A grating structure capable of generating space-time vortex light and a preparation method thereof

Abstract

The present invention discloses a grating structure capable of generating space-time vortex light and a preparation method thereof, which belongs to the field of nano grating technology. The grating structure is composed of a number of one-dimensional photonic crystal cells arranged periodically; the one-dimensional photonic crystal cells include: a high Si grating, a short Si grating and a SiO2 substrate, and the high Si grating and the short Si grating are arranged on the SiO2 substrate. The cross-sections of the high Si grating and the short Si grating are both rectangular. The length of the one-dimensional photonic crystal cell is 400nm, the height of the high Si grating is 150nm, the height of the short Si grating is 55nm, and the interval between the high Si grating and the short Si grating is 50nm. The present invention can enable the passing femtosecond laser pulse to generate a space-time optical vortex, and does not need to pass through multiple optical components, thereby effectively reducing the volume of the equipment.

Description

Technical Field

[0001] The present invention belongs to the technical field of nano gratings, and in particular relates to a grating structure capable of generating space-time vortex light and a preparation method thereof. Background Art

[0002] The space-time optical vortex is a new type of optical vortex. It has the characteristic of carrying transverse orbital angular momentum, which makes it show novel characteristics in optical phenomena and has attracted much attention in recent years. Its characteristic of carrying transverse orbital angular momentum provides an additional degree of freedom for the manipulation of light, which is of great significance for the spatiotemporal control of light-matter interaction. Unfortunately, the existing methods for generating space-time optical vortices are plagued by various limitations, such as low efficiency, bulky size and complexity. In terms of cost, existing equipment, such as 4f pulse shapers, require a large number of high-precision gratings, lenses and phase plates, and the manufacturing cost of each component is high. In addition, the equipment requires professional debugging, and the labor cost is high; the nonlinear photonic crystal generation of space-time optical vortex scheme requires high-purity materials, complex process flow and precision equipment. In terms of size and integration, the Fourier transform-based pulse shaper composed of many optical elements such as gratings requires a larger footprint, which is not conducive to integration into small and micro devices, such as structured light communications, quantum information processing equipment, etc. Therefore, it is necessary to develop a more miniaturized, lower-cost and simpler space-time optical vortex generation device to meet different application requirements. Summary of the invention

[0003] The present invention aims to solve the deficiencies of the prior art and proposes a grating structure that can generate space-time vortex light and a preparation method thereof. The prepared grating structure can generate space-time light vortex in a relatively simple manner and does not require a complex optical lens combination.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] A grating structure capable of generating space-time vortex light, wherein the grating structure is composed of a plurality of one-dimensional photonic crystal cells arranged periodically;

[0006] The one-dimensional photonic crystal unit cell comprises: a tall Si grating, a short Si grating and a SiO2 substrate, wherein the tall Si grating and the short Si grating are arranged on the SiO2 substrate.

[0007] Preferably, the cross-sections of the tall Si grating and the short Si grating are both rectangular.

[0008] Preferably, the length of the one-dimensional photonic crystal unit cell is 400 nm, the height of the tall Si grating is 150 nm, the height of the short Si grating is 55 nm, and the interval between the tall Si grating and the short Si grating is 50 nm.

[0009] The present invention also provides a method for preparing a grating structure capable of generating space-time vortex light, the method being used to prepare any of the above-mentioned grating structures, comprising the following steps:

[0010] S1. Depositing a 150 nm thick amorphous silicon layer on a 500 μm thick molten silicon substrate by plasma enhanced chemical vapor deposition to obtain an amorphous silicon film, and cleaning the surface of the amorphous silicon film using oxygen plasma;

[0011] S2. applying a positive electron beam resist on the amorphous silicon film by dynamic coating, performing soft baking after coating to remove the solvent of the positive electron beam resist, and then applying an anti-charging conductive polymer on the surface of the positive electron beam resist again by dynamic coating, and performing soft baking again;

[0012] S3. using a high-energy accelerated electron beam to photoetch the high Si grating on the surface of the positive-modulation electron beam resist, and developing it in ethyl acetate, then using an electron beam evaporator to deposit an aluminum layer on the surface of the positive-modulation electron beam resist to obtain an aluminum mask, and then using N-methylpyrrolidone to strip the positive-modulation electron beam resist;

[0013] S4. Reactive ion etching is performed using CF4 to transfer the pattern of the aluminum mask to the amorphous silicon film;

[0014] S5. Then, the remaining aluminum mask is removed with an aluminum etchant, and then the S2 is repeated;

[0015] S6. Repeat S3 to expose and photolithography the high Si grating and the short Si grating,

[0016] S7. Repeat S4, transfer the pattern of the aluminum mask to the amorphous silicon film, and photolithographically form a gap between the high Si grating and the low Si grating;

[0017] S8. Cleaning the aluminum mask with an aluminum etchant to obtain the one-dimensional photonic crystal cell;

[0018] S9. Periodically arrange a number of the one-dimensional photonic crystal cells to obtain the grating structure.

