FERMI – Lightsources.org

Ultrafast all-optical spin injection in silicon revealed at FERMI

2022/11/282022/11/29

A revolutionary and energy-efficient information technology encoding digital data in electron spin (spintronics) by combining semiconductors and ferromagnets is being developed worldwide. Merging of memory and logic computing of magnetic based storage devices and silicon-based logic transistors is expected to ultimately lead to new computing paradigms and novel spin-based multifunctional devices. The advantages of this new technology would be non-volatility, increased data processing speed, reduced electric power consumption. All of them are essential steps towards next generation quantum computers.

To create spin-based electronics with potential to revolutionize information technology, silicon, the predominant semiconductor, needs to be integrated with spin functionality. Although silicon is non-magnetic at equilibrium, spin polarized currents can be established in Si by a variety of approaches such as the use of polarized light, hot electrons spin injection, tunnel spin injection, Seebeck spin tunneling and dynamical spin pumping methods, as had been demonstrated recently. In general, spin polarized currents refer to the preferential alignment of the spin angular momentum of the electrons in a particular direction.

If at first you don’t succeed…….

2022/03/072022/03/07

For our final Monday Montage, we focus on the benefits of perseverance and our ability as humans to dig deep and remain determined even when things are going wrong. Maximilian Obst, one of the FELBE (HZDR) users, and Michele Manfredda, from FERMI at Elettra, give honest and insightful accounts of their light source experimental experiences. Light sources are complex scientific tools. They are challenging to build, optimise and utilise. But by continuing to overcome obstacles and working as a team, great and unexpected results can appear. Often these results are obtained in the middle of the night or towards the end of a very long shift on the beamline. However, the lucky few who observe them realise they are the first people in the world to have gleaned this knowledge. Perseverance pays off!

One of the most exciting things is being part of the community

2022/01/192022/01/19

FERMI #LightSourceSelfie

Michele Manfredda is an Italian physicist working at FERMI, the Free Electron laser Radiation for Multidisciplinary Investigations, near Trieste in Italy. Michele words in the PADReS group, which stands for photon analysis delivery and reduction system. The group’s role is to make experiments possible for FERMI users and they look after the optics and diagnostics of the light. As Michele explains, the role involves working in different places and with different teams. His #LightSourceSelfie takes viewers on a fantastic tour of FERMI.

Michele explains that he was first attracted to this field of research by the fact that simple things are done in a very complicated way. When it comes to advice that Michele would give those starting out in their careers, he says, “The advice I would give to someone entering the world of large facilities is go for it. They are crazy environments and you will enjoy it, but remember large facilities can be very time-consuming. So always keep in mind what you can give to science and what science can give you back. Also, find the right people. People you can learn from and people you like to work with because remember, science facilities are wonderful creations but the most wonderful creation is your career, your life. So, as an optical physicist, I tell you don’t be focused on your sample only, be focused mostly on you.”

New technology on the beamlines gifts sleep back to staff and users

2022/01/172022/01/17

Dohyun Moon, Beamline Senior Scientist at Pohang Light Source II in Korea, and Michele Manfredda, Scientist in the Photon Transport Group at FERMI in Italy, talk about new technology that is delivering remote control, automation and robot systems. All of these advances reduce the need for humans to be on the beamlines round the clock.

As Michele says, “The best science that we can do at a light source is the one that we do when we sleep and the machines and computers work.”

Disclosing the time evolution of magnetic chirality after an optical excitation

2021/02/032021/02/08

Chiral magnetic structures, such as spin spirals, chiral domain walls and skyrmions, are intensively investigated due to their fascinating properties such as potentially enhanced stability and efficient spin-orbit torque driven dynamics. These structures are stabilized by the Dzyaloshinskii-Moriya interaction (DMI) that favours a chiral winding of the magnetisation.Cir

In a recent work, circularly polarized light pulses of the FERMI free-electron laser (FEL) has been used to disclose the dynamics of chiral order on ps time scale. After an optical excitation, the researchers observe a faster recovery of the chirality within the domain walls compared to the ferromagnetic order in the domains. The study paves the way for future investigations of fundamental aspects such as, e.g., the dependence of the timescales of the chiral order build-up on the absolute strength of the DMI. The control of the DMI can finally allow the manipulation at ultrafast timescale of chiral topological objects such as skyrmions and pave the path to applications in the field of ultrafast chiral spintronics.

