A. Cavalleri - Academia.edu (original) (raw)
Papers by A. Cavalleri
Physical Review B, 1998
... If the plasma density exceeds a critical value ( 1022 cm 3, 10% of the whole valence populati... more ... If the plasma density exceeds a critical value ( 1022 cm 3, 10% of the whole valence population RAPID COMMUNICATIONS PRB 58 THERMAL AND NONTHERMAL MELTING ... Depending onexcitation strength they identified different regimes in the time evolution of the ...
Nonlinear Optics: Materials, Fundamentals and Applications, 2002
... RW Schoenleina, A. Cavalleria, HHW Chongb, TE Gloverc PA Heimannc, CV Shanka, AA ... are dire... more ... RW Schoenleina, A. Cavalleria, HHW Chongb, TE Gloverc PA Heimannc, CV Shanka, AA ... are directly measured from a bend-magnet beamline at the Advanced Light Source (ALS)[2 ... electron passing through the wiggler matches the laser wavelength (FEL resonance condition)[1 ...
![Research paper thumbnail of Erratum: Enhanced Photosusceptibility near Tc for the Light-Induced Insulator-to-Metal Phase Transition in Vanadium Dioxide [Phys. Rev. Lett. 99, 226401 (2007)]](https://attachments.academia-assets.com/49399042/thumbnails/1.jpg)
](Physical Review Letters, 2008
We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of th... more We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.
Physical Review Letters, 2000
Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thi... more Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times ͑T 2 ͒ for 7-GHz longitudinal acoustic phonons in germanium. PACS numbers: 63.20.Ry, 42.65.Re, 61.10.Nz In semiconductors, the response to impulsive, interband optical excitation generally proceeds through intraband relaxation of hot carriers via phonon emission, radiative and nonradiative recombination, vibrational acoustic transport into the bulk or across interfaces, and eventual thermalization of the phonon modes via lattice anharmonicity . While the processes involving charge carriers and Ramanactive optical phonons have been extensively characterized with optical methods [1], the ultimate steps of transport and thermalization of nonequilibrium acoustic lattice vibrations have remained largely undetected. Moreover, anharmonic lattice effects are of general interest because they are responsible for many other common processes in the solid state, such as thermal expansion, volume-dependent elastic constants, and temperature dependent thermal conductivity in solids . Yet, the measurement of non-Raman-active coherent acoustic phonons is demanding and has been indirectly achieved only at surfaces . By combining the temporal resolution of ultrafast laser spectroscopy [5] with the structural sensitivity of x-ray scattering [6], a number of direct studies [7-13] of atomic motion deep within the bulk of matter have been recently achieved. In this paper, we report on ultrafast x-ray measurements of strain oscillations in an impulsively heated germanium film. Excitation-dependent damping times and vibrational transport across a buried interface are simultaneously measured, thereby identifying individual coherent phonon decay mechanisms.
Nature Materials, 2014
Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magneti... more Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magnetic or orbital orders and to transform insulators into metals. In cuprates, this technique has been used to remove charge stripes and promote superconductivity, acting in a way opposite to static magnetic fields. Here, we show that excitation of large-amplitude apical oxygen distortions in the cuprate superconductor YBa2Cu3O6.5 promotes highly unconventional electronic properties. Below the superconducting transition temperature (Tc = 50 K) inter-bilayer coherence is transiently enhanced at the expense of intra-bilayer coupling. Strikingly, even above Tc a qualitatively similar effect is observed up to room temperature, with transient inter-bilayer coherence emerging from the incoherent ground state and similar transfer of spectral weight from high to low frequency. These observations are compatible with previous reports of an inhomogeneous normal state that retains important properties of a superconductor, in which light may be melting competing orders or dynamically synchronizing the interlayer phase. The transient redistribution of coherence discussed here could lead to new strategies to enhance superconductivity in steady state.
Journal of Molecular Structure, 2003
Fourier Transform Infrared Microscopy enables to study small samples by means of high quality spe... more Fourier Transform Infrared Microscopy enables to study small samples by means of high quality spectra. Biochemical and morphological changes in carotid plaques and thyroid tissues have been displayed by drawing three-dimensional chemical maps of the main vibrational modes in regions of interest. A huge number of spectra were collected that allow to point out even subtle changes in the main components. Micro FT-IR spectroscopy is suited for in vivo clinical diagnostics, immunocytochemical analysis and ex vivo histological determinations to establish a suitable protocol for the study of lesions in human tissues, even at an early stage. q
Summary form only given. Ultrashort x-ray pulses offer a unique combination of atomic-scale spati... more Summary form only given. Ultrashort x-ray pulses offer a unique combination of atomic-scale spatial and temporal resolution, which permits direct measurements of structural transients on an ultrafast time scale. Using time-resolved X-ray diffraction with femtosecond, multi-keV X-ray pulses we have studied transient lattice dynamics in optically excited semiconductors and metals. In an optical pump/X-ray probe configuration transient changes in X-ray diffraction from [111]-oriented, single-crystalline thin films of Ge and Bi have been measured.
