Guojun Gao

Ion-redistribution induced efficient upconversion in β-NaYF_4:20%Yb^3+,2%Er^3+ microcrystals with well controlled morphology and size

We develop an efficient green upconversion (UC) β-NaYF₄:20%Yb³⁺,2%Er³⁺ microcrystal with well controlled morphology and size by hydrothermal method using two different chelating agents of CIT and EDTA-2Na via a simple ion-exchange reaction. Importantly, the UC emission efficiency of newly developed CIT and EDTA-2Na β-NaYF₄:20%Yb³⁺,2%Er³⁺ microcrystals is almost as strong as that of commercial counterpart by solid-state method. A proof-of-concept β-NaYF₄:20%Yb³⁺,2%Er³⁺ microcrystal waveguide is demonstrated to extend their applications in modern micro-optoelectronics. The local ion-redistribution process during the ion-exchange reaction, which effectively disperses the locally clustered Yb³⁺, accounts for the enormously enhanced UC emission in β-NaYF₄:20%Yb³⁺,2%Er³⁺ microcrystals.

Finely-tuned NIR-to-visible up-conversion in La2O3:Yb3+,Er3+ microcrystals with high quantum yield

Up-conversion (UC) materials whose emission color can be finely-tuned while a high UC quantum efficiency is maintained are desirable for many applications. Herein, we report near-infrared-to-visible La2O3:Yb3+,Er3+ (LYE) UC materials with a high internal quantum yield (UCQY) of 3.8%, external UCQY (brightness) of 1.6% and tunable emission color. UC emission colors from pure green to reddish-orange can be precisely tailored by simply controlling synthesis conditions, whilst maintaining the high UCQY. The internal UCQY and external UCQY of LYE yield better performance than both commercially available and other record UC phosphors reported in the literature under the same excitation conditions. The facile preparation combined with the color-tuning and high UCQYs make these materials attractive candidates for solar energy harvesting, sensors, 3D volumetric displays, solid state lasers and bio-imaging.

Monodisperse β-NaYF4:Yb3+,Tm3+ hexagonal microplates with efficient NIR-to-NIR up-conversion emission developed via ion exchange

Monodisperse β-NaYF4:Yb3+,Tm3+ (NYF) hexagonal microplates with efficient near-infrared (NIR)-to-NIR up-conversion (UC) developed via a new ion-exchange modified hydrothermal method are reported. Ion exchange modification (IEM) not only significantly increases the UC intensity by up to 1000 times and prolongs the emission lifetimes of Tm3+ and Yb3+, but also enables the monodisperse morphology and size of hexagonal microplates to be maintained. A high UC quantum efficiency (QE) of 3.1% is obtained for IEM β-NaYF4:20%Yb3+,1%Tm3+ when excited with 980 nm light at a power density of 10 W cm−2. The UC emission properties can be finely tailored by changing the NaF/NYF molar ratio (NYF represents lanthanide doped β-NaYF4) and the doping concentration of Tm3+. The two photon NIR UC emission centered at ∼803 nm (arising from Tm3+: 3H4 → 3H6) dominates the UC emission spectrum and high concentrations of Tm3+ favor NIR UC further. A proof-of-concept optical image of printed patterns is demonstrated to verify their applications in security. These results suggest the promising applications of the newly developed monodisperse β-NaYF4:Yb3+,Tm3+ hexagonal microplates in security, luminescent labels, solid-state lasers, amplifiers, and biomedicine.

Co-precipitation synthesis and photoluminescence properties of BaTiF6:Mn4+:an efficient red phosphor for warm white LEDs

The investigation of efficient red phosphors is highly desired for the development of novel warm white light emitting diodes (WLEDs). In this paper, we report on an efficient red phosphor of Mn4+-activated BaTiF6 by a facile co-precipitation method as a promising candidate for warm white LEDs. BaTi1−xF6:xMn4+ phosphors show efficient pure red emission with a high quantum yield (QY) of 44.5% under 460 nm excitation. The BaTi1−xF6:xMn4+ phosphor exhibits a number of advantages. Firstly, the corresponding excitation/absorption profile matches the commercial blue LED chip well. Secondly, it also exhibits appropriate CIE coordinates (x = 0.694, y = 0.306) with an activation energy of 0.603 eV. The demonstration of a blue chip combined with a blend of yellow-emitting YAG:Ce3+ and newly developed BaTi0.97F6:0.03Mn4+ red phosphor greatly improved the colour rendering index (CRI) from 69.9 to 83.5, while significantly decreasing the correlated colour temperature (CCT) from 5088 to 4213 K, thus validating their application in warm white LEDs.

