A method for generating vortex light based on organic molecular dichroic spherulites

Abstract

The present invention provides a method for generating vortex light based on organic molecular dichroic spherulites. The present invention solves the problem that vortex light cannot be directly obtained through natural materials, and successfully gives the vortex light field specific polarization characteristics based on this technology. An organic molecular material with dichroic characteristics is placed between two transparent sheets of glass material to form a "sandwich" structure; by appropriate heating / cooling treatment, the dichroic organic molecular material is melted / crystallized, and a spherulite structure is obtained in the crystallized material. Monochromatic light or polychromatic light of a certain wavelength is used as incident light, and is converted into circularly polarized light after passing through a specific optical element. The incident light with circular polarization characteristics is incident on the spherulite structure, the center of the incident light spot coincides with the center of the dichroic spherulite, and the obtained output light is vortex light. The method for generating vortex light proposed by the present invention has the following advantages: abundant material sources, strong universal applicability, broadband availability, convenience and efficiency, low cost, etc.

Description

Technical Field

[0001] The present invention relates to a technology for generating a spiral phase light field, and more particularly to a technology for generating vector vortex light with a specific polarization state based on organic molecular dichroic spherulites, and more specifically to a method for generating vortex light based on organic molecular dichroic spherulites. Background Art

[0002] Vortex light belongs to the field of structured light field, and its equal phase surface is spiral. The phase factor of vortex light contains an azimuth term e in φ, where φ is the azimuthal angle, and n represents the topological charge, that is, the phase change in the vertical section along the wave vector direction is 2πn. Since the phase at the center of the light beam is uncertain, the central intensity of the vortex light is zero. The phase distribution of the spiral in the vortex light introduces orbital angular momentum. The orbital angular momentum of vortex light can be used to prepare optical tweezers; orbital angular momentum multiplexing can greatly increase the transmission capacity of optical communications. In addition, vortex light also shows great application value in the fields of quantum computing and super-resolution imaging.

[0003] At present, researchers have explored a variety of methods to generate vector vortex light. Using a spiral phase plate with a spirally varying thickness, Gaussian light can be phase-shaped so that the phase change of the transmitted light / reflected light is proportional to the azimuth angle, thereby obtaining a spiral wavefront. However, this method has high requirements for processing accuracy. Using computational holography technology, the inclined interference pattern of the target vortex light and the reference plane wave is obtained, and the interference pattern is recorded to form a "fork grating" structure. After Gaussian light is irradiated onto the "fork grating", a series of vortex lights with opposite spiral directions can be obtained, and the topological charges of different diffraction orders are different. In addition, the spatial light modulator can use the electro-optical effect of liquid crystal to regulate the wavefront of the incident light to obtain the desired vortex light. However, the pixel points of the spatial light modulator are large, resulting in low light beam conversion efficiency and high price. In addition to the above methods, the wavefront phase can also be regulated by designing a metasurface to obtain vortex light with different topological charges. In addition, vortex light can also be directly generated through a ring resonator. Although the above methods can be used to generate vortex light, these methods mostly involve complex micro-nano processing techniques and it is difficult to balance bandwidth, cost, reliability and efficiency. Summary of the invention

[0004] The purpose of the present invention is to provide a method for generating a vortex light field, which has the advantages of wide applicability, low cost, simple process, high reliability, and wide bandwidth availability.

[0005] The method for generating a vortex light field provided by the present invention is to generate a vortex light field based on a natural organic molecule spherulite structure.

[0006] Specifically, the method for generating a vortex light field of the present invention comprises the following steps:

[0007] 1) Selecting pure dichroic organic molecule materials or mixed materials containing dichroic organic molecules or dichroic materials composed of multiple organic molecular monomers as raw materials;

[0008] 2) placing the above raw materials on a glass sheet (glass slide) and heating them until the temperature reaches a set value and the organic molecules melt;

[0009] 3) Cover the molten sample surface with another glass slide (cover glass) to form a "sandwich" structure;

[0010] 4) subjecting the above-mentioned "sandwich" structure to a cooling treatment, and the organic molecules crystallize during the cooling process, thereby introducing a spherulite structure into the crystallized organic molecular film, and obtaining an organic film containing a dichroic spherulite structure;

[0011] 5) Irradiate the above-mentioned dichroic spherulite with monochromatic or polychromatic circularly polarized light of a certain wavelength, and ensure that the center of the light spot coincides with the center of the spherulite. The transmitted light obtained is the vortex light containing orbital angular momentum.

