An integrated and efficient device and method for generating nonlinear Airy beams

Abstract

The present invention discloses an integrated and efficient device and method for generating nonlinear Airy beams, which relate to the fields of light field control and nonlinear optical technology. The nonlinear crystal of the present invention is polished and cut at a specific angle to meet phase matching conditions to achieve efficient nonlinear frequency conversion, and then a focused ion beam (FIB) is used to process a depth-gradient microstructure on its surface. When fundamental frequency Gaussian light is incident on the interior of the crystal from a non-modulated surface, efficient frequency conversion occurs, and the generated nonlinear harmonics are subjected to cubic phase modulation after passing through the depth-gradient microstructure, and then Fourier transformed through an objective lens, generating a nonlinear Airy beam in the Fourier plane of the objective lens. The structure is simple, compact, highly integrated, and has high nonlinear conversion efficiency. By designing the parameters of the cubic phase structure, any nonlinear Airy beam can be generated.

Description

Technical Field

[0001] The present invention relates to the fields of light field control and nonlinear optics, and in particular to a device and method for integrating and efficiently generating nonlinear Airy beams. Background Art

[0002] Airy beams, due to their non-diffraction, self-accelerating, and self-recovering properties, have important applications in optical manipulation, optical imaging, and optical communications. Siviloglou et al. demonstrated that after Fourier transforming an Airy beam to the frequency domain, its spectrum exhibits a Gaussian distribution with a cubic phase factor. Therefore, experimentally, an Airy beam can be obtained by inverse Fourier transforming a cubic phase-modulated Gaussian beam. In recent years, research on nonlinear frequency conversion of Airy beams has become a hot topic. In 2009, researchers achieved the generation of a one-dimensional frequency-doubled Airy beam using nonlinear photonic crystals, marking the first demonstration of nonlinear frequency conversion of an Airy beam. However, this method only generates one-dimensional nonlinear Airy beams. Methods based on nonlinear Raman-Nyes diffraction have also been used to generate nonlinear Airy beams. However, due to phase mismatch, the nonlinear frequency conversion efficiency of this process is low. Furthermore, there have been reports on the generation of nonlinear Airy beams using nonlinear metasurfaces, a method that offers high integration but low efficiency. The researchers also proposed a method of first generating a fundamental frequency Airy beam and then using a nonlinear crystal to perform nonlinear frequency conversion to generate a nonlinear Airy beam. This scheme has problems such as complex experimental methods, cumbersome experimental optical paths, and low integration.

[0003] Therefore, those skilled in the art are committed to developing an efficient and integrated device and method for generating nonlinear Airy beams to promote the application of Airy beams in the entire optical band. Summary of the Invention

[0004] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present invention is that the existing method for generating nonlinear Airy beams has problems such as complex experimental methods, cumbersome experimental optical paths, low efficiency, and low integration.

[0005] To achieve the above objectives, the present invention provides an integrated and efficient device for generating nonlinear Airy beams, characterized in that it includes a Gaussian light source, a nonlinear crystal that meets phase matching conditions and has a cubic phase microstructure prepared on its surface, an objective lens, a filter, a lens, a tube lens, and a charge-coupled device (CCD); the objective lens is used to perform Fourier transform; the fundamental frequency Gaussian beam undergoes nonlinear frequency conversion through the nonlinear crystal and then undergoes cubic phase modulation through the microstructure; the nonlinear Gaussian beam loaded with the cubic phase is then Fourier transformed through the objective lens, generating a nonlinear Airy beam in the Fourier plane of the objective lens; and the distribution of the Airy beam in different observation planes is verified by moving the CCD to verify the properties of the generated nonlinear Airy beam.

[0006] Preferably, the cutting angle of the nonlinear crystal is calculated according to the nonlinear characteristics of the selected crystal, the wavelength of the nonlinear process and the polarization conditions, and the cubic phase microstructure is designed and prepared by the FIB process after grinding and polishing to ensure the uniformity of the subsequent light field.

[0007] Preferably, the phase distribution and processing depth of the cubic phase satisfy the relationship where h0 = λ SH / (n SH -n air ), λ SH , n SH , n air are the wavelength of the frequency-doubled light and the refractive index of the frequency-doubled light in the crystal and air respectively.

