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Spontaneous Raman spectroscopy is a powerful characterization tool for graphene research. Its extension to the coherent regime, despite the large nonlinear third-order susceptibility of graphene, has so far proven challenging. Due to its gapless nature, several interfering electronic and phononic transitions concur to generate its optical response, preventing to retrieve spectral profiles analogous to those of spontaneous Raman. Here we report stimulated Raman spectroscopy of the G-phonon in single and multi-layer graphene, through coherent anti-Stokes Raman Scattering. The nonlinear signal is dominated by a vibrationally non-resonant background, obscuring the Raman lineshape. We demonstrate that the vibrationally resonant coherent anti-Stokes Raman Scattering peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the vibrationally non-resonant background. We model the spectra, taking into account the electronically resonant nature of both. We show how coherent anti-Stokes Raman Scattering can be used for graphene imaging with vibrational sensitivity., Coherent anti-Stokes Raman Scattering (CARS) accesses the vibrational properties of a material via nonlinear four-wave mixing (FWM); CARS in graphene has not been observed to date despite its high nonlinear third-order susceptibility. Here, the authors devised a FWM scheme to perform stimulated Raman spectroscopy in single and multi-layer graphene through CARS.

Self-action nonlinearity is a key aspect -- either as a foundational element or a detrimental factor -- of several optical spectroscopies and photonic devices. Supercontinuum generation, wavelength converters and chirped pulse amplification are just a few examples. The recent advent of Free Electron Lasers (FEL) fostered building on nonlinearity to propose new concepts and extend optical wavelengths paradigms for extreme ultraviolet (EUV) and X-ray regimes. No evidence for intrapulse dynamics, however, has been reported at such short wavelengths, where the light-matter interactions are ruled by the sharp absorption edges of core-electrons. Here, we provide experimental evidence for self-phase modulation of femtosecond FEL pulses, which we exploit for fine self-driven spectral tunability by interaction with sub-micrometric foils of selected monoatomic materials. Moving the pulse wavelength across the absorption edge, the spectral profile changes from a non-linear spectral blue-shift to a red-shifted broadening. These findings are rationalized accounting for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold, respectively., 20 pages, 6 figures

Excited state vibrations are crucial for determining the photophysical and photochemical properties of molecular compounds. Stimulated Raman scattering can coherently stimulate and probe molecular vibrations with optical pulses, but it is generally restricted to ground state properties. Working under resonance conditions enables cross-section enhancement and selective excitation to a targeted electronic level but is hampered by an increased signal complexity due to the presence of overlapping spectral contributions. Here, we show how detailed information about ground and excited state vibrations can be disentangled by exploiting the relative time delay between Raman and probe pulses to control the excited state population, combined with a diagrammatic formalism to dissect the pathways concurring with the signal generation. The proposed method is then exploited to elucidate the vibrational properties of the ground and excited electronic states in the paradigmatic case of cresyl violet. We anticipate that the presented approach holds the potential for selective mapping of the reaction coordinates pertaining to transient electronic stages implied in photoactive compounds.

Electronic properties and lattice vibrations are supposed to be strongly correlated in metal-halide perovskites, due to the "soft" fluctuating nature of their crystal lattice. Thus, unveiling electron-phonon coupling dynamics upon ultra-fast photoexcitation is necessary for understanding the optoelectronic behaviour of the semiconductor. Here, we use impulsive vibrational spectroscopy to reveal ground and excited state vibrational modes of methylammonium lead-bromide perovskite. We observe a coherent phonon at 106 cm-1 (13 meV), pertaining to the inorganic octahedral, which is peculiar of the electronic excited state and generated via displacive excitation mechanism. This indicates the formation of a new geometry, reached after a quarter of the phonon period T/4=80 fs, and fully equilibrated within the phonon lifetime of about 1 ps. Our observations unambiguously prove that this mode drives the crystalline distortion occurring upon carrier generation, implying the presence of polaronic effects., 8 pages, 2 figures

The "Frenkel line" (FL), the thermodynamic locus where the time for a particle to move by its size equals the shortest transverse oscillation period, has been proposed as a boundary between recently discovered liquid-like and gas-like regions in supercritical fluids. We report a simulation study of isothermal supercritical neon in a range of densities intersecting the FL. Specifically, structural properties and single-particle and collective dynamics are scrutinized to unveil the onset of any anomalous behavior at the FL. We find that (i) the pair distribution function smoothly evolves across the FL displaying medium-range order, (ii) low-frequency transverse excitations are observed below the "Frenkel frequency", and (iii) the high-frequency shear modulus does not vanish even for low-density fluids, indicating that positive sound dispersion characterizing the liquid-like region of the supercritical state is unrelated to transverse dynamics. These facts critically undermine the definition of the FL and its significance for any relevant partition of the supercritical phase.

