Modeling and admittance recursive simulation of anti-reflective coatings for photothermal conversion: synergy between subwavelength structures and gradient refractive index layers.

In the field of photothermal conversion, light-absorbing layers show limitations such as low solar energy utilization and excessive surface reflection. This paper proposes a new anti-reflective coating consisting of a gradient-doped fluorescent glass film covering a subwavelength structural layer for photothermal conversion. Its transmittance was simulated using equivalent medium theory and the admittance recursion method. The subwavelength structure provides a refractive index gradient, and its shape solves the problem of the sharp decrease in transmittance at high angles of incidence. Subsequently, we adjust the material parameters of the gradient refractive layers and control the thickness of each layer to minimize interlayer Fresnel reflections. Finally, the efficient light-trapping ability of the model was verified by calculating and comparing the transmittances of the optimized model and bare glass. Notably, within the visible spectrum, our model achieves an average transmittance of over 95% across wavelength and angle ranges, effectively suppressing surface reflections. At a larger light incident angle, the transmittance increases by 29.7%, and the minimum angle transmittance reaches 92.7%. This study proposes an innovative method to enhance the performance of transmission layers in photothermal conversion devices.

Structures, principles, and properties of metamaterial perfect absorbers.

Metamaterials are a kind of artificial material with special properties, showing huge potential for applications in fields such as infrared measurement, solar cells, optical sensors, and optical stealth. A metamaterial perfect absorber (MPA) is designed based on a metamaterial, featuring strong absorption, small volume, light weight, ultra-bandwidth, tunability and other characteristics. This paper introduces the absorption mechanism of MPAs from microwave to optical wave band, and four directions of absorber design are elaborated. Equivalent impedance matching, plasma resonance and interference effect are the main absorption mechanisms of MPA. Multiband perfect absorption, ultra-wideband and ultra-narrowband perfect absorption, polarization and angle insensitive absorption, and dynamically controllable tunable absorption are the main design aspects. Among them, the proposal of a dynamically tunable absorber realizes the dynamic absorption. Finally, the problems and challenges of metamaterial perfect absorber design are discussed.

Structures, plasmon-enhanced luminescence, and applications of heterostructure phosphors.

Heterostructure phosphor composites have been used widely in the fields of targeted bio-probes and bio-imaging, hyperthermia treatment, photocatalysis, solar cells, and fingerprint identification. The structures, plasmon-enhanced luminescence and mechanism of metal/fluorophore heterostructure composites, such as core-shell nanocrystals, multilayers, adhesion, islands, arrays, and composite optical glass, are reviewed in detail. Their extended applications were explored widely since the surface plasmon resonance effect increased the up-conversion efficiency of fluorophores significantly. We summarize their synthesis methods, size and shape control, absorption and excitation spectra, plasmon-enhanced up-conversion luminescence, and specific applications. The most important results acquired in each case are summarized, and the main challenges that need to be overcome are discussed.

Monte Carlo simulation and experimental evaluation of the quantum efficiency of Eu3+-doped glass at different temperatures.

The quantum efficiency (QE) is a key parameter to evaluate the optical properties of fluorescent glass. However, it is very difficult to measure the QE at high temperatures by the integrating sphere test system. In this paper, we report a new method to calculate the QE of five kinds of Eu3+-doped glasses at different temperatures based on experimental absorption and excitation spectra of Eu3+-doped glasses. The simulated QE values agree well with the experimental values of QE. Furthermore, the influence of the shape, refractive index and temperature on the QE and the spatial light intensity distribution of the Eu3+-doped glass is studied based on the Monte Carlo method. This work presents a simple method to calculate the QE and the spatial light intensity distribution at different temperatures.

Morphology control, spectrum modification and extended optical applications of rare earth ion doped phosphors.

Rare earth ion (RE3+) doped nano-phosphors with controllable morphologies have a wide range of applications in laser crystals, LEDs, bio-probes, photo-catalysis, three-dimensional displays, sensors, and flash memories. This review summarizes the morphology control strategy, phase transfer theory, spectrum modulation, and extended optical applications of RE3+-doped phosphors. The roles of surfactants in the morphology control in the liquid-solid-solution phase transfer process for RE3+-doped fluorides, oxides and other compounds are discussed. The relevant mechanisms of controlling morphologies are illustrated. The size- and shape-dependent optical properties of RE3+ doped phosphors, including the emission intensities, intensity ratios of adjacent emission bands, decay times and thermal stability, are analyzed. The extended optical applications and main challenges of RE3+-doped phosphors are also discussed.

Modifying phase, shape and optical thermometry of NaGdF4:2%Er3+ phosphors through Ca2+ doping.

