Waveguide device including a waveguide member in an inorganic material substrate attached to a support substrate and with a low-loss dielectric portion

A waveguide device, comprising:

• a waveguide member including an inorganic material substrate and coplanar electrodes arranged above the inorganic material substrate, the waveguide member being capable of guiding an electromagnetic wave having a frequency of 30 GHz or more and 20 THz or less;
• a support substrate, which is arranged below the inorganic material substrate, and is configured to support the waveguide member;
• and
• a low-dielectric constant portion, which is arranged below the inorganic material substrate, and has a dielectric constant smaller than a dielectric constant of the inorganic material substrate;
• and
• wherein a thickness “t” of the inorganic material substrate satisfies the following formula (1):
• t
• <
• λ
• a
• ⁢
• ε
• (
• 1
• )
• where λ represents a wavelength of the electromagnetic wave guided by the waveguide member, ε represents a relative dielectric constant of the inorganic material substrate and “a” represents a numerical value of 2.