Tm@NaYF4

Enhancing Multiphoton Upconversion from NaYF4:Yb/Tm@NaYF4 Core−Shell Nanoparticles via the Use of Laser Cavity Li Min Jin,† Xian Chen,‡ Chun Kit Siu,† Feng Wang,*,‡ and Siu Fung Yu*,† †

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China

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‡

S Supporting Information *

ABSTRACT: We discover that emission efficiency of Tm3+doped upconversion nanoparticles can be enhanced through the use of a laser cavity. With suitable control of the lasing conditions, the population of the intermediate excited states of the Tm3+ can be clamped at a required value above the excitation threshold. As a result, upconversion efficiency for the 300−620 nm emission band of the Tm3+-doped nanoparticles under 976 nm excitation can be enhanced by an order of magnitude over the case without a laser cavity. This is because the intrinsic recombination process of the intermediate excited states is suppressed and the surplus of excitation power directly contributes to the enhancement of multiphoton upconversion. Furthermore, our theoretical investigation has shown that the improvement of upconversion emission efficiency is mainly dependent on the cavity loss, so that this strategy can also be extended to other lanthanide-doped systems. KEYWORDS: lanthanide-doped upconversion nanoparticles, cylindrical microcavity, lasing emission, whispering gallery modes

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anthanide (Ln3+)-doped upconversion nanoparticles find enormous applications in data storage,1 multicolor displays,2 bioimaging,3−5 and photovoltaic devices6 due to their unique properties of upconversion luminescence.7 Unfortunately, low quantum yield is still a main obstacle of all the upconversion nanomaterials especially for the emission at short wavelength.8 A general guideline to realize effective multiphoton upconversion emission is minimizing depletion of the intermediate excited states that are critical for subsequent population of the higher-lying emitting states. Several complementary strategies, including surface functionalization, high-power excitation scheme, and host lattice manipulation, have been developed to alleviate depopulation of intermediate states associated with the nonradiative processes (e.g., surface quenching).9−13 However, there is no established approach to suppress undesired radiative recombination of the intermediate states, which represents another important depopulation channel.14 In this paper, we demonstrate the use of a laser cavity to enhance multiphoton upconversion emission of Tm3+-doped core−shell nanoparticles. Using this technique, the difficulty to suppress undesired radiative recombination processes of the ladder-like energy level structures of luminescent Tm3+ ions can be overcome. Hence, the surplus of excitation power can © 2016 American Chemical Society

directly contribute to the enhancement of multiphoton upconversion. It can be shown that the upconversion efficiency of Tm3+-doped core−shell nanoparticles over a spectral bandwidth between 300 and 620 nm (i.e., corresponding to three-, four-, and five-photon upconversion emission) can be increased by an order of magnitude. Our theoretical investigation has deduced the design consideration of the laser cavity to sustain high-efficiency multiphoton upconversion emission and verify that this proposed technique can also be extended to maximize the upconversion emission of other Ln3+doped systems in the short-wavelength regime.

RESULTS AND DISCUSSION Figure 1a and b describe the upconversion process in Yb3+/ Tm3+-codoped upconversion nanoparticles under low and high power excitation, respectively. Under low power excitation, high-lying excited states are difficult to fill as a result of radiative emission from the lower-lying intermediate states. In contrast, as the influence of concentration quenching of luminescence is minimized, the population of higher-lying intermediate states of Received: October 31, 2016 Accepted: December 29, 2016 Published: December 29, 2016 843

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Figure 1. Simplified energy level diagrams show the energy transfer from Yb3+ to Tm3+ in the NaYF4:Yb/Tm@NaYF4 core−shell nanoparticles under (a) low power, (b) high power, and (c) above threshold excitations, respectively. A laser light−light curve is also plotted on the righthand side of (c). Ip and Ith are the pump and threshold energy densities, respectively. n1, n2, n3, n4, n5, and n6 are the populations at the corresponding energy states of Tm3+. S1, S2, S3, and S4 are the radiative recombination emission. The light gray and dark gray stripes represent the population concentration of the energy states and pumping power density, respectively. It is noted that the arrows have different thickness as different amounts of photon flux are used to populate different intermediate excited states. Furthermore, high excitation power will lead to a high possibility of multiphoton upconversion emission, so that the corresponding arrows representing radiative recombination are thicker.

