Applied Physics B最新文献

While time-resolved laser-induced incandescence (TiRe-LII) has become a standard laser-based diagnostic for soot, there remain unexplained observations in some datasets. One such effect is the so-called “anomalous cooling”, in which the pyrometric temperature decays faster than can be explained by conventional heat transfer models immediately following the peak temperature. This work investigates this phenomenon through experiments on soot entrained in different bath gases and irradiated in the low-fluence regime, where particle sublimation is minimal. The anomalous cooling phenomenon is caused by the contribution of particles in the probe volume that have been heated beyond the sublimation threshold to the overall incandescence signal, due to nonuniform laser fluence. Particles in these “hot spot” regions feature a faster cooling rate due to sublimation, contributing to the effect of apparent anomalous cooling. Particle-size polydispersity also plays a notable but minor role. The effect depends on the bath-gas composition, which is attributed to differences in species-specific heat transfer.

Seven-light-screen precision target is an new exterior ballistic parameter testing device that enables non-contact measurement of multiple parameters with a single shot, including impact coordinates, flight velocity vector, pitch and azimuth angles of the flight direction, and velocity attenuation rate for obliquely incident flying targets. Precise calibration of structural parameters for invisible light-screen array of seven-light-screen precision target is crucial for ensuring its accuracy and constitutes an essential step in its development. In this paper, we proposes a high-accuracy calibration method. It uses a light-blocking probe to simulate the entry of a projectile into the invisible light screen for the extraction of the actual light screen center. Dual theodolites are employed in conjunction with the readings of a grating ruler during screen crossing to quickly and accurately locate the coordinate points within the screen. Multiple extracted screen coordinate points are then fitted to reconstruct the actual light screen plane, achieving accurate spatial position calibration of multiple invisible screens. This paper introduces the structure and measurement principle of seven-light-screen target, describes the calibration system's composition and working principle, and derives the specific calibration algorithm formulas. The constructed system was applied to calibrate an actual seven-screen target. Live-fire experiments were conducted based on this calibrated target, and the measured impact coordinates were compared against results from cardboard target measurements. The research results show that the structural parameters obtained by the proposed calibration method are accurate. After calibration, the coordinate measurement accuracy of the seven-light-screen precision target within a 1 m × 1 m target sensor area is better than 1.5 mm. This calibration method is universal for the calibration of light-screen arrays with similar principles and provides an effective solution for calibrating structure parameters of multi-light-screen precision targets.

Understanding the plasmonic behavior of materials with large-scale inhomogeneity (like negative index metamaterial) is crucial because these materials cannot be modeled as simple scatterers. This is due to the emergence of plasmonic materials other than those based on noble metals and the advancement of sophisticated materials processing techniques. Propagation of surface plasmons can be achieved using a metamaterial and dielectric interface. Control of the surface plasmons opens the wide avenues for a broad range of applications. Similarly, the shape of individual surface nanostructures or the phase control of individual elements in an array of such structures can be used to regulate the propagation of surface plasmons at the interface between metal films and dielectric materials. Here, we show how to regulate surface plasmon propagation at the negative index metamaterial-dielectric interface. The negative index metamaterial enables the surface modes, while the adjacent dielectric is what tunes their properties.

Unusual characteristics of graphene as a tunable and nonlinear 2D material embedded in a novel voltage-controlled electromechanical design of the metasurface is implemented to achieve effective tunable third harmonic generation (THG) in the terahertz (THz) regime. This approach offers certain advantages over conventional methods based on tuning the Fermi energy of graphene via electrical or chemical doping, such as a broader tuning range and post-fabrication reconfigurability. We introduce mechanical tunability through a suspended graphene sheet placed over an oxide grating on a gold substrate. Applying a low voltage in the range of (1{-}5:text{V}) between the graphene and gold layers induces mechanical bending of the graphene, resulting in a tunable shift in the resonance frequency. Simulations exhibit a continuous fundamental frequency (FF) shift of (2.9:text{T}text{H}text{z}) with only a (:5:text{V}) potential, enabling a dynamic control of the THG output without modifying the metasurface structure. This compact and efficient platform holds promise for tunable THz sources in spectroscopy and imaging applications.

We demonstrate fast, high-resolution fabrication of in-volume diffractive optical elements (DOEs) by combining galvanometric scanning with a microscope objective ((text {NA}=0.4)) and a 1 ps laser source. Carefully chosen parameters exploit nonlinear absorption in fused silica to create highly localized refractive-index modifications. Estimating the scaling laws of energy deposition, single-pulse writing becomes feasible, yielding a ( Delta n approx 0.5 times 10^{{ - 2}} ) and modification dimensions below the nominal focal spot. Using a layer-stacking scheme in z, we assemble multi-level phase masks: examples include a 4-level, 250 × 250 px DOE (2 µm pixels) and a 10-level, 416 × 416 px DOE (1.2 µm pixels). Phase-contrast microscopy confirms the written phase structure and comparison with theoretical phase data shows strong correspondence. Optical characterization at 532 nm reproduces target intensity distributions with high fidelity (overlap (>80)% against the computed discretized mask in a selected region; >66% vs. the original target). Yet, production times remain short (8–9 min for the high-resolution DOE, 60 s for the 4-level device), demonstrating that we improve trade-off between quality and speed to a level good enough for practical applications.

