JPH04334544A - Laser trapping method - Google Patents

Abstract

PURPOSE:To certainly trap even fine particles with a low refractive index or light reflective fine particles such as metal fine particles by subjecting condensed laser beam to rotary operation at a high speed to catch fine particles or a fine particle group. CONSTITUTION:When condensed laser beam is repeatedly subjected to rotary operation in the shape matched with the shape or gathering of an object to be caught such as a circle, a dark part wherein the bottom surfaces of two cones are bonded is formed inside the operated beam. When fine particles enter said part, they receive repulsive force and are caught at the position where the resultant force of repulsive forces received in all of directions is well- balanced with external force such as gravity. In order to confirm this laser trapping phenomenon, the experimental system shown by drawing is used and trapping laser beam is emitted from a laser beam source 1 to be matched with the number of apertures and focal position of a microscopic optical system. Laser beam is condensed to a sample within a microscope by an oil-immersion lens 5 and the state of trapping is formed into an image on a CCD camera 7 to be observed on a monitor 8.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a laser trapping method. More specifically, the present invention provides a new method for a laser top that can optically trap light-reflecting particles such as low refractive index particles or metal particles, and process and modify these particles. This is related to the processing and modification of fine particles using this technology.

0002

[Prior Art and its Problems] Laser trapping is a technique proposed by Ashkin in 1970 that traps microparticles on the order of micrometers using the radiation pressure of light. In this laser trapping, by narrowing down the laser beam to the wavelength order, it is possible to lift particles against gravity and capture them three-dimensionally. By scanning the beam and moving the sample stage, it is possible to capture only the target particles. It has the feature that non-contact manipulation is possible. Therefore, studies are actively being conducted on its applications in the fields of biology and chemistry, and applications such as manipulation of biological cells, cell sorters, and microsurgery have been reported. The inventor of the present invention is also attempting to apply the invention to microchemistry such as laser ablation of polymer latex.

However, such conventional laser trapping has the limitation that it can only trap fine particles that have a higher refractive index than the surrounding medium and that do not absorb laser light at all. For example, water, which is often used as a solvent, has a low refractive index, making it difficult to trap water droplets. Furthermore, fine metal particles and fine particles made of polymer latex coated with metal cannot be trapped because they reflect laser light, but are instead repelled by the laser beam. The reason is that for these particles, the radiation force acts in a direction away from the laser beam.

[0004] That is, the laser beam is scattered by fine particles, the direction of the wave number vector changes, and the momentum of the photons changes in proportion to the direction. At this time, force (radiation pressure) acts on the particle due to the law of conservation of momentum. When the refractive index of the particles is higher than that of the surrounding medium, the force is directed toward the laser focal point, so the particles are attracted and captured near the focal spot. However, as shown in FIG. 1, for example, in the case of a fine particle whose refractive index is lower than that of its surroundings, the direction is reversed and a force acts in a direction that pushes it away from the laser beam. Therefore, this optical system cannot capture particles with a single beam.

Similarly, FIG. 2 shows the radiation force for a particle that completely reflects laser light. The radiation force of one photon is directed perpendicularly to the reflecting surface, that is, in this case, toward the center of the fine particle, and a force acts on the laser beam as a whole from the direction of high light intensity to the direction of low light intensity. Therefore, in this case as well, a phenomenon occurs in which fine particles cannot be captured and are repelled from the beam.

The laser trapping method is a unique method of light trapping of fine particles, and it can trap not only organic polymer particles but also biological cells, inorganic substances, etc.
This state is extremely useful as a method that enables microfabrication and chemical modification. However, as described above, the conventional methods cannot trap low-refractive-index particles or light-reflecting particles such as metals, so there is a natural limit to the expansion of their applications.

