1. Introduction

Upconversion photoluminescence (UCPL) is a non-linear process producing a photon of higher energy from low-energy photons [1]. The research of rare-earth-based UCPL has attracted much attention because of a broad application in light-emitting diode (LED) technology, anti-counterfeiting, three dimensional display, solar cells, and biological imaging probes [2–9]. Fluoride crystals codoped with Er³⁺ and Yb³⁺ have been studied most extensively, because the efficient energy transfer from Yb³⁺ to Er³⁺ and the low phonon energy of fluoride matrix give a high internal quantum yield in converting infrared light into visible light [10]. However, even after the elaborate choice of rare earth ions and matrix crystals, the external quantum yield of the fluorides codoped with Er³⁺ and Yb³⁺ is intrinsically low, because of the forbidden nature of the 4f- 4f transition involved in the UCPL process.

Metallic nanostructures have been applied to enhance the intensity of UCPL by exploiting localized surface plasmon resonance (LSPR) [11–14]. Upon light illumination, LSPRs on metallic nanostructures significantly enhances the local electromagnetic field around them. This alters the local density of optical states around the rare-earth ions, which accelerates the rates of excitation and/or radiation transitions, resulting in enhanced UCPL [15–18]. Metallic nanostructures including nanoparticles (NPs), nanorods, nanopores, and their assemblies or arrays are employed to show enhanced UCPL. Although the enhancement of UCPL more than 400 times has been reported from the rare-earth-doped NPs embedded in metallic nanostructures [19], they are local enhancement because the LSPRs are the local effect bound to the metallic surface. Therefore, it is difficult to enhance UCPL from a large volume of upconverters, such as films of several micrometers.

Diffractive arrays of metallic NPs support hybrid plasmonic-photonic modes, where optical diffraction in the plane of the array mediates radiative coupling between the LSPRs in neighboring NPs [20–31]. Hybrid plasmonic-photonic modes have proven useful for surface-enhanced Raman scattering (SERS) [32], intensified fluorescence [26,33,34], sensing [35], and solar cells with improved efficiency [36] due to their characteristic electric field distributions. While LSPR is a localized mode, hybrid plasmonic-photonic modes contain electric fields extending to neighboring NPs in the array due to the associated strong radiative coupling. Although some previous works exploit periodic structures to couple collective resonances to UCPL [11,37], there are few reports on plasmonic-photonic hybrid mode to enhance UCPL [38].

In the present work, we demonstrate an enhanced UCPL from an optically thick layer (∼ 5.7 µm) by utilizing a hybrid plasmonic-photonic mode. The sample comprises the Al nanocylinders arranged in square lattice with a period that is tuned to support the hybrid mode at the absorption wavelength of CaF₂ codoped with Er³⁺ and Yb³⁺. We select Al because of the low-cost, abundance in earth’s crust, and compatibility with the nanofabrication processes [39]. We design the size of nanocylinder and the period of the lattice so that the LSPRs and in-plane light diffraction spectrally overlap around the wavelength of 980 nm, which corresponds to ²F_7/2 → ²F_5/2 transition of Yb³⁺. A thick layer of upconverter, a dense and transparent layer of CaF₂:Yb³⁺,Er³⁺ NPs, is deposited on the array. We observe UCPL enhanced by up to 9 times. UCPL intensity varies with the excitation angle of the system, showing the maximum at the angle where the incident light excites the hybrid mode. The result confirms that the enlarged absorption causes the enhanced UCPL.