Nature Physics, 2011
Competition between electron localization and delocalization in Mott insulators underpins the phy... more Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast timescale 1,2 . So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations 3-5 , dominate. However, the underlying quantum dynamics of 'bare' electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices 6,7 where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F 2 TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon-doublon pairs.
Physical Review B, 1998
... If the plasma density exceeds a critical value ( 1022 cm 3, 10% of the whole valence populati... more ... If the plasma density exceeds a critical value ( 1022 cm 3, 10% of the whole valence population RAPID COMMUNICATIONS PRB 58 THERMAL AND NONTHERMAL MELTING ... Depending onexcitation strength they identified different regimes in the time evolution of the ...
Nonlinear Optics: Materials, Fundamentals and Applications, 2002
... RW Schoenleina, A. Cavalleria, HHW Chongb, TE Gloverc PA Heimannc, CV Shanka, AA ... are dire... more ... RW Schoenleina, A. Cavalleria, HHW Chongb, TE Gloverc PA Heimannc, CV Shanka, AA ... are directly measured from a bend-magnet beamline at the Advanced Light Source (ALS)[2 ... electron passing through the wiggler matches the laser wavelength (FEL resonance condition)[1 ...
Physical Review Letters, 2008
We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of th... more We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.
Physical Review Letters, 2000
Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thi... more Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times ͑T 2 ͒ for 7-GHz longitudinal acoustic phonons in germanium. PACS numbers: 63.20.Ry, 42.65.Re, 61.10.Nz In semiconductors, the response to impulsive, interband optical excitation generally proceeds through intraband relaxation of hot carriers via phonon emission, radiative and nonradiative recombination, vibrational acoustic transport into the bulk or across interfaces, and eventual thermalization of the phonon modes via lattice anharmonicity . While the processes involving charge carriers and Ramanactive optical phonons have been extensively characterized with optical methods [1], the ultimate steps of transport and thermalization of nonequilibrium acoustic lattice vibrations have remained largely undetected. Moreover, anharmonic lattice effects are of general interest because they are responsible for many other common processes in the solid state, such as thermal expansion, volume-dependent elastic constants, and temperature dependent thermal conductivity in solids . Yet, the measurement of non-Raman-active coherent acoustic phonons is demanding and has been indirectly achieved only at surfaces . By combining the temporal resolution of ultrafast laser spectroscopy [5] with the structural sensitivity of x-ray scattering [6], a number of direct studies [7-13] of atomic motion deep within the bulk of matter have been recently achieved. In this paper, we report on ultrafast x-ray measurements of strain oscillations in an impulsively heated germanium film. Excitation-dependent damping times and vibrational transport across a buried interface are simultaneously measured, thereby identifying individual coherent phonon decay mechanisms.
Nature Materials, 2014
Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magneti... more Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magnetic or orbital orders and to transform insulators into metals. In cuprates, this technique has been used to remove charge stripes and promote superconductivity, acting in a way opposite to static magnetic fields. Here, we show that excitation of large-amplitude apical oxygen distortions in the cuprate superconductor YBa2Cu3O6.5 promotes highly unconventional electronic properties. Below the superconducting transition temperature (Tc = 50 K) inter-bilayer coherence is transiently enhanced at the expense of intra-bilayer coupling. Strikingly, even above Tc a qualitatively similar effect is observed up to room temperature, with transient inter-bilayer coherence emerging from the incoherent ground state and similar transfer of spectral weight from high to low frequency. These observations are compatible with previous reports of an inhomogeneous normal state that retains important properties of a superconductor, in which light may be melting competing orders or dynamically synchronizing the interlayer phase. The transient redistribution of coherence discussed here could lead to new strategies to enhance superconductivity in steady state.
Journal of Molecular Structure, 2003
Fourier Transform Infrared Microscopy enables to study small samples by means of high quality spe... more Fourier Transform Infrared Microscopy enables to study small samples by means of high quality spectra. Biochemical and morphological changes in carotid plaques and thyroid tissues have been displayed by drawing three-dimensional chemical maps of the main vibrational modes in regions of interest. A huge number of spectra were collected that allow to point out even subtle changes in the main components. Micro FT-IR spectroscopy is suited for in vivo clinical diagnostics, immunocytochemical analysis and ex vivo histological determinations to establish a suitable protocol for the study of lesions in human tissues, even at an early stage. q
Summary form only given. Ultrashort x-ray pulses offer a unique combination of atomic-scale spati... more Summary form only given. Ultrashort x-ray pulses offer a unique combination of atomic-scale spatial and temporal resolution, which permits direct measurements of structural transients on an ultrafast time scale. Using time-resolved X-ray diffraction with femtosecond, multi-keV X-ray pulses we have studied transient lattice dynamics in optically excited semiconductors and metals. In an optical pump/X-ray probe configuration transient changes in X-ray diffraction from [111]-oriented, single-crystalline thin films of Ge and Bi have been measured.
Nature Physics, 2011
Competition between electron localization and delocalization in Mott insulators underpins the phy... more Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast timescale 1,2 . So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations 3-5 , dominate. However, the underlying quantum dynamics of 'bare' electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices 6,7 where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F 2 TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon-doublon pairs.