Optical element and optical apparatus

1. An optical element switchable to an electro-inactive and an electro-active state, the optical element comprising:

a first material; and

a second material,

wherein each of optical properties of the first and second materials changes between the electro-inactive state and the electro-active state,

wherein conditions below are satisfied:

n 1 <n 2, and

v 1 <v 2

where n1 is a refractive index of the first material in the electro-active state, v1 is Abbe's number of the first material in the electro-active state, n2 is a refractive index of the second material in the electro-active state, and v2 is Abbe's number of the second material in the electro-active state, and

wherein a condition below is satisfied:

Δ n 1/Δ v 1<Δ n 2/Δ v 2

where Δn1 and Δn2 are respectively variations of the refractive indices of the first and second materials when the electro-inactive state is changed to the electro-active state, and Δv1 and Δv2 are respectively variations of the Abbe's numbers of the first and second materials when the electro-inactive state is changed to the electro-active state.

2. The optical element according to claim 1 , wherein:

each of the refractive index of the first material and the Abbe's number of the first material changes between the electro-inactive state and the electro-active state, and

each of the refractive index of the second material and the Abbe's number of the second material changes between the electro-inactive state and the electro-active state.

3. The optical element according to claim 1 , wherein at least one of surfaces of each of the first material and the second material is a relief surface.

4. The optical element according to claim 1 , wherein the variation Δn1 of the refractive index of the first material, the variation Δn2 of the refractive index of the second material, the variation Δv1 of the Abbe's number of the first material, and the variation Δv2 of the Abbe's number of the second material satisfy a condition below:

(Δ n 1/Δ v 1)/(Δ n 2/Δ v 2)>1.5.

5. The optical element according to claim 1 , wherein each of the first and second materials has a birefringence index not less than 0.08.

6. The optical element according to claim 1 , wherein in the electro-active state, the refractive index n1 of the first material and the refractive index n2 of the second material satisfy a condition below:

0.006<( n 2− n 1)<0.06.

7. The optical element according to claim 1 , wherein a difference between the refractive indices of the first and second materials in the electro-inactive state is smaller than a difference between the refractive indices of the first and second materials in the electro-active state.

8. The optical element according to claim 1 , wherein a difference between the Abbe's numbers of the first and second materials in the electro-inactive state is smaller than a difference between the Abbe's numbers of the first and second materials in the electro-active state.

9. The optical element according to claim 1 , wherein each of the first and second materials is a cholesteric liquid crystal or a nematic liquid crystal including a chiral twist agent.

10. The optical element according to claim 1 , further comprising:

a pair of first transparent electrodes provided to sandwich the first and second materials; and

a pair of substrates provided to sandwich the pair of first transparent electrodes.

11. The optical element according to claim 10 , wherein a surface located closer to the pair of substrates of surfaces of each of the first and second materials is a relief surface.

12. The optical element according to claim 1 , further comprising a second transparent electrode provided between the first and second materials.

13. The optical element according to claim 1 , further comprising a substrate provided between the first and second materials,

wherein a surface located closer to the substrate of surfaces of each of the first and second materials is a relief surface.

14. The optical element according to claim 1 , wherein the optical element is a liquid crystal diffraction lens having a variable focusing function.

15. An optical apparatus comprising:

the optical element according to claim 1 ; and

a controller configured to set the optical element to be in the electro-inactive stateor the electro-active state.

16. An optical element switchable to an electro-inactive state and an electro-active state, the optical element comprising:

a first material; and

a second material,

wherein each of optical properties of the first and second materials changes between the electro-inactive state and the electro-active state,

wherein conditions below are satisfied:

n 1< n 2, and

v 1< v 2

where n1 is a refractive index of the first material in the electro-active state, v1 is Abbe's number of the first material in the electro-active state, n2 is a refractive index of the second material in the electro-active state, and v2 is Abbe's number of the second material in the electro-active state, and

wherein a difference between the refractive indices of the first and second materials in the electro-inactive state is smaller than a difference between the refractive indices of the first and second materials in the electro-active state.

17. An optical element switchable to an electro-inactive state and an electro-active state, the optical element comprising:

a first material; and

a second material,

wherein each of optical properties of the first and second materials changes between the electro-inactive state and the electro-active state,

wherein conditions below are satisfied:

n 1< n 2, and

v 1< v 2

where n1 is a refractive index of the first material in the electro-active state, v1 is Abbe's number of the first material in the electro-active state, n2 is a refractive index of the second material in the electro-active state, and v2 is Abbe's number of the second material in the electro-active state, and

wherein a difference between the Abbe's numbers of the first and second materials in the electro-inactive state is smaller than a difference between the Abbe's numbers of the first and second materials in the electro-active state.