Session G10: High Harmonic Generation

High-order harmonic generation in a microfluidic device for the investigation of ultrafast dynamics in organic perovskites

Extreme UltraViolet (XUV) sources based on High Harmonic Generation (HHG) in noble gases enable ultrafast spectroscopy with extreme temporal resolutions and site and chemical selectivity, with transient absorption in the XUV allowing access to purely electronic dynamics in molecules and solids. To reduce the complexity of efficient harmonic generation, we designed a HHG scheme in a microfluidic device [1] achieving a high photon-flux and broadband phase matching up to 200 eV. This source is coupled with a beamline for transient absorption, with an XUV spectrometer and a polarimeter recently developed at CNR-IFN. Such a setup allows to study light-matter interaction in solids with an unprecedented temporal and spatial resolution, while enabling an element-selective and oxidation-state specific spectroscopy [2], as photon absorption in the XUV occurs at the atomic cores, locally probing both the electronic and the structural environment of specific atoms.

Metal Halide Perovskites (MHPs) are a class of direct bandgap semiconductors with properties that can be tuned by changing their chemical composition ABX 3 , particularly the metal B (Pb, Sn), and halogen X (I, Br). While their optoelectronic properties, such as long carrier lifetimes and diffusion length and small Stokes shift, are well suited for photovoltaics and light emitting applications [3], the physics of these materials is still not completely understood, e.g. concerning defects from lattice distortions, ionic mobility, the polaronic character of excited free carriers and excitons [5] and the nonradiative losses related to Auger recombination [6]. Typically, MHPs conduction band is composed of orbital states of metal and halogen atoms, making them an interesting system for a transient XUV set up which can access the absorption edges of I and Br. Here we report on developing a beamline for site-sensitive ultrafast XUV spectroscopy to explore nonradiative charge recombination and charge transfer at the femtosecond time-scale in MPHs.   

Extracting correlation length in Mott insulators by strong-field driving

The break-down of a Mott-insulator when subjected to intense laser fields is characterized by the formation of elementary charge excitations known as doublon-hole pairs.  This break-down is furthermore evidenced by the production of high harmonics that can be experimentally measured. Recently, we investigated coupling a metal to the Mott insulator in an interface environment to study the effect this had on the production of these doublon-hole pairs and the resultant high harmonics at the interface.

 

We found that interfacial coupling of a metal to the Mott insulator enhances high harmonic production inside the insulator and suggested an increase in the doublon-hole correlation length as the physical mechanism behind this lowered threshold for dielectric break-down.  Here, we present an approach for extracting the doublon-hole correlation length of a Mott insulator to verify this hypothesis. The method is based on a dynamical calculation of the Mott insulator's rate of charge production in response to an applied strong-field laser pulse. We find that the correlation length does in fact increase drastically with increased interfacial coupling, in support of our hypothesis, which signifies the metal's role in the enhancement of charge fluctuations in the insulator.

 

We confirm our conclusions using Density Matrix Renormalization Group (DMRG) calculations. The proposed method further allows one to experimentally access the doublon-hole correlation length through measurable observables, such as differential reflectivity or the high harmonic generation (HHG) spectrum.   

Quantitative comparison of TDDFT-calculated high harmonic generation yields in ring-shaped organic molecules

We calculate high harmonic generation (HHG) yields in benzene, cyclohexene, and cyclohexane using time-dependent density functional theory (TDDFT). Our goal is to obtain quantitative agreement with experimental yield ratios. We simulate gas-phase experimental conditions by averaging over the molecular orientation angles. By resolving the yield into contributions from individual molecular orbitals, we show that quantitative agreement between theory and experiment is suppressed due to a difference between DFT-computed and experimental ionization potentials. We further show that by reweighting individual orbital contributions through an adjustment in the tunnel-ionization step of HHG, one can account for this difference to obtain more realistic yields in good quantitative agreement with experimental results [1].

[1] A. F. Alharbi, A. E. Boguslavskiy, N. Thiré, B. E. Schmidt, F. Légaré, T. Brabec, M. Spanner, and V. R. Bhardwaj, Phys. Rev. A 92, 041801 (2015).   

