A preparation device and method of a reflective curved forked surface grating

Abstract

The application discloses a preparation method of a reflective curved fork surface grating. The grating prepared by the method is a pure phase grating and can be used for generating perfect vortex light (POV). A high-order Bessel Gaussian beam is interfered with a Gaussian beam to expose a photosensitive material on a substrate, wherein the high-order Bessel Gaussian beam is generated by a reflective pure phase liquid crystal spatial light modulator (SLM), and after development and metal film plating, the reflective curved fork surface grating is prepared. When a Gaussian light of a base mode is used for irradiation, the POV can be obtained on a far field or a lens focal plane in a first-order diffraction direction. The method for obtaining the POV by using the reflective curved fork surface grating has the advantages of a higher damage threshold, a wider working wavelength range and a higher conversion efficiency, and has the advantages of simple structure, low cost and mass production, and has an important prospect in generation of high-power POV and can be used in the fields of optical manipulation and optical processing.

Description

Technical Field

[0001] This invention relates to the field of reflective surface gratings and vortex beam technology, and in particular to an apparatus and method for fabricating a reflective curved fork-shaped surface grating. Background Technology

[0002] Vortex light, as a core research area in singularity optics, has received widespread attention. A vortex beam is a beam exhibiting vortex characteristics; its phase or wavefront is helical, and its complex amplitude contains a helical phase term, which can be expressed as... ,in It is a topological charge. It is angular coordinates. Each photon in the vortex beam carries... The orbital angular momentum (OAM) of vortex beams can be transferred to the radiated particles. Vortex beams exhibit orthogonality, meaning that any two vortex beams of different orders are orthogonal to each other, and vortex beams of different orders can be separated from each other. Due to these unique properties, vortex beams have great potential value in many fields, such as optical communication, detection, optical tweezers and optical wrenches, optical processing, astronomical exploration, and quantum information processing.

[0003] In the aforementioned fields, vortex beams with large topological charges are often required to achieve better performance. However, the loop radius of traditional vortex beams increases with the increase of the topological charge. In optical manipulation, a large topological charge and a small spot size are needed to achieve good capture results. In optical communication, vortex beams with different topological charge values ​​need to be transmitted coaxially, but due to their different radii, it is difficult to couple them into a fixed optical fiber.

[0004] In 2013, Ostrovsky et al. first proposed the concept of perfect vortex beams, whose ring diameters are independent of topological charge values ​​[Opt Lett 38, 534-536 (2013)]. They obtained perfect vortex beams using a spatial light modulator, but multiple secondary bright rings were present, resulting in a low signal-to-noise ratio. In the same year, Chen et al. generated vortex beams using a conical lens in conjunction with a reflective spatial phase modulator (SLM), obtaining higher-quality perfect vortex beams and observing that they could be used to confine particles [Opt Lett 38, 4919-4922 (2013)]. In 2015, Pravin Vaity et al. superimposed the transmittance function of the conical lens and the phase mask onto the SLM, achieving control over the size of the perfect vortex beam by changing the cone angle [Opt Lett 40, 597-600 (2015)]. In 2016, Chaitanya cascaded a spiral phase plate and a conical lens to generate high-energy, high-order perfect vortex light [Opt Lett 41, 1348-1351 (2016)], but the optical path was very complex and the optical center was difficult to align. In practical applications, the most common method is to use a liquid crystal lamp (SLM) to generate perfect vortex light (POV), but this method is costly, the equipment is large, and its liquid crystal structure makes it difficult to increase the damage threshold, thus preventing the generation of high-power perfect vortex light. Summary of the Invention

[0005] This invention addresses the difficulty in fabricating high-power perfect vortex light by proposing a fabrication device and method for a reflective curved fork-shaped grating. This method is simple in principle, has a concise optical path, produces gratings with a high damage threshold, has a simple structure, can be mass-produced, and can generate high-power perfect vortex light.

[0006] The technical solution of the present invention is as follows:

[0007] A method for fabricating a reflective curved fork-shaped grating, characterized in that the method includes the following steps:

[0008] Step 1) Constructing the optical path: A laser, beam expander, linear polarizer, and depolarization beam splitter are coaxially arranged; the depolarization beam splitter splits the incident light into reflected light and transmitted light; a mirror group, a microscope objective with a pinhole, a first collimating lens, and a rotating base for placing the grating sample to be exposed are arranged sequentially along the optical path of the reflected light; an SLM is arranged along the optical path of the transmitted light, and the SLM is connected to the PC control terminal;

[0009] Step 2) Adjusting the optical path: Start the laser and adjust the magnification of the beam expander so that the diameter of the expanded beam spot is smaller than the short side length of the SLM screen. Adjust the angle of the SLM without loading phase information so that the light energy returns along the original path and is reflected by the depolarization beam splitter to form the second reflected light.

