White-Light Whispering-Gallery-Mode Lasing from Lanthanide-Doped

Article

White-light whispering-gallery-mode lasing from lanthanidedoped upconversion NaYF4 hexagonal microrods Ting Wang, Huan Yu, Chun Kit Siu, Jianbei Qiu, Xuhui Xu, and Siu Fung Yu ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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White-Light Whispering-Gallery-Mode Lasing from Lanthanide-Doped Upconversion NaYF4 Hexagonal Microrods Ting Wang1+, Huan Yu2+, Chun Kit Siu1, Jianbei Qiu2, Xuhui Xu1,2*, and Siu Fung Yu1* 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: [email protected], [email protected]

Abstract – We demonstrate simultaneous red, green and blue emission from a Yb3+-Er3+-Tm3+ tri-doped hexagonal β-NaYF4 microrod, which supports whispering gallery modes (WGMs) resonance, to realize white-light lasing under near-infrared excitation at room temperature. This can be done by optimizing the upconversion efficiency and emission intensity balance of the blue, green, and red peaks through the proper tuning of sensitizer (Yb3+) and activators (Er3+, Tm3+) concentration in the host matrix. In addition, we minimize the difference of lasing threshold and maintain stable single-mode operation to achieve simultaneous red, green and blue lasing by optimizing the radius of the hexagonal microrods. Keywords: NaYF4 hexagonal microrods, white-light lasing, whispering-gallery-mode, upconversion emission, lanthanide ions

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A conventional laser emits photons of the same color and phase so that the corresponding output beam is directional and has high power density. A white-light laser, which has optical characteristics similar to that of a conventional laser, however, demonstrates multi-color emission based on the mixture of the three basic colors – red, green and blue. When compared to white-light light-emitting diodes, white-light lasers have the advantages of low beam divergence and high power density emission. Hence, white-light lasers find scientific applications such as the lasing sources of three-dimensional (3D) optical tweezers, fluorescence microscopy, and true-color 3D holograms [1-4]. More importantly, white-light lasers have potential utilization in consumer products such as spotlights and headlights of automobiles (e.g. BMW i8) as well as laser lighting [5,6]. Current technology to realize white-light lasers is based on the phenomena of highly nonlinear optical processes which require an extremely high input power of light to obtain broadening of the emission spectrum. For example, white-light pulses can be generated through the focus of high-power femtosecond pulses on transparent materials

[7,8]

. Using this

technique, white-light lasing sources had been commercialized as a product for scientific applications

[9-11]

. However, drawbacks of using nonlinear optical processes are its bulk in

size, high production and high operation cost. Recently, a monolithic multi-segment ZnCdSSe based semiconductor nanosheet, which lases in the red, green and blue (RGB), has potential to achieve low-cost and compact white-light laser diode [12]. Nevertheless, it is still a challenge to achieve RGB emission simultaneously from the same laser cavity to generate white-light lasing. Recently, lanthanide-doped upconversion luminescence fluoride-based materials have been utilized to produce white-light emission. This is because RGB emission can be obtained simultaneously by either controlling the concentration of activator ions (Er3+,Tm3+,Ho3+) [13,14]

, adjusting pumped power [15, 16] or tuning the concentration of sensitizer ion (Yb3+) [17,18]

within any defined location to generate white light. In addition, low cost and compact near-infrared (NIR) laser diodes can be used as the excitation sources. Here, we propose to use Yb3+-Er3+-Tm3+ tri-doped β-hexagonal-phase NaYF4 microrods to achieve RGB lasing under NIR excitation. RGB emission peaks can be obtained from the doped NaYF4 microrods by optimizing the sensitizer and activator ions. By selecting a suitable size of the microcavity, the different in lasing threshold between the RGB modes can be minimized. Hence, white-light lasing emission based on the three basic colors can be obtained from an individual hexagonal microrod under NIR excitation. 2


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EXPERIMENTAL RESULTS

Figure 1. (a) SEM images of the as-synthesized Yb3+-Er3+-Tm3+ tri-doped β-NaYF4 hexagonal microrods. (b) Elemental mapping of a NaYF4 microrod. (c) HR-TEM image, (d) electron diffraction image and (e) XRD pattern of a NaYF4 microrod.

