Powered by Indico
v3.2.9
Molecular chirality is a central property in Nature. However, as biological functions are homochiral, a central issue in relation to various applications is the separation and identification of enantiomers of chiral molecules. Regarding the latter, the use of short-wavelength spectroscopic methods is gaining momentum, as the sensitivity scales inversely with the wavelength of light.
Both X-ray circular (CD) and X-ray helical dichroism (HD) are being explored. Furthermore, non-linear spectroscopies in the optical domain have been shown to be promising tools for the identification of enantiomers, and are being considered in the extreme ultraviolet (EUV) to X-ray domains, as this would add element-selectivity to the enantio-selectivity. Last, when light carries an orbital angular momentum, it becomes a more reactive chiral reagent and enhances the sensitivity in distinguishing enantiomers.
The goal of the workshop NETLINCS is to capitalise on the momentum created by these various novel experiments and theoretical developments and explore and discuss strategies for their successful outcome, especially at XFELs, but also at synchrotrons and with lab-based HHG sources. It will deal with gas, liquid and solid molecular systems.
NETLINCS is a workshop co-organized by COST Action CA22148 - An international network for Non-linear Extreme Ultraviolet to hard X-ray techniques (NEXT) and Elettra-Sincrotrone Trieste.
Credits for the logo: Ella Maru Studio, Inc.
A molecule is chiral if it lacks both a plane and a center of symmetry, resulting in two non-superimposable mirror-image forms known as enantiomers. Chirality is crucial in the physical and biological sciences because enantiomers, despite having the same chemical structure, often exhibit different chemical and physical properties when interacting with other chiral entities. While chirality is extensively studied in biology, chemistry, and pharmaceutics, the ultrafast dynamics of chiral compounds remain largely unexplored. A promising technique for investigating these dynamics is Time-Resolved Photo-Electron Circular Dichroism (TR-PECD, which uses an ultrashort circular pulse to ionize the molecule from its electronically excited states, revealing time-dependent dichroism through the asymmetry in photoelectron angular distribution. However, interpreting TR-PECD experiments is challenging due to the non-local nature of the initial excited state.
Recently, Time-Resolved X-ray Photoelectron Spectroscopy (TR-XPS) has shown potential in probing local relaxation dynamics of electronically excited states with chemical specificity. TR-XPS can distinguish different atomic sites by analyzing their transient excited-state chemical shifts.
Recently, we combined the chemical and site-specificity of TR-XPS with the enantio-sensitivity of TR-PECD to study the relaxation dynamics of photoexcited fenchone molecules at the carbon K-edge. This experiment demonstrates that femtosecond chiral dynamics can be effectively probed using core-level spectroscopy with circularly polarized XUV light provided by the FERMI free-electron laser. Additionally, core-level PECD spectroscopy of transient excited states enables us to isolate different carbon atoms within the molecule through their distinct chemical shifts, enhancing the site-specificity of the technique. This allows for separating and identifying PECD from various C-atom sites in the molecule that would otherwise be inaccessible.
Furthermore, we will discuss preliminary results of other experiments performed at FERMI and XFEL to extend this approach to other relevant ultrafast chiral dynamics.
Within a bottom/up approach of molecular complexity, the study of isolated, substrate- and solvent-free chiral species is crucial, but classical chiroptical probes such as Circular Dichroism (CD) in absorption are poorly adapted to dilute matter because of their very weak associated asymmetries.
At the opposite, two decades ago was introduced a new chiroptical effect called Photoelectron Circular Dichroism (PECD), allowed in the electric dipole approximation, leading to very intense (up to 40 %) forward/backward asymmetries, with respect to the photon axis, in the angular distribution of photoelectrons produced by circularly polarized light ionization of gas phase pure enantiomers. PECD happens to be a universal, orbital-specific, photon energy dependent chiroptical effect and is a subtle probe of the molecular potential, on which the photoelectron is scattered off, being very sensitive to static molecular structures such as conformers, isomers, clusters, as well as to vibrational motion, much more so than other observables in photoionization such as the cross section (Photoelectron Spectrum-PES) or the usual (achiral) asymmetry parameter (for a review see [1]). Therefore, PECD studies have a dual fundamental and analytical interest. This last aspect is probably the driving force for the recent extension and blooming of PECD studies, beyond the pioneering synchrotron radiation works, towards the laser (and now FEL) community.
