Substrate with dielectric thin film, optical waveguide element, and optical modulation element

A substrate with a dielectric thin film comprising:

• a single-crystal substrate;
• a stress relaxation layer formed in contact with a main surface of the single-crystal substrate;
• and
• a dielectric thin film formed in contact with the stress relaxation layer,
• wherein the stress relaxation layer is made of a c-axis-oriented epitaxial film, and includes:
• a twin crystal structure of LiNbO3 including a first crystal and a second crystal that is rotated 180° about a c-axis with respect to the first crystal;
• and a LiNb3O8 phase,
• the dielectric thin film is made of a lithium niobate film which is a c-axis-oriented epitaxial film, has the twin crystal structure of LiNbO3, has a film thickness of 0.5 μm to 2 μm, and has a maximum domain width greater than a maximum domain width of the stress relaxation layer, and
• a percentage of a film thickness of the stress relaxation layer to the film thickness of the dielectric thin film is 5% to 25%.