A curved forked grating structure and a curved forked grating thereof

Abstract

The present application belongs to the field of optical technology, and discloses a curved fork-shaped grating structure. In the curved fork-shaped grating, the curves formed by all the points in the same period and the same relative position are distributed in a curved fork shape, so that the incident light irradiated on the curved fork-shaped grating is converted into Bessel Gauss light. The curved fork-shaped grating provided by the present application has a simpler light path than the method for obtaining Bessel Gauss light in the prior art, and is suitable for a wide waveband. The curved fork shape can be realized by various gratings, such as metal gratings, dielectric gratings or metal-dielectric hybrid gratings, and meanwhile, the intrinsic characteristics of the gratings, such as diffraction efficiency, used waveband, diffraction angle, polarization characteristics and the like, are not affected, and only the diffracted light field carries a high-order Bessel phase.

Description

Technical Field

[0001] This invention relates to the field of diffraction gratings, and in particular to a grating with a curved fork structure that can convert Gaussian light into Bessel Gaussian light. Background Technology

[0002] A vortex beam is a beam of light exhibiting vortex characteristics. The phase or wavefront of this light is helical, and its complex amplitude contains a helical phase term, which can be expressed as: Where l is the topological charge. It is angular coordinate. Each photon in the vortex beam carries l. The orbital angular momentum (OAM) of a vortex beam can be transferred to the radiated particle. Vortex beams have many potential applications, such as in optical communication, manipulating tiny particles, and optical trapping. However, the ring radius of traditional vortex beams increases with the topological charge, thus limiting their application in many fields. In 2013, Ostrovsky et al. first proposed the concept of perfect vortex beams, whose ring diameter is independent of the topological charge [Opt Lett 38, 534-536 (2013)], which has attracted widespread attention. Perfect vortex beams are obtained through the Fourier transform of Bessel-Gaussian beams.

[0003] Currently, the most common way to generate Bessel-Gaussian light fields is to load a hologram using a liquid crystal spatial light modulator (SLM). However, this method is costly, has low diffraction efficiency, and its liquid crystal structure makes it difficult to improve the damage threshold. There are also methods that cascade spiral phase plates and conical lenses to obtain higher-order Bessel-Gaussian light [Opt Lett 41, 1348-1351 (2016)], thereby generating perfect vortex light. However, the optical path is very complex, the optical center is difficult to align, and it can only be applied to a single wavelength.

[0004] To the best of our knowledge, there are currently no reports on holographic diffraction gratings for mid- and long-wave infrared gratings, including their characteristic structures and fabrication methods. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a diffraction grating that can directly convert Gaussian light into Bessel Gaussian light, which has a characteristic curved fork-shaped grating structure.

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

[0007] A curved fork-shaped grating structure is characterized in that the curve formed by all points at the same relative position in their respective periods is curved, and a fork-shaped structure is distributed between only a pair of adjacent curved curves, so that the incident light illuminating the curved fork-shaped grating is converted into Bessel-Gaussian light.

[0008] The aforementioned curved fork-shaped grating lateral structure is specifically as follows:

[0009] Taking the bifurcation point of the fork-shaped structure as the pole O, the distance between any point M on the grating and point O as ρ, and the direction of the maximum periodic density of the curved fork-shaped grating as the polar axis Ox, the transverse distribution function of the curved fork-shaped grating in the polar coordinate system is expressed as:

[0010]

[0011] Where γ is the curvature factor of the curvature curve. Let be the polar angle, l be the topological charge, Λ be the average period of the grating, n be the difference in the number of periods between points M and O, and x0 be the relative position of point M within any grating period.

[0012] The bending factor γ is any positive real number, the topological charge l is any non-zero integer, the period difference n is any integer within the positive and negative N / 2 closed interval, N is the total number of periods within the entire grating aperture, and the relative position x0 is any real number between [0,1).

[0013] A curved fork-shaped grating of metal is characterized in that it adopts the above-mentioned curved fork-shaped grating structure, and the longitudinal structure from bottom to top consists of a substrate, a top grating layer and a metal coating.

[0014] A curved fork-shaped grating of a medium is characterized in that it adopts the above-mentioned curved fork-shaped grating structure, and the longitudinal structure from bottom to top consists of a substrate, a multilayer dielectric film and a top grating layer.

