JP2947971B2 - Laser trapping method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser trapping method and apparatus for three-dimensionally capturing (trapping) fine particles by using a laser beam, and more particularly to enhancing a trapping force in a direction of an optical axis of a light source. The present invention relates to a laser trapping method and apparatus.

[0002]

2. Description of the Related Art Continuously oscillating laser light is guided into a microscope, condensed by an objective lens, and small particles are guided near the focal point. The particles are attracted and trapped near the focal point. Is done. This laser trapping method is used for non-contact, non-destructive manipulation such as capture and transfer of living cells and microorganisms (Satoichi Sato,
Fumio Inaba: Optics, Vol.19, No.8 (1990) 513-
514). Specifically, in the field of chemistry, those used for capturing a target of laser ablation of fine particles or (H.
Misawa et al. : Chem. Lett. 19
90, 1479. ), In the field of biology, manipulation of living cells (A. Ashkin et al .: Nature, 33).
0,769 (1987)) and those used for moving the target in cell fusion (RW Steubing).
et al. : SPIE, 1202, 272 (199
0)) and the like are known.

[0003] Incidentally, all of the above are operations mainly in a plane (xy) perpendicular to the optical axis direction, and the operation in the optical axis direction (z direction) where the light source is located is not performed.
The trapping force in the z-direction appears to play only an auxiliary role in manipulating in the xy plane. This is mainly because the trapping force in the z direction is smaller than the trapping force in the xy direction. If the trapping force in the z direction is large, various operations are performed in the z direction as in the xy direction. Can be performed freely, and the operation in the xy direction can be performed stably and quickly. Therefore, it is strongly desired to increase the trapping force in the z direction.

[0004] The magnitude of the force in the z direction in laser trapping is experimentally determined using Stokes' law or the like. For example, light traps spherical particles, such as polystyrene latex spheres, and moves the particles or the surrounding medium (eg, water) to measure the speed of movement of the particles or the medium as the particles move out of the trap. Is calculated. From such experiments, it is concluded that the trapping force in the z direction is qualitatively proportional to the incident laser power, increases as the refractive index difference between the fine particles and the surrounding medium increases, and increases as the particle size increases. And these conclusions are common in the art.

[0005] However, in connection with the above, if a high-power laser is used, a large absorption in particles or a medium is caused, the convection and Brownian motion are increased due to a rise in temperature, and the scattering and transmission of laser light are increased. In addition to the difficulty of processing light and scattered light, when the target is a biological sample, there is a problem of damage or destruction due to strong heat or chemical action.

In connection with the above, according to the analysis by the present inventors, it has been obtained that such conclusion cannot always be reached. As will be described later, as a result of the analysis, it has been concluded that depending on the conventional method, if the relative refractive index is 2.0 or more, a positive trapping force does not work in the z direction. . The reasons for the above-mentioned theory are that there are many points that have not been elucidated regarding the radiation pressure of light, that the relationship with the gradient force of light based on the intensity distribution of light is not clear, and that the fine particles are usually in the surrounding medium. However, the behavior of fine particles in a liquid in relation to the radiation pressure of light is unknown.

Further, in the conventional method, in a medium (for example, water), a trapping force works in the z direction.
In a vacuum or when the medium is air, whether the trapping force in the z-direction works on the fine particles and stably traps the particles, and, in relation to gravity, opposes the particles in the direction opposite to the light irradiation direction against the gravity. Whether it can be lifted or not is currently unclear.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to increase the force acting in the direction of an optical axis (z direction) of a light source in laser trapping.

[0009]

The present invention is based on the results of an analysis performed by the present inventors for the first time, not on an experimental basis.
A fundamental feature is that a conical cylindrical laser beam is incident on the fine particles and / or a P-polarized laser beam whose S component of polarized light is reduced is used.

[0010]

According to the first aspect of the present invention, since only the light beam incident on the fine particles at a large angle with respect to the optical axis direction and there is no light beam whose angle with the optical axis direction is from zero to a certain value, it is mainly reflected. As a result, the force pushing the particles in the direction of the optical axis is annihilated, and as a result, the force in the direction of the optical axis increases.

In the second aspect of the present invention, since the S-polarized light having a higher reflectance than the P-polarized light among the light irradiated to the fine particles is reduced, the force for pushing the particles in the anti-optical axis direction by reflection is obtained. On the other hand, while the P-polarized light transmitted through the inside of the particle remains unchanged, the force offset by the reflection becomes apparent, and the force in the optical axis direction increases.