Rad more on the Elettra website

Electron and X‑ray Focused Beam-Induced Cross-Linking in Liquids:

2020/10/262020/10/27

Toward Rapid Continuous 3D Nanoprinting and Interfacing using Soft Materials

Modern additive fabrication of three-dimensional (3D) micron to centimeter size constructs made of polymers and soft materials has immensely benefited from the development of photocurable formulations suitable for optical photolithography,holographic,and stereolithographymethods. Recent implementation of multiphoton laser polymerization and its coupling with advanced irradiation schemes has drastically improved the writing rates and resolution, which now approaches the 100 nm range. Alternatively, traditional electron beam lithography and its variations such as electron-beam chemical lithography, etc. rely on tightly focused electron beams and a high interaction cross-section of 0.1−10 keV electrons with the matter and have been routinely used for complex patterning of polymer resists, self-assembled monolayers, and dried gel films with up to a few nanometers accuracy.

Similarly, a significant progress has been made in deep X-ray lithography, direct writing with zone plate focused X-ray beams for precise, and chemically selective fabrication of high aspect ratio microstructures. Reduced radiation damage within the so-called “water window” has spurred wide biomedical X-ray spectroscopy, microscopy, and tomography research including material processing, for example, gels related controlled swelling and polymerization inside live systems, particles encapsulations,and high aspect ratio structures fabrication.The potential of focused X-rays for additive fabrication through the deposition from gas-phase precursors or from liquid solutions is now well recognized and is becoming an active area of research.

Read more on the Elettra website

Image: The electron/X-ray beam gelation in liquid polymer solution through a SiN ultrathin membrane. Varying the energy and focus of the soft X-rays smaller or larger excitation volumes and therefore finer or wider feature sizes and patterns can be generated.

Orbital angular momentum carried by an optical field can be imprinted onto a propagating electron wave

2020/10/232020/10/23

Photons have fixed spin and unbounded orbital angular momentum (OAM). While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wavefront. The distinctive way in which the photon spin dictates the electron motion upon light–matter interaction is the basis for numerous well-established spectroscopies. By contrast, imprinting OAM on a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable.

We carried out an experiment at the LDM beam line at the FERMI free-electron laser, with the aim of inducing an OAM-dependent dichroic photoelectric effect on photo-electrons emitted by a sample of He atoms. The experiment involved a large international collaboration and surprisingly confirmed that the spatial distribution of an optical field with vortex phase profile can be imprinted coherently on a photoelectron wave packet that recedes from an atom. Our results explore new aspects of light–matter interaction and point to qualitatively novel analytical tools, which can be used to study, for example, the electronic structure of intrinsic chiral organic molecules. The results have been published in Nature Photonics.

Read more on the Elettra website

Image: A VUV free-electron laser (violet) is used to ionize a sample of He atoms, and an infrared beam (red) to imprint orbital angular momentum on photo-emitted electrons. Credit: J. Wätzel (Halle university)

Coherent soft x-ray pulses from an echo-enabled free-electron laser

2019/05/092019/05/13

The free-electron laser (FEL) FERMI is a unique facility, providing users with laser-like pulses in the XUV spectral range. At FERMI, the generation of highly coherent pulses, with tunable spectro-temporal properties, relies on the so-called high-gain harmonic generation (HGHG) technique. In the latter, a (single) infrared seed laser is used to shape the electron-beam properties and trigger the amplification process. Amplification occurs at one selected harmonic, h, of the seed. However, in HGHG, the seed energy required to prepare the electron beam for FEL emission becomes larger and larger for higher harmonics (i.e., shorter FEL wavelengths). For high harmonics, the resulting strong electron-beam energy modulation reduces the FEL gain, limiting the scheme to h<15 (wavelengths of about 10-20 nm) for a single HGHG scheme, or to h of the order of 60-70 (i.e., 4-5 nm), in case of two-stage HGHG. Moreover, at such high h, the sensitivity to the shape of the electron-beam phase space becomes critical and may severely affect the FEL radiation in terms of longitudinal coherence, pulse energy, and shot-to-shot stability. In addition, the HGHG scheme cannot cover the whole harmonic range, as the final harmonic number is a product between the harmonic numbers of the individual stages. Last, but not least, the two-stage setup uses a relatively large portion of the e-beam to accommodate the double seeding process, which makes the implementation double-pulse operation difficult.
The drawbacks of the two-stage HGHG can be overcome by using a recently proposed technique called echo-enabled harmonic generation (EEHG), where the electron-beam is shaped using two seed lasers to enable FEL emission at high harmonics. The method requires a much weaker energy modulation compared to HGHG and is also intrinsically less sensitive to the initial electron-beam imperfections, making it a strong candidate for producing highly stable, nearly fully coherent, and intense FEL pulses, down to soft x-ray wavelengths.

