arXiv subjects

Niclas Sven Mueller

Publications and source records attributed to Niclas Sven Mueller.

3 recordsLinked to original sources

arXiv2026

Non-equilibrium state during proton-deuteron exchange at a liquid-liquid interface

Proton-deuteron exchange is a very fast process, even across macroscopic length scales. Here we directly and quantitatively measure the formation of HDO within the first 100 microseconds of the reaction at the liquid-liquid interface between D$_2$O and H$_2$O, using a fast-flowing liquid flat jet combined with infrared spectroscopic imaging. We demonstrate that, at early stages HDO formation is reaction-limited, set by the low concentration of the hydroxide and hydronium ions that mediate the exchange. As the ion concentration rises, the rate rapidly approaches the diffusion limit. The reaction rate constant we extract is consistent with the picosecond timescale of the elementary proton-deuteron exchange. Access to these microsecond kinetics reveals a non-equilibrium state in the early H$_2$O/D$_2$O interface: the two liquids are fully mixed by diffusion, yet the HDO concentration remains well below equilibrium. Quantitative imaging of reactant and product concentrations at well-defined liquid-liquid interfaces, as introduced here, will enable the study of fast kinetics across a wide range of chemical reactions.

physics.chem-ph↗

arXiv2024

Surface Phonon Polariton Ellipsometry

Surface phonon polaritons (SPhPs) have become a key ingredient for infrared nanophotonics, owing to their long lifetimes and the large number of polar dielectric crystals supporting them. While these evanescent modes have been thoroughly characterized by near-field mapping or far-field intensity measurements over the last decade, far-field optical experiments also providing phase information are less common. In this paper, we study surface phonon polaritons at the gallium phosphide (GaP)-air interface in the momentum domain using the Otto-type prism coupling geometry. We combine this method with spectroscopic ellipsometry to obtain both amplitude and phase information of the reflected waves across the entire reststrahlen band of GaP. By adjusting the prism-sample air gap width, we systematically study the dependence of the ellipsometry parameters on the optical coupling efficiency. In particular, we show that the combined observation of both ellipsometry parameters - amplitude and phase - provides a powerful tool for the detection of SPhPs, even in the presence of high optical losses. Finally, we theoretically study how surface phonon polariton ellipsometry can reveal the emergence of vibrational strong coupling through changes in the topology of their complex plane trajectories, opening up a new perspective on light-matter coupling.

physics.optics↗

arXiv2022

Carbon nanotubes for the optical far-field readout of processes that are mediated by plasmonic near-fields

As science progresses at the nanoscopic level, it becomes more and more important to comprehend the interactions taking place at the nanoscale, where optical near-fields play a key role. Their phenomenology differs significantly from the propagative light we experience at the macroscopic level. This is particularly important in applications such as surface-enhanced spectroscopies for single-molecule detection, where often the optimization of the plasmonic structures and surfaces relies on far-field characterizations. The processes dominating in the far-field picture, though, are not the same dominating in the near-field. To highlight this, we resort to very simple metallic systems: Isolated gold nanorods in solution. We show how single-walled nanotubes can be exploited to read out processes occurring at the near-field level around metallic nanoparticles and make the information accessible in the far-field region. This is implemented by monitoring the spectral profile of the enhancement of the photoluminescence and Raman signal of the nanotubes for several excitation wavelengths. Through this excitation-resolved study, we show that the far-field optical read-out detects the transversal and longitudinal dipolar plasmonic oscillations of gold nanorods, whereas the near-field read-out through the nanotubes reveals other mechanisms to dominate. The spectral position of the maximum enhancement of the optical near-field mediated signals are located elsewhere than the far-field bands. This dichotomy between near-field and far-field response should be taken into account when optimizing plasmonic nanostructures for applications such as surface-enhanced spectroscopies.

physics.optics↗