RU2488905C1 - Method of moving opaque microobjects - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: according to the method of moving a group of opaque microobjects, a light beam with closed regions of zero intensity is formed from multiple beams. First, three coaxial, zero-order Bessel beams with different propagation constants are formed, thereby forming a stable beam in form of a circular spot. These beams are then arranged in space so as to form one or more closed regions for capturing and moving opaque microparticles.

EFFECT: high efficiency owing to automation of the process.

11 dwg

Description

The invention relates to the field of optical microscopy and optical micromanipulation.

One of the applications of devices that are commonly called “optical tweezer” is the movement and assembly of micromechanical elements by a specially formed light beam. The light beam due to the gradient forces in the light beam moves a single or a group of micro-objects of a special shape (US patent 7622710, IPC G01B 21/06, published on November 24, 2009, US patent 6995351, IPC G01N 30/00, published on 08/11/2005 ., US patent 7678222, IPC G03H 1/00, publ. 12/28/2006).

The disadvantage of all these rotation methods is the vortex nature of the light beams for capturing opaque microobjects. This causes unnecessary rotation of the microobject when moving.

The closest technical solution, selected as a prototype, is a method of moving microobjects, which consists in the formation of a vortex light beam (US patent 6995351, IPC G01N 30/00, published on 08/11/2005).

The main disadvantage of this method is the presence of forces rotating the micro-object, which complicates the assembly of micromechanical systems by this method and the automation of the assembly process.

The basis of the invention the task is to increase productivity due to the ability to automate the process.

This problem is achieved by the fact that in the method of moving a group of opaque microobjects, which consists in forming a light beam with closed regions of zero intensity, according to the invention, the light beam is formed from several beams, first use three coaxial Bessel beams of zero order with different propagation constants, forming a stable beam in the shape of a round spot, then these beams are placed in space so as to form one or more closed areas for capture and movement not rozrachnyh microparticles.

The claimed technical solution differs from the prototype in that a superposition of several 0-order Bessel beams is used, which allows one to form a group of three-dimensional optical traps for opaque microobjects without limiting the distance between the traps and without a polar phase gradient in each individual trap. Thus, the drawback of the group of traps in the form of ordinary vortex beams, which cannot be brought closer to a distance shorter than their own diameter, and which in addition, when capturing a micro-object begin to rotate it, is removed.

These differences allow us to conclude that the claimed technical solutions meet the criterion of "novelty." Signs that distinguish the claimed technical solution from the prototype are not identified in other technical solutions when studying this and related areas of technology and, therefore, provide the claimed solutions with the criterion of "significant differences".

Figure 1 presents the optical circuit of the device that implements the method.

The device consists of a solid-state laser with a wavelength of 532 nm and a maximum average power of 500 mW, a rotary mirror 2 and a translucent mirror of the lighting system, a diffractive optical element 4, a cuvette with microobjects 5, depicting a micro-lens 6, a rotary mirror 7 of the imaging system, CCD video camera 8, the control computer 9, the lamp of the illuminator 10, the capacitor of the illuminator 11, the focusing micro-lens 12.

The light trap for opaque micro-objects in English-language scientific articles is usually called optical "bootle" (optical "bottle") or light "bootle" (light "bottle"). It is proposed to create a group of optical “bottles” using a superposition of Bessel beams (modes) having different propagation axes parallel to each other.

The method is as follows.

A laser beam is sent to the fabricated diffractive optical element. After diffraction on the element, a double optical trap closed in three coordinates of FIG. 11 is formed.

As can be seen from Fig. 11, a double light trap begins to form at a distance of 775 mm from the element and closes at a distance of 975 mm. When focusing with the micro-lens indicated in FIG. 1, the longitudinal length of such a trap is only 40 μm.

To form a group of optical “bottles”, we consider a superposition of N spatially separated Bessel modes of different orders with different numbers of the roots of the Bessel function, i.e. different values of m used in the calculation of the phase function of the DOE forming the n-th order Bessel beam

$$\tau \left(x,y\right)=\mathrm{sgn}\left(J_n\left({\alpha }_{m}r\right)\right)\exp \left(in\varphi \right),\left(\mathrm{one}\right)$$

, σ₀=cosθ, n - порядок функции Бесселя, m - номер корня функции Бесселя, k - волновое число, θ - средний угол наклона плоских волн пространственного спектра для заданного поля, х, y - декартовы координаты.where α _m = kρ _m , $${\rho }_m=\sqrt{\mathrm{one}-{\left({\mathrm{<figure-callout\; id="0"\; label="\sigma "\; filenames="00000008.png,00000011.png"\; state="\{\{state\}\}">\sigma </figure-callout>}}_0+\frac{m\lambda }{z_0}\right)}^2}$$

, σ ₀ = cosθ, n is the order of the Bessel function, m is the root number of the Bessel function, k is the wave number, θ is the average slope of the plane waves of the spatial spectrum for a given field, x, y are the Cartesian coordinates.

