Optics and Laser Technology最新文献

Surface-enhanced Raman scattering (SERS) using noble metal complexes in nanomaterials has been extensively explored in many fields. Here, a novel bimetallic nanocavity SERS substrate of closely spaced Ag@Au core–shell nanoparticles combined with smooth gold films called metal particle-on-film nanocavity were prepared by physical deposition and chemical self-assemble.The limit of detection for rhodamine 6G (R6G) on the SERS substrate has been extended to an impressively low concentration of 10⁻¹⁰ M, with commendable sensitivity and uniformity, corresponding to an analytical enhancement factor of 2 × 10⁷. In addition, the bimetallic nanocavity SERS substrates combine the beneficial plasmon properties of Au and Ag. The Raman intensity of R6G on this bimetallic nanocavity substrate is more than 20 times that of only Ag nanoparticles. Our results of finite-different time-domain (FDTD) simulation and experiment show that such a nanocavity substrate supports strong plasmonic resonance which results in excellent SERS activity, high spatial homogeneity and chemical stability. This work provides an effective SERS substrate for imaging and detection in the chemical and biological fields.

This work compares two ultra-high sensitive refractive index (RI) sensors based on metal–insulator-metal (MIM) waveguides coupled with an elliptical cavity for label-free, low-cost, and fast gas sensing for biomedical and industrial applications. The first sensor can be called a Side-Coupled Elliptical Cavity (SCEC), and the second sensor can be called a novel Ring Encapsulated Elliptical Cavity (REEC), which has an elliptical cavity inside an elliptical ring with a small gap. The transmission spectra of both sensors are investigated by finite element method (FEM) simulations. By optimizing the structural parameters and enhancing the light-matter interaction, the REEC sensor exhibits a maximum sensitivity (S) of 7078.12 nm/RIU and a figure of merit (FOM) of 16.3 RIU⁻¹, which are 91.06 % and 46.94 % higher than the SCEC sensor, respectively. The sensors are tested with dielectric materials of different RIs from 1 to 1.02. Notably, both sensors exhibit nearly identical resonant wavelengths. To evaluate efficiency, we introduce a new parameter: sensitivity per resonant wavelength (S/RW). A higher S/RW indicates superior sensitivity at a lower resonant wavelength, desirable for compact and cost-effective devices. The REEC sensor outperforms existing plasmonic MIM waveguide-based sensors in terms of S/RW. Furthermore, owing to its exceptional accuracy, the REEC sensor can detect gases such as helium, carbon dioxide, gaseous methanol, and gaseous ethanol. This makes it a promising candidate for diverse biomedical and industrial applications, raising exciting possibilities for real-world implementation.

Processing CuCr1Zr copper alloy using the L-PBF process is extremely difficult due to its high reflectivity at the common L-PBF wavelength of 1064 nm and high thermal conductivity. Therefore, previous studies utilized a high laser power energy density to perform the 3D printing of CuCr1Zr parts. However, at high laser energy densities, the physics that lead to the formation of the melt pool, such as the laser absorption, Marangoni force, and recoil pressure, are extremely complex. Notably, these phenomena have both individual and interactive effects on the stability of the melt pool. Thus, identifying the processing conditions (i.e., laser power, scanning speed, and hatching space) that lead to stable scan tracks and smooth surface scanning through experimental trial-and-error methods is costly and time-consuming. Accordingly, this study develops a Computational Fluid Dynamics (CFD) simulation model that considers the effects of all three factors on the formation of the CuCr1Zr melt pool. The simulation model is verified with experimental data reported in the literature. The verified model is then utilized to determine the L-PBF processing conditions that lead to stable scan tracks and surface scanning. The numerical and experimental results reveal that the laser power of 500 W, scanning speed of 600 mm/s, and hatching spaces between 80 and 100 µm ensure the stability of both single-scan tracks and surface scanning and yield a smooth surface morphology as a result.

Six-temperature model is a main model to simulate dynamics of molecular gas lasers including transversely excited atmospheric-pressure (TEA) CO₂ lasers. We propose a method to predict the effect of hydrogen on the dynamics of TEA CO₂ laser by the six-temperature model. A new term was added to the six-temperature model to allow for the interaction of hydrogen with the gas mixture. We also determined some relaxation times of the molecular energy levels from the vibrational energy exchange rates between hydrogen and the gas mixture. The system of ordinary differential equations including five energy densities, ambient temperature and light intensity was simulated using the standard Runge-Kutta method. The simulation results were also compared with the experimental results in the literature to verify the accuracy of an improved six-temperature model and simulation method. Using the proposed method, the optimum gas mixture ratio, total pressure, laser cavity and discharge gap geometry can be theoretically determined and these values will be very useful for high-power TEA CO₂ laser design.

