Nonlinear optical device for generating annular beam and preparation method thereof

Abstract

This application provides a nonlinear optical device for generating an annular beam and a method for preparing the same. The method includes the following steps: designing a shaping structure for a hollow cylindrical quasi-phase-matching grating based on the distribution characteristics of a pump beam and the quasi-phase matching mechanism; modifying a ferroelectric crystal using a femtosecond laser to induce the spatial distribution of the ferroelectric crystal's second-order nonlinear coefficient to adopt the shaping structure, thereby preparing a nonlinear optical device; and generating an annular second harmonic beam when the pump beam is incident on the prepared nonlinear optical device. The nonlinear optical device prepared by the preparation method provided in this application can simultaneously generate an annular beam and double frequency. The preparation method provided in this application reduces processing difficulty and effectively controls the dark spot ratio at the center of the beam, providing a competitive nonlinear optical device product for generating annular beams.

Description

Technical Field

[0001] The present application relates to the technical field of nonlinear beam shaping, and in particular to a nonlinear optical device for generating an annular beam and a preparation method thereof. Background Art

[0002] An annular beam is a structured beam whose amplitude distribution is characterized by a central aperture and annular radiation. The linear generation of an annular beam is achieved by adjusting the phase and amplitude of the incident beam through special optical elements (such as diffractive optical elements, spatial light modulators, phase plates, and metamaterials). In contrast, nonlinear beam shaping has the advantage of achieving both frequency conversion and beam intensity conversion, but its related research is still very limited. Nonlinear photonic crystals achieve quasi-phase matching by periodically changing the second-order nonlinear optical coefficient (χ(2)), which has attracted attention in the field of nonlinear annular beam shaping.

[0003] The technique of spatially modulating the second-order nonlinear coefficients with a near-infrared femtosecond laser is used to fabricate three-dimensional nonlinear photonic crystals. Compared to two-dimensional nonlinear photonic crystals, three-dimensional nonlinear photonic crystals provide an extra dimension to ensure that quasi-phase matching conditions are met.

[0004] Currently, there are two main structures of three-dimensional nonlinear photonic crystals used for annular beam shaping:

[0005] One is a three-dimensional fork-shaped grating nonlinear photonic crystal derived from binary computer-generated hologram theory. For example, in 2019, a femtosecond laser was used to fabricate a three-dimensional fork-shaped nonlinear photonic crystal in lithium niobate. Compared with the two-dimensional case, the conversion efficiency of nonlinear beam shaping was improved by two orders of magnitude.

[0006] Another approach is to use three-dimensional helical nonlinear photonic crystals based on nonlinear volume holography theory. In 2020, Imbrock et al. demonstrated the concept of using helical nonlinear structures for second harmonic beam shaping.

[0007] However, the complex cross-sectional structures of three-dimensional nonlinear photonic crystals remain a challenge. The optical resolution of femtosecond laser processing along the depth direction is relatively low. In addition, the use of motorized stages in the processing system can introduce errors when manufacturing continuously varying smooth structures. As a result, the structural errors of three-dimensional nonlinear photonic crystals are generally more severe than those of low-dimensional nonlinear photonic crystals. This can disrupt the phase matching of nonlinear photonic crystals, making it impossible to achieve the desired nonlinear optical functions.

[0008] To simplify the fabrication process and reduce manufacturing errors for three-dimensional nonlinear photonic crystals, Liu et al. proposed a discretized structure consisting of four units. This universal, simplified structure addresses the fabrication challenges associated with complex three-dimensional structures while retaining the functionality of pre-designed three-dimensional nonlinear photonic crystals. However, as the topological charge of the annular beam increases, the complexity of the structure and the number of corresponding discretized units also increase. To reduce the fabrication difficulty, a larger sampling period is required, but this approach still faces the potential problem of disrupting the phase matching of the nonlinear photonic crystal. The central dark spot ratio, denoted by "c," represents the ratio of the inner to outer beam diameters. It reflects the ratio of the central dark spot size to the beam width and is a key characteristic parameter of annular beams. Therefore, it is necessary to provide an easily fabricated three-dimensional nonlinear photonic crystal structure that can generate annular beams with a larger central dark spot ratio. Summary of the Invention

[0009] The present application provides a nonlinear optical device for generating an annular light beam and a preparation method thereof, which can solve the technical problem of difficulty in manufacturing a three-dimensional nonlinear photonic crystal structure for generating an annular light beam in the prior art.

