Time-resolved study of the spectral characteristics of supercontinuum pulses propagating in scattering media

The spectral and temporal characteristics of supercontinuum pulses propagating through monodisperse scattering media consisting of sub-lambda-sized particles have been measured with a broad-bandwidth cross-correlated frequency-resolved optical gating (XFROG) technique. The results show that the ballistic component of the supercontinuum preserves a phase relationship among its spectral components, which acquire a temporal shift in relation to propagation in a non-scattering medium.     

Microstructure fibers as frequency-tunable sources of ultrashort chirped pulses for coherent nonlinear spectroscopy

Microstructure fibers are shown to allow the creation of new tunable sources for femtosecond nonlinear spectroscopy. These fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr:forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp. These pulses from a microstructure fiber were used to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene solution by cross-correlating these pulses with the femtosecond second-harmonic output of the Cr:forsterite laser in boxcars geometry (XFROG CARS). PACS 42.65.Wi; 42.81.Qb     

Extraordinary scattering diagram for nanoparticles near plasmon resonance frequencies

Light scattering by a small spherical particle with low dissipation rate is discussed according to the Mie theory. It is shown that near plasmon (polariton) resonance frequencies one can see non-Rayleigh anomalous light scattering with quite unusual scattering diagrams. PACS 42.25.Fx; 42.65.Es; 46.40.Ff; 78.67.Bf     

Particle on surface: 3D-effects in dry laser cleaning

Previous analysis of dry laser cleaning within the frame of a one-dimensional (1D) model with homogeneous surface heating shows that this model disagrees with experiments by one–two orders of magnitude. The particle on the surface produces an inhomogeneous intensity distribution in its vicinity due to scattering and diffraction. This produces a nonstationary 3D distribution of the temperature and nonstationary 3D thermal deformations of the surface. If one uses the Mie theory for calculation of inhomogeneous intensity then in some region of the parameters, the 3D model predicts results close to the experimental ones. The next step was done when the scattering effects for radiation reflected from the surface was taken into account (so-called “particle on surface” problem). This approach yields results close to the experimental one within the wide range of parameters. PACS 42.25.Fx; 42.25.Hz; 81.65.CfAn erratum to this article can be found at .     

Coherence

The concept of coherence is widely used in different areas of physics like in optics, in quantum optics or in neutron and X-ray scattering, however with subtle differences in meaning for the different communities. In quantum optics it is mainly the source of photons and its characterization in terms of coherence functions which is of concern. In a scattering experiment, on the other hand, the source is supposed to be characterized and it is the internal degrees of freedom of the sample which are studied via their influence on the detected interference pattern. It is one of the purposes of this paper to clarify the different concepts and to show how they are interrelated. The paper is organized as follows. First, we will discuss what interferes in a physical event. This will be treated according to the Feynman formulation of quantum mechanics in terms of probability amplitudes and we will describe nine rules on how to calculate with these amplitudes. Then we will discuss what destroys interference. The main part of the paper treats a number of applications from quantum optics and X-ray and neutron scattering. These include quantum beats, Hanbury Brown and Twiss interferometry, entangled states, Einstein–Podolsky–Rosen paradox and speckle from coherently illuminated samples. PACS 42.25.Hz; 42.25.Kb     

Amplitude and phase measurement of mid-infrared femtosecond pulses by using cross-correlation frequency-resolved optical gating

We describe a cross-correlation-based frequency-resolved optical gating (XFROG) technique for simultaneously measuring the amplitude and phase of two ultrashort pulses that have different wavelengths but are derived from a common mode-locked oscillator. A measurement is presented in which 4.0-mum mid-IR pulses from a synchronously pumped femtosecond optical parametric oscillator (OPO) are characterized by mixing with the 770-nm OPO pump pulses. Details of the pulse-retrieval algorithm are included, together with examples of pulse data retrieved from the experimentally measured XFROG trace.     

