real data acquisition at the beamline BW4.     

Recent results on K ? multinucleon absorption by FINUDA

K₂Fe₃(OH)₂(SO₄)₃(H₂O)₂ was prepared by hydrothermal synthesis. The crystal structure is the isomorphous phase of K₂Co₃(OH)₂(SO₄)₃(H₂O)₂. M?ssbauer spectra of K₂Fe₃(OH)₂(SO₄)₃(H₂O)₂ were measured at low temperatures between room temperature and 14?K, and the hyperfine interactions were analyzed. The Neel temperature is 39?K. Two paramagnetic Fe^2?+? species were observed in the M?ssbauer spectrum at room temperature, and M?ssbauer spectra measured below 38?K were decomposed into four magnetic subspectra. From the temperature dependence, it is found that the local electron density at each site is largely deviating at low temperatures, which may induce the giant coercivity.     

Ultrasonic absorption with infrared divergence in cluster spin glass

The infrared divergence response theory is applied toreorientational relaxation process of magnetic clusters in cluster spinglass(CoF_2)_(0.5)(BaF_2)_/(0.2)(NaPO_3)_(0.3)to study the behaviour of low temper-ature ultrasonic absorption in this system.Our calculation results showedthat the reorientational relaxation process makes the main contributions tothe magnetic part of the ultrasonic absorption for T<4.0K;as forT>4.0K,the contributions of the internal process inside the clustersmust be considered.     

Bragg suppression for optical absorption in multi-quantum well structures

Direct calculations of absorption spectra for multi-quantum well structures by extracting field distributions at well positions are performed. Results demonstrate the previously reported Bragg suppression, and agree exactly with the indirect calculation by linear dispersion theory. This reveals that Bragg suppression effect in fact originates from the remarkably decreased intensities at well positions by Bragg interference condition, rather than from the formation of supperradiant modes.     

Two- and three-photon absorption in a novel fluorene-based compound

A novel symmetrical charge transfer fluorene-based compound 2,7-bis (4-methoxystyryl)-9, 9-bis (2-ethylhexyl)-9H-fluorene (abbreviated as BMOSF) was synthesized and its nonlinear absorption was investigated using two different laser systems: a 140-fs, 800-nm Ti:sapphire laser operating at 1-kHz repetition rate and a 38-ps, 1064-nm Nd:YAG pulsed laser operating at 10-Hz repetition rate, respectively. Unique nonlinear absorption properties in this new compound were observed that rise from multiphoton absorption. The nonlinear absorption coefficients were measured to be 6.02×10-3 cm/GW (due to two-photon absorption, exciting wavelength is 800 nm) and 3.6×10-20 cm3/W2 (due to three-photon absorption, exciting wavelength is 1064 nm). This new compound possesses strong fluorescence induced by two-photon absorption and obvious three-photon absorption optical limiting effects.