T-ReX Laboratory

Welcome To The T-ReX Laboratory Group

T-ReX (Time Resolved X-Ray spectroscopy) Lab is a set of facilities born inside the Fermi Project at Elettra under the supervision of Prof. Fulvio Parmigiani. 

The members of the group are associated with Fermi and Elettra, but also with the italian National Research Council (CNR) and with the University of Trieste.
Our research is devoted to the study of ultrafast or non-equililbrium processes in condensed and soft matter and their applications in technology through the use of femtosecond laser pulses. Our goal is to study transient states and photo-induced phase transitions in superconductors, magnetic materials, and electron correlations in hard- and soft- condensed matter (charge transfer and phonon assisted excitations).
The Laboratory is conceived around a set of sources and set-ups that offer different spectroscopic techniques. It is also a facility that is open to the public through collaborations.

Research Highlights | Publications | Thesis Works 

Photon Number Statistics uncover the fluctuations in non-equilibrium lattice dynamics

Lattice dynamics fluctuations in quartz have been revealed for the first time by adopting a photon number statistics non-equilibrium optical experiment, combined with a fully quantum description of the interaction between photonic and phononic fields. Quantum fluctuations of atomic positions are related to several intriguing materials properties such as quantum para-electricity, charge density waves, or high temperature superconductivity.

Figure 1 Histogram plot of repeated shot noise limited pump and probe experiments. For every time step, the histogram represents the distribution of the outcome for 4000 experiments. (a) Measured distribution and (b) distribution centered at their mean value. A time dependent noise is revealed by the modulation of the width of the distribution at positive times.

The vibrations in a solid can be analyzed in terms of collective modes of motion of the atoms which are dubbed phonons. In a classical description the displacement of the atoms along the phonon eigenmodes of a crystal can be measured with unlimited precision. Conversely, in the quantum formalism positions and momenta of the atoms can be determined simultaneously only within the boundary given by the Heisenberg uncertainty principle. For this reason, in real materials, in addition to the thermal disorder, the atomic displacements are subject to fluctuations which are intrinsic to their quantum nature.
The motivation of studying the quantum proprieties of phonons in crystals comes from various evidences, suggesting that quantum fluctuations of the atoms in solids may be of relevance in determining the onset of intriguing and still not completely understood material properties, such as quantum para-electricity, charge density waves, or high temperature superconductivity.
The time evolution the atomic position in crystals is usually addressed in the framework of ultrafast optical spectroscopy by means of pump-probe experiments. In these experiments the phonon dynamics is driven by an intense ultrashort laser pulse (the pump), and then the collective excitation is investigated in time domain through the interaction with a weaker pulse (the probe). Unfortunately this method typically provides information only about the average position of the atoms at a certain time after the excitation. On the other hand different static techniques give access (indirectly) to a time integrated statistical distribution of the atomic position.
The possibility of measuring the time evolution of fluctuations of the atomic positions (quantum or thermal), beyond a time integrates statistical dystribution, is the subject of an intense scientific debate.
In our recent research a new approach to investigate quantum fluctuations of collective atomic vibrations in crystals is proposed. An original experimental apparatus that allows for the measurement of the photon number quantum fluctuations of the probe pulses in a pump and probe setup has been developed. The connection between the measured photon number uncertainty and the fluctuations of the atomic positions is given by a fully quantum mechanical theoretical description of the time domain process. Overall we prove that, in appropriate experimental conditions, the fluctuations of the lattice displacements can be directly linked to the photon number quantum fluctuations of the scattered probe pulses. Our methodology, which combines non-linear spectroscopic techniques with a quantum description of the electromagnetic fields, is benchmarked on the measurement of phonon squeezing in α-quartz.
The experimental layout is similar to standard pump and probe experiments. The sample is excited by an ultrashort pump pulse and the time evolution of the response is measured by means of a second much weaker probe pulse, that interacts with the photo-excited material at a given delay time. Both pump and probe come from the same laser source, a 250 kHz mode-locked amplified Ti:Sapphire system.

The unique characteristics of our setup are:

unlike standard experiments, where the response is integrated over many repeated measurements, our system can measure individual pulses;
the apparatus operates in low noise conditions (shot noise limited) allowing for the measurement of intrinsic photon number quantum fluctuations.

