Determination of the refractive index of BaY₂F₈:Er³⁺ (0.5 mol% to 30 mol%) in the 300 nm to 1800 nm range by ellipsometry; a record-breaking upconversion material

This work reports the ellipsometric study of trivalent erbium (Er ³⁺) doped monocrystalline barium yttrium fluoride (BaY ₂F ₈), which has recently been shown to be one of the best photon upconversion (UC) materials available. This spans the BaY ₂F ₈ applications in a large range of wavelengths, from ultraviolet (UV) to near-infrared (NIR). We detail the optical properties of BaY ₂F ₈: Er ³⁺ (0.5 mol%, 10 mol%, and 30 mol%), measured via variable angle spectroscopic ellipsometry over a spectral range from 300 nm to 1800 nm, reporting for the first time the indices of refraction for BaY ₂F ₈:Er ³⁺. The upconversion external photoluminescence quantum yield (ePLQY) of the BaY ₂F ₈:Er ³⁺ samples have also been studied by exciting at [JG]λ=1588nmλ=1493 nm. The highest ePLQY of BaY ₂F ₈ was found for the largest dopant concentration 30 mol% Er ³⁺ reaching the value of [JG]1.07% ± 0.12%, at an irradiance of 9.512 ×10 ⁻²W/cm ²3.62% ± 0.01%, at an irradiance of (6.23 ± 0.45) ×10 ⁻² W/cm ². The refractive index (λ= 589.3 nm) was determined to be 1.4808± 0.014 for 0.5 mol%, 1.4980± 0.003 for 10 mol%, and 1.5022± 0.006 for 30 mol%. Increasing Er ³⁺ doping concentration increased the refractive index. All samples decreased monotonically with increasing wavelength. The Brewster angle of BaY ₂F ₈:Er ³⁺ is observed to be ≈ 56 ^∘, whilst the Abbe number of the samples was found to be as high as 124.62. These findings provide valuable insight into the optical properties of BaY ₂F ₈:Er ³⁺ in the wide range of frequencies that is has proven useful.