CN111816343A - A method and apparatus for realizing multi-position optical trap using sinusoidal phase modulation - Google Patents

Abstract

The invention discloses a device for realizing multi-position optical traps by using sinusoidal phase modulation. The laser beam emitted by the laser is expanded and then reflected to the liquid crystal spatial light modulator, and loaded into the liquid crystal spatial light modulator through the sinusoidal phase modulation of the graphics controller. The liquid crystal spatial light modulator modulates the beam by sinusoidal phase modulation processing. The latter sinusoidal Gaussian beam is divided into two paths, one path is input to the first high-focus objective lens through the reflector in turn and enters the sample stage; A plurality of particles, a detector is arranged near the sample stage, and the detector is connected to a computer. The invention can realize multi-position high-precision, non-destructive optical capture and far-field manipulation, can capture and control particles in multiple positions simultaneously, and solves the problem that traditional optical tweezers cannot capture multiple particles independently.

Description

A method and apparatus for realizing multi-position optical trap using sinusoidal phase modulation

technical field

The invention relates to a device for capturing multi-position optical traps, in particular to a device and method for realizing multi-position optical traps by using sinusoidal phase modulation, which can be used in the fields of optical particle manipulation for capturing multiple particles, cell biomedicine and the like, It belongs to the technical field of optical tweezers.

Background technique

In 1873, Maxwell demonstrated the existence of light pressure according to the electromagnetic wave theory. As an electromagnetic wave, light not only has energy, but also has momentum, and deduced the calculation formula of light pressure. In 1986, A. Ashkin et al. found that a single strongly focused laser beam is sufficient to form a three-dimensional stable optical potential well, which can attract particles and confine it near the focal point. Optical tweezers are the study of light suspension, light trapping and light manipulation of micro-nanoparticles. When the size of the object is in the range of tens of nanometers to tens of microns, the thrust of the particles in the laser light field due to the light pressure will move along the direction of light propagation. Using this principle, the two beams of light are propagating in opposite directions to clamp the particles. Particles can be suspended in a specific position in space to achieve optical suspension of particles. Light trapping is formed in a certain area centered on the optical field of the optical tweezers. Once the object falls into this area, it will automatically move to the center of the optical field. Due to various external effects or the Brownian motion of the particle itself, when the particle deviates slightly The center of the light field will also quickly return to its original position, which is the gradient force potential well. When the kinetic energy of the object is not enough to overcome the potential barrier, it will continue to stay in the potential well. The optical manipulation of optical tweezers is to make the target object move relative to its environment and move the captured sample to a new location in the sample chamber. Compared with the traditional micro-needle or atomic force microscope for manipulating micro-nano particles, optical tweezers are a non-contact remote control method for particle manipulation, which will not cause mechanical damage to the object. Moreover, the optical tweezers can not only manipulate the particles, but also measure the tiny force. The distance of the particles away from the capture center is proportional to the restoring force. Similar to a spring, the optical tweezers can be used as an extremely sensitive force sensor during the manipulation process. Quantitative measurement of interaction forces between biomacromolecules and cells.

In the application of modern optical technology, the control of the wavefront phase of light plays a crucial role in the optical information processing system. It is not only used in beam deflection, beam shaping, dynamic holography, optical tweezers technology, but also in adaptive It plays an important role in optical wavefront correction technology. The spatial light modulator (SLM) used for wavefront correction is constantly being developed. SLM is a device that can write the information contained in the signal source into the light wave, that is, the light wave can be controlled by controlling the signal source. Phase, amplitude, frequency, polarization state and other one-dimensional or two-dimensional transformation of space and time. According to the way of reading out light, it can be divided into transmissive spatial light modulators and reflective spatial light modulators.

Spatially modulated optical tweezers are optical tweezers that change the incident optical field through specific optical elements to form a continuously changing optical field in space. The function of the spatial light modulator is to convert the complex amplitude distribution or light intensity distribution on the object plane into the complex amplitude distribution or light intensity distribution on the image plane. The optical information of a complex optical picture is represented by its complex-amplitude transmission coefficient, which can be decomposed into a linear superposition of elementary periodic structures with continuous spatial frequencies. When the graphics processor loads the SLM with sinusoidal phase modulation, and the Gaussian light vertically illuminates the optical image on the SLM, the primitive periodic structure of these complex amplitude distributions is similar to the diffraction grating with different grating constants, due to the effect of diffraction Instead, the incident light wave is decomposed into a series of plane waves. Therefore, each plane wave acts as the carrier of the basic information of the optical pattern to propagate the entire pattern information.