[0010] In a first aspect, the present application provides a method for preparing a nonlinear optical device for generating an annular beam, comprising the following steps:

[0011] Step S1, designing a shaping structure of a hollow cylindrical quasi-phase matching grating according to the distribution characteristics of the pump beam and the quasi-phase matching mechanism;

[0012] Step S2, using a femtosecond laser to modify the ferroelectric crystal, inducing the spatial distribution of the second-order nonlinear coefficient of the ferroelectric crystal to be the shaped structure, thereby preparing a nonlinear optical device;

[0013] When the pump beam is incident on the prepared nonlinear optical device, a ring-shaped second harmonic beam is generated.

[0014] In combination with the first aspect, in one embodiment, in step S1, when the distribution characteristics of the pump beam satisfy the Gaussian distribution, the outer diameter of the shaping structure is the same as the outer diameter of the pump beam; by changing the hollow ratio of the shaping structure of the hollow cylindrical quasi-phase matching grating, the generated annular beam can achieve linear adjustment of the central dark spot ratio on the two-dimensional full-wave surface.

[0015] In combination with the first aspect, in one embodiment, in step S2, the focus energy of the femtosecond laser used is greater than the damage threshold of the ferroelectric crystal at the focus position.

[0016] In combination with the first aspect, in one embodiment, in step S2, the ferroelectric crystal is transparent to the wavelength of the femtosecond laser.

[0017] In combination with the first aspect, in one embodiment, in step S2, the femtosecond laser is a near-infrared femtosecond laser.

[0018] In combination with the first aspect, in one embodiment, in step S2, as the processing depth increases during modification, the pulse energy of the femtosecond laser is adjusted to compensate for focal spot distortion caused by aberration and absorption loss.

[0019] In combination with the first aspect, in one embodiment, adjusting the pulse energy of the femtosecond laser specifically includes the following steps:

[0020] The pulse energy of the femtosecond laser is adjusted from bottom to top starting from a preset depth below the crystal surface through an attenuator.

[0021] In conjunction with the first aspect, in one embodiment, the method further includes the following steps:

[0022] Characteristic measurements are performed on the generated annular second harmonic beam, and the performance of the nonlinear optical device is verified based on the obtained measurement results.

[0023] In a second aspect, a nonlinear optical device for generating an annular light beam is prepared using the above method for preparing a nonlinear optical device for generating an annular light beam.

[0024] The beneficial effects of the technical solutions provided in the embodiments of the present application include at least:

[0025] The nonlinear optical device prepared by the preparation method of the present application can generate a ring-shaped second harmonic beam after the pump beam is incident, and simultaneously realize the ring beam and double frequency;

[0026] The present application provides a method for preparing a nonlinear optical device using a hollow cylinder to generate an annular light beam. Compared with the preparation process of traditional fork-shaped or spiral-shaped nonlinear optical devices, it not only reduces the processing difficulty, but also can linearly adjust and effectively control the dark spot ratio of the center of the light beam by changing the hollow ratio of the cylindrical structure, providing a competitive nonlinear optical device product for generating annular light beams. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the structure and design principle of the nonlinear optical device provided in the embodiment of the present application;

