Strategy for achieving spectral flatness of low crystal symmetry nanoparticles doped polymer based optical waveguide amplifier by constructing a heterogeneous core-shell structure

Due to the low crystal symmetry of LiYF₄:Yb,Er nanocrystals (NCs), polymer-based optical waveguide amplifiers (OWAs) doped with these NCs can amplify signals across most of the S-band and the entire C+L band. This broadband amplification arises from the splitting of Er³⁺ energy levels (⁴I_13/2 and ⁴I_15/2) induced by the crystal field in low-symmetry LiYF₄. However, the strong splitting also leads to poor spectral flatness, causing gain dip in the C+L band. To achieve broadband amplification with flat gain, we propose a heterogeneous core-shell structure, coating a low-symmetry tetragonal LiYF₄ core with a high-symmetry hexagonal NaYF₄ shell. Er³⁺ ions in the core broaden the operating band through level splitting, while those in the shell contribute narrowband, flat emission around 1.5 µm due to reduced splitting. An OWA was fabricated using SU-8 doped with LiYF₄:Yb,Er@NaYF₄:Yb,Er nanoparticles as the gain medium. Under 980 nm excitation at 300 mW, the device provided a relative gain of 3.68-8.58 dB over 1500-1625 nm, covering part of the S-band and the full C+L band. This approach enhances the applicability of low-symmetry NCs in OWAs and offers a strategy for fabricating broadband polymer OWAs with flat gain.