A diffractive optical device for generating a tunable hollow beam

Abstract

The present invention discloses a diffractive optical device for generating a tunable hollow beam, which relates to the field of optical field regulation. The device consists of periodic concentric rings, with bright intensities on the rings and dark outside the rings. The phase of each ring is periodically distributed between -π and π and satisfies a complex variable function. As demonstrated by simulations, by focusing angularly polarized light, a highly tunable hollow beam can be obtained. By adjusting the parameters of the structure, we can flexibly manipulate the size and length of the hollow beam. The length of the beam can be increased from 80λ to 280λ, and the maximum full width at half maximum increases from 0.43λ to 0.61λ. In addition, the light intensity and side lobes of the hollow beam can also be adjusted by the designed diffractive optical element. Potential applications of this tunable hollow beam include optical nano-manipulation, microscope imaging, and nano-lithography.

Description

Technical Field

[0001] The present invention relates to the field of optical field regulation, and particularly to a diffractive optical device capable of generating tunable hollow beams. Background Art

[0002] Hollow beams have broad application prospects in fields such as nanomachining, microimaging, lithography, particle manipulation, and optical wrenches. So far, several methods for generating hollow beams have been proposed, including modulating polarized beams to generate light needles or hollow beams with adjustable lengths; using a 4π confocal system to back-propagate the radiation field to generate multi-segment hollow beams or create a spatial light tube with specified characteristics; focusing vortex beams through a phase-type spatial light modulator to generate a segment of hollow beam with uniform intensity.

[0003] Although the methods reported above generate hollow beams with uniform light intensity, the properties such as the length and full width at half maximum (FWHM) of these hollow beams still need to be improved. Hollow beams with longer lengths and smaller FWHMs will have better effects in practical applications. In recent years, circular diffractive optical devices with subwavelength structures have provided a new solution for generating hollow beams with sub-diffraction characteristics. However, the adjustable parameters of such diffractive optical devices are often only structural parameters such as ring width and ring pitch, and there is less phase adjustment for the device; at the same time, although this method can effectively reduce the FWHM of the hollow beam, the length of the hollow beam generated by the current circular DOE is still short (the longest known is 100λ), and the tunability of the hollow beam is often limited, and its length, size, intensity, sidelobes and other parameters cannot be flexibly adjusted. Summary of the Invention

[0004] In view of the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a diffractive optical device, which combines a complex variable phase with an amplitude-type ring grating to solve the problems of short length and low tunability of the current hollow beam.

[0005] To achieve the above purpose, the present invention can adopt the following technical solutions:

[0006] A diffractive optical device for generating tunable hollow beams, comprising:

[0007] A plurality of periodic concentric rings, wherein the phase ψ on each ring periodically jumps between -π and π, and the periodic function is is the azimuth angle, the regulation factor n is used to regulate the light intensity of the generated hollow beam, and the regulation factor m is used to regulate the sidelobes of the generated hollow beam.

[0008] The diffractive optical device for generating a tunable hollow beam as described above, further: the period P of a plurality of periodic concentric rings is the sum of the ring width w and the ring pitch d, wherein the rings are light-transmissive and the space between two adjacent rings is light-impermeable.

[0009] The diffractive optical device for generating a tunable hollow beam as described above, further: the amplitude distribution a(θ) of a plurality of periodic concentric rings is a (0, 1) type distribution.

[0010] The diffractive optical device for generating a tunable hollow beam as described above, further: the transmission function of the light source of the diffractive optical device is where θ is the angle between each ring and the optical axis.

