Multi-wavelength Stellar Polarimetry of the Filamentary Cloud IC5146: I.
  Dust Properties
  We present optical and near-infrared stellar polarization observations toward
the dark filamentary clouds associated with IC5146. The data allow us to
investigate the dust properties (this paper) and the magnetic field structure
(Paper II). A total of 2022 background stars were detected in $R_{c}$-, $i'$-,
$H$-, and/or $K$-bands to $A_V \lesssim 25$ mag. The ratio of the polarization
percentage at different wavelengths provides an estimate of $\lambda_{max}$,
the wavelength of peak polarization, which is an indicator of the small-size
cutoff of the grain size distribution. The grain size distribution seems to
significantly change at $A_V \sim$ 3 mag, where both the average and dispersion
of $P_{R_c}/P_{H}$ decrease. In addition, we found $\lambda_{max}$ $\sim$
0.6-0.9 $\mu$m for $A_V>2.5$ mag, which is larger than the $\sim$ 0.55 $\mu$m
in the general ISM, suggesting that grain growth has already started in low
$A_V$ regions. Our data also reveal that polarization efficiency (PE $\equiv
P_{\lambda}/A_V$) decreases with $A_V$ as a power-law in $R_c$-, $i'$-, and
$K$-bands with indices of -0.71$\pm$0.10, -1.23$\pm$0.10 and -0.53$\pm$0.09.
However, $H$-band data show a power index change; the PE varies with $A_V$
steeply (index of -0.95$\pm$0.30) when $A_V < 2.88\pm0.67$ mag but softly
(index of -0.25$\pm$0.06) for greater $A_V$ values. The soft decay of PE in
high $A_V$ regions is consistent with the Radiative Aligned Torque model,
suggesting that our data trace the magnetic field to $A_V \sim 20$ mag.
Furthermore, the breakpoint found in $H$-band is similar to the $A_V$ where we
found the $P_{R_c}/P_{H}$ dispersion significantly decreased. Therefore, the
flat PE-$A_V$ in high $A_V$ regions implies that the power index changes result
from additional grain growth.