[0015] A metal-dielectric hybrid curved fork grating is characterized by employing the aforementioned curved fork grating structure, wherein the longitudinal structure consists of a substrate, a metal-dielectric hybrid layer, and a top grating layer from bottom to top.

[0016] The technical effects of this invention are as follows:

[0017] 1) Compared with conventional straight-striped gratings, the curved fork-shaped grating of the present invention can directly diffract incident Gaussian light into Bessel Gaussian light.

[0018] 2) The curved fork grating designed in this invention can effectively utilize the longitudinal structure of various conventional gratings and inherit their performance characteristics. It can achieve a more arbitrary and wider operating band compared to spiral conical lenses, and a higher damage threshold and diffraction efficiency compared to SLMs.

[0019] 3) This invention defines the stripe distribution of the top grating layer of the designed curved fork grating, which can more effectively guide the design and fabrication of the curved fork grating.

[0020] 4) This invention is compatible with various optical systems that use conventional straight-striped gratings, such as ultra-intense and ultra-short laser compressors and laser beam combining systems, and can generate Bessel-Gaussian light simply and directly. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the curved fork-shaped grating structure of the present invention. The horizontal structure is a curved fork-shaped structure, and the vertical structure can directly adopt the vertical structure of a conventional grating. The dashed lines represent the fork-shaped structures distributed between a pair of adjacent curved curves.

[0022] Figure 2 This is a schematic diagram of grating stripes obtained using the curved fork-shaped expression provided by the present invention.

[0023] Figure 3 This is a cross-sectional view of the longitudinal structure of the metal curved fork grating used in this embodiment of the invention, with the top gold layer having a thickness of 150 nanometers.

[0024] Figure 4 This example uses the curved fork-shaped expression provided by the present invention to simulate the height distribution at various locations on the surface profile of the top grating layer within a 2.7 mm side length around the center of the gold grating. Λ = 675.68 nm, l = 1, γ = 9.5 × 10⁻⁶. -4 .

[0025] Figure 5 The following is a simulation and experimental result of the intensity distribution of a metal curved fork grating actually fabricated using the curved fork distribution formula provided by the present invention, after the diffracted light passes through a lens with a focal length of 50 cm, and the intensity distribution changes with the propagation distance. The wavelength of the test light source is 413.1 nm, the incident angle is 15°45′, and the diffraction angle is 62°.

[0026] Figure 6 In this embodiment, the cylindrical mirror method was used to measure the intensity distribution of the topological charge value of the diffraction field of the actually fabricated metal curved fork grating. There is an inclined dark fringe between the two bright spots, which proves that the topological charge value of the diffraction field is 1. Detailed Implementation

[0027] To better understand the above technical solutions, the following will provide a more detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific embodiments.

[0028] This embodiment provides a metal curved fork grating, comprising: a substrate 2, a top grating layer 3, and a metal coating 4; the curved fork grating is used to receive p-polarized fundamental mode Gaussian light and obtain higher-order Bessel Gaussian light with a topological charge of l at the +1st diffraction order.

[0029] Figure 3This is a cross-sectional view of the metal curved fork-shaped grating structure of Embodiment 1 of the present invention. The substrate 2 is a quartz substrate, the grating layer 3 is a photoresist with a refractive index of 1.6 and a height of 200 nanometers, and the metal layer 4 is a gold (Au) metal with a thickness of 150 nanometers.

[0030] Design a curved fork-shaped grating. Define the outline of the grating over one complete cycle, with the relationship between its height h and its relative position x0 within the cycle taken as a trigonometric function:

[0031] h(x0)=[sin(2πx0-π / 2)+1]×h0

[0032] The trench depth h0 is set to 200 nanometers.

[0033] With the center of the fork-shaped structure as the pole O, and the distance between any point M on the grating and point O as ρ, and with the direction of maximum periodic density of the curved fork-shaped grating as the polar axis Ox, the transverse distribution function of the grating structure can be expressed in polar coordinates as:

[0034]

[0035] In this embodiment, the grating period Λ is 675.68 nanometers, corresponding to 1480 lines, and the total length of the grating fringe direction is 50 millimeters, with a total of 37,000 periods. n takes any integer value within the closed interval of ±18,500. x0 = 0, meaning the curve is drawn connecting the starting points of each period; in this embodiment, the starting point of the period is the center of the grating groove. The topological charge value l = 1, meaning the number of forks at the grating center is 1, and the bending parameter γ = 9.5 × 10⁻⁶. -4 .