[0011] The diffractive optical device for generating a tunable hollow beam as described above, further: according to the wavelength of the incident light source, the focal length and radius of the diffractive optical device, and the regulation factors m and n, the complex amplitude distribution and the electric field distribution in the focal plane are calculated using the Debye-Wolf diffraction law. According to the obtained electric field distribution, the full width at half maximum function FWHM(w, d) of the hollow beam is obtained. Taking this full width at half maximum function as the objective function, the ring width w and the ring pitch d are optimized using a genetic algorithm, and the constraint condition is set as FWHM(w, d) < 0.5λ to obtain the optimal ring width w and ring pitch d.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention proposes a diffractive optical device with a novel complex variable phase. By combining the diffractive optical device with an annular grating, more variables are introduced into the annular diffractive optical device (DOE), and at the same time, more possibilities are provided for this diffractive optical device (DOE) in optical field regulation. Further, the longest length of the hollow beam generated by the novel diffractive optical device (DOE) proposed by the present invention can reach 248λ (264 μm), and this length of the hollow beam is also the longest known length at present. The length, light intensity, full width at half maximum function (FWHM), and side lobes of the hollow beam can be adjusted within a certain range by changing the period P of the structure and the magnitudes of the two regulation factors. Furthermore, the uniformity of the full width at half maximum function (FWHM) of the hollow beam generated by the present invention is relatively high, basically remaining above 96%. Based on this, such a flexibly adjustable hollow beam has potential applications in different fields because, as an optical potential well, they can trap high refractive index particles in a high-intensity region and confine low refractive index particles along their length. Therefore, the tunable hollow beam generated by the DOE provided by the present invention may provide a new approach for realizing optical micro- and nano-manipulation of particles. Description of the Drawings

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings required for use in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0014] Figure 1 This is a diffractive optical device with complex variable phase in the embodiments of the present invention.

[0015] Figure 2 This is the curve of the complex variable phase change of the DOE within the range of 0 to 180 degrees when the regulation factors n = 3 and m = 1 in the embodiments of the present invention.

[0016] Figure 3 This is the intensity distribution diagram of the hollow beam propagating along the z-axis numerically simulated in the embodiments of the present invention when n = 3, m = 3, and P = 2.6 μm.

[0017] Figure 4 This is the intensity distribution diagram of the hollow beam propagating along the z-axis numerically simulated in the embodiments of the present invention when n = 3, m = 6, and P = 2.6 μm.

[0018] Figure 5 This is the intensity distribution of the hollow beam at z = 200.8 μm in the x-y plane numerically simulated in the embodiments of the present invention when n = 3, m = 3, and P = 2.6 μm.

[0019] Figure 6 This is the enhanced distribution of the hollow beam at z = 200.8 μm in the x-y plane numerically simulated in the embodiments of the present invention when n = 3, m = 6, and P = 2.6 μm.

[0020] Figure 7 This is the intensity distribution diagram of the hollow beam propagating along the z-axis numerically simulated in the embodiments of the present invention when n = 1, m = 6, and P = 2.6 μm.

[0021] Figure 8 This is the relationship curve of the FWHM and length of the hollow beam varying with the period P numerically simulated in the embodiments of the present invention.

[0022] Figure 9 This is the relationship curve of the uniformity u of the FWHM of the hollow beam with different lengths varying with P numerically simulated in the embodiments of the present invention.

[0023] Figure 10 This is a schematic diagram of the incident light source generating a tunable hollow beam through a diffractive optical device in the embodiments of the present invention. Detailed implementation manners

[0024] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present application.

[0025] Embodiment:

[0026] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned accompanying drawings are used to distinguish similar objects, and do not necessarily need to be used to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described here can be implemented in an order other than those illustrated or described here. In addition, the terms "comprising" and "having" in the embodiments of the present invention and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device including a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0027] It should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0028] In the embodiments of the present invention, first, according to the number of rings, ring width, ring spacing, and phase distribution of each ring of the initial diffractive optical element (DOE), a constrained optimization problem of the light field intensity at the focus after the angularly polarized light passes through the diffractive optical element (DOE) proposed in the embodiments of the present invention is designed to determine the final structural parameters.