Multiple particles can be captured and manipulated simultaneously using multi-position optical traps. The generation methods of multiple optical traps include multi-beam interference method, time division multiplexing method and spatial modulation optical tweezers. Multi-beam interferometry can only produce symmetrical structures, but can only achieve two-dimensional trapping, and its axial scattering force needs to be overcome by other factors. Time division multiplexing can also generate multiple optical traps, but this multiple optical traps are a combination of effects in time. Using acousto-optic modulators or scanning galvanometers, the position of the laser can be changed very quickly in a very short time, so that particles at different positions can experience the time-averaged optical potential well. But systems based on both scanning devices cannot produce three-dimensional arrays of optical traps. Spatially modulated optical tweezers can modulate the amplitude or phase of the beam, and can realize manipulation methods such as dynamic capture, optical transport, and photostretching of multiple particles in three-dimensional space, which greatly expands the functionality of optical micromanipulation. and application areas. The existing spatial modulation optical tweezers do not have a formula for determining that the same plane has multiple independent light intensity distribution centers, which causes difficulties in practical use.

SUMMARY OF THE INVENTION

The problem to be solved by the present invention is to overcome the deficiencies of the prior art, and to provide a multi-position optical trap generating device and method with simple principle, easy implementation and practical application value, aiming at the situation that the existing optical tweezers have only one main capture area. , and according to the invention principle of the method, a novel optical tweezers system with simple structure and instructive significance is provided.

For achieving the above object, the technical scheme adopted in the present invention is as follows:

The invention includes a laser, a beam expander group, a first reflecting mirror, a liquid crystal spatial light modulator, a graphic controller, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a first high-focus objective lens, a second high-focusing Focusing the objective lens, sample stage and computer; the laser emits a laser beam, which is expanded by the beam expander group and then reflected to the liquid crystal spatial light modulator by the first reflection mirror, and loaded into the liquid crystal through the sinusoidal phase modulation of the graphics controller. The liquid crystal spatial light modulator performs sinusoidal phase modulation on the beam to obtain a modulated sinusoidal Gaussian beam, and the output is divided into two paths. A high-focus objective lens is incident on the sample stage after passing through the first high-focus objective lens; the other path is reflected by the fourth mirror and then input to the second high-focus objective lens, and then incident on the sample stage after being focused by the second high-focus objective lens. The focal points of the first high-focus objective lens and the second high-focus objective lens coincide and are located on a sample stage, a plurality of particles are placed on the sample stage, a detector is arranged near the sample stage, and the detector is connected with the computer.

The liquid crystal spatial light modulator is a liquid crystal transmission spatial light modulator or a liquid crystal reflection spatial light modulator.

The graphic controller loads the preset phase information map on the liquid crystal spatial light modulator to realize the sinusoidal phase modulation of the passing beam. The beam reflected by the first mirror is used as the incident Gaussian beam, and the incident Gaussian beam passes through the liquid crystal type. The spatial light modulator becomes a beam with four light intensity distribution centers after sinusoidal phase modulation, as a sinusoidally modulated Gaussian beam, and each light intensity distribution center and its vicinity serve as a light trap, so that the four light intensity distribution centers are respectively Correspondingly, four independent optical traps are formed;

Taking the optical axis direction of the liquid crystal spatial light modulator as the z-axis direction, a three-dimensional Cartesian coordinate system is established. The x-axis and the y-axis are perpendicular to each other and perpendicular to the z-axis. The preset phase information map is calculated according to the following formula to obtain the sine The transmission coefficient T of the phase modulation is formed by the pixel plane setting of the liquid crystal spatial light modulator:

Among them, g is the sinusoidal modulation coefficient, i is an imaginary number, x and y are the coordinate positions of a single pixel in the liquid crystal spatial light modulator on the x-axis and y-axis, e is a natural constant, and w ₀ is the incident Gaussian beam. Waist Radius.