[0012] In step 1) of the above method, the raw material exhibits strong dichroism, that is, after crystallization, the raw material exhibits strong anisotropic absorption to a pair of orthogonal polarized lights, with strong absorption in one polarization direction and relatively weak absorption in the other polarization direction;

[0013] The dichroic organic molecules include, but are not limited to: azo dichroic molecules, anthraquinone dichroic molecules, and naphthylimide dichroic molecules.

[0014] Based on the absorbance-wavelength relationship of these dichroic molecules, one or more combinations can be selected as dyes and added to an organic molecule host (including various organic small molecules, organic macromolecules, high molecular polymers, etc. that can crystallize at room temperature. The characteristic of the organic molecule host here is that it can form a spherulite structure under room temperature conditions), and dichroic spherulites can be obtained after crystallization.

[0015] By introducing various heterocyclic rings and functional groups (such as pyridine ring, thiadiazole ring, piperazine ring, ester bond, amide bond, imidazole ring, etc.) into dye molecules, dichroic molecules with different dichroic ratios and different colors can be obtained.

[0016] Dichroic materials composed of two monomers are mainly based on the charge transfer effect between the two monomers, which will strongly absorb the incident electromagnetic waves. Since the charge transfer effect occurs in the stacking direction of the two monomer molecules, the strong absorption caused by the charge transfer is sensitive to the polarization of the incident electromagnetic wave, thus showing dichroism.

[0017] Of the two monomers, one monomer acts as a charge donor and the other monomer acts as a charge acceptor. Charge donors include, but are not limited to, alkoxyanthracene molecules, dihexylnaphthalene molecules, neurotransmitter melatonin, dialkoxynaphthalene, oligothiophene molecules, benzodithiophene molecules, etc.; charge acceptors include, but are not limited to, naphthalene diimide molecules, bipyridine derivatives, naphthalenetetracarboxylic acid diimide, condensed ring molecules, porphyrin molecules, etc.

[0018] The selection of the dichroic organic molecules in the raw materials is not limited to the size of the molecular weight, is not limited to a single dichroic molecule, is not limited to the dichroism induced by the mixture of multiple monomer molecules, and is not limited to introducing the dichroic molecules as dye dopants into other organic molecules (the same as the above organic molecule main body);

[0019] In step 2), the setting value is significantly greater than the melting point of the raw material to ensure that the raw material is fully melted and its composition is fully uniform;

[0020] In step 3), the upper and lower glass sheets of the "sandwich" structure have high transparency, but are not limited to a certain material or thickness;

[0021] In step 4), the above-mentioned "sandwich" structure is subjected to a cooling treatment by setting a certain cooling system, and the cooling system is set to introduce a spherulite structure into the organic molecular film after crystallization;

[0022] The specific cooling system is determined by the composition of the raw materials used, the quality of the raw materials, the pressure introduced by the glass sheets in the "sandwich" structure, and the size of the glass sheets in the "sandwich" structure;

[0023] The method may further include the following operations between steps 4) and 5): using a polarizing microscope to observe the "sandwich" structure after the above-mentioned temperature rise and fall treatment, confirming that a spherulite structure appears inside the organic molecular film after cooling, and observing and recording the morphological characteristics of the spherulites such as size, position, number, etc.;

[0024] The size and number of spherulites are mainly related to the composition of the raw materials and the cooling rate.

[0025] The size of the spherulite structure of dichroic organic molecules can be regulated by temperature regime and / or spherulite nucleating agents;

[0026] The thickness of the organic molecular film formed after cooling is mainly related to the mass of the raw materials weighed, the compressive stress introduced by the upper glass sheet in the "sandwich" structure, and the surface area of ​​the upper glass sheet in the "sandwich" structure;

[0027] By controlling the weight of the raw materials and the pressure applied to the upper and lower glass surfaces of the "sandwich" structure during the heating and cooling process, the thickness of the film formed after the heating and cooling treatment can be adjusted. The film thickness can vary from a few hundred nanometers to several millimeters.

[0028] The organic molecular film finally formed is not limited to whether or not spherulite nucleating agents, metal micro-nano particles, inorganic micro-nano particles, etc. are added;

[0029] It is not limited to whether a polymerization reaction is introduced into the finally obtained organic molecular film;

[0030] The edge micro-region of the spherulite structure shows strong anisotropic absorption for a pair of orthogonal polarized incident light; one polarization direction has strong absorption, with an absorption rate of about 100%; the other polarization direction has relatively weak absorption; the greater the difference in absorption rate, the higher the efficiency of generating vortex light, and the theoretical maximum efficiency is 50%;

[0031] Due to the spherical symmetry of the spherulite, any edge microregion of the spherulite exhibits the above-mentioned dichroic absorption relationship.