[0007] For this reason, the laser trapping method is applied to various types of fine particles in a wider range of fields.
It has been desired to realize new means that enable fine processing, modification, etc. of these fine particles. This invention was made in view of the above-mentioned circumstances, and is a new method that improves the shortcomings of conventional methods and is capable of optically trapping even low-refractive-index particles or light-reflecting particles such as metals. The purpose is to provide a laser trapping method.

[0008]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides a laser trapping method characterized in that fine particles or groups of fine particles are captured by scanning a condensed laser beam in a circular motion at high speed. I will provide a. The present invention also provides methods for processing and modifying the captured fine particles, as well as for forming patterns, transporting them, and the like.

That is, the present invention proposes a method of creating a so-called optical capsule by scanning a condensed laser beam circularly or the like at high speed, and trapping fine particles three-dimensionally within the capsule. Using this method not only expands the range of applications of laser trapping, but also eliminates the possibility that particles other than the ones you want to trap will be attracted by the radiation pressure, unlike conventional laser trapping. It is also advantageous for spectroscopy of single particles.

The method of this invention and its principle are shown in FIG.
As shown in , a focused laser beam is repeatedly scanned at high speed in a circular shape or other shape that matches the shape or group of objects to be captured. At this time, the time-averaged spatial intensity distribution is an incoherent summation of intensity distributions at each time. Therefore, when considered geometrically, a dark area (a part not illuminated by light) is created inside the scanning beam in the shape of two conical bottoms joined together. When particles or groups of particles enter this area, they receive a repulsive force no matter which direction they move, creating what is called a wall of light. In reality, according to wave optics, the light intensity does not reach zero even in dark areas, and the particle receives repulsive forces from all directions, and particles are captured at the position where the resultant force balances the external force such as gravity. .

FIG. 4(a) shows the focal plane (
This is a schematic representation of the mechanical potential on the axis passing through the center of the surface scanned by the focused spot. The two peaks correspond to the scanning position of the laser beam and exist at the equilibrium position of the dip between them. Outside the tops of the two mountains, the potential decreases and an outward force acts. Therefore, particles outside the light wall cannot enter the equilibrium position. Therefore, when performing trapping, it is necessary to first move the particles near the trapping position by Brownian motion or stage scanning without irradiating the laser beam, and then irradiate the particle with the beam to trap it. This point differs from conventional laser trapping which has a cone-shaped mechanical potential as shown in FIG. 4(b). However, in conventional laser trapping, particles other than the target particles gather at the bottom of the potential over time, which poses a problem when performing spectroscopy, etc. However, with the method of this invention, Complete capture of single particles is possible.

[0012] In the method of the present invention having the above-mentioned principle characteristics, the target fine particles include various types of fine particles with a low refractive index, which have been difficult to optically trap.
It can also be applied to light-reflecting fine particles such as metals and alloys. There is no restriction on the type of these fine particles, and various laser beams can be used in consideration of the type.

[0013] Furthermore, the fine particles (including their aggregated state) trapped by the method of the present invention can be processed, processed, or Qualifications can be made. A variety of processing and modifications are possible, from changing the composition and properties of fine particles to altering their surface properties. In addition, patterning and transportation are also possible by using movement of laser light, reflection and diffraction, etc.

[0014] There are no particular restrictions on the type of dispersion medium. Water, alcohol, organic solvents such as ether, and various other media can be used.

[0015]

EXAMPLES Hereinafter, the laser trapping method of the present invention will be explained in more detail by way of examples. FIG. 5 shows an example of the configuration of the present invention and the configuration of an experimental system for confirming its effects. The trapping laser beam used in this example is CWNd:YA
G laser (SpectronSL902T, wavelength 10
64 nm). This laser light from the laser light source (1) is transmitted through two galvanometer mirrors (GSI G325D).
T) (2) deflects in two axial directions, and the two lens system (3
) to match the numerical aperture and focus position of the microscope optical system. Inside the microscope (Nikon Optiphoto
00, NA=1.30) (5) to focus the light onto the sample. The size of the focused spot is approximately 1 μm. The two galvano mirrors (2) are both located at the aperture pupil and imaging position of the microscope, and the focal position scans the sample two-dimensionally by deflection by the galvano mirrors (2). The galvanometer mirror (2) is controlled by a controller (Marubun) (6), and for example, in the case of a circular pattern described below,
It is possible to repeat drawing 33 times per second. The shape and size of the drawing pattern is determined by a computer (NEC PC980
1RA) gives instructions to the controller. The state of particle trapping can be observed with CC by illuminating from below the sample.
The image is formed on a D camera (NEC NC-15M) (7) and observed on a monitor (8). The power of the laser light was 145 mW on the sample.

