Highly selective chiral discrimination in high harmonic generation by dynamical symmetry breaking spectroscopy

We propose and numerically demonstrate a new very robust and highly selective method for femtosecond time-resolved chiral spectroscopy using high harmonic generation (HHG). The method is based on dynamical symmetry breaking from chiral media, and relies only on intense electric-dipole transitions, and not on the interplay of electric and magnetic dipoles. The symmetry breaking results in the emission of a strong chiral signal in the form of otherwise 'forbidden' harmonics (i.e., that are not emitted from achiral media). The intensity of these symmetry-forbidden harmonics is directly correlated to the media's enantiomeric excess, yielding chiral selectivity. On the contrary, the strength of the 'allowed' harmonics is chiral-independent, hence they can be used as a reference to provide chiral selectivity from a single measurement, unlike previous time-resolved schemes that require multiple measurements. We demonstrate numerically 96% discrimination level from microscopic gas phase emission, outperforming by far previous time-resolved methods (the selectivity should be further enhanced when the HHG process is phase matched). We expect the new method to give rise to precise table-top characterization of chiral media in the gas-phase, and for highly sensitive time-resolved ultrafast probing of dynamical chiral processes

Optical chirality in high harmonic generation

Optical chirality (OC) - one of the fundamental quantities of electromagnetic fields - corresponds to the instantaneous chirality of light. It has been utilized for exploring chiral light-matter interactions in linear optics, but has not yet been applied to nonlinear processes. Motivated to explore the role of OC in the generation of helically polarized high-order harmonics and attosecond pulses, we first separate the OC of transversal and paraxial beams to polarization and orbital terms. We find that the polarization-associated OC of attosecond pulses corresponds to that of the pump in the quasi-monochromatic case, but not in multi-chromatic pump cases. We associate this discrepancy to the fact that the polarization OC of multi-chromatic pumps vary rapidly in time along the optical cycle. Thus, we propose new quantities, non-instantaneous polarization-associated OC, and timescale-weighted polarization-associated OC, that link the chirality of multi-chromatic pumps and their generated attosecond pulses. The presented extension to OC theory should be useful for exploring various nonlinear chiral light-matter interactions. For example, it stimulates us to propose a tri-circular pump for generation of highly elliptical attosecond pulses with a tunable ellipticity

Unambiguous definition of handedness for locally-chiral light

Synthetic chiral light fields were recently introduced as a novel source of chirality [Ayuso et al. Nat. Phot. 13, 866 (2019)]. This locally-chiral light spans a three-dimensional polarization that plots a chiral trajectory in space-time, leading to huge nonlinear chiral signals upon interactions with chiral media. The degree of chirality of this new form of light was defined, characterized, and shown to be proportional to the chiral signal conversion efficiency. However, the sign of the light's chirality - its 'handedness' - has not yet been defined. Standard definitions of helicity are inapplicable for locally-chiral light due to its complex three-dimensional structure. Here, we define an unambiguous handedness for locally-chiral fields and employ it in practical calculations

Platinum-Doped α‑Fe₂O₃ for Enhanced Water Splitting Efficiency: A DFT+<i>U</i> Study

Hematite (α-Fe₂O₃) is commonly considered for converting solar energy into hydrogen fuel through water splitting. Recent experiments performed in 2013 reached a maximum efficiency in Fe₂O₃ photoelectrochemical cells while using platinum-doped Fe₂O₃. In order to understand how platinum increases efficiency, we use the density functional theory + <i>U</i> (DFT+<i>U</i>) method to model the bulk and the (0001) surface of platinum-doped Fe₂O₃. We also give a unique ligand field theory combined with Bader charge analysis to explain changes resulting from symmetry breaking by the dopant. First, we find that, although platinum has a lower oxidation state than usual n-type dopants, platinum donates electrons. We find a theoretical ideal doping range of 0.64–2.96 atom % for enhanced electron conductivity, which is within the optimal range obtained by previous experiments. Second, we find that the energy gap decreases upon doping, improving solar energy absorption. Third, in agreement with previous experiments, we calculate an unfavorable increase in overpotential for oxidizing water upon platinum doping. Since platinum has both good and bad effects, we recommend bypassing this duality by platinum doping with a gradient-based strategy: high doping in the bulk for enhanced conductivity and low doping at the surface to not interfere with catalysis. We anticipate that experimentally testing our proposed strategy will advance the development of better electrodes for photoelectrochemistry

Novel High-Throughput Screening Approach for Functional Metal/Oxide Interfaces

Metal/oxide interfaces have long been studied for their fundamental importance in material microstructure as well as their broad applicability in electronic devices. However, the challenge involved in characterizing the relation between structure and electron transport of a large number of metal/oxide combinations inhibits the search for interfaces with improved functionality. Therefore, we develop a novel high-throughput screening approach that combines computational and theoretical techniques. We use a Density Functional Theory + U (DFT+U) quantum mechanical formalism to produce effective Schrödinger equations, which are solved by wave packet propagation to simulate charge transport across the metal/oxide interface. We demonstrate this method on α-Fe₂O₃/Mt interfaces, for Mt = Ag, Al, Au, Ir, Pd, or Pt metals. We use this novel method to screen for binary alloys of these metals at the α-Fe₂O₃/Mt interface and perform a successful validation test of the methodology. Finally, we correlate the interface potential energy and the charge transport permeability through the interface. Counterintuitively, among the interfaces studied, we find that higher mismatch interfaces have better charge transport permeability. We anticipate that this method will be useful as a computationally tractable strategy to perform high-throughput screening for new metal/oxide interfaces