Wide-range non-contact fluorescence intensity ratio thermometer based on Yb3+/Nd3+ co-doped La2O3 microcrystals operating from 290 to 1230 K

Non-contact ratiometric thermometry has applications ranging from in situ physiological measurements to industrial process monitoring. The technique is based on the optical detection of the fluorescence intensity ratio (FIR) of two thermally-coupled levels (TCLs). Here, we report a Yb3+/Nd3+ co-doped La2O3 microcrystal ratiometric thermometer based on upconverted emission in the near-infrared (NIR) after excitation with a 980 nm laser diode, which operates effectively over the wide temperature range from 290–1230 K. The thermometer uses the TCLs of Nd3+:4F7/2 (emission peak 762 nm) and Nd3+:4F5/2 (emission peak 825 nm). The chosen levels combine desirable characteristics to act as a sensitive temperature sensor over a wide range of elevated temperatures: namely a suitable energy gap (ΔE = 950 cm−1); and weak thermal quenching effects (maximum photoluminescence at 803 and 853 K respectively for two Nd3+ emission peaks). This leads to a high relative sensitivity (SR) of 1334/T2, and low temperature uncertainty ΔTmin of 0.1 K (<400 K), 1 K (400–853 K) and 3 K (853–1233 K). In addition to these characteristics, the excellent repeatability of FIR of the two Nd3+ emission peaks makes Yb3+/Nd3+ co-doped La2O3 microcrystals a promising non-contact NIR ratiometric thermometer for temperatures up to 1230 K.

Manipulating refractive index, homogeneity and spectroscopy of Yb 3+ -doped silica-core glass towards high-power large mode area photonic crystal fiber lasers

Output power scaling of single mode large mode area (LMA) photonic crystal fiber (PCF) amplifiers urgently requires the low refractive index of Yb3+-doped silica glasses whilst maintaining high optical homogeneity. In this paper, we report on a promising alternative Yb3+/Al3+/F-/P5+-co-doped silica core-glass (YAFP), which is prepared by modified sol-gel method developed by our group and highly suitable for fabricating high power LMA PCF amplifiers. By controlling the doping combinations of Al3+/F-/P5+ in Yb3+-doped silica glass,it not only ensures low refractive index (RI) but also maintains the excellent optical homogeneity and spectroscopic properties of Yb3+. The spectroscopic properties of Yb3+ ions have not deteriorated by the co-doping of F- and P5+ in YAFP glass compared with that of Yb3+/Al3+ co-doped silica glass. A large-size (⌀5 mm × 90 mm) YAFP silica-core glass rod with low average RI difference of 2.6 × 10-4 (with respect to pure silica glass), and low radial and axial RI fluctuations of ~2 × 10-4, was prepared. A LMA PCF with 50 µm core diameter was obtained by stack-capillary-draw techniques using YAFP core glass. Its core NA is 0.027. An average amplified power of 97 W peaking at 1030 nm and light-light efficiency of 54% are achieved from a 6.5 m long PCF in the pulse amplification laser experiment. Meanwhile, quasi-single-mode transmission is obtained with laser beam quality factor M2 of 1.4.

Highly efficient La2O3:Yb3+,Tm3+ single-band NIR-to-NIR upconverting microcrystals for anti-counterfeiting applications

Efficient single-band NIR-to-NIR upconversion (UC) emission is strongly desired for many applications such as fluorescent markers, plastic recycling, and biological imaging. Herein, we report highly efficient single-band NIR-to-NIR UC emission in La2O3:Yb3+,Tm3+ (LYT) microcrystals. Under 980 nm laser excitation, LYT exhibits a NIR UC emission at ∼795 nm (Tm3+: 3H4 → 3H6) and blue UC emission at ∼476 nm; the NIR UC emission is dominant, with the intensity ratio of the NIR to blue INIR/ Iblue > 100. Remarkably, a high absolute UC quantum yield (UCQY) of 3.4% is obtained for the single-band NIR UC emission of LYT at a relatively low excitation power density of 7.6 W/cm2. This value is much higher than the reported values of a single-band NIR UC for rare-earth-based UC materials in literature, such as the well-known benchmark UC materials of β-NaYF4:Yb3+,Er3+ (∼0.9%, with a excitation power density of 9 W/cm2) and Gd2O2S:Yb3+,Er3+ (∼1.9%, with a excitation power density of 20 W/cm2). The high absolute UCQY of single-band NIR UC emission combined with their facile preparation hints at their potential application in anti-counterfeiting, verified by the proof-of-concept demonstration of fluorescent labeling of a transparent IMT pattern.