[0032] In step 5), the monochromatic or polychromatic circularly polarized light is obtained by the following operation: selecting monochromatic light or polychromatic light of a certain wavelength as a light source, constructing an optical path on an optical platform, and converting the incident light into circularly polarized light after passing through a linear polarizer and a quarter wave plate respectively;

[0033] The wavelength range corresponding to the selected monochromatic or polychromatic light is determined by the absorption spectrum characteristics of the dichroic spherulite microregion to polarized light;

[0034] According to the absorption spectrum of the spherulite, the wavelength that can meet the conditions for generating vector vortex light is confirmed; specifically, if the difference in the absorption rate under two orthogonal polarization states corresponding to a certain wavelength is greater than 5%, and one of the polarization states is strongly absorbed (absorption rate is close to 100%), the vortex light at this wavelength can be generated by the spherulite;

[0035] The irradiation direction is normal incidence or oblique incidence;

[0036] The wavelength range allowed by the above circularly polarized light (polychromatic and monochromatic circularly polarized light) satisfies the absorption relationship of the above dichroic spherulite microregion; specifically, the spherulite microregion exhibits strong anisotropic absorption for two orthogonal polarization states at a certain wavelength, that is, one polarization direction is strongly absorbed with an absorption rate of about 100%; the other polarization direction is relatively weakly absorbed.

[0037] The obtained vortex light field has specific polarization characteristics, and the polarization characteristics of the outgoing vortex light field are determined by the strongest absorption polarization direction corresponding to the spherulite micro-region; specifically, if the spherulite micro-region has strong absorption for radial polarization, the obtained vortex light is tangential polarization; if the spherulite micro-region has strong absorption for tangential polarization, the obtained vortex light is radial polarization; otherwise, the obtained vortex light is the superposition of radial polarization and tangential polarization.

[0038] Compared with the prior art, the present invention proposes a principle of generating vortex light based on dichroic absorption, and realizes the use of spherulites after natural crystallization of dichroic organic molecules as a generator of vortex light. Its advantages are reflected in the following four aspects: first, the source of materials with dichroic absorption is abundant; second, spherulites are a common crystallization phenomenon of organic molecular materials, so the technology has wide universality; third, the wavelength range that satisfies the strong anisotropy of dichroic absorption can be obtained within a large bandwidth, which is conducive to the acquisition of broadband vortex light; fourth, the preparation process is convenient and efficient, and no complex and expensive equipment is required. In particular, this principle and method of generating vortex light based on dichroic spherulites can give the vortex light field specific polarization characteristics by regulating the orientation relationship of dichroic absorption, thereby obtaining vector vortex light, which has broad application fields and development prospects. Compared with the currently commonly used vortex light generation method, the present invention does not involve any micro-nano processing means, and its advantages are significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the "sandwich" structure of the organic molecular film in the present invention.

[0040] Figure 2 Optical microscope photograph of spherulites.

[0041] Figure 3 (a) Transmission spectrum of radially polarized incident light.

[0042] Figure 3 (b) Transmission spectrum of incident light with tangential polarization.

[0043] Figure 4 Schematic diagram of the interference light path.

[0044] Figure 5 Interference fringes of vortex light after passing through a spherulite. DETAILED DESCRIPTION

[0045] The present invention is described below by means of specific embodiments, but the present invention is not limited thereto.

[0046] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the reagents, materials, etc. used in the following examples, unless otherwise specified, can be obtained from commercial channels.

[0047] The present invention provides a method for generating a vortex light field, comprising the following steps:

[0048] 1) Selecting pure dichroic organic molecule materials or mixed materials containing dichroic organic molecules or dichroic materials composed of multiple organic molecular monomers as raw materials;

[0049] 2) placing the above raw materials on a glass sheet (glass slide) and heating them until the temperature reaches a set value and the organic molecules melt;

[0050] 3) Cover the molten sample surface with another glass slide (cover glass) to form a "sandwich" structure;

[0051] 4) subjecting the above-mentioned "sandwich" structure to a cooling treatment, and the organic molecules crystallize during the cooling process, thereby introducing a spherulite structure into the organic molecular film after crystallization, and obtaining an organic molecular film containing a dichroic spherulite structure;

[0052] 5) Irradiate the above-mentioned dichroic spherulite with monochromatic or polychromatic circularly polarized light of a certain wavelength, and ensure that the center of the light spot coincides with the center of the spherulite. The transmitted light obtained is the vortex light containing orbital angular momentum.