[0008] Preferably, the size of the cubic phase microstructure is on the order of micrometers; and the phase distribution is a gradual phase from 0 to 2π.

[0009] The present invention also provides a method for generating a nonlinear two-dimensional Airy beam, comprising the following steps:

[0010] Step 1: Select a nonlinear crystal material, and polish the upper and lower surfaces of the crystal, with the upper and lower surfaces being the light-transmitting surfaces;

[0011] Step 2: Use FIB to process cubic phase microstructure. The phase distribution and processing depth of the cubic phase satisfy the relationship where h0 = λ SH / (n SH -n air ), λ SH , n SH , n air The wavelength of the frequency-doubled light and the refractive index of the frequency-doubled light in the crystal and air are respectively used. The deepest FIB etching depth is calculated based on the refractive index of the selected crystal. Different height information structures can be created by different grayscale colors. Then, etching is performed to obtain a nonlinear crystal that meets the phase matching conditions and has a cubic phase microstructure prepared on its surface.

[0012] Step 3: Focus the fundamental frequency Gaussian light onto the nonlinear crystal through the objective lens for nonlinear frequency conversion to obtain nonlinear Gaussian light. Where A is the amplitude of the nonlinear Gaussian light, a is the cutoff factor of the Airy beam, and k is the Fourier spectrum coordinate. The nonlinear Gaussian beam that has undergone cubic phase modulation passes through a filter to remove the fundamental frequency component. The expression of the nonlinear Gaussian beam loaded with cubic phase modulation is: Two lenses are used to form a 4f system. The focal plane of the collection lens is moved outside the objective lens, and then the image is formed on the CCD through the tube lens for observation. The expression of the nonlinear Airy beam generated by Fourier transform is:

[0013] E=Ai[s-(ξ / 2) 2 +iaξ]exp[as-(aξ 2 / 2)-i(ξ 3 / 12)+i(a 2 ξ / 2)+i(sξ / 2)]; where s is the dimensionless length coordinate and ξ is the normalized propagation distance; the efficiency of generating nonlinear Airy beam is in and is the average power of the fundamental frequency and the doubled frequency light and the peak power of the fundamental frequency light, A is the effective spot area, and L is the nonlinear action length.

[0014] Preferably, the nonlinear crystal in step 1 needs to be ground and polished, and the polishing accuracy is much smaller than the wavelength scale to ensure the uniformity of the wavefront of the subsequently generated light field and facilitate the processing of the microstructure.

[0015] Preferably, in step 2, when performing microstructural processing, the nonlinear crystal sample is first cleaned with acetone, isopropyl alcohol, SPM solution and deionized water; and the crystal surface is processed using a dual-beam FIB system.

[0016] Preferably, in step 2, in order to avoid charge accumulation during the manufacturing process, a gold film is deposited on the surface of the nonlinear crystal using a sputtering coating device before processing, and finally the remaining gold layer is removed using a gold removal solution.

[0017] Technical Effects

[0018] The nonlinear Airy beam generating device and method provided by the present invention have a simple, compact and highly integrated structure and high nonlinear conversion efficiency. By designing the cubic phase structure parameters, any nonlinear Airy beam can be generated. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of a cubic phase microstructure processed by FIB according to a preferred embodiment of the present invention;

[0020] Figure 2 FIG1 is an experimental optical path diagram for generating a nonlinear Airy beam according to a preferred embodiment of the present invention;

[0021] Figure 3 This is the distribution of the nonlinear Airy beam generated by a preferred embodiment of the invention. DETAILED DESCRIPTION

[0022] The following describes several preferred embodiments of the present invention with reference to the accompanying drawings to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms of embodiments, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0023] In the drawings, components with identical structures are denoted by the same reference numerals, and components with similar structures or functions are denoted by similar reference numerals. The size and thickness of each component shown in the drawings are arbitrary and are not limited by the present invention. For clarity, the thickness of components in some places in the drawings is appropriately exaggerated.

