  $1T$-TiSe$_2$ is believed to posses a soft electronic mode, i.e., plasmon or
exciton, that might be responsible for the exciton condensation and
charge-density-wave (CDW) transition. Here, we explore collective electronic
excitations in single-layer $1T$-TiSe$_2$ by using the ab-initio
electromagnetic linear response and unveil intricate scattering pathways of
two-dimensional (2D) plasmon mode near the CDW phase. We found the dominant
role of plasmon-phonon scattering, which in combination with the CDW gap
excitations leads to the anomalous temperature dependence of the plasmon
linewidth across the CDW transition. Below the transition temperature $T_{\rm
CDW}$ a strong hybridization between 2D plasmon and CDW excitations is
obtained. These optical features are highly tunable due to
temperature-dependent CDW-related modifications of electronic structure and
electron-phonon coupling and make CDW-bearing systems potentially interesting
for applications in optoelectronics and low-loss plasmonics.
