Influence of Plasmonic Effect on the Upconversion Emission

Jun 25, 2018 - The influence of plasmonic effect on the upconversion emission characteristics of Yb3+–Er3+–Tm3+ tridoped β-NaYF4 hexagonal micror...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of Plasmonic Effect on the Upconversion Emission Characteristics of NaYF4 Hexagonal Microrods Ting Wang,† Chun Kit Siu,† Huan Yu,‡ Yunfeng Wang,† Siqi Li,† Wei Lu,§ Jianhua Hao,† Hong Liu,∥ Jing Hua Teng,∥ Dang Yuan Lei,† Xuhui Xu,‡ and Siu Fung Yu*,†,⊥ †

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming Yunnan China § University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, Hong Kong, China ∥ Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore ⊥ Shenzhen Research Institute, The Hong Kong Polytechnic University, Shenzhen, P. R. China

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S Supporting Information *

ABSTRACT: The influence of plasmonic effect on the upconversion emission characteristics of Yb3+−Er3+−Tm3+ tridoped β-NaYF4 hexagonal microrods is studied. Upconversion spontaneous emission can be improved by 10 times if the microrod is deposited on an Ag-coated substrate. The enhancement is also dependent on the emission wavelength and the polarization of the excitation source. Furthermore, upconversion lasing is supported by the geometry of the microrods via the formation of whispering gallery modes. The corresponding excitation threshold can also be reduced by 50% through the influence of plasmonic effect, the coupling between the whispering gallery modes and the surface plasmonic resonance modes.

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INTRODUCTION Wide emission bandwidth (from ultraviolet to infrared wavelength), which can be obtained from the lanthanide ion (Ln3+)-doped upconversion materials, is a necessary condition to realize broadband-tunable lasers. However, the relatively low upconversion emission efficiency1,2 leads to the high excitation threshold of the Ln3+-doped upconversion lasers.3 If the lasing threshold can be further reduced, the wide bandwidth emission characteristics of upconversion lasers may find enormous applications such as all-optical on-chip information processing, biomedical imaging and optogenetics.4−7 Recently, extensive investigations have shown that the enhancement of upconversion emission from the Ln3+-doped nanoparticles (NPs) can be achieved via the surface plasmon resonance (SPR).8 This is due to the effective coupling between the excited fluorophores of the upconversion NPs and the SPR of the metal NPs. It is reported that the matching of the metal NPs plasmon resonance to the emission wavelengths of the upconversion NPs can improve spontaneous upconversion emission intensity by at least two to more than 30 times.9−13 Stimulated emission of surface plasmon polaritons is also observed from a long metal stripe embedded in Er3+doped phosphate glass.14 However, there is no direct proof if the use of SPR can reduce the lasing threshold of the Ln3+doped upconversion lasers.15,16 © XXXX American Chemical Society

In this article, we show unambiguously that the use of SPR can reduce the excitation threshold of blue, green, and red lasing emission from a Yb3+−Er3+−Tm3+ tridoped β-NaYF4 hexagonal microrod deposited on an Ag-coated substrate. Here, the use of hexagonal microrods allows us (1) to obtain sufficient optical amplification of the whispering gallery modes to sustain lasing emission and (2) to minimize the absorption loss from the Ag coating. The use of an Ag-coated substrate is to support the generation of SPR modes under high-power excitation.17 As the interaction of whispering gallery modes and the SPR modes is demonstrated outside the laser cavity through the coupling from the Ag layer, we can clearly show that the enhancement of lasing emission and the reduction of the lasing threshold are mainly due to the plasmonic effect.