In impulsive stimulated Raman scattering, vibrational oscillations, coherently stimulated by a femtosecond Raman pulse, are monitored in real time and read out as intensity modulations in the transmission of a temporally delayed probe pulse. Critically, in order to retrieve broadband Raman spectra, a fine sampling of the time delays between the Raman and probe pulses is required, making conventional ISRS ineffective for probing irreversible phenomena and/or weak scatterers typically demanding long acquisition times, with signal-to-noise ratios that crucially depend on the pulse fluences and overlap stabilities. To overcome such limitations, here we introduce the chirped-based impulsive stimulated raman scattering (CISRS) technique. Specifically, we show how introducing a chirp in the probe pulse can be exploited for recording the Raman information without the need to scan over the Raman-probe pulse delay. We then experimentally demonstrate with a few examples how to use the introduced scheme to measure Raman spectra.

Author(s): Batignani, G; Pontecorvo, E; Bossini, D; Ferrante, C; Fumero, G; Cerullo, G; Mukamel, S; Scopigno, T | Abstract: Illuminating a magnetic material with femtosecond laser pulses induces complex ultrafast dynamical processes. The resulting optically detectable response usually contains contributions from both the optical properties and the magnetic degrees of freedom. Disentangling all the different components concurring to the generation of the total signal is a major challenge of contemporary experimental solid-state physics. Here, this problem is tackled, addressing the purely optical, nonmagnetic artifacts on the time resolved two-magnon stimulated Raman spectrum of an antiferromagnet, rationalizing the recent observation on the exchange energy modification upon photo-excitation. It is demonstrated how the genuine dynamics of the magnetic eigenmode can be disentangled from the nonlinear optical effects, generated by cross phase modulation, on the femtosecond timescale. The introduced approach can be extended for the investigation of l100nfs dynamic processes by means of coherent Ramannscattering.

Femtosecond stimulated Raman scattering is a time-resolved vibrational spectroscopic technique able to access sub-picosecond dynamics and providing accurate structural information. Thanks to an appropriate combination of three laser pulses, triggering vibrational coherences delayed with respect to the photoinduced event of interest, it is capable of uncompromised temporal precision (down to 50 fs) and spectral resolution (a few wavenumbers), better than spontaneous Raman. Reaching such extreme time scales requires significant temporal overlap of pulses, which gives rise to undesired nonlinear artifacts, often hampering an immediate interpretation of the Raman spectra. Building on a perturbative expansion of the density matrix, we identify the origin of such artifacts in cross phase modulation effects and show how they can be theoretically evaluated and factored out from the signals. We experimentally benchmark the theoretical predictions in a nonreactive model system, namely cyclohexane.

In a previous letter and other papers, we have interpreted the supercritical region of fluids as being parted into a gas-like and a liquid-like regimes. The boundary between these two distinct regions of the P-T phase diagram of substances is represented by the Widom line, extending the subcritical liquid-gas 1st order transition line above the critical point. Out authors have been commenting on our interpretation, claiming that the two regions are parted by the so called Frenkel line, which does not reconnect to the subcritical liquid-gas phase boundary. In this work, we show on a solid theoretical basis that the Frenkel line is incorrect and that indeed the Widom line is the relevant boundary between to two supercritical regimes.

© the Owner Societies 2015. We introduce a general theoretical description of non resonant impulsive femtosecond stimulated Raman spectroscopy in a multimode harmonic model. In this technique an ultrashort actinic pulse creates coherences of low frequency modes and is followed by a paired narrowband Raman pulse and a broadband probe pulse. Using closed-time-path-loop (CTPL) diagrams, the response on both the red and the blue sides of the broadband pulse with respect to the narrowband Raman pulse is calculated, the process couples high and low frequency modes, which share the same ground state. The transmitted intensity oscillates between the red and the blue side, while the total number of photons is conserved. The total energy of the probe signal is periodically modulated in time by the coherence created in the low frequency modes.

