Control of Ultrafast (Attosecond and Strong Field) Processes Using Structured Light

Although it was observed almost 30 years ago, the dynamics of magnetization at ultimate femtosecond and attosecond time scale remains a hot topic. In part, this is due to the lack of dedicated and suitable experimental insights. While most of ultrafast measurements have been carried out with visible lasers, progress of attosecond science, which offers ultrashort light pulses in the eXtreme UltraViolet spectral range, let envision radically new approaches. In particular, contrary to visible light, it is suitable to address specific edges of many magnetic materials in magnetic alloys and heterostructures. However, up to recent days, control of its angular momenta was difficult, if possible at all. The situation has changed lately, with the demonstration of reliable attosecond light sources carrying either spin or orbital angular momentum. When interacting with magnetic matter, SAM can be modified through magnetic circular dichroism (MCD). As for OAM, we recently showed that the beam’s content depends on the material’s spin texture upon reflection, a process that we called magnetic helicoidal dichroism (MHD). Here we present recent developments in building a high harmonic generation-based beamline dedicated to the study of both MCD and MHD in solids with attosecond resolution in a transient spectroscopy scheme. First commissioning experiments were performed on gaseous and solid samples with linear polarization. Finally, our efforts towards the more difficult task of magnetic dichroisms will be presented.

In 1996, Beaurepaire et al. discovered that femtosecond lasers could drive the demagnetization of Nickel films in a sub-picosecond range, opening up a new field of femtomagnetism. Combined with ultrafast laser pulses, ultrafast electron microscopy can achieve sub-picosecond time resolution and nanometer spatial resolution, which is demonstrated to be a promising characterization method in ultrafast electronic, spin, and structural dynamics. Here, we apply the optical grating technique to the study of demagnetization by ultrafast Lorentz transmission electron microscopy. Through the Fresnel mode, we observed a sub-picosecond demagnetization process in the initially magnetically uniform permalloy thin film. Secondly, we observed the enhanced demagnetization amplitude caused by the plasmonic effect by combining the Photon-induced Near field Microscope (PINEM) technique. Last but not least, we found that the plasmonic effect may accelerate the demagnetization speed. Our research proves that it is applicable to study the ultrafast demagnetization process by introducing optical grating technology in the ultrafast Lorentz TEM, which can enrich the research objects of the ultrafast Lorentz TEM in the field of ultrafast demagnetization.

Chiral molecules have attracted significant multidisciplinary attention for many decades, from pharmaceuticals and the life sciences to quantum chemistry and optics. Of particular importance is the ability to identify and separate molecules of different chirality (known as enantiomers of each other), a process usually referred to as chiral discrimination. Recently, the progress on laser cooling has opened the possibility of cooling chiral molecules to the ultracold regime in the following years. This will possibly open exciting new directions in the study of chiral molecules, much in analogy to the already well-established cold atom gases. Anticipating these developments, in this work we theoretically study the quantum dynamics and phase diagram of cold and interacting chiral molecules immersed in recently proposed \emph{helicity lattices. The lattices have homogeneous mean squared values of the electric field, but spatially varying helicity, and thus, they exert a discriminatory force on chiral molecules with different handedness. We model these lattices with an extended Bose-Hubbard model, where the chiral molecules are represented as structureless bosonic particles which interact through repulsive on-site and dipolar long-range interactions. We provide a detailed presentation of the different quantum phases shown by the helicity lattices. In particular, we find that a strong dipole-dipole repulsion between molecules results in the polarization of left/right enantiomers. In an experiment, this would produce a phase separation of enantiomers, acting as an alternative route towards chiral discrimination..

High Harmonic Generation (HHG) is a key tool for modern ultrafast science. While a simple photon model of HHG can predict the properties of the emitted photons (energy, direction, spin...), it fails to explain the non-perturbative yield, typical of HHG. This is particularly clear when studying HHG driven by two non-collinear beams. In that instance, instead of a single beam at angular frequency q\omega, a series of beamlets corresponding to the absorption of p photons from one beam and q-p photons from the second beam are emitted. Their emission directions correspond to the wavevectors $\vec{k}_{q,p}=p\vec{k}_1+q\vec{k}_2$, with $\vec{k}_1$ and $\vec{k}_2$ the wavevectors of the two driving beams. All these beamlets, at angular frequency q\omega, show properties like their orbital angular momentum, spin angular momentum, energy… following algebraic sum analogous to the one explicated for the wavevector. Conversely, through dedicated experiments, we recently showed that their yields show much more complex behavior: each of them dominates in turn, before fading away. Moreover, this behavior is accurately reproduced by an analytical development of a field-based model. To explain this behavior through photon pathways, we propose a new interpretation of HHG in which harmonic q is produced by interferences of pathways involving the absorption of q+n photons, and the stimulated emission of n photons. By including enough supplementary absorptions and stimulated emissions from one of the driving fields, we manage to retrieve accurate scaling laws of the harmonics beamlets intensities for any ratio of intensity between the two driving beams. Our interpretation bridges the gap between field and photon interpretation of HHG. Furthermore, we show that this could be very useful in tailoring the spatial profile of XUV field.

The study of high-order harmonic generation (HHG) in solids by virtue of intense laser pulses provides a fascinating platform to study ultrafast electron dynamics as well as material properties. We theoretically investigate HHG on the basis of massive Dirac Fermions, serving as a prototypical model for topologically non-trivial matter. The high harmonic spectra resulting from a numerical solution of the equations of motion for the density-matrix are supplemented and compared to a semiclassical saddle-point analysis known as the Lewenstein or simple- man model[1,2]. We illustrate that HHG can be interpreted as a result of interfering classical trajectories generated at different half- cycles of the laser pulse. The semi-classical method is leveraged to analyze the influence of few-cycle pulses as compared to long signals close to the plane-wave limit. [1]M. Lewenstein, P. Balcou, M. Y. Ivanov, A. L’Huillier, and P. B. Corkum, Physical Review A 49, 2117 (1994). [2]G. Vampa, C. R. Mcdonald, G. Orlando, D. D. Klug, P. B. Corkum, and T. Brabec, Physical Review Letters 113, (2014).