[0010] Step 3) Constructing the exposure optical path: Set up a Fourier lens, an adjustable aperture stop, an attenuator, a third reflecting mirror, and a second collimating mirror along the optical path of the second reflected light;

[0011] Step 4) Calculate the hologram loaded by the SLM, i.e., the phase increment Ф(x,y) provided by the incident fundamental mode Gaussian light, as shown in the following formula:

[0012]

[0013] In the formula, (x,y) are the position coordinates. For the topological charge value of a perfect vortex beam, The arctangent of the coordinates in the four quadrants Let λ be the wavenumber of the exposure beam, and λ be the wavelength of the laser. Let n be the phase parameter of the conical lens, where... The equivalent angle is the base angle of the cone lens, and n is the equivalent refractive index of the cone lens. and Let be the number of periods of the blazed grating in the x and y directions within the screen area, respectively. This causes the diffraction spots of each order to be distributed laterally;

[0014] , n and The following relationship exists between them:

[0015]

[0016] In the formula, D is the diameter of the aperture (8), and r POV The radius of the POV on the focal plane:

[0017]

[0018] L2 is the distance between the second-order diffracted beam and the 0th-order beam:

[0019]

[0020]

[0021] In the formula, f is the focal length of the lens, m is the diffraction order, and H is the actual x-direction length of the SLM display screen;

[0022] Under the control of the PC control terminal, the phase increment Ф(x,y) is mapped to the gray value range of (0, 255) using the following formula.

[0023]

[0024] The grayscale distribution map is loaded onto the SLM display screen. The object light is modulated by the SLM, and the light field of the first-order diffracted light is the Bessel-Gaussian light field.

[0025] Step 5) Adjust the angle φ between the object beam and the reference beam: Temporarily remove the vortex beam phase loaded by the SLM, retaining only the blazed grating phase; temporarily replace the sample to be exposed with a mirror; adjust the angle of the rotating base so that the object beam path returns along its original path; rotate the rotating base by φ / 2 angle, adjust the position and angle of the mirror group so that the reference beam path coincides with the object beam path, then the angle between the object beam path and the reference beam path is φ, and the grating period is:

[0026]

[0027] Step 6) Adjust the position and angle of the Fourier lens so that the front focal plane coincides with the SLM screen and the center is perpendicular to the first-order diffracted light. Adjust the position of the aperture stop so that it is coplanar with the rear focal plane of the lens. Adjust the aperture size so that only the first-order diffracted light passes through the aperture stop.

[0028] Step 7) Rotate the polarizer to select the polarization direction with the strongest relative intensity of the first-order diffracted light. The diffracted light is reflected by the mirror. Adjust the position of the collimating lens so that the front focal plane of the lens coincides with the plane of the aperture stop. Adjust the angle of the collimating lens so that the first-order diffracted light passes perpendicularly through the center of the lens. At this time, the light field on the back focal plane of the collimating lens is amplified by the light field on the front focal plane of the lens, and the magnification factor is the ratio of the focal lengths of the two lenses.

[0029] Step 8) The light field interferes with the reference light to form an interference light field. The intensity distribution of the interference light field is a curved fork-shaped fringe distribution. The grating sample to be exposed in the interference region is exposed and developed, and then a metal film is deposited to prepare a reflective curved fork-shaped grating.

[0030] The aforementioned depolarization beam splitter splits the incident light into reflected light and transmitted light without changing the polarization state of the incident beam, with a splitting ratio of 1:1.

[0031] The phase map loaded by the SLM is a grayscale map with a grayscale value range of 0~255, corresponding to a phase increment of 0~2π.

[0032] The PC control terminal is connected to the SLM and controls the phase diagram loaded by the SLM.

[0033] The grating sample to be exposed includes a substrate and a photosensitive material on the substrate, and the photosensitive material is uniformly covered on the substrate.

[0034] The substrate is a thin film or substrate made of fused silica, silicon carbide, silicon, or metal.

