  We theoretically revealed that a weak photoexcitation achieves the electric
polarization-inversion with approximately $18\%$ of all the charges, which was
interpreted as a superimposition of multi-exciton states, from the
charge-ordered ferroelectric ground state of (TMTTF)$_{2}$PF$_{6}$ at absolute
zero temperature. Regarding a relative change of electric polarization ($\Delta
P/P$), the photoexcitation corresponds to $36\%$, which is much larger than
$\Delta P/P$ of other typical organic materials. The value of $\Delta P/P\sim
36\%$ can be enlarged by a strong photoexcitation. This fact is useful not only
for applications of this material and other analogous materials in optical
devices but also for researches toward controlling electric polarizations by
light, which is one of the recent attracting issues on photoinduced phase
transition phenomena. The photoexcitation of $\Delta P/P\sim 36\%$ corresponds
to the single peak of the optical conductivity in the low-energy region, which
was also observed at 10 K. Theoretical calculations are based on a
quarter-filled one-dimensional effective model with appropriate parameters and
50 unit cells.
