NETLINCS - New Trends in Linear and Non-LInear Spectroscopic Studies of Natural Chirality

Dec 3, 2024, 9:00 AM → Dec 4, 2024, 6:20 PM Europe/Rome

Sala Kastler (Adriatico Guesthouse, ICTP, Trieste, Italy)

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.} 

Tuesday, December 3

12:30 PM → 2:00 PM registration

2:00 PM → 2:10 PM Welcome

2:00 PM Welcome to the NETLINCS Workshop and Introductory Remarks

2:10 PM → 5:10 PM Gas Phase Studies: 1

2:10 PM High Resolution Spectroscopy and Molecular Chirality: Symmetries and primary processes between less than yoctoseconds and more than days

Speaker: Prof. Martin Quack (ETH Zurich Department of Chemistry and Applied Biosciences Vladimir Prelog Weg 2 CH-8093 Zurich Switzerland)

2:50 PM Recent results on static valence-shell Photo-Electron Circular Dichroism (PECD) on gas phase systems

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).

Speaker: Dr Laurent Nahon (Synchrotron SOLEIL)

3:30 PM Advances in time-resolved photoelectron circular dichroism of chiral molecules in the gas phase

PhotoElectron Circular Dichroism (PECD) is an effect driven by electric dipole interactions between an ensemble of chiral molecules and ionizing circularly polarized light [1,2]. It results in a preferential emission direction of photoelectrons along the light propagation axis, leading to an asymmetry that can be observed by recording their angular distribution with an electron detector. Established as a benchmark method to study the static properties of chiral molecules in the gas phase with high sensitivity [3,4], it was extended to the time domain for the investigation of chiral dynamics less than a decade ago [5].

After reviewing the achievements of time-resolved PECD in molecular spectroscopy, I will present the capabilities of our few-femtosecond UV light sources [6,7] to reveal the contribution of ultrafast electron dynamics to the chiral properties of photoexcited neutral molecules [8]. Our results show an ultrafast inversion of the molecular chiroptical response in less than 10 fs after UV excitation, which is captured by modulations of photoelectron angular distributions produced by a time-delayed, circularly polarized ionizing pulse with few-optical-cycle duration. Our theoretical modelling shows that the electronic coherences induced by the UV radiation also generate chiral currents along the molecular structure, offering important prospects for the enantiosensitive, charge-driven reactivity of chiral molecules [9]. As a proof of principle, I will present a case where chiral electron currents can be used as a tool to select the photodissociation direction of chiral molecules with few-fs resolution. Finally, some concluding remarks will be made on the new opportunities made possible by few-femtosecond PECD.

References

[1] B. Ritchie, Phys. Rev. A. 13, 1411–1415 (1976).
[2] N. Böwering et al., Phys. Rev. Lett. 86, 1187–1190 (2001).
[3] L. Nahon et al., J. Elec. Spectr. Relat. Phen. 204, 322 (2015).
[4] L. Nahon et al., Phys. Chem. Chem. Phys. 18, 12696 (2016).
[5] A. Comby et al., J. Phys. Chem. Lett. 7, 4514–4519 (2016).
[6] M. Galli et al., Opt. Lett. 44, 1308-1311 (2019).
[7] V. Wanie et al., Rev. Sci. Instrum. 95, 083004 (2024).
[8] V. Wanie et al., Nature, 630, pp. 109-115 (2024).
[9] A. F. Ordonez et al., Commun Phys. 6, 257 (2023).

Speaker: Dr Vincent Wanie (Deutsches Elektronen-Synchrotron DESY)

4:10 PM Cofee break

4:30 PM Core-level photoelectron spectroscopy for the investigation of ultrafast chiral dynamics in molecules

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.