Scattering of shaped beam by an arbitrarily oriented spheroid having layers with non-confocal boundaries

An approach to express an incident shaped beam with respect to an arbitrarily oriented spheroidal particle having layers with non-confocal boundaries is presented. To overcome the difficulty of non-confocal boundary conditions connected with different spheroidal coordinate systems, a theoretical procedure is developed to deal with the non-confocal boundary conditions by virtue of a transformation for vector wave functions. The unknown coefficients of scattered and internal electromagnetic fields are determined by solving a system of linear equations derived from the boundary conditions and relations between the spheroidal vector wave functions and spherical ones. Numerical results of the normalized scattering cross section for a two-layered non-confocal prolate spheroid are evaluated. PACS 42.25.Fx; 42.25.Bs     

SASE free electron lasers as short wavelength coherent sources

During the last few years free electron lasers (FELs) based on self-amplified spontaneous emission (SASE) have been demonstrated at wavelengths of 12 m [1], 830 nm [2], 530 nm [3] and 385 nm [3], and around 100 nm [4]. Recently, saturation has been observed in the vacuum ultraviolet (VUV) spectral region between 82 nm and 125 nm at the TESLA Test Facility (TTF) at DESY. The radiation pulses have been characterized with respect to pulse energy, statistical fluctuations, angular divergence and spectral distribution, both in the linear gain and in the saturation regime of the FEL [5-6]. The results are in good agreement with theoretical simulations, providing a solid basis for other projects aiming at still shorter wavelengths down to the 0.1 nm range [7-8].Received: 8 January 2003, Published online: 24 April 2003PACS: 41.60.Cr Free-electron lasers - 42.25.Fx Diffraction and scattering - 42.25.Kb Coherence - 29.17.+w Electrostatic, collective, and linear accelerators - 41.75.Lx Other advanced accelerator concepts - 36.40.Qv Stability and fragmentation of clusters     

Structure of boundary diffraction wave revisited

The physically appealing boundary diffraction wave theory which suggests that diffraction patterns arise due to interference of an undisturbed (geometrical) wave and the boundary diffraction wave generated by edge of the diffracting aperture, simplifies the solution of diffraction problems by reducing the Fresnel–Kirchhoff surface integral into a line integral over the illuminated boundary of the diffracting aperture. The present work reports experimental investigations carried out on the structure of the boundary diffraction wave. It has been shown that the boundary diffraction wave is continuous behind the diffracting aperture and apparently there does not exist any discontinuity at the geometrically light to shadow transition boundary, as is required by the theory. PACS 42.25.-p; 42.25.Fx; 42.25.Hz     

Butterfly inclusions in Van Schrieck masterpieces. Techniques and optical properties

Dutch painter Otto Marseus Van Schrieck (1619–1678) is famous for his invention of “sottobosco”. These specific still-life paintings are characterized by the presence of various living organisms (mainly insects and plants) directly on the canvas. We will focus our attention on the painting kept in the museum of Grenoble, France, where a real butterfly is pasted on the canvas. The actual butterfly is a common Nymphalidae, Inachis io, presented in a static position on the dorsal side, without any perspective, compared to the neighboring butterflies. The colors of this butterfly are mainly due to pigments, melanin (black to brown) and ommochromes (yellow, orange, red) often in granules configuration that introduce scattering of light superimposed to the classical selective absorption, except in the ocelli of the hind wings where the blue coloration is due to interferential effects. The nearly perfect refraction index equality between the varnish and the chitin, the main constituent of the butterfly wings, deeply affects its colors. This leads the artist to a final intervention in some parts of the wings, revealed by microscope observation. PACS  42.25.Fx; 42.25.Hz; 78.40.Me; 87.64.Cc; 87.80.Dj     

Separation of changes in light scattering and chromophore concentrations during cortical spreading depression in rats

We used near-infrared spectroscopy to separate tissue scattering changes from changes in cerebral oxyhemoglobin and deoxyhemoglobin and the redox state of cytochrome- c -oxidase. A separate term of the transport scattering coefficient (micro(s)(?)) was included in a modified Lambert-Beer equation. It is shown by diffusion equation analysis that there is a simple relationship between the differential path-length factor (D(a)) and its scattering equivalent (D(s)) . The method was applied to cortical spreading depression (CSD) data recorded through the skulls of rats. Biphasic changes in micro(s)(?)of +/-0.1mm(-1)were observed during CSD's that spread with a velocity of ~5mm/min . The method proposed has the promise to permit monitoring of scattering changes noninvasively in humans during cortical activation or pathophysiological conditions.     