[0019] Preferably, the method of applying positive-modulated electron beam resist comprises: applying 200 nm thick resist on the amorphous silicon film at a rotation speed of 4000 rpm, and baking at 180° C. for 3 minutes.

[0020] Preferably, the method of coating the anti-charging conductive polymer comprises: coating the anti-charging conductive polymer on the surface of the positive-modulated electron beam resist at a rotation speed of 4000 rpm, and baking at 90° C. for 90 seconds.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention comprises two Si gratings, one high and one low, and a SiO2 substrate to form a group of one-dimensional photonic crystal cells, which are repeatedly formed periodically. By adopting the technical solution of the present invention, the passing femtosecond laser pulse can generate a spatiotemporal optical vortex without passing through multiple optical components, thereby effectively reducing the size of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0024] Figure 1 is a schematic diagram of a grating structure according to an embodiment of the present invention;

[0025] Figure 2 A schematic diagram of a one-dimensional photonic crystal cell structure according to an embodiment of the present invention;

[0026] Figure 3 Schematic diagram of the simulation structure of the fourth embodiment of the present invention, wherein (a) is a schematic diagram of four ports of the resonant mode, and (b) is a schematic diagram of a single cell model in the simulation. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 the present invention.

[0028] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Embodiment 1

[0030] In this embodiment, a grating structure capable of generating space-time vortex light is provided. The grating structure is composed of a number of one-dimensional photonic crystal cells arranged periodically, such as Figure 1 As shown; the one-dimensional photonic crystal cell is as follows Figure 2As shown, it includes: a tall Si grating, a short Si grating and a SiO2 substrate, and the tall Si grating and the short Si grating are arranged on the SiO2 substrate. The cross-sections of the tall Si grating and the short Si grating are both rectangular. The length of the one-dimensional photonic crystal unit cell is 400nm, the height of the tall Si grating is 150nm, the height of the short Si grating is 55nm, and the interval between the tall Si grating and the short Si grating is 50nm.

[0031] Embodiment 2

[0032] In this embodiment, a method for preparing a grating structure capable of generating space-time vortex light comprises the following steps:

[0033] S1. A 150 nm thick amorphous silicon layer is deposited on a 500 μm thick molten silicon substrate using plasma enhanced chemical vapor deposition to obtain an amorphous silicon film, and the surface of the amorphous silicon film is cleaned using oxygen plasma.

[0034] S2. Apply positive-modulation electron beam resist on the amorphous silicon film by dynamic coating. Specifically, apply 200nm thick resist on the amorphous silicon film at a rotation speed of 4000rpm. After coating, soft baking is performed to remove the solvent of the positive-modulation electron beam resist to enhance its adhesion. Specifically, it is baked at 180°C for 3 minutes. In order to reduce the charging effect during electron beam lithography, an anti-charging conductive polymer is again applied on the surface of the positive-modulation electron beam resist by dynamic coating. Specifically, an anti-charging conductive polymer is applied on the surface of the positive-modulation electron beam resist at a rotation speed of 4000rpm, and soft baking is performed again. Specifically, it is baked at 90°C for 90 seconds.

[0035] S3. Use a 30keV high-energy accelerated electron beam to photolithograph a high Si grating on the surface of the positive-modulation electron beam resist and develop it in ethyl acetate. Then use an electron beam evaporator to deposit a 30nm thick aluminum layer on the surface of the positive-modulation electron beam resist to obtain an aluminum mask to protect the high Si grating part. Subsequently, use N-methylpyrrolidone to strip the positive-modulation electron beam resist.

[0036] S4. Reactive ion etching is performed using CF4 to transfer the pattern of the aluminum mask to the amorphous silicon film. Specifically, optical emission spectroscopy technology is used to track the etching process in real time during the etching process to ensure that the amorphous silicon has a thickness of 55nm after etching.

[0037] S5. Then, the remaining aluminum mask is removed using an aluminum etchant, and then S2 is repeated.

[0038] S6. Repeat S3 to expose and photolithography the tall Si grating and the short Si grating to ensure that the subsequently deposited aluminum mask can protect the etched grating.

[0039] S7. Repeat S4 to transfer the pattern of the aluminum mask to the amorphous silicon film and photolithography the space between the tall Si grating and the short Si grating.