Speaker: Dr Caterina Vozzi (CNR-IFN)

5:10 PM → 6:00 PM Solution Phase Studies: 1

5:10 PM Photoelectron Circular Dichroism of Aqueous Solutions

Photoelectron Circular Dichroism of Aqueous Solutions
Bernd Winter
Fritz-Haber-Institut, Berlin, Germany

Chirality-sensitive, large-cross section Photoelectron Circular Dichroism (PECD) — the forward-backward asymmetry in photoemission from chiral molecules photoionized by circularly polarized light — uniquely connects molecular electronic structure to chirality. In addition, core-level PECD is site-specific and sensitive to chemical environment and especially to the conformation of small (bio)molecules.[1] The application of this technique to chiral molecules in an aqueous environment, with hydration and chiral recognition being fundamental biochemical processes, is desirable but remains experimentally challenging. As PECD is a near-threshold effect, one major complication for condensed-phase experiments is the unavoidable convolution of the PECD signal with the secondary electron background within the energy range of interest (<25 eV).[2] Specifically, strong quasi-elastic vibrational scattering of low-energy photoelectrons (<10 eV), for which PECD is anticipated to be most pronounced, by water molecules in the bulk solution currently complicates resolution of the primary photoelectron features. We report on the feasibility of PECD measurements using liquid-jet photoelectron spectroscopy, exemplified for carbon core-level ionization of aqueous-phase alanine. We will present and discuss experimental results, obtained for all three protonation states of alanine in water, survey the instrumental developments that have enabled these measurements,[4] and highlight opportunities and challenges in the evolving field of liquid-phase PECD.

[1] L. Nahon, G.A. Garcia, & I. Powis, J. Electron Spectrosc. Relat. Phenomena 204, 322 (2015).
[2] S. Malerz et al., Phys. Chem. Chem. Phys. 23, 8246 (2021).
[3] M. Pohl et al., Phys. Chem. Chem. Phys. 24, 8081 (2022).
[4] S. Malerz et al., Rev Sci Instrum. 93, 015101 (2022).

Speaker: Bernd Winter (Fritz-Haber-Institut, Berlin, Germany)

6:00 PM → 7:30 PM Poster Session

Wednesday, December 4

9:00 AM → 11:00 AM Solution Phase Studies: 1

9:00 AM Time-resolved circular dichroism : what can we learn on conformational changes ?

Time-resolved circular dichroism (CD) is a powerful technique to investigate the dynamics of conformational changes in molecules and in biomolecules. Starting from a pump-probe configuration, it consists in measuring the CD of the pump-excited molecules to gain information on the relevant timescales. Complementary experiments on short (picoseconds) or longer (microseconds) timescales have been set-up based on various polarization-sensitive measurements. However, in order for this technique to be quantitative, it must rely on a priori knowledge of the CD spectra before and after photoexcitation or on some simple models. In this talk, such issues will be discussed for several experimental studies : ultrarapid conformational changes in Binaphthol molecules or in the chromophore of Photoactive Yellow Protein on the one hand ; determination of thermodynamic and kinetic parameters in denaturation of poly(Glutamic acid) and DNA G-quadruplexes on the other hand.emphasized text

Speaker: Francois Hache (Laboratoire d'Optique et Biosciences)

9:40 AM Ultrafast chiral spectroscopy for stereocontrolled photochemistry

The incorporation of chiral structures into photochemical systems is a powerful strategy to control their functions [1]. For example, uni-directional molecular motors, chiral photocatalysts, and chiral metal nanostructures have achieved exceptional levels of stereocontrol over mechanical motion, energy transfer, and electric charge-carriers on the nanoscale. However, the direct characterization of the underlying chiral excited states remains a formidable experimental challenge, due to a lack of analytical techniques that combine high chiral sensitivity in solution phase with ultrafast time resolution [2].

To address this challenge, we have developed an ultrafast electronic circular dichroism set up that measures the absorption difference of left- and right-circularly polarized laser pulses in photoexcited chiral molecules [3]. Through an ultra-sensitive broadband detection scheme, we are now able to capture ultrafast changes in molecular chirality and follow the stereochemical evolution of the associated photoexcited states with sub-picosecond time resolution. This development has extended the scope of ultrafast chiral spectroscopy to new classes of chiral photochemical phenomena, which I will illustrate via two examples: 1) the identification and control of a new reaction coordinate in the spin-crossover dynamics of Fe(II) complexes [4], and 2) the investigation of the energy and chirality transfer dynamics in a chiral lanthanide-based OLED complex with circularly polarized luminescence (CPL) [5].