Tm3+ is achievable since the low-lying states are readily saturated under high power excitation.15,16 Unfortunately, this type of upconversion process is less effective, as substantially high pumping power is required. Figure 1c explains how multiphoton upconversion can be enhanced via the use of a laser cavity. It is noted that the injection carriers of an ideal laser will clamp at a threshold and the emission intensity will jump to a high value with the increase of injection current above a threshold. Hence, the onset of lasing emission clamps the population (i.e., at Ith), which is far from its saturation value, of the intermediate excited states at a pumping level equal to the laser pump threshold.17 This is because the intermediate states (e.g., 3F4 and 3H4) have to remain at an unsaturated population in order to avoid self-termination of lasing.18 Further increase of Ip will no longer contribute to the population at these intermediate states, and the surplus of carriers (i.e., Ip − Ith) will then populate the upper energy levels. As a result, population of higher-lying excited states can be established through the appropriate design of the laser cavity. It is expected that effective multiphoton upconversion can be realized at a largely reduced excitation power density. In the experiment, NaYF4:Yb/Tm@NaYF4 core−shell nanoparticles were chosen to demonstrate the enhancement of multiphoton upconversion via the use of a laser cavity. This is because (1) Tm3+ offers a long ladder-like energy level structure with a suitable energy gap between two adjacent levels to endure multiphoton upconversion, and (2) Yb3+ provides a stronger near-infrared absorption cross-section and favors effective energy transfer to the plentiful intermediate states of Tm3+. The nanoparticles were fabricated by a layer-by-layer epitaxial growth approach.19 In order to minimize the influence of concentration quenching of fluorescence under near-infrared excitation, emission spectra of the nanoparticles with different doping concentrations of Tm3+ ions were measured. Figure 2a illustrates that the emission intensity is maximized at a Tm3+ concentration of 1 mol % under near-infrared laser excitation at room temperature. Figure 2b gives the X-ray diffraction (XRD)

Figure 2. (a) Upconversion emission spectra of the NaYF4:Yb/ Tm@NaYF4 core−shell nanoparticles comprising different concentrations of Tm3+ under 976 nm excitation (continuous wave laser diode, ∼60 W cm−2) at room temperature. Doping concentration of Yb3+ is kept constant at 40 mol %. (b) XRD spectrum of the assynthesized NaYF4:Yb/Tm(40/1 mol %)@NaYF4 core−shell nanoparticles. The line spectrum of hexagonal phase NaYF4 crystal (JCPDS standard file number 16-0334) is also included as a reference. (c) TEM image of as-synthesized nanoparticles. (d) High-resolution TEM image reveals the single-crystalline nature of the nanoparticle with a d-spacing in the (100) planes of hexagonal NaYF4 (0.515 nm).

spectrum of the as-synthesized NaYF4:Yb/Tm(40/1 mol %)@ NaYF4 nanoparticles. Figure 2c displays the corresponding transmission electron microscope (TEM) image. It is noted that the as-synthesized nanoparticles have a highly uniform morphology with an elongated shape of average length and width of ∼46 and ∼25 nm, respectively. The high-resolution TEM image of a single nanoparticle in Figure 2d reveals lattice fringes of {100} with a d-spacing of 0.515 nm, which value matches with hexagonal NaYF4 (0.515 nm, JCPDS #16-0334). 844

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Figure 3. Spectroscopic analysis of multiphoton upconversion of the NaYF4:Yb/Tm(40/1 mol %)@NaYF4 core−shell nanoparticles dispersed in cyclohexane held inside a quartz cuvette under 5-pulse 976 nm excitation. (a) Upconversion emission spectra of the nanoparticles at varying power densities, Ip. (b) Plot of peak intensity ratios, S2/S1, S3/S2, and S4/S3 versus Ip. Inset shows a photo of the sample under excitation at room temperature. (c) Optical gain at peak wavelength of the four dominant peaks, S4, S3, S2, and S1, versus Ip. Differential gains, ∂G/∂Ip (i.e., slope of curves), of S1, S2, S3, and S4, are found to be 0.3435, 0.6497, 0.3998, and 0.6216 cm MW−1 respectively. (d) Relative peak intensity (in percentage) of S1, S2, S3, and S4 versus Ip.