After a broad introduction to PECD, several recent results regarding static one VUV-photon valence-shell PECD will be presented, including a striking example of the specific sensitivity of PECD to conformations [2] as well as a first evidence of induced-PECD onto an achiral chromophore within a molecular complex [3]. Then the demonstration of conformer-specific PECD via two-photon ns-laser ionization will be highlighted [4].
The conclusion will stress the universality of PECD, which may be at play in numerous environments including the interstellar medium where it has been suggested as a possible symmetry-breaking process in link with the origin of life’s homochirality [5].
[1] L. Nahon, G. A. Garcia, and I. Powis, J. Elec. Spectr. Relat. Phen. 204, 322 (2015).
[2] J. Dupont, V. Lepere, A. Zehnacker, S. Hartweg, G. A. Garcia and L. Nahon, J. Phys. Chem. Lett. 13, 2313 (2022).
[3] E. Rouquet, M. Roy Chowdhury, G. A. Garcia, L. Nahon, J. Dupont, V. Lepère, K. Le Barbu-Debus,and A. Zehnacker, Nature Communications 14, 6290 (2023).
[4] E. Rouquet, J. Dupont, V. Lepere, G. A. Garcia, L. Nahon, and A. Zehnacker, Angew. Chem. Int. Ed. Engl., e202401423 (2024).
[5] R. Hadidi, D. Bozanic, G. Garcia, and L. Nahon, Adv. Phys. X 3, 1477530 (2018).
Since the pioneering work by French scientists in the early 1800s, optical activity and chiral light-matter interactions have been produced via the chirality of light stemming from a degree of ellipticity in its local polarisation state. The mechanism is well understood: the polarisation state can rotate in a left or right-handed fashion and leads to differential interactions with left and right-handed materials. Structured light can be chiral in degrees of freedom beyond local polarisation state. Optical vortices, for example, possess a chiral wavefront (a twisting tornado structure) due to their azimuthal phase. This chirality is completely independent of that associated with the local state of polarisation. Early studies in the 2000s concluded this chirality of optical vortices can play no role in chiral light-matter interactions. Fast forward to 2018 and all of this changed, with multiple studies proving it was in fact possible.This talk aims to lay out a ‘past, present, and future’ perspective on this rapidly emerging field of ‘chiral structured light’. We will provide a narrative of the field of research starting from its origin in those initial negative results all the way through to experiments highlighting the new and enhanced chiral light-matter interactions provided by structured light. Along the way we will develop and present the underlying theoretical breakthroughs. We will present and discuss a selection of current start-of-the-art results in the area, and conclude with some thoughts for future endeavours.
This presentation will give an overview about the generation and applications of X-ray beams that exhibit an orbital angular momentum (OAM) using helical zone plates, focusing particularly on their ability to probe chiral properties in matter. Helical (or spiral) zone plates represent an advanced class of diffractive optical lenses that can modulate the phase of light while focusing it at the same time. The resulting beams with twisted wavefronts carry OAM, a property of light that is different from the spin angular momentum (SAM) that is widely known from X-ray circular dichroism.
In the first part of the presentation, we will hear about the principle on OAM-carrying radiation and shed light on how helical zone plates create those beams. The great advantage of this method lays in the well-defined topology, which allows for careful controlling of the phase gradient, and as such the topological charge. We will also discuss the limitations, the stringent need for coherent and astigmatic illumination and the break-down of vortices with high topological charges in many smaller sub-vortices.