By adjusting the sinusoidal modulation coefficient g of the liquid crystal spatial light modulator, the distance R from the center of the light intensity distribution to the optical axis is changed. The specific change method formula is:

In the present invention, the optical information of the optical picture is the transmission coefficient T of the sinusoidal phase modulation. And for the transmissive spatial light modulator and the reflective spatial light modulator, their transmission coefficients T are the same.

The laser is a power-adjustable semiconductor-pumped solid-state laser with a wavelength of 1064 nanometers.

The liquid crystal spatial light modulator is a liquid crystal transmission spatial light modulator or a liquid crystal reflection spatial light modulator, and the liquid crystal material adopts twisted nematic TN, which can adjust the phase, polarization state, amplitude or intensity of the light beam. Real-time spatial modulation of 1D or 2D distributions.

The laser emitted from the liquid crystal spatial light modulator is divided into two paths. After passing through the first high-focus objective lens and the second high-focus objective lens respectively, it is irradiated on the sample stage symmetrically and oppositely from both sides. With microparticles, the microparticles are light trapped on the sample stage.

The numerical apertures of the first high-focus objective lens and the second high-focus objective lens are not less than 0.85.

The microparticles are in an aqueous environment.

In the present invention, the spatial light modulator realizes the spatial modulation of the wavefront phase of the incident Gaussian beam mainly according to the design parameters.

The advantages of a device of the present invention for realizing multi-position optical traps by using sinusoidal phase modulation:

1. The laser processing of the present invention captures the particles and realizes high-precision, non-contact and non-destructive manipulation.

2. The sinusoidal modulation coefficient of the sinusoidally modulated Gaussian beam proposed by the present invention can be changed in size according to requirements, and four independent beams with variable distance from the center of the optical field are generated, and the optical tweezers device is highly flexible.

3. The optical system of the device of the present invention is simple in principle, high in conversion rate, easy to implement, and has high practical value and promotion value in the field of novel optical tweezers.

The invention can realize multi-position high-precision, damage-free optical capture and far-field manipulation, and specifically utilizes a liquid crystal spatial light modulator controlled by an incident Gaussian beam through a graphic controller to generate a spatially distributed sinusoidally modulated Gaussian beam, thereby The modulated light field has four central beams of light intensity distribution, forming four independent light traps, which can simultaneously capture and control the particles in four positions.

The method of the invention solves the problem that traditional optical tweezers cannot capture multiple particles independently, and has high practical value and promotion value in the technical field of novel optical tweezers.

Description of drawings

FIG. 1 is a structural diagram of an apparatus for realizing a multi-position optical trap by using sinusoidal phase modulation according to an embodiment of the present invention.

FIG. 2 In an embodiment of the present invention, a graphic controller loads a sinusoidal modulation coefficient on a spatial light modulator, and generates a light intensity distribution diagram of a sinusoidally modulated Gaussian beam when g=1.

Fig. 3 In an embodiment of the present invention, a graphic controller loads a sinusoidal modulation coefficient on the spatial light modulator, and generates a light intensity distribution diagram of a sinusoidally modulated Gaussian beam when g=2.

FIG. 4 is a graph of the light intensity distribution of a new multi-position optical trap using a sinusoidally modulated Gaussian beam obtained by simulation in an embodiment of the present invention, and the light intensity distribution in the x-z direction to the focal point of the focusing objective lens.

In the figure: laser (1), beam expander group (2), first reflector (3), liquid crystal spatial light modulator (4), graphic controller (5), second reflector (6), Three reflecting mirrors (7), a fourth reflecting mirror (8), a first high-focus objective lens (9), a second high-focus objective lens (10), a sample stage (11), and a computer (12).

Detailed ways

The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments

As shown in FIG. 1 , the specifically implemented device includes a laser 1 , a beam expander group 2 , a first mirror 3 , a liquid crystal spatial light modulator 4 , a graphics controller 5 , a second mirror 6 , and a third mirror 7 , a fourth mirror 8 , a first high focus objective lens 9 , a second high focus objective lens 10 , a sample stage 11 and a computer 12 .