FIGS. 6(a) and 6(b) show water droplets (refractive index 1.33) with a particle size of approximately 4 μm dispersed in liquid paraffin (refractive index 1.46-1.47, viscosity 25 cP) that are captured by laser. It appears that they are doing so. The laser beam rotates and scans around the water droplet (solid arrow in the figure) with a diameter of about 6 μm. Although this water droplet remains stationary even when the stage of the microscope is moved in the x and y directions, it can be seen that the water droplets in the vicinity (dashed line arrows in the figure) are moving. Furthermore, since the image did not blur even when the stage was moved up and down, it was confirmed that the image was captured three-dimensionally. Furthermore, by using a computer program to move the center of the circular scan within the x and y planes, it was possible to observe how the particles were transported accordingly. If you stop laser scanning and illuminate only one point,
Since the water droplets moved in the direction of escaping from that point, it was confirmed that the radiation pressure acts on the particles as a repulsive force, as shown in Figure 1.

FIGS. 7(a) and 7(b) show how iron powder (particle size of approximately 2 μm) is trapped in water (solid arrows in the figures). Particles that are not captured are moving from right to left in the figure (dashed arrows in the figure), but the wall of light causes the particles to flow around the captured particles. In this case, it was not possible to trap in the z-axis direction, but it was possible to move freely in the x and y directions. When this sample was directly irradiated with a focused beam, it was instantly bounced out of the field of view.

[0018]

[Effects of the Invention] As explained in detail above, according to the present invention, by scanning a laser beam, it becomes possible to capture and manipulate light-reflecting particles such as particles with a refractive index lower than that of the medium and metals. . This method not only increases the degree of freedom of modification and processing by trapping, which the inventor has conventionally performed, but also expands the scope of its application. Furthermore, since it can manipulate silicon, etc., it can also be used in the assembly of micromachines and as a driving device. Furthermore, the method of the present invention can be extended to manipulations such as particle pattern formation and transportation proposed by the inventors.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the radiation effect of fine particles according to a conventional method.

FIG. 2 is a schematic diagram showing the radiation effect of fine particles according to a conventional method.

FIG. 3 is a schematic diagram showing the principle of the laser trapping method of the present invention.

FIGS. 4(a) and 4(b) are schematic phase diagrams showing the mechanical potential state at the focal plane.

FIG. 5 is a diagram showing the configuration of an apparatus as an embodiment of the present invention.

FIGS. 6(a) and 6(b) are plan views showing laser trapping of water particles dispersed in liquid paraffin as an example of the present invention.

FIGS. 7(a) and 7(b) are plan views showing laser trapping of iron powder dispersed in water as an example of the present invention.

[Explanation of symbols]

1 Laser light source 2 Galvano mirror 3 Lens system 4 Dichroic mirror 5 Objective lens 6 Controller 7 CCD camera 8. Monitor

Claims (3)

[Claims] 1. A laser trapping method comprising trapping fine particles or groups of fine particles by scanning a condensed laser beam in a circular motion at high speed. 2. A laser trapping processing/modification method, which comprises processing and modifying the fine particles or a group of fine particles captured by the method according to claim 1. 3. A laser trapping dynamic formation method, which comprises patterning or transporting the fine particles or a group of fine particles captured by the method according to claim 1.