Three-photon Upconversion Process Research Articles

Localized three-photon upconversion enhancement in silver nanowire networks and its effect in thermal sensing.

The quest for enhancing the upconversion luminescence (UCL) efficiency of rare-earth doped materials has been a common target in nanophotonics research. Plasmonic nanoarchitectures have proven potential for amplifying UCL signals, prompting investigations into localized enhancement effects within noble metal nanostructures. In this work we investigate the localized enhancement of UCL in silver nanowire (AgNW) networks coated with upconversion nanoparticles (UCNPs) by employing hyperspectral microscopy to unveil distinctive regions of local enhancement. Our study reveals that three-photon upconversion processes predominantly occur at hot-spots in nanowire junctions, contributing to heightened luminescence intensity on AgNW networks. Intriguingly, our findings demonstrate that enhancement on AgNWs introduces significant artifacts for thermometry based on ratiometric analysis of the emission spectra, resulting in the observation of artificial thermal gradients. To address this challenge, we developed correction methods that were successfully applied to mitigate this effect, enabling the generation of accurate thermal maps and the realization of dynamic thermal measurements. We quantified the distance-dependent enhancement profiles and studied the effect of temperature by exploiting the heat dissipation under varying electrical voltages across the electrically percolated AgNW networks. The observations were confirmed through numerical calculations of the enhancement factor and the energy transfer rates. This comprehensive investigation sheds light on the complex interplay between plasmonic nanostructures, three-photon upconversion processes, and their influence on thermal sensing applications. The presented hyperspectral method not only allows a direct visualization of plasmonic hot-spots but also advances our understanding of localized enhancements. The correction methods applied to analyze the emission spectra also contribute to the refinement of accurate temperature mapping using UCNPs, thereby enhancing the reliability of this thermal sensing technology.

Fabrication and characterization of up-converting \u03b2-NaYF4:Er3+,Yb3+@NaYF4 core\u2013shell nanoparticles for temperature sensing applications

This paper presents the use of soft template method to synthesize core and core–shell up-converting nanoparticles usefull for temperature sensing applications. Based on the stock solutions of core β-NaYF4:Er3+,Yb3+ nanoparticles and involving soft template method without any additional process of surface functionalization, it is possible to directly design the core–shell β-NaYF4:Er3+,Yb3+@NaYF4 nanoparticles, which can be perfectly dispersed in cyclohexane and surfactants like oleic acid (OA), triethanolamine (TEA) or Cetyltrimethylammonium bromide (CTAB). The morphological, crystalline and elemental characteristics of samples were investigated by Field Emission Scanning Electron Microscopy, X-Ray Diffraction, High Resolution Transmission Electron Microscopy, Selected Area Electron Diffraction patterns and Energy-Dispersive X-Ray Spectroscopy (EDX) measurements. The results showed that the synthesized NaYF4:Er3+,Yb3+@NaYF4 core–shell nanoparticles have roughly spherical shape, pure hexagonal β phase with core size of about 35 ± 5 nm and shell thickness of about 40 ± 5 nm. It has been shown that the coating of the β-NaYF4:Er3+,Yb3+ core with NaYF4 shell layer enables to enhance the green upconversion (UC) emission intensities in respect to red one. Under 976 nm excitation, the synthesized β-NaYF4:2%Er3+,19%Yb3+@NaYF4 core–shell nanoparticles revealed three strong emission bands at 520 nm, 545 nm and 650 nm corresponding to 2H11/2, 4S3/2 and 4F9/2 to 4I15/2 transitions of Er3+ ions with the lifetimes of 215, 193 and 474 µs, respectively. The calculated CIE chromaticity coordinates proved that the emission colour of core–shell nanoparticles was changed from red into yellowish green upon increasing the power density of the 976 nm laser from 0.73 to 9.95 W/cm2. The calculated slopes indicated that in the β-NaYF4:2%Er3+,19%Yb3+@NaYF4 core–shell nanoparticles, two-photon and three-photon UC processes took place simultaneously. Although the former one is similar as in the case of β-NaYF4:Er3+,Yb3+ bare core nanoparticles, the latter one, three-photon UC process for green emission occurs, due to cross relaxation processes of two Er3+ ions only within nanoparticles with core–shell architecture. Moreover, the energy difference between the 2H11/2 and 4S3/2 levels and associated constant of NaYF4@NaYF4 host lattice were determined and they reached ~ 813 cm−1 and 14.27 (r2 = 0.998), respectively. In order to investigate the suitability of nanoparticles for optical temperature sensing, the emission spectra were measured in a wide temperature range from 158 to 298 K. An exceptionally high value of relative sensitivity was obtained at 158 K and it amounted to 4.25% K−1. Further temperature increase resulted in gradual decrease of relative sensitivity, however, it maintained a high value > 1% K−1 in the entire analyzed temperature range.