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EXPERIMENTAL RESULTS Figure 1a shows the X-ray diffraction (XRD) pattern, which resembles the β-NaYF4 crystal (JCPDS file number 16−0334), of the Yb3+−Er3+−Tm3+ tridoped β-NaYF4 hexagonal microrods.18 This result suggests the crystallinity and high purity of the microrods. The inset shows a typical scan electron microscopy (SEM) image of the microrods. The hexagonal Received: March 15, 2018

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DOI: 10.1021/acs.inorgchem.8b00690 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

inserted between the NaYF4 microrod (with radius r = 4 μm) and the Ag layer. Figure 2a plots the simulation results (i.e., using finite element method) of the Q-factor versus h of the proposed optical resonator for three emission wavelengths (i.e., 450, 540, and 654 nm). The results indicate that the optical resonator with high Q-factor requires no protective layer. This is because the microrod has a dimension of micrometers so that the profile of the plasmon modes can extend toward the microrod and avoid penetrating to the Ag layer. As a result, the protective layer has no contribution to the reduction of the absorption loss of the Ag layer.10 Figure 2b plots the spontaneous emission spectra of a NaYF4:Yb3+, Er3+, Tm3+ hexagonal microrod versus h. The samples were excited by a 980 nm continuous wave (CW) laser at room temperature. It is observed that the relative intensity of the emission peak at 450 nm increases from 3.95 to 65 (i.e., increases by 16 times), the emission peak at 540 nm increases from 3.6 to 48 (i.e., increases by 13 times), and the emission peak at 654 nm increases from 3.25 to 44 (i.e., increases by 13 times) for the case h = 0 nm. The increase is more than 10 times, and the blue emission peak intensity has the highest improvement. However, the enhancement reduces with the increase of h, and this is consistent with the simulation results. It is also noted that the enhancement is independent of the concentration and the type of the dopants (Figures S2 and S3). However, the influence of plasmonic effects can be amplified by the size of the microrods but not the excitation power (Figures S4 and S5). In the following experiment, h ≈ 5 nm was selected to protect the Ag layer from oxidization. In the experimental setup given in Figure 1c, the polarization of the 980 nm excitation laser can be controlled through the λ/ 2 waveplate and polarizer (i.e., on the right-hand-side of the objective lens). Figure 3a plots the measured emission spectra from the NaYF4:Yb3+, Er3+, Tm3+ hexagonal microrod deposited on an Ag-coated substrate under the excitation of 980 nm CW laser with two orthogonal polarizations, θin (i.e., θin = 90° represents that the polarization of the 980 nm CW laser is in the direction perpendicular to the length of the microrod). It is shown that the emission intensity can be increased by 30% for θin change from 0° to 90°. Similar optical characteristics are also observed from the hexagonal microrods with different dopants (Figure S6). Figure 2b shows the nearfield profile of the microrod excited by the 980 nm CW laser at the middle of the microrod with θin = 0° and 90°. It is observed that the emission intensity is enhanced for θin = 90°. In addition, the emission intensity is scattered from the edges of

Figure 1. (a) XRD pattern of the as-synthesized Yb3+−Er3+−Tm3+ tridoped β-NaYF4 hexagonal microrods. The inset shows the corresponding SEM images. (b) HR-TEM and FFT image of the βNaYF4 hexagonal microrod. (c) Experimental setup for the PL and lasing spectra measurement of a NaYF4 microrod.

microrods, which have radius varying from 0.5 to 4.5 μm, have six flat surfaces and two sharp ends (Figure S1). Figure 1b shows the high-resolution transmission electron microscopy (HR-TEM) image of a microrod. The lattice structure of the βNaYF4 crystal can be indexed as a (100) plane, which has a space group of P63/m. The corresponding FFT image suggests that there is a single crystalline nature of the microrods. Figure 1c plots the schematic of a homemade microscopy setup to excite and measure the upconversion emission characteristics of a single microrod at room temperature. A 980 nm excitation laser is focused onto a surface of the microrod through a 50× objective lens. Light emitting from the surface is collected through the same objective lens. The collected light is then coupled to either a spectrum analyzer or a CCD camera. To realize the plasmonic effect, we place a Yb3+−Er3+−Tm3+ tridoped β-NaYF4 hexagonal microrod on a SiO2 substrate coated with a 50 nm thick Ag layer (this thickness of Ag has the best surface smoothness). The inset of Figure 2a shows the schematic diagram of the optical resonator. In the diagram, a thin MgF2 protective layer of thickness h is assumed to be

