Visible and near infra-red up-conversion in Tm3+/Yb3+ co-doped silica fibers under 980 nm excitation

Hydrothermal Synthesis and Properties of Yb3+/Tm3+ Doped Sr2LaF7 Upconversion Nanoparticles

We report the procedure for hydrothermal synthesis of ultrasmall Yb³⁺/Tm³⁺ co-doped Sr₂LaF₇ (SLF) upconversion phosphors. These phosphors were synthesized by varying the concentrations of Yb³⁺ (x = 10, 15, 20, and 25 mol%) and Tm³⁺ (y = 0.75, 1, 2, and 3 mol%) with the aim to analyze their emissions in the near IR spectral range. According to the detailed structural analysis, Yb³⁺ and Tm³⁺ occupy the La³⁺ sites in the SLF host. The addition of Yb³⁺/Tm³⁺ ions has a huge impact on the lattice constant, particle size, and PL emission properties of the synthesized SLF nanophosphor. The results show that the optimal dopant concentrations for upconversion luminescence of Yb³⁺/Tm³⁺ co-doped SLF are 20 mol% Yb³⁺ and 1 mol% Tm³⁺ with EDTA as the chelating agent. Under 980 nm light excitation, a strong upconversion emission of Tm³⁺ ions around 800 nm was achieved. In addition, the experimental photoluminescence lifetime of Tm³⁺ emission in the SLF host is reported. This study discovered that efficient near IR emission from ultrasmall Yb³⁺/Tm³⁺ co-doped SLF phosphors may have potential applications in the fields of fluorescent labels in bioimaging and security applications.

Mathematical Model of Photodarkening in Rare-Earth-Doped Fiber

In this paper, an improved mathematical model is proposed by taking the factors of high-energy photons and temperature into consideration, which is verified and explained by the experimental data in our experiments and other papers. By fitting and analyzing the experimental data, we can quantitatively determine the relationship between the pump power Pλ and the photon frequency ν in the fiber core, the core area A and the temperature T of the fiber core and PD loss, and explain the mechanism of the PD phenomenon to a certain extent. We believe that the excitation of color centers by high-energy photons is the main reason for photodarkening. Furthermore, there is a positive correlation between the power of high-energy photons and the photodarkening rate, and the temperature is positively correlated with the saturated photodarkening absorption.

Progress and Summary of Photodarkening in Rare Earth Doped Fiber

In this paper, we summarize the research on photodarkening in optical fibers. The causes of photodarkening in fiber, the influence of photodarkening on fiber laser, the experimental device of photodarkening, and the mathematical model used to study the phenomenon of photodarkening are described in detail. At the end of the paper, we summarize the means and methods to suppress photodarkening.

Upconversion Luminescence of Silica-Calcia Nanoparticles Co-doped with Tm3+ and Yb3+ Ions

Looking for upconverting biocompatible nanoparticles, we have prepared by the sol-gel method, silica-calcia glass nanopowders doped with different concentration of Tm³⁺ and Yb³⁺ ions (Tm³⁺ from 0.15 mol% up to 0.5 mol% and Yb³⁺ from 1 mol% up to 4 mol%) and characterized their structure, morphology, and optical properties. X-ray diffraction patterns indicated an amorphous phase of the silica-based glass with partial crystallization of samples with a higher content of lanthanides ions. Transmission electron microscopy images showed that the average size of particles decreased with increasing lanthanides content. The upconversion (UC) emission spectra and fluorescence lifetimes were registered under near infrared excitation (980 nm) at room temperature to study the energy transfer between Yb³⁺ and Tm³⁺ at various active ions concentrations. Characteristic emission bands of Tm³⁺ ions in the range of 350 nm to 850 nm were observed. To understand the mechanism of Yb³⁺-Tm³⁺ UC energy transfer in the SiO₂-CaO powders, the kinetics of luminescence decays were studied.

Investigation of photodarkening in tandem-pumped Yb-doped fibers

Investigation of photodarkening (PD) in Yb-doped fibers tandem-pumped at 1018 nm is reported. For a homemade Yb-doped aluminosilicate double-clad fiber (YADF), the transmitted power of a 633 nm probe beam is reduced by 2.4% over 2 hours for the tandem pumping configuration at 1018 nm, which is significantly smaller than 33.3% for a laser diode (LD) pumping at 976 nm. A tandem-pumped Yb fiber amplifier also shows a much smaller decrease in the amplified output power over time than a LD-pumped Yb fiber amplifier. Based on fluorescence spectra of the YADF, we can not only associate PD of the YADF to intrinsic oxygen deficiency centers or Tm³⁺ impurities but also confirm the impact of the excited Yb³⁺ ion density on PD. The benefits of the tandem pumping in a high-power Yb fiber laser system will be discussed.

