  We investigate an interesting anisotropic van der Waals material,
Ta$_{2}$NiSe$_{5}$, using optical spectroscopy. Ta$_{2}$NiSe$_{5}$ has been
known as one of the few excitonic insulators proposed over 50 years ago.
Ta$_{2}$NiSe$_{5}$ has quasi-one dimensional chains along the $a$-axis. We have
obtained anisotropic optical properties of a single crystal Ta$_{2}$NiSe$_{5}$
along the $a$- and $c$-axes. The measured $a$- and $c$-axis optical
conductivities exhibit large anisotropic electronic and phononic properties.
With regard to the $a$-axis optical conductivity, a sharp peak near 3050
cm$^{-1}$ at 9 K, with a well-defined optical gap ($\Delta^{EI} \simeq$ 1800
cm$^{-1}$) and a strong temperature-dependence, is observed. With an increase
in temperature, this peak broadens and the optical energy gap closes around
$\sim$325 K($T_c^{EI}$). The spectral weight redistribution with respect to the
frequency and temperature indicates that the normalized optical energy gap
($\Delta^{EI}(T)/\Delta^{EI}(0)$) is $1-(T/T_c^{EI})^2$. The
temperature-dependent superfluid plasma frequency of the excitonic condensation
in Ta$_{2}$NiSe$_{5}$ has been determined from measured optical data. Our
findings may be useful for future research on excitonic insulators.
