An example device in accordance with an aspect of the present disclosure includes a first layer and a second layer to be bonded to the first layer. The first and second layers are materials that generate gas byproducts when bonded, and the first and/or second layers is/are compatible with photonic device operation based on a separation distance. At least one bonding interface layer is to establish the separation distance for photonic device operation, and is to prevent gas trapping and to facilitate bonding between the first layer and the second layer.