Figure 4. Upconversion lasing characteristics of cylindrical microcavities with diameter d equal to 60 μm under 5-pulse 976 nm excitation. (a) Lasing spectra at different values of Ip. (b) Peak intensity ratios, S2/S1 and S4/S3, versus Ip. The optical image of cylindrical microcavity without (top) near-infrared (NIR) excitation illuminated under a daylight lamp and with (down) NIR pumping in the dark. (c) Light−light curves of the four dominant peaks. (d) Relative emission intensity (in percentage) of the four dominant lasing peaks versus Ip.

peak@345 nm).21 As expected at low Ip (i.e., 49 MW cm−2), the emission intensities of low-lying excited states are higher than that of higher-lying emitting states (i.e., S1 > S2 > S3 > S4). However, S2 and S4 are dominant over S1 and S3, respectively, at high Ip (i.e., ≥ 174 MW cm−2). As there is no absorption of lasing light between 300 and 500 nm detected, this transition can be interpreted as the increase in population density at 1D2 and 1I6 states. In this case, the lower-energy excited states are saturated so that the surplus of excitation power populates the higher-lying excited states. Similar PL characteristics are also observed from the sample under continuous-wave pumping. Figure 3b gives the dependence of peak intensity ratios S2/S1, S3/S2, and S4/S3 on Ip. It is observed that the minimum value of Ip to obtain S2/S1 and S4/S3 greater than 1 is about 61 and 115 MW cm−2, respectively. This is related to the variation of optical gain, G, of the four emission peaks with Ip as shown in Figure 3c.22 On the other hand, as the value of S3/S2 remains insensitive to Ip, both S2 and S3 saturate simultaneously with the increase of Ip. This is expected as G of S2 and S3 consumes the same pool of upconversion population (i.e., 1D2 excited state). However, their emission intensity is different due to the

These measurements confirm the single-crystalline hexagonal phase of the NaYF4:Yb/Tm(40/1 mol %)@NaYF4 core−shell nanoparticles. This optimized core−shell nanoparticle will be used throughout our following experiment unless otherwise specified. Figure 3 studies the room-temperature upconversion optical characteristics of the as-optimized core−shell nanoparticles in cyclohexane solution. The nanoparticles, which were dispersed in cyclohexane with an optimized concentration (i.e., with maximizing emission intensity) of 6.5 wt %, were held by a quartz cuvette for the photoluminescence (PL) experiment. A 5-pulse pumping scheme, which reduces the thermal effect as well as provides sufficient peak power to establish a population of the emitting states, was used to excite the sample.20 This excitation method was employed throughout our investigation unless otherwise specified. Figure 3a plots the upconversion emission spectra of the nanoparticles versus excitation energy density, Ip. Four dominant peaks, which match the transitions of Tm3+, are the low-energy pair 1G4 → 3H6 (S1, peak@475 nm) and 1D2 → 3F4 (S2, peak@451 nm), as well as the highenergy pair 1D2 → 3H6 (S3, peak@361 nm) and 1I6 → 3F4 (S4, 845

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Figure 5. Upconversion lasing characteristics of cylindrical microcavities with a diameter d equal to 970 μm under 5-pulse 976 nm excitation. (a) Lasing spectra at different Ip. (b) Peak intensity ratios, S2/S1 and S4/S3, versus Ip. (c) Light−light curves of the four dominant peaks. (d) Relative emission intensity (in percentage) of the four dominant lasing peaks versus Ip.

Figure 4b. It is noted that the dependence of S2/S1 and S4/S3 on Ip is similar to that given in Figure 3 but at a much lower value of Ip (i.e., reduces from ∼174 to ∼1.04 MW cm−2). This low value of Ip can be attributed to the formation of lasing emission that suppresses the undesired radiative recombination from the nanoparticles. Pump thresholds, Ith, of S1, S2, S3, and S4 are found to be about 0.86, 0.67, 0.39, and 0.30 MW cm−2, respectively; see Figure 4c. As explained in Figure 1c, populations of all the intermediate excited states are clamped at their threshold values due to the onset of lasing. The surplus of pump power, Ip − Ith, directly contributes to the establishment of the population of 1 G4, 1D2, and 1I6 excited states, so that the effective multiphoton upconversion is enhanced. Figure 4d displays the relative emission intensity (in percentage) of the four dominant lasing peaks versus Ip. Compared with that given in Figure 3d, a similar trend for the four lasing peaks is observed except the intersection values of Ip are much lower than that displayed in Figure 3d. Ratio of upconversion efficiency between the cylindrical microcavity and cuvette, ηmicrocavity/ηcuvette, for a 300−620 nm upconversion emission band under 976 nm laser excitation can be approximated by

