Patent ID: 7724997

Claim:
A waveguide element comprising: a photonic crystal waveguide including a core that is constituted by a photonic crystal having a refractive index periodicity at least in one direction, and that propagates an electromagnetic wave in a direction in which the photonic crystal does not have the refractive index periodicity, wherein at least one of materials forming the core constituted by the photonic crystal, and at least a portion of a cladding in contact with a side face of the core are constituted by a fluid, wherein the at least a portion of the cladding in contact with the side face of the core is in contact with at least one side face of the core that is parallel to the direction in which the photonic crystal has the refractive index periodicity, and when the period in a traveling direction of an electromagnetic wave propagating through the core is λ Z , the refractive index of the fluid is n F , the period of the core constituted by the photonic crystal is a, and the vacuum wavelength of an electromagnetic wave propagating through the core is λ 0 , then the following condition is satisfied: aλ Z /(λ Z 2 /4 +a 2 ) 0.5 <λ 0 /n F.