[0035] The metal material is gold or silver (Au / Ag), and the thickness of the metal film is 100~220 nanometers.

[0036] The photosensitive material is a photoresist, which undergoes chemical decomposition or polymerization reaction after being exposed to light of a specific wavelength.

[0037] Beneficial technical effects of the present invention:

[0038] This invention presents for the first time a method and system for generating perfect vortex light based on a reflective curved fork-shaped grating. Compared with the technology of generating POV using SLM, the optical path of this invention has the advantages of high conversion efficiency, low cost, high damage threshold, wide spectrum, and small size. It can obtain high-power perfect vortex light and has important application prospects in the fields of optical communication and optical manipulation. Attached Figure Description

[0039] Figure 1 : Schematic diagram of the optical path for fabricating the reflective curved fork-shaped grating of the present invention.

[0040] The components include: 1. Laser; 2. Beam expander; 3. Linear polarizer; 4. Depolarizing beam splitter; 5. Reflective pure phase liquid crystal spatial light modulator (SLM); 6. PC control terminal; 7. Fourier lens; 8. Adjustable aperture stop; 9. Attenuator; 10. Third mirror; 11. Second collimating mirror; 12. First mirror; 13. Second mirror; 14. Microscope objective with pinhole; 15. First collimating mirror; 16. Grating sample to be exposed; 17. Rotating base.

[0041] Figure 2 In this embodiment of the invention, the pattern loaded on the reflective pure phase liquid crystal spatial light modulator... Take 37.5. Setting it to 0 corresponds to a blazed grating period of 32 micrometers and a topological charge value. The value is set to 3, the refractive index n of the cone lens is set to 1.5, and the bottom angle parameter of the cone lens is... It is 0.8°.

[0042] Figure 3 The overall distribution of the diffraction field of the curved fork-shaped grating obtained by simulation in this embodiment of the invention. Figure 4 The +1 order distribution of the diffraction field of the curved fork-shaped grating obtained by simulation in this embodiment of the invention.

[0043] Figure 5The light intensity distribution at the plane where the aperture is located, obtained by the experiment in the embodiment of the present invention, shows a series of POV rings.

[0044] Figure 6 The +1 order distribution of the diffraction field of the curved fork-shaped grating obtained in the experiment of this embodiment of the invention. Detailed Implementation

[0045] The present invention will now be further described with reference to the embodiments and accompanying drawings. The following examples are only for explaining the present invention and do not limit its content. If specific experimental conditions are not specified in the embodiments, conventional conditions or conditions recommended by the sales company are generally followed.

[0046] Please see Figure 1 , Figure 1 This is a schematic diagram of the fabrication apparatus for the reflective curved fork-shaped grating of the present invention. As shown in the figure, the fabrication apparatus for the reflective curved fork-shaped grating of the present invention includes: 1. a laser; 2. a beam expander; 3. a linear polarizer; 4. a depolarizing beam splitter; 5. a reflective pure phase liquid crystal spatial light modulator (SLM); 6. a PC control terminal; 7. a Fourier lens; 8. an adjustable aperture stop; 9. an attenuator; 10. a third reflecting mirror; 11. a second collimating mirror; 12. a first reflecting mirror; 13. a second reflecting mirror; 14. a microscope objective with a pinhole; 15. a first collimating mirror; 16. a grating sample to be exposed; and 17. a rotating base.

[0047] A method for fabricating a reflective all-dielectric curved fork-shaped grating includes the following steps:

[0048] 1) Start laser 1, adjust the magnification of beam expander 2 so that the diameter of the expanded beam spot is slightly smaller than the short side length of the SLM (5) screen. Adjust the angle of SLM 5 without loading phase information so that the light energy returns along the original path. In this embodiment, the laser wavelength is 413 nanometers, the radius of the Gaussian beam spot after beam expansion is 3 millimeters, and the SLM screen size is 8.64 millimeters * 15.36 millimeters.

[0049] 2) Calculate the computational hologram required for SLM5. The phase increment provided by the computational hologram for the incident fundamental Gaussian light is:

[0050]

[0051] in, The topological charge value for a perfect vortex beam is set to 3. The arctangent of the coordinates in the four quadrants Let λ be the wavenumber of the exposure beam, and λ be the wavelength of laser 1, which is 413 nanometers. And n is the phase parameter of the conical lens. The equivalent angle is the base angle of the cone lens, and n is the equivalent refractive index of the cone lens, set to 1.5. and Let be the number of periods of the blazed grating in the x and y directions within the screen area, respectively. It is 37.5. A value of 0 corresponds to a blazed grating period of 32 micrometers. The lateral distribution of diffraction spots at each order and the intensity distribution at the focal plane are shown in the attached diagram. Figure 5 As shown.