Measuring Activation and Luminescence Time Scales of Upconverting NaYF4:Yb,Er Nanocrystals

Accurate determination of upconversion and luminescence lifetimes requires kinetic modeling of the complete time-resolved response for upconversion luminescence of NaYF4:Yb,Er nanocrystals. Prior investigations typically perform exponential fitting of the tail in the time profile or employ complex systems of differential equations to extract lifetimes. To simplify analysis while fitting the entire time-resolved response, this work introduces a set of simplified models that model the response as a convolution of upconversion and fluorescence processes. Models for two- and three-photon upconversion processes are developed and tested for NaYF4:Yb,Er nanocrystals excited by a 980 nm laser. The results are presented for the transitions 2H11/2, 4S3/2 → 4I15/2 (530 nm, green emission), 4F9/2 → 4I15/2 (650 nm, red emission), and 2H9/2 → 4I15/2 (400 nm, blue emission). Even with the same number of fitting parameters, the two- and three-photon models resulted in better fitting than the simpler models of single and ...

Efficient NIR-NIR upconversion in Na5Lu9F32: Yb3+, Tm3+ single crystals

Na5Lu9F32: Yb3+, Tm3+ single crystals prepared by an improved Bridgman method are expected to be incorporated into solar cells as an upconversion layer to expand the absorption range. Upconversion (UC) with weak emission of 488 nm blue, 704 nm red and strong 805 nm near-infrared fluorescence upon 964 nm excitation as well as its pump power dependence on UC emission were obtained in the single crystals. Furthermore, a high external quantum efficiency (EQE) of 2.65% is obtained for Na5Lu9F32: 8%Yb3+, 1%Tm3+ under 1 W/cm2 980 nm excitation. Meanwhile, the temperature-dependent UC emission spectra is studied as a function of temperature in the range of 250–370 K. The cubic dependence of pump power on UC emission reveals that three-photon UC processes are mainly responsible for the 488 nm, 704 nm emissions and the quadratic dependence indicates two-photon process for the 805 nm emission in Tm3+/Yb3+ co-doped Na5Lu9F32 single crystals, respectively. The energy transfer processes were studied based on energy level diagram, and the results indicated that the cross-relaxation (CR) between Tm3+ and Tm3+ affect the luminous intensity. Moreover, it can infer that the energy transfer from Tm3+ to Yb3+ exists from the decay curves of Tm3+ at 805 nm under excitation by 964 nm.

Upconversion emissions from high energy levels of Tb^3+ under near-infrared laser excitation at 976 nm

Ultraviolet upconversion emissions from high energy levels (5D1, 5L8, 5L9,5G5, and 5L10) of Tb3+ were observed in the NaYF4:Tb3+, Yb3+ microcrystals under excitation of a 976 nm laser diode at room temperature. These energy levels were populated by energy transfer and excited state absorption processes. Power dependence measurements confirmed it was caused by a three-photon upconversion process. Concentration quenching did not strongly affect the population of these levels.

Three- and two-photon upconversion luminescence switching in Tm3+/Yb3+-codoped sodium niobate nanophosphor

Intense infrared-to-visible upconversion (UC) emission in Tm 3+ /Yb 3+ -codoped sodium niobate (NaNbO 3 ) nanocrystals under resonant excitation at 976 nm is presented. The results showed that by increasing the pump power/intensity, a strong reduction is observed at the 800/480 nm emitted intensity ratio, characterizing what can be denominated as laser pump power-induced color tunability or luminescent switching. The physical origin is discussed with a focus on tailoring of luminescent switchers to operate at a large pump power range and, indeed, it is intrinsically associated with the competition of the two- and three-photon UC processes and with highly efficient UC emissions in the investigated material. The effect of Yb 3+ -ion concentration along with the theoretical aspects on luminescence switching has been investigated. The results obtained here could be useful in the field of sensors and networks for optical processing and optical communications.