2. Experimental section

2.1 Synthesis of CaF₂:Yb³⁺,Er³⁺ nanoparticles

CaF₂:Yb³⁺,Er³⁺ (Ca/Yb/Er = 94/5/1 mol%) NPs were prepared via hydrothermal method. Appropriate amounts of cation sources (CaCO₃, Er₂O₃, Yb₂O₃) were dissolved in concentrated nitric acid, and the solution of the corresponding nitrate (Ca(NO₃)₂, Yb(NO₃)₃, Er(NO₃)₃) was obtained by removing the excess of nitric acid by evaporation under heat. They were dissolved in appropriate amounts of deionized water under sonification. A homogeneous mixture of 2.6 mmol ammonium fluoride, 19 ml lactic acid and 1 ml deionized water was prepared by stirring for 1 h. The solution of nitrates was added to the mixture and stirred for several minutes, then the mixture was transferred to a 100 ml reactor and sealed, then heated at 230 °C for 5.5 h. After the reaction, the reactor was cooled to room temperature. The suspension was centrifuged at 15000 rpm for 30 min. After the precipitates were washed with ethanol and acetone twice, they were dispersed in 10 ml lactic acid. The crystalline phases of the precipitates were characterized via X-ray diffraction (XRD) using Cu Kα radiation (SmartLab Rigaku Corp., Japan). The samples were prepared by placing a drop of a dilute suspension of NPs onto SiO₂ glass and drying at 250 °C for 2.5 h. The morphology of the synthesized NPs was examined using scanning electron microscopy (SEM, SU8000, Hitachi).

2.2 Fabrication of Al nanocylinder arrays

Al nanocylinder array arranged in a square lattice with a period of 820 nm was fabricated using nanoimprint lithography in combination with reactive ion etching (RIE) [40]. The height of the nanocylinders was 180 nm, and their diameter was 440 nm. The fabrication procedure is as follows. First, a resist layer was deposited on a thin film of polycrystalline Al. Then, the surface of the resist was nanostructured by nanoimprint techniques, replicating the surface morphology of an Si mold. The sample was then structured by RIE. The size, periodicity and height of the Al nanocylinders were determined via SEM.

2.3 Deposition of CaF₂:Yb³⁺,Er³⁺ NPs layer on the array

For the preparation of a UCPL layer, a 1 µl lactic acid solution containing the CaF₂:Yb³⁺,Er³⁺ NPs was dropped on arrays and glass substrate and then they were heated at 250 °C for 2 h, so as to evaporate the lactic acid. The refractive index of the CaF₂:Yb³⁺,Er³⁺ NPs layer was examined using a spectroscopic ellipsometry setup (FE-5000, Otsuka Electronics, Japan). The thickness of the CaF₂:Yb³⁺,Er³⁺ NPs layer was determined by a Dektak 150 surface profilometer (Veeco Instruments, Inc., USA).

2.4 Optical measurement

Zeroth-order optical transmission spectra (p-polarized component) were measured as a function of the angle of incidence θ_in. For the measurement, the sample was placed on a rotation stage and white light from a halogen lamp (HL-2000, Ocean Optics, USA) was incident from the backside (substrate side). The transmitted light was collected by a detector (visible: FLAME-S, near infrared: NIRQUEST, Ocean Optics, USA) through an optical fiber. The zeroth-order optical transmission spectra were obtained by normalizing the transmission of the incident light through the sample to that through the glass substrate. The optical transmittance at θ_in = 0° in the range λ = 200-2000 nm (λ is the wavelength) was also recorded using a ultraviolet–visible–near infrared (UV-Vis-NIR) spectrophotometer (V-770, JASCO, Japan).

The UCPL properties of the samples were examined using a variable-power 980 nm diode laser (L980P300J, THORLABS, NJ, USA) as the excitation source. The sample was irradiated with a laser (spot size = 0.15 mm) from the side of CaF₂:Yb³⁺, Er³⁺ NPs layer and the UCPL was detected in reflection configuration by a multichannel photodetector (MCPD-7700:311C, Otsuka Electronics Co., Ltd., Osaka, Japan).

