arXiv subjects

D. Fausti

Publications and source records attributed to D. Fausti.

10 recordsLinked to original sources

arXiv2025

Transient laser-induced periodic surface structures revealed by time-resolved EUV diffuse scattering

The formation of permanent laser-induced periodic surface structures (LIPSS) on solid surfaces under impulsive laser irradiation above the damage threshold has been subject of extensive research. We demonstrate the formation of transient surface displacement patterns under femtosecond laser irradiation at fluences well below this threshold. Time-resolved extreme ultraviolet scattering measurements reveal distinct reciprocal-space features similar to those observed for permanent LIPSS but dissipating on the hundreds-of-picoseconds time scale. We show that the transient surface displacement patterns responsible for these features are produced via thermal expansion by the spatial modulation of absorbed laser intensity caused by scattering of the laser radiation by surface roughness and present a model accounting for the experimental observations. We suggest that our experiment revealed a universal phenomenon that will be observed on any strongly absorbing material under ultrafast laser irradiation.

physics.optics↗

arXiv2025

Time-domain extreme ultraviolet diffuse scattering spectroscopy of nanoscale surface phonons

We report the observation of dynamic fringe patterns in the diffuse scattering of extreme ultraviolet light from surfaces, following femtosecond optical excitation. At each point on the detector, the diffuse scattering intensity exhibits oscillations at well-defined frequencies that correspond to surface phonons with wave vectors determined by the scattering geometry, indicating that the optical excitation generates coherent surface phonons propagating in all directions and spanning a wavelength range from 60 to 300 nm. This phenomenon is observed on a variety of samples, including single-layer and multilayer metal films, as well as bulk semiconductors. The measured surface phonon dispersions show good agreement with theoretical calculations. By comparing signal amplitudes from samples with different surface morphologies, we find that the excitation mechanism is linked to the natural surface roughness of the samples. However, the signal is still detectable on extremely smooth surfaces with sub-nanometer roughness. Our findings demonstrate a simple and effective method for optically exciting coherent surface phonons with nanoscale wavelengths on a wide range of solid samples and establish a foundation for surface phonon spectroscopy in a wave vector range well beyond the limit of conventional surface Brillouin scattering.

cond-mat.mtrl-sci↗

arXiv2021

Transient measurement of phononic states with covariance-based stochastic spectroscopy

We present a novel approach to transient Raman spectroscopy, which combines stochastic probe pulses and a covariance-based detection to measure stimulated Raman signals in alpha-quartz. A coherent broadband pump is used to simultaneously impulsively excite a range of different phonon modes, and the phase, amplitude, and energy of each mode are independently recovered as a function of the pump-probe delay by a noisy-probe and covariance-based analysis. Our experimental results and the associated theoretical description demonstrate the feasibility of 2D-Raman experiments based on the stochastic probe schemes, with new capabilities not available in equivalent mean-value-based 2D-Raman techniques. This work unlocks the gate for nonlinear spectroscopies to capitalize on the information hidden within the noise and overlooked by a mean-value analysis.

physics.optics↗

arXiv2018

Femtosecond Covariance Spectroscopy

The success of non-linear optics relies largely on pulse-to-pulse consistency. In contrast, covariance based techniques used in photoionization electron spectroscopy and mass spectrometry have shown that wealth of information can be extracted from noise that is lost when averaging multiple measurements. Here, we apply covariance based detection to nonlinear optical spectroscopy, and show that noise in a femtosecond laser is not necessarily a liability to be mitigated, but can act as a unique and powerful asset. As a proof of principle we apply this approach to the process of stimulated Raman scattering in alpha-quartz. Our results demonstrate how nonlinear processes in the sample can encode correlations between the spectral components of ultrashort pulses with uncorrelated stochastic fluctuations. This in turn provides richer information compared to the standard non-linear optics techniques that are based on averages over many repetitions with well-behaved laser pulses. These proof-of-principle results suggest that covariance based nonlinear spectroscopy will improve the applicability of fs non-linear spectroscopy in wavelength ranges where stable, transform limited pulses are not available such as, for example, x-ray free electron lasers which naturally have spectrally noisy pulses ideally suited for this approach.