Symmetries and selection rules in Floquet systems: application to harmonic generation in nonlinear optics

Symmetry is one of the most generic and useful concepts in physics and chemistry, often leading to conservation laws and selection rules. For example, symmetry considerations have been used to predict selection rules for transitions in atoms, molecules, and solids. Floquet systems also demonstrate a variety of symmetries which are spatiotemporal (i.e. dynamical symmetries (DSs)). However, the derivation of selection rules from DSs has so far been limited to several ad hoc cases. A general theory for deducing the impact of DSs in physical systems has not been formulated yet. Here we explore symmetries exhibited in Floquet systems using group theory, and discover novel DSs and selection rules. We derive the constraints on a general system's temporal evolution, and selection rules that are imposed by the DSs. As an example, we apply the theory to harmonic generation, and derive tables linking (2+1)D and (3+1)D DSs of the driving laser and medium to allowed harmonic emission and its polarization. We identify several new symmetries and selection rules, including an elliptical DS that leads to production of elliptically polarized harmonics where all the harmonics have the same ellipticity, and selection rules that have no explanation based on currently known conservation laws. We expect the theory to be useful for manipulating the harmonic spectrum, and for ultrafast spectroscopy. Furthermore, the presented Floquet group theory should be useful in various other systems, e.g., Floquet topological insulators and photonic lattices, possibly yielding formal and general classification of symmetry and topological properties

Mapping light-dressed Floquet bands by highly nonlinear optical excitations and valley polarization

Ultrafast nonlinear optical phenomena in solids have been attracting major interest as novel methodologies for femtosecond spectroscopy of electron dynamics and control of material properties. Here, we theoretically investigate strong-field nonlinear optical transitions in a prototypical two-dimensional material, hBN, and show that the k-resolved conduction band charge occupation patterns induced by an elliptically-polarized laser can be understood in a multi-photon resonant picture; but remarkably, only if using the Floquet light-dressed states instead of the undressed matter states. Consequently, our work establishes a direct measurable signature for band-dressing in nonlinear optical processes in solids, and opens new paths for ultrafast spectroscopy and valley manipulation

Mapping Light-Dressed Floquet Bands by Highly Nonlinear Optical Excitations and Valley Polarization

Ultrafast nonlinear optical phenomena in solids have been attracting a great deal of interest as novel methodologies for the femtosecond spectroscopy of electron dynamics and control of the properties of materials. Here, we theoretically investigate strong-field nonlinear optical transitions in a prototypical two-dimensional material, hBN, and show that the k-resolved conduction band charge occupation patterns induced by an elliptically polarized laser can be understood in a multiphoton resonant picture, but, remarkably, only if using the Floquet light-dressed states instead of the undressed matter states. Our work demonstrates that Floquet dressing affects ultrafast charge dynamics and photoexcitation even from a single pump pulse and establishes a direct measurable signature for band dressing in nonlinear optical processes in solids, opening new paths for ultrafast spectroscopy and valley manipulation

A First-Principles Study on the Role of an Al₂O₃ Overlayer on Fe₂O₃ for Water Splitting

Understanding the role of an overlayer material on a catalyst is crucial for improving catalytic activity. Iron­(III) oxide (α-Fe₂O₃) is a widely studied catalyst commonly used for solar water splitting. Recently, the water splitting efficiency with α-Fe₂O₃ was enhanced by deposition of an α-Al₂O₃ overlayer. In order to understand the origin of this improvement, we perform first-principles calculations with density functional theory + <i>U</i> on the α-Fe₂O₃(0001) surface with an α-Al₂O₃ surface overlayer. We find catalysis is unfavorable directly over α-Al₂O₃ and rather takes place over α-Fe₂O₃ exposed areas. In agreement with experiment, we find that α-Al₂O₃ coverage decreases the overpotential required for water oxidation on α-Fe₂O₃. We explain this improvement through the decrease in the work function of α-Fe₂O₃ upon α-Al₂O₃ coverage that aids in extracting electrons during the water oxidation reaction. We suggest that selecting an overlayer with a smaller work function than that of the catalyst as a strategy for future development of better catalysts

Toward Settling the Debate on the Role of Fe₂O₃ Surface States for Water Splitting

Understanding the chemical nature and role of electrode surface states is crucial for improved electrochemical cell operation. For iron­(III) oxide (α-Fe₂O₃), which is one of the most widely studied anode electrodes used for water splitting, surface states were related to the appearance of a dominant absorption peak during water splitting. The chemical origin of this signature is still unclear, and this open question has provoked tremendous debate. In order to pin down the origin and role of surface states, we perform first-principles calculations with density functional theory + U on several possible adsorbates at the α-Fe₂O₃(0001) surface. We rule out the existence of a stable peroxo Fe–O–O–Fe adsorbate and show that the origin of the surface absorption peak could be a Fe–O· type bond that functions as an essential intermediate of water oxidation