A waveguide structure is provided. The waveguide structure includes: a slot channel waveguide including first and second patterns, which are spaced apart from each other to define a slot; a first upper layer covering at least a portion of the slot channel waveguide; and a second upper layer covering the remaining portion of the slot channel waveguide. A thermo-optic coefficient (TOC) of the channel waveguide times a TOC of the second upper layer is a negative number.