The laser 1 emits a laser beam, which is a Gaussian beam, that is, a beam with Gaussian distribution. The laser beam is expanded by the beam expander group 2 and then reflected by the first mirror 3 to the liquid crystal spatial light modulator 4. The control terminal of the light modulator 4 is electrically connected to the graphic controller 5 and is loaded and controlled by the graphic controller 5, and is loaded to the liquid crystal spatial light modulator 4 through the sinusoidal phase modulation of the graphic controller 5, and the liquid crystal spatial light modulator 4 sinusoidally modulates the light beam. The modulated sinusoidal Gaussian beam is obtained by the phase modulation process and is output into two paths. 9 is focused and incident on the sample stage 11.

The other way is reflected by the fourth mirror 8 and then input to the second high-focus objective lens 10, and is also focused by the second high-focus objective lens 10 and then incident on the sample stage 11. The first high-focus objective lens 9 and the second high-focus objective lens The focus of the focusing objective lens 10 is coincident and is located on the sample stage 11. A plurality of particles are placed on the sample stage 11. A detector is arranged near the sample stage 11. Image and Processing of Particles Manipulate the particles. The CCD detector is used to detect the capture situation of the particles on the sample stage 11, and sends the collected data to the computer 12, and the computer 12 completes the data collection and processing.

The laser light emitted from the liquid crystal spatial light modulator 4 is divided into two paths. After passing through the first high-focus objective lens 9 and the second high-focus objective lens 10 respectively, it is irradiated on the sample stage 11 symmetrically and oppositely from both sides. There are particles on the sample stage 11 , and the particles are optically suspended on the sample stage 11 . In the implementation of the experiment, many particles are placed on the sample stage 11, the device is turned on, and the two beams emitted by the first high-focus objective lens 9 and the second high-focus objective lens 10 are coaxially projected to form four optical traps, each of which captures One particle, a total of four particles were captured.

Particles are in an aqueous environment. In a specific implementation, the sample stage 11 is placed in a water environment.

The liquid crystal spatial light modulator 4 is a liquid crystal transmission spatial light modulator or a liquid crystal reflection spatial light modulator. The preset phase information map is loaded onto the liquid crystal spatial light modulator 4 by the graphic controller 5 to realize sinusoidal phase modulation of the passing beam, and the beam reflected by the first mirror 3 is the incident Gaussian beam, and the incident Gaussian beam After sinusoidal phase modulation by the liquid crystal spatial light modulator 4, it becomes a beam with four light intensity distribution centers as a sinusoidally modulated Gaussian beam, and each light intensity distribution center and its vicinity serve as a light trap, so that four light The strong distribution centers respectively form four independent optical traps;

Taking the optical axis direction of the liquid crystal spatial light modulator 4 as the z-axis direction, a three-dimensional Cartesian coordinate system is established. The x-axis and the y-axis are perpendicular to each other and perpendicular to the z-axis. The preset phase information map is calculated according to the following formula. The transmission coefficient T of the sinusoidal phase modulation is formed for the pixel plane setting of the liquid crystal spatial light modulator 4:

Among them, g is a sinusoidal modulation coefficient, i is an imaginary number, x and y are the coordinate positions of a single pixel in the liquid crystal spatial light modulator 4 on the x- and y-axes, e is a natural constant, and w ₀ is the incident Gaussian beam waist radius. By reducing the sinusoidal modulation factor g by an order of magnitude, the optical trap gradient force can be improved by three orders of magnitude.

By adjusting the sinusoidal modulation coefficient g of the liquid crystal spatial light modulator 4, the distance R from the center of the light intensity distribution to the intersection point between the optical axis and the pixel plane of the liquid crystal spatial light modulator 4 is changed. The specific change method formula is:

where a new sinusoidal modulation factor g corresponds to a new distance R.

In a specific implementation, the laser 1 is a power-adjustable semiconductor-pumped solid-state laser with a wavelength of 1064 nanometers.