physics.optics↗

arXiv2015

Quantum interferences reconstruction with low homodyne detection efficiency

Standard quantum state reconstruction techniques indicate that a detection efficiency of $0.5$ is an absolute threshold below which quantum interferences cannot be measured. However, alternative statistical techniques suggest that this threshold can be overcome at the price of increasing the statistics used for the reconstruction. In the following we present numerical experiments proving that quantum interferences can be measured even with a detection efficiency smaller than $0.5$. At the same time we provide a guideline for handling the tomographic reconstruction of quantum states based on homodyne data collected by low efficiency detectors.

quant-ph↗

arXiv2011

Bi-directional ultrafast electric-field gating of interlayer transport in a cuprate superconductor

In cuprate superconductors, tunneling between planes makes possible three-dimensional coherent transport. However, the interlayer tunnelling amplitude is reduced when an order-parameter phase gradient between planes is established. As such, c-axis superconductivity can be weakened if a strong electric field is applied along the c axis. We use high-field single-cycle terahertz pulses to gate interlayer coupling in La1.84Sr0.16CuO4. We induce ultrafast oscillations between superconducting and resistive states and switch the plasmon response on and off, without reducing the density of Cooper pairs. Indeed, in-plane superconductivity remains unperturbed throughout, revealing a non-equilibrium state in which the dimensionality of the superconductor is time dependent. The gating frequency is determined by the electric field strength, in the spirit of the ac Josephson effect. Non-dissipative, bi-directional gating of superconductive coupling is of interest for device applications in ultrafast nanoelectronics. It is also a new example of nonlinear terahertz physics, applicable to nanoplasmonics and active metamaterials.

cond-mat.supr-con↗

arXiv2008

Phonon and crystal field excitations in geometrically frustrated rare earth titanates

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations of the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R2Ti2O7 pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb2Ti2O7 in Raman spectroscopy for the first time, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb3+ sites in the low temperature structure. Furthermore, the crystal field level at approximately 13 cm-1 is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb2Ti2O7. These findings call for a careful reassessment of the low temperature structure of Tb2Ti2O7, which may serve to improve its theoretical understanding.

cond-mat.mtrl-sci↗

arXiv2007

A Raman study of the Charge-Density-Wave State in A$_{0.3}$MoO$_3$ (A = K,Rb)

We report a comparative Raman spectroscopic study of the quasi-one-dimensional charge-density-wave systems \ab (A = K, Rb). The temperature and polarization dependent experiments reveal charge-coupled vibrational Raman features. The strongly temperature-dependent collective amplitudon mode in both materials differ by about 3 cm, thus revealing the role of alkali atom. We discus the observed vibrational features in terms of charge-density-wave ground state accompanied by change in the crystal symmetry. A frequency-kink in some modes seen in \bb between T = 80 K and 100 K supports the first-order lock-in transition, unlike \rb. The unusually sharp Raman lines(limited by the instrumental response) at very low temperatures and their temperature evolution suggests that the decay of the low energy phonons is strongly influenced by the presence of the temperature dependent charge density wave gap.

cond-mat.str-el↗

arXiv2006

Coherent amplitudon generation in K_0.3MoO_3 through ultrafast inter-band quasi particle decay

The charge density wave system K_0.3MoO_3 has been studied using variable energy pump-probe spectroscopy, ellipsometry, and inelastic light scattering. The observed transient reflectivity response exhibits quite a complex behavior, containing contributions due to quasi particle excitations, coherent amplitudons and phonons, and heating effects. The generation of coherent amplitudons is discussed in terms of relaxation of photo-excited quasi particles, and is found to be resonant with the interband plasmon frequency. Two additional coherent excitations observed in the transients are assigned to zone-folding modes of the charge density wave state.

cond-mat.str-el↗

arXiv2005

Evidence for differentiation in the iron-helicoidal-chain in GdFe$_{3}$(BO$_{3}$)$_{4}$

We report on a single-crystal X-ray structure study of $GdFe_{3}(BO_{3})_{4}$ at room temperature and at T=90 K. At room temperature $GdFe_{3}(BO_{3})_{4}$ crystallizes in a trigonal space group R32 (No. 155), the same as found for other members of iron-borate family $RFe_{3}(BO_{3})_{4}$. At 90 K the structure of $GdFe_{3}(BO_{3})_{4}$ has transformed to the space group $P3_{1}2_{1}$ (No. 152). The low-temperature structure determination gives new insight into the weakly first-order structural phase transition at 156 K and into the related Raman phonon anomalies. The discovery of two inequivalent iron chains in the low temperature structure provide new point of view on the low-temperature magnetic properties.

cond-mat.str-el↗