[0024] Example 1:

[0025] This embodiment provides an integrated and efficient device for generating a nonlinear Airy beam, comprising a Gaussian light source, a nonlinear crystal that satisfies phase matching conditions and has a cubic phase microstructure fabricated on its surface, an objective lens, a filter, a lens, a tube lens, and a charge-coupled device (CCD). The nonlinear crystal has a cutting angle calculated based on the nonlinear characteristics of the selected crystal, the wavelength of the nonlinear process, and polarization conditions, and is then polished to ensure uniformity of the subsequent light field. The cubic phase microstructure is fabricated using a FIB process based on the crystal properties and structural parameters. The objective lens is used to perform a Fourier transform. To generate a nonlinear Airy beam, a fundamental frequency Gaussian beam undergoes nonlinear frequency conversion through the nonlinear crystal and then undergoes cubic phase modulation through the microstructure. The nonlinear Gaussian beam loaded with the cubic phase then undergoes a Fourier transform through the objective lens, generating a nonlinear Airy beam in the Fourier plane of the objective lens. The properties of the generated nonlinear Airy beam can be verified by moving the CCD to observe the distribution of the Airy beam in different observation planes.

[0026] Example 2:

[0027] This embodiment provides a method for generating a nonlinear two-dimensional Airy beam. In this embodiment, the nonlinear material selected is preferably a 0.5 mm thick lithium niobate (LN) crystal containing 5 mol% MgO. The upper and lower surfaces of the crystal are well polished, and the upper and lower surfaces serve as light-transmitting surfaces. In the experiment, a laser with a wavelength of 1064 nm, a repetition rate of 1 kHz, and a pulse width of 10 ns was selected as the light source to output a horizontally polarized Gaussian beam. To meet the oo-e phase matching condition, calculations show that the birefringence phase matching angle of the nonlinear crystal is 75° at room temperature.

[0028] Cubic phase microstructures processed by FIB Figure 1As shown, in order to better characterize the quality of the processed cubic phase microstructure, the right side is an enlarged view of a portion of the microstructure. The size of the cubic phase microstructure is 60μm×60μm. The phase distribution is a gradual phase from 0 to 2π. The phase distribution of the cubic phase and the processing depth satisfy the relationship where h0 = λ SH / (n SH -n air ), λ SH , n SH , n air h0 is the wavelength of the frequency-doubled light and the refractive index of the frequency-doubled light in the crystal and air, respectively. Based on the refractive index of the LN crystal, it can be calculated that the deepest FIB etching depth is h0 = 0.42 μm. The LN sample is first cleaned with acetone, isopropyl alcohol, SPM solution, and deionized water. A dual-beam FIB system with a 30 kV gallium beam is used to treat the LN crystal surface. Different grayscale colors can be used to create structures with different height information, which are then etched. During the etching process, the beam current is set to 4 nA. To prevent charge accumulation during the fabrication process, a gold film approximately 50 nm thick is deposited on the LN crystal surface using sputtering equipment before processing. Finally, a gold removal solution is used to remove the remaining gold layer.

[0029] The experimental optical path for generating nonlinear Airy beams is shown in the figure below: Figure 2 As shown. Fundamental frequency Gaussian light First, the 20× objective lens (OL1) is focused onto the sample. The nonlinear Gaussian beam after cubic phase modulation passes through the filter to remove the fundamental frequency component. The expression of the nonlinear Gaussian beam loaded with cubic phase modulation is: Since the focal plane of the collecting lens for Fourier transform cannot be observed inside the objective lens, two lenses with a focal length of f = 300mm are used to form a 4f system. The focal plane of the collecting lens is moved outside the objective lens and then imaged onto the CCD through a tube lens for observation. The expression of the nonlinear Airy beam generated by Fourier transform is E = Ai[s-(ξ / 2) 2 +iaξ]exp[as-(aξ 2 / 2)-i(ξ 3 / 12)+i(a 2 ξ / 2)+i(sξ / 2)]. Figure 2 Where z0 is the initial position of the nonlinear Airy beam. The distribution of the generated nonlinear Airy beam is as follows: Figure 3 The efficiency of generating nonlinear Airy beam is in and is the average power of the fundamental frequency and the doubled frequency light and the peak power of the fundamental frequency light, A is the effective spot area, and L is the nonlinear action length.