We introduce a novel configuration for stimulated Raman scattering (SRS) microscopy, called In-line Balanced Detection (IBD), which employs a birefringent plate to generate a time-delayed polarization-multiplexed collinear replica of the probe, acting as a reference. Probe and reference cross the sample at the same position, thus maintaining their balance during image acquisition. IBD can be implemented in any conventional SRS setup, by adding a few simple elements, bringing its sensitivity close to the shot-noise limit even with a noisy laser. We tested IBD with a fiber-format laser system and observed signal-to-noise ratio improvement by up to 30 dB.

Summary form only given. Electron-phonon scattering and anharmonicity are the dominant mechanisms, that enable to describe the equilibrium phonon properties in graphene and Raman scattering is the main tool for their characterization. In the first tens fs after the photoexcitation, an out of equilibrium distribution of (hot) electron is induced with respect to the (cold) phonon bath. Within a few picoseconds, the fast electron-electron and electron-phonon non radiative recombination channels determine the equilibrium between the electronic distribution and the lattice. Therefore, on the laboratory timescale, continuous wave laser sources, commonly used for high resolution spontaneous Raman scattering, examine already equilibrated carrier-phonon distributions. A way to impulsively localize energy into graphene's electronic subsystem is provided by sub picosecond photoexcitation. While the behaviour of hot charge carriers to such ultrafast perturbation has been thoroughly elucidated unraveling the nature of optical phonons under a strongly out of equilibrium regime is a challenge. We perform spontaneous Raman measurements in graphene by using a 3-ps laser excitation, which is revealed to be a good agreement between impulsive stimulation and the necessity of spectral resolution. Furthermore, we show how the Raman response of graphene can be detected in presence of an electronic subsystem temperature largely exceeding that of the phonon bath. We find a peculiar behaviour of the period and lifetime of both G and 2D phonons as function of the carriers' temperature in the range 1700-3100 K, which is strongly suggestive of a smearing out of the Dirac cones. We rationalize such behaviour by accordingly revisiting the traditional theoretical modeling of the electron-phonon coupling in this highly excited transient scenario, which is critical in the emerging field of graphene-based nanophotonic and optoelectronic devices operating at THz rates.

We demonstrate a wavelength-tunable graphene-synchronized all-fiber laser with Yb and Er cavities spanning 1040–1080 and 1535–1560 nm, corresponding to ~2750–3200 cm−1 frequency detuning. We apply the laser to coherent anti-Stokes Raman spectroscopy.

We perform a back-to-back comparison between two nonlinear vibrational imaging techniques: stimulated Raman scattering (SRS) and balanced detection Raman-induced Kerr effect (BD-RIKE). Using a compact fiber-based laser system for generation of pump and Stokes signals, we image polymer beads as well as human hepatocytes under the same experimental conditions. We show that BD-RIKE, despite the slightly lower signal levels, consistently offers an improved signal-to-noise ratio with respect to SRS, resulting in significantly higher image quality. Importantly, we observe that such quality is not affected by the static birefringence of the sample, which makes BD-RIKE a robust and attractive alternative to SRS. We also highlight a unique advantage of the technique, which is its capability to easily access both the real and imaginary parts of the nonlinear susceptibility, thus allowing for vibrational phase imaging. The phase information can be readily obtained from BD-RIKE with minimal experimental effort and provides an additional chemical selectivity channel for coherent Raman microscopy. Copyright © 2014 John Wiley & Sons, Ltd.

International audience; Determining the initial pathway for ultrafast energy redistribution within biomolecules is a challenge, and haem proteins, for which energy can be deposited locally in the haem moiety using short light pulses, are suitable model systems to address this issue. However, data acquired using existing experimental techniques that fail to combine sufficient structural sensitivity with adequate time resolution have resulted in alternative hypotheses concerning the interplay between energy flow among highly excited vibrational levels and potential concomitant electronic processes. By developing a femtosecond-stimulated Raman set-up, endowed with the necessary tunability to take advantage of different resonance conditions, here we visualize the temporal evolution of energy redistribution over different vibrational modes in myoglobin. We establish that the vibrational energy initially stored in the highly excited Franck–Condon manifold is transferred with different timescales into low- and high-frequency modes, prior to slow dissipation through the protein. These findings demonstrate that a newly proposed mechanism involving the population dynamics of specific vibrational modes settles the controversy on the existence of transient electronic intermediates.

Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.

Spontaneous Raman spectroscopy is a formidable tool to probe molecular vibrations. Under electronic resonance conditions, the cross section can be selectively enhanced enabling structural sensitivity to specific chromophores and reaction centers. The addition of an ultrashort, broadband femtosecond pulse to the excitation field allows for coherent stimulation of diverse molecular vibrations. Within such a scheme, vibrational spectra are engraved onto a highly directional field and can be heterodyne detected overwhelming fluorescence and other incoherent signals. At variance with spontaneous resonance Raman, however, interpreting the spectral information is not straightforward, due to the manifold of field interactions concurring to the third order nonlinear response. Taking as an example vibrational spectra of heme proteins excited in the Soret band, we introduce a general approach to extract the stimulated Raman excitation profiles from complex spectral lineshapes. Specifically, by a quantum treatment of the matter through density matrix description of the third order nonlinear polarization, we identify the contributions which generate the Raman bands, by taking into account for the cross section of each process.

The pressure dependence of the glass-transition temperature, Tg(P), of the ionic glass-former 2Ca(NO3)2 x 3KNO3, CKN, has been obtained by molecular dynamics (MD) simulations The liquid-glass difference of thermal expansivity, deltaalpha, heat capacity, deltaCp, and isothermal compressibility, deltak, have been calculated as a function of pressure. It has been found that the Ehrenfest relation dTg/dP = TVdeltaalpha/deltaCp predicts the pressure dependence of Tg, but the other Ehrenfest relation, dTg/dP = deltakappa/deltaalpha, does not. Consequently, the Prigogine-Defay ratio, pi = deltaCpdeltakappa/TVdeltaalpha2, is n pi approximately 1.2 at low pressures, but increases 1 order of magnitude at high pressures. The pressure dependence of the Prigogine-Defay ratio is interpreted in light of recent explanations for the finding pi > 1.

Acoustics dynamics of the molecular glass-former indomethacin (IMC) have been investigated by Brillouin light scattering (BLS) at GHz frequencies. Elastic response of the system has been tracked from the melting temperature down to the glass transition through the supercooled liquid. Both the structural arrest and the vibrational dynamics are described by modeling the experimentally determined dynamic structure factor within the framework of the Langevin equation, through a simplified choice of memory function which allows one to determine sound velocity and the acoustic attenuation coefficient as a function of temperature. The density fluctuation spectra in the glassy phase, as probed by BLS, are compared with time-domain results from photoacoustics experiments. The arising scenario is discussed in the context of current literature reporting inelastic X-ray scattering and BLS in platelet geometry. The link between the probed elastic properties and the non-ergodicity factor of the glass phase is finally scrutinized.

The sound attenuation in the THz region is studied down to T 16 K in glassy glycerol by inelastic x-ray scattering. At striking variance with the decrease found below 100 K in the GHz data, the attenuation in the THz range does not show any T dependence. This result (i) indicates the presence of two different attenuation mechanisms, active, respectively, in the high- and low-frequency limits, (ii) demonstrates the nondynamic origin of the attenuation of THz sound waves, and confirms a similar conclusion obtained in SiO2 glass by molecular dynamics, and (iii) supports the low-frequency attenuation mechanism proposed by Fabian and Allen [ Phys. Rev. Lett. 82, 1478 (1999)].

Spectral compression of femtosecond pulses by second harmonic generation in the presence of substantial group velocity dispersion provides a convenient source of narrowband Raman pump pulses for femtosecond stimulated Raman spectroscopy (FSRS). We discuss here a simple and efficient modification that dramatically increases the versatility of the second harmonic spectral compression technique. Adding a spectral filter following second harmonic generation produces narrowband pulses with a superior temporal profile. This simple modification i) increases the Raman gain for a given pulse energy, ii) improves the spectral resolution, iii) suppresses coherent oscillations associated with slowly dephasing vibrations, and iv) extends the useful tunable range to at least 330-750 nm.