In the combination of the first invention and the second invention, the above functions work synergistically.
The force in the optical axis direction becomes extremely large.

[0013]

FIG. 1 is a sectional view of an optical path for explaining an embodiment of the present invention. On the optical axis 1, a first axicon prism 2, a second axicon prism 3, and a focusing lens 4 are arranged. The laser beam 5 traveling on the optical axis 1 is incident on the plane 21 of the first axicon prism 2, travels straight inside, and exits from the conical surface 22 when the two beams 51 and 52 have a constant refraction angle. Is separated into Luminous flux 51 and 5
The light 2 enters the conical surface 32 of the second axicon prism 3, is refracted at the same constant refraction angle, travels straight inside the optical axis 1, and exits from the plane 31. The outgoing light beam 53 has a cylindrical shape, and its cross section perpendicular to the optical axis is an annular zone as shown in FIG.

When the light beam 53 enters the focusing lens 4, the focusing lens 4 focuses the cylindrical light beam 53 as a spot 54 at the focal position. When the fine particles 6 are guided near the spot 54, the collected particles 6 move so as to be drawn into the spot 54 and stop at a certain position.

Since only the laser beam 54 that forms a large angle with respect to the optical axis 5 is incident on the fine particles 6, a large trapping force is generated in the optical axis direction.

In the embodiment shown in FIG. 1, the first axicon prism 2 and the second axicon prism 3 each have a cone 22 and 32 whose apex angle is the Brewster angle θb. I have. Therefore, a part of the S-polarized light component of the polarized light component of the laser light flux 1 is reflected by the conical surface 22 and emitted out of the optical path, and the remaining S-light fluxes 51 and 52 remain.
The polarization component is also reflected by the conical surface 32 and emitted out of the optical path, and the light beam 53 is a light having a strong P polarization component. In FIG. 2, the polarization direction of P-polarized light (the vibration direction of the electric field) is indicated by an arrow. In this way, by causing the P-polarized laser beam 54 to enter the fine particles 6, the reflection on the particle surface can be reduced. This is based on the fact that the intensity reflectance in Fresnel's formula is different between P-polarized light and S-polarized light. As the reflection at the particle surface decreases, the surface that penetrates inside the particle increases relatively. A decrease in the reflected light results in a decrease in the force for pushing the particles in the light incident direction, and an increase in light transmitted through the inside of the particles results in an increase in the radiation pressure generated in the particles. in this way,
By making the P-polarized light incident on the fine particles 6, the trapping force generated in the optical axis direction can be additively increased.

When a laser beam having a strong P polarization is used, as shown in FIG.
Is not required to be a conical cylindrical shape, and a conical light beam can be used similarly to the conventional method, whereby the trapping force in the optical axis direction can be sufficiently increased.

In the embodiment shown in FIG. 1, two axicon prisms are used to make the cross section of the light beam 53 into a ring shape or an annular shape. A band iris can be used. It is best to provide it on the pupil plane of the optical system. Further, instead of the two axicon prisms, two axicon zone plates may be used, or a combination of one axicon prism and one axicon zone plate may be used. The use of a zone plate has the advantages that the optical system can be made compact and lightweight, and that it can be manufactured in large quantities at low cost by press molding or the like.

Each of the axicon prisms 2 and 3 shown in FIG. 1 has a positive conical surface, but it is also possible to use a negative one having a concave conical surface. Further, a combination of the positive and the negative may be used as a means for forming a light flux orbicular zone. Further, FIG. 1 shows a configuration in which the vertices of the cone face each other, but a configuration in which the vertices face the outward-facing plane may be adopted. In this case, we eliminate the plane,
Although it is possible to form an integral body, which has an advantage in the configuration of the optical system, there is a drawback that the degree of freedom is restricted when the diameter of the ring zone is desired to be variable. In a configuration in which two axicon elements are combined, it is possible to easily change the diameter of the orbicular zone by changing the distance between them. The axicon element includes an axicon diffraction grating.

In the embodiment shown in FIG. 1, an axicon prism whose apex angle is set to the Brewster angle is used in order to obtain a laser beam having strong P polarization, but the S polarization component is almost completely eliminated. In order to do so, a polarizing beam splitter, as shown in FIG. 3, in which triangular prisms 8 and 9 having a high refractive index are sandwiched between transparent dielectric thin films 7 may be used. In FIG. 3, the symbols P and S indicate the polarization components P and S, and indicate that the S polarization component is almost completely deflected at the boundary surface in a direction perpendicular to the traveling direction of light.

