  We observed the gravitationally lensed blazar JVAS B0218+357 with the KVN and
VERA Array (KaVA) at 22, 43, and 86 GHz. The source has recently been
identified as an active gamma-ray source up to GeV/TeV energy bands, rendering
a unique target for studying relativistic jets through gravitational lensing.
Here we report the first robust VLBI detection and imaging of the lensed images
up to 86 GHz. The detected mas-scale/parsec-scale morphology of the individual
lensed images (A and B) is consistent with that previously seen at 22 and 15
GHz, showing the core-jet morphology with the jet direction being the same as
at the low frequencies. The radio spectral energy distributions of the lensed
images become steeper at higher frequencies, indicating that the innermost jet
regions become optically thin to synchrotron emission. Our findings confirm
that the absorption effects due to the intervening lensing galaxy become
negligible at millimeter wavelengths. These results indicate that
high-frequency VLBI observations are a powerful tool to better recover the
intrinsic properties of lensed active galactic nucleus jets, which therefore
allow us to study the interplay between the low- and high-energy emission.
