Numerical Simulation of the Photobleaching Process in Laser-Induced Fluorescence Photobleaching Anemometer

At present, a novel flow diagnostic technique for micro/nanofluidics velocity measurement—laser-induced fluorescence photobleaching anemometer (LIFPA)—has been developed and successfully applied in broad areas, e.g., electrokinetic turbulence in micromixers and AC electroosmotic flow. Nevertheless, in previous investigations, to qualitatively reveal the dynamics of the photobleaching process of LIFPA, an approximation of uniform laser distribution was applied. This is different from the actual condition where the laser power density distribution is normally Gaussian. In this investigation, we numerically studied the photobleaching process of fluorescent dye in the laser focus region, according to the convection–diffusion reaction equation. The profiles of effective dye concentration and fluorescence were elucidated. The relationship between the commonly used photobleaching time constant obtained by experiments and the photochemical reaction coefficient is revealed. With the established model, we further discuss the effective spatial resolution of LIFPA and study the influence of the detection region of fluorescence on the performance of the LIFPA system. It is found that at sufficiently high excitation laser power density, LIFPA can even achieve a super-resolution that breaks the limit of optical diffraction. We hope the current investigation can reveal the photobleaching process of fluorescent dye under high laser power density illumination, to enhance our understanding of fluorescent dynamics and photochemistry and develop more powerful photobleaching-related flow diagnostic techniques.

Elucidating Cooperative Luminescence and Cooperative Sensitisation Upconversion of Nonanuclear Lanthanide Complexes in Solution

Upconversion nanoparticles have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work we explore, for the first time, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state and investigate the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/ Tb systems. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with an unprecedented quantum yield of 2.8 × 10-6 at a low laser power density of 2.86 W/cm2.

Substrate Cleaning Threshold for Various Coated Al Alloys Using a Continuous-Wave Laser

In this study, different coatings (gray epoxy primer, white epoxy varnish and red alkyd paint) of 7075 aluminum alloy are cleaned with a 500 W continuous-wave (CW) fiber laser. We analyzed the influence of the laser power density on the temperature evolution and target surface morphology. Under the condition of continuous laser irradiation for 1 s, the experimental results indicated that the suitable cleaning thresholds of epoxy primer, epoxy primer and epoxy varnish, as well as epoxy primer, epoxy varnish and alkyd paint were 177.74, 192.89 and 147.44 W/mm2. The results show that the cleaning threshold of thicker three-layer paint target was smaller than the single-layer paint layer, and we analyze the mechanism of this phenomenon.

Yb to Tb Cooperative Upconversion in Supramolecularly Assembled Complexes in a Solution

The podand-type ligand L, based on a tertiary amine substituted by three pyridyl-6-phosphonic acid functions, forms hydrated complexes with Ln3+ cations. The luminescence properties of the YbL complex were studied in D2O as a function of the pD and temperature. In basic conditions, increases in the luminescence quantum yield and the excited state lifetime of the Yb centered emission associated with the 2F5/2 → 2F7/2 transition were observed and attributed to a change in the hydration number from two water molecules in the first coordination sphere of Yb at acidic pH to a single one in basic conditions. Upon the addition of TbCl3 salts to a solution containing the YbL complex in D2O, heteropolynuclear Yb/Tb species formed, and excitation of the Yb at 980 nm resulted in the observation of the typical visible emission of Tb as a result of a cooperative upconversion (UC) photosensitization process. The UC was further evidenced by the quadratic dependence of the UC emission as a function of the laser power density.

Effect of Laser Shock Peening on Fretting Fatigue Life of TC11 Titanium Alloy

The purpose of this paper is to investigate the performance of laser shock peening (LSP) subjected to fretting fatigue with TC11 titanium alloy specimens and pads. Three laser power densities (3.2 GW/cm2, 4.8 GW/cm2 and 6.4 GW/cm2) of LSP were chosen and tested using manufactured fretting fatigue apparatus. The experimental results show that the LSP surface treatment significantly improves the fretting fatigue lives of the fretting specimens, and the fretting fatigue life increases most when the laser power density is 4.8 GW/cm2. It is also found that with the increase of the laser power density, the fatigue crack initiation location tends to move from the surface to the interior of the specimen.

Comparison of laser irradiation behavior of plasma-sprayed Ba2-xSrxSmTaO6 coatings

Ba2 − xSrxSmTaO6 (x = 0–2), a series of low thermal conduction and good thermal stability materials, is believed to have potential use as thermal insulation laser protection materials, however, is limited by their unclear laser damage mechanism. Thus, in this paper, we investigated and compared their laser damage behaviors as well as their protection thresholds. The Ba2 − xSrxSmTaO6 (x = 0, 0.5, 1, 1.5 and 2) powder were prepared by solid state reaction and the coatings were prepared by air plasma spray. During the laser irradiation process, we observed that the coatings underwent irradiation center turning bright, grains recrystallization and melting process. When the laser power density was 500W/cm2, no damage was observed in all five components coatings and the ablation surface of x = 1 and 2 coatings turned bright. However, the melting phenomenon occurred to x = 0.5, 1, 1.5 and 2 coatings while the laser power density reached 1000W/cm2, and the ablation area of x = 0 coatings started to turn bright. At 1500W/cm2 laser power density, coatings of all five components melted and the melting area of x = 0 and 1 was obviously smaller than the coatings of other three components. Ba2SmTaO6 (x = 0) showed the highest protection threshold that was 2000W/cm2 for 10 s, and x = 0.5 coatings showed the lowest protection threshold that was 1000W/cm2 for 26 s. In terms of back surface temperature, BaSrSmTaO6 (x = 1) presented the lowest value and Ba2SmTaO6 was only slightly higher than it. To sum up, Ba2SmTaO6 has the best comprehensive laser protection properties and is believed to be the most potential laser protection materials among the five components.