JP2714305B2 - Optical trap method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the technical field of an optical trap for optically trapping particles suspended in a medium.

[0002]

2. Description of the Related Art When a laser beam is focused on spherical fine particles, the momentum of light changes. At this time, radiation pressure (light pressure) acts on the fine particles according to the law of conservation of momentum. When a particle is transparent to the wavelength of the irradiating laser beam and its refractive index is higher than that of the surrounding medium, the particle is oriented in the direction of the focal point of the laser, and this force balances with an external force such as gravity. Then, the fine particles are trapped. Laser trapping (laser trapping) is a particle manipulation method utilizing this phenomenon.
is there. This laser trapping technique is expected to be applied in, for example, a medical field that handles cells and the like.

[0003]

However, if the particles have a lower refractive index than the surroundings or reflect or absorb light, the radiation pressure acts as a direction away from the laser beam, that is, as a repulsive force. In the configuration, the fine particles could not be trapped. For example, water, which is generally used as a solvent, has a low refractive index, so that it is difficult to trap water droplets in another liquid. Further, metal fine particles that reflect light, carbon fine particles that absorb light, and the like are also splashed by the laser beam and cannot be trapped.
Such a restriction is a major problem in applying laser trapping to microchemistry.

[0004] In order to trap fine particles having the above-mentioned properties, a condensed laser beam is scanned in an annular shape so as to surround the fine particles. There has been proposed a method that allows the user to be confined to a computer. [Symposia'91 (Tokyo) Abstract Report on Creative Science and Technology Research Report by New Technology Agency PART4 16]

[0005] However, this method requires a mechanism for constantly rotating the laser beam by a galvanometer mirror or the like, which is not preferable in terms of the complexity of the device configuration and durability. Further, it is necessary to appropriately adjust the radius of rotation of the laser beam depending on the specific gravity and size of the fine particles, and it is difficult to trap the particles with high stability. Furthermore, when it is desired to move and convey the fine particles trapped by the above method in a desired direction, a very complicated device configuration is required.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and has as its object to provide a method capable of optically trapping particles, which are affected by radiation pressure as a repulsive force, by a simple method.

[0007]

According to the optical trapping method of the present invention for solving the above-mentioned problems, a plurality of light beams having an intensity distribution are converged around particles whose radiation pressure acts as a repulsive force. It is characterized in that particles are optically trapped.

[0008]

【Example】

<Embodiment 1> FIG. 1 is a drawing for explaining a first embodiment of the present invention. A laser light source not shown (for example, H
A laser beam 1 having a Gaussian intensity distribution emitted from an e-Ne laser enters an optical element 2 having a beam splitting function. The optical element 2 has a thin cylindrical shape, one surface of which is cut obliquely in three directions around a central portion, and has a shape in which one surface of the cylinder is formed into a triangular pyramid. The laser beam incident by the action of the optical element 2 is divided into three light beams.
The light is condensed separately into 1, 1-2 and 1-3.

The right side of the drawing shows an enlarged view of the focal plane 4. The fine particles 5 have a lower refractive index than the surrounding medium or have a property of reflecting or absorbing light, that is, fine particles whose radiation pressure acts as a repulsive force. Three laser beams 1
The fine particles 5 at the position surrounded by the convergence positions of 1, 1-2 and 1-3 can be moved from the center by being pushed back because the radiation pressure of the laser beam acts as a repulsive force even if they move outward. Instead, it is trapped at a position surrounded by the laser beam.

<Embodiment 2> FIG. 2 is a drawing for explaining a second embodiment of the present invention. In this embodiment, fine particles can be transported by moving the laser beam. A laser beam emitted from a laser light source 6 enters a polarization conversion element 9 having a light splitting function via an optical pickup 8. The details of the polarization conversion element 9 are described in JP-A-3-191318.

The laser beam 7 incident on the polarization conversion element 9
Is separated into two waves of an S wave (reflection) and a P wave (transmission) by the polarization beam splitter 9-1. Reflected S
The optical axis of the wave is slightly bent by the wedge-shaped optical element 9-2. Then, after passing through the λ / 4 plate 9-3 and being reflected by the mirror 9-4, when passing through the λ / 4 plate 9-3 again, the phase changes by λ / 2 to become a P wave. This P wave transmits through the beam splitter 9-1, is reflected by the mirror 9-5, and emerges from the lower surface of the polarization conversion element 9. On the other hand, the P-wave that has entered the laser beam 7 and transmitted through the beam splitter 9-1 exits from the lower surface of the polarization conversion element 9 as it is. These two emitted light beams are collected by the lens 10 and collected on the focal plane as 7-1 and 7-2.