Microwave hydrothermal synthesis and upconversion properties of BiVO4 nanoparticles

Nanomaterials are the subject of extensive investigations due to their applications in medicine, multimodal imaging, volumetric displays, and photonics. Here, lanthanide-doped bismuth vanadate (BiVO₄) upconverting nanoparticles (UCNPs) have been reported. The nanoparticles have been synthesized by a microwave hydrothermal method. As-synthesized nanoparticles are highly crystalline in the tetragonal zircon phase with particles about 200 nm in size. Under 980 nm excitation, intense multicolor visible and near-infrared upconversion emissions are observed. Moreover, broadband infrared downshifting emissions are also observed. Time-resolved emission measurements have been carried out to investigate the involved upconversion and energy transfer mechanism. The BiVO₄-based UCNPs may provide a new class of nanomaterials for multifunctional applications.

Anti-escaping of incident laser in rare-earth doped fluoride ceramics with glass forming layer

Adaptive fluoride ceramic with glass forming layer (GC_ZBL-Er) used in laser anti-escaping has been prepared by one-step synthesis, and the thickness of glass layer is identified as ~0.41 mm. Blue, green and red emissions of Er³⁺/Yb³⁺ codoped fluoride ceramic (C_ZBL-Er) and glass layer (G_ZBL-Er) have been investigated under ~980 nm laser pumping. With the forming of thin glass layer on ceramic surface, the absorption intensities on diffuse reflection of GC_ZBL-Er at 974 nm and 1.53 μm increase by 48% and 53% than those of C_ZBL-Er. Excited by a 979 nm laser, the presence of the glass layer increases the absolute absorption rate in spectral power from 75% in C_ZBL-Er to 83% in GC_ZBL-Er, which is consistent with the improvement in the absorbed photon number. In addition, the quantum yield of GC_ZBL-Er complex is raised by 28.4% compared to the case of ceramic substrate by photon quantification. Intense absorption-conversion ability and efficient macroscopical anti-escaping effect confirm the superiority of ingenious structure in the fluoride ceramics with glass forming layer, which provides a new approach for developing the absorption-conversion materials of anti-NIR laser detection.

Multifunctional NaLnF4@MOF-Ln Nanocomposites with Dual-Mode Luminescence for Drug Delivery and Cell Imaging

Multifunctional nanomaterials for bioprobe and drug carrier have drawn great attention for their applications in the early monitoring the progression and treatment of cancers. In this work, we have developed new multifunctional water-soluble NaLnF₄@MOF-Ln nanocomposites with dual-mode luminescence, which is based on stokes luminescent mesoporous lanthanide metal-organic frameworks (MOFs-Y:Eu³⁺) and anti-stokes luminescent NaYF₄:Tm³⁺/Yb³⁺ nanoparticles. The fluorescence mechanism and dynamics are investigated and the applications of these nanocomposites as bioprobes and drug carriers in the cancer imaging and treatment are explored. Our results demonstrate that these nanocomposites with the excellent two-color emission show great potential in drug delivery, cancer cell imaging, and treatment, which are attributed to the unique spatial structure and good biocompatibility characteristics of NaLnF₄@MOF-Ln nanocomposites.

Thulium-Doped Silica Fibers with Enhanced Fluorescence Lifetime and Their Application in Ultrafast Fiber Lasers

In this work we report on the thulium-doped silica-based optical fibers with increased fluorescence lifetime of the 3F4 level thanks to the modification of the local environment of thulium ions by high content of alumina. The determination of the cross-relaxation energy-transfer coefficients from the measurements of the fluorescence lifetimes of the 3F4 and 3H4 energy levels of Tm3+ ions in the experimentally prepared optical fiber is provided as well. Preforms of optical fibers were prepared either by conventional solution-doping of Tm3+ and Al3+ ions or by dispersion-doping of Tm3+ ions with alumina nanoparticles. Optical fibers were characterized by means of Tm, Al, and Ge concentrations, refractive index profiles, optical spectral absorption and luminescence, and by time-resolved fluorescence spectroscopy. Highly aluminium-codoped thulium silicate optical fibers exhibited fluorescence lifetimes of over ~500 μs with maximum value of 756 μs, which means a fluorescence lifetime enhancement when compared to the thulium-doped fibers reported elsewhere. We show an application of the thulium-doped fiber in a compact all-fiber ring laser that is passively mode-locked by using graphene-based saturable absorber. The output pulsewidth and repetition rate were 905 fs and 32.67 MHz, respectively.