difference in differential gain (i.e., stimulated emission crosssection or the slope of curves as shown in Figure 3c). As the optical gain of S1 saturates faster than that of the other peaks, saturation of the 3F4 and 3H4 states is expected; see Figure 1b. In this case, the surplus of excitation energy facilitates establishing the population of 1G4, 1D2, and 1I6 excited states. Figure 3d plots the relative emission intensity (in percentage), Rk, of the four dominant peaks (S1, S2, S3, and 4 S4) versus Ip, where R k = Sk /∑i = 1 Si × 100% for k = 1, 2, 3, and 4. It is observed that the intensity of S1 and S3 gradually decreases, while that of S2 and S4 increases with the increase of Ip. The intersection of both low- and high-energy pairs is consistent with that shown in Figure 3b and c. All four emission peaks appear to be a tendency to plateau at a high Ip, and this characteristic is consistent with the saturation phenomena of optical gain. Upconversion efficiency, η, can be determined by η = ∫ Lemission/(∫ Esolvent − ∫ Esample), where Lemission is the luminescence emission intensity of the sample and Esample and Esolvent are the intensity of the pumping light in the presence of the sample and solvent, respectively.23 It must be noted that the integration sphere was not employed in the measurement of Lemission, Esample, and Esolvent due to the use of a 5-pulse pumping scheme. It is found that η for the emission band between 300 and 620 nm is about 0.17% for the sample excited at 174 MW cm−2. This measured value of η is consistent with the recent report on the upconversion emission characteristics of Ln3+doped core−shell nanoparticles.24,25 The core−shell nanoparticles can be used as the gain medium to achieve upconversion lasing under near-infrared excitation at room temperature. Cylindrical microcavities, which support resonance of whispering gallery modes (WGMs), were fabricated by directly drawing a mixture of poly(methyl methacrylate) (PMMA), the core−shell nanoparticles (6.5 wt %), and epoxy resins.26 As the microcavities support coherent optical feedback over a wide range of wavelengths,27 lasing emission of the four dominant peaks can be excited simultaneously. Figure 4 depicts upconversion lasing characteristics of a cylindrical microcavity with a diameter, d, equal to 60 μm. The microcavity, which was incorporated with as-optimized upconversion nanoparticles, was pumped under 5-pulse 976 nm excitation at room temperature. Figure 4a shows the upconversion lasing spectra of a cylindrical microcavity versus Ip. The corresponding values of S2/S1 and S4/S3 are also shown in

ηmicrocavity ηcuvette

≈ 9.9112 ×

Ip(cuvette) ∫ Smicrocavity(λ) dλ × Ip(microcavity) ∫ Scuvette(λ) dλ (1)

where Scuvette (Smicrocavity) is the upconversion emission intensity recorded from the sample dispersed in the cuvette (microcavity) and Ip(cuvette) and Ip(microcavity) are the excitation energy density for the cases without and with laser cavity, respectively. This equation has taken into consideration the influence of size and geometry for both the cuvette and cylindrial microcavity as well as the parameters used for the experimental setup. From Figures 3a and 4a, it can be shown that Ip(cuvette)/Ip(microcavity) ≃ 174/1.04 ≅ 167 and ∫ Smicrocavity(λ) dλ/∫ Scuvette(λ) dλ ≈ 5.8 × 10−3; hence ηmicrocavity/ηcuvette ≈ 9.6. This implies that the upconversion efficiency of a cylindrical microcavity with d = 60 μm (i.e., with a peak ratio S2/S1 ≈ 2.4 and S4/S3 ≈ 1.2 at Ip ≈ 1.04 MW cm−2) is about an order of magnitude higher than that without a laser cavity (i.e., with peak ratio S2/S1 ≈ 2.3 and S4/S3 ≈ 1.4 at Ip ≈ 174 MW cm−2; see Figure 3a). The significant reduction of Ip due to the onset of upconversion lasing emission is the main reason for the huge enhancement of 846

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Figure 6. Lasing characterization. (a) Peak intensity ratios, S2/S1 and S4/S3, versus Ip for some values of d. (b) Plot of mode spacing, Δλ, and excitation threshold, Ith, versus 1/d for the four dominant peaks.