Separately, positive and negative axicon prisms 10 and 11 having a Brewster angle at the apex angle are provided.
As shown in FIG. 4, a thin film 12 may be used with the thin film 12 interposed therebetween. If the refractive index of the material of this prism is large, as in the case of the polarization beam splitter, the extinction ratio can be increased accordingly.

Next, the outline of the analysis by the present inventors and the results that led to the construction of the above embodiment will be described.

Although the calculation method is based on geometrical optics approximation,
A ray tracing method that follows Fresnel's formula and Snell's law, which is not an approximation, is applied. The calculations assume that the laser spot is sufficiently small compared to the particle, the particle is sufficiently large compared to the laser wavelength, the effects of interference from scattered light are eliminated, surface waves are not considered on the surface, The light is ideal uniform convergent light or annular convergent light,
The particles are assumed to be ideally spherical and have a uniform refractive index.

As shown in FIG. 5, the spot position of the incident light I on the spherical particle R having a radius r is defined as the origin F, the distance l from the origin F to the center O of the particle, and the force in the direction of the optical axis AX is positive. I do. As shown in FIG. 6, the magnitude of the intensity reflectance and the magnitude of the intensity transmittance at each boundary surface determine in which direction and how much force the particles R are pushed. By repeating about 1000 times, each component of force was integrated for each direction. FIG. 7 shows that the relative refractive index of the particles R is n ₁ =
A force (× 10 ⁻¹² N) acting on a particle R at a particle position 1 (unit is r) with a laser power of 1 mW at 1.2
3 characteristic curves are shown. The solid curve represents the numerical aperture NA.
Is from 0 to 0.95 (conical luminous flux), and the broken curve is NA
Is 0.94 to 0.95 (conical cylindrical light beam), and the dashed line curve shows the case where NA is 0.94 to 0.95 (conical cylindrical light beam) and 100% P-polarized light is used.

FIG. 8 shows that the relative refractive index of the particles R is n ₂ = 2.0.
At 1 mW laser (λ = 1.6 μm)
Force acting on particle R in (unit is r) (× 10
^-12 N) are shown. The solid line curve indicates that the numerical aperture NA is 0 to 0.95 (conical luminous flux),
The dashed curve has an NA of 0.94 to 0.95 (conical cylindrical light beam), and the dashed line curve has an NA of 0.94 to 0.95.
(Conical cylindrical light beam) and 100% P-polarized light,
This corresponds to the characteristic in FIG.

From the curve C1, it can be seen that when the relative refractive index is as high as 2.0, a positive force does not work in the optical axis direction in the conventional method. This means that no trapping force is generated even if the power is 100 mW. From both figures, it can be seen that the P-polarized light method works strongly when the relative refractive index is high.

9 and 10 show trapping characteristics in air, and FIG. 9 is for glass beads having an absolute refractive index n ₀₃ = 1.6, and the conditions are the same except that the range of the numerical aperture NA is different. It is the same as FIG. The characteristics in FIG. 10 are for silicon beads having an absolute refractive index n ₀₄ = 3.4710, and show the characteristics under the same conditions as those given for obtaining the characteristics in FIG.

From the curve C2, according to the U.S.A. method at this numerical aperture, a positive force does not act in the optical axis direction.
It can be seen that glass beads cannot be trapped in the direction of the optical axis, and cannot be lifted upward against gravity, of course. From the curves C3 and C4, it can be seen that, when trying to lift silicon particles having a large refractive index in the air, the P-polarized light technique and the annular convergence technique must be used together. It is clear from the characteristics of these figures that the P-polarized light method works extremely effectively for trapping in the air.

FIG. 11 shows the trapping force characteristics of the optical glass BK7 having an absolute refractive index n ₀₅ = 1.5.
The peak height in the case of P polarization and NA of 0.94 to 0.94 is 2.01 × 10 ⁻¹² N / mW. BK7 has a specific gravity of 2.51 g / cm ³ , a particle diameter r of 1 μm,
When changed to 10 μm and 100 μm, the gravity acting on the spherical beads is 12.6 × 10
⁻¹⁵ N, 12.6 × 10 ⁻¹² N 12.6 × 10 ⁻⁹
N. Under the above-mentioned conditions, in order to make 10 μm particles of BK7 levitate against the gravity, it is required to have a particle diameter of about 10 mW.
It can be seen that a laser power of about 10 W is required for a laser of 0 μm. Conversely, if a laser power of 10 W is used, BK7 beads having a particle size of several tens of μm can be lifted in a direction opposite to the light irradiation direction against the gravity by laser trapping in air. You can.

