Notch filter

1. 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 two 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 G IR is within one of two ranges: 0.1λ≤G IR <0.5λ or 0.5λ<G IR ≤0.9λ, where λ is a wavelength determined by an electrode finger pitch of the interdigital transducer electrode, and the IR gap G IR is a distance between electrode finger centers of an electrode finger of the interdigital transducer electrode closest to at least one of the reflectors, of the electrode fingers of the interdigital transducer electrode, and an electrode finger of the at least one reflector closest to the interdigital transducer electrode, of the electrode fingers of the at least one reflector.

2. The notch filter according to claim 1 , wherein the IR gap G IR is within one of two ranges: 0.4λ≤G IR <0.5λ or 0.5λ<G IR ≤0.6λ.

3. The notch filter according to claim 1 , wherein the IR gap G IR is within one of two ranges: 0.2λ≤G IR ≤0.4λ or 0.6λ≤G IR ≤0.8λ.

4. The notch filter according to claim 1 , wherein number of the electrode fingers of the at least one reflector is 21 or less.

5. The notch filter according to claim 1 , wherein 0.91λ≤λ R ≤5λ is satisfied, where λ R is a wavelength determined by an electrode finger pitch of the at least one reflector.

6. The notch filter according to claim 1 , wherein

the high-acoustic-velocity member includes a supporting substrate and a high-acoustic-velocity film provided on the supporting substrate; and

an acoustic velocity of a bulk wave propagating in the high-acoustic-velocity film is higher than an acoustic velocity of an acoustic wave propagating in the piezoelectric thin film.

7. The notch filter according to claim 1 , wherein the piezoelectric thin film includes LiTaO 3 .

8. The notch filter according to claim 1 , wherein the low-acoustic-velocity film includes SiO 2 .

9. The notch filter according to claim 1 , wherein the high-acoustic-velocity member is a high-acoustic-velocity substrate including Si.

10. The notch filter according to claim 1 , wherein

the interdigital transducer electrode and the reflectors define a first acoustic wave resonator; and

the first acoustic wave resonator is a one-port acoustic wave resonator.

11. The notch filter according to claim 10 , further comprising:

additional interdigital transducer electrodes that define a second acoustic wave resonator and a third acoustic wave resonator; wherein

the first acoustic wave resonator and the second acoustic wave resonator are electrically connected to each other in series; and

the third acoustic wave resonator is electrically connected between a ground potential and a connection point between the first acoustic wave resonator and the second acoustic wave resonator.

12. The notch filter according to claim 11 , wherein an inductor is electrically connected in parallel with the third acoustic wave resonator.

13. The notch filter according to claim 1 , wherein the interdigital transducer electrode and the reflectors are defined by a multilayer metal film in which a plurality of metal layers are stacked or by a single layer metal film.