RU2566371C1 - Method of making polymer-based diffraction grating - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method includes forming a given periodic diffraction microstructure on a polymer substrate by implanting metal ions with energy of 4-1200 keV, with a radiation dose which provides concentration of implanted metal atoms of 2.5·10²⁰ - 6.5·10²² atoms/cm³ in the irradiated substrate. As the substrate a light-insensitive polymer with ion beam current density of 1.5·10¹² - 3.5·10¹³ ions/cm²·s through a surface mask is used.

EFFECT: making diffraction gratings for the visible range based on light-insensitive types of polymers with nanoparticles of different metals.

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Description

The invention relates to optics, and in particular to methods for manufacturing diffraction gratings for the visible range, made on the basis of polymeric materials. Polymers represent a special separate specific class of materials, which can be characterized by various dielectric, semiconductor and conductor properties, while fundamentally differing in structure from inorganic materials. Polymeric materials are currently actively used to build various types of optical waveguides and photon elements controlling light, such as prisms, lenses, etc. In practice, polymer-based lattices and periodic structures are used:

- in the elements of optical communication for introducing laser radiation into the thin-film waveguides or filtering the optical signal in the waveguide (periodic structures — Bragg gratings);

- as resonators with distributed feedback in waveguide lasers, diffraction elements used to control light [1], etc.

A known method of manufacturing a holographic diffraction grating based on a photosensitive polymer, for example methyl polyacrylate, selected as an analogue, which consists in the formation of an interference pattern using two intersecting recording light beams from two coherent light sources transmitted through a prism from an optically transparent material or light beam from one a radiation source passing simultaneously through two faces of the prism (RF patent No. 2199769, published on 02.27.2003).

The disadvantage of this method by analogy is that diffraction gratings can be obtained only on a limited type of polymers, namely in photosensitive organic media.

There is a known [2] method for manufacturing an optical diffraction grating, in which the formation of a given periodic structure (polymer alternating with regions of polymer / Ag nanoparticles) is carried out by restoring a metal precursor periodically distributed in a polymer matrix. This method is a combination of two methods: holographic ordering of photopolymer nanocomposites and photoreduction of silver nanoparticles in a spatial periodic field. For this, a specially developed photopolymerizing composition [3] is used, which provides irreversible volumetric diffuse redistribution of the monomer component and the precursor of silver nanoparticles in the interference field (holographic method). As a result of the photochemical reaction, stable periodic spatial structures of the polymer are formed, alternating with local regions of the polymer / Ag nanoparticles.

This manufacturing technology of the diffraction grating [2] is the closest to the claimed method and is therefore selected as a prototype.

The disadvantages of the prototype:

- for the possible implementation of the technology for producing the lattice described in the prototype [2], a prerequisite is the use of only a special photopolymerizing composition, which reduces the available set of types of polymers that are possible for the construction of various optical elements (waveguides, prisms, lenses, etc.) and which do not coincide with the polymers most widely used in practice at present, such as, for example, polymethyl methacrylate (PMMA), polyimide, epoxy resins, etc. [4];

- the photochemical reaction technique used in the method [2] provides for the reduction in this way of only metal nanoparticles — silver, and does not allow the use of other types of metals to form periodic structures and diffraction gratings based on a polymer matrix.

The technical problem to be solved in the claimed method consists in providing the possibility of manufacturing diffraction gratings based on non-photosensitive types of polymers with nanoparticles of various metals.

The technical problem posed in the proposed method for manufacturing a polymer-based diffraction grating, including the formation of a given periodic diffraction microstructure on a polymer substrate, is achieved by the formation of a given periodic diffraction microstructure using implantations with metal ions with an energy of 4-1200 keV, a radiation dose that provides a concentration metal atoms introduced 2.5 × 10 ²⁰ - 6.5 · October ²² atoms / cm ^3, the irradiated substrate, which is used as nesvetochuvstvite ny polymer ion beam current density of 1.5 10 ¹² - 3.5 10 ¹³ ions / cm ² through the mask surface.

In FIG. 1 shows an isometric drawing of a diffraction grating (product) comprising: 1 — a substrate of a non-photosensitive polymer; 2 - implanted cells; 3 - unirradiated partitions between cells.

In FIG. Figure 2 shows the calculated depth distribution of implanted silver in PMMA at an irradiation energy of 30 keV.

In FIG. Figure 3 shows the optical transmission spectra of unirradiated PMMA (a) and PMMA with ion-synthesized silver nanoparticles (b).