References

[1] J. R. Brandt, F. Salerno, and M. Fuchter, Nat. Rev. Chem. 1, 0045 (2017)
[2] M. Oppermann, CHIMIA 78, 45-49 (2024)
[3] M. Oppermann, B. Bauer, T. Rossi, F. Zinna, J. Helbing, J. Lacour, and M. Chergui, Optica 6, 56 (2019)
[4] M. Oppermann F. Zinna, J. Lacour, and M. Chergui., Nat. Chem. 14, 739-745 (2022)
[5] J. L. Lunkley, D. Shirotani, Y. Kazuaki, K. Sumio, and G. Muller, J. Am. Chem. Soc. 130, 13814-13815 (2008)

Speaker: Malte Oppermann (University of Basel, Department of Chemistry)

10:20 AM Thirty-five years of trying – why are we still „developing“ chrial spectroscopy with ultrafast lasers?

Since the pioneering transient circular dichroism (CD) experiments of Xie and Simon$^1$ and Kliger$^2$ more than three decades ago, laser technology has developed enormously, and highly stable, ultrashort broad-band pulses for extremely sensitive differential absorption measurements at wavelengths from the mid-IR to the far UV have become routinely available. Yet, progress in time-resolved CD has been much more difficult and slower, and laser-based measurements are still less sensitive and reliable than conventional CD spectrometers, which use much weaker incoherent light sources such as lamps or glow bars.
In the first part of my presentation I will provide an overview of our attempts to boost the sensitivity of laser-based CD detection in the mid-IR and the UV, both via signal enhancement and artifact mitigation$^{3–5}$, in order to illustrate the difficulty of transferring the measurement principles of non-linear spectroscopy to chirality detection. Artefact reduction in particular requires significantly better laser stability and longer averaging to reach the signal to noise levels of today’s achiral transient absorption measurements$^6$.
Another limitation facing transient CD-experiments with ultrashort lasers is the short time-window they can usually cover. Photo-induced changes in molecular reactions often occur on a large range of timescales. While electronic and vibrational relaxation is usually ultrafast, subsequent biomolecular catalytic processes or conformational changes in biomolecules often require hundreds of nanoseconds or microseconds. For example, we seek direct information on local transition dipole couplings in model peptides$^7$ by combining transient UV-circular dichroism and site-specific labelling. We will discuss how to couple two femtosecond laser systems to detect absorption and chirality changes under identical excitation and probing conditions over all relevant timescales$^8$.

1. Xie, X. & Simon, J. D. Rev. Sci. Instrum. 60, 2614–2627 (1989).
2. Milder, S. J., Gold, J. S. & Kliger, D. S. J Am Chem Soc 108, 8295–8296 (1986).
3. Helbing, J. & Bonmarin, M. J. Chem. Phys. 131, (2009).
4. Bonmarin, M. & Helbing, J. Chirality 21 (2009).
5. Dutta, B. & Helbing, J. Opt Express 23, 16449 (2015).
6. Oppermann, M. et al. J. Phys. Chem. Lett. 10, 2700–2705 (2019).
7. Spekowius, J., Pfister, R. & Helbing, J. Phys. Chem. B 125, 7662–7670 (2021).
8. Helbing, J. & Hamm, P. J. Phys. Chem. A 127, 6347–6356 (2023).