2.5 Simulation

The optical characteristics of the arrays were simulated using the finite-element method (COMSOL Multiphysics). Three-dimensional models were used under periodic boundary conditions on the lateral coordinates to realize the square lattice with a periodicity of 820 nm. The simulated structures consisted of a SiO₂ glass substrate and Al nanocylinders with a CaF₂:Yb³⁺,Er³⁺ NPs layer on the top of them. The thickness of the layer was set to 5700 nm. The Al nanocylinders were modeled as the tapered cylinder with the height of 180 nm and top and bottom diameters of 360 and 440 nm, respectively. Refractive indices (n) and extinction coefficients (k) were deduced from the fits to the spectroscopic ellipsometry data for Al, and the n of the SiO₂ glass substrate was set to 1.46. For the CaF₂:Yb³⁺,Er³⁺ layer, we used the ellipsometry-derived n while k being set to 0. This choice of k is justified because the CaF₂:Yb³⁺,Er³⁺ is transparent in the spectral range of interest and the k value obtained in ellipsometry largely comes from light scattering. A plane wave with an electric field oscillating in the x-direction (p-polarization) was incident from the top boundary at a defined θ_in to investigate the optical response of the model.

3. Results and discussion

Figure 1(a) shows the X-ray diffraction pattern of the CaF₂:Yb³⁺,Er³⁺ NPs, confirming that they are single phase of cubic CaF₂. Figures 1(b) and 1(c) show the UCPL spectrum under λ = 980 nm laser excitation for the suspension and the energy diagram of Yb³⁺ - Er³⁺ system, respectively. In the present system, Yb³⁺ acts as a sensitizer that absorbs the 980 nm near infrared excitation light via ²F_7/2 → ²F_5/2 transition. Subsequent multiple and consecutive energy transfers from Yb³⁺ to the activator Er³⁺ are responsible for the UC emission. Under λ = 980 nm laser excitation, CaF₂:Yb³⁺,Er³⁺ NPs exhibit bright UCPL, including green (λ = 520 and 540 nm) and red (λ = 658 nm) bands originating from (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively [41,42].

 

Fig. 1. (a) Powder X-ray diffraction pattern of CaF₂:Yb³⁺,Er³⁺ NPs. (b) The UCPL spectrum of the NPs suspended in lactic acid. (c) Energy level diagram for the Er³⁺-Yb³⁺ system.

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Top-view SEM images of the fabricated Al nanocylinder array are shown in Figs. 2(a) and 2(b). The array consists of well-defined nanocylinders with diameter of 440 nm and height of 180 nm arranged in a square lattice with a period of 820 nm over the large area of 6 mm × 6 mm. The period of the lattice is designed to tune the in-plane diffraction at λ = 980 nm. The height (180 nm) and the diameter (440 nm) of nanocylinder are designed to support the LSPR around the near infrared region (see Appendix Fig. 6 for the simulated extinction spectrum for a single Al nanocylinder). Figure 2(c) shows the SEM image of CaF₂:Yb³⁺, Er³⁺ NPs layer on the Al array. The CaF₂:Yb³⁺, Er³⁺ NPs have a diameter ranging from 30 to 70 nm and show faceted faces. This faceted surfaces, together with the sharp peaks in the XRD pattern (Fig. 1(a)), confirm the high crystallinity of the NPs. The thickness of the layer is 5.7 µm. Thanks to the dense packing of NPs, the layer is optically transparent as will be confirmed by the optical transmission measurement. The refractive index of the layer measured by the spectroscopic ellipsometry is ∼1.4 at λ = 800 nm, which is close to the value of bulk CaF₂ crystal ( = 1.43) [43].

 

Fig. 2. Top-view SEM images of arrays at (a) low magnification and (b) high magnification. The coordinate axes used in this study are also denoted. (c) SEM image of CaF₂:Yb³⁺, Er³⁺ NPs on the array.