[0028] Figure 2 (a) A partially hollow cylindrical three-dimensional nonlinear photonic crystal corresponding to a ring beam with a central dark spot ratio of 0.7 provided in an embodiment of the present application Second harmonic generation confocal microscope characterization diagram; Figure 2(b) is a characteristic end view of a portion of a hollow cylindrical three-dimensional nonlinear photonic crystal xz plane ring unit corresponding to a ring beam with a central dark spot ratio of 0.7 provided by an embodiment of the present application; Figure 2 (c) Raman spectra of the processed and unprocessed areas provided in the embodiments of the present application; Figure 2 (d) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0 provided in an embodiment of the present application; Figure 2 (e) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.1 provided in an embodiment of the present application; Figure 2 (f) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.2 provided in an embodiment of the present application; Figure 2 (g) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.3 provided in an embodiment of the present application; Figure 2 (h) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.4 provided in an embodiment of the present application; Figure 2 (i) Characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.5 provided in an embodiment of the present application; Figure 2 (j) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.6 provided in an embodiment of the present application; Figure 2 (k) is a characterization image of a hollow cylindrical three-dimensional nonlinear photonic crystal with a central dark spot ratio of 0.7 provided in an embodiment of the present application;

[0029] Figure 3 (a) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (b) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.1 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (c) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.2 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (d) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.3 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (e) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.4 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (f) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.5 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (g) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.6 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application; Figure 3 (h) Theoretical and experimental normalized intensity distribution diagrams of the circular second harmonic with a center dark spot ratio of 0.7 obtained when the pump fundamental frequency power is 0.1 mW, provided in an embodiment of the present application;

[0030] Figure 4 (a) is a normalized intensity map along a horizontal pixel line obtained from a single column of pixels of a CCD camera at the center of the light spot provided by an embodiment of the present application by generating a circular double frequency; Figure 4 (b) is a normalized intensity map along a vertical pixel line obtained from a single column of pixels of a CCD camera at the center of the light spot provided by an embodiment of the present application by generating annular double frequency; Figure 4 (c) shows the relationship between the ring second harmonic power and the pump power when the center dark spot ratio is 0.3; Figure 4 (d) shows the relationship between the ring second harmonic power and the fundamental wavelength when the center dark spot ratio is 0.3, as theoretically simulated and experimentally measured at an input power of 0.75 mW and a temperature of 20°C;

[0031] Figure 5 (a) A diagram of a device for optically manipulating a glass powder solution (diameter 4 to 10 μm) using an annular beam (c = 0.3) according to an embodiment of the present application; Figure 5 (b) is a capture image of the glass powder particle at position A under annular beam operation; Figure 5 (c) is a capture image of a glass frit particle moving laterally from position A to position B under an annular beam operation; Figure 5 (d) is a capture image of the glass powder particle at position C under annular beam operation; Figure 5 (e) is a capture image of a glass powder particle moving longitudinally from position C to position D under an annular beam operation; Figure 5 (f) is a captured image of glass powder particles in the observation window under annular beam operation;

[0032] Figure 5 (g) is a capture image of a glass powder particle being moved outside the observation window under an annular beam operation. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0034] The terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally includes steps or units that are not listed, or optionally includes other steps or units inherent to these processes, methods, products or devices. The terms "first", "second" and "third" are used to distinguish different objects, etc., and do not represent a sequence, nor do they limit the "first", "second" and "third" to different types.

[0035] In the description of the embodiments of this application, the words "exemplary," "for example," or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary," "for example," or "for example" in the embodiments of this application should not be construed as being preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary," "for example," or "for example" is intended to present the relevant concepts in a concrete manner.

[0036] In the description of the embodiments of the present application, unless otherwise specified, “ / ” means or, for example, A / B can mean A or B; “and / or” in the text is merely a description of the association relationship of associated objects, indicating that three relationships may exist, for example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, in the description of the embodiments of the present application, “multiple” refers to two or more than two.

[0037] In some processes described in the embodiments of the present application, multiple operations or steps are included that appear in a specific order. However, it should be understood that these operations or steps may not be performed in the order in which they appear in the embodiments of the present application or may be performed in parallel. The sequence numbers of the operations are only used to distinguish between different operations, and the sequence numbers themselves do not represent any order of execution. In addition, these processes may include more or fewer operations, and these operations or steps may be performed in sequence or in parallel, and these operations or steps may be combined.

[0038] In order to make the objectives, technical solutions and advantages of this application clearer, the implementation methods of this application will be further described in detail below with reference to the accompanying drawings.