Image: The EEHG scheme together with the e-beam phase space at different stages of the evolution. The 1st seed laser with a wavelength λ₁ imprints a sinusoidal energy modulation with an amplitude ΔE<3σ_E, σ_E is the initial uncorrelated energy spread, onto the relativistic e-beam in the 1st modulator. After passing through a strong 1^st chicane, the electrons with different energies move relative to each other, resulting in a striated phase space with multiple energy bands. Importantly, the energy spread within a single band is much smaller than σ_E. The electrons then pass through the 2^nd modulator, where their energy is again periodically modulated using a 2^nd seed laser with λ₂= λ₁ and ΔE₂≈ΔE₁. After traversing a weaker 2^nd chicane, the e-beam phase space is rotated, transforming the sinusoidal energy modulation into a periodic density modulation, with high-frequency components. As the energy spread within a single band is much smaller than σ_E, only a moderate ΔE₂ is required to reach very high harmonics. The e-beam is then injected into the radiator, tuned to emit light at a high harmonic of the 2^nd seed laser.

Signatures of enhanced superconducting phase coherence in cuprates

2019/03/142019/03/21

The capability to control material properties on short timescales is one of the key challenges of modern condensed matter physics. This possibility becomes even more attractive in the case of intriguing material phases, such as superconductivity. As a matter of fact, despite the evolution of non-equilibrium spectroscopies of the last two decades have increased our understanding of the physics of strongly correlated materials, after more than 30 years from its discovery, High Temperature Superconductivity is still discussed and a clear and unanimous explanation of the origin of the phenomenon is still lacking. Moreover, the understanding of the phenomena at the basis of this effects could affect several technological applications, from the need for fast digital circuits and for speeding up computer performances, to the detection of very low magnetic fields, with implication in geology (mineral exploration and earthquake prediction), medical sciences (neuron activity and magnetic resonance), oil prospecting and, of course, research.
We focused our research on cuprates, a class of materials known for displaying unconventional superconductivity at relatively temperatures, and on which various studies have shown the possibility of turning off (and, to some extent, on) superconductivity by ultrashort light pulses. In our work, we reveal that light pulses characterized by long wavelength (and a peculiar polarization) can induce, for a very short time interval (1-2 ps), a state displaying superconductivity even above the critical temperature, i.e. in conditions where superconductivity is not observed at equilibrium.

Figure: Difference between the transient reflectivity due to Cu-Cu and Cu-O polarized pump in time and temperature, induced by excitations with (a) 70 and (b) 170 meV pump photon energies. The dashed lines highlight the critical temperature Tc.