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Figure 3(a) compares the transmittance at θ_in= 0° for the Al nanocylinder array with and without the CaF₂:Yb³⁺, Er³⁺ NPs layer on the top, and the same layer on the glass substrate. LSPRs of Al nanocylinder give a broad spectral dip ranging from the visible to near infrared region. When arranged in the periodic lattice, the LSPRs are modulated by the in-plane diffractions. Prominent modulations appear at λ = 820 and 1200 nm, which are degenerated (±1, 0) and (0, ±1) diffraction orders on the air and substrate sides, respectively, as explained below. After the deposition of the NPs layer, two main dips slightly redshift with a clear sign of light extinction at λ < 500 nm. The NPs layer is transparent at λ > 500 nm and dips due to Yb³⁺ absorption are not obvious, reflecting a weak oscillator strength of f-f transition. The assignment of the dips in the near infrared is better visualized in Fig. 3(b), which shows the p-polarization incident angle-dependent (θ_in= 0-60°) transmittance spectra from λ = 350 to 1600 nm of the CaF₂:Yb³⁺,Er³⁺ NPs layer on the Al array. In-plane diffraction condition, known as Rayleigh anomaly, follows the relation [34]: k₀² = [k_|| + m₁(2π/a)]² + m₂²(2π/a)², where k₀ ( = 2πn/λ) and k_|| = (2π/λ)sinθ_in are the wave vectors of the scattered and incident light, a is the periodicity of the lattice, n is the refractive index of the surrounding medium, and (m₁, m₂) are the diffraction orders in the x- and y-directions, respectively.

 

Fig. 3. (a) UV-Vis-NIR optical transmittance at θ_in= 0° for the Al array (top panel) and the CaF₂: Yb³⁺, Er³⁺ NPs layer on the Al array (bottom). The transmittances of the flat glass substrate and the CaF₂ NPs layer on the flat glass substrate are shown as dotted line. (b) Experimental angle-dependent (θ_in= 0-60°) optical transmittance spectra ranging from λ = 350 to 1600 nm for the CaF₂: Yb³⁺, Er³⁺ NPs layer on the Al array with p-polarization incident wave. The plane of incidence is in the zx plane. (c) Angle-dependent UCPL spectra of CaF₂:Yb³⁺,Er³⁺ NPs layer on the array (denoted by solid lines) and that on the glass substrate (solid areas) in the range of 0 to 60° with an angle interval of 10°. Inset is a sketch of PL measurement. Positions of the laser head and the detector are fixed while the samples are rotated to vary the angle of incidence. (d) Photographs of the of CaF₂: Yb³⁺, Er³⁺ NPs layer on the glass substrate (No array) and on the array (With array), under white light (left), and upon irradiation with a λ = 980 nm laser in dark (right).

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The spectral positions of the dips follow the Rayleigh anomaly, manifesting the coupling between the LSPR and in-plane diffraction to excite plasmonic-photonic hybrid modes [34]. In addition, several fringe patterns arise between the two (-1, 0) diffraction orders on the air and the substrate sides due to Fresnel reflections at the top and bottom surface of the 5.7 µm layer of CaF₂:Yb³⁺, Er³⁺ NPs.

In order to verify the UCPL enhanced by the plasmonic-photonic hybrid mode in Al arrays, we measured the UCPL spectra as a function of incident angle of the λ = 980 nm laser (θ_in=0-60°, with a 10° interval). The UCPL intensity varies with θ_in as illustrated in Fig. 3(c). The setup of the PL measurement is shown in the inset. Figure 3(d) depicts the photographs of the CaF₂:Yb³⁺,Er³⁺ NPs layers on the array and the SiO₂ glass substrate, respectively, under while light illumination (left panel) and λ = 980 nm excitation at θ_in = 10° (right). The layer of CaF₂:Yb³⁺, Er³⁺ NPs is transparent, as is shown in the optical transmission spectrum in Fig. 3(a). The UCPL is significantly enhanced by the array, recognizable by the eyes. The mixture of green and red makes a yellowish color at the excitation spot. Also the green and red components diffracted into different directions are noticeable. In the measurement, we collect UCPL from the excitation beam spot and the diffracted components are not examined.

