Abstract Hybrid spin-exchange optical pumping (SEOP) using multiple alkali-metal species is commonly employed to improve the spatial homogeneity of alkali spin polarization in atomic magnetometers and comagnetometers. We investigate the dependence of spin polarization and its spatial homogeneity on the alkali density ratio \(D_r=[\mathrm{Rb}]/[\mathrm{Cs}]\) in Rb--Cs hybrid vapor cells. A series of cells with controlled \(D_r\) were fabricated and characterized using absolute electron paramagnetic resonance (EPR) polarimetry at multiple positions along the cell axis. The results show that the density ratio strongly influences both the magnitude and spatial uniformity of alkali polarization through the competition among optical pumping, spin exchange, and spin-destruction processes. An optimal density ratio is observed near \(D_r \approx 6.59 \pm 0.47\), where the polarization reaches its maximum while maintaining high spatial homogeneity (end-to-end ratio $\sim$89\% and RMS homogeneity $>95\%$). Under typical operating conditions, the optimal density ratio lies in the range \(D_r \approx 4\)--\(10\). These results clarify the role of the alkali density ratio in Rb--Cs hybrid SEOP and provide practical guidance for optimizing hybrid vapor cells used in precision atomic magnetometers and comagnetometers.

Abstract A novel model for dissipation of energy in a classical-quantum realm, and in a dedicated nanoenvironment, incorporating the effects of contamination or doping, is presented. At first glance, the theoretical framework behind it aims to merge some basic physical concepts of a statistical-mechanical and quantum nature, all of which point to a straightforward derivation of the coefficient of friction (COF) at a minute thermomechanical scale, somewhere between the meso- and nanoscale. As a motivation for our theoretical construct, we rely upon a certain number of superlubricity-related experiments for which the COF is supposed to be distinctly lower than 0.01. The principal novelty offered by the present study lies in introducing the environmental contamination and/or lubrication, to some extent also doping effects, that cause the friction process to transition from dry to wet characteristics. Building upon our previous modelling, we have decided to introduce an extension that accounts for practical lubrication. The extension rests upon employing the 2D Van der Waals (2D-VdW) equation of state as an approximation of a flat and somewhat squeezed interlayer between the rubbing incommensurate surfaces. The interlayer per se is a fluid layer at well-defined criticality conditions, where the criticality arises from the corresponding internal vs. external pressure-impact coming out from an efficient liaison of the Coulomb-Amontons (CA) friction and 2D-VdW thermodynamic laws together. This way, the form of the COF is adapted to a virtual change in environmental friction conditions. This adaptation offers primarily an intriguing possibility to address superlubric effect in the contamination and/or interlayer-formation conditions. In addition, certain plausible signatures of the nanoscale friction at the edge of the classical-quantum vacuum fluctuations’ involving edge, are disclosed. As a result, the ultralow COF values are gained and adjusted to primarily explaining selected experimental data of vital interest for recent advanced (bio)nanotechnology, energy saving and avoidance-of-wear or other severe dissipation circumstances.

Abstract As a classic frequency-doubling crystal for the visible spectra range, heavily magnesium-doped lithium niobate (LN:Mg) has long been hindered in its application to compact and thermally stable laser systems by its intrinsically high phase-matching temperature. In this study, a series of MgO-doped LiNbO3 crystals were grown from the ternary MgO-Li2O-Nb2O5 system, and their optical and nonlinear optical properties were systematically characterized. Compared with crystals grown from the conventional congruent binary Li2O-Nb2O5 system, the as-grown crystals from the ternary system exhibit markedly improved optical quality, enhanced optical damage resistance, and superior frequency-doubling performance. In particular, the LiNbO3 crystal doped with 6.0 mol.% MgO achieves an optical damage resistance threshold of 6.5 MW/cm2, which is an order of magnitude higher than that of the 5.0 mol.% MgO-doped LiNbO3 crystal grown from the binary system. Meanwhile, a high SHG conversion efficiency of 30.7% for a 1064 nm pulsed laser is realized at a reduced phase-matching temperature of 45.1 °C, with a corresponding temperature bandwidth of 1.5 °C. These results demonstrate that compositional modulation via the ternary system is an effective strategy to simultaneously improve the crystal quality and nonlinear optical performance of MgO-doped LiNbO3 crystals. The greatly optimized frequency-doubling properties underscore the significant application potential of the as-prepared LN:Mg crystals as efficient frequency converters for advanced compact green laser systems.

Abstract Magnetic particle spectroscopy (MPS) has emerged as a powerful biosensing modality due to its high sensitivity, matrix-free detection, and compatibility with point-of-care diagnostics. However, the lack of robust multiplexing capability remains a major bottleneck that limits its broader application in complex bioassays. To address this challenge, spatial and spectral separation strategies have been explored in recent years. While spatial separation approaches enable multiplexing without sacrificing sensitivity, they require physically distinct assay regions and substantially increase assay complexity, cost, and reagent consumption. In contrast, spectral separation offers a more resource-efficient solution by decoding multiple analyte signals from a single MPS measurement, but its performance critically depends on the choice of harmonic components used for decoding. In practical multiplexed MPS bioassays, numerical instability in solving harmonic equations directly translates into analyte misquantification and degrades assay performance. Despite significant progress, there is currently no systematic framework to guide the selection of harmonic sets that maximize spectral separation accuracy across different mixture complexities. Herein, we systematically investigate truncated spectral separation across binary, ternary, and quaternary mixtures of distinct magnetic nanoparticle (MNP) labels by evaluating a wide range of harmonic component combinations under identical experimental conditions. We demonstrate that the absolute determinant of the truncated reference matrix serves as a robust and effective criterion for identifying harmonic sets that yield the lowest decoding error under the evaluated experimental conditions. The proposed harmonic selection framework is demonstrated to be effective across multiple decoding strategies and provides practical design guidance for robust and scalable multiplexed MPS bioassays.