$In-situ single-shot diffractive fluence mapping for x-ray FEL pulses$

In-situ single-shot diffractive fluence mapping for x-ray FEL pulses

2018/12/072018/12/19

Free-electron lasers (FEL) for the extreme-ultraviolet (XUV) and x-ray regime opened up the possibility to investigate and exploit non-linear processes in the interaction of x-rays with matter. Such processes are of considerable interest in numerous research fields, owing to the huge impact of non-linear techniques on optics and spectroscopy in the visible and near-visible spectral range. Generating and understanding non-linear effects requires sophisticated control of the sample illumination. This is especially challenging at FEL sources, where variations of the spatial fluence distribution on a single-shot basis are common. Moreover, the focused spot often exhibits a complex internal structure due to diffraction artefacts from the focusing optics. These factors cause considerable uncertainties with respect to the effective fluence on a solid sample for scattering experiments in the forward direction.
We demonstrate a flexible solution for true in-situfluence monitoring on solid samples in transmission-type diffraction experiments. Our concept measures the detailed beam footprint on the actual sample under study. The image of the illumination is recorded simultaneously with the specimen’s primary scattering signal on a two-dimensional detector. This is facilitated by a shallow grating structure of only a few nanometer depth that is lithographically fabricated into the sample carrier membrane. Such membranes are routinely used in transmission-type diffraction experiments as a transmissive structural support for thin-film or sparsely dispersed samples. The grating structure forms a diffractive optical element that maps the spatial fluence distribution on the sample to a configurable position on the detector.

Image: Figure 1. a) Single-shot diffraction image of a sample with grating-based fluence monitor and ferromagnetic domains on a logarithmic false-color scale. The ring-shaped structure is due to the magnetic domains, while the fluence monitor grating gives rise to the brighter patterns on the image diagonals. Both grating patterns are equivalent images of the beam footprint on the sample. b) Enlarged detail of the diffracted fluence map on the sample on a linear false-color scale. c) AFM image of a single-shot damage crater in the sample’s silicon substrate. The pattern observed matches the in-situ measured beam footprint very well, but belongs to a different FEL shot. Scale bars are 10µm. Adapted from M. Schneider et al., Nature Communications 9, 214 (2018)

When is a laser a real laser?

2018/10/312018/11/04

Pulsed lasers are intense and coherent light sources, and the latest category is that of Free Electron Lasers, such as FERMI. First order coherence is a familiar phenomenon, and is manifested for example in diffraction phenomena. This represents the correlation between the amplitudesof a wave at different points in space (transverse coherence) or time (longitudinal coherence.) However, a high degree of first order coherence is not enough to define a laser, according to the Nobel laureate Roy Glauber, who stated that a laser can be defined as a source that is coherent in all orders. The higher order correlations are between intensityat different points in time and space. How are these correlations measured? For this one has to look at the statistics of the photons.
Glauber’s work was inspired by the famous Hanbury Brown and Twiss experiment, in which coincidences of photons (i.e. correlations) were measured of photons coming from distant stars. By varying the distance between two detectors, they were able to determine the degree of coherence of the star, and extract other information. This is the key to measuring the second order coherence of a light source: the intensity of light at different points is measured in coincidence, and statistical analysis is made. This experiment is considered by many as initiating the whole field of quantum optics. Now a team led by Ivan Vartaniants (DESY, Hamburg, and the National Research Nuclear University, Moscow) has performed a Hanbury Brown and Twiss experiment at FERMI. Instead of the two discrete photodetectors used originally, a CCD detector was used. Since all of the photons arrive in less than 100 fs, there is no need to use coincidence methods: the signal is naturally synchronised.

Figure 1. Difference between chaotic and coherent light sources. (a) photon correlation map for FERMI operated in seeded mode. (b) corresponding spectrum. (c) correlation map for FERMI operated in Self Amplified Stimulated Emission mode (the mode of operation of most Free Electron Lasers). (d) corresponding spectrum.
Credit: Reprinted from O. Yu. Gorobtsov et al, Nature Communications 9 (2018) 4498. (Copyright Nature Publishing Group)

