Advanced Research Center for Nanolithography (Netherlands)

Quasi-bound states in the continuum in dielectric metasurfacessupport sharp Fano resonances that emerge from the interference betweenbrightand dark modes. We exploit this modal interplay to demonstrate tunablethird-harmonic emission, controlled through the driving pulse’swavelength and intensity. Our experiments show imbalances in third-harmonicdiffraction patterns and non-Gaussian third-harmonic spectral featuresthat exhibit strong variations near the Fano resonance. We explainthe observations via a coupled-oscillator model that captures theinterplay between the driving field and the nonlinear response ofthe modes, explaining our observations and providing a predictiveframework for optimizing the third-harmonic diffraction efficiency.These results establish pulse-engineered metasurfaces as a powerfulplatform for nonlinear wavefront shaping and frequency conversionapplications while simultaneously serving as a warning that pulseproperties play a vital role in metasurface function design.

Nickelate materials offer diverse functionalities for energy and computing applications. Lithium nickel oxide (LiNiO2) is an archetypal layered nickelate, but the electronic structure of this correlated material is not yet fully understood. Here we investigate the temperature-dependent speciation and spin dynamics of Ni ions in LiNiO2. Ab initio simulations predict that Ni ions disproportionate into three states, which dynamically interconvert and whose populations vary with temperature. These predictions are verified using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and resonant inelastic x-ray scattering at the Ni L3,2-edge. Charge-transfer multiplet calculations consistent with disproportionation reproduce all experimental features. Our results support a model of dynamic disproportionation that explains diverse physical observations of LiNiO2, including magnetometry, thermally activated electronic conduction, diffractometry, core-level spectroscopies, and the stability of ubiquitous antisite defects. This unified understanding of the material properties of LiNiO2 is important for applications of nickelate materials as battery cathodes, catalysts, and superconductors.

Fano lineshapes associatedwith quasi bound-state-in-the-continuumresonances, that are supported by dielectric metasurfaces, have theadvantageous properties of being extremely sensitive to minute geometricalchanges in the meta-atoms. We show an approach to determine deep subwavelengthfeature sizes, comparable to semiconductor critical dimension metrology,by structurally infilling a void of a dielectric disk-hole metasurfacedesign. Our simulated results show a sensitivity of 40.5 nm resonantwavelength shift for a 1 nm feature width (i.e., critical dimension)change, at an optical line width of 1.8 nm. We present both experimentaland simulated results of different void infillings and attribute thespectral change of the resonance to the sensitivity to an effectiveindex in the void of the meta-atom, which arises from the filled volumefraction and the material boundaries orientation relative to the localpolarization. Treating our metasurface as an effective index sensor,the sensitivity is 262 nm·RIU–1 and the figureof merit is 146 RIU–1, which underlies the pronouncedresonant wavelength shift driven by similarly large changes in theeffective index caused by extremely tiny critical dimension variations.This approach could impact critical dimension measurements in semiconductormetrology, as it works at the high throughput of optical measurementswhile performing at the high resolution of scanning electron microscopy.

Pump-probe spectroscopy is commonly used to study ultrafast phenomena [1]. An ultrashort laser pulse (pump) induces dynamics, which are then measured with another, delayed laser pulse (probe). The probe signal is often weak, making efficient measurements with good signal-to-noise ratio (SNR) challenging. If long integration times are needed to achieve sufficient SNR, applications requiring high speed acquisition, such as 2D pump-probe imaging, become impractical. In this contribution, we optimize three crucial aspects of pump-probe spectroscopy to enable high-speed and low-noise measurements. We show measurements of ultrafast photoacoustics to demonstrate shot-noise-limited performance.

Label-free quantitative phase imaging is vital for optical microscopy and metrology applications. A multimode fiber stands out as a desirable platform for imaging. Here, we propose and experimentally demonstrate a non-interferometric non-iterative approach for high-speed high-resolution label-free quantitative phase imaging via a random light scattering in a multimode fiber.