[0039] In a first aspect, the present application provides a method for preparing a nonlinear optical device for generating an annular beam, comprising the following steps:

[0040] Step S1: designing a simple hollow cylindrical quasi-phase matching grating shaping structure based on the distribution characteristics of the pump beam and the quasi-phase matching mechanism to determine the position of the constituent unit three-dimensional modified domain;

[0041] Step S2: using a femtosecond laser to modify the interior of the ferroelectric crystal, inducing the spatial distribution of the second-order nonlinear coefficient of the ferroelectric crystal to be the shaped structure, thereby preparing a hollow cylindrical three-dimensional nonlinear optical crystal as a nonlinear optical device;

[0042] When the pump beam is incident on the prepared nonlinear optical device, a ring-shaped second harmonic beam is generated.

[0043] The nonlinear optical device prepared by the preparation method of the present application can generate a ring-shaped second harmonic beam after the pump beam is incident, and simultaneously realize the ring beam and double frequency;

[0044] The present application provides a method for preparing a nonlinear optical device using a hollow cylinder to generate an annular light beam. Compared with the preparation process of traditional fork-shaped or spiral-shaped nonlinear optical devices, it not only reduces the processing difficulty, but also can linearly adjust the dark spot ratio of the center of the light beam by changing the hollow ratio of the cylindrical structure, thereby achieving controllable changes in the dark spot area in the center of the annular light beam, providing a competitive nonlinear optical device product for generating annular light beams.

[0045] The structural diagram and design principle of the nonlinear optical device prepared in this application are as follows: Figure 1 As shown, the design principle applies to the undepleted pump approximation. The period length of the quasi-phase-matched grating determines the intensity and efficiency of second harmonic generation. Therefore, a hollow cylindrical periodic quasi-phase-matched grating structure with a transversely varying period length can be used to convert the ideal ring-shaped second harmonic intensity distribution and achieve the conversion of the ring-shaped second harmonic intensity distribution. By changing the interaction length of the transverse and longitudinal periods through a three-dimensional hollow cylindrical nonlinear photonic crystal, the conversion efficiency at the edge is higher, thus generating a ring-shaped second harmonic. In regions without periodicity, the second harmonic conversion efficiency is negligible.

[0046] In one embodiment, in step S1, when the distribution characteristics of the pump beam satisfy the Gaussian distribution, the outer diameter of the shaping structure is determined at different propagation distances, that is, it is the same as the outer diameter of the pump beam, quasi-phase matching is performed along the propagation direction y-axis, and the annular beam is shaped in two dimensions of the x-axis and the z-axis.

[0047] In one embodiment, in step S2, the focus energy of the femtosecond laser used is greater than the damage threshold of the ferroelectric crystal at the focus position, and the femtosecond laser is used to generate a modified region on the surface or inside the ferroelectric crystal.

[0048] In one embodiment, in step S2 , the ferroelectric crystal is transparent to the wavelength of the femtosecond laser.

[0049] In one embodiment, in step S2, the femtosecond laser is a near-infrared femtosecond laser.

[0050] In one embodiment, in step S2, as the processing depth increases during modification, the pulse energy of the femtosecond laser is adjusted to compensate for focal spot distortion caused by aberration and absorption loss, thereby ensuring the overall uniformity of the prepared nonlinear photonic crystal.

[0051] In one embodiment, the following steps are further included:

[0052] The pulse energy of the femtosecond laser is adjusted from bottom to top starting from a preset depth below the crystal surface through an attenuator.

[0053] In one embodiment, the following steps are further included:

[0054] Characteristic measurements are performed on the generated annular second harmonic beam, and the performance of the nonlinear optical device is verified based on the obtained measurement results.

[0055] In one embodiment, the following steps are further included:

[0056] Glass powder solutions with different particle sizes were optically manipulated using the generated annular second harmonic beam to verify the feasibility of the generated annular second harmonic beam in optical trapping.

[0057] In a second aspect, a nonlinear optical device for generating an annular light beam is prepared using the above method for preparing a nonlinear optical device for generating an annular light beam.