Second-harmonic generation in strongly scattering porous gallium phosphide

Second-harmonic generation in a strongly scattering macroporous GaP layer with typical sizes of GaP nanocrystals ranging from 200 to 500 nm is studied using nanosecond pulses of an optical parametric oscillator. As the pump wavelength decreases from 1.6 to 1.0 m the second-harmonic yield from porous GaP increases more than one order of magnitude relative to the second-harmonic yield from crystalline GaP. This enhancement of the second-harmonic generation for shorter wavelengths correlates with the increase in the efficiency of light scattering, indicating the importance of scattering effects in nonlinear-optical processes in strongly scattering random media . PACS 42.25.Fx; 42.65.Ky     

Frequency response of second-order extremely asymmetrical scattering in wide uniform holographic gratings

Second-order extremely asymmetrical scattering (EAS) and grazing-angle scattering (GAS) are types of Bragg scattering in slanted wide periodic gratings. They occur when the second diffracted order satisfies the Bragg condition and has a wavevector parallel (for EAS) and almost parallel (for GAS) to the grating boundaries. In this paper, for the first time, a rigorous numerical study of the frequency responses of second-order EAS and GAS is presented for bulk TE electromagnetic waves in planar holographic gratings. A highly unusual pattern of strong optical resonances in the side-lobe structure of these frequency responses is predicted. A relationship between these resonances and the previously predicted GAS resonances (at zero detunings of the Bragg condition) is established and analysed. A special new type of eigenmodes in slanted wide periodic gratings with strong frequency detunings are predicted in the case of second-order EAS and GAS. The eigenmodes are shown to be guided by the grating alone without any conventional guiding effect in the structure. The typical field structure in such eigenmodes is investigated and discussed. PACS 42.25Fx; 42.79Dj; 42.40Eq     

Modeling of regular gold nanostructures arrays for SERS applications using a 3D FDTD method

We study the localized surface plasmon resonance (LSPR) and the surface-enhanced Raman scattering (SERS) of arrays of gold cylindrical and ellipsoidal nanoparticles with different diameters or major axes. The LSPR and SERS gains are calculated with the three dimensional Finite-Difference Time-Domain method using the Drude–Lorentz dispersion model. We find that the maximum of the extinction spectrum and the average SERS gain of each investigated nanostructures are shifted whatever their size and their shape. PACS 42.25.Fx; 71.45.Gm; 78.30.-j     

Coupling mode-based nanophotonic circuit device

Coupling mode-based nanophotonic U-shaped circuit devices is proposed. Furthermore, the coupling of TM mode between adjacent air channels on metal and propagation of surface plasmon polariton (SPP) waves was demonstrated. Different output intensities were derived by modifying the U-shaped air channel width, and the binary logical value “1” or “0” is obtained after definition of threshold. A spacial coupling mode theory is used to explain the computational simulation results. The theory and the numerical results are well fitted to each other. The structure can be used to obtain all four binary arrays applying to the highly integrated optical circuit. PACS 02.60.Cb; 42.25.Fx; 42.25.Bs     

Use of diffuse reflections in tunable diode laser absorption spectroscopy: implications of laser speckle for gas absorption measurements

The use of diffusely scattering materials as a means of eliminating interference fringes has been investigated. Their use introduces laser speckle that can contribute a random, rather than periodic, uncertainty to gas measurements. We have established a method for quantifying the uncertainty due to speckle and investigated ways of reducing it. We characterised the speckle at 823 nm allowing the use of low-cost CCD cameras. We have tested the principle of the model by making gas absorption measurements on the 1650-nm methane absorption line using wavelength-modulation spectroscopy, for which preliminary results are presented. PACS 42.62.Fi; 42.25.Hz; 42.30.Ms     

