Spectral non-hermitian quantization line shape controlled by phonon dressing in various phases of Eu³⁺:BiPO₄

We report the relationship between phonon dressing and spectral line shape in different phases of Eu3+-doped BiPO4 nanocrystals. We depict how the pure hexagonal phase exhibited a narrow line shape in contrast to mixed phases (more hexagonal phase (HP) with less low-temperature monoclinic phase (LTMP), and less HP with more LTMP). We exhibited spectral-line-shape controllability through non-Hermitian exceptional points, phase-transition phonon dressing and thermal phonon dressing. We observed that the spectral line shape in half HP + half LTMP was approximately 7 times wider at 300 K than at 77 K. We also realized that the line shape of a nonclassical spontaneous four-wave mixing (SFWM) signal could be tuned by changing the phases, time gates, detector position, and lasers. Furthermore, we showed that thermal phonons influence the in-phase SFWM line shape but not the out-of-phase fluorescence line shape. These results imply a spectral notch filter analogy with a maximum bandwidth contrast of about 74.6%.