In such an optical configuration, when the optical pickup 8 is moved in the direction of an arrow perpendicular to the optical axis by a driving mechanism (not shown), the two laser beams 7-1 and 7-
2 moves as shown by the arrow 11-1. Then, along with this, the fine particles 5 can be moved as shown by the arrow 11-2 by pushing the fine particles by the repulsive force of the radiation pressure of the laser beam.

<Embodiment 3> FIG. 3 is a view for explaining a third embodiment of the present invention. The same reference numerals as those in FIG. 2 denote the same members. The difference from the configuration of FIG. 2 is that four beams are irradiated around the fine particles using two laser light sources. Two laser light sources 6-1 and 6-2 are arranged as shown in the figure, and four laser beams 6-1-1, 6-1-2 and 6-2-1 by an optical system similar to FIG. 6-2-2
Get. The fine particles 5 are trapped by these laser beams focused on the focal plane.

In this embodiment, the optical pickup 8 is moved in any direction perpendicular to the optical axis or in the optical axis direction in order to converge and irradiate the microparticles 5 with laser beams in four directions around the microparticles 5 and trap the microparticles inside the laser beam. By moving the convergence position of the laser beam three-dimensionally, the fine particles 5 can be transported in an arbitrary three-dimensional direction. Further, trapping and release of the fine particles can be freely performed by controlling on / off of the two laser light sources 6-1 and 6-2.

<Embodiment 4> Next, a fourth embodiment of the present invention will be described. In each of the embodiments described above, one fine particle is targeted. However, in this embodiment, two fine particles can be individually moved and, for example, two fine particles can be adhered and fused.

In order to divide one laser beam into four,
An optical element 10 having a quadrangular pyramid on one side as shown in FIG. 4 is prepared and configured similarly to FIG. Further, an optical pickup 8 is provided on the light source side on the optical axis. The optical pickup 8 can be moved in the optical axis direction. Another similar optical system is arranged so as to be symmetrical on the optical axis as shown in FIG. 4, and a total of two particles can be independently trapped and moved on the same optical axis.

By moving the two optical pickups 8 along the optical axis in this configuration, the laser convergence position can be moved in the optical axis direction, and the fine particles trapped therein can be moved. it can. FIG. 5 shows this state, in which the two particles are attached and fused by shifting from the state of (a) to the state of (b).

<Embodiment 5> Next, a fifth embodiment of the present invention will be described. In this embodiment, a four-divided optical shutter 11 is provided in front of a four-divided optical element 10 as shown in FIG. The optical shutter 11 is a PLZT optical shutter, has a circular shape divided radially into four so as to correspond to four surfaces of a quadrangular pyramid, and can turn on and off four beams on a focal plane independently. . Such an optical system is configured as shown in FIG.
And place them side by side. Each optical pickup 8 can be moved in a plane orthogonal to the optical axis.

FIG. 7 shows a control method in which two fine particles are attached to each other as a time-series change. In the state of (a), four beams are irradiated around each fine particle to move the fine particles so as to approach each other. When the two particles approach each other, the optical shutter 11
To turn off the irradiation of only the beam closer to the partner particle among the laser beams as shown in FIG. As a result, two fine particles can be stably adhered as shown in FIG.

[0020]

According to the present invention, the fine particles whose radiation pressure acts as a repulsive force can be optically trapped by a simple method.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a first embodiment.

FIG. 2 is a diagram for explaining a second embodiment.

FIG. 3 is a diagram for explaining a third embodiment.

FIG. 4 is a diagram for explaining a fourth embodiment.

FIG. 5 is a diagram for explaining a fourth embodiment.

FIG. 6 is a diagram for explaining a fifth embodiment.

FIG. 7 is a diagram for explaining a fifth embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser beam 2 optical element 3 lens 6 laser light source 8 optical pickup 9 polarization conversion element 10 optical element 11 four-part optical shutter

Claims (3)

(57) [Claims] 1. An optical trapping method, wherein a plurality of light beams having an intensity distribution are converged around a particle on which radiation pressure acts as a repulsive force to optically trap the particle. 2. The optical trapping method according to claim 1, wherein the trapped particles are conveyed by simultaneously moving the convergence positions of the plurality of light beams. 3. The optical trapping method according to claim 1, wherein the light beam is a laser beam.