[0029] See Figure 1 , a diffractive optical element for generating a tunable hollow beam. The diffractive optical element (DOE) is composed of periodic concentric rings. Compared with the existing ring-type diffractive optical element (DOE), the embodiments of the present invention propose a new angular phase distribution, and the phase on each ring is The phase ψ periodically jumps between -π and π, where is the azimuth angle, n and m are two special control factors. The control factor n is used to control the light intensity of the generated hollow beam, and the control factor m is used to control the sidelobes of the generated hollow beam, so that the diffractive optical element (DOE) has tunable properties through the generated hollow beam.

[0030] Due to the introduction of these two control factors m and n and the adoption of the phase distribution, the diffractive optical element (DOE) of the embodiment of the present invention can flexibly control the phase distribution of each ring. Among them, when n increases, the interpolation response of this function will increase, and the number of jumps of this function between -π and π within one period will increase; when m increases, within the range of 0 to 180 degrees, the number of periods of this complex function will increase.

[0031] In the above embodiment, the period P of the diffractive optical element is the sum of the ring width w and the ring pitch d, that is, P = w + d. The number of periods (number of rings) is set to 40. The ring is a light-transmitting structure, and the space between rings is light-impermeable. That is, the amplitude distribution a(θ) of the structure is a (0, 1) type distribution.

[0032] In the above embodiment, the transmission function of the diffractive optical element (DOE) is where

[0033]

[0034] In the above formula, θ is the angle between the focused beam passing through each ring and the optical axis. This (0, 1) type amplitude distribution indicates that the ring is a light-transmitting structure, and the space between rings is light-impermeable.

[0035] When performing simulation, set the focal length to z, the device radius R and the sampling resolution N, and take m = n = 3. The incident light source uses angularly polarized light with a wavelength λ = 1.064 μm. According to the known transmission function and structural parameters, use the Debye-Wolf diffraction law to calculate the complex amplitude distribution U(x, y) on the focal plane, and the electric field distribution is |U(x, y) 2 |. At this time, when n increases, as the summation term, it will cause the transmission function of the structure to become larger, and finally the light intensity of the light field will increase. n plays a role in controlling the light intensity of the hollow beam; when m increases, the number of periods of the phase change will increase, and this drastic frequency change will push the sidelobes away from the center of the light field. m plays a role in controlling the sidelobes of the hollow beam.

[0036] According to the obtained electric field distribution, the full width at half maximum function FWHM(w, d) of the hollow beam is obtained. Taking this function as the objective function, the genetic algorithm is used to optimize w and d, which is transformed into a constrained optimization problem. The constraint condition is set as FWHM(w, d) < 0.5λ. When optimizing, the population number M of the genetic algorithm is set to 300, the crossover probability is 0.8, the mutation probability is 0.4, and the number of iterations is 500 times. After operations such as selection, crossover, and mutation, the final optimal solution is w = d = 1.3μm, the radius R = 104μm. At this time, the length of the hollow beam is 140λ, and the FWHM is 0.48λ.

[0037] Thus, based on the existing parameters and using the genetic algorithm to optimize and iterate the structural ring width and ring spacing, and taking the length function and full width at half maximum function (FWHM) of the generated hollow beam as the optimization objectives, the Debye diffraction theory is used to calculate the target optical field. When optimizing, both the regulation factors m and n are taken as 3. Finally, the optimized structural parameters are w = d = 1.3μm. At this time, the period P of the DOE is 2.6μm, the radius R = 104μm, the length of the hollow beam is 140λ, and the FWHM is 0.48λ, breaking through the diffraction limit.

[0038] As Figure 1 shown, the diffractive optical device of the present invention is composed of bright and dark alternating rings. The phase on each bright ring is distributed in the form of a complex variable function along the angular direction, while the dark ring between two bright rings is opaque, and the phase distribution thereon is all 0.

[0039] As Figure 2 shown, the change curve of the phase distribution on the ring within the range of 0 to 180 degrees jumps continuously between -π and π. When n increases, the interpolation response of this function will increase, and the number of jumps of this function between -π and π within one period will increase; while when m increases within the range of 0 to 180 degrees, the number of periods of this complex variable function will increase. This drastic phase change frequency plays a role in pushing the side lobes away.