[0053] Compared with the prior art, the present invention proposes a principle of generating vortex light based on dichroic absorption, and realizes the use of spherulites after natural crystallization of dichroic organic molecules as a generator of vortex light. Its advantages are reflected in the following four aspects: first, the source of materials with dichroic absorption is abundant; second, spherulites are a common crystallization phenomenon of organic molecular materials, so the technology has wide universality; third, the wavelength range that satisfies the strong anisotropy of dichroic absorption can be obtained within a large bandwidth, which is conducive to the acquisition of broadband vortex light; fourth, the preparation process is convenient and efficient, and no complex and expensive equipment is required. In particular, this principle and method of generating vortex light based on dichroic spherulites can give the vortex light field specific polarization characteristics by regulating the orientation relationship of dichroic absorption, thereby obtaining vector vortex light, which has broad application fields and development prospects. Compared with the currently commonly used vortex light generation method, the present invention does not involve any micro-nano processing means, and its advantages are significant.

[0054] Example 1: Dichroic spherulites generate vector vortex light based on charge transfer

[0055] In this example, two organic molecules with charge transfer effects are selected as monomers. Due to the strong charge transfer effect between the two organic molecular monomers, the two molecules will form an orientation feature of alternating arrangement after crystallization, and this alternating molecular orientation will produce dichroism. Specifically, when the polarization state of the incident light is along the arrangement direction of the two molecules, the incident light will be strongly absorbed; when the polarization state of the incident light is perpendicular to the arrangement direction of the two molecules, the incident light will not be strongly absorbed.

[0056] Monomer molecule 1 (charge donor): N1, N5-dihexylnaphthalene-1, 5-diamine.

[0057] Monomer molecule 2 (charge acceptor): N,N-dioctyl-naphthalenediimide (NDI,2). The molecular structure is shown in the figure below:

[0058]

[0059] The preparation methods of the above two monomer molecules are commercially available. The detailed preparation process can be found in the literature: DOI:10.1002 / adma.201706787.

[0060] The above two organic molecular monomers are weighed separately according to the molecular molar ratio of 1:1 and then mechanically mixed. Take 1 mg of the above mixture and place it on top of a glass slide, and place the glass slide on a heating table for heating treatment. For this example, the two organic molecular monomers begin to absorb heat and melt significantly at 130 degrees Celsius. In order to ensure that the two organic molecular monomers are mixed at the molecular level, the final heating temperature is set to 180 degrees Celsius in this example. After reaching this temperature, the organic molecular monomer mixture is transformed into a molten state. Due to the charge transfer effect between the two molecular monomers, the color before and after melting changes significantly. The color before melting is light yellow, and the color after melting is dark green. Further, a cover glass is slowly placed on the surface of the organic molecule melt to form a sandwich structure, such as Figure 1 After the above operation is completed, the temperature is maintained at 180 degrees Celsius for 10 minutes to ensure that the two organic molecular monomers are fully mixed at the molecular level.

[0061] Further, a cooling treatment is performed, during which a crystallization process will occur. In order to produce a spherulite structure in the organic molecular film after cooling and crystallization, this example adopts a faster cooling rate. Specifically, the "sandwich" structure after the above-mentioned full melting treatment is removed from the heating table, placed on the workbench desktop at room temperature, and after standing for 5 minutes, an organic molecular film containing a spherulite structure can be obtained, with a thickness of 10μm (variable within the range of 1-20μm). The optical microscope photo of the spherulite structure is shown in Figure 2 .

[0062] In order to confirm the dichroic characteristics of the above-mentioned spherulite structure, a transmission spectrum test was carried out on the edge micro-area of ​​the spherulite. The transmission spectrum test device was built on the optical platform. The specific test process is: use a certain linear polarization state of incident light to irradiate the sample-free blank area of ​​the "sandwich" structure to obtain the transmission spectrum T0; use the above-mentioned incident light to irradiate the edge micro-area of ​​the spherulite to obtain the transmission spectrum T1. This example uses two linear polarization states of incident light, namely: 1. Linear polarization state incident light along the tangential direction of the above-mentioned spherulite micro-area; 2. Linear polarization state incident light along the radial direction of the above-mentioned spherulite micro-area. The transmission spectra of the spherulite micro-area for the two linear polarization states of incident light are shown in Figure 2. Figure 3 It can be found that for incident light in the 520nm-620nm band, the transmittance of the radial polarization state is almost 0%, while the transmittance of the tangential polarization state is 5%-10%. This shows that the light polarized along the radial direction of the spherulite is fully absorbed, while the light polarized along the tangential direction of the spherulite is transmitted to a certain extent.

[0063] Based on the tangential-radial dichroic characteristics of the above-mentioned spherulites, it can be learned through theoretical analysis that when circularly polarized incident light is used to illuminate the spherulites, the output light is vortex light containing a spiral phase distribution, and the polarization state is along the tangential direction.

