JP2004138906A - Photo tweezer system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical tweezer system which makes real time control targeting a plurality of points possible. <P>SOLUTION: The optical tweezer system forms a plurality of condensing points on a trap surface by displaying phase patterns on a spatial light modulator, in which the modulation surface of the spatial light modulator is divided to a plurality of regions and the positions of the plurality of the condensing points are adjusted by independently forming the patterns corresponding to the condensing points in the respective regions. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical tweezer device that captures or moves a minute substance by using a radiation pressure of light, and more particularly to an optical tweezer device that collects light at a plurality of positions on a light collecting surface using a spatial light modulator. About.
[0002]
[Prior art]
2. Description of the Related Art An optical tweezer device is known that uses a radiation pressure of light to capture and move a minute object such as a cell, a chromosome, or a nanomachine in a non-contact manner. For example, in Patent Document 1, a plurality of optical traps are formed by forming a plurality of output beams using an optical diffraction element such as a hologram or a diffraction grating.
[0003]
[Patent Document 1]
JP-T-2002-502043 (pages 10 to 12, FIG. 3)
[0004]
[Problems to be solved by the invention]
When a plurality of optical traps are formed at predetermined positions in this manner, it is sufficient to use a predetermined diffraction grating. However, when a plurality of objects are to be captured and moved independently, it is necessary to change the position of each optical trap in real time. To do so, the diffraction characteristics of the optical diffraction element must be changed in real time.
[0005]
As a method of changing the diffraction characteristics, there is a method of displaying a computer generated hologram (CGH: Computer Generated Hologram) using a spatial light modulator. However, performing CGH calculation in real time requires a great deal of computer resources and is not realistic. Thus, a method of preparing a pattern that can obtain a desired output in advance is conceivable. However, when trying to control multiple points, the number of patterns to be prepared becomes enormous, and the handling becomes complicated.
[0006]
Therefore, an object of the present invention is to provide an optical tweezer device capable of real-time control of a plurality of points.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, an optical tweezer device according to the present invention condenses light emitted from a light source by a spatial light modulator to a plurality of points located at desired positions on a predetermined surface, and emits the light. In an optical tweezer device that captures and moves minute substances using pressure, the grating pattern is spatially divided into a plurality of regions corresponding to each of the points where the modulation surface of the spatial light modulator is focused, and a grating pattern is formed in each region. By forming, the position of the focal point is adjusted.
[0008]
In this way, by dividing the modulation surface of the spatial light modulator into a plurality of regions, forming a grating pattern on each of them, and adjusting the position of the focal point, each point can be controlled independently. Since multipoint control can be performed by a combination of patterns, even when a pattern is created by calculation, a computer resource or a memory is small in any case where a necessary pattern is stored in advance. Since it can be performed at a high speed, real-time control becomes possible.
[0009]
When dividing the modulation surface of the spatial light modulator, it is preferable to adjust the area of the region to adjust the intensity of the condensed light. For example, in the case of a fine grating pattern, the light intensity to be condensed becomes weak, so a large area is allocated to a region where such a pattern is to be formed, and a small area is allocated to a region where a relatively coarse grating pattern is formed. Then, the light intensity at each light condensing point can be made equal.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram of an optical tweezer device 1 according to the present invention. This device 1 forms an optical trap that condenses light on an optical trap surface 14 using coherent light such as laser light that is incident light. As a trap forming means, a spatial light modulator (optical diffractive means) is used. SLM) 11 and the diffracted light is condensed on an optical trapping surface 14 by a relay lens system 12 and an objective lens 13. Here, the configuration of the lens of the relay lens system 12 is designed so that the position of the SLM 11 and the position of the objective lens 13 are conjugate surfaces. As the SLM 11, a transmission type SLM having a number of pixels of 512 × 512, for example, of a phase modulation type is used. The control device 10 controls the SLM 11 to switch the displayed grating pattern (CGH).