Steering the outcome of photoionization in a molecule

2018/10/152018/10/25

An important step towards the understanding and control of photoinduced fragmentation processes in molecules has been achieved in an experiment on the H₂ molecule taking advantage of the unique properties of the FERMI free-electron laser source in the vacuum ultraviolet (VUV) photon energy range.
Molecular dissociation, i.e., the breaking of a chemical bond, is governed by the coupling of electronic and nuclear motion and, once understood and controlled in large systems, e.g., by utilizing ultrashort light pulses, has the potential to impact tremendously photochemical and biochemical applications. A team of both experimentalists and theoreticians from France (CNRS, Université Paris-Sud, Université de Bordeaux), Spain (Universidad Autónoma de Madrid), Germany (European XFEL), and Italy (Elettra-Sincrotrone Trieste) has demonstrated that the outcome of dissociative (DI) and nondissociative (NDI) photoionization in the simplest of all molecules, H₂, can be controlled exploiting nonlinear two-photon ionization with intense femtosecond pulses in the VUV.
The FERMI seeded free-electron laser is currently the only light source worldwide that provides external users access to bright femtosecond pulses at wavelengths in the VUV up to 100 nm, the energy regime required for studying nonlinear two-photon single-ionization in H₂. The high spectral resolution and precise tunability of the 100-fs pulses provided by FERMI made it possible to selectively excite single vibrational levels in the neutral intermediate B state of H₂ (blue line in Fig. 1). Absorption of a second VUV photon then leads to NDI or DI into the ionic H₂⁺ ground state (green in Fig. 1) or to DI into the first excited H₂⁺2p continuum (orange in Fig. 1). In single-photon single-ionization of H₂, the yield of DI is very low – less than 2%. By contrast, recent ab initiocalculations suggest that the ratio of DI/NDI can be increased significantly in resonance-enhanced two-photon ionization and that it can be controlled by varying the pulse duration between 2 and 10 fs.

>Read more on the Elettra website

Image: (a) Schematic of resonant two-photon ionization viathe B intermediate state (12.51 eV). The grey shaded area shows the Franck-Condon region for one-photon absorption from the H2electronic ground state. The dashed purple arrows visualize the range for the absorption of the second FEL photon. The green (red) horizontal line shows the ionization threshold at 15.43 eV (dissociation limit at 18.08 eV). (b) The experimental photoelectron spectrum shows a clear separation of electrons correlated to NDI and DI. For DI, it is close to the prediction of the Condon-reflection approximation, i.e., the projection of the vibrational wavefunction onto the dissociative 2p continuum state. The infinite-time limit calculation (grey line for the convolution of the contributions from the two first ionization continua) reproduces the main features of the spectrum. The differences between experiment and calculation indicates that at FERMI a timescale between ultrafast dynamics and steady-state excitation is probed.

Ultralow-fluence for phase-change process

2018/09/142018/09/19

Ultrafast active materials with tunable properties are currently investigated for producing successful memory and data-processing devices. Among others, Phase-Change Materials (PCMs) are eligible for this purpose. They can reversibly switch between a high-conductive crystalline state (SET) and a low-conductive amorphous state (RESET), defining a binary code. The transformation is triggered by an electrical or optical pulse of different intensity and time duration. 3D Ge-Sb-Te based alloys, of different stoichiometry, are already employed in DVDs or Blu-Ray Disks, but they are expected to function also in non-volatile memories and RAM. The challenge is to demonstrate that the scalability to 2D, 1D up to 0D of the GST alloys improves the phase-change process in terms of lower power threshold and faster switching time. Nowadays, GST thin films and nanoparticles have been synthetized and have beenshown to function with competitive results.
A team of researchers from the University of Trieste and the MagneDyn beamline at Fermi demonstrated the optical switch from crystalline to amorphous state of Ge2Sb2Te5nanoparticles (GST NPs) with size <10 nm, produced via magnetron sputtering by collaborators from the University of Groeningen. Details were reported in the journal Nanoscale.
This work aims at showing the very low power limit of an optical pulse needed to amorphize crystalline Ge2Sb2Te5 nanoparticles. Particles of 7.8 nm and 10.4 nm diameter size were deposited on Mica and capped with ~200nm of PMMA. Researchers made use of a table-top Ti:Sapphire regenerative amplified system-available at the IDontKerr (IDK) laboratory (MagneDyn beamline support laboratory) to produce pump laser pulses at 400 nm, of ~100 fs and with a repetition rate from 1kHz to single shot.

Image (extract): Trasmission Electron Microscopy image of the nanoparticles sample. Ultafast single-shot optical process with fs-pulse at 400 nm. Microscope images of amorphized and amorphized/ablated areas obtained on the nanoparticles sample. Comparison of amorphization threshold fluences between thin films and nanoparticles cases.
Please see here the entire image.