Figure 2. (a) Plot of calculated Q-factor of a NaYF4:Yb3+, Er3+, Tm3+ hexagonal microrod deposited on an Ag-coated (50 nm) substrate, which is also protected by a MgF2 dielectric layer with different thickness. (b) Measured spontaneous emission spectra from a microrod deposited on an Agcoated substrate with a different thickness of MgF2 protection layer. B

DOI: 10.1021/acs.inorgchem.8b00690 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Plot of emission spectra of a NaYF4:Yb3+, Er3+, Tm3+ hexagonal microrod at different output polarization, θout. The microrod is deposited on an Ag-coated substrate and under 980 nm laser excitation with two orthogonal orientations (θin = 90° perpendicular and θin= 0° is parallel to the length of the microrod). (b) Photos of microrod under 980 nm laser excitation with two orthogonal orientations. (c) Twodimensional plot of emission intensity versus θin and emission wavelength.

Figure 4. (a) Light−light curves of the hexagonal microrods with and without deposition on the Ag-coated substrate for the three-color emission peaks (i.e., ∼450, ∼540 and ∼654 nm). (b) Corresponding emission spectra of the hexagonal microrods with and without deposition on the Agcoated substrate at the pumped power of 3.5 mJ/cm2.

and 2.3 to 0.89, 1.16, and 1.16 mJ/cm2). The emission spectra for the two laser configurations under the excitation power of 3.5 mJ/cm2 are also plotted in Figure 4b. It is observed that narrow peaks of line width less than 0.4 nm emerge from the emission spectra. Hence, it is demonstrated that the microrods can sustain lasing emission and that the presence of plasmonic effect reduces the lasing threshold by 50%. Previous investigation on the modal profile of hexagonal microrods under lasing action has suggested the support of whispering gallery modes.19 Resonant oscillation is obtained through total internal reflection at the center of the flat surface of the hexagonal structure, see Figure 5a. In fact, experimental measurement has shown the near-field image of the hexagonal microrod under lasing operation has shown an intense spot appears at the center of the flat surface, see Figure 5b. To understand the influence of plasmonics effect on the modal characteristics of the hexagonal microrods under lasing operation, we performed computer simulation on the modal characteristics of the hexagonal microrod with r = 4 μm deposited on an Ag-coated substrate, see Figure 5c. The resonant wavelengths are selected to be around 450, 540, and 654 nm in the calculation. As we can see, only TM modes

the microrod. Similar emission characteristics are also observed from the samples with excitation at the edge of the microrod (see Figure S7). This implies that the excitation laser light and upconversion PL light can travel along the length of the microrod with the help of plasmonic effect. This is because light propagation is not observed from the microrods deposited on a substrate without Ag coating. Hence, our theoretical studies and experimental observation have verified that plasmonic effect, which leads to the enhancement of spontaneous emission from the NaYF4:Yb3+, Er3+, Tm3+ hexagonal microrods, is due to the presence of Ag-coated substrate. Figure 4 shows the lasing characteristics of a microrod (r = 4 μm) deposited on a substrate with and without Ag-coating at room temperature. The microrod was excited at the middle location by a 980 nm pulsed (6 ns) laser with polarization θin = 90°. Figure 4a plots the corresponding light−light curves for three emission wavelengths measured at around 450, 540, and 654 nm. Kinks (i.e., excitation threshold) appear at the light− light curves, and the case with the Ag-coated substrate shows excitation threshold lower than that without Ag-coating by 50% (i.e., red, green, and blue threshold reduces from 2.0, 2.0, C

