Multilayer minus filter and fluorescence microscope

1. A multilayer minus filter for reflecting a light having at least one reflection wavelength λ 0 and transmitting a light having a wavelength other than the reflection wavelength λ 0 , the filter comprising:

a transparent substrate; and

a repetition layer formed on a surface of the substrate including a plurality of high refractive index layers having an average optical thickness H per layer and a plurality of low refractive index layers having an average optical thickness L per layer, the repetition layer having an alternative stack of high and low refractive index layers, and satisfying the following formulas:

1.1>( H+L )/λ 0 >0.9  (1)

2> H/L >0.5  (2)

wherein the repetition layer is formed by an ion assisted process technology, and

wherein a reflection band utilized with respect to a light incident obliquely on the surface of the filter, the reflection band appearing in reflection wavelength λ 0 with respect to a light incident vertically on the surface of the filter, utilizes a reflection band with respect to a light incident obliquely on the surface of the filter formed to be shifted on a short wavelength side.

2. The multilayer minus filter according to claim 1 , wherein the repetition layer is formed such that average optical thickness of the high refractive index layer is from 0.51λ 0 to 0.62λ 0 and such that average optical thickness of the low refractive index layer is from 0.38λ 0 to 0.49λ 0 .

3. The multilayer minus filter according to claim 1 , wherein the repetition layer is formed such that average optical thickness of the high refractive index layer is from 0.39λ 0 to 0.48λ 0 and such that average optical thickness of the low refractive index layer is from 0.52λ 0 to 0.62λ 0 .

4. A fluorescence microscope comprising the multilayer minus filter according to claim 1 .

5. The fluorescence microscope according to claim 4 , further comprising a laser as an excitation light source.

6. The multilayer minus filter according to claim 1 , wherein an incident angle of the obliquely incident light is used as 45°, the filter including the repetition layer formed such that average optical thickness of the high refractive index layer is from 0.49λ 0 to 0.63λ 0 and such that average optical thickness of the low refractive index layer is from 0.37λ 0 to 0.51λ 0 .

7. The multilayer minus filter according to claim 1 , wherein an incident angle of the obliquely incident light is used as 45°, the filter including the repetition layer formed such that average optical thickness of the high refractive index layer is from 0.32λ 0 to 0.47λ 0 and such that average optical thickness of the low refractive index layer is from 0.53λ 0 to 0.68λ 0 .

8. The multilayer minus filter according to claim 1 , wherein the reflection wavelength is selected as a wavelength of a laser light of a laser light source, and only a laser light is reflected.

9. The multilayer minus filter according to claim 1 , wherein the multilayer minus filter is a shiftless filter having a small characteristic change caused by an ambient environment, a high refractive index film material configuring the multilayer minus filter is Ta 2 O 5 , TiO 2 , Nb 2 O 5 , HfO 2 , Al 2 O 3 , or a mixture including them, and a low refractive index film material is SiO 2 or a mixture including it.

10. The multilayer minus filter according to claim 9 , wherein the transparent substrate is made of a quartz glass or a glass having a linear expansion coefficient of 0.8×10 −6 or less.

11. The multilayer minus filter according to claim 1 , wherein the transparent substrate is heated while applying the ion assisted process technology.

12. The multilayer minus filter according to claim 11 , wherein a temperature of the substrate exceeds 200° C. while applying the ion assisted process technology.