An efficient soft chemistry method to modify the phase, shape and optical thermometry of NaGdF4:2%Er3+ nano-phosphors through doping Ca2+ ion is reported. With the introduction of Ca2+, the phase changes from the GdF3:2%Er3+ to NaGdF4:2%Er3+ was achieved, and the shapes of NaGdF4:2%Er3+ were modified from irregular particles to pure hexagonal NaGdF4 microtubes. These modifications derive from the charge redistribution on the nucleus surface through internal electron charge transport between Gd3+ in a lattice and co-doped Ca2+ ion. An obvious enhancement of the total fluorescence intensity was observed after doping the Ca2+ ion. Moreover, an interesting phenomenon was observed that the fluorescence intensity of the mixed GdF3:2%Er3+ and NaGdF4:2%Er3+ was not be quenched at the high temperature more than 473 K. A maximum relative sensitivity of 0.00213/K (416 K) was obtained at 20%Ca2+ doping. These results indicate that NaGdF4:Er3+/Ca2+ can be applied in optical temperature sensor.

Dual-mode infrared laser-excited synergistic effect in NaGdF4:Er3+ nano-glass ceramics: a kinetic model.

Realization of the absorption and conversion of wide band infrared light have been a challenge in the field of upconversion luminescence. Herein, a facile physical approach is reported to realize the cooperative absorption and conversion of dual-band infrared light by NaGdF4:Er3+ nano-glass ceramics by employing a dual-mode excitation source (980 nm + 1545 nm). A synergistic effect of infrared photons induced by dual-wavelength infrared excitation is observed. The dual-mode excited red emission intensity is 2.76 times the total red emission intensities from 980 nm and 1545 nm single excitation. This upconversion synergistic effect can be modulated by adjusting the single excitation power, and it is proved to originate from ground and excited state absorption, in which the Er3+ ions in metastable states excited by 980 nm (or 1545 nm) photons are excited again by the 1545 nm (or 980 nm) infrared photons. A rate equation model is established to simulate the dynamic process in the dual-mode infrared upconversion process. The synergistic effect provides us with a way to convert two low-energy infrared photons into middle-energy visible photons to enhance the upconversion efficiency of rare earth ion doped glass ceramics.

A novel optical thermometry based on the energy transfer from charge transfer band to Eu3+-Dy3+ ions.

Optical thermometry based on the up-conversion intensity ratio of thermally coupled levels of rare earth ions has been widely studied to achieve an inaccessible temperature measurement in submicron scale. In this work, a novel optical temperature sensing strategy based on the energy transfer from charge transfer bands of W-O and Eu-O to Eu3+-Dy3+ ions is proposed. A series of Eu3+/Dy3+ co-doped SrWO4 is synthesized by the conventional high-temperature solid-state method. It is found that the emission spectra, emission intensity ratio of Dy3+ (572 nm) and Eu3+ (615 nm), fluorescence color, lifetime decay curves of Dy3+ (572 nm) and Eu3+ (615 nm), and relative and absolute sensitivities of Eu3+/Dy3+ co-doped SrWO4 are temperature dependent under the 266 nm excitation in the temperature range from 11 K to 529 K. The emission intensity ratio of Dy3+ (572 nm) and Eu3+ (615 nm) ions exhibits exponentially relation to the temperature due to the different energy transfer from the charge transfer bands of W-O and Eu-O to Dy3+ and Eu3+ ions. In this host, the maximum relative sensitivity Sr can be reached at 1.71% K-1, being higher than those previously reported material. It opens a new route to obtain optical thermometry with high sensitivity through using down-conversion fluorescence under ultraviolet excitation.

Tm3+ Modified Optical Temperature Behavior of Transparent Er3+-Doped Hexagonal NaGdF4 Glass Ceramics.

Er3+-doped and Er3+-Tm3+-co-doped transparent hexagonal NaGdF4 glass ceramics are fabricated via melt-quenching method. The emissions of Er3+-doped NaGdF4 glass ceramics are adjusted from the green to red by varying the concentration of Tm3+ ion under the excitation of 980 nm. The spectrum, thermal quenching ratio, fluorescence intensity ratios, and optical temperature sensitivity of the transparent glass ceramics are observed to be dependent on the pump power. The maximum value of relative sensitivity reaches 0.001 K-1 at 334 K in Er3+-doped NaGdF4, which shifts toward the lower temperature range by co-doping with Tm3+ ions, and has a maximum value of 0.00081 K-1 at 292 K. This work presents a method to improve the optical temperature behavior of Er3+-doped NaGdF4 glass ceramics. Moreover, the relative sensitivity SR is proved to be dependent on the pump power of 980-nm lasers in Er3+-doped NaGdF4 and Er3+-Tm3+-co-doped NaGdF4.

Enhance the Er3+ Upconversion Luminescence by Constructing NaGdF4:Er3+@NaGdF4:Er3+ Active-Core/Active-Shell Nanocrystals.

