  We report the ultrafast optical pump-probe spectroscopy measurements on the
recently discovered quantum critical ferromagnet CeRh$_6$Ge$_4$. Our
experimental results reveal the two-stage development of the hybridization
between localized $f$ moments and conduction electrons with lowering
temperature, as evidenced by (1) the presence of hybridization fluctuation for
temperatures from $\sim$85 K ($T^*$) to $\sim$140 K ($T^\dagger$), and (2) the
emergence of collective hybridization below the coherence temperature, $T^*$,
marked by the opening of an indirect gap of 2$\Delta$ $\approx$12 meV. We also
observe three coherent phonon modes being softened anomalously below $T^*$,
reflecting directly their coupling with the emergent coherent heavy electrons.
Our findings establish the universal nature of the hybridization process in
different heavy fermion systems.