Compound Glass Microsphere Resonator Devices

In recent years, compound glass microsphere resonator devices have attracted increasing interest and have been widely used in sensing, microsphere lasers, and nonlinear optics. Compared with traditional silica resonators, compound glass microsphere resonators have many significant and attractive properties, such as high-Q factor, an ability to achieve high rare earth ion, wide infrared transmittance, and low phonon energy. This review provides a summary and a critical assessment of the fabrication and the optical characterization of compound glasses and the related fabrication and applications of compound glass microsphere resonators.

Achieving high-efficiency emission depletion nanoscopy by employing cross relaxation in upconversion nanoparticles

Stimulated emission depletion microscopy provides a powerful sub-diffraction imaging modality for life science studies. Conventionally, stimulated emission depletion requires a relatively high light intensity to obtain an adequate depletion efficiency through only light–matter interaction. Here we show efficient emission depletion for a class of lanthanide-doped upconversion nanoparticles with the assistance of interionic cross relaxation, which significantly lowers the laser intensity requirements of optical depletion. We demonstrate two-color super-resolution imaging using upconversion nanoparticles (resolution ~ 66 nm) with a single pair of excitation/depletion beams. In addition, we show super-resolution imaging of immunostained cytoskeleton structures of fixed cells (resolution ~ 82 nm) using upconversion nanoparticles. These achievements provide a new perspective for the development of photoswitchable luminescent probes and will broaden the applications of lanthanide-doped nanoparticles for sub-diffraction microscopic imaging.

Upconversion nanoparticles, which do not suffer from the photophysical artifacts that limit fluorescent molecules, offer an exciting opportunity for biological super-resolution imaging. Here, Zhan et al. develop an efficient STED mechanism using optimized lanthanide upconversion nanoparticles, enabling cytoskeleton nanoscopic imaging.

Enhancing Multiphoton Upconversion from NaYF4:Yb/Tm@NaYF4 Core-Shell Nanoparticles via the Use of Laser Cavity

We discover that emission efficiency of Tm³⁺-doped upconversion nanoparticles can be enhanced through the use of a laser cavity. With suitable control of the lasing conditions, the population of the intermediate excited states of the Tm³⁺ can be clamped at a required value above the excitation threshold. As a result, upconversion efficiency for the 300-620 nm emission band of the Tm³⁺-doped nanoparticles under 976 nm excitation can be enhanced by an order of magnitude over the case without a laser cavity. This is because the intrinsic recombination process of the intermediate excited states is suppressed and the surplus of excitation power directly contributes to the enhancement of multiphoton upconversion. Furthermore, our theoretical investigation has shown that the improvement of upconversion emission efficiency is mainly dependent on the cavity loss, so that this strategy can also be extended to other lanthanide-doped systems.

Optimization of Bi3+ in Upconversion Nanoparticles Induced Simultaneous Enhancement of Near-Infrared Optical and X-ray Computed Tomography Imaging Capability

Bioimaging probes have been extensive studied for many years, while it is still a challenge to further improve the image quality for precise diagnosis in clinical medicine. Here, monodisperse NaGdF₄:Yb³⁺,Tm³⁺,x% Bi³⁺ (abbreviated as GYT-x% Bi³⁺, x = 0, 5, 10, 15, 20, 25, 30) upconversion nanoparticles (UCNPs) have been prepared through the solvothermal method. The near-infrared upconversion emission intensity of GYT-25% Bi³⁺ has been enhanced remarkably than that of NaGdF₄:Yb³⁺,Tm³⁺ (GYT) with a factor of ∼60. Especially, the near-infrared upconversion emission band centered at 802 nm is 150 times stronger than the blue emission band of GYT-25% Bi³⁺ UCNPs. Such high ratio of NIR/blue UCL intensity could reduce the damage to tissues in the bioimaging process. The possibility of using GYT-25% Bi³⁺ UCNPs with strong near-infrared upconversion emission for optical imaging in vitro and in vivo was performed. Encouragingly, the UCL imaging penetration depth can be achieved as deep as 20 mm. Importantly, GYT-25% Bi³⁺ UCNPs exhibit a much higher X-ray computed tomography (CT) contrast efficiency than GYT and iodine-based contrast agent under the same clinical conditions, due to the high X-ray attenuation coefficient of bismuth. Hence, simultaneous remarkable enhancement of NIR emission and X-ray CT signal in upconversion nanoparticles could be achieved by optimizing the doping concentration of Bi³⁺ ions. Additionally, Gd³⁺ ions in the UCNPs endow GYT-25% Bi³⁺ UCNPs with T₁-weighted magnetic resonance (MR) imaging capability.