[0036] Figure 4 The simulation results show the height distribution at various locations on the surface profile of the top grating layer within a side length of 2.7 mm. Obvious curved moiré fringes can be observed.

[0037] One way to obtain the designed metal curved fork-shaped stripe grating is to prepare the grating using holographic interference exposure.

[0038] First, a layer of photoresist is coated on a fused silica substrate, with a thickness of about 200 nanometers.

[0039] A holographic exposure interferometry method is used to record curved fork-shaped fringes on the surface of a grating.

[0040] By developing, a photoresist grating layer with a high sine function distribution is prepared on the photoresist.

[0041] A 150-nanometer-thick gold thin film was deposited on the photoresist grating layer to obtain a metal curved fork-shaped gold grating with a side length of 50 millimeters.

[0042] During testing, Bessel-Gaussian light is easier to observe its properties when passed through a lens. The light field produces an extremely fine, perfect vortex light ring on the focal plane that is independent of the topological charge value, while at positions far from the focal plane, it exhibits a concentric ring-shaped light intensity distribution. Figure 5 a(1,2) represents the simulation results of the first-order diffracted light obtained by passing the fundamental Gaussian light through a metal curved fork grating with the above parameters, and the light field intensity distribution in front of and on the focal plane after passing through a lens with a focal length of 50 cm.

[0043] Figure 5 b(1,2) represents the experimental results of the first-order diffracted light obtained by the fundamental Gaussian light passing through a fabricated metal curved fork grating, and then through a lens with a focal length of 50 cm, showing the light field intensity distribution in front of and on the focal plane. The laser source used in the test had a wavelength of 413.1 nm, an incident angle of 15°45′, and a diffraction angle of 62°.

[0044] Figure 6 To measure using a cylindrical lens with a focal length of 80 cm. Figure 5 The topological charge value of the perfect vortex light shown in b(2). In the intensity distribution diagram, there is a dark stripe between the two bright spots, indicating that the topological charge value of this perfect vortex light is 1, that is, the topological charge value of the Bessel-Gaussian light generated by the metal curved fork grating in this embodiment is 1.

[0045] In addition to being applicable to metal curved fork gratings, the curved fork grating structure in this invention can also be applied to dielectric or metal-dielectric hybrid gratings.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention.

Claims

1. A curved fork-shaped grating structure, characterized in that, The curves formed by all points in the same relative position in their respective periods are distributed in a curved pattern, and there is a fork structure between only a pair of adjacent curved curves, so that the incident light illuminating the curved fork grating is converted into Bessel Gaussian light. Taking the bifurcation point of the fork-shaped structure as the pole O, the distance between any point M on the grating and point O as ρ, and the direction of the maximum periodic density of the curved fork-shaped grating as the polar axis Ox, the transverse distribution function of the curved fork-shaped grating in the polar coordinate system is expressed as: Where γ is the curvature factor of the curve, φ is the polar angle, l is the topological charge, Λ is the average period of the grating, n is the difference in the number of periods between point M and point O, and x0 is the relative position of point M in any grating period. The topological charge value l is any non-zero integer, the period difference n is any integer within the positive and negative N / 2 closed interval, N is the total number of periods within the entire grating aperture, and the relative position x0 is any real number between [0,1).

2. The curved fork-shaped grating structure as described in claim 1, characterized in that: The curved fork-shaped grating is a metal, dielectric, or metal-dielectric hybrid grating.

3. A curved fork-shaped grating made of metal, characterized in that, The curved fork-shaped grating structure described in any one of claims 1-2 is adopted, and the longitudinal structure consists of a substrate, a top grating layer, and a metal coating layer from bottom to top.

4. A curved fork-shaped grating for a medium, characterized in that, The curved fork-shaped grating structure described in any one of claims 1-2 is adopted, and the longitudinal structure consists of a substrate, a multilayer dielectric film, and a top grating layer from bottom to top.

5. A metallic dielectric hybrid bent fork-shaped grating, characterized in that, The curved fork-shaped grating structure described in any one of claims 1-2 is adopted, and the longitudinal structure from bottom to top consists of a substrate, a metal-dielectric hybrid layer, and a top grating layer.

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