The time domain response is shown in Figure for a representative pump fluence of 14 mJ cm-2.
  Our novel experimental approach allows for the direct measurement of the photon number quantum fluctuations of the probing light in the shot-noise regime and our fully quantum model for time domain experiments maps the phonon quantum fluctuations into such photon number quantum fluctuations, thereby providing an absolute reference for the vibrational quantum noise. A quantitative analysis of noise (see Figure) and mean values allowed for a comparison of the experimental results with the predictions of the model unveiling non classical vibrational states (squeezed states) produced by photo-excitation. In particular, we demonstrated that the observation of an oscillating component in the variance of the optical transmittance at twice the phonon frequency is indicative of a squeezed phonon state. 

Figure 2 Time domain transmittance mean and variance. (a) Mean (blue curve) and variance (red curve) of the transmittance calculated over 4000 acquired pulses. The zero time is the instant in which pump and probe arrive simultaneously on the sample. In the inset a zoom of the variance for the first 3 ps is shown. (b) Wavelet analysis (Morlet power spectrum) of the variance oscillating part. (c) Fourier transforms of the oscillating parts of mean (blue curve) and variance (red curve). The dashed lines indicate the phonon frequency and twice the phonon frequency.

This research put at test a new spectroscopic approach based on the photon number statistics by investigating quantum fluctuations of simple excitations, Raman active atomic vibrational modes, in a prototype transparent system. The approach can be in principle generalized to the study of quantum fluctuations of any collective excitations in crystals, included - for example - excitations of electronic origin.

M. Esposito, K. Titimbo, K. Zimmermann, F. Giusti, F. Randi, D. Boschetto, F. Parmigiani, R. Floreanini, F. Benatti, and Fausti D. “Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics” Nature Communication, 6, 10249 (2015).

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Revealing cuprates high energy electronic excitations

In strongly correlated systems the electronic properties at the Fermi energy are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity is to understand how the high-energy scale physics is correlated to Mott-like excitations. By using a novel time-resolved optical spectrosocpy we faced the problem and give clear answers in this very hot topic.

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Ultrafast Optical Spectroscopy of Retinal Chromophore

The branching ratio of the excited-state population at the conical intersection between the S1 and S0 energy surfaces (ΦCI) was studied by multipulse ultrafast transient absorption spectroscopy. Pump–dump–probe experiments allowed to measure the photoisomerization time constant with unprecedented precision. Starting from these results, we demonstrate that the polarity of the solvent is the key factor influencing the ΦCI and the photoisomerization yield.

Article Published on J. Am. Chem. Soc., 2012, 134 (2), pp 955–961 ( Link: http://pubs.acs.org/doi/abs/10.1021/ja205763x )

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Disentangling Phononic and Eletronic Glue in HTSC

Unveiling the nature of the bosonic excitations mediating the formation of Cooper pairs is a key issue for understanding superconductivity in copper-oxyde based superconductors (cuprates). We performed optical spectroscopy on Bi2Sr2Ca0.92Y0.08Cu2O8+δ crystals with simultaneous time and frequency resolution. This technique allowed us to disentangle the electronic and phononic contributions by their different temporal evolution.

A fundamental step in understanding the mechanisms of HTC superconductivity would be to identify the relative weight of the electronic and phononic contributions to the overall frequency-dependent bosonic function Π(Ω). 
The spectral distribution of the electronic excitations and the strength of their interaction with fermionic quasiparticles fully account for the high critical temperature of the superconducting phase transition. Science 335, 1600 (2012) and http://arxiv.org/abs/1203.0588

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Evidence for Hubbard-type excitons in YVO3

By means of  broadband ultrafast pump-probe spectroscopy in the visible range we have been able to disentangle the thermal and nonthermal contributions to the optical transients across the Mott-Hubbard gap for YVO3. [http://prb.aps.org/pdf/PRB/v86/i16/e165135]

Revealing the nature of charge excitations in strongly correlated electron systems is crucial to understanding their exotic properties. Here we have used broadband ultrafast pump-probe spectroscopy in the visible range to study low-energy transitions across the Mott-Hubbard gap in the orbitally ordered insulator YVO3. Separating thermal and nonthermal contributions to the optical transients, we showed that the total spectral weight of the two lowest peaks is conserved, demonstrating that both excitations correspond to the same multiplet.

These observations, otherwise not conceivable using spectroscopies in the frequency domain, suggests that the optically pump-driven spin disorder can be used to quantify the kinetic energy gain of the excitons in the FM phase.

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Effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

By time- and angle-resolved photoemission spectroscopy we determined the evolution of the out-of-equilibrium electronic structure of the topological insulator Bi2Se3.
We found that the energy dependence of the nonequilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.

