  We use time-domain terahertz spectroscopy to measure the low energy
conductivity and magnons in RuCl$_3$ under external magnetic field. At zero
field, an oscillation with a frequency of 0.62 THz is clearly observed in
time-domain spectrum below T$_N$, which is identified as a magnon excitation in
the magnetic order state. The magnon excitation is not affected by the external
magnetic field $\textbf{H}_{DC}$ when it is applied along the c-axis, but is
clearly suppressed when $\textbf{H}_{DC}$ is applied within ab plane. More
interestingly, when the magnetic component of THz wave $\textbf{h}(t)$ is
perpendicular to the applied in-plane magnetic field, we observe another
coherent oscillation at slightly higher energy scale at the field above 2 T,
which is eventually suppressed for $H_{DC}>$5 T. The measurement seems to
indicate that the in-plane magnetic field can lift the degeneracy of two
branches of low energy magnons at $\Gamma$ point. The low energy optical
conductivity calculated from the measured transmission spectrum is dominated by
a broad continuum contribution, which is not affected by changing either
temperature or external magnetic field. The continuum is likely to be related
to the fractional spin excitation due to dominated Kitaev interaction in the
material.