Single and double electron capture cross-sections for collisions of 118Sn4+ with molecular hydrogen have been measured in an energy range of 1 keV to 16 keV using a crossed-beam setup. The cross-sections are determined from measurements of charge-state-resolved ion currents obtained through a retarding field analyser. Remarkably, the single electron capture cross-sections for Sn4+ are more than a factor 3 smaller than the previously determined single electron capture cross-sections for Sn3+–H2 collisions and the double electron capture cross-sections are only about 20% smaller than the single electron capture cross-sections. These results are understood on the basis of potential energy curve crossings. The first active curve crossings for the Sn4+–H2 system happen at a relatively small internuclear distance of about 5.5 a.u., which should be compared to 8 a.u. for Sn3+ ions. Multi-channel Landau–Zener calculations have been performed for single electron capture and confirm these low cross-sections. The curve crossing for double electron capture by Sn4+ lies very close to the one for single electron capture, which may explain the single and double electron capture cross-sections being of similar magnitude.

We introduce an alternative method for switching-magnetization magnetic force microscopy that utilizes planar tip-on-chip probes. Unlike conventional needle-like tips, the planar probe technique incorporates a microdevice near the tip apex on a 1×1mm2 chip, which allows for thin-film engineering and micro/nano-customization aimed at application-specific tip functionalization. In this study, we establish a microscale current pathway near the tip end to manage the tip magnetization state. This planar probe was used to investigate the intricate disordered magnetic domain structure of an epitaxial thin film of the transition metal oxide perovskite LaMnO3, a material previously demonstrated to exhibit complex domains related to superparamagnetism, antiferromagnetism, and ferromagnetism. We successfully visualized an inhomogeneous distribution of magnetic islands near the Curie temperature, with a resolution exceeding 10nm.

The emission of high-order harmonics from solids under intense laser-pulse irradiation is revolutionizing our understanding of strong-field solid-light interactions, while simultaneously opening avenues towards novel, all-solid, coherent, short-wavelength table-top sources with tailored emission profiles and nanoscale light-field control. To date, broadband spectra in solids have been generated well into the extreme-ultraviolet (XUV), but the comparatively low conversion efficiency in the XUV range achieved under optimal conditions still lags behind gas-based high-harmonic generation (HHG) sources. Here, we demonstrate that two-color high-order harmonic wave mixing in a fused silica solid is more efficient than solid HHG driven by a single color. This finding has significant implications for compact XUV sources where gas-based HHG is not feasible, as solid XUV wave mixing surpasses solid-HHG in performance. Moreover, our results enable utilizing solid high-order harmonic wave mixing as a probe of structure or material dynamics of the generating solid, which will enable reducing measurement times compared to the less efficient regular solid HHG. The emission intensity scaling that follows perturbative optical wave mixing, combined with the angular separation of the emitted frequencies, makes our approach a decisive step for all-solid coherent XUV sources and for studying light-engineered materials.

A universal power-law scaling z[over ¯]∝E^{0.4} in the correlation between the average ion charge state z[over ¯] and kinetic energy E in expanding laser-driven tin plasmas is identified. Universality here refers to an insensitivity to all experimental conditions: target geometry, expansion direction, laser wavelength, and power density. The power law is accurately captured in an analytical consideration of the dependence of the charge state on temperature and the subsequent transfer of internal to kinetic energy in the expansion. These analytical steps are individually, and collectively, validated by a two-dimensional radiation-hydrodynamic simulation of an expanding laser-driven plasma. This power-law behavior is expected to hold also for dense plasma containing heavier, complex ions such as those relevant to current and future laser-driven plasma light sources.

In a joint experimental and modeling effort, we have studied 15 keV Ar2+, Kr2+, and Xe2+ projectiles scattering off a polycrystalline Ru surface to assess the possible role of surface roughness in the apparent near-absence of the single-collision (SC) peak for Xe, the heaviest of the ions. The surface roughness of the Ru sample was determined by atomic force microscopy (AFM). The AFM image is used as input to simulations by means of the ray tracing code SPRAY. The observed spectra display a significant SC peak in the energy distributions of reflected Ar and Kr ions, which is not the case for Xe. The energy spectrum produced by the reflection of Xe and recoiled Ru ions closely corresponds to the results obtained from our SPRAY simulations. It is evident that surface roughness plays a crucial role in the visibility of the SC peak. From simulations for different target roughnesses, it is clear that a concentrated distribution of inclination angles centered around 2° effectively reduces the intensity of the single-collision peak for Xe ion scattering. The presence of a distinct SC peak for Ar and Kr ions, unlike Xe ions, supports the notion that the absence of a prominent SC peak in Xe ion scattering is not primarily due to geometric blocking of trajectories by surface roughness. This suggests that for slow, heavy ions like Xe, scattering effects arising from the nearest neighbors to the binary collision partner are significant and should be carefully examined.