In the second part, we delve into recent findings on how OAM beams interact with chiral molecules and magnetic nanostructures, revealing helical dichroism effects that are sensitive to the handedness of the structures. Especially for the realm of naturally chiral molecules, this method is a valuable alternative to the rotation of the polarization plane or the differential absorption of circularly polarized light in the visible light domain. Additionally, we demonstrate how these beams enhance the resolution of high-precision imaging techniques such as ptychography.
Potential future applications in the X-ray domain lay in the observation of 3-dimensional magnetic structures – for instance Hopfions – or in super-resolution fluorescence microscopy, dynamic observation of chiral entities, such as Skyrmions, as well as in more systematic investigation of chiral molecules, crystals and metamaterials.
The growing interest in chirality represents a notable example of convergence between different areas of research within the realm of condensed matter. The observation of skyrmions in compounds of group B20 [1] represents only the first report of a wealth of magnetic ordering [2] and collective excitations [3] that can be realized only in the absence of inversion and mirror symmetry operations. Topology predicts the existence of new Weyl fermions [4], and chiral symmetry is responsible for unique spin arrangements in momentum space [5, 6], along with large Berry curvature that is key ingredient for non-linear optical [7] and transport properties [8].
Elemental tellurium is one of the simplest chiral crystals and it is an ideal playground to study the interplay between symmetries and other interesting physical properties. In my talk I will discus how intense laser light can be used to excite coherent phonons, both total symmetric Ag mode and symmetry breaking Eg mode. By using time and angle-resolved photoelectron spectroscopy (trARPES) we can track the dynamical change in the band structure, and the comparison with state-of-the-art TDDFT+U calculation reveals the microscopic origin of the in-phase oscillations of the edge of the band-gap, due to phonon-induced modulation of the effective Hubbard U term [9]. All optical techniques, including second (and higher order) harmonic generation provide complementary information about the light-induced change in symmetry, which indicates a promising route to alter the topological phase and the spin texture in Te.
Finally, I will give an overview of our activity on other chiral semiconductors, CdAs2 and (TaSe4)2I [10], in which intense the laser light is responsible for a change in the electronic population that persists on time scale ranging from microsecond to minutes, of potential interest for opto-electronics and light harvesting based on the bulk photovoltaic effect [11].
References:
[1] A. Fert et al., Nat. Rev. Mater. 2 17031 (2017)
[2] M. Bode et al., Nature 447 190 (2007)
[3] Zhu, H. et al., Science 359 579 (2018)
[4] G. Chang et al., Nat. Mater. 17 978 (2018)
[5] G. Gatti et al., Phys. Rev. Lett. 125 216402 (2020)
[6] D. Gosálbez-Martinez et al., Phys. Rev. B 108 L201114 (2023)
[7] F. de Juan et al., Nat. Commun. 8 15995 (2017)
[8] Z.Z. Du et al., Nat. Rev. Phys. 3 744 (2021)
[9] G. Gatti et al., in review.
[10] A. Crepaldi et al., J. Phys.: Mater. 5 044006 (2022)
[11] Y. Zhang et al., Phys. Rev. B 100 245206 (2019)
This presentation will review our recent progress studying chiral media with Mueller matrix polarimetry. A comprehensive presentation of our polarimetric method will be provided, detailing the instrumentation used, a few remarks about data analysis as well as some basic concepts about light propagation in chiral anisotropic media. We will discuss how optical characterization methods based only on circularly polarized light can often render inconclusive or erroneous results for chiral assessment. We will show that Mueller matrix spectroscopy and imaging techniques are suitable characterization tools to unveil the ever-growing complexity of anisotropic chiral media.
Mueller matrix polarimetry allows the measurement of circular dichroism (CD) together with other optical effects, e.g. linear birefringence, linear dichroism, and circular birefringence. These measurements are critical when studying the optical activity of supramolecular assemblies, nanomaterials, or, more in general, any solid-state system. Examples of chiroptical spectroscopy and chiroptical imaging based on complete polarimetry will be provided.