Sensitive Detection of ssDNA Using an LRET-Based Upconverting Nanohybrid Material

Water-dispersible, optical hybrid nanoparticles are preferred materials for DNA biosensing due to their biocompatibility. Upconverting nanoparticles are highly desirable optical probes in sensors and bioimaging owing to their sharp emission intensity in the visible region. We herein report a highly sensitive ss-DNA detection based on an energy transfer system that uses a nanohybrid material synthesized by doping NaYF4:Tm(3+)/Yb(3+) upconverting nanoparticles (UCNPs) on silica coated polystyrene-co-acrylic acid (PSA) nanoparticles (PSA/SiO2) as the donor, and gold nanoparticles (AuNPs) decorated with Ir(III) complex as the acceptor. UCNPs tagged on PSA/SiO2 and the cyclometalated Ir(III)/AuNP conjugates were then linked through the ss-DNA sequence. Sequential addition of the target DNA to the probe molecular beacon complex resulted in the separation of the optical nanohybrid material and the quencher, leading to a measurable increase in the blue fluorescence emission intensity. Our results have shown a linear relationship between the fluorescence intensity and target DNA concentration down to the picomolar.

Simultaneous influence of Zn2+/Mg2+ on the luminescent behaviour of La2O3:Tm3+–Yb3+ phosphors

Appreciable enhancement of the UC emission bands due to incorporation of Zn²⁺ and Mg²⁺ ions in Tm³⁺–Yb³⁺ codoped La₂O₃ phosphor can be visualized from the figure.

Evidence of Tm impact in low-photodarkening Yb-doped fibers

In contrast to Yb/Al-doped fibers, the influence of very low Tm(2)O(3) concentrations (≥ 0.1 mol-ppm) on photodarkening (PD) is clearly detectable in Yb/P-doped fibers that are known to show little degradation effects. For Tm(2)O(3) additions of more than 50 mol-ppm, the measured PD loss is even similar to Yb/Al-doped fibers with comparable rare earth concentrations. Our work reveals the risk of color center generation by pumping at wavelengths of 915 nm or 976 nm even in Al-free Yb-doped fibers and emphasizes the importance of high purity of raw materials for the preparation of Yb laser fibers with expected very low PD.

Upconversion luminescence with tunable lifetime in NaYF4:Yb,Er nanocrystals: role of nanocrystal size

Despite recent achievements to reduce surface quenching in NaYF(4):Yb,Er nanocrystals, a complete understanding of how the nanocrystal size affects the brightness of upconversion luminescence is still incomplete. Here we investigated upconversion luminescence of Yb,Er-doped nanocrystals in a broad range of sizes from 6 nm to 45 nm (cubic or hexagonal phases), displaying an increasing red-to-green luminescence intensity ratio and reduced luminescence lifetimes with decreasing size. By analyzing the upconversion process with a set of rate equations, we found that their asymptotic analytic solutions explain lower decay rates of red compared to green upconversion luminescence. Furthermore, we quantified the effect of the surface on luminescence lifetime in a model where nanocrystal emitters are divided between the near-surface and inside regions of each nanocrystal. We clarify the influence of the four nonradiative recombination mechanisms (intrinsic phonon modes, vibration energy of surface ligands, solvent-mediated quenching, and surface defects) on the decay rates for different-size nanocrystals, and find that the defect density dominates decay rates for small (below 15 nm) nanocrystals. Our results indicate that a defect-reduction strategy is a key step in producing small upconversion nanocrystals with increased brightness for a variety of bioimaging and biosensing applications.

