Dangli Gao

Simultaneous quasi-one-dimensional propagation and tuning of upconversion luminescence through waveguide effect

Luminescence-based waveguide is widely investigated as a promising alternative to conquer the difficulties of efficiently coupling light into a waveguide. But applications have been still limited due to employing blue or ultraviolet light as excitation source with the lower penetration depth leading to a weak guided light. Here, we show a quasi-one-dimensional propagation of luminescence and then resulting in a strong luminescence output from the top end of a single NaYF4:Yb3+/Er3+ microtube under near infrared light excitation. The mechanism of upconversion propagation, based on the optical waveguide effect accompanied with energy migration, is proposed. The efficiency of luminescence output is highly dependent on the concentration of dopant ions, excitation power, morphology, and crystallinity of tube as an indirect evidence of the existence of the optical actived waveguide effect. These findings provide the possibility for the construction of upconversion fiber laser.

Rare-earth doped LaF3 hollow hexagonal nanoplates: hydrothermal synthesis and photoluminescence properties

A facile hydrothermal route at designated pH values has been developed to synthesize a series of well-dispersed LaF3 colloidal nanocrystals (NCs) with a rich variety of morphologies, including nanoparticles, hexagonal nanoplates and fullerene-like nanoparticles. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, and photoluminescence spectroscopy were used to characterize the samples. It is found that the formation of monodispersed fluoride NCs not only closely correlates with the pH values of the mother solutions, but also depends on the basicity of the base employed to adjust the pH value. The strong alkaline solution in the absence of any organic additive in this system was found to be a prerequisite for producing hexagonal fullerene-like LaF3 nanodisks. A mechanism for the formation of the fullerene-like LaF3via the local Ostwald ripening process has been proposed based on observations of time-dependent experiments. The multicolor upconversion (UC) emission was successfully realized in a series of Yb3+/Er3+ doped LaF3 NCs by excitation in the near-infrared region. The UC emission ratios of red to green for a series of LaF3 NCs can be tuned by adjusting the pH values of the mother liquids, and an UC mechanism activated by high-energy phonons inherent in the hollow LaF3 nanoplates is proposed. It is expected that these rare-earth fluoride NCs might have potential applications in photocatalysis, biolabelling and drug-delivery.

The novel upconversion properties of LiYbF4:Er microcrystals compared to the Na counterpart

Erbium-doped ytterbium fluoride compounds with different crystal phases and morphologies have been synthesized via a facile hydrothermal route assisted with EDTA. Tunable upconversion emissions can be obtained by replacing Na+ with Li+ in NaYbF4 microcrystals. An interesting blue shift is observed and a mechanism based on a weakened polarization effect is proposed.

Up/down conversion switching by adjusting the pulse width of red laser beams in LaF₃:Tm³⁺ nanocrystals.

We demonstrate a versatile approach to fine-tuning the ratio of blue to near-infrared emission intensity from Tm3+ ions in LaF3 nanocrystals by adjusting the pulse widths and excitation wavelengths of red laser beams. The mechanism of color-tunable Tm3+ emission by pulse widths is explored, and a mechanism based on promoting the population of some luminescence levels and cutting off the population of others by suitably adjusting pulse duration is proposed. The underlying reason of excitation wavelength-modulated emission is ascribed to tuning absorption probability ratio of ground state absorption to excited state absorption by tuning the matching degree between the energies of excitation wavelength and ground (excited) state absorption of Tm3+. The ability of our LaF3:Tm3+ nanocrystals to emit variable emissions on demand in response to pulse width and excitation wavelength provides keen insights into controlling the population processes of luminescent levels and offers a versatile approach for tuning the spectral output.

The effects of structural characterization on the luminescence of Eu3+-doped fluoride nano/microcrystals

The pure tetragonal crystal phase LaOF:Eu3+ (2 mol%) nanocrystals (NCs) are obtained by annealing ultrasmall-sized LaF3:Eu3+ (2 mol%) NCs assisted with the surfactant oleic acid. Spectroscopic characterization of Eu3+ in various host materials, including hexagonal-phase LaF3, NaYF4 and NaLaF4, cube-phase NaYF4, tetragonal-phase LiYF4 and LaOF crystals, is performed to tune the luminescence by controlling the host matrices with different compositions or different phase structures of the same inorganic compound. The excitation and emission spectra arising from LaOF:Eu3+ (2 mol%) clearly show that the excitation and emission profiles are different from those of the other matrices. By combining the equation of the spectral conversion with the emission and excitation spectra, it is evident that the efficiency of the spectral conversion in the LaOF host material is superior to that in others, indicating a potential application in solar cells. The underlying reason for the enhanced efficiency of the spectral conversion in the LaOF host material is explored, and a mechanism based on the low local symmetry induced by the shortened bond distance and the asymmetric spatial distribution of the chemical bonds of anions coordinating the rare-earth ions is proposed.

Spatial control of upconversion emission in a single fluoride microcrystal via the excitation mode and native interference effect

Lanthanide (Ln)-doped upconversion (UC) micro/nanomaterials have attracted significant attention in various applications. However, spatial control of UC emission from micro/nanomaterials has not been developed to date due to the ineffective quantum yield and weak luminescence intensity. In this study, the fascinating luminescent patterns, including a dual-colour dumbbell, sunflower, rod-shaped candy, dichroic plate, and coloured ring from a series of single NaYF4:Yb/Ln (Ln = Er, Ho, and Tm) microcrystals, that exhibit different distinctive distributions on luminescence intensity and colour in space are obtained by varying the excitation modes. The overall integral UC intensity of a single microtube by waveguiding-excitation is invariably stronger relative to that in the case of the spot-excitation approach, and the enhancement factor increases 10 times. We explained the underlying physical mechanisms behind the changes in the colour and intensity of the luminescent pattern based on the native diffraction and interference effect of microcrystals. These findings provide a new way to improve UC luminescence and facilitate a basic understanding of the use of UC micro/nanocrystals as natural interference devices, which will result in new applications in displays, optical waveguides, and anti-counterfeiting.