1 Introduction

Lanthanide-doped upconversion nanoparticles (UCNPs) demonstrate many superior properties of broadly-tunable multicolor emission and long emission lifetimes making them well-suited for many applications including super-resolution imaging [1,2,3,4,5], optical multiplexing [6, 7], three-dimensional displays [8], and low-threshold lasing [9, 10]. Despite many advantages, the further progress and practical applications of UCNPs face significant challenges due to low upconversion efficiencies. Polarization, being another important characteristic of fluorescence, provides orientation and structural information in an additional dimension, and it has been extensively employed in the fluorescence polarization imaging technique [11, 12]. Unfortunately, the inherent upconversion emission processes associated with the 4f electronic transition in UCNPs are typically weak, and they do not exhibit polarization characteristics [13].

A majority of the previous studies focused primarily on enhancing the upconversion luminescence by coupling UCNPs with plasmonic nanostructures [14,15,16,17,18,19,20,21,22]. The concentrated electromagnetic field in the vicinity of these nanostructures can significantly boost both the absorption cross-sections and the upconversion radiation rates. Metallic structures supporting surface plasmons are effective in localizing light and creating strong electric fields, which can enhance upconversion fluorescence. However, metallic nanostructures generally have low values of the quality factor (Q factor) due to the intrinsic loss and may cause quenching effect due to the direct contact to the emission materials. The Q value is a crucial parameter for resonant excitation. The typical range of Q factor for plasmonic resonances is up to a few tens [23]. Although some special designs like metal-insulator-metal (MIM) structure can obtain the high Q factor, the emitters are close to the metallic surface, the quenching effect makes the quantum efficiency drop rapidly. In the meantime, there are also a few works reporting the controllability of luminescence’s polarization state with plasmonic structures [24], but the fluorescence enhancement is very weak due to their low Q factor. In addition, these studies have concentrated on the polarization properties of the overall luminescence, which cannot achieve selective modulation of polarization characteristics for multi-color upconversion bands.

In comparison to plasmonic nanostructures, all-dielectric resonant metasurfaces composed of high-index dielectric nanostructures with low losses at visible frequencies support much richer multipolar Mie-resonances [25,26,27,28,29,30], which provide an excellent alternative to fluorescence enhancement. Furthermore, collective high-Q resonances such as lattice resonances and bound states in the continuum (BICs) emerge due to a coupling of these Mie multipoles in periodic arrays [31,32,33,34]. Among them, BICs, which are completely decoupled from radiative modes, have recently attracted considerable attention due to their infinitely-high Q factors [33, 35]. Breaking symmetry of meta-atoms can transform a BIC into a quasi-BIC with finite but extraordinarily high Q factors [36,37,38,39]. These emerging concepts have recently been well-exploited and demonstrated for fluorescence enhancement and low-threshold lasing applications [40,41,42].

Here, we utilize the emerging concept of high-Q collective modes of resonant metasurfaces for polarization-controlled dual-band upconversion burst as illustrated in Fig. 1. We design and fabricate high-Q resonant metasurfaces composed of Titanium dioxide (TiO2) diatomic nanobricks (Fig. 1a, b), which support a quasi-BIC mode (with Q ~ 1031) at the wavelength of 660 nm for the y-polarized incidence, and another high-Q Mie resonance mode (with Q ~ 1044) at the wavelength of 540 nm for the x-polarized incidence, respectively. Consequently, NaYF4:Yb/Er UCNPs deposited on the metasurface exhibit ultrabright upconversion luminescence at dual bands of 540 and 660 nm, with enhancement factors up to approximately 51-fold and 43-fold. By controlling the rotation of the polarization analyzer, we demonstrate linearly- and cross-polarized emission at dual bands with ultra-high degrees of polarization (DoPs). Our demonstration paves the way for efficient enhancements and polarization control of the UCNP emission with potential applications in low-threshold polarization upconversion lasers and hyperspectral imaging/sensing.

