Light-wave-controlled Haldane model in monolayer hexagonal boron nitride.

In recent years, the stacking and twisting of atom-thin structures with matching crystal symmetry has provided a unique way to create new superlattice structures in which new properties emerge1,2. In parallel, control over the temporal characteristics of strong light fields has allowed researchers to manipulate coherent electron transport in such atom-thin structures on sublaser-cycle timescales3,4. Here we demonstrate a tailored light-wave-driven analogue to twisted layer stacking. Tailoring the spatial symmetry of the light waveform to that of the lattice of a hexagonal boron nitride monolayer and then twisting this waveform result in optical control of time-reversal symmetry breaking5 and the realization of the topological Haldane model6 in a laser-dressed two-dimensional insulating crystal. Further, the parameters of the effective Haldane-type Hamiltonian can be controlled by rotating the light waveform, thus enabling ultrafast switching between band structure configurations and allowing unprecedented control over the magnitude, location and curvature of the bandgap. This results in an asymmetric population between complementary quantum valleys that leads to a measurable valley Hall current7, which can be detected by optical harmonic polarimetry. The universality and robustness of our scheme paves the way to valley-selective bandgap engineering on the fly and unlocks the possibility of creating few-femtosecond switches with quantum degrees of freedom.

Propagation effects in polarization-gated attosecond soft-X-ray pulse generation.

Accurate estimation of the duration of soft-x-ray pulses from high-harmonic generation (HHG) remains challenging given their higher photon energies and broad spectral bandwidth. The carrier-envelope-phase (CEP) dependence of generated soft-x-ray spectra is indicative of attosecond pulse generation, but advanced simulations are needed to infer the pulse duration from such data. Here, we employ macroscopic propagation simulations to reproduce experimental polarization-gated CEP-dependent soft-x-ray spectra. The simulations indicate chirped pulses, which we theoretically find to be compressible in hydrogen plasmas, suggesting this as a viable compression scheme for broadband soft-x-rays from HHG.

Anomalous formation of trihydrogen cations from water on nanoparticles.

Regarded as the most important ion in interstellar chemistry, the trihydrogen cation, [Formula: see text], plays a vital role in the formation of water and many complex organic molecules believed to be responsible for life in our universe. Apart from traditional plasma discharges, recent laboratory studies have focused on forming the trihydrogen cation from large organic molecules during their interactions with intense radiation and charged particles. In contrast, we present results on forming [Formula: see text] from bimolecular reactions that involve only an inorganic molecule, namely water, without the presence of any organic molecules to facilitate its formation. This generation of [Formula: see text] is enabled by "engineering" a suitable reaction environment comprising water-covered silica nanoparticles exposed to intense, femtosecond laser pulses. Similar, naturally-occurring, environments might exist in astrophysical settings where hydrated nanometer-sized dust particles are impacted by cosmic rays of charged particles or solar wind ions. Our results are a clear manifestation of how aerosolized nanoparticles in intense femtosecond laser fields can serve as a catalysts that enable exotic molecular entities to be produced via non-traditional routes.

Few-cycle laser driven reaction nanoscopy on aerosolized silica nanoparticles.

Nanoparticles offer unique properties as photocatalysts with large surface areas. Under irradiation with light, the associated near-fields can induce, enhance, and control molecular adsorbate reactions on the nanoscale. So far, however, there is no simple method available to spatially resolve the near-field induced reaction yield on the surface of nanoparticles. Here we close this gap by introducing reaction nanoscopy based on three-dimensional momentum-resolved photoionization. The technique is demonstrated for the spatially selective proton generation in few-cycle laser-induced dissociative ionization of ethanol and water on SiO2 nanoparticles, resolving a pronounced variation across the particle surface. The results are modeled and reproduced qualitatively by electrostatic and quasi-classical mean-field Mie Monte-Carlo (M3C) calculations. Reaction nanoscopy is suited for a wide range of isolated nanosystems and can provide spatially resolved ultrafast reaction dynamics on nanoparticles, clusters, and droplets.

Millijoule femtosecond micro-Bessel beams for ultra-high aspect ratio machining.

We report on a functional experimental design for Bessel beam generation capable of handling high-energy ultrashort pulses (up to 1.2 mJ per pulse of 50 fs duration). This allows us to deliver intensities exceeding the breakdown threshold for air or any dielectric along controlled micro-filaments with lengths exceeding 4 mm. It represents an unprecedented upscaling in comparison to recent femtosecond Bessel beam micromachining experiments. We produce void microchannels through glass substrates to demonstrate that aspect ratios exceeding 1200∶1 can be achieved by using single high-intensity pulses. This demonstration must lead to new methodologies for deep-drilling and high-speed cutting applications.