Speaker: Dr Jan Helbing (University of Zurich)

11:00 AM → 11:20 AM Coffee Break

11:20 AM → 12:00 PM Solution Phase Studies

11:20 AM Selective and tunable absorption of helical light beams in matter

Selective absorption of light at specific frequencies determines the color of an object and results from the discrete energy levels of atoms or molecules in the material. This selective absorption also occurs when the handedness of polarized light interacts with the non-superimposable mirror image of a chiral object, as in circular dichroism (CD). Tunability of light absorption, it is typically achieved through fabrication of optoelectronic devices made of plasmonic nanostructures or via electrical tuning of a thin film absorber. In this presentation, I will demonstrate how the helical phase of light can be used to achieve selective and dynamic tuning of absorption in liquids, solids, plasmonic metasurfaces, and gas-phase atoms and molecules.

First, I will introduce a novel chiroptical detection technique based on absorption of linearly polarized helical light beams. In chiral systems, two enantiomers, which are non-superimposable mirror images, can only be distinguished when they interact with another chiral system. A typical optical probe exploits the broken symmetry of circularly polarized light, resulting in differential absorption of left- and right-handed polarizations, leading to CD. By transferring this broken symmetry to the helical phase of the light rather than its polarization, I will demonstrate enhanced and scalable enantioselectivity in chiral systems. The handedness of helical light beams is determined by the twisting of the wavefront undergoing l intertwined rotations in one wavelength resulting in an orbital angular momentum (OAM) of lħ, where l value is theoretically unbounded.

Second, I will show that dichroism also exists in achiral systems and can be precisely controlled by displacing the phase singularity present in helical beams. According to conventional understanding within dipole approximation, dichroism is not expected in achiral systems due to mirror symmetry. However, breaking this symmetry spatially in the beam profile and using the helical phase of light leads to dichroism, which arises from coupling of the electric dipole and electric quadrupole terms in the multipole expansion of the interaction Hamiltonian. Helical dichroism can be tubed by changing the l value, position of the singularity, polarization and superposition of Gaussian and OAM beams.

Third, I will discuss the control and tunability of strong field ionization of atoms and molecules. Ionization can either be enhanced or suppressed by changing the handedness of helical light beams and displacing the phase singularity, leading to dichroism. The OAM dependence of tunnel ionization, beyond dipole approximation, is induced by the phase singularity, causing variations in the distortion of the atomic potential depending on the handedness of helical light. This effect is further amplified by the field gradient in asymmetric LG beams, enabling control and tunability over ionization and electron energy, which opens new opportunities in spectroscopy, attosecond science, plasma physics and imaging.

References
1. Begin, J-L, et al “Nonlinear helical dichroism in chiral and achiral molecules,” Nature Photonics, 17, 82, 2023.
2. Jain, A. et al “Intrinsic dichroism in amorphous and crystalline solids with helical light, Nature Comm, 15, 1350, 2024.
3. Jain, A. et al “Selective and tunable absorption of twisted light in achiral and chiral plasmonic metasurfaces,” ACS Nano, 18, 27383, 2024

Speaker: Ravi Bhardwaj (University of Ottawa)

12:00 PM → 12:40 PM Theory

12:00 PM Molecular high-harmonic generation spectroscopy with two-colour bicircular fields: a theoretical chemist's point of view

We present here our recent work on the development of ab initio time-domain methods for studying the contribution of inner molecular orbitals to the HHG spectrum [1] of aligned or non-aligned molecules [2-4]. First, we show how, in the presence of a linearly polarised pulse, the selection rules for harmonic generation can be different depending on the molecular orbital considered: an example is given by ammonia [5], with a specific pulse polarisation. As a second house, we show how to modulate the selection rules in molecules such as methane and fluoromethane [6], interacting with two-colour bicircular (BCR) fields, by changing the relative orientation and geometry of the spectroscopic target. In conclusion, results on HHG spectra of a chiral molecule, such as methyloxirane, with BCR pulses [6] are shown and discussed. The work on BCR fields is in collaboration with Vozzi's experimental group.

