Context : 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 ( \mu m - size ) and large ( mm - size ) dust grains have recently been pointed out .

Aims : 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 .

Methods : We obtained VLT/NACO near-IR high-resolution polarimetric differential imaging observations of SAO 206462 ( { HD 135344 B } ) .
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 \sim 0.1 \arcsec .

Results : A face-on disk is detected in H and K _ { s } bands between 0.1 \arcsec and 0.9 \arcsec .
No significant differences are seen between the H and K _ { s } images .
In addition to the spiral arms , these new data allow us to resolve for the first time an inner disk cavity for small dust grains .
The cavity size ( \simeq 28 AU ) is much smaller than what is inferred for large dust grains from ( sub- ) mm observations ( 39 to 50 AU ) .
This discrepancy can not be ascribed to any resolution effect .

Conclusions : 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 { M _ { J } } ) at 17 to 20 AU from the star is consistent with the observed cavity sizes .