[0040] S8. Clean the aluminum mask with an aluminum etchant to obtain a one-dimensional photonic crystal cell.

[0041] S9. Arrange a number of one-dimensional photonic crystal cells periodically to obtain a grating structure. In this embodiment, 2500 one-dimensional photonic crystal cells are arranged periodically in the horizontal direction and stretched directly in the vertical direction to obtain a grating with a specification of 1 mm×1 mm, such as Figure 1 shown.

[0042] Embodiment 3

[0043] In this embodiment, when the grating structure is working, the intrinsic topological singularity caused by the breaking of the C2 symmetry and z-mirror symmetry of the nanograting is used to perform Fourier transform on the spiral phase in the momentum-frequency space to the space-time domain to generate a space-time optical vortex. When the laser pulse passes through the nanograting, a part of the wavefront of the incident pulse is changed by the dispersion relation of the optical resonance. Specifically: the interaction between the pulse and the nanograting can be decomposed into two processes: the first part of the pulse energy is transmitted directly through the nanograting without interacting with the resonant mode; the second part excites the resonance and radiates outward, which theoretically produces two different resonant interactions at ±kx. The overall transmitted light field g(ω,k x ) is transmitted to the pulse g(τ,x) in the final observed time-space domain, where ω represents the angular frequency of the incident pulse light, k x represents the wave vector, τ represents time, and x represents the one-dimensional space coordinate. Then, through two interlaced Fourier transform relationships, the far field is obtained as g(τ,x)={g(ω,k x )}, indicating that in ω-k x A field with a spiral phase in the domain can be transformed into a field with a spiral phase in the τ-x domain, which is the required space-time vortex light.

[0044] Nanogratings refer to gratings with nanoscale periodic structures. These structures can produce optical phenomena such as diffraction and interference when light waves pass through, thereby realizing the manipulation of light waves. The working principle of nanogratings is based on the diffraction and interference principles of light, and light waves are modulated through periodic structures. In this embodiment, by manipulating the height of two Si gratings, one high and one low, the frequency corresponding to the spiral phase center can be changed, thereby generating space-time vortices for light of different wavelengths. Because changing the grating height can change the overall symmetry, and the generation of space-time vortex light is related to the symmetry of the grating.

[0045] The embodiment of the present invention manipulates the symmetry of the grating, that is, the height of the two Si gratings, so that the incident laser pulse interacts with the resonant mode, thereby generating space-time vortex light. The simulation results confirm that the complex transmission coefficient of the system is x The spiral phase and zero-value singularity are generated in the domain. Since the electromagnetic field is invalid at the center of the vortex, and the transmission coefficient reflects the characteristics of the outgoing electromagnetic field, ω-k x The field with a spiral phase in the τ-x domain can be transformed into a field with a spiral phase in the τ-x domain, indicating that the outgoing electromagnetic field also has a spiral phase and a zero-value singularity in the τ-x domain, which is the required space-time vortex light.

[0046] Embodiment 4

[0047] In this embodiment, a theoretical derivation for generating space-time vortex light is given:

[0048] The interaction between the pulse and the nanograting can be decomposed into two processes: the first part of the pulse energy is transmitted directly through the nanograting without interacting with the resonant mode; the second part excites the resonance and radiates outward. In theory, two different resonant interactions will be generated at ±kx, with 4 ports, such as Figure 3 As shown in (a) in the figure, the leakage rates are denoted as γ1, γ2, γ3 and γ4.

[0049] Express the transmission coefficient s as a complex number:

[0050] ,

[0051] ,

[0052] ,

[0053] Where Re(s) represents the real part of the transmission coefficient, Im(s) represents the imaginary part of the transmission coefficient, r represents the reflection coefficient, t represents the transmission coefficient, and satisfies r 2 +t 2 =1, ω0 represents the resonant frequency.

[0054] The phase φ is obtained as:

[0055] ,

[0056] In this embodiment, we focus on the two-dimensional frequency-momentum domain. x and ω read together from P 1 To P 2 When changing, along the path P 1 →P 2 The accumulated phase Ψ in the complex plane [Re(s),Im(s)] can be expressed as:

[0057] ,

[0058] In ω-k x In the domain, the transmission coefficient s is eliminated at the winding center of the phase spiral, that is:

[0059] ,

[0060] Point P that meets this condition s It is called a zero-value singular point. If an isolated singular point P s , then along the circle P s The phase accumulation of any closed path is 2π, and the number of turns of the path is:

[0061] ,

[0062] Therefore, the zero-value singular point shows a topological charge l = -1 in the transmission of the grating. Combining the above formula, it can be obtained by Re[s(P s )]=Im[s(P s )]=0Find the x The conditions satisfied by the zero-value singular point in the domain are:

[0063] ,

[0064] In addition, in order to generate a space-time optical vortex, the zero-value singularity of the transmission parameter must be isolated, so as to meet the requirements of the space-time optical vortex "light intensity hollow, phase spiral". In order to achieve this, the present invention breaks the C2 symmetry and z-mirror symmetry in the plane, which makes γ1=γ2 not always true, and then introduces two degrees of freedom for the singularity of the transmission coefficient, so in ω-k x Only one singularity can be found in the domain, and this is the required space-time light vortex.

