Four-wave mixing (FWM) processes, based on third-order nonlinear light-matter interactions, can combine ultrafast time resolution with energy and wavevector selectivity, and enable the exploration of ...dynamics inaccessible by linear methods. The coherent and multi-wave nature of the FWM approach has been crucial in the development of advanced technologies, such as silicon photonics, subwavelength imaging and quantum communications. All these technologies operate at optical wavelengths, which limits the spatial resolution and does not allow the probing of excitations with energy in the electronvolt range. Extension to shorter wavelengths--that is, the extreme ultraviolet and soft-X-ray ranges--would allow the spatial resolution to be improved and the excitation energy range to be expanded, as well as enabling elemental selectivity to be achieved by exploiting core resonances. So far, FWM applications at such wavelengths have been prevented by the absence of coherent sources of sufficient brightness and of suitable experimental set-ups. Here we show how transient gratings, generated by the interference of coherent extreme-ultraviolet pulses delivered by the FERMI free-electron laser, can be used to stimulate FWM processes at suboptical wavelengths. Furthermore, we have demonstrated the possibility of observing the time evolution of the FWM signal, which shows the dynamics of coherent excitations as molecular vibrations. This result opens the way to FWM with nanometre spatial resolution and elemental selectivity, which, for example, would enable the investigation of charge-transfer dynamics. The theoretical possibility of realizing these applications has already stimulated ongoing developments of free-electron lasers: our results show that FWM at suboptical wavelengths is feasible, and we hope that they will enable advances in present and future photon sources.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
The extension of nonlinear optical techniques to the extreme-ultraviolet (EUV), soft and hard x-ray regime represents one of the open challenges of modern science since it would combine chemical ...specificity with background-free detection and ultrafast time resolution. We report on the first observation of a four-wave-mixing (FWM) response from solid-state samples stimulated exclusively by EUV pulses. The all-EUV FWM signal was generated by the diffraction of high-order harmonics of the FERMI free-electron laser (FEL) from the standing wave resulting from the interference of two crossed FEL pulses at the fundamental wavelength. From the intensity of the FWM signal, we are able to extract the first-ever estimate of an effective value of ∼6×10^{-24} m^{2} V^{-2} for the third-order nonlinear susceptibility in the EUV regime. This proof of principle experiment represents a significant advance in the field of nonlinear optics and sets the starting point for a manifold of techniques, including frequency and phase-resolved FWM methods, that are unprecedented in this photon-energy regime.
Ultrafast optical-domain spectroscopies allow to monitor in real time the motion of nuclei in molecules. Achieving element-selectivity had to await the advent of time resolved X-ray spectroscopy, ...which is now commonly carried at X-ray free electron lasers. However, detecting light element that are commonly encountered in organic molecules, remained elusive due to the need to work under vacuum. Here, we present an impulsive stimulated Raman scattering (ISRS) pump/carbon K-edge absorption probe investigation, which allowed observation of the low-frequency vibrational modes involving specific selected carbon atoms in the Ibuprofen RS dimer. Remarkably, by controlling the probe light polarization we can preferentially access the enantiomer of the dimer to which the carbon atoms belong.
The recent construction of free electron lasers allows extending laboratory-based laser experiments to shorter wavelengths, accessing wavevectors typical of nanoscale dynamics and adding element and ...chemical state specificity by exploiting electronic transitions from core levels. The high pulse energies available ensure that this new wavelength range can be advantageously used for nonlinear optics, as in the pioneering case of transient grating spectroscopy: a time-resolved four-wave mixing technique in which two pump pulses are crossed at the sample to generate a spatially periodic excitation whose dynamics is monitored via diffraction of a probe pulse. We will show how extreme ultraviolet photon pulses have been successfully deployed in the last seven years to carry out transient grating experiments, mainly performed at the FERMI free electron laser, addressing a variety of scientific questions, ranging from the study of thermal transport in semiconductors approaching the ballistic regime to the modelling of ultrafast demagnetization at the nanoscale. We will also discuss possible future developments of the transient grating method specifying the impact this could have in various fields of scientific research ranging from molecular chirality to spintronics.
Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the ...observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.
A liquid carbon (l-C) sample is generated through constant volume heating exposing an amorphous carbon foil to an intense ultrashort laser pulse. Time-resolved x-ray absorption spectroscopy at the C ...K edge is used to monitor the dynamics of the melting process revealing a subpicosecond rearrangement of the electronic structure associated with a sudden change of the C bonding hybridization. The obtained l-C sample, resulting from a nonthermal melting mechanism, reaches a transient equilibrium condition with a temperature of about 14 200 K and pressure in the order of 0.5 Mbar in about 0.3 ps, prior to hydrodynamic expansion. A detailed analysis of the atomic and electronic structure in solid-density l-C based on time-resolved x-ray absorption spectroscopy and theoretical simulations is presented. The method can be fruitfully used for extending the experimental investigation of the C phase diagram in a vast unexplored region covering the 103–104 K temperature range with pressures up to 1 Mbar.
Collective lattice dynamics determine essential aspects of condensed matter, such as elastic and thermal properties. These exhibit strong dependence on the length-scale, reflecting the marked ...wavevector dependence of lattice excitations. The extreme ultraviolet transient grating (EUV TG) approach has demonstrated the potential of accessing a wavevector range corresponding to the 10s of nm length-scale, representing a spatial scale of the highest relevance for fundamental physics and forefront technology, previously inaccessible by optical TG and other inelastic scattering methods. In this manuscript we report on the capabilities of this technique in the context of probing thermoelastic properties of matter, both in the bulk and at the surface, as well as discussing future developments and practical considerations.
An extreme ultraviolet pump and visible-light probe transmission experiment in crystalline LiF, carried out at the Free Electron Laser facility FERMI, revealed an oscillating time dependence of the ...plasmon mode excited in the high-density high-temperature electron plasma. The effect is interpreted as a fingerprint of the electron-ion interaction: the ion motion, shaped by the electron dynamic screening, induces, in turn, electron density fluctuations that cause the oscillation of the plasmon frequency at the timescale of the ion dynamics. Fitting the high resolution transmission data with an RPA model for the temperature-dependent dielectric function, which includes electron self-energy and electron-ion coupling, confirms the interpretation of the time modulation of the plasmon mode.
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