LEAPS and FELs of Europe meetings at Elettra

2018/03/192018/03/28

On March 12-13 Elettra-Sincrotrone Trieste hosted the 2nd meeting of General Assembly (GA) of the League of European Accelerator-based Photon Sources (LEAPS), a strategic consortium that includes 16 Synchrotron Radiation and Free Electron Laser (FEL) user facilities in Europe based in 10 different European countries .
This followed the LEAPS Launch Event in Brussels on November 13, 2017. The main topics of the GA meeting were the LEAPS Governance Structure and the LEAPS Strategy Paper to be forwarded to the EU Commission during the Bulgarian Presidency Conference on Research Infrastructures in Sofia, 22-23 March.

Image: LEAPS General Assembly and Coordination Board group picture.
Credit: Fotorolli

High coherence and intensity at FERMI enables new X-ray interfacial probe

2018/02/052018/03/14

Interfaces are involved in a wide range of systems and have significant implications in many fields of scientific and technological advancement, often determining device performance or chemical reactivity. Vital examples include solar cells, protein folding, and computer chips. A class of commonly used surface science techniques are comprised of even-ordered nonlinear spectroscopies (i.e., second harmonic and sum frequency generation) which exhibit no response in centrosymmetric media due to symmetry constraints.As a result, they have been widely used at optical wavelengths to explore physical and chemical properties of interfaces, where centrosymmetry is broken. Extending this to x-ray wavelengths would effectively combine the element specificity and spectral sensitivity of x-ray spectroscopy with the rigorous interfacial/surface specificity of optical even-ordered nonlinear spectroscopies. Unfortunately, at hard x-ray energies (x-ray wavelength order of the spacing between atoms) these even-ordered nonlinear spectroscopies are effectively bulk probes, as each individual atom breaks inversion symmetry. As soft x-ray wavelengths fall in between the UV and hard x-ray regimes, there has been uncertainty regarding the interface specificity of soft x-ray second harmonic generation.

>Read more on the FERMI webpage

Figure: (extract) Experimental Design. X-ray pulses are passed through a 2 mm iris and focused onto the graphite sample at normal incidence. The transmitted beam is then passed through a 600 nm aluminum filter and onto a spectrometer grating, spatially resolving the second harmonic signal from the fundamental. Inset: A schematic energy level diagram of the second harmonic generation process. (entire figure here)

Extreme-ultraviolet vortices from a free-electron laser

2018/02/052018/02/14

Extreme-ultraviolet vortices may be exploited to steer the magnetic properties of nanoparticles, increase the resolution in microscopy, and gain insight into local symmetry and chirality of a material; they might even be used to increase the bandwidth in long-distance space communications. However, in contrast to the generation of vortex beams in the infrared and visible spectral regions, production of intense, extreme-ultraviolet (XUV) and x-ray optical vortices still remains a challenge. Here, we present an in-situ and an ex-situ technique for generating intense, femtosecond, coherent optical vortices with tunable topological charge at a free-electron laser (FEL) in the XUV.

The first method takes advantage of nonlinear harmonic generation in a helical undulator and exploits the fact that such harmonics carry a topological charge of l = n-1, where n is the harmonic number. The experiment was performed at the FERMI FEL. An ultraviolet (250-nm) seed laser was used to energy modulate the electron beam (e-beam) in the first undulator (modulator), as shown in the top panel of Figure 1. The e-beam was then sent through a dispersive section (a four-dipole-magnet chicane), where the energy modulation was transformed into a current-density modulation (bunching) with Fourier components spanning many harmonics of the seed laser frequency. Such a bunched e-beam entered the helical radiator tuned to a fundamental wavelength of 31.2 nm (i.e., the 8th harmonic of the seed), producing coherent light in the XUV. The FEL was operated in the high-gain regime, close to the saturation point. Under these conditions, the interaction between the radiation at the fundamental FEL wavelength and the e-beam induced bunching at the second harmonic (15.6 nm), resulting in emission of coherent XUV vortices carrying unit topological charge (l = 1) at intensities on the order of 10⁻³ of the fundamental FEL emission; see bottom panel in Figure 1.

>Read more on the FERMI website

Image:
Top: The scheme to generate optical vortices at harmonics (in the present case at the 2^nd harmonic) of the fundamental FEL wavelength. The optical vortex is separated from the fundamental FEL emission using a Zr filter.
Bottom: Intensity profile of the generated optical vortex with a topological charge of l =1 (left), and interference with a Gaussian beam revealing the twisted nature of the vortex (right).