Figure 7. Theoretical analysis of a five-level laser system. (a) Photon densities and (b) power ratio, S2/S1, versus cavity loss, αcav, for Ip = 0.5 and 1 × 1029 cm−3 s−1. S2/S1 = 1 occurs when αcav ≈ 85 cm−1, and it is independent of the value of Ip. (c) Ratios of population, n3/n4 and n4/n5, versus αcav. Inset plots the variation of n2, n3, n4, and, n5 versus cavity loss, αcav. Please note that the population is independent of Ip.

emission peaks. As expected, Δλ (i.e., Δλ = λo2/πdneff, where λo is the lasing wavelength and neff ≈ 1.58 is the effective refractive index of the fiber) is inversely proportional to d. Besides, it is observed that Ith of the four dominant peaks is different especially for S3 and S4. This is because cavity loss, stimulated emission cross-section, and carrier lifetime of each lasing peak are different.28,29 As Ith is directly proportional to 1/d, it is believed that cavity loss, αcav, is also proportional to 1/d. If d ≤ 140 μm is the necessary condition to sustain the suppression, this implies that αcav of all the lasing peaks should be greater than 74 cm−1. We also fabricated a random laser, which has resonant conditions different from that of the cylindrical microcavity, by using the as-optimized nanoparticles as the gain media and scatterers.30,31 The measurement has shown that αcav of S1 is ∼34 cm−1, which is lower than that of the microcavity with d = 970 μm (i.e., S1 has αcav equal to ∼50 cm−1). This explains the reason for S2/S1 < 1 and S4/S3 < 1 observed from the random laser. Hence, a microcavity with a relatively high cavity loss is preferred for the suppression of S1 and S3, as the value of the population clamped at the intermediate excited states should not be too small.

ηmicrocavity over ηcuvette. As ηcuvette was found to be ∼0.17% at Ip ≈ 174 MW cm−2, the value of ηmicrocavity is larger than 1.7% at Ip ≈ 1.04 MW cm−2 due to the presence of a laser cavity. For a microcavity with d equal to 970 μm, upconversion lasing spectra, peak ratios (i.e., S2/S1 and S4/S3), light−light curves, and relative emission intensity (in percentage) for the four lasing peaks (i.e., S1, S2, S3, and S4) versus Ip are shown in Figure 5. Ith of S1, S2, S3, and S4 are recorded to be around 0.62, 0.46, 0.17, and 0.14 MW cm−2, respectively. It should be pointed out that S1 and S3 are the dominant lasing peaks of the low- and high-energy pairs, respectively, for all the values of Ip. Hence, the suppression of undesired radiative recombination failed for this value of d. This indicates that the design of microcavities plays an important role in obtaining S2/S1 > 1 and S4/S3 > 1. Figure 6a shows the peak intensity ratios S2/S1 and S4/S3 versus Ip for microcavities with a range of d. S2/S1 > 1 and S4/S3 > 1 appear simultaneously for Ip ≥ 0.37 and ≥ 0.76 MW cm−2, respectively, under the condition of d ≤ 140 μm. This implies that the suppression of undesired radiative recombination is dependent on the size of the microcavities. Figure 6b plots mode spacing, Δλ, and Ith versus 1/d for the four dominant 847

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ACS Nano From Figure 6b (bottom), as αcav of S3 and S4 is at least 2 times larger than that of S1 and S2, we can ignore the influence of S3 and S4 in the theoretical study of S1 and S2 under the condition that Ip < Ith of S3 and S4. Hence, a simplified five-level model, in which only S1 and S2 are taken into consideration, is developed to understand the multiphoton upconversion mechanism in cylindrical microcavities.32−36 Figure 7a plots S1 and S2 versus cavity loss, αcav, for Ip (= 0.5 and 1 × 1029 cm−3 s−1) larger than Ith of both S1 and S2 over a range of αcav. The values of αcav for both S1 and S2 are assumed to be the same as indicated in Figure 6b. The corresponding power ratio, S2/S1, versus αcav for the two values of Ip is also plotted in Figure 7b. It is observed that S2 and S1 are an increasing function of Ip; however, the value of S2/S1 is mainly determined by αcav (i.e., defined by the resonance conditions). The independence of S2/ S1 on Ip implies that the population of intermediate excited states clamps at its threshold value for Ip > Ith (see also Figure 1c). These calculation results explain why S2/S1 > 1 and S4/S3 > 1 in Figures 3a and 4a. This is because both the quartz cuvette (i.e., can be considered as a laser cavity with very low optical feedback) and microcavities with small d have a large value of αcav for all the emission peaks so that S2/S1 > 1 is expected. On the other hand, S2/S1 < 1 is observed from Figure 5a because microcavities with large d have a small value of αcav for both S1 and S2. The inset of Figure 7c shows the variation of population n2, n3, n4, and n5 at the 3F4, 3H4, 1G4, and 1D2 excited states, respectively, versus αcav. Note that the values of n2, n3, n4, and n5 are independent of Ip, as they are clamped at their threshold values for Ip > Ith. Furthermore, the values of n4 and n5 increase with the increase of αcav. This is because microcavities with large αcav require an increase in optical gain (i.e., an increase of population at the 1G4 and 1D2 excited states) to sustain lasing emission. Switching of S2/S1 from less than 1 to larger than 1 can be understood from the steady-state rate equations of n4 and n5. By approximating that S2/S1 is independent of Ip, we can write −1 σ σ a ⎡ n ⎤⎡ n ⎤ S2 ≈ 45 41 41 ⎢ 4 ⎥⎢ 3 ⎥ + constant σ34 σ52a52 ⎣ n5 ⎦⎣ n4 ⎦ S1