This paper examines the effects of horizontal airflow on laser welding of galvanized steels. A uniform laminar flow was delivered using a specially designed external airflow device, and its impact on vapor plume dynamics and keyhole behavior was analyzed with synchronized high-speed cameras. The analysis showed that, under Follow airflow conditions, penetration depth was notably reduced compared to Against conditions, with increased surface defects observed in galvanized steel. Keyhole instability, as observed in the frequency spectrum (≤500 Hz), was more pronounced under Follow conditions. Theoretical analysis identified two main effects: 1) Airflow in the Against condition helps maintain keyhole opening by dragging melt away, while in the Follow condition, it drags melt toward the keyhole, leading to shrinkage. 2) Airflow affects the plasma plume, with Follow conditions increasing laser energy attenuation and resulting in shallow penetration. A hollow rectangular block fixture was designed to shield the molten pool and keyhole region from airflow effects. CFD modeling and experiments with a 5 mm block demonstrated reduced airflow impacts, improved process stability, and defect-free welds in both bare and galvanized steel.

This study presents the results of an investigation on the influence of powder feed rate on the microstructure and composition of laser-deposited cobalt–based alloy (MetcoClad21). Optical emission spectroscopy (OES), metallographic analysis, and energy-dispersive X-ray spectroscopy (EDX) was used to characterize the samples and the process. The OES analysis was used to identify element specific atomic emission lines (peaks) within the captured optical process emissions. The elemental composition of the deposited material was observed and peak intensity ratio of certain elements were calculated in situ. The metallographic and EDX analyses were used to measure the cross–sectional–dimensions of the deposition tracks and to analyse the elemental composition of the deposited material. The results showed that the powder feed rate had a significant influence on the microstructure, the cross–sectional–dimensions and the composition within the deposition tracks. Specifically, the authors found that the Fe/Cr peak intensity ratio decreased with increasing powder feed rate, indicating a decrease in the Fe content and an increase in the Cr content of the deposited material. Hence, the peak intensity ratios could have been correlated with the track compositions and the dilution with the Fe-based substrate material. The results of this study have implications for the optimization of laser deposition processes for cobalt–based alloys by an in situ control by OES.

Miniaturization of erbium fiber lasers is a crucial task, which implies a use of heavily doped fibers. In dense ensemble, interaction of erbium ions changes a configuration of quantum levels of gain medium and leads to pulsed generation. As a result, heavily doped erbium lasers demonstrate two thresholds, the first one associated with an onset of lasing in the pulsed regime, and the second with a transition to CW. Operation features near these two thresholds have been established experimentally. For the first time, a power-law behavior of the system parameters – pulses frequency, duration and peak intensity – was revealed in a wide range of pump rates around both thresholds. The power exponents were associated with critical indices of phase transition. Their values were convincingly determined different from integers and half-integers. Critical indexes were shown weakly dependent on the Fabry-Perot and distributed feedback (DFB) laser cavity parameters, which made it possible to experimentally establish the universal dependence of the pulse frequency and duration on the lasing power. The results of the work are extremely useful for determining and predicting the parameters of the designed erbium lasers, due to universality of the critical indices.

A maiden attempt to synthesize ternary nanocomposites of Bismuth Ferrite-Graphene-Sodium Niobate (BiFeO₃-Graphene-NaNbO₃) has been made by utilizing conventional solution method. Systematic increase in the loading concentrations of graphene on BiFeO₃-NaNbO₃ heterogeneous composites found to have influenced their structural parameters, optical properties, photocatalytic dye degradation and their dielectric properties and they were assessed from powder X-ray diffraction (XRD), Raman spectral measurements, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), optical absorption spectra in UV–Visible-Near IR region, Photoluminescence (PL) studies, photocatalysis and dielectric analyses. As such, admixture of graphene concentration of 0.025 percent with BiFeO₃-NaNbO₃ composite powders capable of degrading the methylene blue (MB) dye upto a maximum efficiency of 92% within the time frame of 120 min. Strikingly, at the same graphene concentration of 0.025 percent, the synthesized ternary composite sample exhibits an increased dielectric constant value of ‘37′. A close parallelism between the dielectric behaviour and photocatalytic dye degradation properties could be predicted from this study. Further, among various loading concentrations of graphene with BiFeO₃-NaNbO_3, lower concentration of 0.025 percent of graphene has been found as the optimum one for the improved MB dye degradation characteristics of the ternary composites of BiFeO₃-Graphene-NaNbO₃ with enhanced suppression of carrier recombination. The other interesting results have been discussed in detail.

Temperature-dependent anisotropic thermal and polarized spectroscopic properties of Nd:YVO₄ crystal from liquid hydrogen temperature (20 K) to 300 K have been investigated. Thermal properties, such as thermal conductivity and thermal expansion coefficient (TEC) were presented. The maximum thermal conductivity along a-axis and c-axis are 218.3 W/mK and 238.9 W/mK at ∼ 20 K, respectively, both are over one order of magnitude than that at room temperature. The TEC decreases progressively as temperature drops from room temperature to 20 K. Moreover, the polarized spectroscopic properties including absorption cross section, stimulated emission cross section and fluorescence lifetime have been measured to evaluate laser performance. Furthermore, we experimentally explore continuous wave laser performances under various temperatures. A maximum output power of 1.67 W is obtained at 20 K, corresponding to an optical–optical efficiency of 55.7 %, both are about 3.2 times better than that at room temperature. The near-diffracted-limited operation is also observed at 20 K. Thus, based on the large thermal conductivity, low thermal expansion and high emission cross section, high-power, high-beam-quality and highly efficient Nd:YVO₄ lasers can be developed at 20 K.