[0065] This embodiment uses COMSOL to perform simulation, and the specific steps are as follows:

[0066] Step 1: Draw a single cell and build a model, such as Figure 3 As shown in (b) in the figure. The model from top to bottom is: air layer, air layer (for integral calculation), Si grating, SiO2 substrate (for integral calculation), SiO2 substrate. The specific parameters are shown in Table 1.

[0067] Table 1

[0068] ;

[0069] Step 2: The mesh size is set to the physical field control size, and the simulation is performed using the "Electromagnetic Waves, Frequency Domain (ewfd)" research module in COMSOL. Periodic boundary conditions are set in the x-direction, and scattering boundary conditions are set in the y-direction.

[0070] Step 3: After calculation, the transmission coefficient produces an isolated zero-value singular point in the modulus value and a spiral in the phase.

[0071] The above embodiments are only descriptions of the preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should fall within the protection scope determined by the claims of the present invention.

Claims

1. A grating structure capable of generating space-time vortex light, characterized in that: The grating structure is composed of a number of one-dimensional photonic crystal cells arranged periodically; The one-dimensional photonic crystal unit cell comprises: a tall Si grating, a short Si grating and a SiO2 substrate, wherein the tall Si grating and the short Si grating are arranged on the SiO2 substrate.

2. A grating structure capable of generating space-time vortex light according to claim 1, characterized in that: The cross sections of the tall Si grating and the short Si grating are both rectangular.

3. The grating structure capable of generating space-time vortex light according to claim 1, characterized in that: The length of the one-dimensional photonic crystal unit cell is 400 nm, the height of the tall Si grating is 150 nm, the height of the short Si grating is 55 nm, and the interval between the tall Si grating and the short Si grating is 50 nm.

4. A method for preparing a grating structure capable of generating spatiotemporal vortex light, the method being used to prepare the grating structure according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Depositing a 150 nm thick amorphous silicon layer on a 500 μm thick molten silicon substrate by plasma enhanced chemical vapor deposition to obtain an amorphous silicon film, and cleaning the surface of the amorphous silicon film using oxygen plasma; S2. applying a positive electron beam resist on the amorphous silicon film by dynamic coating, performing soft baking after coating to remove the solvent of the positive electron beam resist, and then applying an anti-charging conductive polymer on the surface of the positive electron beam resist again by dynamic coating, and performing soft baking again; S3. using a high-energy accelerated electron beam to photoetch the high Si grating on the surface of the positive-modulation electron beam resist, and developing it in ethyl acetate, then using an electron beam evaporator to deposit an aluminum layer on the surface of the positive-modulation electron beam resist to obtain an aluminum mask, and then using N-methylpyrrolidone to strip the positive-modulation electron beam resist; S4. Reactive ion etching is performed using CF4 to transfer the pattern of the aluminum mask to the amorphous silicon film; S5. Then, the remaining aluminum mask is removed with an aluminum etchant, and then the S2 is repeated; S6. Repeat S3 to expose and photolithography the high Si grating and the short Si grating, S7. Repeat S4, transfer the pattern of the aluminum mask to the amorphous silicon film, and photolithographically form a gap between the high Si grating and the low Si grating; S8. Cleaning the aluminum mask with an aluminum etchant to obtain the one-dimensional photonic crystal cell; S9. Periodically arrange a number of the one-dimensional photonic crystal cells to obtain the grating structure.

5. The method for preparing a grating structure capable of generating space-time vortex light according to claim 4, characterized in that: The method for applying positive-modulated electron beam resist comprises: applying 200 nm thick resist on the amorphous silicon film at a rotation speed of 4000 rpm, and baking at 180° C. for 3 minutes.

6. The method for preparing a grating structure capable of generating space-time vortex light according to claim 4, characterized in that: The method for coating the anti-charging conductive polymer comprises: coating the anti-charging conductive polymer on the surface of the positive-modulated electron beam resist at a rotation speed of 4000 rpm, and baking at 90° C. for 90 seconds.

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