The liquid crystal spatial light modulator 4 generates the optical path difference by changing the orientation of the liquid crystal molecules, that is, the change of the inclination angle of the liquid crystal molecules causes the change of the extraordinary light refractive index of the liquid crystal, which in turn causes the phase delay of the light beam after propagating in the liquid crystal, so as to realize the incidence of the incident light. Phase modulation of light. The liquid crystal spatial light modulator is regarded as a variable phase grating element, and the programmable control of the incident light wave phase plane is realized by controlling the voltage drive.

The phase information diagram of the sinusoidal phase modulation is obtained through the graphic controller 5, and the graphic controller loads the phase information diagram of the sinusoidal phase modulation onto the liquid crystal spatial light modulator 4, and the liquid crystal spatial light modulator 4 only detects the incident Gaussian beam Phase modulation is pure phase modulation. When the incident Gaussian beam is vertically illuminated to the pixel optical information on the liquid crystal spatial modulator, the elementary periodic structure of the complex amplitude distribution of the pixel optical information is similar to the diffraction grating with different grating constants. The wave is decomposed into a series of plane waves, and finally a controllable light intensity distribution pattern is generated after passing through the liquid crystal spatial light modulator 4 to form a phase information map, which is used for capturing a multi-position optical trap.

The electric field distribution E of the sinusoidally modulated Gaussian beam after passing through the liquid crystal spatial light modulator of the present invention at the transmission distance z=0 (that is, at the pixel plane of the liquid crystal spatial light modulator) is:

Among them, E(x1, y1, 0) represents the electric field of the pixel at the coordinates of x1 and y1 in the liquid crystal spatial light modulator 4, and E ₀ is the initial electric field intensity.

在(x1-y1)方向上空间周期为

在(-x1+y1)方向上空间周期为

在(-x1-y1)方向上空间周期为

According to the above electric field, a sinusoidally modulated Gaussian beam with light intensity distribution in four directions is obtained, generating a plurality of optical traps. On the plane perpendicular to the z-axis, the space period in the x1+y1 direction is

The space period in the (x1-y1) direction is

The space period in the (-x1+y1) direction is

The space period in the (-x1-y1) direction is

Through the above-mentioned device and processing, the present invention finds that the sinusoidally modulated Gaussian beam output in this way can generate four central beams with light intensity distribution, form four independent optical traps, and change the new light intensity by adjusting the sinusoidal modulation coefficient g The distance R from the center of the distribution to the center focus. Moreover, due to pure phase modulation, that is, only the phase of the Gaussian beam is modulated, the amplitude of the Gaussian beam remains unchanged, and the total light intensity after modulation remains unchanged, maintaining the energy concentration and long-distance propagation of the Gaussian beam.

The embodiment of the present invention and its implementation process are as follows:

In the following, taking a laser beam with a Gaussian beam waist radius of 5 mm and a laser incident power of 100 mW as an example, the method proposed in this patent can realize multi-position optical traps in detail with reference to the accompanying drawings.

Fig. 2 and Fig. 3 respectively show the light intensity distribution diagrams of the sinusoidally modulated Gaussian beam generated by simulation when the graphic controller loads the spatial light modulator with sinusoidal modulation coefficients g=1 and g=2. A sinusoidally modulated Gaussian beam can generate a beam with four centers of intensity distribution, each at the same distance from the center focal point. By comparing Fig. 1 and Fig. 2, we find that when the sinusoidal modulation factor g=1, the distances between the four light intensity distributions are closer to the focal point, which is consistent with the analysis results.

FIG. 4 is a graph of the light intensity distribution in the x-z direction obtained by changing the distance of the high-focus objective lens at the focal point of the new multi-position optical tweezers using a sinusoidally modulated Gaussian beam. It can be seen from the figure that by reducing the distance between the optical axis of the optical trap and the focal point of the high-focus objective lens, the intensity of the light field can be increased.

It can be seen from the above implementation experiments that the sinusoidally modulated Gaussian beam output by the liquid crystal spatial light modulator in this technical solution can generate four central beams with light intensity distribution, forming four independent optical traps, and by adjusting the sinusoidal modulation coefficient g And change the distance R from the center of the new light intensity distribution to the central focus.

