Manipulating the macroscopic phases of solids using ultrashort light pulses has resulted in spectacular phenomena, including metal-insulator transitions, superconductivity and subpicosecond ...modification of magnetic order. The development of this research area strongly depends on the understanding and optical control of fundamental interactions in condensed matter, in particular the exchange interaction. However, disentangling the timescales relevant for the contributions of the exchange interaction and spin dynamics to the exchange energy, Eex , is a challenge. Here, we introduce femtosecond stimulated Raman scattering to unravel the ultrafast photo-induced dynamics of magnetic excitations at the edge of the Brillouin zone. We find that femtosecond laser excitation of the antiferromagnet KNiF3 triggers a spectral shift of the two-magnon line, the energy of which is proportional to Eex . By unravelling the photo-induced modification of the two-magnon line frequency from a dominating nonlinear optical effect, we find that Eex is increased by the electromagnetic stimulus.
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 v(3/2) 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.
We investigate the out-of-equilibrium switching dynamics of a molecular Fe(III) spin-crossover solid triggered by a femtosecond laser flash. The time-resolved x-ray diffraction and optical results ...show that the dynamics span from subpicosecond local photoswitching followed by volume expansion (nanosecond) and thermal switching (microsecond). We present a physical picture of the consecutive steps in the photoswitching of molecular materials.
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.
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.
Spontaneous Raman is a well‐established tool to probe molecular vibrations. Under resonant conditions, it is a largely used method for characterizing the structure of heme‐proteins. In recent years, ...advances in pulsed laser sources allowed to explore vibrational features with complex techniques based on nonlinear optical interactions, among which is stimulated Raman scattering (SRS). Building on its combined spectral–temporal resolutions and high chemical sensitivities, SRS has been largely applied as a probe for ultrafast, time‐resolved studies, as well as an imaging technique in biological systems. By using a frequency tunable, narrowband pump pulse jointly with a femtosecond white light continuum to initiate the SRS process, here we measure the Raman spectrum of a prototypical heme‐protein, namely deoxy myoglobin, under two different electronic resonances. The SRS results are compared with the spontaneous Raman spectra, and the relative advantages, such as the capability of our experimental approach to provide an accurate mapping of Raman excitation profiles, are discussed.
Combining a frequency tunable, narrowband pump pulse, and a femtosecond white light continuum, the stimulated Raman spectrum of a prototypical heme‐protein, namely, deoxymyoglobin, is measured under two different resonance conditions. Comparison with spontaneous resonant Raman is discussed.
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.
The time-resolved diffraction signal from a laser-excited solution has three principal components: the solute-only term, the solute-solvent cross term, and the solvent-only term. The last term is ...very sensitive to the thermodynamic state of the bulk solvent, which may change during a chemical reaction due to energy transfer from light-absorbing solute molecules to the surrounding solvent molecules and the following relaxation to equilibrium with the environment around the scattering volume. The volume expansion coefficient alpha for a liquid is typically approximately 1 x 10(-3) K(-1), which is about 1000 times greater than for a solid. Hence solvent scattering is a very sensitive on-line thermometer. The decomposition of the scattered x-ray signal has so far been aided by molecular dynamics (MD) simulations, a method capable of simulating the solvent response as well as the solute term and solute/solvent cross terms for the data analysis. Here we present an experimental procedure, applicable to most hydrogen containing solvents, that directly measures the solvent response to a transient temperature rise. The overtone modes of OH stretching and CH3 asymmetric stretching in liquid methanol were excited by near-infrared femtosecond laser pulses at 1.5 and 1.7 microm and the ensuing hydrodynamics, induced by the transfer of heat from a subset of excited CH3OH* to the bulk and the subsequent thermal expansion, were probed by 100 ps x-ray pulses from a synchrotron. The time-resolved data allowed us to extract two key differentials: the change in the solvent diffraction from a temperature change at constant density, seen at a very short time delay approximately 100 ps, and a term from a change in density at constant temperature. The latter term becomes relevant at later times approximately 1 mus when the bulk of liquid expands to accommodate its new temperature at ambient pressure. These two terms are the principal building blocks in the hydrodynamic equation of state, and they are needed in a self-consistent reconstruction of the solvent response during a chemical reaction. We compare the experimental solvent terms with those from MD simulations. The use of experimentally determined solvent differentials greatly improved the quality of global fits when applied to the time-resolved data for C2H4I2 dissolved in methanol.
Bistable arches for morphing applications Pontecorvo, Michael E; Barbarino, Silvestro; Murray, Gabriel J ...
Journal of intelligent material systems and structures,
02/2013, Letnik:
24, Številka:
3
Journal Article, Conference Proceeding
Recenzirano
This article examines the bistable behavior of an arch for morphing applications. The arch has a cosine profile, is clamped at both ends, and is restrained axially by a spring at one end. Fabrication ...and testing of several Delrin and NiTiNOL arch specimens (with varying arch height, thickness, and spring stiffness) were followed by ANSYS finite element modeling, and the ANSYS simulation results showed good overall agreement with the test results. A parametric study was conducted using the ANSYS model to assess the influence of arch thickness, height, and spring stiffness on the bistable behavior. The results indicated that lower arch thickness, larger arch height, and higher spring stiffness tend to promote bistability; lower arch thickness and height reduce peak strains as the arch moves between equilibrium states, but increasing spring stiffness has a smaller effect; and higher arch thickness, height, and spring stiffness increase the snap-through force, which in turn increases the actuation force requirement as well as load carrying capability of the bistable morphing arch. If the arch slenderness ratio is unchanged, change in arch span (size) does not change the maximum stress while increasing the peak snap-through force proportionally.
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