[0064] This example uses the interference method to characterize the spiral phase characteristics of light passing through the spherulite. The schematic diagram of the interference light path is shown in Figure 4 . Specifically, a 633nm laser is selected as the light source of incident light, and the incident light is passed through a polarizer so that the outgoing light is linearly polarized light; a polarization-insensitive beam splitter is used to split the outgoing linearly polarized light into light A and light B; wherein, light A is converted into circularly polarized light after passing through a 1 / 4 wave plate, and the circularly polarized light is focused onto the surface of the spherulite, and the center of the focused light spot coincides with the center of the spherulite. The light after passing through the spherulite is recorded as light C, which becomes vortex light, and the polarization state of light C is tangential polarization; for light B, its role is to act as a reference light and interfere with light C. Light B is passed through two reflectors and a beam splitter to interfere with light C, and the interference fringes are recorded with a CCD camera. See the interference fringes photo. Figure 5 . Figure 5 The interference fringes in the figure show a fork-shaped grating pattern, proving that the emitted C light is vortex light.

Claims

1. A method for generating a vortex light field, comprising: generating a vortex light field based on a natural organic molecular spherulite structure; The method for generating the vortex light field comprises the following steps: 1) Select pure dichroic organic molecule materials or mixed materials containing dichroic organic molecules or dichroic materials composed of multiple organic molecular monomers as raw materials; 2) Placing the above raw materials on a glass sheet and heating them until the temperature reaches a set value and the organic molecules melt; 3) Cover the molten sample surface with another glass sheet to form a "sandwich" structure; 4) subjecting the above-mentioned "sandwich" structure to a cooling treatment, and the organic molecules crystallize during the cooling process, thereby introducing a spherulite structure into the crystallized organic molecular film, and obtaining an organic film containing a dichroic spherulite structure; 5) Irradiate the above-mentioned dichroic spherulite with monochromatic or multi-color circularly polarized light of a certain wavelength, and ensure that the center of the light spot coincides with the center of the spherulite. The transmitted light obtained is the vortex light containing orbital angular momentum.

2. The method according to claim 1, characterized in that: In step 1), the raw material exhibits strong dichroism, that is, after crystallization, the raw material exhibits strong anisotropic absorption to a pair of orthogonal polarized lights, with strong absorption in one polarization direction and relatively weak absorption in the other polarization direction; The raw material is a dichroic molecule, a mixture having dichroicity obtained by mixing a plurality of monomer molecules, or a mixture obtained by introducing a dichroic molecule as a dye dopant into other organic molecules.

3. The method according to claim 1, characterized in that: In step 2), the setting value is significantly greater than the melting point of the raw material, In step 3), the upper and lower glass sheets of the "sandwich" structure have high transparency.

4. The method according to claim 1, characterized in that: In step 4), the cooling system is determined by the composition of the raw materials used, the quality of the raw materials, the pressure introduced by the glass sheets on the "sandwich" structure, and the size of the glass sheets on the "sandwich" structure.

5. The method according to claim 1, characterized in that: The following operation is further included between steps 4) and 5): using a polarizing microscope to observe the "sandwich" structure after the temperature rise and fall treatment, and confirming that a spherulite structure appears inside the organic molecular film after cooling.

6. The method according to claim 1, characterized in that: The size of the dichroic organic molecule spherulite structure is regulated by temperature regime and / or spherulite nucleating agent; The thickness of the organic molecular film formed after cooling is related to the weight of the raw materials weighed, the compressive stress introduced by the upper glass sheet in the "sandwich" structure, and the surface area of ​​the upper glass sheet in the "sandwich" structure. The thickness of the film formed after the temperature rise and fall treatment can be adjusted by controlling the weighed mass of the raw materials and the pressure applied to the upper and lower glass surfaces of the "sandwich" structure during the temperature rise and fall process.

7. The method according to claim 1, characterized in that: In step 5), the monochromatic or polychromatic circularly polarized light is obtained by the following operation: monochromatic light or polychromatic light of a certain wavelength is selected as the light source, and an optical path is constructed on an optical platform so that the incident light is converted into circularly polarized light after passing through a linear polarizer and a quarter wave plate respectively; The wavelength range corresponding to the selected monochromatic or polychromatic light is determined by the absorption spectrum characteristics of the dichroic spherulite microregion to polarized light.

8. The method according to claim 1, characterized in that: In step 5), the irradiation direction is normal incidence or oblique incidence; The obtained vortex light field has specific polarization characteristics, and the polarization characteristics of the outgoing vortex light field are determined by the strongest absorption polarization direction corresponding to the spherulite microregion.

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