Fig. 1
figure 1

Polarization-controlled upconversion emission enhancement. a Schematic presentation of a resonant metasurface with high-Q quasi-BIC and Mie resonant modes. The metasurface is composed of an array of TiO2 nanobricks loaded with UCNPs. b Diagram of the designed unit cell depicting dual TiO2 nanobricks with a tilt angle of δ = ±10° relative to the y-axis, placed on a SiO2 substrate. Geometrical parameters: wx=100 nm, wy = 240 nm, h = 450 nm. The rectangular lattices periods are px = 430 nm and py = 345 nm. c TEM image of NaYF4:Yb/Er upconversion nanoparticles. d Energy level diagram illustrating the upconversion transition of Yb3+/Er3+. e x- and y-polarization controlled reflection spectra of the resonant metasurfaces overlapping with the emission spectra of the NaYF4:Yb/Er upconversion nanoparticles. Yellow curves denote Mie resonance under x-polarization and magenta curves denote quasi-BIC resonance under y-polarization incidence

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2 Results and discussion

The unit-cell of the proposed dielectric resonant metasurface is shown in Fig. 1b. It is constructed by a pair of TiO2 (refractive indexes refer to Additional file 1: Fig. S1) nanobricks placed on a SiO2 substrate. The nanobricks are of height (h) 450 nm, length (wy) 240 nm and width (wx) 100 nm. The periods along the x- and y-directions are px = 430 nm, and py = 345 nm, respectively. A BIC can be formed in such metasurface without any radiation due to symmetry protection or destructive interference between different radiation channels. In our design, the introduction of the asymmetric parameter δ (the orientation angle δ of the nanobrick with respect to the y axis) breaks the rotational symmetry protection of BICs, transferring them into high Q-factor quasi-BICs that are accessible with free-space excitations at normal incidence. By varying the orientation angle between the two nanobricks and the background refractive index, we can adjust the resonant wavelength to match the emission wavelength of the UCNPs. As depicted in Fig. 1c–e, NaYF4:Yb/Er (20/2 mol %) nanoparticles with size of around 15 nm (Additional file 1: Fig. S2) were selected to be spin-coated onto the metasurface to adjust the background refractive index. The designed metasurface exhibits a quasi-BIC mode at λ ≈ 660 nm for y-polarization, and another Mie resonance mode at λ ≈ 540 nm for x-polarized light. Those two high-Q resonance modes overlap with the emission band of the Er3+ upconversion transition (4H11/2 to 4I15/2 and 4F9/2 to 4I15/2).

To further get insights into the resonance behavior, we calculate the reflectance spectra of the metasurface with respect to asymmetry parameter δ under x- and y- polarized normal incidence (Fig. 2a–b). Under y-polarized normal incidence, the metasurface supports a symmetry-protected BIC at δ = 0o, which has an infinite Q-factor indicated by the red circle. A slight symmetry breaking (δ ≠ 0 o) can transfer the symmetry-protected BIC into a quasi-BIC. Additional file 1:  Fig. S3 shows the Q-factor and the resonance wavelength of the quasi-BIC mode on different asymmetry parameter δ. As the δ increases, the Q factor remains high and the resonance wavelength red-shifts. A spectrally narrow resonance at ∼660 nm can be observed for the case of δ = 10°, in which case the Q factor is calculated to be 1031.

Fig. 2
figure 2

a–b The evolution of reflection spectra by continuously varying the geometry asymmetry parameter δ under y-polarization a and x-polarization b. The vanishing point indicated by the red circle denotes the BIC mode. The green circle denotes the Mie resonance. c–d Electric field distributions at resonant wavelength 660 nm for y polarization, and 540 nm for x polarization. e Slant view SEM image of the resonant metasurface unloaded with UCNPs. f Top view SEM image of the resonant metasurface loaded with UCNPs. Scale bar: 500 nm. g The measured reflection spectra of the resonant metasurface under x- (green) and y- (red) polarizations, respectively

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For x-polarized normal incidence, multiple linewidth-vanishing BIC modes are identified in Fig. 2b, with different resonant wavelengths. Among them, a Mie resonance at 540 nm sustains as indicated by the green circle, regardless of variations in the asymmetry parameter δ. Moreover, the linewidth and resonance wavelength keep almost intact during the variation, as shown in Fig. 2b and Additional file 1:  Fig. S3. We calculate the electromagnetic near field distributions and the multipole mode expansion spectra of the Mie resonance. It reveals that the Mie resonance is dominated by the toroidal (TD) mode, with smaller contributions from other dipole modes, considering the lattice interaction and the influence of periodicity (Additional file 1: Fig. S4). Additional file 1:  Fig. S5 illustrates that variations in other structural parameters (px, py, wx, wy, h and n) only affect the peak position of the Mie resonance. To match the emission wavelengths of the UCNPs, we judiciously adjust the asymmetric parameters to introduce high-Q resonances at 540 and 660 nm, respectively, responding to x- and y-polarizations. Along with resonance wavelength and polarization tunability, both the high-Q quasi-BIC and the Mie resonance mode strongly amplify the electric near fields around the resonators with intensity enhancement factors I/I0 up to 1600 (Fig. 2c–d). This enables a strong confinement of the radiative emission field inside the metasurface, which exhibits a high field intensity for efficient upconversion luminescence enhancement.