Harmonic Generation with Optical Vortex Beams

Vortex beams carry two types of angular momentum: spin angular momentum (SAM) due to the time-dependence of the light polarization, and orbital angular momentum (OAM) due to the spatial phase variation of their wavefront. When a noble gas is illuminated by an optical vortex beam, odd harmonics of the driven laser frequency are generated. For long and weak laser pulses, the generated harmonics are known to possess a well-defined topological number that scales linearly with the harmonic order. In this work, we consider high-order harmonic generation (HHG) of a helium gas exposed to an optical Bessel vortex beam. The single-atom response is computed through time-dependent calculations, and the collective response of the atomic gas is simulated by the Fraunhofer diffraction formula. We find that the harmonics produced by an intense few-cycle optical vortex beam exhibit a mixture of OAMs, indicating the emergence of high-order processes and the breakdown of the perturbative regime as we increase the pulse intensity. The produced far field exhibits the fingerprint of these non-perturbative processes and lead to a complex light intensity profile and phase variation, which could be observed experimentally. We discuss the possible applications of this approach.   

Solid-state high harmonic generation spectroscopy of quantum materials in complex environments

Solid-state High Harmonic Generation (sHHG) spectroscopy offers significant potential for the characterization of nanomaterials and 2D quantum materials with sensitivity to lattice and electronic dynamics, including correlated phenomena and properties such as topology. sHHG spectroscopy is particularly promising for studying material properties and dynamics in challenging sample environments, including liquid-phase and high-pressure diamond cells, where traditional tools of condensed matter physics fail. In this presentation, I will discuss our recent work where we explore the sHHG polarization anisotropy in monolayer MoS₂, specifically examining how it varies with the crystal's orientation relative to the polarization of the mid-infrared laser field. Our experiments, conducted across several laser wavelengths, revealed a notable angular shift in the parallel-polarized odd harmonics at energies above approximately 3.5 eV. We determined that this shift is linked to differences in the recombination dipole strengths that involve multiple conduction bands. This finding is not only specific to the material in question but also augments the angular dependence dictated by the dynamical symmetry properties of the crystal in interaction with the laser field. Expanding on these results, we incorporated a diamond anvil cell (DAC) into our setup to investigate pressure-induced phase transitions in MoS₂. Our high-pressure experiments on MoS₂, both in bulk and at the monolayer limit, revealed a structural phase transition at approximately 25 GPa, characterized by significant changes in symmetry and a closure of the bandgap, indicating a transition to a metallic phase. These results underscore the versatility and efficacy of sHHG spectroscopy in probing quantum materials in complex environments. Finally, I will briefly discuss recent advancements, including sHHG on resonant dielectric metasurfaces for efficient optical harmonic generation, ellipticity-dependent high-order harmonic generation in ZnO, size-dependent suppression of high-order harmonics in CdSe quantum dots, and the observation of momentum-dependent electron-phonon scattering as predominant mechanism for dephasing in sHHG.   

Benchmark calculations for high-order harmonic generation in helium.

We report a detailed study of high-order harmonic generation (HHG) in helium. When comparing predictions from a single-active-electron model with those from all-electron simulations [1], such as R-matrix with time-dependence (RMT) [2] and the ATTOMESA code currently under development [3], both of which can include different numbers of states in the close-coupling expansion, it seems imperative to generate absolute numbers for the HHG spectrum in a well-defined framework. While qualitative agreement in the overall frequency dependence of the spectrum, including the cut-off frequency predicted by a semi-classical model [4,5], can be achieved by many models in arbitrary units, only absolute numbers can be used for benchmark comparisons between different approaches. This is particularly important when obtaining appropriate estimates for the conversion efficiency, i.e., the area under the HHG plateau compared to the peaks due to the fundamental frequency and the low-order harmonics.

 [1] A. T. Bondy et al., https://arxiv.org/abs/2311.03677 (2023)

[2] A. C. Brown et al., Comput. Phys. Comm. 250 (2020) 107062.

[3] N. Douguet, unpublished (2024)

[4] K. J. Schafer, B. Yang, L. F. DiMauro, and K. C. Kulander, Phys. Rev. Lett. 70 (1993) 1599.

[5] P. B. Corkum, Phys. Rev. Lett. 71 (1993) 1994.