In FIG. Figure 4 shows a TEM image obtained by a transmission electron microscope on the surface of PMMA with silver nanoparticles synthesized by ion implantation.

In FIG. Figure 5 shows an AFM image obtained using an atomic force microscope of a microstructured PMMA (diffraction grating fragment) implanted with silver ions through a surface mask.

In FIG. Figure 6 shows the image of the diffraction scattering pattern obtained on the screen when transmitting light from microstructured PMMA with ion-synthesized silver nanoparticles probed by a laser at a wavelength of 527 nm.

In FIG. 7 shows a TEM image obtained by a transmission electron microscope, the surface of an epoxy resin with silver nanoparticles synthesized by ion implantation.

In FIG. 8 shows the optical absorption spectrum of (a) epoxy and epoxy with ion-synthesized silver nanoparticles (b).

FIG. 9. TEM image obtained on a transmission electron microscope, the surface of an epoxy resin with cobalt nanoparticles synthesized by ion implantation. The bottom left of the inset shows electron microdiffraction of cobalt nanoparticles.

In FIG. 10. The optical transmission spectrum of (a) polyimide and polyimide with ion-synthesized copper nanoparticles (b) is shown.

Consider the implementation of the proposed method with specific examples.

Example 1. Consider a method of manufacturing a diffraction grating on a polymer basis, including the formation of a given diffraction periodic microstructure on a polymer substrate by the proposed method, which consists in the formation of a given diffraction periodic microstructure using implantation on the ILU-3 accelerator with metal ions - Ag ⁺ with an energy of E = 30 keV , the irradiation dose D = 2.5 · 10 ¹⁶ ion / cm ² providing a concentration of introduced metal atoms of 1.0 · 10 ²² atoms / cm ³ in the irradiated substrate, which is used as non-light an appreciable polymer (PMMA) with a current density in the ion beam of J = 1 · 10 ¹³ ion / cm ² s through a surface mask — a metal mesh with a mesh size of 25 μm.

In FIG. 1 is an isometric drawing of a polymer-based diffraction grating (product) containing a substrate 1 (made of a non-photosensitive polymer material) with a diffractive periodic microstructure on its surface, the elements of which are regions subjected to ion irradiation — implanted cells 2, and characterized by a different dielectric constant relative to the substrate material 1. The diffraction periodic microstructure of the implanted cells 2 contains ion-synthesized metal ochastitsy dispersed in the surface region of the substrate 1 to a layer thickness of from 20 to 500 nm at a concentration of metal atoms of 2.5 × 10 ²⁰ - 6.5 · October ²² atoms / cm ^3. Unirradiated partitions 3 located between the implanted cells 2 have the same dielectric constant as the optically transparent substrate 1.

Modeling the concentration profiles of the distribution of implanted silver with an energy of 30 keV in PMMA in depth using the computer algorithm SRIM-2013 [5] (Fig. 2) showed that silver atoms accumulate in the surface of the implanted polymer layer, leading to the nucleation and growth of metal nanoparticles. The total thickness of the implanted layer with silver nanoparticles, and therefore the thickness of the active layer in the formed diffraction grating in PMMA, for these implantation conditions, does not exceed 100 nm.

In FIG. Figure 3 shows the experimental linear optical transmission spectra for the initial PMMA and also implanted with silver ions (Ag: PMMA), measured on a Hitach-330 two-beam spectrometer. Unlike the original PMMA matrix of FIG. 3 (a), implanted Ag sample: PMMA of FIG. 3 (b) is characterized by the presence in the visible region of the spectrum of a selective absorption band with a maximum of ~ 500 nm. This band indicates the formation of silver nanoparticles in PMMA, and it is due to the manifestation of the effect of surface plasma resonance in metal nanoparticles [6].

The image obtained by transmission electron microscope - TESLA BS-500, (TEM image) of the PMMA surface in the region of the polymer not covered with a mesh mask, after implantation with silver ions, is shown in FIG. 4. Unlike unirradiated polymer, spherical nanoparticles are visible in the micrograph. From the analysis of microdiffraction (photograph not shown) it follows that the spherical formations have a face-centered cubic lattice with a constant corresponding to metallic silver. Microdiffraction images are characterized by a set of thin rings of a polycrystalline type from silver nanoparticles, observed against the background of wide diffuse diffraction-diffused low-intensity rings corresponding to an amorphous polymer matrix. Comparison of experimental diffraction patterns with standard international tabular x-ray ASTM data allows us to conclude that the formation of any chemical compounds with silver ions does not occur during ion implantation.