Typical scan electron microscopy (SEM) images of Yb3+-Er3+-Tm3+ tri-doped β-NaYF4 hexagonal microrods are shown in figure 1(a). The figure reveals that the microrods, which have two sharp ends, have an average radius of ~3 µm (see supplementary information). The close-up view (see the inset of figure 1(a)) displays that the microrods have six flat surfaces and the ends are of the hexagonal pyramid structure. Figure 1b shows the mapping results of Yb, Er and Tm elements which are uniformly distributed inside the microrod. Figure 1(c) shows a high-resolution transmission electron microscopy (HR-TEM) image of a β-NaYF4 hexagonal microrod. The image was taken at the edge of one of the sharp ends of the microrod. The lattice structure of the β-NaYF4 crystal can be indexed as (100) and (001) planes which have a space group of P63/m. This implies that the growth direction of the microrods is along the [001] direction. [19] The electron diffraction image shown in figure 1(d) suggests that the single crystalline nature of the microrods. X-ray diffraction (XRD) pattern shown in figure 1(e) resembles the β-NaYF4 crystal (JCPDS file number 16-0334) closely. This suggests that the crystallinity and high purity of the microrods. Such highly crystalline microstructure results from the preferential growth along the [001] direction (c-axis)

[19]

.

Hence, we conclude that high-quality idiomorphic β-NaYF4 crystals are successfully prepared.

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Figure 2 (a) Energy level diagram of the Yb3+-Er3+-Tm3+ tri-doping β-NaYF4 hexagonal microrod under NIR excitation. The mechanisms of up-conversion population, radiative and non-radiative recombination as well as cross-relaxation transitions, are indicated in the diagram. (b) Room-temperature emission spectra of the doped β-NaYF4 hexagonal microrods under 980 nm CW excitation at a pumped power of 1 MW/cm2. The inset photos show the corresponding emission profile.

Figure 2(a) shows the energy level diagrams of Yb3+-Er3+-Tm3+ tri-doped β-NaYF4 microrods. The presence of Er3+ leads to 4F9/2→4I15/2 (654 nm - red) as well as 2H11/2→4I15/2 (520 nm – green) and 4S3/2→4I15/2 (540 nm – green) transitions. These red and green transitions are of 2-photon absorption process. It is expected that high concentration of Yb3+ will favor the red emission peak and the reduction of Yb3+ will enhance the green emission peak due to the influence of cross-relaxation process. On the other hand, the presence of Tm3+ leads to 1

D2→3F4 (450 nm – blue) and 1G4→3H6 (475 nm – blue) transitions [20,21]. However, the blue

transition is of 3-photon absorption process so its upconversion efficiency is less than that of the red and green peaks. In order to obtain high-power white-light emission at a high excitation power, the doping concentration of both Yb3+ and Tm3+ is selected to maximize blue emission intensity. Then, the doping concentration of Er3+ is selected to simultaneously optimize red, green and blue (RGB) emission with the same level of intensity (see supplementary information). Figure 2(b) shows the emission spectra of the doped β-NaYF4 hexagonal microrods. In the experiment, the continuous wave (CW) 980 nm laser beam was focused to an 8 µm diameter laser spot through an objective lens so that high excitation pumped power of 1 MW/cm2 can be obtained. Red, green and blue emission intensities are maximized via the use of 100%Yb3+-1%Er3+, 20%Yb3+-1%Er3+, and 40%Yb3+-2%Tm3+, respectively as the co-doping concentration. Furthermore, 40%Yb3+-2%Tm3+-0.5%Er3+ tri-doped β-NaYF4 hexagonal 4


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microrods are found to achieve effective white-light emission

[22]

. As shown in figure 2(b),

the relative intensity of the RGB emission peaks is just slightly reduced from their maximum values so that the upconversion efficiency of the white-light emission has been optimized.

Figure 3 Room-temperature emission spectra of three individual ~3 µm radius β-NaYF4 microrods with (a) 100%Yb3+-1%Er3+, (b) 20%Yb3+-1%Er3+, and (c) 40%Yb3+-2%Tm3+ doping concentration under 980 nm ns-pulsed laser excitation. The insets show the corresponding optical images with emission wavelength, λ, of 654, 540 and 450 nm. (d) The corresponding light-light curves. Numerical simulation of optical field distribution inside a 100%Yb3+-1%Er3+ co-doped β-NaYF4 microrod emitted at 654 nm is shown in the inset of figure 3(d). The yellow dashed-line indicates the formation of one round-trip.