[0040] As Figure 3 、 4 shown, when n = 3, P = 2.6μm and m are 3 and 6 respectively, the intensity diagrams of the hollow beam when propagating along the z-axis are only different in the side lobe part. When m = 3, the side lobe intensity near the hollow beam is relatively strong and basically equal to the main peak intensity. Such high-intensity side lobes will greatly limit the practical application of the hollow beam. Therefore, according to the characteristics of the above complex variable function, when we increase the value of m to 6, the side lobes in the central region of the hollow beam are effectively suppressed, and this result verifies our theoretical analysis.

[0041] From Figure 5 , Figure 6It can be seen that when m = 3, the light intensity distribution of the hollow beam in the x-y plane is consistent with the light intensity distribution in the z-axis direction. There is a petal-shaped high-intensity sidelobe around the central region. When m = 6, it has a good suppression effect on this high-intensity sidelobe.

[0042] Compared with Figure 4 the contrast shows that when m and P remain unchanged, when n increases from 1 to 3, the maximum light intensity value of the hollow beam also increases from 1 to 4. Figure 7 As shown in

[0043] As shown in the figure, the black line part shows that when the period P increases from 2.2 μm to 3.1 μm, the FWHM of different hollow beams generated by the DOE also increases from 0.43λ to 0.61λ; while the red line part shows that the length of the hollow beam increases from 98λ to 248λ. This figure reflects the regulation effect of P on the FWHM and length of the hollow beam. Figure 8

[0044] Figure 9

[0045]

[0046]

[0047] shows the uniformity of the FWHM of hollow beams of different lengths. When P increases from 2.2 μm to 3.1 μm, the uniformity u of the hollow beam remains above 96%.

[0048] Parameters such as the period and regulation factor of the diffraction optical device proposed in the embodiments of the present invention can be adjusted to manipulate the light intensity, size, length, and sidelobes of the generated hollow beam. Such flexibly adjustable hollow beams have potential applications in different fields. Because they can act as optical traps to capture high-refractive-index particles in high-intensity regions and confine low-refractive-index particles along their length. Therefore, the tunable hollow beams generated by the diffraction optical device (DOE) provided in the embodiments of the present invention may provide a new way to achieve optical micro- and nano-manipulation of particles.

[0048] In the description of this specification, the description referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0049] The above embodiments are only used to illustrate the technical concept and features of the present invention, and the purpose is to enable those of ordinary skill in the art to understand the content of the present invention and implement it accordingly. It is not intended to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A diffractive optical device for generating a tunable hollow beam, characterized in that, Including: A number of periodic concentric rings, where the phase ψ on each ring jumps periodically between -π and π, and the periodic function is the azimuth angle, the modulation factor n is used to modulate the intensity of the generated hollow beam, and the modulation factor m is used to modulate the sidelobes of the generated hollow beam; Among them, the period P of several periodic concentric rings is the sum of the ring width w and the ring pitch d. The rings are light-transmissive, while the area between two adjacent rings is light-opaque. The amplitude distribution a(θ) of several periodic concentric rings follows the distribution of formula (0, 1), that is θ is the angle between the focused light beam passing through each ring and the optical axis; the transmission function of the light source of the diffractive optical device is where 2. The diffractive optical device for generating a tunable hollow beam according to claim 1, characterized in that, According to the wavelength of the incident light source, the focal length and radius of the diffractive optical device, and the regulation factors m and n, the complex amplitude distribution and electric field distribution on the focal plane are calculated using the Debye-Wolf diffraction law. Based on the obtained electric field distribution, the full width at half maximum function FWHM(w, d) of the hollow beam is obtained. Taking this full width at half maximum function as the objective function, the ring width w and ring distance d are optimized using a genetic algorithm, and the constraint condition is set as FWHM(w, d) < 0.5λ to obtain the optimal ring width w and ring distance d.

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