[0058] In a specific embodiment, a method for preparing a nonlinear optical device for generating an annular beam is described by taking a lithium niobate crystal and a pump beam having a Gaussian distribution as an example. The method includes the following steps:

[0059] Step 1: According to the distribution characteristics of the pump beam and the quasi-phase matching mechanism, a simple hollow cylindrical quasi-phase matching grating shaping structure is designed to determine the component units (from the attached Figure 1 It can be seen that the prepared hollow cylindrical three-dimensional nonlinear photonic crystal is a structure that is periodically distributed along the propagation direction with a fixed propagation distance. The component units are as follows: Figure 2 (b) The location of the three-dimensional modified domains of the ring unit of the periodic structure shown in Figure 1. This step utilizes the maximum nonlinear coefficient d of the lithium niobate crystal. 33Since the distribution characteristics of the pump beam satisfy the Gaussian distribution, the outer diameter of the annular unit structure of each periodic structure of the shaping structure along the propagation direction is determined, that is, the same as the outer diameter of the pump beam. Specifically, the outer diameter is 100 μm;

[0060] Step 2: Using a femtosecond laser to modify the ferroelectric crystal, inducing the spatial distribution of the second-order nonlinear coefficient of the ferroelectric crystal to be the shaping structure, and preparing a hollow cylindrical three-dimensional nonlinear photonic crystal. The three-dimensional nonlinear photonic crystal is used as a nonlinear optical device, that is, focusing the femtosecond laser into the interior of the ferroelectric crystal to process a nonlinear photonic crystal that converts a three-dimensional Gaussian beam into a ring beam for beam shaping.

[0061] During femtosecond laser modification processing:

[0062] A femtosecond laser writing system was used to fabricate a three-dimensional ring-shaped nonlinear photonic crystal in a z-cut 5% MgO-doped lithium niobate crystal sample.

[0063] The light source is a fs pulse laser (pulse width: 190 fs; repetition rate: 200 kHz) from a regeneratively amplified Yb:KGW laser system, emitting at a wavelength of 1026 nm;

[0064] The sample is placed on a high-precision displacement stand;

[0065] The laser was focused by a 50× microscope objective (NA=0.42), and the spot size of the processing focal plane was about 1 μm.

[0066] During the processing, the femtosecond laser is polarized along the x-direction of the crystal. The key to preparing three-dimensional nonlinear photonic crystals is to maintain the overall uniformity of the structure.

[0067] In step 2, in order to compensate for the focal spot distortion caused by aberrations and absorption losses, the pulse energy needs to be adjusted as the depth increases. The pulse energy can be continuously adjusted by an attenuator consisting of a half-wave plate and a polarization plate, starting from a depth of 120 microns below the crystal +z surface and processing 20 layers upwards. The pulse energy from the bottom to the top layer is controlled according to the depth, ranging from 10μJ to 2.5μJ, and the scanning speed of the writing process is 100μm·s -1 .

[0068] Figure 2 This is a characterization diagram of a hollow cylindrical three-dimensional nonlinear photonic crystal. Figure 2 The scale line length is 20 microns, and the total length of the hollow cylindrical three-dimensional nonlinear photonic crystal is 630 microns, including Second harmonic generation confocal microscopy and Raman spectroscopy were used for structural observation, Figure 2 (a)-2(b) and Figure 2 (d)-2(k) is given by The images were observed using a second harmonic generation confocal microscope (FVMPE-RS, Olympus).

[0069] Figure 2 (a) shows a portion of a hollow cylindrical three-dimensional nonlinear photonic crystal corresponding to the generated annular beam with a central dark spot ratio of 0.7; the outer diameter of the xz plane annular unit is 100 μm, and its characteristic end is as follows Figure 2 (b) Figure 2 (c) shows the Raman spectra of the processed and unprocessed areas. The processed modified area shows significantly weaker Raman spectral characteristic peaks, which indicates a significant decrease due to physical structural changes. According to the Raman spectrum, the depletion rate is 0.275. The modification of the femtosecond laser pulse causes changes in the χ(2) and refractive index of the crystal. By measuring the intensity of the central spot and the intensity of the non-zero-order diffraction spot, it can be calculated that the change in refractive index is about 8×10 -3 . Figure 2 (d)-2(k) shows the radial cross section of the nonlinear photonic crystal corresponding to the ring beam with a central dark spot ratio of 0 to 0.7. According to the quasi-phase matching condition, the basic beam wavelength is 1030nm and the period is Λ y =6.3μm, the duty cycle is 1 / 6.3, the modified area presents an obvious periodic structure and a uniform morphology. As the ratio of the central dark spot increases, the hollow area of ​​the nonlinear photonic crystal also expands.