[1] M. Lewenstein, P. Balcou, P., M. Y. Ivanov, A. L’Huillier, and P. B. Corkum, Phys. Rev. A 49, 2117 (1994);
[2] E. Luppi and E. Coccia, J. Phys. Chem. A, 127, 7335 (2023)
[3] C. Morassut, A. Ravndran, A. Ciavardini, E. Luppi, G. De Ninno and E. Coccia, J. Phys. Chem. A, 128, 2015 (2024)
[4] M. Marchetta, C. Morassut, J. Toulouse, E. Coccia and E. Luppi, accepted.
[5] M. Marchetta, E. Luppi and E. Coccia, submitted.
[6] In preparation.

Speaker: Emanuele Coccia (University of Trieste)

12:40 PM → 2:00 PM Lunch

2:00 PM → 3:20 PM Theory

2:00 PM Ultrafast molecular chirality: a topological connection

An object is said to be chiral if it cannot be superimposed on its mirror image by any rotation. The two mirror images of the same chiral molecule are called enantiomers and are often referred to as “left”- and “right”-handed. While the physical properties of the two enantiomers of the same chiral molecule are nearly identical, the geometric property of chirality leads to vastly different chemical properties of the two enantiomers. The need for rigorous selection of a specific enantiomer, a now standard requirement in drug design, is one of the many reasons behind the ever-greater need for improving sensitivity of chiral sensing.
Yet, standard optical methods of chiral detection still use the same principles as the method discovered by Louis Pasteur in the XIX century: the linear interaction between chiral molecules and light, which becomes chiral-sensitive due to the magnetic field component of the light wave.
Ultrafast non-linear spectroscopies promise to increase the enantio-sensitive signal by three orders of magnitude [1] by removing the need to rely on the interaction with the magnetic field component of light. The second important feature of non-linear light-matter interactions is the opportunity to imprint topological properties of light on matter, presenting an opportunity to achieve topologically robust enantio-sensitive observables.
I will describe our very recent results [2,3] on marrying chiral and topological properties in ultrafast electronic response of chiral molecules in the gas phase, enabling highly efficient and robust chiral observables. I will present three vignettes where topological connection appears in optical or electronic chiral response:
(i) Chiral topological light: a new concept enabling chiral-sensitive and topologically robust properties of high harmonic emission, generated by such light in chiral molecular gases [2]
(ii) Geometry of temporal chiral structures: a concept encompassing the emergence of geometric fields in electronic response of chiral molecules [3]
(iii) Enantio-sensitive exceptional points: chiral topology in non-Hermitian chiral systems [4]

References:
[1] “Ultrafast chirality: the road to efficient chiral measurements” D Ayuso, A F Ordonez, O Smirnova, (Perspective) Phys. Chem. Chem. Phys., 2022, 24, 26962-26991, (2022)
[2] “Chiral topological light for detecting robust enantio-sensitive observables” N Mayer, D Ayuso, M Ivanov, M. Khokhlova, E Pisanty, O Smirnova, Nature Photonics 2024, https://doi.org/10.1038/s41566-024-01499-8
[3] Geometry of temporal chiral structures, A. F. Ordonez, A. Roos, P. Mayer, D.Ayuso, O. Smirnova, arXiv preprint arXiv:2409.02500, 2024
[4] “Enantiosensitive exceptional points” N Mayer, N Moiseyev, O Smirnova, arXiv preprint arXiv:2306.12293, 2023

Speaker: Olga Smirnova

2:40 PM A perspective on chiral structured light

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.

Speaker: Kayn Forbes (University of East Anglia)

3:20 PM → 4:00 PM Molecular powders, films, nanostructure and solids

3:20 PM Twist and Shine: Helical Zone Plates for Probing Chirality

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.

Speaker: Benedikt Rösner (Paul Scherrer Institut)

4:00 PM → 4:20 PM Coffee Break

4:20 PM → 5:40 PM Molecular powders, films, nanostructure and solids

4:20 PM Electron dynamics in chiral semiconductors

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)

Speaker: Alberto Crepaldi (Politecnico di Milano)

5:00 PM Mueller matrix polarimetry for chiral detection in the solid state

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.

Speaker: Oriol Arteaga (UB)

5:40 PM → 6:00 PM Concluding remarks