[0011]
FIG. 2 is a diagram illustrating a divided phase grating pattern according to the present invention, and FIG. 3 is a diagram illustrating a corresponding output pattern, that is, a condensing pattern on the optical trap surface 14. Here, the grating pattern is shown by converting the phase modulation amount of 0 to 2π in each pixel into a luminance value of 0 to 255. 2 (a) and 3 (a) show the case of four-point output, and FIGS. 2 (b) and 3 (b) show the case of 16-point output. In each case, the phase pattern surface is divided into a number corresponding to the output points. FIGS. 4 and 5 show a grating pattern and its output pattern when 8 × 8 condensing points are formed at regular intervals by a conventional method for comparison.
[0012]
In the present invention, the grating pattern of each divided region is completely independent of the grating patterns of other regions, and is not affected by them. Therefore, for example, if the control coordinates on the optical trapping surface 14 are 100 × 100, a phase table (100 × 100 sets) corresponding to each output point coordinate is prepared, and these are arbitrarily combined. An entire phase pattern can be created. Furthermore, even when the number of output points increases, the same phase table may be used as long as the range of output point coordinates is the same, and control of multiple points becomes easy. Further, since the grating in each area is simple as shown in FIG. 2, the calculation time is short, and real-time control is possible. On the other hand, in the conventional method, since calculation is performed in all 512 × 512 regions, the calculation time is extremely long and there is no real-time property. Even if a table method in which the phase patterns corresponding to all the output patterns are calculated in advance is employed, each point on the output surface cannot be regarded as independent, so that an enormous number of tables need to be prepared. For example, even when four points are output, the number of tables that must be prepared reaches 100 × 100 to the fourth power, that is, 10 ^16, and the number of tables increases when the number of output points increases, which is not practical. .
[0013]
Next, a specific control method will be described. Here, a case where four points are output at the output plane coordinates of 100 × 100 will be described as an example. The following control is performed by the control device 10.
[0014]
First, the phase pattern area is divided according to the number of output points. Here, for simplicity, equal division is performed, and the number of pixels in each area is 256 × 256.
[0015]
Next, a phase table corresponding to the output plane coordinates is created based on the number of divided pixels. The number of phase tables is 100 × 100, and a phase grating for outputting from (0,0) to coordinates (99,99) is created. The time required for this calculation is within a few minutes.
[0016]
Subsequently, the previously created phase tables (256 × 256 pixels) corresponding to the coordinates of the four points that are actually desired to be output are read out and pasted together to create a 512 × 512 pixel phase pattern. By displaying the created phase pattern on the SLM 11 as a hologram, four points can be condensed at desired output coordinate positions on the optical trap surface 14 to form an optical trap.
[0017]
In the case of moving the focal point, a phase table is created in advance, stored in a memory or the like, the phase table is read out according to the movement position, and the phase table is attached to form a phase pattern. It can respond by forming. Since the calculation of the bonding itself takes little time, real-time processing becomes possible.
[0018]
By the way, a laser light source usually has a Gaussian distribution intensity distribution in which the center is bright and the light amount becomes weaker toward the periphery. Therefore, in the phase pattern formed on the SLM 11, the light intensity of the light condensing point formed by the region of the central portion is strong, and the light intensity of the light condensing point formed by the region of the peripheral portion is weak. Normally, in an optical tweezer device, it is desired to make the light intensity on the optical trapping surface 14 as constant as possible. Therefore, as shown in FIG. It is preferable to correct the intensity of each point on the trap surface 14. In this figure, if the area of the central 3 × 3 area A is 1, the area of the four 1 × 3 areas B adjacent to it is 1.4, and the area of the four corner areas C is 1.4. Is set to 2. This area ratio is preferably determined in accordance with the distribution of the amount of light incident on the SLM 11.