[0070] Step 3: Use the nonlinear optical device prepared in this application to shape the beam from Gaussian to annular, generate the annular second harmonic beam, and measure its characteristics. Verify the performance of the nonlinear optical device based on the measurement results. Specifically, inject a pump beam into the nonlinear photonic crystal prepared in Step 2 and measure the characteristics of the emitted target annular beam to verify the performance of the nonlinear optical device in this application.

[0071] In order to demonstrate the controllable linear change of the annular intensity distribution, the present application generates annular second harmonics with a central dark spot ratio ranging from 0 to 0.7. The present application uses a set of laser beam reduction groups and uses the nonlinear optical device prepared by the present application to perform shaping from Gaussian to annular beams to obtain a pump beam with a spot diameter of 100 microns. The laser beam reduction group includes a concave lens (focal length 15 mm) and a convex lens (focal length 750 mm). The basic beam wavelength is 1030 nm, the pulse duration is 270 fs, and the repetition frequency is 100 kHz. The y-axis of the sample is set in the propagation direction of the input light, and the z-axis of the sample is set perpendicular to the propagation direction. A CCD camera is used to capture images. The second harmonic emitted by the crystal is expanded by a 4x beam expander. The CCD collects data at a distance of 100 mm from the emission end of the crystal. When measuring power, the CCD is replaced by a power meter with a resolution accuracy of 0.1 microwatts. The pump light is independently incident on Figure 2 (d)-2(k) structure.

[0072] Figure 3 The theoretical and experimental normalized intensity distributions of the second harmonic beam at a pump power of 0.1 mW are shown. Figure 3 (a)-3(h) show the ring-shaped second harmonic with a central dark spot ratio between 0 and 0.7. The experimentally obtained second harmonic intensity is in good agreement with the corresponding simulated distribution.

[0073] Figure 4 (a) shows the pixel distribution along the horizontal line ( Figure 3 The normalized intensity of Figure 4 (b) shows the pixel values ​​along the vertical pixel line ( Figure 3 The theoretical peak intensities of the annular second harmonic (corresponding to the central dark spot ratio: 0.1-0.7) are 0.961, 0.850, 0.695, 0.521, 0.368, 0.237 and 0.140 respectively. Figure 4 (a), the experimentally obtained horizontal average peak intensities (i.e., the average of the two peaks) are 0.863, 0.779, 0.575, 0.383, 0.260, 0.081, and 0.047, respectively. Figure 4As shown in (b), the average peak intensity values ​​along the vertical direction are 0.825, 0.759, 0.540, 0.406, 0.250, 0.096, and 0.063, respectively. The experimental values ​​of the central dark spot ratios (CDRs) of the annular second harmonic (corresponding to theoretical values: 0.1-0.7) are 0.0997, 0.1986, 0.2884, 0.3625, 0.471, 0.6325, and 0.6827 horizontally, and 0.1131, 0.1904, 0.2903, 0.3794, 0.4507, 0.6259, and 0.6704 vertically. A CDR c value closer to 1 indicates a larger annular dark spot region. The deviations between the experimental and theoretical values ​​are due to slight changes in the refractive index caused by laser-induced changes in the lithium niobate. However, these errors are within acceptable limits and do not affect the agreement between the experimental and theoretical values. Figure 4 (c) shows the relationship between the ring second harmonic power and the pump power. The effective nonlinearity is 0.223pm / V. The theoretical simulation curve is in good agreement with the experimental results. When the fundamental power is 1mW (peak power ≈ 37kW), the power of the ring second harmonic reaches 0.0107mW with c = 0.3, and the conversion efficiency is about 1.07×10 -2 Given the low conversion efficiency, the small signal approximation is sufficient. Increasing the quasi-phase-matched grating length, the fundamental frequency input energy, and adjusting the duty cycle of the periodic structure can help improve the conversion efficiency. Figure 4 (d) shows the relationship between the theoretically simulated and experimentally measured c = 0.3 ring second harmonic power and fundamental wavelength when the input power is 0.75 mW and the temperature is 20 °C.