The possibilities to support RGB lasing emission from the hexagonal microrods are investigated. Figure 3(a) plots the emission spectra of a 3 µm radius 100%Yb3+-1%Er3+ doped β-NaYF4 microrod under 980 ns-pulsed excitations at room temperature. In general, a board emission spectrum is observed at low excitation power. Furthermore, a stable sharp peak of linewidth less than 0.4 nm emerges from the board emission spectrum for the increase of excitation power larger than a threshold Pth. The inset shows the images of the hexagonal microrods through an optical microscope. Figure 3(b) and 3(b) show the emission spectra of the 3 µm radius β-NaYF4 microrod with doping profile of 20%Yb3+-1%Er3+ and 40%Yb3+-2%Tm3+ respectively. It is noted that their emission characteristics are similar to that of given in figure 3(a). The light-light curves of these RGB emission spectra are also plotted in Figure 3(d). A kink (i.e. pump threshold, Pth) occurs in each of the light-light curves. Pth is found to be equal to 3.0, 3.78 and 4.8 mJ/cm2 for the emission peak λ at 654, 540 and 450 nm respectively. This verifies that the microrods support lasing at these wavelengths. The inset of figure 3(d) shows the numerical investigation of optical field profile (cross-section) of the 100%Yb3+-1%Er3+ co-doped β-NaYF4 microrod emitted at 654 nm.

It is noted

that the

six flat

surfaces

of

the

microrod

support

solely

whispering-gallery-mode (WGM) via total internal reflection (see the dashed line). However, quasi-WGMs and Fabry-Perot modes are not supported inside the hexagonal microrods due 5


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to their relatively high cavity losses. Furthermore, it is observed that the linewidth of the blue lasing mode is in general larger than that of the red and green. It was shown that the linewidth of a single-mode semiconductor laser is inversely proportional to its output power [23]. As the blue lasing mode has relatively low output power due to its relatively higher threshold, the corresponding linewidth is relatively larger than that of the green and red.

Figure 4 (a) Plots of the threshold, Pth, versus radius, R, of the doped β-NaYF4 hexagonal microrods with λ equal to 654, 540 and 450 nm. (b) Light-light curves, (c) emission spectra and (d) lasing linewidth of the RGB modes for a 40%Yb3+-2%Tm3+- 0.5%Er3+ tri-doping microrod with R equal to 4 µm. The inset in figure 4(c) shows the corresponding microscopy image under 980 nm ns-pulsed excitation at room temperature.

From the above results, the use of 40%Yb3+-2%Tm3+-0.5%Er3+ tri-doped β-NaYF4 hexagonal microrods should achieve white-light lasing emission. This is because the similar intensity of RGB lasing emission can be excited simultaneously within the same microcavity. Fig. 4(a) shows Pth of the RGB lasing modes (at 654, 540 and 450 nm) versus radius, R, of the 40%Yb3+-2%Tm3+-0.5%Er3+ tri-doped β-NaYF4 hexagonal microrods under 980 nm ns-pulsed excitation. It is observed that the different of Pth between the RGB lasing modes decreases with the increase of R. Therefore, it is preferred to use a large value of R so that the RGB lasing modes can be excited simultaneously. Here, we select R = 4 mm because this value of R can also maintain stable single-mode lasing (see supplementary information). Figure 4(b) plots the light-light curves of the 40%Yb3+-2%Tm3+-0.5%Er3+ tri-doped β-NaYF4 microrod with R equal to 4 µm for the RGB modes. The microrod was excited by a 980 nm ns-pulsed laser at room temperature. Figure 4(c) shows the corresponding lasing spectra of the microrod for pumped power density, P, equal to 1.0 and 21.6 mJ/cm2. The inset gives the microscopy image of the sample. It is observed that the sample sustain single-mode lasing emission at λ equal to 654, 540 and 450 nm. In addition, white-light lasing is observed from the flat surface of the hexagonal microrod, see the inset of figure 4(c). Figure 4(d) plots the corresponding emission linewidth versus pumped power density, P, for the RGB lasing modes. Figures 4b to 4d show a kink from the light-light curves and demonstrate narrowing 6


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of emission spectra simultaneously from the RGB lasing modes. As these are the primary colors of white light, the observation of blue, green and red lasing emission implies that the microrod supports white-light lasing.

Figure 5 The plot of CIE1931 color coordinates of the doped microrods under lasing emission with different (a) R with pumped power kept at 21.5 mJ/cm2 and (b) pumped power with R kept at 4 µm.