The difference in harmonic intensity distribution during frequency up-conversion of 800 nm, 65 fs pulses in a few laser-induced plasmas (Li, Ge, Yb, and As) at variable concentrations of ions is demonstrated. At small fluencies of heating pulses that create laser-induced plasmas, the prevailing presence of neutrals leads to a gradual decrease in harmonic yield. The stronger laser ablation caused the appearance of the singly and doubly charged ions, which led to the enhancement of the single harmonic or small group of harmonics in the vicinity of the ionic transitions possessing large oscillator strengths. These resonance-enhanced harmonics become significantly larger than the neighboring harmonics. The peculiarities of this process are analyzed at different fluencies of the ablating pulses.

This study systematically quantifies the surface figure error (SFE) of the first reflection mirror (M1) in SHINE’s FEL-I beamline via numerical calculations and experiments, analyzing contributions of thermal SFE (from X-ray heat loads) and non-thermal SFE (mechanical-related: clamping force, mirror self-gravity, In-Ga liquid gravity; manufacturing-induced). Results demonstrate that thermal SFE dominates at (ge )50 kHz (over 96% of total SFE); at (le )10 kHz, non-thermal SFE accounts for up to 38% (manufacturing-induced SFE leads in effective footprint zone, In-Ga liquid gravity impacts mechanical-related SFE more than other factors). This work clarifies thermal/non-thermal SFE dynamics across repetition rates, providing a basis for M1 optimization and high-repetition-rate XFEL mirror design.

This work examines the second harmonic generation of a Hermite-cosh-Gaussian (HChG) laser beam with circular polarisation in a magnetised plasma. In order to explain relativistic electron motion and the distinctive transverse structure of the Hermite-cosh-Gaussian beam, it is assumed that the nonlinear current density is the cause of second harmonic generation. By using the paraxial approximation, analytical formulas for the second harmonic field amplitude are derived, reflecting dependencies on plasma density and beam characteristics (mode indices, intensity, and beam width). The wiggler magnetic field enhances the phase-matching conditions to permit resonant SHG by giving the second-harmonic photons some more velocity. Furthermore, the wiggler field will contribute to the maintenance of the cyclotron frequency, which confines the plasma electrons and increases SHG efficiency. While the HChG profile delivers mode-selective gain in SHG efficiency, circular polarisation improves azimuthal symmetry in the nonlinear current. The findings demonstrate how structured beams may be used to generate high-harmonics in laser-plasma interactions in a customised manner.

The manipulation of quantum states via mechanical strain offers a pathway to engineer topological excitons in soft semiconductors. Here, we present a theoretical framework that shows that helical strain transforms lead halide perovskite quantum dots (QDs) into a platform for topological excitonics. Using a first-principles-informed framework combining strain-modulated Lamé eigenstates and non-perturbative Coulomb interactions, we identify a strain-driven topological transition at critical ellipticity (k = 0.59 pm 0.02), (corresponding to ≈ 2% torsional strain) marked by inversion of the exciton Chern number ((C = 0 to 1)) and π-Berry phase accumulation. Quantitative calculations yield an exciton binding-energy enhancement up to 45 meV and photoluminescence (PL) redshifts of ≈ 72 meV, in agreement with experimental data. The computed deformation potential (− 0.8 eV/% strain), group-velocity scaling (v ∝ k^1.7), and Chern-number inversion confirm a strain-driven topological crossover supported by Berry-phase accumulation. Comparison with reported PL and diffusion measurements validates the predictive accuracy of the Lamé-Coulomb formalism, which bridges continuum elasticity with quantum confinement. These findings proposes perovskite QDs as experimentally accessible hosts of strain-tunable topological excitons, enabling reconfigurable quantum-photonic and optoelectronic devices based on mechanically programmable excitonic states.

The laser-induced fluorescence of oxygen in organic solvents was investigated as a potential contactless method for quantifying its concentration. The fluorescence was excited directly by a diode laser radiation in the 1270 nm oxygen absorption band. Neither a sensitizer nor singlet oxygen traps were added to the solvent. An uncooled photodiode was used as a detector. A simple kinetic model was used for assessing the oxygen concentration from the signal magnitude and decay time. An advanced model explaining the observed deviation of fluorescence kinetics from single exponential decay is presented. The method was tested with pure and contaminated Freon-113 samples, chloroform, acetone and n-heptane.