To calculate the superposition of N spatially separated Bessel modes, the following formula was used:

$$T\left(x,y\right)={\displaystyle \sum _{k=\mathrm{one}}^N{C}_k}\cdot \mathrm{sgn}\left(J_{n_k}\left({\alpha }_{m_k}\left({\overrightarrow{r}}_k-{\overrightarrow{r}}_{k0}\right)\right)\right)\exp \left(i{n}_k\varphi \right)\exp \left[i\left(x{u}_x+y{\nu }_y\right)\right]\left(2\right)$$

with complex coefficients C _k for each individual mode.

Although there are simpler methods for forming a single optical trap for opaque microobjects, for example, using a composite axicon, you can create such a trap by the method described above to demonstrate the universality of the proposed approach.

These 27 modes are arranged according to the scheme shown in figure 2.

In this and subsequent schemes, the minimum size of the diffraction spot formed by a given element at a distance z ₀ from the input plane at a wavelength of λ = 532 nm was taken as unity.

In each of the positions in the diagram, there is a superposition of three Bessel modes of the 0th order with the same values of z ₀ = 800 mm, but with different numbers of the roots of the Bessel functions m = 8, 9, 10. For all modes except for the central position, the coefficients C _k = 1; for modes located in the central position C _k = 1 · e ^iπ .

Figure 3 presents the amplitude and phase of a diffractive optical element designed to form a single trap.

The amplitude-phase distribution of the element forming such a superposition is shown in Fig.4.

If we ignore the amplitude component and consider only the phase, then at distances of 780-880 mm with an element diameter of 6 mm and a diameter of the illuminating beam of 4.4 mm, the following light field distributions are obtained, shown in Fig. 4. Distributions are obtained at equal distances from each other (20 mm).

As can be seen from figure 4, really formed a classic optical "bottle", closed in all three coordinates. Moreover, the relatively large size of the trap along the propagation axis (100 mm) refers only to its formation in free space. When focusing such a beam with a 90 × micro lens, its length is reduced to 20 μm. Trap efficiency is about 67%. Let us examine successively several increasingly complex configurations of optical “bottles”. We will consistently increase their number.

To form a double light “bottle”, a superposition of a larger number of Bessel modes is required. In this case, the 45 Bessel modes are arranged according to the scheme shown in Fig. 5.

In each of the positions in the diagram, there is a superposition of the same three Bessel modes of the 0th order with the same values of z ₀ = 800 mm, but with different numbers of the roots of the Bessel functions m = 8, 9, 10 (as for a single bottle). For all modes, the coefficients C _k were real numbers and were equal to:

- for modes located at points with coordinates [1; -eleven; eleven; -one]; [-one; 1] C _k = 2.75;

- for modes located at a point with coordinates [0; 0] C _k = 2.25;

- for other modes C _k = 2.0;

The amplitude-phase distribution of the element forming such a superposition is shown in Fig.6.

If we ignore the amplitude component and consider only the phase, then at distances of 780-880 mm with an element diameter of 6 mm and a diameter of the illuminating beam of 4.4 mm, the following light field distributions are obtained (Fig. 7). Distributions are obtained at equal distances from each other (20 mm).

The trap efficiency, in comparison with a single trap, decreased slightly and amounted to about 45%.

To form three adjacent traps, 66 Bessel modes must be arranged according to the scheme shown in Fig. 8.

In each of the positions in the diagram, there is a superposition of the same three Bessel modes of the 0th order with the same values of z ₀ = 800 mm, but with different numbers of the roots of the Bessel functions m = 8, 9, 10 (as for a single bottle). For all modes, the coefficients C _k were real numbers and were equal to:

- for modes located at points with coordinates [5; -fifteen; fifteen; -one]; [-5; 1] C _k = 1.75;

- for modes located at a point with coordinates [3; -one]; [3; 12; 0], [-2; -0]; [-3; -13; 1] C _k = 2.25;

- for modes located at a point with coordinates [6; 0]; [-6; 0] C _k = 1.5;

- for modes located at a point with coordinates [-1; -one]; [0; -2]; [one; -one]; [one; one]; [-one; one]; [0; 2] C _k = 2.5;

- for the rest of the mod.

The amplitude-phase distribution of the element forming such a superposition is shown in Fig.9.

If we ignore the amplitude component and consider only the phase, then at distances of 780-880 mm with an element diameter of 6 mm and a diameter of the illuminating beam of 4.4 mm, the following light field distributions are obtained (Fig. 10). Distributions are obtained at equal distances from each other (20 mm). The efficiency of this light trap is 29%.

Claims (1)

The method of moving a group of opaque micro-objects, which consists in forming a light beam with closed regions of zero intensity, characterized in that the light beam is formed from several beams, first use three coaxial Bessel beams of zero order with different propagation constants, forming a stable beam in the form of a round spot, then these beams are arranged in space so as to form one or more closed regions for capturing and moving opaque microparticles.

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