[0074] Step 4: If Figure 5 As shown, the present application uses the generated annular beam (c=0.3) to optically manipulate a glass powder solution (4 to 10 microns in diameter), demonstrating the feasibility of the generated annular beam in optical trapping. Figure 5 (a) Demonstration of the optical manipulation device. Figure 5 (b) and 5(c) show the lateral movement of the particle from position A to B, while Figure 5 (d) and 5(e) show the longitudinal movement of the particle from position C to D. Figure 5 (f) and 5(g) show that the particles in the red dashed box are moved outside the observation window.

[0075] It should be noted that the serial numbers of the above-mentioned embodiments of the present application are for description only and do not represent the advantages or disadvantages of the embodiments.

[0076] Through the description of the above implementation methods, those skilled in the art can clearly understand that the above-mentioned embodiment methods can be implemented by means of software plus the necessary general hardware platform, of course, it can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art, can be embodied in the form of a software product, which is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk) as described above, and includes a number of instructions for enabling a terminal device to execute the methods described in each embodiment of the present application.

[0077] The above are only preferred embodiments of the present application and do not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A method for preparing a nonlinear optical device for generating an annular beam, characterized in that: The following steps are involved: Step S1: designing a shaping structure of a hollow cylindrical quasi-phase matching grating according to the distribution characteristics of the pump beam and the quasi-phase matching mechanism; Step S2, using a femtosecond laser to modify the ferroelectric crystal, inducing the spatial distribution of the second-order nonlinear coefficient of the ferroelectric crystal to be the shaped structure, thereby preparing a nonlinear optical device; When the pump beam is incident on the prepared nonlinear optical device, a ring-shaped second harmonic beam is generated.

2. The method for preparing a nonlinear optical device for generating an annular light beam according to claim 1, wherein: In step S1, when the distribution characteristics of the pump beam satisfy the Gaussian distribution, the outer diameter of the shaping structure is the same as the outer diameter of the pump beam, and by changing the hollow ratio of the shaping structure of the hollow cylindrical quasi-phase matching grating, the generated annular beam can achieve linear adjustment of the central dark spot ratio on the two-dimensional full-wave surface.

3. The method for preparing a nonlinear optical device for generating annular light beam according to claim 1, wherein: In step S2, the focus energy of the femtosecond laser used is greater than the damage threshold of the ferroelectric crystal at the focus position.

4. The method for preparing a nonlinear optical device for generating annular light beam according to claim 1, wherein: In step S2, the ferroelectric crystal is transparent to the wavelength of the femtosecond laser.

5. The method for preparing a nonlinear optical device for generating annular light beam according to claim 1, wherein: In step S2, the femtosecond laser is a near-infrared femtosecond laser.

6. The method for preparing a nonlinear optical device for generating an annular light beam according to claim 1, wherein: In step S2, as the processing depth increases during modification, the pulse energy of the femtosecond laser is adjusted to compensate for focal spot distortion caused by aberration and absorption loss.

7. The method for preparing a nonlinear optical device for generating an annular light beam according to claim 6, wherein: The step of adjusting the pulse energy of the femtosecond laser specifically comprises the following steps: The pulse energy of the femtosecond laser is adjusted from bottom to top starting from a preset depth below the crystal surface through an attenuator.

8. The method for preparing a nonlinear optical device for generating an annular light beam according to claim 1, wherein: The following steps are also included: Characteristic measurement is performed on the generated annular second harmonic beam, and the performance of the nonlinear optical device is verified based on the obtained measurement results.

9. A nonlinear optical device for generating an annular beam, characterized in that: The nonlinear optical device for generating annular light beam is prepared by the preparation method of the nonlinear optical device for generating annular light beam as described in claims 1 to 8.

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