Surface microstructures implanted with silver ions through a PMMA mask, observed with a FastScan Brucker atomic force microscope (ACM), are shown in FIG. 5. The AFM image shows that the surface of the sample is an ordered lattice with 25 μm cells, which are formed upon implantation of PMMA with silver ions in a given mode. In this case, the square region of the cells is ion-irradiated PMMA, i.e. the structure of the polymer with silver nanoparticles observed in FIG. 4 characterized by selective plasma absorption. The walls between the square lattice cells are composed of unirradiated polymer. The diffraction grating shown in FIG. 5 is formed in accordance with a predetermined drawing, an isometric image of which is shown in FIG. one.

Since it is known that implantation of metal ions into a dielectric increases its refractive index up to ~ 1.7–1.9 for the visible spectral region (especially at the plasma resonance frequencies of metal nanoparticles) [7], it is obvious that a microstructure is formed as a result of PMMA implantation through a mask with a periodically variable distribution of the optical constants of the material, i.e. between the lattice cells and its walls (n _PMMA = 1.5).

Thus, the microstructure formed by implantation with a periodically variable refractive index (dielectric constant) is a diffraction grating. In FIG. Figure 6 shows the diffraction image recorded by probing the formed grating with a laser at a wavelength of 527 nm.

Example 2. As a substrate, a non-photosensitive polymer is used - an epoxy resin, which is characterized by a relatively high optical transparency in a wide spectral range from 350 to 900 nm.

The implantation is carried out by singly charged Ag ⁺ ions with an energy of E = 30 keV, a dose of D = 3 · 10 ¹⁶ ion / cm ² (concentration - 2.2 · 10 ²² atoms / cm ³ ) and a current density in the ion beam of J = 3.2 · 10 ¹³ ion / cm ² · s. Other technological operations and modes of ion implantation associated with irradiation through a surface mask, such as in the first example of a specific implementation of the proposed diffraction grating on a polymer basis.

A TEM image of the surface of an epoxy resin implanted with silver ions is shown in FIG. 7, on which the spherical synthesized silver nanoparticles are clearly visible.

The optical absorption spectrum of the synthesized sample, measured on a Hitachi-330 spectrometer, is shown in FIG. 8. As a result of ion implantation of the polymer, a selective plasma absorption band appears in the spectrum of the composite material with a maximum near 495 nm, corresponding to ion-synthesized silver nanoparticles [6].

Example 3. As a substrate, a non-photosensitive polymer is used - epoxy resin. which is characterized by a relatively high optical transparency in a wide spectral range from 350 to 900 nm.

The implantation is carried out by singly charged Co ⁺ ions with an energy of E = 40 keV, a dose of D = 1 · 10 ¹⁷ ion / cm ² (concentration - 0.7 · 10 ²² atoms / cm ³ ) and a current density in the ion beam of J = 3.2 · 10 ¹³ ion / cm ² · s. Other technological operations and modes of ion implantation associated with irradiation through a surface mask, such as in the first example of a specific implementation of the proposed diffraction grating on a polymer basis.

A TEM image of the surface of an epoxy resin implanted with cobalt ions is shown in FIG. 9, on which the spherical synthesized cobalt nanoparticles are clearly visible. The same figure in the inset shows electron microdiffraction, which confirms the formation of cobalt nanoparticles. Comparison of experimental diffraction patterns with standard international tabular x-ray ASTM data allows us to conclude that the formation of any chemical compounds with cobalt ions does not occur during ion implantation.

Example 4. As a substrate, a non-photosensitive polymer, a polyimide, is used, which is characterized by high transparency (about 90%) in a wide spectral range from 200 to 1000 nm.

The implantation is carried out by singly charged Cu ⁺ ions with an energy of E = 40 keV, a dose of D = 5.0 · 10 ¹⁶ ion / cm ² (concentration - 2.0 · 10 ²² atoms / cm ³ ) and a current density in the ion beam of J = 3.5 · 10 ¹³ ion / cm ² · s. Other technological operations and modes of ion implantation associated with irradiation through a surface mask, such as in the first example of a specific implementation of the proposed diffraction grating on a polymer basis.

The optical transmission spectrum of the synthesized sample, measured on a Hitachi-330 spectrometer, is shown in FIG. 10. As a result of ion implantation of the polymer, a selective plasma absorption band appears in the spectrum of the composite material with a maximum near 630 nm, corresponding to ion-synthesized copper nanoparticles [6].