facilitated. It is verified that the upconversion efficiency (i.e., for 300−620 nm emission band refers to three-, four-, and fivephoton upconversion emission) of Tm3+-doped nanoparticles can be improved by an order of magnitude provided that a laser cavity is established to support lasing emission. This improvement can be explained as the clamping of population at a required threshold value, which allows the suppression of undesired radiative recombination of the intermediate states. In addition, there is a minimum value of αcav (≥74 cm−1) to sustain the suppression. Hence, our understanding of upconversion lasing emission induced enhancement of multiphoton upconversion emission can extend to the development of other Ln3+-doped upconversion nanoparticles with high quantum yield, especially in the short-wavelength regime.

METHODS Nanoparticle Synthesis. The core−shell upconversion nanoparticles were synthesized by using the method described in ref 19. Additional experimental details are given in the Supporting Information. Five-Pulse Excitation Scheme. A 5-pulse excitation technique was developed by using the method described in ref 20. Detailed explanations of the measurement setup are given in the Supporting Information. Optical Gain Measurement. Optical gain of the core−shell upconvesion nanoparticles was measured by using the variable stripe length method as described in ref 16. The longer side of a UV quartz cuvette, which was filled with the upconversion nanoparticles, was pumped by an excitation stripe with width and length of ∼100 μm and L, respectively. Light intensity emitted from the shorter side of the cuvette, Itot(λ), was measured by a monochromator setup. The optical gain, g(λ), at a particular wavelength, λ, was deduced by fitting Itot(L, λ) = Isp(λ) [exp(g(λ)L) − 1]/g(λ) with the measured values of Itot(λ), where Isp(λ) is the spontaneous emission intensity. Fabrication of the Cylindrical Microcavities. An optical fiber embedded in the upconversion nanoparticles was fabricated by using the technique described in ref 26. Additional experimental details can be found in the Supporting Information.

ASSOCIATED CONTENT S Supporting Information *

(2)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07322. Detailed information for the synthesis of nanoparticles, excitation setup for 5-pulse excitation scheme, optical gain measurement, fabrication of cylindrical microcavities, measurement of random laser action, optical characteristics of the nanoparticles, derivation of upconversion efficiency of the nanoparticles with and without a laser cavitiy, relationship between exctiation threshold and cavity loss of lasers, and five-level rate equation model of Tm3+ (PDF)

where the definition of the material parameters, which are all independent of αcav, can be found in the Supporting Information. Figure 7c shows that the value of n3/n4 decreases with the increase of αcav, but that of n4/n5 remains constant. Hence, the reason for S2/S1 > 1 can be related to the reduction of n3/n4 for a large value of αcav. This is equivalent to saying that the increase (decrease) in population at the 1G4 and 1D2 (3H4 and 3F4) excited states by the increase in αcav contributes to the switching of S2/S1 from less than 1 to larger than 1.

CONCLUSIONS In conclusion, we demonstrate a significant enhancement in upconversion emission efficiency of Tm3+-doped nanoparticles via the use of a laser cavity. As a proof-of-principle study, NaYF4:Yb/Tm(40/1 mol %)@NaYF4 core−shell nanoparticles were utilized as the gain medium. Tm3+ was selected as the activator because Tm3+ has (1) a long ladder-like configuration of energy level structures with roughly equal energy spacing to obtain successive absorption of photons and (2) a suitable carrier lifetime at the intermediate energy states to support multiphoton upconversion emission so that five-photon upconversion emission under near-infrared excitation can be

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Feng Wang: 0000-0001-9471-4386 Siu Fung Yu: 0000-0003-0354-3767 Notes

The authors declare no competing financial interest. 848

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DOI: 10.1021/acsnano.6b07322 ACS Nano 2017, 11, 843−849