[0052] Depend on , n and The relationship between them:

[0053]

[0054] Where D is the diameter of the aperture, which is 1 mm, and r POV The radius of the POV on the focal plane:

[0055]

[0056] L2 is the distance between the second-order diffracted beam and the 0th-order beam:

[0057]

[0058]

[0059] f is the focal length of lens 7, which is 30 mm; m is the diffraction order; H is the actual x-direction length of the SLM display screen, which is 15.36 mm. In this embodiment... The value range is 0.8.

[0060] 3) Map the phase increment Ф(x,y) to the grayscale range of (0, 255) using the following formula.

[0061]

[0062] The grayscale distribution map is loaded onto the SLM display screen, and the light field of the first-order diffracted light is the Bessel-Gaussian light field.

[0063] 4) Adjust the angle φ between the object beam and the reference beam. In this example, the value is 33.6°, corresponding to a grating line density of 1400. Temporarily remove the vortex beam phase loaded by the SLM and retain only the blazed grating phase. Temporarily replace the sample to be exposed with a mirror. Adjust the angle of the rotating base so that the object beam path returns along the original path. Rotate the rotating base by 16.8° and adjust the position and angle of the mirror so that the reference beam path coincides with the object beam path.

[0064] 5) Adjust the position and angle of the Fourier lens 7 so that the front focal plane coincides with the SLM screen and the center is perpendicular to the first-order diffracted light. Adjust the position of the aperture stop so that it is coplanar with the rear focal plane of the lens 7. Adjust the aperture size to 1 mm so that only the first-order diffracted light passes through the aperture stop.

[0065] 6) Rotate polarizer 3 to select the polarization direction with the strongest relative intensity of the first-order diffracted light. The diffracted light is reflected by the mirror. Adjust the position of the second collimating lens 11 so that the focal planes of the two lenses in the object light path coincide at the aperture. Adjust the angle of the second collimating lens 11 so that the light path passes perpendicularly through the center. At this time, the light field on the back focal plane of the collimating lens is a magnified version of the light field on the front focal plane of the first lens, and the magnification is the ratio of the focal lengths of the two lenses. In this embodiment, the focal length of lens 7 is 30 mm, the focal length of the second collimating lens 11 is 500 mm, and the magnification is 16.67 times.

[0066] 7) A 1.5 mm fused silica substrate was used as the substrate. A photoresist layer of approximately 200 nanometers thick was coated on the substrate.

[0067] 8) Set up the grating sample 16 to be exposed in the interference area. After exposure and development, deposit a 200-nanometer thick metal film on the photoresist grating layer. The material is gold (Au). The gold film and the photoresist grating layer are tightly bonded to prepare a reflective curved fork-shaped grating.