In the manufacture of a polymer-based diffraction grating, the ion implantation regimes according to the parameters have the following limitations: E = 4-1200 keV, D must provide a concentration of introduced metal atoms in the irradiated substrate of 2.5 · 10 ²⁰ - 6.5 · 10 ²² atoms / cm ³ , J = 1.5 · 10 ¹² - 3.5 · 10 ¹⁴ ion / cm ² · s. Beyond the boundaries of these regimes, the necessary technical result is not achieved, and the quality of the manufactured polymer-based diffraction gratings will not meet the necessary requirements.

The radiation dose is determined by the required number of atoms of the metal substance in order, firstly, to provide high contrast in the reflection coefficients of the formed elements of the diffraction grating, i.e. sufficiently large metal nanoparticles should be synthesized, exhibiting selective plasma reflection and absorption for noble metal nanoparticles or Rayleigh scattering, for example, for transition metal nanoparticles. This condition, according to our studies of the dependence of the appearance of the optical signal in metal nanoparticles on the implantation dose, is fulfilled at metal atom concentrations in the irradiated polymer of the order of 2.5 · 10 ²⁰ atoms / cm ³ . Secondly, the amount of embedded impurity should not exceed the dose at which the adhesion of growing metal nanoparticles begins, leading to the formation of a continuous metal film, and according to our estimates, is not more than 6.5 · 10 ²² atoms / cm ³ .

The current density in the ion beam J determines, on the one hand, the time it takes for the implantation dose to be set, and on the other hand, the degree of heating of the irradiated material. It was experimentally established that at J = 3.5 · 10 ¹³ ion / cm ² · s, the temperature of the irradiated surface of the sample increases and leads to its destruction (change). Irradiation with a low ion current density does not expediently increase the implantation time. Therefore, the minimum ion current density is limited to J = 1.5 · 10 ¹² ion / cm ² · s.

The energy of the ion E determines the value of its average projection range, which determines the depth of the implanted ion, and therefore the thickness of the modified layer and diffraction grating. From above, the ion acceleration energy is limited to E = 1200 keV, since with an increase in this implantation energy and a reasonable duration of irradiation, the required concentration of metal atom impurity is not reached, which is necessary for nucleation of metal nanoparticles at a great depth. The lower limit for E = 4 keV, according to our experiments, is due to the fact that with a further decrease in E, it is not possible to obtain such large elements of the lattice structure on which light diffraction is observed.

Compared with the prototype, the proposed method allows to manufacture diffraction gratings on a polymer basis, based on non-photosensitive polymers using various types of metals.

List of references

1. Diffraction nanophotonics. Ed. Soifer V.A. M .: Fizmatlid 2011.

2. Kokhtich L.M., Smirnova T.N., Kutsenko A.S. A new method of forming periodic polymer structures - silver nanoparticles. Proceedings of the scientific-practical conference “Holography. Science and Practice ”and the 6th international conference“ HOLOEXPO-2009 ″, July 1-2, 2009, Kiev, Ukraine. S.223-224.

3. Smirnova A. L., Kokhtich L. M., Sakhno O. V., Stump I. Holographic nanocomposites for recording periodic polymer-nanoparticle structures. I. General approach to the selection of components of nanocomposites and their holographic properties. / Optics and spectroscopy. 2011.V. 110. No. 1. S.135-142.

4. Serova V.N. Polymer materials for optics. M .: Scientific foundations and technologies 2011.

5. Ziegel J.F., Biersak J.R., Littmark U. The stopping and range of ions in solids. N.Y .: Pergamon, 1996.

6. Kreibig U., Vollmer M. Optical properties of metal clusters. Berlin: Springer. 1995.

7. Faik A., Allen L., Etcher C., Gagola A., Townsend P.D. Dispersion and luminescence measurements of optical waveguides / J. Appl. Phys. 1983. V. 54. P.2597-2601.

Claims (1)

A method of manufacturing a polymer-based diffraction grating, including the formation of a given periodic diffraction microstructure on a polymer substrate, characterized in that the formation of a given periodic diffraction microstructure is carried out by implantation with metal ions with an energy of 4-1200 keV, an irradiation dose that provides a concentration of introduced metal atoms of 2.5 · 10 ²⁰ - 6.5 · 10 ²² atoms / cm ³ in the irradiated substrate, which is used as a non-photosensitive polymer, with an ion beam current density of 1.5 · 10 ^{1 2} - 3.5 · 10 ¹³ ion / cm ² · s through a surface mask.

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