Claims (8)

1. a device utilizing sinusoidal phase modulation to realize multi-position optical trap, is characterized in that: comprise laser (1), beam expander group (2), first reflecting mirror (3), liquid crystal type spatial light modulator (4) ), graphics controller (5), second mirror (6), third mirror (7), fourth mirror (8), first high focus objective (9), second high focus objective (10) , a sample stage (11) and a computer (12); the laser (1) emits a laser beam, which is expanded by the beam expander group (2) and then reflected by the first reflecting mirror (3) to the liquid crystal spatial light modulator On (4), the liquid crystal spatial light modulator (4) is loaded through the sinusoidal phase modulation of the graphic controller (5), and the liquid crystal spatial light modulator (4) performs sinusoidal phase modulation processing on the beam to obtain a modulated sinusoidal Gaussian The light beam is divided into two paths, one path is reflected by the second reflecting mirror (6) and the third reflecting mirror (7) in turn, and then input to the first high-focus objective lens (9), and then incident through the first high-focus objective lens (9) onto the sample stage (11); the other path is reflected by the fourth reflecting mirror (8) and then input to the second high-focus objective lens (10), and then incident on the sample stage (11) after being focused by the second high-focus objective lens (10). The focal points of the first high-focus objective lens (9) and the second high-focus objective lens (10) coincide and are located on the sample stage (11), a plurality of particles are placed on the sample stage (11), and a There is a detector, which is connected to a computer (12). 2 . The device for realizing multi-position optical traps by sinusoidal phase modulation according to claim 1 , wherein the liquid crystal spatial light modulator (4) is a liquid crystal transmission spatial light modulator or a liquid crystal spatial light modulator. 3 . reflective spatial light modulator. 3. a kind of device that utilizes sinusoidal phase modulation to realize multi-position optical trap according to claim 1, it is characterized in that: by graphic controller (5) preset phase information map is loaded into liquid crystal type spatial light modulator ( 4) On, the sinusoidal phase modulation of the passing beam is realized, and the beam reflected by the first reflecting mirror (3) is the incident Gaussian beam, and the incident Gaussian beam is modulated by the liquid crystal spatial light modulator (4) after sinusoidal phase modulation. A beam of four light intensity distribution centers is used as a sinusoidally modulated Gaussian beam, and each light intensity distribution center and its vicinity are used as an optical trap, so that the four light intensity distribution centers respectively form four independent optical traps; The optical axis direction of the type spatial light modulator (4) is the z-axis direction, and a three-dimensional Cartesian coordinate system is established. The x-axis and the y-axis are perpendicular to each other and perpendicular to the z-axis. The preset phase information map is calculated according to the following formula. The transmission coefficient T of the sinusoidal phase modulation is formed for the pixel plane setting of the liquid crystal spatial light modulator (4):

Among them, g is the sinusoidal modulation coefficient, i is an imaginary number, x and y are the coordinate positions of a single pixel in the liquid crystal spatial light modulator (4) on the x- and y-axes, e is a natural constant, and w ₀ is the incident The waist radius of the Gaussian beam. 4. A device for realizing multi-position optical traps using sinusoidal phase modulation according to claim 1, wherein the light intensity distribution is changed by adjusting the sinusoidal modulation coefficient g of the liquid crystal spatial light modulator (4). The distance R from the center to the optical axis, the specific change method formula is:

5 . The device for realizing multi-position optical traps using sinusoidal phase modulation according to claim 1 , wherein the laser ( 1 ) is a power-adjustable semiconductor-pumped solid-state laser with a wavelength of 1064 nanometers. 6 . 6. A device for realizing multi-position optical traps by sinusoidal phase modulation according to claim 1, characterized in that: the laser light emitted from the liquid crystal spatial light modulator (4) is divided into two paths, respectively passing through The first high-focusing objective lens (9) and the second high-focusing objective lens (10) are then symmetrically irradiated onto the sample stage (11) from both sides, and the sample stage (11) has particles, which are opposite to the sample stage (11). ) on the microparticles for light trapping. 7. a kind of device that utilizes sinusoidal phase modulation to realize multi-position optical trap according to claim 1, it is characterized in that: the numerical aperture of described first high focus objective lens (9) and second high focus objective lens (10) Not less than 0.85. 8 . The device for realizing multi-position optical traps using sinusoidal phase modulation according to claim 1 , wherein the particles are in a water environment. 9 .

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