  The c-axis optical response of the underdoped cuprates is qualitatively
different from its in-plane counterpart. The features of the pseudogap show
themselves more prominently in the c-axis than in-plane. We compute both the
c-axis and in-plane optical conductivity using the Yang-Rice-Zhang model of the
underdoped cuprates. This model combined with coherent interlayer tunnelling is
enough to explain the qualitative differences between the in-plane and c-axis
data. We show how pseudogap features manifest themselves in the infrared and
microwave conductivity within this model.