Exact relation for contributions to heat capacity of liquids is obtained from hydrodynamic theory. It is shown from analysis of the long-wavelength limit of heat density autocorrelation functions that the heat capacity of simple liquids is represented as a sum of two contributions due to “phonon-like” collective excitations and heat relaxation. The ratio of both contributions being the analogy of Landau-Placzek ratio for heat processes depends on the specific heats ratio. The theory of heat density autocorrelation functions in liquids is verified by computer simulations. Molecular dynamics simulations for six liquids having the ratio of specific heats γ in the range 1.1–2.3, were used for evaluation of the heat density autocorrelation functions and predicted Landau-Placzek ratio for heat processes. The dependence of contributions from collective excitations and heat relaxation process to specific heat on γ is shown to be in excellent agreement with the theory.

The lack of long-range structural order in amorphous solids induces well known thermodynamic anomalies, which are the manifestation of distinct peculiarities in the vibrational spectrum. Although the impact of such anomalies vanishes in the long wavelength, elastic continuum limit, it dominates at length scales comparable to interatomic distances, implying an intermediate transition regime still poorly understood. Here we report a study of such mesoscopic domains by means of a broadband version of picosecond photo-acoustics, developed to coherently generate and detect hypersonic sound waves in the sub-THz region with unprecedented sampling efficiency. We identify a temperature-dependent fractal v3/2 frequency behaviour of the sound attenuation, pointing to the presence of marginally stable regions and a transition between the two above mentioned limits. The essential features of this behaviour are captured by a theoretical approach based on random spatial variation of the shear modulus, including anharmonic interactions., Characterizing the acoustic dynamics of glasses at mesoscopic wavelengths is challenging. Here, Ferrante et al. use the broadband picosecond acoustics technique to study attenuation at such length scales, where they find temperature-dependent, fractal frequency behaviour.

International audience; Sudden changes in the dynamical properties of a supercritical fluid model have been found as a function of pressure and temperature (T/Tc52-5 and P/Pc510-103), striving with the notion of a single phase beyond the critical point established by thermodynamics. The sound propagation in the Terahertz frequency region reveals a sharp dynamic crossover between the gas like and the liquid like regimes along several isotherms, which involves, at sufficiently low densities, the interplay between purely acoustic waves and heat waves. Such a crossover allows one to determine a dynamic line in the phase diagram which exhibits a very tight correlation with a number of thermodynamic observables, showing that the supercritical state is remarkably more complex than thought so far

The high frequency dynamics of Indomethacin and Celecoxib glasses has been investigated by inelastic x-ray scattering, accessing a momentum-energy region still unexplored in amorphous pharmaceuticals. We find evidence of phonon-like acoustic dynamics and determine the THz behavior of sound velocity and acoustic attenuation. Connections with ordinary sound propagation are discussed, along with the relation between fast and slow degrees of freedom as represented by non-ergodicity factor and kinetic fragility, respectively.

The behaviour of high-frequency acoustic excitations in disordered materials is one of the most provocative and less understood aspects of glass science. The frequency dependence of sound damping, in particular, is the result of the interplay of several physical mechanisms crucial for many theoretical models. The emerging picture suggests the existence of a frequency crossover in sound attenuation identifying a transition from a macroscopic continuum-like behavior, due to anharmonic damping effects, to a microscopic regime in which the dynamics is dominated by topological disorder, characteristic of the amorphous phase. The very nature of this hypothetic crossover, its frequency position and its relation to other anomalies of the amorphous state is highly debated.

In contrast to crystalline solids in which structural order governs dynamics and thermodynamics, the lack of long-range periodicity in amorphous materials is responsible for several anomalies. Although the relation between these anomalies and the 'bulk structure' is generally understood, the surface structure and the corresponding vibrational spectrum of amorphous solids is practically an unexplored theme. In this study, we resolve the differences in vibrational dynamics and atomic structure between bulk and surface (top 5 nm) atoms of amorphous selenium. We combine experimental (grazing incidence inelastic X-ray scattering) and computational (ab initio and semiempirical molecular orbital theoretical calculations) methods to scrutinize a variety of possible structural models. We find that a high concentration of particular types of 'coordination defects' in the surface layer is responsible for the observed differences. Resolving the structure of amorphous surfaces is, for example, important for understanding nanoparticles' properties where the surface-to-bulk ratio has a crucial role.

