SUBSTRATE WITH THIN DIELECTRIC FILM AND LIGHT MODULATION ELEMENT USING SAME

The invention provides a substrate with a thin dielectric film wherein cracking does not easily occur even when the thickness of a lithium niobate film is, for example, 1 [um]m or greater. Provided isa substrate with a thin dielectric film that is provided with a monocrystalline substrate 2 and an epitaxially formed thin dielectric film 3 formed from lithium niobate with a c axis orientation on amain surface 2a of the monocrystalline substrate 2. The thin dielectric film 3 has a twin structure that includes a first crystal and a second crystal obtained by rotating the first crystal 180 degrees centered on the c axis. In establishing poles by x-ray diffraction, the ratio of first diffraction intensity corresponding to the first crystal and second diffraction intensity corresponding to thesecond crystal is 0.5 - 2.0. Thus, the strain accumulating within the lithium niobate film is easily relaxed; therefore, it is possible to suppress occurrences of cracks accompanying increases in film thickness. 本发明提供种带介电薄膜的基板