DOI: 10.1021/acs.inorgchem.8b00690 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

horizontally, were generated by an optical parametric oscillator. The sample was placed on an X-Y-Z translation stage of a light-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 the sample. 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. Computer Simulation. Two-dimensional finite element method (Comsol Multiphysics 5.0) is applied to determine the supported whispering-gallery modes (WGMs) of the lasing cavities. A regular hexagon of radius 4 μm is used to represent the cross-section of the NaYF4 hexagonal rod. In the case of cavities with the Ag-coated substrate, rectangles of length (x-dimension) 10 μm are used to represent the MgF2, Ag, and substrate (SiO2) layers. The rectangle widths (y-dimension) are the thicknesses of Ag and SiO2. All systems are surrounded by air with scattering boundary conditions. Using the eigenfrequency solver, the WGMs are solved in the form of complex eigenfrequency, f (= f r + if i), which results in a mode quality factor given by Q = f r/(2f i). The refractive index of NaYF4 (= 1.623) is assumed to be wavelength-independent, whereas those of MgF2, Ag, and SiO2 are, respectively, 1.3815, 0.04−2.6484i, and 1.4656 at 450 nm; 1.3787, 0.056895−3.5047i, and 1.4603 at 540 nm; and 1.3767, 0.051288−4.4404i, and 1.4564 at 654 nm.

Figure 5. (a) Numerical simulation results of resonant modes inside a hexagonal microrod with different wavelengths (450, 540, and 654 nm). (b) Near-field photo of a hexagonal microrod under 980 nm excitation. (c) Numerical simulation results of resonant modes inside a hexagonal microrod deposited on an Ag-coated substrate with different wavelengths (450, 540, and 654 nm). (d) Near-field photo of a hexagonal microrod deposited on an Ag-coated substrate under 980 nm excitation.

supported inside the microrods, and it seems that optical feedback is established between the two flat surfaces parallel to the Ag layer (i.e., it is more obvious for the green and red modes, see the white arrows). The near-field on the flat surface opposite to the Ag-coated substrate shows the two edges of the flat surface have intense spots. Figure 5d displays the near-field profile of the microrod deposited on Ag-coated substrate under lasing operation. It is clearly observed that two spots appear at the edge of the flat surface. Hence, it is believed that the plasmonic mode on the interface between the microrod and Ag layer coupled to the original whispering gallery modes form modified whispering gallery modes with lower cavity loss to support lasing emission.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00690. Fabrication method of β-NaYF4 microrods, physical properties of the β-NaYF4 microrods, investigation on the influence of dopant concentration and the type of dopants on the emission characteristics of the microrods, study of the influence of size of the microrods and the excitation intensity on the emission characteristics of the microrods, and study on the influence of the excitation polarization and excitation location on the emission characteristics of the microrods (PDF)

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CONCLUSION The possibility to enhance lasing emission from the Yb3+− Er3+−Tm3+ tridoped β-NaYF4 upconversion hexagonal microrods via the use of plasmonic effect is studied. This can be done by placing the microrod on an Ag-coated substrate. For the microrod excitations below the lasing threshold, we observe the enhancement of upconversion spontaneous emission through the presence of SPR by 10 times. For the microrods under lasing operation, we show that the presence of SPR reduces the lasing threshold by 50%. This is because of the formation of low-loss whispering gallery modes, which arise from the coupling between the whispering gallery modes of the bare microrod and the SPR modes propagating on the surface of the Ag layer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ting Wang: 0000-0002-7549-3888 Jing Hua Teng: 0000-0001-5331-3092 Siu Fung Yu: 0000-0003-0354-3767 Notes

METHODS

The authors declare no competing financial interest.

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Microrods Synthesis. We synthesized the microrods using the method described in ref 18. Additional experimental details are provided in the Supporting Information. Materials Characterization. XRD data of the NaYF4 power was measured on a D8 Focus diffractometer (Bruker) with Cu−Kα radiation (λ = 0.15405 nm) in the 2θ range from 15° to 60. SEM analysis was performed on a field emission scanning electron microscope. Transmission electron microscopy (TEM) analysis was performed on a JEOL-JEM 2100F transmission electron microscope operating at an acceleration voltage of 200 kV. Optical Measurement. Lasing characteristics of NaYF4 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. Laser beams (980 nm), which were polarized

ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (61775187), Science and Technology Projects of Shenzhen (JCYJ20170818105010341), Research Grants Council of Hong Kong (PolyU 153036/14P and 15301414), and University Research Grant (1-BBA5, GYBHG).

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DOI: 10.1021/acs.inorgchem.8b00690 Inorg. Chem. XXXX, XXX, XXX−XXX