Figure Caption:
ARPES band dispersion of Bi2Se3 acquired with the 4th harmonic of our laser system, at 6.3 eV. In the figure the topological surface state (SS) and conduction band (CB) are clearly visible. The chemical potential energy is marked with μ and a green line.
The graph shows the snapshot for one particular delay time of the the pump-probe tr-ARPES. The signal is obtained as the difference between the ARPES image at +600 fs and an ARPES image at a negative delay. Red (blue) represents an increase (decrease) of the spectral weight.

The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modeling the dynamics of hot electrons after optical excitation. We disentangled a large increase in the effective temperature (T*) from a shift of the chemical potential (μ*), which is consequence of the ultrafast photodoping of the conduction band. We demonstrated that the relaxation dynamics of T* and μ* are k independent and these two quantities uniquely define the evolution of the excited charge population. 

Physical Review B 86, 205133 (2012)

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Franz-Keldysh Effect in Bulk GaAs reveals a mixed regime of light-matter interaction

The study of the FKE in bulk GaAs showed that the phase content of the selected electromagnetic pulses can be used to measure the timescales characteristic for the different regimes of matter-light interactions. This allowed us to identify a novel regime of saturation where memory effects are of relevance.

Scientific Reports 3: 1227 doi:10.1038/srep01227

Figure Caption:
(a) THz field detected by electro-optical sampling.

(b) Time-resolved variation of the transmission in GaAs as a function of pump-probe delay and probed energy.

(c) Wavelength-dependent Franz-Keldysh effect. Vertical section at t = 0 (red curve) and simulation based on the static FK effect with an applied field of 100 kV/cm (black curve).

Significant changes of the optical properties of semiconductors can be observed by applying strong electric fields capable to modify the band structure at equilibrium. This is known as the Franz-Keldysh effect (FKE). Here we study the FKE in bulk GaAs by combining single cycle THz pumps and broadband optical probes. The experiments show that the phase content of the selected electromagnetic pulses can be used to measure the timescales characteristic for the different regimes of matter-light interactions. Furthermore, the present phase-resolved measurements allow to identify a novel regime of saturation where memory effects are of relevance

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Ultrafast optical control of the ZrTe5 electronic properties

Ultrafast optoelectronics consists in the capability to manipulate electronic transport properties via light at the sub picosecond (10-12 s) time scale. In this letter, we have addressed the origin of the resistivity anomaly in ZrTe5 and we have proven the possibility to manipulate its electronic properties at the ultra-short time scale via optical excitation with laser light

Nowadays, optical switches are realized in oxides by exploiting phase transitions between metallic and insulating states. However, to meet the full integration with the current technology, optical control of semiconductors electronic properties is of pivotal importance.
In this respect, ZrTe5 represents an ideal system, which is fascinating the condensed matter community with its amazing set of transport properties. A resistivity peak is accompanied by the switch of the charge carriers, from holes to electrons. Magneto-resistivity is observed with both positive and negative sign, as a result of either the presence of three-dimensional Dirac particles or spin polarized two-dimensional Dirac particles.
Angle resolved photoemission spectroscopy (ARPES) and Time resolved ARPES measurements have been carried out at the T-Rex laboratory giving a thorough insight in the origin of the unique behaviour of ZrTe5 band structure at the Fermi level.

  We report an energy shift of the band structure across the Fermi level by varying the temperatures.
We prove the capability to control it at the ultrafast scale by changing the material (electronic and lattice) temperature with a pulsed laser pulse. Therefore, by optically controlling the band structure binding energy and the charge carriers' lifetime, we unlock the route for a unique platform for magneto, optical and thermoelectric transport applications.

This research was conducted by the following research team:

Giulia Manzoni, Università degli studi di Trieste, Trieste, Italy
Andrea Sterzi, Università degli studi di Trieste, Trieste, Italy
Alberto Crepaldi, Sincrotrone Trieste S.C.p.A., Trieste, Italy
Michele Diego, Università degli studi di Trieste, Trieste, Italy
Federico Cilento, Sincrotrone Trieste S.C.p.A., Trieste, Italy
Michele Zacchigna, CNR-IOM Trieste, Trieste, Italy
Philippe Bougnon, EPFL Lausanne, Switzerland
Helmuth Berger, EPFL Lausanne, Switzerland
Arnaud Magrez, EPFL Lausanne, Switzerland
Marco Grioni, EPFL Lausanne, Switzerland
Fulvio Parmigiani, Università degli studi di Trieste, Trieste, Italy; Sincrotrone Trieste S.C.p.A., Trieste, Italy; International faculty, University of Köln, Germany.

