Photoacoustic microscope objective lens unit and photoacoustic microscope including the same
A photoacoustic microscope objective lens unit includes: an objective lens which irradiates a sample with excitation light L; a photoacoustic wave detection unit which detects a photoacoustic wave U generated from the sample; and a photoacoustic wave guide system. The photoacoustic wave guide system includes: a photoacoustic wave separation member; and an acoustic lens that is disposed between the photoacoustic wave separation member and the sample and has a focus position that substantially matches with a focus position of the objective lens. The acoustic lens is obtained by cementing a main acoustic lens and a correction acoustic lens to each other. The main acoustic lens and the correction acoustic lens satisfy predetermined Conditional Expressions.
1. A photoacoustic microscope objective lens unit, comprising:
an objective lens which irradiates a sample with excitation light having an absorption wavelength band of an object to be observed;
a photoacoustic wave detection unit which detects a photoacoustic wave generated from the sample through irradiation of the excitation light; and
a photoacoustic wave guide system which guides the photoacoustic wave to the photoacoustic wave detection unit, wherein
the photoacoustic wave guide system comprises:
a photoacoustic wave separation member which transmits the excitation light traveling from the objective lens and reflects the photoacoustic wave generated from the sample; and
an acoustic lens that is disposed between the photoacoustic wave separation member and the sample and has a focus position that substantially matches with a focus position of the objective lens,
the acoustic lens is formed by cementing a main acoustic lens to a correction acoustic lens, the main acoustic lens being disposed between the photoacoustic wave separation member and the correction acoustic lens and the correction acoustic lens being disposed between the sample and the main acoustic lens to suppress reflection of the photoacoustic wave, and
the main acoustic lens and the correction acoustic lens satisfy the following Conditional Expressions (1), (2) and (3):
0.8< T 0≤1.0 (1)
Z 2< Z 0< Z 1 (2)
s×d <( Z 2− Z 1) 2 /( Z 2+ Z 1) 2 (3)
where
T0 is a transmittance of the correction acoustic lens with respect to the excitation light on an optical axis,
Z0 is acoustic impedance of a medium of the correction acoustic lens,
Z1 is acoustic impedance of a medium of the main acoustic lens,
Z2 is acoustic impedance of a medium that is brought into contact with the correction acoustic lens on the sample side during detection of the photoacoustic wave;
s is a scattering coefficient of the medium of the correction acoustic lens with respect to the excitation light, and
d is a thickness of the correction acoustic lens on an optical axis.
2. The photoacoustic microscope objective lens unit according to claim 1 , wherein the following Conditional Expression (2-1) is satisfied:
1.5×10 6 (kg/(m 2 s))< Z 0< Z 1 (2-1)
where
Z0 is the acoustic impedance of the medium of the correction acoustic lens, and
Z1 is the acoustic impedance of the medium of the main acoustic lens.
3. The photoacoustic microscope objective lens unit according to claim 1 , wherein the medium of the main acoustic lens comprises glass.
4. The photoacoustic microscope objective lens unit according to claim 1 , wherein the correction acoustic lens satisfies the following Conditional Expression (4):
α× d <( Z 2− Z 1) 2 /( Z 2+ Z 1) 2 (4)
where a is an absorption coefficient of the medium of the correction acoustic lens with respect to the excitation light, d is the thickness of the correction acoustic lens on an optical axis, and Z1 is the acoustic impedance of the medium of the main acoustic lens, and Z2 is the acoustic impedance of the medium that is brought into contact with the correction acoustic lens on the sample side during detection of the photoacoustic wave.
5. The photoacoustic microscope objective lens unit according to claim 1 , wherein the correction acoustic Ions satisfies the following Conditional Expression (5):
1.0 <d/λ (5)
where d is the thickness of the correction acoustic lens on the optical axis, and X is the wavelength of the photoacoustic wave in air.
6. The photoacoustic microscope objective lens unit according to claim 1 , wherein a difference between the thickness of the correction acoustic lens on the optical axis and the thickness of the correction acoustic lens at a peripheral part is equal to or larger than a half of a wavelength X of the photoacoustic wave in air.
7. The photoacoustic microscope objective lens unit according to claim 1 , wherein a surface of the correction acoustic lens on the sample side is a concave surface or a flat surface.
8. The photoacoustic microscope objective lens unit according to claim 1 , wherein the correction acoustic lens satisfies at least one of the following Conditional Expression (6-1) and Conditional Expression (6-2):
| n 0− n 2|<0.1 (6-1)
| n 0− n 1|<0.1 (6-2)
where
n0 is a refractive index of the medium of the correction acoustic lens with respect to the excitation light,
n1 is a refractive index of the medium of the main acoustic lens with respect to the excitation light, and
n2 is a refractive index of a medium that is brought into contact with the correction acoustic lens on the sample side with respect to the excitation light during detection of the photoacoustic wave.
9. The photoacoustic microscope objective lens unit according to claim 1 , wherein the correction acoustic lens is formed of a material obtained by solidifying water with a coagulant into a gel.
10. A photoacoustic microscope, comprising:
the photoacoustic microscope objective lens unit according to claim 1 ; and
a scanning unit which scans the sample with the excitation light while moving an irradiation position of the excitation light.