Investigation on Yb3+∕Er3+∕Tm3+ tri-doped phosphate glass ceramic for white-light-emitting diode

Yb3+∕Er3+∕Tm3+ tri-doped phosphate glass ceramics were prepared by a high-temperature melting method and thermal treatment technology. Upconversion (UC) emissions of the Yb3+∕Er3+∕Tm3+ tri-doped phosphate glass ceramic samples were studied under 975 nm excitation. The glass ceramic samples can simultaneously generate blue, green, and red emissions. The multicolor emission obtained was tuned to white light by adjusting the Er3+ ion concentration. The emission color of the sample doped with 8 mol.% Er3+ ion is white to the naked eye, and CIE coordinates (x=0.316, y=0.354) of the sample are close to the standard equal energy white-light illumination (x=0.333, y=0.333). The material will be useful in developing the white-light-emitting diode.

Intense red upconversion emission of Yb/Tm/Ho triply-doped tellurite glasses

By conventional melting and quenching methods, 3Yb2O3-0.2Tm2O3-xHo2O3 (wt%, x=0.2~1.2) was doped into an easily fiberized tellurite glass with composition of 78TeO2-10ZnO-12Na2O (mol%) to form YTH-TZN78 glasses. Under 976 nm excitation, the direct sensitizing effect of Yb ions (Yb→Ho) and indirect sensitizing and self-depopulating effects of Tm ions (Yb→Tm→Ho) were found to present intense red upconversion emission at 657 nm (Red, Ho:5F5→5I8) and were responsible for the absence of the usually observed 484 nm emission (Blue, Tm:1G4→3H36). Regardless of the dopant concentration of Ho ions, the intensity of the red emission at 657 nm (Red, Ho:5F5→5I8) is about three times stronger than that of the green one at 543 nm (Green, Ho:5S2→5I8). For this certain red emission at 657 nm, 0.4 wt% Ho2O3-doped YTH-TZN78 glass was found to present the highest emission intensity and is therefore determined as a promising active tellurite glass for red fiber laser development.

Influence of Tm- or Er-codoping on the photodarkening kinetics in Yb fibers

We investigated photodarkening (PD) parameters of Yb/Al-doped silica fibers as a function of the concentration of additional rare earth ions like Tm or Er. It was found that both Tm and Er cause a decrease in Yb inversion followed by a reduction of PD in the case of Er, whereas Tm-codoping with more than 10 mol-ppm can strongly accelerate the process and also increase the PD loss. However, contrary to [1], we conclude that the typical PD behavior of Yb/Al fibers is an intrinsic feature of this fiber type and not caused by trace impurities of Tm (< 1 mol-ppm) unintentionally incorporated by the raw materials during fiber preparation.

Theoretical modeling of fiber laser at 810 nm based on thulium-doped silica fibers with enhanced 3H4 level lifetime

A compact upconversion fiber laser operating around 810 nm is proposed using thulium-doped silica-based fiber with locally modified thulium environment by high alumina codoping. Using a comprehensive numerical model of thulium doped fiber we investigate performance of the proposed laser. Comparison with two other thulium hosts, fluoride glass and standard silica, is presented. Efficient lasing can be expected even for silica based fiber for specific ranges of the fiber and laser cavity parameters, especially when 3H4 lifetime is enhanced. With moderate pump power of 5 W at wavelength of 1064 nm, the predicted output power of the upconversion laser is about 2 W at 810 nm.

Rare-earth ions doped heavy metal germanium tellurite glasses for fiber lighting in minimally invasive surgery

In Er(3+)/Yb(3+) codoped Na(2)O-ZnO-PbO-GeO(2)-TeO(2) (NZPGT) glass fiber, a clear and compact green upconversion amplified spontaneous emission (ASE) trace is observed, and the NZPGT glasses are proved to be a desirable candidate in fabricating low-phonon energy fiber. Intense green upconversion luminescence of Er(3+), balanced green and red upconversion emissions of Ho(3+), and dominant three-photon blue upconversion fluorescence of Tm(3+) have been represented. By varying the excitation power of 974 nm wavelength laser diode, a series of green and white fluorescences have been achieved in Tm(3+)/Er(3+)/Yb(3+) and Tm(3+)/Ho(3+)/Yb(3+) triply doped glass systems, respectively. These results reveal that high-intensity blue, green, and white upconversion ASE fluorescences, which can be adopted for lighting in minimally invasive photodynamic therapy and minimally invasive surgery, are reasonable to be expected in rare-earth doped NZPGT glass fibers.