A strategy for preparing broadband polymer based optical waveguide amplifiers: doping low crystal field symmetric nanocrystals in polymer matrix as gain media

Optical waveguide amplifiers are essential devices in integrated optical systems, with their gain bandwidths directly influencing the operating wavelengths of optical circuits. Previous Er³⁺-doped polymer optical waveguide amplifiers have been limited to amplifying signals within the C-band. To achieve broadband polymer optical waveguide amplification, we propose the use of nanocrystals with low crystal field symmetry to extend the working bandwidth. Our approach utilizes LiYF₄: Yb, Er nanoparticles embedded in poly(methyl methacrylate) as the gain medium, enabling signal amplification from most of the S-band to the whole (C + L) band. The low crystal field symmetry of the LiYF₄ host significantly splits the ⁴I_13/2 and ⁴I_15/2 levels of Er³⁺ ions owing to the crystal field effect, facilitating broadband down-conversion luminescence under 980-nm excitation. Furthermore, a fluorescence kinetic analysis confirms that the broadband luminescence of Er³⁺ arises from significant energy-level splitting caused by the crystal field effect. Under 980-nm excitation, the amplifiers exhibited relative gains of approximately 12.6 dB at 1535 nm, 7.4 dB at 1480 nm, and 3.7 dB at 1610 nm. The Er³⁺-doped broadband polymer optical waveguide amplifier was successfully prepared.