[0030] The preferred embodiments of the present invention have been described in detail above. It should be understood that numerous modifications and variations based on the concepts of the present invention are possible without inventive effort by those skilled in the art. Therefore, any technical solution that can be derived by one skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. An integrated and efficient device for generating nonlinear Airy beams, characterized in that: The invention comprises a Gaussian light source, a nonlinear crystal that satisfies phase matching conditions and has a cubic phase microstructure prepared on its surface, a first objective lens, a second objective lens, a filter, a lens, a tube lens, and a charge-coupled device (CCD); the second objective lens is used for Fourier transform; the fundamental frequency Gaussian beam undergoes nonlinear frequency conversion through the nonlinear crystal and then undergoes cubic phase modulation through the microstructure; the nonlinear Gaussian beam loaded with the cubic phase undergoes Fourier transform through the second objective lens, and a nonlinear Airy beam is generated in the Fourier plane of the second objective lens; the distribution of the Airy beam in different observation planes is verified by moving the CCD to verify the properties of the generated nonlinear Airy beam.

2. The device according to claim 1, wherein The cutting angle of the nonlinear crystal is calculated based on the nonlinear characteristics of the selected crystal, the wavelength of the nonlinear process, and the polarization conditions. After grinding and polishing to ensure the uniformity of the subsequent light field, the cubic phase microstructure is designed and prepared by the FIB process.

3. The device as claimed in claim 1, wherein the size of the cubic phase microstructure is on the order of micrometers; and the phase distribution is a gradual phase change from 0 to 2π.

4. A method for generating a nonlinear two-dimensional Airy beam using the device according to any one of claims 1 to 3, characterized in that: The following steps are involved: Step 1: Select a nonlinear crystal material, and polish the upper and lower surfaces of the crystal, with the upper and lower surfaces being the light-transmitting surfaces; Step 2: Use FIB to process cubic phase microstructure. The phase distribution and processing depth of the cubic phase satisfy the relationship , , , The wavelength of the frequency-doubled light and the refractive index of the frequency-doubled light in the crystal and air are respectively used to calculate the deepest FIB etching depth based on the refractive index of the selected crystal. By using different grayscale colors, structures with different height information are created, and then etching is performed to obtain a nonlinear crystal that meets the phase matching conditions and has a cubic phase microstructure prepared on its surface. Step 3: Focus the fundamental frequency Gaussian light onto the nonlinear crystal through the first objective lens for nonlinear frequency conversion to obtain nonlinear Gaussian light. , where A is the amplitude of the nonlinear Gaussian light, a is the cutoff factor of the Airy beam, and k is the Fourier spectral coordinate. The nonlinear Gaussian beam subjected to cubic phase modulation passes through a filter to remove the fundamental frequency component. The expression of the nonlinear Gaussian beam subjected to cubic phase modulation is: Two lenses are used to form a 4f system. The focal plane of the second objective lens is moved outside the objective lens, and then imaged onto the CCD through the tube lens for observation. The expression of the nonlinear Airy beam generated by Fourier transform is: ; where s is the dimensionless length coordinate, is the normalized propagation distance; the efficiency of generating nonlinear Airy beam is in and is the average power of the fundamental and doubled frequency light and the peak power of the fundamental frequency light, is the effective spot area, and L is the nonlinear action length.

5. The method according to claim 4, wherein In step 1, the nonlinear crystal needs to be ground and polished, and the polishing accuracy is much smaller than the wavelength scale to ensure the uniformity of the subsequent light field wavefront and facilitate the processing of microstructures.

6. The method according to claim 4, wherein In the step 2, when performing microstructure processing, the nonlinear crystal sample is first cleaned with acetone, isopropyl alcohol, SPM solution and deionized water; and the crystal surface is processed using a FIB system with a dual beam.

7. The method according to claim 4, wherein In step 2, in order to avoid charge accumulation during the manufacturing process, a gold film is deposited on the surface of the nonlinear crystal using a sputtering coating device before processing, and finally the remaining gold layer is removed using a gold removal solution.

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