[0019]
Further, in the present invention, the light-converging point is formed by utilizing the diffraction phenomenon caused by the grating. Here, the diffraction efficiency is high in the region where the grating is coarse, so that the light intensity at the converging point is high. However, the diffraction efficiency is low in the region where the grating is fine, so the light intensity at the converging point is low.
[0020]
In order to suppress the variation in point intensity on the trap surface due to this factor, the area of each area may be made different in advance, and a wide area may be allocated to a pattern with a fine grating. FIG. 7 shows an example of such area division. The 1 × 4 region E on the right side has 1.4 times the area of the 3 × 4 region D on the left side of the drawing. On the optical trapping surface 14, the grating required to form the focal point becomes finer and farther away from the center, resulting in lower diffraction efficiency. Therefore, the light intensity on the output surface is corrected by allocating a wide area such as the area E to such a pattern. Here, an example in which the area of each region is fixed in advance has been described, but the area may be changed each time according to the fineness of the grating.
[0021]
Further, it may be combined with the correction for the light intensity distribution of the incident light described above. In this case, if a fine pattern of the grating is arranged in the center part and a coarse pattern of the grating is arranged in the periphery, the fluctuation of the light intensity due to the density of the grating and the fluctuation of the light intensity due to the intensity distribution of the incident light can be offset. Therefore, the variation of the area can be reduced.
[0022]
In the above case, the control for maintaining the light intensity at the focal point constant has been described, but it is also possible to control the light intensity at the focal point by positively changing the area of the focal point. is there. In an optical tweezer device, there is a method of moving a target while holding it, other than simply capturing the target, but the latter requires a higher light intensity than the former. When a plurality of types of different targets are present, the light intensity required for capturing differs for each target. Therefore, it is possible to adjust the light intensity at the output surface by allocating a large area to a phase pattern forming a trap requiring a high light intensity and allocating a small area to a phase pattern forming a trap requiring a low light intensity. it can.
[0023]
In the case where the area of the area varies as described above, a combination of phase patterns in one or several basic pixel sizes is calculated, and each divided area has a similar shape or a basic pixel size. If the shape is formed by bonding the sizes, the calculation of the phase pattern according to the change in the area can be simplified, and the real-time property is not impaired.
[0024]
【The invention's effect】
As described above, according to the present invention, the modulation surface of the SLM is divided into a plurality of regions, and a grating pattern is formed on each of the regions to adjust the position of the focal point, thereby controlling each point independently. can do. In addition, since multipoint control can be performed by combining patterns, even when a pattern is created by calculation, necessary computer resources and memory etc. are reduced in any case where a necessary pattern is stored in advance, and the Can be performed at high speed, so that real-time control is possible.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a preferred embodiment of an optical tweezer device according to the present invention.
FIG. 2 is an illustration of a divided phase grating pattern in the device of FIG.
FIG. 3 is a diagram showing an output pattern based on the grating pattern of FIG. 2;
FIG. 4 is an example of a conventional grating pattern.
FIG. 5 is a diagram showing an output pattern based on the grating pattern of FIG.
FIG. 6 is an illustration of a division pattern in the device of FIG. 1;
FIG. 7 is another example of a division pattern in the apparatus of FIG. 1;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Optical tweezer device, 10 ... Control device, 11 ... Spatial light modulator (SLM), 12 ... Relay lens system, 13 ... Objective lens, 14 ... Optical trap surface.

Claims (2)

An optical tweezer device that condenses light emitted from a light source by a spatial light modulator to a plurality of points located at desired positions on a predetermined surface, and uses the light radiation pressure to capture and move minute substances. At
An optical tweezer device that adjusts the position of a light-converging point by spatially dividing the modulation surface of the spatial light modulator into a plurality of regions corresponding to light-condensing points and forming a grating pattern in each region. 2. The optical tweezers device according to claim 1, wherein when the modulation surface of the spatial light modulator is divided into regions, the area of the region is adjusted to adjust the intensity of the condensed light.

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