NaGdF4:12%Er3+@NaGdF4:x%Er3+ (x = 0, 6, 8, 10, and 12) active-core/active-shell nanoparticles (NPs) were peculiarly synthesized via a delayed nucleation pathway with procedures. The phase, shape, and size of the resulting core-shell NPs are confirmed by transmission electron microscopy and X-ray diffraction. Coated with a NaGdF4:10%Er3+ active shell around the NaGdF4:12%Er3+ core NPs, a maximum luminescent enhancement of about 336 times higher than the NaGdF4:12%Er3+ core-only NPs was observed under the 1540 nm excitation. The intensity ratio of green to red was adjusted through the construction of the core-shell structure and the change of Er3+ concentration in the shell. By analyzing the lifetimes of emission bands and exploring the energy transition mechanism, the giant luminescence enhancement is mainly attributed to the significant increase in the near-infrared absorption at 1540 nm and efficient energy migration from the shell to core.

Influence of Doping and Excitation Powers on Optical Thermometry in Yb3+-Er3+ doped CaWO4.

Optical thermometry has been widely studied to achieve an inaccessible temperature measurement in submicron scale and it has been reported that the temperature sensitivity depends mainly on host types. In this work, we propose a new method to improve the optical temperature sensitivity of Yb3+-Er3+ co-doped CaWO4 phosphors by doping with Li+, Sr2+, and Mg2+ ions and by controlling excitation powers of 980 nm laser. It is found that the thermometric parameters such as upconversion emission intensity, intensity ratio of green-to-red emission, fluorescence color, emission intensity ratios of thermally coupled levels (2H11/2/4S3/2), and relative and absolute temperature sensitivity can be effectively controlled by doping with Li+, Sr2+, and Mg2+ ions in the Yb3+-Er3+ co-doped CaWO4 system. Moreover, the relative sensitivity SR and the absolute sensitivity SA are proved to be dependent on the pump power of 980 nm laser. The sensitivities of SR and SA in Yb3+-Er3+ co-doped CaWO4 increase about 31.5% and 12%, respectively, by doping with 1 mol% Sr2+.

Detecting the origin of luminescence in Er3+-doped hexagonal Na1.5Gd1.5F6 phosphors.

Understanding site-selective fluorescence is one of valuable importance for spectrum modulation. In this Letter, we observed the existence of two non-equivalent Gd-activated crystallographic sites in an Er3+-doped hexagonal Na1.5Gd1.5F6 phosphor. It is proved that two green emissions from the S3/24 level separately originate from the Gd1 (540 nm) and Na2/Gd2 (550-555 nm) crystallographic sites, and the 657 nm red emission from the F9/24 level only originates from Na2/Gd2 site through using the time-resolved luminescence spectra. The 142.2% absolute enhancement of the red emission is realized through the synergistic effect of ultraviolet downconversion and infrared upconversion induced by the 370 nm and 1.54 µm dual-mode excitation.

Shape-controlled tunable homochromatic luminescence and inner photoelectric effect of hexagonal Na1.23Ca0.12Y1.28Er0.24F6 phosphors.

Novel hexagonal Na1.23Ca0.12Y1.28Er0.24F6 nanodisks, microtubes, and nanorods were synthesized hydrothermally for the first time. Time-dependent morphology evolution showed that the resulting nanodisks, microtubes, and nanorods were synthesized by a dissolution-reconstruction formation mechanism. With the help of a 980 nm semiconductor laser, tunable homochromatic luminescence was observed by exciting single hexagonal Na1.23Ca0.12Y1.28Er0.24F6 nanodisks, microtubes, and nanorods, respectively. Simultaneously, the inner photoelectric effect was witnessed in the hexagonal Na1.23Ca0.12Y1.28Er0.24F6 nanodisks, microtubes, and nanorods under 980 nm excitation. We attributed anomalous inner photoelectric current to the presence of a laser stepped resonant excitation energy migration in the Na1.23Ca0.12Y1.28Er0.24F6 matrix when a laser was applied to it. Simultaneous control of homochromatic luminescence and inner photoelectric effect were achieved by modifying morphological shapes of single Na1.23Ca0.12Y1.28Er0.24F6 phosphors.

A novel screening model for the molecular drug for diabetes and obesity based on tyrosine phosphatase Shp2.

Tyrosine phosphatase Src-homology phosphotyrosyl phosphatase 2 (Shp2) was identified as a potential molecular target for therapeutic treatment of diabetes and obesity. However, there is still no systematic research on the enhancers for the Shp2 enzyme. The present study established a novel powerful model for the high-throughput screening of Shp2 enhancers and successfully identified a new specific Shp2 enhancer, oleanolic acid, from Chinese herbs.

Visible photon-avalanche upconversion in Ho3+ singly doped beta-Na(Y1.5Na0.5)F6 under 980 nm excitation.

Visible IR-to-green photon-avalanche upconversion is reported in an Ho3+ singly doped beta-Na(Y1.5Na0.5)F6 crystal under 980 nm excitation. Upconverted green, red, and IR emissions are observed at 540, 645, and 751 nm, respectively. Temporal evolution and excitation power dependent upconversion intensity are measured, suggesting that a photon-avalanche mechanism is responsible for the upconversion process. It is believed that an efficient cross relaxation (5S2,5I8)-->(5I6,5I6) mainly performs the population of 5I6 excited state, resulting in the intense photon-avalanche upconversion emission in the synthesized samples.