The laboratory is a located in a newly renovated infrastructure with stabilized temperature and humidity, with vibration isolation, an independent thermal regulation unit, and a connected to a back-up generator [https://luxem.unipv.it/facilities/]. The laboratory features:
- The ultrafast laser source.
A Ti:sapphire ultrafast regenerative amplifier that employs a patented cryogenically-cooled amplifier technology with tunable average pulse energy: > 3 mJ at 5 kHz, > 2 mJ at 10 kHz, > 0.6 mJ at 20 kHz, with 35 fs average pulse duration and 785 nm center wavelength (KMLabs). This source feeds two beamlines for: (i) ultrafast soft X-ray spectro-microscopy and (ii) ultrafast electron diffraction imaging. In each beamline, the amplified femtosecond laser beam is divided into two paths:- The pump laser pulse, which initiates the sample’s dynamics and serves as a reference point in time.
- The probe – in our case extreme ultraviolet/soft x-ray photons or electrons pulses, which record image snapshots of the samples’ dynamics as a function of the time delay
- The Ultrafast EUV/soft X-ray Microscopy beamline.
Ultrafast EUV microscopy is primarily carried-out (albeit not exclusively) with a technique for coherent diffractive imaging called Ptychography, in which multiple diffraction patterns from overlapping fields of view are processed by iterative algorithms to recover amplitude and phase images of sample and beam, separately.
The probe, i.e. EUV/soft X-ray light pulses, is generated via a non-linear process called High-Harmonic Generation (HHG). In HHG, the laser light from the tabletop femtosecond laser is coherently upconverted to shorter wavelengths while retaining excellent spatial coherence when implemented in a phase-matched geometry, as well as temporal coherence, with durations of <100 attoseconds. In our microscope, we generate bright, phase-matched beams in a hollow-core waveguide at 13 nm (1010 photons/sec, He at 400-700 torr) or 30 nm (1012 photons/sec, Ar at 30-60 torr). The fundamental driving frequency is eliminated before the sample with rejecter mirrors and metallic filters. The ultrafast microscope chamber is custom-designed for full remotely controlled in-vacuum positioning of all optical elements, the sample, and the detector, allowing for a flexible range of Numerical Apertures (NA) from 0.04-0.5, or diffraction-limited spatial (transverse) resolutions from 150 nm down to 13 nm with 13 nm illumination.
The EUV light is focused and overlapped with the pump pulses nearly collinearly onto the sample; their arrival time is controlled by a motorized delay-line. At each delay-time, EUV light is scattered by the sample and it is collected on an in-vacuum EUV CMOS detector with 11 µm pixel size at a maximum rate of 24 fps (AXIS Photonique). The dynamic response of each sample is captured by collecting stroboscopic images as a function of time delay between pump and probe pulses, acquiring amplitude and phase snapshots of a nanoscale movie. - The Ultrafast Electron Diffraction Imaging beamline.
In this beamline for time-resolved electron diffraction, we also develop new methods for image reconstruction and speckle detection from compact sources with partial spatial coherence.
The amplified femtosecond pulse trains are divided into the pump (optical) and the probe (non-relativistic electron bunches) paths with a beam-splitter. The probe beam is frequency-tripled by third harmonic generation in nonlinear BBO crystals, and it is used to generate the probing electron pulses inside a high-voltage (0-100 kV) DC gun. The electron probe pulses are directed to the sample with two steering plates for the orthogonal deflection of the beam, and a solenoid, to focus the beam onto the sample. The sample is contained in the experimental chamber, which is designed to have openings for the incoming probe pulses, the pump optical access, the detector and a cryogenic system that allows to explore a range of temperatures from 1.8 K up to 300 K. Additional openings are reserved for sample vision & alignment, and pressure gauges. A high-precision manipulator allows the motion of the sample along three axis (x, y, z) and one angle (the rotation around the cryostat axis). The angular rotation of the sample around its surface normal is also possible through a piezo-electric sample holder, which is in thermal contact with the cryostat.
An optical delay line allows to change the time of arrival on the sample between pump and probe pulses. The electron pulses scatter from the sample and diffraction images are detected by the Hybrid Particle Counting pixelated detector QUADRO (DECTRIS), which is capable of single-shot detection and one-to-one conversion efficiency between electrons and output counts.
The beamlines are fully custom designed within LUXEM, in collaboration with international academic and industrial partners. They both are implemented in ultra-high vacuum [10-7 to 10-10 mbar] on a highly-stabilized vibration damped optical table, which also hosts the ultrafast laser source. The set-ups are designed to run in parallel at 5 kHz or, separately, at higher rep-rates. Both beamlines feature capabilities for experiments in transmission or reflection geometry with high samples throughput and full remotely controlled actuation systems. Their operating configurations are selected depending on the signal-to-noise, flux, jitter, dose, required for each experimental campaign.
Responsible: Giulia Fulvia Mancini