FIG. 12 shows the above results.
1 shows a schematic cross-sectional view of a laser trapping device of a size that can be held in a hand. The configuration is the same as that shown in FIG.
As the light source, a commercially available semiconductor laser 70 having a CW oscillation output of 10 W is used. Semiconductor laser 70, axicon prism 2,
3. The converging lens 4 is fixed to the lens barrel 80, and power is supplied to the semiconductor laser 70 via the cable 90. Spherical, elliptical, flat, and rod-shaped particles 61, 62, 63, and 64 having a particle size of several tens of μm on the surface 60 in the air are freely lifted into the air and trapped at arbitrary positions. Can be.

[0031]

As described above, according to the present invention, the trapping force in the optical axis direction can be increased without increasing the intensity of the laser beam applied to the particles, and the operation of moving the particles in the xy directions can be performed stably and quickly. At the same time, the moving operation in the optical axis direction can be stably performed.

While it has been found that the high refractive index particles cannot be captured in the optical axis direction by the conventional method, according to the present invention, even the high refractive index particles generate a force in the optical axis direction to capture the force. In addition, a trapping force in the optical axis direction can be generated even in a non-medium environment in which particles are not placed in a liquid, and laser trapping of fine particles in a vacuum or in the air is possible. .

[Brief description of the drawings]

FIG. 1 is an optical path cross-sectional view illustrating an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a cross section perpendicular to the optical axis of a laser beam and a polarization direction.

FIG. 3 is a sectional view of a polarizing beam splitter.

FIG. 4 is a sectional view of a polarization beam splitter using an axicon prism.

FIG. 5 is an explanatory diagram provided for analysis of ray tracing according to the present invention.

FIG. 6 is an explanatory diagram of reflection and transmission by fine particles.

FIG. 7 is a graph showing the magnitude of a trapping force at a particle position 1;

FIG. 8 is a graph showing the magnitude of a trapping force at a particle position 1;

FIG. 9 is a graph showing the magnitude of a trapping force at a particle position 1;

FIG. 10 is a graph showing the magnitude of a trapping force at a particle position 1;

FIG. 11 is a graph showing the magnitude of a trapping force at a particle position 1;

FIG. 12 is an explanatory diagram of a portable laser trapping device that can be operated in the air.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical axis 2, 3 ... Axicon prism 4 ... Focusing lens 5 ... Laser beam 54 ... Conical cylindrical laser beam 6 ... Fine particle 7 ... Semiconductor laser θb ... Brewster angle

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) G02B 21/32 G02B 21/00

Claims (11)

(57) [Claims] 1. A laser trapping method, wherein a laser beam having a conical cylindrical shape is incident on particles in a vacuum or a medium, and a force in a direction opposite to a light irradiation direction generated on the particles is increased. 2. A laser beam having a reduced S component of polarized light is focused, and the focused laser beam is radiated to a particle in a vacuum or a medium to increase a force generated in the particle in a direction opposite to a light irradiation direction. A laser trapping method, comprising: 3. The laser trapping method according to claim 2, wherein the focused laser beam has a conical shape. 4. The focused laser beam has a conical cylindrical shape.
3. A method for laser trapping according to claim 2. 5. An optical device which irradiates a focused laser beam to a particle in a vacuum or a medium to generate a radiation pressure of light on the particle so that the particle can be trapped in a space. A laser trapping device for converting a laser beam into a beam having a ring shape or an annular shape in a cross section perpendicular to the optical axis. 6. A laser trapping apparatus according to claim 5, wherein said light beam converting means comprises a single or a plurality of axicon elements. 7. The laser trapping device according to claim 6, wherein the axicon element is an axicon zone plate. 8. The laser trapping device according to claim 6, wherein said axicon element is a conical prism. 9. The light beam converting means is a combination of two conical prisms, and is disposed in front of a condenser lens for emitting a focused laser beam toward particles. Laser trapping equipment. 10. The laser trapping apparatus according to claim 8, wherein the conical prism has a vertex angle of Brewster angle. 11. An optical device which irradiates a focused laser beam to a particle in a vacuum or a medium to generate a radiation pressure of light on the particle so that the particle can be trapped in a space. A laser trapping device, comprising: a polarization optical means for transmitting substantially P-polarized light of a laser beam while emitting most of S-polarized light out of an optical path.