Supplementary Information. White-Light Whispering-Gallery-Mode Lasing from Lanthanide-Doped Upconversion NaYF 4 Hexagonal Microrods

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1 Supplementary Information White-Light Whispering-Gallery-Mode Lasing from Lanthanide-Doped Upconversion NaYF 4 Hexagonal Microrods Ting Wang 1+, Huan Yu 2+, Chun Kit Siu 1, Jianbei Qiu 2, Xuhui Xu 1,2*, and Siu Fung Yu 1* 1 Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China 2 College of Materials Science and Engineering, Kunming University of Science and Technology, Kuming Yunnan, China. + these authors contributed equally to this work *corresponding authors: xuxuh07@126.com, sfyu21@hotmail.com A) Synthesis of the rare-earth doped hexagonal β-nayf 4 microrods Reagents: The NaYF 4 :Ln 3+ (Ln 3+ =Yb 3+ /Er 3+ /Tm 3+ ) microparticles were prepared by a hydrothermal method. All reagents used in the experiment were purchased and unpurified rare-earth (Ln =Yb,Er,Tm) oxides (99.99% purity obtained from Aladdin, China), ethylenediamine tetraacetic acid (EDTA > 99.5%, obtained from Tianjin Fengchuan Chemical Reagent Co., Ltd., China), sodium hydroxide (NaOH > 96.0%, Tianjin Fengchuan Chemical Reagent Co., Ltd., China), ammonium fluoride (NH 4 F > 96.0%, Tianjin Fengchuan Chemical Reagent Co., Ltd., China) and hydrochloric acid (HCL,12M, need to be diluted with deionized water for 1M). All chemicals are of analytical grade and were used directly without further purification. Synthesis of β-nayf 4 microrods: The rare-earth doped NaYF 4 (1 m mol) microrods were prepared by a hydrothermal method. Firstly, Y 2 O 3, Yb 2 O 3 and Er 2 O 3 powder were dissolved in dilute nitrate solution and the residual nitrate was removed by heating and evaporation, resulting in the formation of a clear solution of Ln(NO 3 ) 3 (Ln=Y, Yb, Er). Subsequently, 12.5 ml of aqueous solution containing 0.4g EDTA and 1.05 ml of NaOH aqueous solution (5.0 M) were mixed under stirring until the solution becomes clear. After that, 5 ml of Ln(NO 3 ) 3 (Ln = Y, Yb, Er) aqueous solutions (0.2 M), 8 ml of NH 4 F (2.0 M) aqueous solutions and 7 ml dilute hydrochloric acid (1 M) were added to the mixture. After additional agitation for 90 min on a magnetic stirrer, the resultant solution was transferred to a 50 ml autoclave and maintained at 200 for 40 hours. Finally, the autoclave was cooled down to room temperature. The precipitates were separated by centrifugation, washed with deionized water and ethanol for three times, and then dried in air at 80 C for 8 hours. Yb 3+ -Tm 3+ doped NaYF 4 microrods and Yb 3+ -Er 3+ -Tm 3+ doped NaYF 4 microrods can also be prepared by the similar process as described above with the change of rare-earth ions. 1

2 B) Physical characteristics of the Yb 3+ -Er 3+ and Yb 3+ -Tm 3+ doped hexagonal β-nayf 4 microrods Figure S1 Scan electron microscopy images of (a) 100%Yb 3+ -1%Er 3+, (b) 20%Yb 3+ -1%Er 3+ and (c) 4-%Yb 3+ -2%Tm 3+ co-doped hexagonal β-nayf 4 microrods. (d) X-ray diffraction (XRD) patterns of the corresponding co-doped hexagonal β-nayf 4 microrods. 2

3 C) Size distribution of the Yb 3+ -Er 3+ -Tm 3+ tri-doped β-nayf 4 microrods Figure S2 Size distribution of Yb 3+ -Er 3+ -Tm 3+ tri-doped β-nayf 4 microrods. 3

