  Transitional disks represent a short stage of the evolution of circumstellar
material. Studies of dust grains in these objects can provide pivotal
information on the mechanisms of planet formation. Dissimilarities in the
spatial distribution of small (micron-size) and large (millimeter-size) dust
grains have recently been pointed out. Constraints on the small dust grains can
be obtained by imaging the distribution of scattered light at near-infrared
wavelengths. We aim at resolving structures in the surface layer of
transitional disks (with particular emphasis on the inner 10 - 50 AU), thus
increasing the scarce sample of high resolution images of these objects. We
obtained VLT/NACO near-IR high-resolution polarimetric differential imaging
observations of SAO 206462 (HD135344B). This technique allows one to image the
polarized scattered light from the disk without any occulting mask and to reach
an inner working angle of 0.1''. A face-on disk is detected in H and Ks bands
between 0.1'' and 0.9''. No significant differences are seen between the H and
Ks images. In addition to the spiral arms, these new data allow us to resolve
for the first time an inner cavity for small dust grains. The cavity size
(about 28 AU) is much smaller than what is inferred for large dust grains from
(sub)mm observations (39 to 50 AU). The interaction between the disk and
potential orbiting companion(s) can explain both the spiral arm structure and
the discrepant cavity sizes for small and large dust grains. One planet may be
carving out the gas (and, thus, the small grains) at 28 AU, and generating a
pressure bump at larger radii (39 AU), which holds back the large grains. We
analytically estimate that, in this scenario, a single giant planet (with a
mass between 5 and 15 Jupiter masses) at 17 to 20 AU from the star is
consistent with the observed cavity sizes.
