  Solar coronal shocks are very common phenomena in the solar atmosphere and
are believed to be the drivers of solar type II radio bursts. However, the
microphysical nature of these emissions is still an open problem. This paper
proposes that electron cyclotron maser (ECM) emission is responsible for the
generation of radiations from the coronal shocks. In the present model, an
energetic ion beam accelerated by the shock excites first Alfv\'en wave (AW)
and then the excited AW leads to the formation of a density-depleted duct along
the foreshock boundary of the shock. In this density-depleted duct, the
energetic electron beam produced via the shock acceleration can effectively
excite radio emission by the ECM instability. Our results show that this model
may have potential application to solar type II radio bursts.