4 páginas, 3 figuras.-- PACS number(s): 64.70.kj, 61.05.C-, 63.50.-x, 64.70.Q-.-- et al., The connection between viscous flow and vibrational properties in glass-forming materials is scrutinized examining the fragility of a wide set of liquids and the nonergodicity factor of the corresponding glasses. Building on the same line of reasoning which allows us to extend the connection between viscosity and thermodynamics in complex systems, we show here how the two quantities are strongly correlated once the effect of those secondary relaxation processes due to internal degrees of freedom is correctly accounted for. This result provides a missing thermodynamic rationale for the recently debated universality of the correlation between fast and slow degrees of freedom., T.S. acknowledges support from the European Research Council under the European Community Seventh Framework Program (FP7/2007-2013)/ERC IDEAS Grant No. 207916.

The approach of generalized collective modes is applied to the study of dispersion curves of collective excitations along isothermal lines of supercritical pure Lennard-Jones fluid. An effect of structural relaxation and other nonhydrodynamic relaxation processes on the dispersion law is discussed. A simple analytical expression for the dispersion law in the long-wavelength region of acoustic excitations is obtained within a three-variable viscoelastic model of generalized hydrodynamics. It is shown that the deviation from the linear dependence in the long-wavelength region can be either "positive" or "negative" depending on the ratio between the high-frequency (elastic) and isothermal speed of sound. An effect of thermal fluctuations on positive and negative dispersion is estimated from the analytical solution of a five-variable thermoviscoelastic model that generalizes the results of the viscoelastic treatment. Numerical results are reported for a Lennard-Jones supercritical fluid along two isothermal lines T(*)=1.71,4.78 with different densities and discussed along the theoretical expressions derived.

We report on the study of the slow dynamics of supercooled and molten Selenium by means of dynamic light scattering. To achieve this in the present case, where the opaqueness of Se prevents visible light scattering measurements, we have developed a novel experimental setup using infrared radiation as the light source. By measuring the scattered intensity autocorrelation function we have been able to extract the average size of the Se clusters in the liquid state and determine the temperature dependence of the related diffusion coefficient which was found to exhibit Arrhenius behavior.

We present an experimental setup to study terahertz dynamics in fluids under high pressure, employing inelastic x-ray scattering and diamond anvil cell techniques. The use of a carefully designed vacuum chamber and the minimization and control of sources of parasitic scattering allowed circumventing previous limitations due to important empty cell contributions to the scattering signal. The successful implementation of our setup is demonstrated in the case of supercritical fluid argon, for which a full viscoelastic analysis yields the dispersion relation of sound waves, the generalized heat capacity ratio, and longitudinal viscosity. Our results are in excellent agreement with available experimental observables and molecular dynamics simulations.

Multi-µJ narrow-bandwidth (≈ 10 cm(-1)) picosecond pulses, broadly tunable in the visible-UV range (320-520 nm), are generated by spectral compression of femtosecond pulses emitted by an amplified Ti:sapphire system. Such pulses provide the ideal Raman pump for broadband femtosecond stimulated Raman spectroscopy, as here demonstrated on a heme protein.

We study via self-consistent Born approximation a model for sound waves in a disordered environment, in which the local fluctuations of the shear modulus G are spatially correlated with a certain correlation length The theory predicts an enhancement of the density of states over Debye's omega(2) law (boson peak) whose intensity increases for increasing correlation length, and whose frequency position is shifted downwards as lg. Moreover, the predicted disorder-induced sound attenuation coefficient r(k) obeys a universal scaling law F(k) = f (ke) for a given variance of G. Finally, the inclusion of the lowest-order contribution to the anharmonic sound damping into the theory allows us to reconcile apparently contradictory recent experimental data in amorphous SiO(2).

Using spectral compression by second-harmonic generation we generate pulses with 10÷15 cmμ1 linewidth, multi-μJ energy and broad tunability from 330 to 510 nm. This source is ideally suited as Raman pump for Stimulated Raman Scattering.