Figure 4(a) compares UCPL enhancement factor and optical extinction (1-T/T₀, where T and T₀ refer to the transmission through the sample and the glass substrate) at λ = 980 nm as a function of θ_in. UCPL enhancement factor is defined as the UCPL intensity integrated over the green or red branches from the NPs layer on the array divided by that from the layer on the glass substrate. We find that the enhancement of the UCPL signal is strongly related to θ_in and reaches the maximum when θ_in= 10°, at which the integrated UCPL enhancement factor is about 9. When θ_in further increases, the UCPL decreases, hits a minimum at θ_in= 40°, and increases again. This behavior is similar to angle dependence of extinction at λ = 980 nm, demonstrating that UCPL enhancement mainly originates from the increased absorption of excitation laser by the layer of CaF₂: Yb³⁺, Er³⁺ NPs via ²F_7/2 → ²F_5/2 transition of Yb³⁺. It is noted at θ_in= 10°, the (-1, 0) diffraction occurs at λ = 980 nm, meaning the excitation laser resonantly couples to the hybrid mode at θ_in= 10° and resulting in the highest UCPL enhancement. We also show the results for the arrays with larger periods (900 and 980 nm) to see the effect of in-plane diffraction (see Appendix Fig. 7). The UCPLs for the layers on these arrays are less amplified because of the mismatch of the period with the 980 nm light.

 

Fig. 4. (a) Incident angle-dependent UCPL enhancement factor (symbols, left axis) and the optical extinction, 1-T/T₀, where T and T₀ refer to the transmission through the sample and the glass substrate, at λ = 980 nm (dotted line, right) in the range θ_in = 0-60°. Excitation intensity dependence of UCPL intensity for the (b) green and (c) red branches both for the array and the reference. (d) UCPL enhancement factors for the red (658 nm) and green (540 nm) branches as a function of excitation intensity, respectively.

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We estimate the number of photons involved in the UCPL process by plotting the integrated UCPL intensity at λ = 540 and 658 nm as a function of laser pump measured for the array sample and the reference (Figs. 4(b) and 4(c)). The UCPL intensity increases in accordance with the n_p-th power of the pumping intensity P, as I_UCPL ∝ P^np, where I_UCPL is the emission intensity and n_p is the number of photons required. At lower excitation intensities, the slope is close to three, while at higher intensities the slope is two, with the transition at 314 W/cm² for the green branch. The transition means that the UCPL is from three to two photon process. A further increase in the excitation intensity causes the transition to linear (n_P = 1) region, which is presumably outside the range of our data. The transition to the linear range is reported to occur at much lower intensities in previous reports [44]. This is due to the very thick UC layer we use; because our layer is as thick as 5.7 µm and consists of the densely-packed CaF₂ NPs without the dispersion polymer, the incident laser excites not only the NPs on the focus spot but also a number of NPs in the volume under the spot. This effectively reduces the excitation intensity. It is found that for both the λ = 540 and 658 nm branches, the slope is slightly less for the array sample than that for the reference, which indicates that the array increases the local excitation intensity to modify the UCPL processes. Preceding works suggest that the local field enhancement associated with the surface plasmons saturates several excited states of Er³⁺ and can reduce the slope [44–46]. The slope reduction observed in the present study can be explained by the same scenario. Figure 4(d) shows the enhancement factor for the λ = 540 and 658 nm branches as a function of the excitation intensity. A larger enhancement occurs for the 658 nm branch. This is in agreement with the preceding work [44] and suggests more photons are involved for the 658 nm branch compared to the 540 nm branch.

It is noted that not only the excitation light but also the UCPL can couple to the optical modes excited in the system because the transmission spectra (Fig. 3) indicate the excitation of the modes at the wavelengths of emission. These modes may change the decay rate via the modification of the local density of optical state and contribute to the UCPL enhancement. [38].

We simulate transmittance and the integrated light energy inside the CaF₂: Yb³⁺, Er³⁺ NPs layer at λ = 980 nm as a function of θ_in, as presented in Fig. 5(a). Assuming that the UCPL is a two-photon process, the square value of light energy, i.e., the fourth power of the electric field intensity, is proportional to the UCPL intensity. The dips in transmittance correspond to the local maxima of the light energy, at which the incident light energy is coupled into the CaF₂: Yb³⁺, Er³⁺ NPs layer and consequently is converted to UCPL. Figure 5(b) shows simulated spatial distribution of the light energy squared (λ = 980 nm) at θ_in= 0, 10, 40, and 60°, respectively. The contrast shows that the light energy is stored inside the CaF₂ layer in addition to the proximity to the Al nanocylinders. Figure 5(c) shows the experimental UCPL enhancement factor and the simulated integral light energy squared. The agreement of angular profiles between the UCPL enhancement and λ = 980 nm light energy squared confirms that the enhancement occurs via efficient coupling of excitation light into the layer.

