  In heavy fermions the relaxation dynamics of photoexcited carriers has been
found to be governed by the low energy indirect gap, E$_{g}$, resulting from
hybridization between localized moments and conduction band electrons. Here,
carrier relaxation dynamics in a prototype Kondo insulator YbB${}_{12}$ is
studied over large range of temperatures and over three orders of magnitude. We
utilize the intrinsic non-linearity of dynamics to quantitatively determine
microscopic parameters, such as electron-hole recombination rate. The extracted
value reveals that hybridization is accompanied by a strong charge transfer
from localized 4f-levels. The results imply the presence of a hybridization gap
up to temperatures of the order of E$_{g}$/k$_{B}\approx200$ K, which is
extremely robust against electronic excitation. Finally, below 20 K the data
reveal changes in the low energy electronic structure, attributed to
short-range antiferromagnetic correlations between the localized levels.