The metasurface was fabricated using electron beam lithography (EBL) technology. Figure 2e shows slant-view scanning electron microscopy (SEM) images of the fabricated sample. NaYF4:Yb/Er (20/2 mol %) nanoparticles were uniformly spin-coated onto the metasurface, as shown in the top-view SEM image (Fig. 2f), and some of them were filled in TiO2 nanobricks. The spectral measurement reveals that UCNPs/metasurface sample exhibit resonance peaks at ∼540 and 660 nm under different polarizations, which perfectly overlap with the luminescence peaks of UCNPs (Fig. 2g). The experimentally achieved Q-factor of the Mie resonance mode at 540 nm is 151, and that of the quasi-BIC mode is around 268. The smaller values of Q-factors compared with the theoretical ones are due to the finite array size and fabrication imperfections.

We further experimentally validated the feasibility of our design to achieve polarization-controlled upconversion emission enhancement. Figure 3a and b show the optical band structure of the resonant metasurface when the incident light is along the x- and y-direction, respectively. We note that x- and y-polarization incidences show a clear response difference at the Γ point. The x-polarization incidences show strong reflection while y-polarization excitation shows nearly zero reflection at 540 nm, as indicated by the white circle. In contrast, the strong reflection in y-polarization excitation happens at 660 nm. The polarized upconversion luminescence from the UCNPs/metasurface sample was then collected by using home-made confocal system with discrimination capability (Additional file 1:  Fig. S6). Under laser excitation power of 1 mW at wavelength 980 nm, the luminescence spectra of UCNPs on metasurface and glass are shown in Fig. 3c–d. Er3+-activated UCNPs showed two characteristic bands centered around 540 and 660 nm. Compared to the luminescence intensity on the glass substrate, the UCNPs/metasurface sample exhibit sharp green emission in x-polarization detection. In contrast to glass, the luminescence intensity at 660 nm is significantly increased when detected in y-polarization. In the meantime, the Fourier-plane upconversion luminescence images at 540 and 660 nm were measured using a bandpass filter are shown in insets of Fig. 3c–d. The upconversion luminescence images display characteristic angle dispersion at dual bands, which implies that emissions of UCNPs are polarized and coupled to Mie resonance mode and the quasi-BIC mode of the resonant metasurface, respectively.

Fig. 3
figure 3

a–b Simulated angle-resolved reflectance spectra under a) x-polarized beam and (b) y-polarized beam for the wavelength range 530–700 nm. The ratio between the projected in-plane momentum kx and free-space momentum k0 indicates the reflection angle sinθ = kx/k0. c–d Upconversion luminescence spectra from UCNPs deposited on the metasurface and on glass substrate under x-polarization detection (c) and y-polarization detection d. Inset: Fourier-plane upconversion photoluminescence images. (e)The linewidth of luminescence spectra of UCNPs on the metasurface and glass substrate. f The upconversion luminescence intensities from UCNPs deposited on glass and metasurface, as a function of the excitation power density for the green and red emissions, respectively. g Luminescence enhancement factors at emission wavelengths of 540 nm (green) and 660 nm (red) as a function of the 980 nm incident excitation power. h Emission lifetime measurements at 540 nm for UCNPs deposited on resonant metasurface supporting Mie resonant mode (green) and on a glass slide (black) under x-polarization detection. i Time-resolved photoluminescence measurements of UCNPs deposited on a glass slide (black) and a metasurface supporting quasi-BIC mode (red) at an emission wavelength of 660 nm under y-polarization detection