Claims

1. A method for fabricating a reflective curved fork-shaped grating, characterized in that, The method includes the following steps: Step 1) Constructing the optical path: A laser (1), a beam expander (2), a linear polarizer (3), and a depolarizing beam splitter (4) are coaxially arranged; the depolarizing beam splitter (4) divides the incident light into reflected light and transmitted light; a mirror group, a microscope objective with a pinhole, a first collimating lens (15), and a rotating base (17) for placing the grating sample (16) to be exposed are arranged sequentially along the optical path of the reflected light; an SLM (5) is arranged along the optical path of the transmitted light, and the SLM (5) is connected to the PC control terminal (6); Step 2) Adjust the optical path: Start the laser (1), adjust the magnification of the beam expander (2) so that the diameter of the expanded beam spot is smaller than the short side length of the SLM (5) screen. Adjust the angle of the SLM (5) without loading phase information so that the light energy returns along the original path and is incident on the depolarized beam splitter (4) to reflect and form the second reflected light. Step 3) Constructing the exposure optical path: Set up a Fourier lens (7), an adjustable aperture stop (8), an attenuator (9), a third reflecting mirror (10), and a second collimating mirror (11) along the optical path of the second reflected light. Step 4) Calculate the hologram loaded by SLM (5), that is, the phase increment Ф(x,y) provided by the incident fundamental mode Gaussian light, as shown in the following formula: In the formula, (x,y) are the position coordinates. For the topological charge value of a perfect vortex beam, The arctangent of the coordinates in the four quadrants Let λ be the wavenumber of the exposure beam, and λ be the wavelength of the laser (1). Let n be the phase parameter of the conical lens, where... The equivalent angle is the base angle of the cone lens, and n is the equivalent refractive index of the cone lens. and Let be the number of periods of the blazed grating in the x and y directions within the screen area, respectively. This causes the diffraction spots of each order to be distributed laterally; , n and The following relationship exists between them: In the formula, D is the diameter of the aperture (8), and r POV The radius of the POV on the focal plane: L2 is the distance between the second-order diffracted beam and the 0th-order beam: In the formula, f is the focal length of the Fourier lens (7), m is the diffraction order, and H is the actual x-direction length of the SLM display screen. Under the control of the PC control terminal (6), the phase increment Ф(x,y) is mapped to the gray value range of (0, 255) using the following formula. The grayscale distribution map is loaded onto the SLM (5) display screen. The object light is modulated by the SLM (5), and the light field of the first-order diffracted light is the Bessel-Gaussian light field. Step 5) Adjust the angle φ between the object beam and the reference beam: Temporarily remove the vortex beam phase loaded by the SLM, retaining only the blazed grating phase; temporarily replace the sample to be exposed with a mirror; adjust the angle of the rotating base (17) so that the object beam path returns along the original path; rotate the rotating base (17) by φ / 2 angle, adjust the position and angle of the mirror group (12, 13) so that the reference beam path coincides with the object beam path, then the angle between the object beam path and the reference beam path is φ, and the grating period is: Step 6) Adjust the position and angle of the Fourier lens (7) so that the front focal plane coincides with the SLM screen and the center is perpendicular to the first-order diffracted light. Adjust the position of the aperture (8) so that it is coplanar with the rear focal plane of the Fourier lens (7). Adjust the aperture size so that only the first-order diffracted light passes through the aperture. Step 7) Rotate the polarizer (3) to select the polarization direction with the strongest relative intensity of the first-order diffracted light. The diffracted light is reflected by the mirror (10). Adjust the position of the second collimating mirror (11) so that the front focal plane of the second collimating mirror (11) coincides with the plane where the aperture (8) is located. Adjust the angle of the second collimating mirror (11) so that the first-order diffracted light passes perpendicularly through the center of the second collimating mirror (11). At this time, the light field on the back focal plane of the second collimating mirror (11) is the magnification of the light field on the front focal plane of the Fourier lens (7). The magnification factor is the ratio of the focal lengths of the two lenses. Step 8) The light field interferes with the reference light to form an interference light field. The intensity distribution of the interference light field is a curved fork-shaped fringe distribution. The grating sample (16) to be exposed in the interference area is exposed and developed, and then a metal film is deposited to prepare a reflective curved fork-shaped grating.

2. The method for fabricating a reflective curved fork-shaped grating according to claim 1, characterized in that, The depolarization beam splitter (4) splits the incident light into reflected light and transmitted light without changing the polarization state of the incident beam, with a splitting ratio of 1:

1.

3. The method for fabricating a reflective curved fork-shaped grating according to claim 1, characterized in that, The phase map loaded by the SLM (5) is a grayscale map with a grayscale value range of 0~255 and a corresponding phase increment of 0~2π.

4. The method for fabricating a reflective curved fork-shaped grating according to claim 1, characterized in that, The PC control terminal (6) is connected to the SLM (5) and controls the phase diagram loaded by the SLM (5).

5. The method for fabricating a reflective curved fork-shaped grating according to claim 1, characterized in that, The grating sample (16) to be exposed includes a substrate and a photosensitive material on the substrate, and the photosensitive material is uniformly covered on the substrate.

6. The method for fabricating a reflective curved fork-shaped grating according to claim 5, characterized in that, The substrate is a thin film or substrate made of fused silica, silicon carbide, silicon, or metal.

7. The method for fabricating a reflective curved fork-shaped grating according to claim 6, characterized in that, The metal material is gold or silver (Au / Ag), and the metal film thickness is 100~220 nanometers.

8. The method for fabricating a reflective curved fork-shaped grating according to claim 5, characterized in that, The photosensitive material is a photoresist, which undergoes chemical decomposition or polymerization reaction after being exposed to light of a specific wavelength.

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