 

Fig. 5. (a) Simulated transmittance (black line, left axis) and the fourth power of the electric field normalized to that of the reference, |E^film|⁴/|E^film_ref|⁴, integrated over the layer (blue area, right) as a function of θ_in for p-polarized λ = 980 nm light. (b) The spatial distribution of light energy squared for p-polarization incident light (λ = 980 nm) at θ_in= 0, 10, 40, and 60°, respectively. In the simulation, the thickness of the NPs layer is set to 5.7 µm, but the plots only show the layer up to 2 µm above the array for the clear visualization of the field distribution around the nanocylinders. (c) Experimental UCPL enhancement factor (black, left axis) and simulated integral light energy squared (|E^film|⁴/|E^film_ref|⁴) (blue, right). The dotted lines are the guides for the eyes.

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Finally, we examine the effect of the surface oxidation on the optical properties of the array. We simulate the array of Al nanocylinder with a 10 nm surface oxide layer (see Appendix Fig. 8). The extinction spectrum and the spatial field distribution are almost the same as the unoxidized model. Because the volume of nanocylinder is considerably large with respect to the typical volume of the oxide layer, the effect is negligibly small.

4. Conclusion

We demonstrated a strategy of enhancing UCPL by coupling the excitation light to the plasmonic-photonic hybrid modes. The plasmonic array facilitates the absorption of CaF₂:Yb³⁺,Er³⁺ NPs by funneling the excitation light into the layer through the excitation of the hybrid mode. The mechanism of UCPL enhancement is confirmed by the angle-variant UCPL measurement and the simulation. A maximum enhancement of 9-fold was observed for the 5.7 µm thick layer of NPs, which is far thicker than the Al nanocylinders with 180 nm height. It is noted that this strategy of enhancing UCPL is universal: By designing the lattice that supports the hybrid mode at the wavelength of interest, it can enhance the UCPL of any UCPL materials. Therefore, we believe that this strategy is very useful in extending the application of UCPL in various fields.

Appendix

 

Fig. 6. Simulated extinction spectrum of the single Al nanocylinder on the SiO₂ glass substrate with the CaF₂: Yb³⁺, Er³⁺ NPs layer on the top. The height and the diameter of the nanocylinder are 180 and 440 nm, respectively. This simulation was conducted by using a commercial finite-differential time-domain (FDTD) software (Lumerical).

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Fig. 7. (a) Optical transmittance spectra and (b) UCPL spectra of CaF₂:Yb³⁺,Er³⁺ NPs layer on the arrays with the period of 820, 900 and 980 nm. The transmittance and UCPL spectra of the same layer on the flat glass substrate are also shown.

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Fig. 8. (a) Simulated transmission at θ_in= 0° for the CaF₂: Yb³⁺, Er³⁺ NPs layer on the Al nanocylinder array (denoted by the red line) and the same layer on the array with a 10 nm-thick oxidized layer on the surface of the Al nanocylinders (black). (b) The spatial distribution of light energy squared for p-polarization incident light (λ = 980 nm) at θ_in= 0° for the CaF₂: Yb³⁺, Er³⁺ NPs layer on the Al nanocylinder array (left panel) and the same layer on the array with a 10 nm-thick oxidized layer on the surface of the Al nanocylinders (right). In the simulation, the thickness of the NPs layer is set to 5.7 µm, but the plots only show the layer up to 2 µm above the array for the clear visualization of the field distribution around the nanocylinders.

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Funding

Japan Society for the Promotion of Science (18J14889, 19H02434); Kazuchika Okura Memorial Foundation; Asahi Glass Foundation; Japan Science and Technology Agency (Nanotech CUPAL).

Acknowledgments

This work was partly supported by the Nanotechnology Hub, Kyoto University and NIMS Nanofabrication Platform (JPMXP09F19NMC042).

Disclosures

The authors declare no conflicts of interest.

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