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We found that the linewidth of fluorescence luminescence was significantly narrower compared to that on glass substrate. Figure 3e displays the statistical chart of the linewidths of the upconversion emission for UCNPs on the metasurface and on the glass substrate. The luminescence bandwidths at emission wavelengths 540 and 660 nm on the metasurface are 3.18 and 3.67 nm, respectively, which are close to the reflection spectrum bandwidth of the metasurface with UCNPs coating. In comparison to the upconversion fluorescence bandwidth on the glass substrate, the bandwidth on the metasurface is reduced by approximately four times. This finding offers new insights into narrowing linewidth of fluorescence emission for increasing fluorescence detection sensitivity and reducing background noise. Figure 3f illustrates the excitation power dependence of the UCNPs coupled metasurface and the reference samples. At low power excitation, both green and red emissions are present with slopes of 2.2 and 2.3, respectively, indicating that the upconversion emission is basically a two-photon process. It was observed that luminescence intensity increases linearly with incident power under strong excitation. However, the UCNPs coupled with the resonant metasurface exhibit a smaller slope and significant fluorescence enhancement due to the local field enhancement of the quasi-BIC mode and the Mie resonant mode. The upconversion emission rapidly reaches a saturation state, resulting in a reduced slope. Moreover, we quantified the enhancement factor for the upconversion luminescence intensity of each emission band under different excitation intensities (Fig. 3g). In high-power excitation, the enhancement factor is 7-fold for 540 nm and 5-fold for 660 nm due to the saturation, while it increases to 51-fold and 43-fold as the incident power is reduced to 92 Wcm−2. The enhancement factor is lower than that predicted mainly because of fabrication imperfections and uneven distribution of the nanoparticles.

The upconversion luminescence enhancements are typically attributed to a combination of multiple factors, namely the increased excitation rate result from both the absorption of the pump laser, and the radiative emission rate, which is enhanced by the Purcell effect at the emission wavelength. The physical mechanism behind the enhancement of upconversion fluorescence stems from the near-field enhancement induced by the quasi-BIC and Mie-resonant modes. The alterations in the electromagnetic field near the UCNPs may result in an excited flux, while the increased local density of optical states (LDOS) offers additional decay channels for the UCNPs to radiate via the quasi-BIC and Mie resonant modes into the far-field. To characterize the enhanced spontaneous-emission processes, we measured the radiative lifetime for the UCNPs-BIC sample and UCNPs on the reference glass (Fig. 3h–i). UCNPs on the resonant metasurface exhibit a decay rate enhancement of 3.2 at 540 nm. Simultaneously, the enhancement of the decay rate is 2.7 times greater than the glass sample at 660 nm.

The enhancement of the radiative decay rate of the resonant metasurface can also be explained through Purcell factor analysis, which was investigated by simulating a finite resonant metasurface with 30 × 30 unit-cells, consistent with the illumination spot diameter of ∼12 μm in the experiment. The corresponding Purcell factor of at 540 nm in x-polarized dipole orientation and 660 nm in y-polarized dipole orientation revealed the decay rate enhancement for green and red emissions, respectively (Additional file 1:  Fig.S7). Only dipoles with a specific orientation will experience the Purcell effect, which is consistent with the polarization response of reflectance spectrum. It can be observed that the Purcell factor is very close to the changes in the emission decay rate. Furthermore, the Purcell factors at the electric field maxima at different positions are similar, suggesting that the quasi-BIC and Mie resonance with a large LDOS provide additional decay channels to enhance the decay rate and improve upconversion emission.

In addition to the radiative emission rate, we calculated the spectra for both x and y polarizations and the field enhancements in the wavelength range around 980 nm (Additional file 1:  Fig. S8). It can be seen that no resonance peak appears around the 980 nm for both x and y polarization excitations. Most importantly, at excitation polarization angles varying from 0° to 90°, upconversion luminescence spectra of UCNPs coupled to the resonant metasurface as shown in Fig. Additional file 1: Fig. S8e–f. The fluorescence intensity does not change with the polarization of the 980 nm pumping light, suggesting that the quasi-BIC mode and Mie resonance dominate the luminescence enhancement.

The inherent polarization-controlled high Q resonant modes in such resonant metasurface enables the selective manipulation of the dual-band emission polarizations. Figure 4a shows the polarized upconversion emission spectra of UCNPs on the metasurface, recorded by rotating an analyzer before the spectrometer. As the analyzer varies from 0° to 180°, the upconversion luminescence intensity of 540 and 660 nm on metasurface cross-change sinusoidally. The polar plot of the up-conversion luminescence intensity as a function of the polarization angle shows evident “8”-shaped convergence (Fig. 4c). The luminescence intensity at 540 nm shows a distinct x-polarized emission with the weakest and strongest emission at 0 and 90°, respectively. Conversely, the red emission at 660 nm shows the opposite case, indicating their countercyclical polarization anisotropy. On the glass substrate, the intensity of upconversion luminescence remains almost unchanged, and an approximate circle is drawn in polar plates, indicating no polarization response. The degree of polarization (DoP) of the upconversion luminescence can be evaluated as DoP = (Imax− Imin)/(Imax + Imin), where Imax and Imin represent the maximum and minimum values of upconversion luminescence intensity. The luminescence emission of UCNPs on the reference glass shows no polarization sensitivity at all. In stark contrast, the luminescence emission DoP of UCNPs coupled with the resonant metasurface achieves ~ 0.86 and ~ 0.91 for emissions at 540 and 660 nm, respectively. The luminescence intensities of x- and y-components for green and red colors are selectively enhanced for UCNPs on the metasurface, respectively, providing a unique approach to achieve luminescence polarization control in nanophotonic devices.

