Multi-wavelength UV Upconversion in Lanthanides assisted by Photonic Crystals

Abstract

Upconversion luminescence consists of the absorption of low-energies photons followed by the emission of a higher energy photon. The process has mainly been studied in lanthanides to upconvert monochromatic near-infrared excitation to near-infrared or visible light, and has been exploited only to a limited extent to upconvert broad excitations to ultra-violet. In addition, upconverting near-infrared and visible light to ultra-violet is crucial for applications such as solar-to-fuel conversion or environmental remediation. However, upconversion luminescence is limited by the low absorption cross-sections of lanthanides. In this work, we engineered Bloch modes in a photonic crystal to assist a multi-wavelength upconversion mechanism and demonstrated a 28-fold enhancement of ultra-violet upconversion luminescence of Yb3+-Tm3+ doped thin films. Materials were selected and optimized to design nanostructures without parasitic absorption losses. The geometric parameters of the photonic crystals were scanned to match a slow-light resonance with an excited-state transition of Tm3+ and thus enhance incident visible light absorption. Ultra-violet light extraction was also enhanced by photonic crystal Bloch modes. Each of these two contributions were quantified and the measured photonic band structures were well reproduced by electromagnetic simulations.

Figures (17)

• Figure 1: Multi-wavelength UV UCL mechanism in Yb3+-Tm3+. It involves 1 NIR photon absorption by Yb3+ (7F7/2$\rightarrow$7F5/2), whose energy is transferred to Tm3+ (3F4), followed by 1 visible excited state absorption in Tm3+ (3F4$\rightarrow$1D2). UV emission is obtained from spontaneous emission to the ground-state (1D2$\rightarrow$3H6).
• Figure 2: Presentation of the PhC structure. (a) 3D scheme of the PhC design and operational principle: NIR and visible light absorption close to normal incidence and UV emission in a controlled direction. (b) SEM top view image of a fabricated PhC with the following parameters: period p = 267.3 $\pm$ 0.4 nm; air filling factor ff = 47.9% $\pm$ 0.2%; etching depth H = 138 $\pm$ 1 nm.
• Figure 3: UV UCL experimental characterisation. (a-b) Measurements were done under NIR excitation ($\lambda$NIR = 995 nm | NANIR = 0.40), with (top purple plots) or without (bottom red plots) the visible excitation ($\lambda$vis = 447 nm | NAvis = 0.04). UV emission is collected with a microscope objective of numerical aperture NAUV = 0.75. Considered irradiance are INIR = 966.5 kW/cm2 and Ivis = 18.2 kW/cm2. (a) UV emission of 22 PhCs integrated with the composite trapezoidal rule depending on the PhC lattice parameter, and compared to references. Gray bands are reference values at 95% confidence interval considering a normal distribution in the population of 5 measured unpatterned areas. PhC structures A, B, C and D are identified. (b) Emission spectra of the PhC D is compared to the averaged UCL signal of the references (gray curves). UCL data considered for the reference are obtained by averaging responses of 5 unpatterned areas. UCL data of the reference under IR and visible excitations is multiplied by a factor of 10 for the sake of visibility. (c) Evolution of the integrated UV emission of the structure D depending on the excitation conditions: NIR irradiance varies while visible one is fixed at 25 kW/cm2 (red dots), or visible irradiance varies while NIR one is fixed at 3.5 MW/cm2 (blue triangles). Dashed lines results from linear regressions.
• Figure 4: Band structure measurements and analysis along the high symmetry direction $\Gamma$X. Reflectivity measurements of PhC slabs have been conducted using a halogen-deuterium white light source and a microscope objective of numerical aperture NABS = 0.28. White doted lines delimit incidence angles covered by NAvis from the UCL measurement setup. Blue doted lines correspond to the normalized absorption cross-section of the 3F4$\rightarrow$1D2 transition. Blue areas correspond to the spectral range of the transition excited by the laser used in the UCL setup. (a) Measured band structure of the PhC D (left-side) compared to the RCWA simulation (right-side). (b), (c), and (d) are experimental band structure truncations centered on the $\Gamma$-point at 3F4$\rightarrow$1D2 transition of B, C and D PhC structures respectively. TE0-like and TM0-like slow-light resonances are identified on the band structure (c). (e), (f) and (g) are reflectance spectra averaged over NAvis, denoted RPhC, of B, C and D PhC structures respectively, overlapped by the normalized absorption cross-section $\sigma_{Tm^{3+}}$.
• Figure 5: UV emission enhancement factor under NIR and visible excitation depending on the overlap factor $\gamma$.