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Formation and relaxation of dressed quasi-particles in La2CuO4+d

For Epump > D (e), the dynamics is consistent with a ‘thermal dressing’ scenario; for Epump< (f), the ultrafast drop of kinetic energy goes in pair with the reaction of the bosonic field and a ‘coherent dressing’ mechanism takes place.

The non-equilibrium approach to correlated electron systems is often based on the paradigm that different degrees of freedom interact on different timescales. In this context, photo-excitation is treated as an impulsive injection of electronic energy that is transferred to other degrees of freedom only at later times. Here, by studying the ultrafast dynamics of quasi-particles in an archetypal strongly correlated charge-transfer insulator (La2CuO4+d), we show that the interaction between electrons and bosons manifests itself directly in the photo-excitation processes of a correlated material. With the aid of a general theoretical framework (Hubbard–Holstein Hamiltonian), we reveal that sub-gap excitation pilots the formation of itinerant quasi-particles, which are suddenly dressed by an ultrafast reaction of the bosonic field.

" Witnessing the formation and relaxation of dressed quasi-particles in a strongly correlated electron system"
Nature Communications 5, 5112 (2014)

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Speed limit of the insulator–metal transition in magnetite

The workings of the Verwey transition revealed by a pump-probe X-ray diffraction and optical reflectivity techniques.

Nature Materials

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator– metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low- temperature insulating electronically ordered phase . Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5 ± 0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

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TReX Lab Is Now Operational Inside The Fermi Building & Open To External Users


Jan: Pharos Laser is installed and operating
Jan: RegA Laser and all sources will be installed
February: The dedicated optical setups will be completed
March: First Test Experiments in the new Setup

April: Setup for 4th Harmonic Generation is operational


Last Publication
Ultrafast Ge-Te bond dynamics in a phase-change superlattice"
M. Malvestuto, A. Caretta, B. Casarin, F. Cilento, M. Dell'Angela, D. Fausti, R. Calarco, B. J. Kooi, E. Varesi, J. Robertson, and F. Parmigiani
Phys. Rev. B 94, 094310 (2016)


Notable 2016 Publications

M. Malvestuto, A. Caretta, B. Casarin, F. Cilento, M. Dell'Angela, D. Fausti, R. Calarco, B. J. Kooi, E. Varesi, J. Robertson, and F. Parmigiani
Ultrafast Ge-Te bond dynamics in a phase-change superlattice"
Phys. Rev. B 94, 094310 (2016)
G. Manzoni, A. Crepaldi, G. Autès, A. Sterzi, F. Cilento, A. Akrap, I. Vobornik, L. Gragnaniello, Ph. Bugnon, M. Fonin, H. Berger, M. Zacchigna, O.V. Yazyev, and F. Parmigiani
"Temperature dependent non-monotonic bands shift in ZrTe5"
J. Electr. Spectrosc. Relat. Phenom. (2016)

A. Sterzi, A. Crepaldi, F. Cilento, G. Manzoni, E. Frantzeskakis, M. Zacchigna, E. van Heumen, Y. K. Huang, M. S. Golden, and F. Parmigiani 
SmB6 electron-phonon coupling constant from time- and angle-resolved photoelectron spectroscopy 
Phys. Rev. B 94, 081111(R) (2016)

M. Esposito, F. Randi, K. Titimbo, G. Kourousias, A. Curri, R. Floreanini, F. Parmigiani, D. Fausti, K. Zimmermann and F. Benatti
“Quantum interferences reconstruction with low homodyne detection efficiency” 
EPJ Quantum Technology 2016, 3:7

F. Cilento, A. Crepaldi, G. Manzoni, A. Sterzi, M. Zacchigna, Ph. Bugnon, H. Berger, and F. Parmigiani
"Advancing non-equilibrium ARPES experiments by a 9.3 eV coherent ultrafast photon source"
J. Electr. Spectrosc. Relat. Phenom. 207, 7 (2016)


Prof. Fulvio Parmigiani has been awarded with the Zernike Chair 2012. The Zernike Chair is a temporary honorary professorship awarded to a world-class scientist.
Alberto Simoncig has been Awarded with the "Premio Emilio Zavattini 2009--2010" (Best Ph.D. Thesis)
Giacomo Coslovich has been Awarded with the "Premio Emilio Zavattini 2010--2011" (Best Ph.D. Thesis)
Giulio Vampa has been awarded with the "Best Graduate Poster" prize at the"Cross Border Workshop in Laser Science 2011"


Last Updated on Thursday, 23 February 2017 12:50