Notch filter

A notch filter comprising:

• a substrate having piezoelectricity, the substrate including a high-acoustic-velocity member, a low-acoustic-velocity film provided on the high-acoustic-velocity member, and a piezoelectric thin film provided on the low-acoustic-velocity film;
• an interdigital transducer electrode provided on the piezoelectric thin film of the substrate having piezoelectricity;
• and
• reflectors provided on both sides of the interdigital transducer electrode in an acoustic wave propagation direction on the piezoelectric thin film of the substrate having piezoelectricity;
• wherein
• an acoustic velocity of a bulk wave propagating in the low-acoustic-velocity film is lower than an acoustic velocity of an acoustic wave propagating in the piezoelectric thin film;
• an acoustic velocity of a bulk wave propagating in the high-acoustic-velocity member is higher than the acoustic velocity of an acoustic wave propagating in the piezoelectric thin film;
• the interdigital transducer electrode and the reflectors each include electrode fingers;
• and
• an IR gap GIR is within one of two ranges:
• 0.1λ≤GOR<0.5λ or 0.5λ<GIR≤0.9λ, where λ is a wavelength determined by an electrode finger pitch of the interdigital transducer electrode, and the IR gap GIR is a distance between electrode finger centers of an electrode finger of the interdigital transducer electrode closest to the reflector, of the electrode fingers of the interdigital transducer electrode, and an electrode finger of the reflector closest to the interdigital transducer electrode, of the electrode fingers of the reflector.