Fig. 4
figure 4

a Upconversion luminescence spectra of UCNPs coupled to the resonant metasurface, recorded with a rotating analyzer at various angles from 0 to 180° in front of the spectrometer. b Upconversion emission spectra from UCNPs on a glass substrate, recorded at different detection polarization angles ranging from 0 to 180°. c–d Polar plots illustrating the luminescence intensity of UCNPs coupled to the metasurface c and UCNPs on a glass substrate d as functions of the emission polarization angle. e Degree of polarization (DoP) for upconversion luminescence of UCNPs on both metasurface and glass substrate

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In summary, we have demonstrated dual-band upconversion luminescence with simultaneously substantial augmentation and high degree of polarization from upconversion nanoparticles coupled to all-dielectric resonant metasurfaces. Supporting polarization-controlled quasi-BIC and Mie modes with ultra-high Q factors at dual upconversion bands, such resonant metasurfaces provides an enabling platform for significantly amplified upconversion emissions, achieving 51- and 43-fold enhancements, as well as simultaneous cross-polarized luminescence with DoP reaching 0.86 and 0.91, for green and red emissions, respectively. We notice that such NaYF4:Yb/Er are not restricted to a special type of UCNPs, but they can be generalized to other types of UCNPs such as NaYF4:Yb/Tm, as long as the resonance wavelengths of the metasurface are tailored to match the emission spectra of the chosen UCNPs. High-doped UCNPs (such as Tm3+) have multiple energy levels, which is conducive to population establishment and cross-relaxation effect, and easier to realize population inversion. Coupling with polarization selective resonant metasurface is promising for efficient upconversion lasing with polarization controllability. We believe our results provide an excellent platform for ultrabright and polarized nanoscale emission sources holding great potential for the further advances in upconversion polarization lasing, biosensing, and fluorescence imaging.

3 Methods

3.1 Fabrication of samples

The metasurfaces were fabricated on a fused quartz substrate using a combination of processes, including electron beam lithography (EBL), atomic layer deposition (ALD), patterning, lift-off, and etching. First, the polymethyl methacrylate (PMMA) resist 950-A7 was spun on fused quartz substrate and baked at 180 °C for 2 min. The designed structures were patterned by using electron beam lithography system (6300FS, JEOL) and then developed in a mixture of MIBK and IPA with a ratio of 1:3. Then the TiO2 film was deposited by the atomic layer deposition method. The ALD system is under continuous 20 sccm flow of N2 carrier gas and maintained at 105 °C throughout the process. Finally, a dry etching process was performed in the ICP-RIE system (Plasmalab System 100 ICP180, Oxford) with a mixed reactive gas of BCl3 and Cl2 to remove the TiO2 film on the top of the resist. And dry etching process with Oxygen also was used to remove the residual resist.

3.2 Characterization of metasurfaces

The broadband source (Fianium-WL-SC480) through a polarizer and half-wave plate generates different linear polarizations. The excitation beam is transmitted by beam splitters, then focused on the samples through an objective lens (NA = 0.4). The reflection spectra of the resonant metasurface were measured using an Ocean spectrometer (USB4000).

3.3 Characterization of luminescence spectra and lifetime

Luminescence spectra were measured using a CW fiber-coupled laser diode (λex = 980 nm) as the excitation source. The objective lens with a numerical aperture (NA) of 0.4 was used for both excitation and emission collection. Then, the incident light was focused on the sample and the emission through polarizer and filter was captured by a spectrometer (Shamrock 303i, Andor) equipped with an electron-multiplying charge-coupled device camera (EMCCD, Newtown, Andor). And time-resolved photoluminescence measurements of UCNPs were performed by modulating the 980 nm laser with a signal generator. The luminescence is collected by a single-photon avalanche diode (SPAD; SPCM-AQRH-14-FC) and synchronized with a modulated excitation signal by a data acquisition (DAQ).