Influence of core size on the upconversion luminescence properties of spherical Gd2O3:Yb3+/Er3+@SiO2 particles with core-shell structures

Spherical SiO2 particles with different sizes (30, 80, 120, and 180 nm) have been coated with Gd2O3:Yb3+/Er3+ layers by a heterogeneous precipitation method, leading to the formation of core-shell structural Gd2O3:Yb3+/Er3+@SiO2 particles. The samples were characterized by using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, upconversion (UC) emission spectra, and fluorescent dynamical analysis. The obtained core-shell particles have perfect spherical shape with narrow size distribution. Under the excitation of 980 nm diode laser, the core-shell samples showed size-dependent upconversion luminescence (UCL) properties. The inner SiO2 cores in core-shell samples were proved to have limited effect on the total UCL intensities of Er3+ ions. The UCL intensities of core-shell particles were demonstrated much higher than the values obtained in pure Gd2O3:Yb3+/Er3+ with the same phosphor volume. The dependence of the specific area of a UCL shell on the size of its inner SiO2 particle was calculated and analyzed for the first time. It was confirmed that the surface effect came from the outer surfaces of emitting shells is dominant in influencing the UCL property in the core-shell samples. Three-photon UC processes for the green emissions were observed in the samples with small sizes of SiO2 cores. The results of dynamical analysis illustrated that more nonradiative relaxation occurred in the core-shell samples with smaller SiO2 core sizes.

Facile synthesis of β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) microcrystals with down- and up-conversion luminescence

β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) hexagonal microrods have been successfully synthesized through a facile molten salt method without any surfactant. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra were used to characterize the samples. It is found that at a preferred reaction temperature of 400 °C, the structure of β-NaYF4 can gradually transform from microtubes to microrods as reaction time extends from 0.5 to 4 h. Furthermore, as the molar ratio of NaF:RE3+ (RE represents the total amount of Y3+ and the doped rare earth elements such as Eu3+, Tb3+, Yb3+/Er3+, or Yb3+/Tm3+) increased, the phase of sample transforms from YF3 into NaYF4. Under the excitation of 395 nm ultraviolet light, β-NaYF4:5 %Eu3+ shows the emission lines of Eu3+ corresponding to 5D0-3 → 7FJ (J = 1–4) transitions from 400 to 700 nm, resulting in red down-conversion (DC) light emission. When doped with 5 % Tb3+ ions, the strong DC fluorescence corresponding to 5D4 → 7FJ (J = 6, 5, 4, 3) transitions with 5D4 → 7FJ (green emission at 544 nm) being the most prominent group that has been observed. Moreover, upon 980 nm laser diode excitation, the Yb3+/Er3+- and Yb3+,Tm3+- co-doped β-NaYF4 samples exhibit bright yellow and blue upconversion (UC) luminescence, respectively, by two- or three-photon UC process. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.

Flux growth of honeycomb-like β-NaYF4:Yb3+, Er3+/Tm3+ crystals with multicolor upconversion luminescence

Highly crystalline NaYF4:Ln3+ (Ln=Yb, Er, Tm) crystals with honeycomb structure were successfully synthesized for the first time by using a mixed NaCl–KCl flux cooling method at a holding temperature of 800°C for 2h in air. The results indicated that the crystals of NaYF4:Ln3+ belonged to hexagonal system and three-dimensional open pores had an average diameter of 1μm interconnected with each other. Upon 980nm laser diode excitation, yellow green and sky blue upconversion fluorescences of β-NaYF4:20%Yb3+, 2%Er3+ crystals and β-NaYF4:20%Yb3+, 0.5%Tm3+ crystals, which was caused by two- or three-photon upconversion processes, was observed clearly.

Flux Growth of Highly Crystalline NaYF4:Ln (Ln = Yb, Er, Tm) Crystals with Upconversion Fluorescence

Highly crystalline, NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the cooling of the solo NaF flux at a holding temperature of 1100 °C and cooling rate of 5 °C·h−1. The basic forms of NaYF4 and NaYF4:Ln crystals were a sphere. The crystal system, form, and size were affected by cooling rate. The red, green, and blue upconversion fluorescence of NaYF4:Ln crystals were clearly observed under 980 nm laser irradiation by a two- or three-photon upconversion process. The upconversion fluorescence of NaYF4:Ln crystals was successfully controlled by changing the cooling rate and type of dopant. Furthermore, the NaYF4:Yb,Er crystals were successfully grown using a mixed NaF-KF flux cooling method at a relatively low temperature of 800 °C.

Infrared to ultraviolet upconversion luminescence in Nd 3+ doped nano-glass-ceramic

Nd 3+ doped transparent oxyfluoride glass ceramic containing β-YF 3 nanocrystals was prepared and the upconversion luminescence behaviors of Nd 3+ in the precursor glass and glass ceramic were investigated. Under 796 nm laser excitation, ultraviolet upconversion emissions of Nd 3+ ions at 354 nm ( 4D 3/2 → 4I 9/2) and 382 nm ( 4D 3/2 → 4I 11/2) were observed at room temperature. Power dependence analysis demonstrated that three-photon upconversion processes populated the 4D 3/2 excited state. In comparison with those of the precursor glass, the ultraviolet emissions were enhanced by a factor of 500 in the glass ceramic, which was attributed to the change in the ligand field of Nd 3+ ions and the decrease in phonon energy because of the partition of Nd 3+ ions into the β-YF 3 nanocrystals after crystallization.

Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y 2O 3 nanocrystals

Room-temperature blue and violet upconversion (UC) emissions of Er 3+ at 409 nm ( 2H 9/2 → 4I 15/2) and 390 nm ( 4G 11/2 → 4I 15/2) were observed in Y 2O 3:Yb 3+–Er 3+ nanocrystals by diode laser excitation of 980 nm. At low Yb 3+ concentration both wavelengths result from three-photon UC processes. Unexpectedly, a four-photon and a decreased three-photon UC processes were observed at high Yb 3+ concentration for the violet and the blue emissions, respectively. The four-photon process supposably results from the cross relaxation 4D 7/2 + 4I 15/2 → 4G 11/2 + 4I 13/2 of Er 3+ ions with the 4D 7/2 state populated by the 2 2F 5/2(Yb) + 4F 9/2(Er) → 2 2F 7/2(Yb) + 4D 7/2(Er) process, whereas the decreased photon process arises from the larger UC rate at high Yb 3+ concentration.

Concentration enhanced red upconversion in nanocrystalline ZrO2 : Er under IR excitation

The effects of Er3+ concentration in ZrO2 on the red (680 nm) and green (545 nm) upconversion emission from Er3+ under near infrared radiation (NIR) excitation (at both 1438 and 962 nm) are presented. For the highest Er3+ concentration used, the red band is almost quenched under VIS (489 nm) excitation, whereas it is enhanced under NIR excitation. Such a difference between photoluminescence and upconversion is explained taking into account the concentration dependent cross-relaxation process (2H11/2 + 4I15/2) → (4I9/2 + 4I13/2). Experimental results suggest that two- and three-photon upconversion processes are taking place under 962 nm and 1438 nm excitation, respectively.

Infrared to visible upconversion in Er 3+-doped lead oxyfluorosilicate glasses

The upconversion properties and mechanism of Er 3+ ions have been studied for lead oxyfluorosilicate glasses containing Er 2O 3 concentrations from 0.2 to 2 mol%. Under 975 nm excitation, four emission bands centered at 411, 525, 543, and 655 nm corresponding to the 2H 9/2→ 4I 15/2, 2H 11/2→ 4I 15/2, 4S 3/2→ 4I 15/2, and 4F 9/2→ 4I 15/2 transitions of Er 3+ ions, respectively, were simultaneously observed at room temperature. The bright green and red emissions are due to a two-photon upconversion process while the weak blue emission is determined by a three-photon upconversion process. Raman spectrum indicates that fluoride ions in the formation of glass network have important influence on upconversion luminescence of Er 3+ in lead oxyfluorosilicate glasses. The intense upconversion luminescence of Er 3+-doped lead oxyfluorosilicate glasses may be a potentially useful material for developing upconversion optical devices.

Upconversion-pumped infrared erbium laser

A study of upconversion processes in Er3+ ions in CaF2 has led to an unambiguous demonstration of IR lasing produced by upconversion pumping alone. The study has shown: (1) strong upconversion lines originating from the 4I11/2 state; (2) the existence of both two-photon and three-photon upconversion processes; (3) an optimum concentration of 5–10 mol % for Er3+; and (4) upconversion efficiencies for the visible bands as high as 10% at 77 K and 1% at 300 K. Measurements of lasing between the 4I11/2 and 4I13/2 levels show that even though the lifetime of the upper (4I11/2) state is only half that of the lower state, self-quenching does not occur. Upconversion is implicated for this transition both by the nonpersistence of a ‘‘red shift’’ during a lasing pulse, and by the opposite dependence on concentration of laser threshold energy and upconversion-produced fluorescent intensity. A simple theoretical model shows that a 2.8-μm laser operating between the 4I11/2 and 4I13/2 levels can be pumped solely by upconversion of ions in the excited 4I13/2 state produced by absorption of a 1.54-μm pulse. In addition, (1) a minimum of 1.54-μm pump energy is required; (2) lasing occurs with a delay inversely proportional to the pump energy; and (3) lasing can persist for as long as the lifetime of the lower (4I13/2) state. These predictions have been experimentally verified. The lasing experiments give a lower limit for the upconversion coefficient of α=10−16 cm3 s−1.