4 D) Selection of doping concentration of Yb 3+, Er 3+, and Tm 3+ ions to achieve white-light emission at room temperature Figure S3 (a) Red and green emission spectra of x%yb 3+ -1%Er 3+ co-doped β-nayf 4 microrods (x = 15 to 100) under CW 980 nm excitation with pumped power of 1 MW/cm 2. (b) Corresponding ratio of emission intensity between red and green light (i.e. I 654nm /I 540nm ), and the emission intensities of red and green light versus x - the concentration of Yb 3+. (c) Blue emission spectra of y%yb 3+ -2%Tm 3+ co-doped β-nayf 4 microrods (y = 20 to 50) under CW 980 nm excitation with pumped power of 1 MW/cm 2. (d) Emission spectra of 40 %Yb %Tm 3+ -z %Er 3+ tri-doped β-nayf 4 (z = 0.3 to 2) microrods under CW 980 nm excitation with pumped power of 1 MW/cm 2. The corresponding (e) ratio of emission intensity between green and blue light as well as red and blue light and (f) CIE chromaticity versus the concentration of Er 3+. The above experiment was carried out at room temperature. Figure S3(a) gives the green and red emission spectra of x%yb 3+ -1%Er 3+ co-doped β-nayf 4 microrods versus the concentration of Yb 3+, x (x=15 to 100) under continuous wave (CW) 980 nm excitation at 1 MW/cm 2. The emission peaks are attributed to 4 F 9/2 4 I 15/2 (peak at 654 nm) as well as 2 H 11/2 4 I 15/2 and 4 S 3/2 4 I 15/2 (peaks at 520 and 540 nm respectively) transitions of Er 3+. It is noted that the red peak (peak at 654 nm) is dominated for x equal to 100. This is because the population of the 4 F 9/2 state is enhanced through the cross-relaxation process (i.e. 4 F 7/2 4 F 9/2 and 4 I 11/2 4 F 9/2 transitions between the two nearby Er 3+ ions) at high concentration of Yb 3+ [S1 S4]. Green peaks (peaks at 520 and 540 nm) start to dominate the emission spectra for the reduction of x. It is shown in figure 2S(b) that the green peak at 540 nm can be maximized for x = 20. On the other hand, blue emission can be obtained from Yb 3+ -Tm 3+ co-doped β-nayf 4 microrods. Figure 3S(c) plots the emission spectra of y%yb 3+ -2%Tm 3+ co-doped β-nayf 4 microrods versus y (y =20, 30, 40 and 50). Two strong 4

5 blue peaks and 475 nm) are observed from the emission spectra. The 450 and 475 nm emission peaks are related to 1 D 2 3 F 4, and 1 G 4 3 H 6 transitions of Tm 3+ respectively. However, the influence of emission peaks at 647 nm (due to 1 G 4 3 F 4 transition of Tm 3+ ) is negligible. The weak red emission peak is due to the concentration quenching effect of the fluorescence [S5]. From the figure, it is noted that intensity of the blue peak at 450 nm is maximized for y = 40. White-light emission can be obtained from Yb 3+ -Tm 3+ -Er 3+ tri-doped β-nayf 4 microrods under CW 980 nm excitation at 1 MW/cm 2. As the upconversion efficiency of the blue emission peaks (i.e. 3-photon upconversion process) is less than that of the red and green emission peaks (i.e. 2-photon upconversion process), we kept the concentration of Yb 3+ and Tm 3+ to 40% and 2% respectively in order to maintain the maximum emission efficiency of the blue light. Then, the concentration of Er 3+ is used to control the relative emission intensity between the red, green and blue peaks in order to deduce a relatively high-efficiency white-light emission. Figure S3(d) studies the emission spectra of 40%Yb 3+ -2%Tm 3+ -z%er 3+ tri-doped β-nayf 4 microrods versus the concentration of Er 3+, z (z = 0.3, 0.5, 1, 2). Figure S3(e) also shows the intensity peak ratios of green/blue and red/blue light versus z. In addition, the corresponding 1931 CIE chromaticity coordinates for these range of z are calculated in figure S3(f). At low Er 3+ concentration, the blue emission intensity is relatively high. However, with the increase of Er 3+ concentration, the green and red emission peaks dominate the emission spectra. For z is equal to 0.5, white-light emission can be obtained with chromaticity coordinate of (x=0.3047, y=0.3417) and the emission intensity is optimized. This coordinate falls exactly within the white region of 1931 CIE diagram and is very close to the standard equal energy white-light emission (x=0.33 and y=0.33) [S6]. Hence, the concentration of Yb 3+, Tm 3+, and Er 3+ are selected to 40%, 2%, and 0.5% respectively to achieve white-light emission from the β-nayf 4 microrods. 5

6 E) Computer simulation of electric field profile of a β-nayf 4 hexagonal microrod supporting multi-mode resonance Figure S4 (a) Emission spectra of a 40%Yb 3+ -2%Tm 3+ co-doped β-nayf 4 hexagonal microrod with a radius R of ~4.5 µm under 980 nm ns-pulses excitation at room temperature. (b) Computer simulation of electric field profile supported inside a hexagonal resonator with a radius R of ~4.5 µm. Figure S4(a) shows the lasing spectra of a hexagonal microrod with radius R = 4.5 µm. It is noted that the microcavity supports multimode operation with mode spacing λ equal to ~5 nm. For the hexagonal microcavity supporting whispering gallery modes (WGMs), λ can be calculated from λ dn λ λ= n λ L dλ Ln g 5.3 nm (S1) where L( = 3 3 R ) ~ 23.4 µ m is the cavity length, λ (= 450 nm) is the emission wavelength and n (=1.498) is the refractive index and n g (= 1.623) is group refractive index [S7]. As the calculated λ is roughly matched with that measured from the experiment, this verifies that our microrods mainly supporting WGMs. Figure S4(b) simulate the electric field profile inside the hexagonal microcavity with R = 4.5 µm. It is observed that optical feedback is achieved through total internal reflection from the six flat surfaces of the microrods (see the yellow dashed-line). 6

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