The goal of the laboratory is the generation and application of attosecond pulses to ultrafast molecular dynamics.
The laboratory, originally realized under the ERC (European Research Council) Advanced Grant “ELYCHE” (Electron-scale dynamics in chemistry), is currently funded by the ERC Synergy Grant “TOMATTO” (The ultimate time scale in organic molecular opto-electronics, the attosecond).
The main research directions are the following:
- Organic optoelectronic materials: study and coherent control of the electron dynamics and photo-induced charge transfer processes in molecular materials with high interest for optoelectronic applications, such as photovoltaic devices, molecular wires, artificial photosynthesis.
- Attochemistry: study of the interaction of light with polyatomic molecular systems by triggering ultrafast electronic motions, with the goal of controlling the chemical properties of molecules through a time-resolved manipulation of the electronic degrees of freedom.
- Ultrafast processes in bio-relevant molecules: study of the electron dynamics in biomolecules like amino acids or DNA bases. The main research focus is the understanding of the elementary processes which play a fundamental role in many relevant biological mechanisms, e.g. photodamage and the transmission of bio-signals in proteins and DNA.
LASER SOURCE
Spectral broadening inside the hollow-core fiber.
The driving source is a Ti:Sapphire amplified laser system which generates 25 fs pulses with an energy of 10 mJ, repetition rate of 1 kHz and stabilized carrier envelope phase (Coherent Legend Elite DUO). The radiation can be converted from 800 nm to the 1 – 2.5 um range thanks to a High-Energy Optical Parametric Amplifier (Light Conversion TOPAS HE PRIME), allowing for ponderomotive scaling of HHG.
A portion of the laser beam is post-compressed by using the hollow fiber technique (in a pressure gradient scheme) in combination with ultrabroadband chirped mirrors (PC1332, Ultrafast Innovations) down to 4 fs (1.5 optical cycles) with an energy of 1.5 mJ. The spectrum covers the entire visible (VIS) range, extending in the near-infrared (NIR) up to 1000 nm.
SH-DSCAN trace (1.8 bar of Helium) measured at the output of the HCF compressor and retrieved pulse duration.
Laser output spectrum and broadened spectrum at the HCF output.
UV/VIS/NIR PUMP – EUV PROBE ATTOSECOND BEAMLINE
The XUV attosecond beamline for molecular spectroscopy.
The 1.5 mJ 4 fs pulses centered at 800 nm are the seed of the EUV attosecond beamline. A fraction of the beam is used for generating Extreme UltraViolet (EUV) radiation through High Order Harmonic generation (HHG) in a semi-infinite gas cell, offering a higher photon yield compared to a standard short generation medium. Recombination of the residual 4 fs beam with the generated EUV in an actively-stabilized interferometer enables VIS/NIR – EUV attosecond spectroscopy and metrology. In particular, temporal characterization of the EUV pulses generated in Argon is achieved by Attosecond Streaking in an Argon gas target: the resulting pulse duration is as short as 180 as (FWHM).
Experimental attosecond streaking trace in Argon and the retrieved attosecond pulse
Spectrum of the isolated attosecond pulse.
We extend the excitation wavelength range at our avail to the ultraviolet (UV) via Resonant Dispersive Wave (RDW) emission, a nonlinear process allowing to obtain few-fs pulses with tuneable central wavelength from Deep UV (200 nm – 300 nm) to UV (300 nm – 400 nm) and high efficiency. RDW in gas-filled HCFs allows us to resonantly address specific molecular excitations in the systems under investigation while preserving the high temporal resolution of the experiment.
Wavelength tuneability of the RDW (generated in Neon) seeded at 800 nm.
All-in-vacuum SD-FROG trace of the RDW generated in Neon (5 bars) and retrieved pulse duration.
The RDW emission process is driven by the residual 4 fs beam in an HCF integrated in beamline. The HCF is filled with noble gas at the input and connected to the vacuum chamber to avoid UV dispersion after generation, allowing to obtain pulses with wide central wavelength tuneability. The time duration of such pulses has been characterized ex situ in a dedicated setup, being shorter than 3 fs. After the spectral separation of the generated UV from the driving VIS-NIR, the measured UV energy is in the order of hundreds of nJs. It is then recombined with the EUV attosecond pulses on target.
CHARGED-PARTICLES DETECTORS AND MOLECULAR SOURCES
To follow the UV/VIS/NIR-induced dynamics, the EUV probe ionizes the sample, typically producing an electron and different ions, which are obtained by the fragmentation of the parent ion. Depending on the type of information desired from the experiment, the properties of these charged particles can be measured.
A Time-Of-Flight (TOF) electron spectrometer (KAESDORF ETF15) can be used to measure photoelectron kinetic energy distribution (or PES). A Velocity Map Imaging (VMI) accesses also the photoelectron angular distribution by projecting the photoelectron distribution on a 2D detector. Both instruments can be used to perform time-resolved valence photoelectron spectroscopy. Additionally, they can be employed for attosecond measurements such as RABBITT or Streaking spectroscopy. Alternatively, an ion TOF (KAESDORF ETF15) can be used to measure the mass spectrum of the emitted ions. This gives information regarding the different fragmentation pathways and how they are modified by the pump. Currently, we are developing a new spectrometer which will allow to measure electrons and ions simultaneously, in order to perform covariance analysis.
Typically, the samples are injected into our experimental setup effusively . We can inject molecular gases, liquid samples and solid samples (provided that their vapor pressure is sufficiently high) which are evaporated directly into vacuum.
A self-built molecular oven can also be exploited to sublimate molecules which are found in powder form at room temperature. They are then injected through vacuum using a continuous flow of noble gas as a buffer.
Alternatively, a Parker valve can be used to pulse the molecular jet, in order to attain supersonic expansion, with a repetition rate of 100 Hz. We are currently developing a new source setup to integrate an Even-Lavie valve, which would allow 1 kHz repetition rate and higher sample density.
Recent publications:
- F. Vismarra, M. Fernández-Galán, D. Mocci, L. Colaizzi, V. W. Segundo, R. Boyero-García, J. Serrano, E. Conejero-Jarque, M. Pini, L. Mai, Y. Wu, H. J. Wörner, E. Appi, C. L. Arnold, M. Reduzzi, M. Lucchini, J. San Román, M. Nisoli, C. Hernández-García, R. Borrego-Varillas “Isolated attosecond pulse generation in a semi-infinite gas cell driven by time-gated phase matching,” Light: Science&Applications 13:197, (2024).
- M. Reduzzi, M. Pini, L. Mai, F. Cappenberg, L. Colaizzi, F. Vismarra, A. Crego, M. Lucchini, C. Brahms, J. C. Travers, R. Borrego-Varillas, M. Nisoli “Direct temporal characterization of sub-3-fs deep UV pulses generated by resonant dispersive wave emission,” Opt. Express 31, 26854(2023).