The corresponding CIE 1931 color diagram coordinate is located in the (x=0.3440, y=0.3573). These values are very close to that of the white point CIE standard illuminant coordinate (x=0.33 and y=0.33). Figure 5(a) and (b) show the CIE 1931 color diagram coordinate of the doped β-NaYF4 with different values of R (with P = 21.5 mJ/cm2) and P (with R = 4 µm), respectively. It is observed that the CIE standard illuminant coordinate has little change over these ranges of operation conditions. DISCUSSIONS AND CONCLUSIONS The optimized concentration of the lanthanide ions for achieving white-light emission from the β-NaYF4 hexagonal microrods described above are their initial amount used in the fabrication process. The actual amount of lanthanide ions doped into the microrods should be deduced through the inductively coupled plasma - optical emission spectrometry (ICP-OES) [24]

. It is found that the actual concentration of Yb3+, Er3+, and Tm3+ doped inside the

β-NaYF4 microrods is about 32.13%, 0.315% and 0.765% respectively which are less than that of the initial concentration of the lanthanide ions. This is expected as some of the lanthanide ions were consumed during the fabrication process. Nevertheless, we kept using the initial doping concentrations to describe the tri-doped β-NaYF4 microrods because these values are more meaningful to initialize the doping process. On the other hand, the quantum efficiency of the tri-doped β-NaYF4 microrods was measured through the use of an integrating sphere. It was found that the quantum efficiency for the white-light emission via 980 nm laser excitation at room temperature can be approaching 0.01% which is as good as our recent reported lanthanide-doped NaYF4 core-shell nanoparticles [25]. 7


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In conclusion, we realize upconversion white-light lasing from lanthanide-doped β-NaYF4 hexagonal microrods under near-infrared excitation at room temperature. In order to obtain effective white-light emission through the mixture of the three basic colors, we use 40% of Yb3+ and 2% of Tm3+ as the co-doping concentration to maximize blue emission intensity at 450 nm. We further introduce Er3+ into the microrods to excite red and green emission intensity with peaks at 654 and 540 nm, respectively. It is found that 0.5% of Er3+ is sufficient to excite RGB emission peaks simultaneously with similar emission intensity. On the other hand, we verify that the hexagonal microrods can support single WGM lasing at 654, 540 and 450 nm with linewidth less than 0.4 nm. In order to simultaneously excite RGB lasing within the same microcavity, the radius of the microrods is set to 4 µm so that the Pth of all the RGB WGMs have a similar range of values. Under 980 nm excitation, we found that the lasing spectra of the microrods have CIE 1931 color diagram coordinate x=0.3440, y=0.3573. These values are very close to that of the white point CIE standard illuminant coordinate (x=0.33 and y=0.33). Hence, these results suggest a simple approach which has potential to obtain high-efficient and compact white-light lasers suitable for consumer applications. Acknowledgements. This work was supported by the Research Grants Council of Hong Kong (PolyU 153036/14P) and the National Nature Science Foundation of China (61378071, 61565009, 11664022).

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METHODS Microrods synthesis: We synthesized the microrods using the method described in Ref. [21, 26]. Additional experimental details are provided in the Supplementary Information.

Materials Characterization: A powder X-ray diffraction (XRD) data were recorded on a Bruker AXS D2 phaser with a graphite-monochro-matized Cu Kα radiation (1.5406 Å). Transmission electron microscopy (TEM) and Elemental mapping analysis were performed on a JEOL-JEM 2100F transmission electron microscope operating at an acceleration voltage of 200 kV. Teledyne Leeman Labs - Prodigy Inductively coupled plasma - optical emission spectrometry (ICP-OES) with 1.1 kW power was used to analyze the doped concentration of lanthanide ions in 0.1 g of the 40%Yb3+-2%Tm3+-0.5%Er3+ tri-doped β-NaYF4 microrods.

Optical measurement: Lasing characteristics of the hexagonal microrods were studied by using a frequency-tripled 355 nm Q-switched Nd:YAG pulsed laser (6 ns, 10 Hz) with a beam diameter of 0.8 cm as the main excitation source. 980 nm laser beams, which were polarized horizontally, were generated through the use of an optical parametric oscillator. The sample (i.e. quartz plate coated with rare earth doped NaYF4 microrods) was placed on an X-Y-Z translation stage of a dark-field optical microscope (Leica DM1000 LED) for the optical (PL and lasing spectra) characterizations. A 50× 0.75 N.A. objective lens was used to excite and to collect light emission from a hexagonal microrod. The recorded light was then analyzed by an Oriel MS257 monochromator (i.e., spectral resolution is 0.1 nm) attached to a photomultiplier tube. The measurement was carried out at room temperature in an atmospheric ambiance. The quantum efficiency measurement was performed by studying the emission spectra of the microrods inside an integrating sphere under CW 980 nm excitation at room temperature. Hence, the quantum efficiency is calculated from the ratio between the number of photons emitted from the microrods and the number of photons absorbed by the microrods.

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