A waveguide structure (200) according to the invention comprises a core layer (210), a cladding layer (206) and a buffer layer (208). Sub-regions (204) are formed in the cladding layer (206) but not in the core layer (210). In one dimensional applications the sub-regions are slots: in two dimensional applications the sub-regions are rods. The rods or slots may be air-filled or filled with an in-fill material, (e.g. Si). The in-fill material, if present, enhances the contrast in dielectric constant between sub-regions (204) and the core layer (210).Sub-regions (204) may furthermore be formed in the portion of the buffer layer neighbouring the core layer. Slots or rods in the buffer may be air-filled or filled by an in-fill material.Substantially complete confinement of the mode in the core can be assured while still maintaining the ability to interact with the field within the photonic band structure region. Rather than perturb the majority of the field (which is confined in the core), it is possible to interact with the evanescent field in the cladding layer (or the buffer layer) without substantially affecting mode confinement.