Quantitative evaluation of pigment particles in organic layers by fibre-optics diffuse-reflectance spectroscopy

Fibre-optics diffuse-reflectance spectroscopy was used to evaluate pigment quantity in art glazes. Measurements are non-invasive, without any contact, and can be performed in situ, without moving the work of art under investigation from its conservation place. Reflectance spectra are processed using Kubelka–Munk theory in order to obtain scattering and absorption parameters of the samples. Assuming a linear dependance of these optical properties with the pigment volume concentration of the paint layers, we were able to evaluate pigment quantities in the samples from reflectance measurements. Results are in excellent agreement with those obtained by X-ray diffraction and micro-X-ray fluorescence. PACS 42.25.Dd; 78.40.Ha; 78.70.Ck; 78.70.En; 81.70.Fy     

Approach to optical interference fringes reduction in diode laser absorption spectroscopy

The advantage of a new scheme for balanced detection has been investigated to reduce the influence of optical interference fringes when performing diode laser gas absorption spectroscopy employing lock-in amplifiers and pigtailed lasers. The influence of the fringes has been reduced by comparing the lock-in 2 f signal due to the gas sample with that of a reference beam. The frequency regions outside the absorption feature have been used to obtain information on the interference fringe impact on the signal of interest. We have demonstrated an efficient way to reduce the influence of such fringes by employing this technique combined with non-linear signal processing methods. The different steps of the algorithm are presented. In the experimental arrangement presented, a reduction of the optical interference fringes by about 10 times is achieved, as demonstrated in measurements on molecular oxygen around 761 nm. The new technique is compared with an analog technique for balanced detection and certain advantages of the computer algorithm are pointed out. In particular, the emerging field of gas spectroscopy in scattering solid media strongly benefits from the technique presented. PACS 42.55.Px; 39.30.+w; 42.25.Hz     

Quantitative characterisation of pigment mixtures used in art by fibre-optics diffuse-reflectance spectroscopy

Fibre-optics diffuse reflectance spectroscopy (FORS) was used to characterise pigment mixtures in paints used in art. Measurements are non invasive, without any contact with the sample. The experimental device is portable, therefore measurements can be performed in situ, without moving the work of art under investigation from its conservation place. The protocol was validated thanks to modern gouache paints: 10 pure gouaches were used as references and 27 binary mixtures of these pure gouaches were studied. Reflectance spectra are processed using the Kubelka–Munk theory in order to get scattering and absorption coefficients of the references. Assuming a linear dependance of these optical properties with the pigment volume concentration (PVC) of the components of paint layers, the protocol enables qualitative as well as quantitative interpretation of the reflectance spectra measured on binary mixtures of references. Indeed, for most cases, numerical processing of FORS-measurements performed on a mixture leads to the identification of its components. Besides, once the components are identified, it is possible to compute their respective proportions with an accuracy of 5%. PACS 42.25.Dd; 78.40.Ha; 78.20.Bh; 78.20.Ci; 81.70.Fy     

A Monte Carlo calculation of the voltage-current curves for GaxIn1−xSb solid solutions with allowance for scattering in the alloy

Monte Carlo calculations have been performed for the electron drift velocity in relation to electric field strength for certain Ga_xIn_1–xSb compositions with allowance for scattering in the alloy. A three-valley model for the band structure is used. A study has been made of the effects of scattering in the alloy and at ionized impurities on the V_dr(E) dependence. It is shown that scattering in the alloy in some cases can increase the ratio of the threshold and saturation drift velocities. An ionized impurity increases the saturation velocity. The reasons for these effects are discussed. The optimum composition from the viewpoint of using the Gunn effect is Ga_0.6In_0.4Sb, and the characteristics of these are substantially better than that of GaAs.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 10–15, September 1981.