A new non-linear optics technique developed at FERMI

Non-linear optics (NLO) techniques are experimental approaches in which two or more light fields interact with matter to detect properties that are not accessible by means of linear methods. Remarkably, despite their complexity, NLO methods frequently provide enhanced selectivity and sensitivity, especially when implemented in the so called non-collinear geometry. In this configuration, the interacting light beams come from different directions, and the energy-momentum conservation imposes that the nonlinear signal is emitted in a different direction than that of the input beams. This creates what is known as the background free condition, which is ideal for detecting faint signals.

NLO experiments in the extreme ultraviolet (EUV) and X-ray spectral range hold the promise of unraveling the secrets of matter at the atomic level and on ultrafast scales, as pointed out by theoretical studies. However, EUV/X-ray NLO had to wait for photon sources with sufficient peak brightness, i.e., free electron lasers (FELs). Despite significant progress, non-collinear EUV NLO techniques, until now, have been limited to detecting only “elastic signals”, i.e., processes where the light beams neither lose nor gain energy when interacting with the sample.

In this study, researchers from Elettra-Sincrotrone Trieste utilized the mini-TIMER setup at the DiProI end-station and the “twin-seed” scheme available at the FERMI FEL to bring two EUV pulses, differing in photon energy by approximately 180 meV, into interaction with a diamond sample. This interaction can be envisaged as a “moving grating” (see Figure 1a), which slides on the sample surface with a velocity of about 100 km/s.

Figure 1: a): Sketch of the experimental geometry; the interaction between the two EUV pulses generates a “moving grating”, revealed by inelastic (“bluer pulse”) scattering from the optical pulse. Panel b): time dependence of the blue-shifted inelastic signal as a function of the EUV-optical delay (black dots) and of the delay between the two EUV pulses with the optical one at time zero (red dots); lines through the data are Gaussian curves.

The occurrence of the moving grating was experimentally probed by a variably-delayed optical probe pulse, with energy E_pr, which investigated the excited sample generating a fourth beam through a third order NLO process called “four-wave-mixing”. This fourth beam serves as experimental signal. As expected, it exhibits a blue-shifted photon energy E = E_pr + 180 meV and propagates in a different direction than that of the probing beams. Both of these key features are governed by energy and momentum conservation, and are predetermined by the setup used in this demonstrative experiment.

The blue-shifted inelastic signal from our moving grating is present only when the two EUV pulses and the optical one are time-coincident, as evident from the Gaussian time-delay dependence shown in Figure 1b. This indicates that the dynamics of the system is hindered by the pulse duration (50 fs) used in the experiment. Nonetheless, the results of the present work indicate how to practically implement such long-theorized experiments. For instance, the use of shorter pulses and improved setups could enable access to ultrafast electronic dynamics and correlations between the different atomic species present in the sample. This has fundamental implications in fields such as organic photovoltaics, where the elemental steps of energy transfer still remain elusive. In the quest for a deeper understanding of material properties, researchers now have a sharper arrow in their quivers.

This research was conducted by the following research team:

R. Mincigrucci¹, A. Cannizzo², F. Capotondi¹, P. Cinquegrana¹, R. Cucini³, F. Dallari⁴, M. B. Danailov¹, G. De Ninno^1,5, S. Di Mitri¹, T. Feurer², L. Foglia¹, H.-M. Frei², M. Manfredda¹, A. A. Maznev⁶, G. Monaco⁴, D. Naumenko¹, I. Nikolov¹, Z. Ollmann², E. Paltanin¹, G. Pamfilidis², E. Pedersoli¹, E. Principi¹, J. Rehault³, A. Simoncig¹, C. Svetina⁷, G. Knopp⁸, C. Masciovecchio¹, F. Bencivenga^{1 4
1} Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy.
² Institute of Applied Physics, University of Bern, Bern, Switzerland
³ CNR-IOM, Trieste, Italy
⁴ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università degli Studi di Padova, Padova, Italy
⁵ Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia
⁶ Massachusetts Institute of Technology, Cambridge, MA, United States.
⁷ IMDEA Nanociencia, Madrid, Spain
⁸ Paul Scherrer Institute, Villigen, Switzerland

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