We consider the dressing of chiral molecules rotational states by off-resonance, circularly polarised, non-monochromatic light. We assume that the intensity of light used is sufficiently weak such that the multipole expansion of the interaction remains valid. In doing so, we investigate the modulation of the THz refractive index.

The interaction of matter with strong-field light leads to highly nonlinear electron dynamics in the material. The electric field of the light accelerates electrons through the band structure and drives non-perturbative transitions, leading to strongly anharmonic electron velocities and the emission of high-order harmonics. We study these electron dynamics for the surface states of topological insulators by solving the time-dependent Schrödinger equation for wave packets. The separation of bulk and surface high harmonics has recently been demonstrated experimentally [1]. Furthermore, an alternating polarisation rotation between odd and even order harmonics was found and attributed to the hexagonal warping present in the surface states of $\text{Bi}_2\text{Te}_3$. Using effective Dirac models and taking into account the effects of the Fermi sea, we can reproduce these experimental features with our wave packet approach. Additionally, our method gives new insight into the interplay of higher-harmonics generation and the distribution of states in momentum space. [1] C. Schmid, L. Weigl, P. Grössing, V. Junk, C. Gorini, S. Schlaud- erer, S. Ito, M. Meierhofer, N. Hofmann, D. Afanasiev, J. Crewse, K. Kokh, O. Tereshchenko, J. Güdde, F. Evers, J. Wilhelm, K. Richter, U. Höfer and R. Huber, Tunable non-integer high-harmonic generation in a topological insulator, Nature 593, 385-390 (2021)

R. Picciuto1*, J. Broughton1, K. Kowalczyk1, H. Allegre1, Misha Ivanov1,2,3, O. Smirnova2,4, J. W. G. Tisch1, J. P Marangos1, D. Ayuso1,2, M. Matthews1 1 Department of Physics, Imperial College London, SW7 2BW London, United Kingdom 2 Max-Born-Institut, 12489 Berlin, Germany 3 Institute für Physik, Humboldt-Universität zu Berlin, Berlin, Germany 4 Technische Universität Berlin, 10623 Berlin, Germany Chirality in molecules occurs due to the spatial arrangement of their atoms forming structures that are non-superimposable upon their mirror image. These two mirror-image forms are called enantiomers and possess identical physical and chemical properties, except when interacting with something that is also chiral. Biological systems are inherently chiral and hence their interactions with chiral molecules are enantiosensitive, making chiral recognition vital, especially in the liquid phase.[1,2] Chiral light has long served as a preferred tool for imaging the chirality of matter. Traditional optical methods use circularly polarised light (CPL), the standard chiroptical tool, to determine the molecular handedness. However, the interaction between CPL and chiral molecules is barely enantiosensitive due to the large disparity between the pitch of the light’s helix and the comparatively tiny size of the molecules.[1] Synthetic chiral light has recently been proposed as an efficient alternative to CPL.3 This new type of chiral light is locally chiral; the tip of the electric field vector traces a 3D chiral Lissajous figure in time, which can drive ultrafast chiral electron currents on the scale of the molecule. As a result, the nonlinear response of chiral molecules to such fields can be orders-of-magnitude more enantiosensitive than with CPL.[3] Here we will present our recent experimental achievement: we have created locally chiral light in our laboratory, using a non-collinear optical configuration. In our setup, we used two ultrashort (50fs) non- collinear beams each carrying 800nm with linear polarisation in the plane of the optical table. In one of the beams an additional, orthogonally polarised, 400nm component is generated from a type I BBO and propagated collinearly with its fundamental. The two non-collinear beams were then focussed individually and overlapped, both spatially and temporally. To demonstrate this locally chiral field, we recorded the nonlinear response of a BBO crystal. The presence of a non-collinear second harmonic (400nm) shows spatial and temporal overlap of the two 800nm components. We also observe, simultaneously, a non-collinear sum frequency generation (266nm) signal, which indicates spatial and temporal of the 800nm and 400nm in the different beams. Thus, these nonlinear emissions indicate temporal and spatial overlap of both beams and all three components, hence unambiguously demonstrating the generation of a locally chiral field.[3] We aim to demonstrate the unique interactions of our chiral field with chiral molecules, in the liquid phase, by measuring the nonlinear response from opposite molecular enantiomers. Not only does experimental realisations of locally chiral fields create new methods for imaging chirality but ultimately paves the way for controlling, manipulating and even separating chiral molecules with extreme enantio- efficiency.[3] 1. P. L. Polavarapu, Chiroptical spectroscopy: fundamentals and applications, Taylor & Francis, Boca Raton, 2017. 2. N. Ananthi, Org. Med. Chem. Int. J., , DOI:10.19080/OMCIJ.2018.05.555661. 3. D. Ayuso, O. Neufeld, A. F. Ordonez, P. Decleva, G. Lerner, O. Cohen, M. Ivanov and O. Smirnova, Nat. Photonics, 2019, 13, 866–871. *r.picciuto21@imperial.ac.uk

We present a numerical library for the simulation of mesoscopic systems and materials, and developed in the Julia language. Its focus is performance and generality and expressibility. The highlights of the current release are: (a) the efficient construction of arbitrary single-particle, tight-binding Hamiltonians, regardless of dimensionality, (b) the computation of bandstructures with advanced interpolation and (c) the computation of Green functions for periodic or composite systems using a set of general solvers.