CN101872081B - Three-dimensionally tunable photonic crystal of photodeformable liquid crystal polymer and its preparation method - Google Patents

Abstract

The invention belongs to the technical field of the liquid crystal polymer material, particularly discloses a photo-deformable liquid crystal polymer three-dimensional tunable photonic crystal and a preparation method thereof. In the invention, the three-dimensional tunable photonic crystal graph structure in the visible light is designed; a SiO2 template is manufactured and then the photonic crystal structure on the SiO2 is transferred onto the liquid crystal by the nano impressing technology. The liquid crystal photonic crystal prepared by the invention is arranged on a glass substrate; the light intensity, the incident angle, the exposure time and the change of the external temperature are controlled and modulated, so that the liquid crystal is transformed from the nematic phase to the magnafacies, the three-dimensional structure parameter of the photonic crystal is changed and the position of the photonic band gap is biased. The photonic crystal can be used to design polarizing films, all-optical switches and all-optical variable optical attenuators and the like, so that the detection precision of the microfluidic and biological monitoring sensor is improved.

Description

Three-dimensionally tunable photonic crystal of photodeformable liquid crystal polymer and its preparation method

technical field

The invention belongs to the technical field of liquid crystal polymer materials, and in particular relates to a photonic deformable liquid crystal polymer tunable photonic crystal and a preparation method thereof

Background technique

Photonic crystals are artificially designed and fabricated crystals with periodic dielectric structures on the optical scale. Theory and experiments have confirmed that photons with frequencies in the forbidden band cannot propagate inside photonic crystals, so people can use photonic crystals to manipulate photons. The bandgap of photonic crystals leads to many new properties not found in ordinary optics, such as photonic energy gap, localized state of photons, superprism dispersion, suppressed spontaneous emission, etc. These properties make photonic crystals a basic material in photonic technology and have a wide range of application values.

Generally, the structure of the prepared photonic crystal is not adjustable, and its structural parameters are fixed, so the position of the photonic band gap is also unchanged. If the dielectric constant, lattice structure and other parameters of the material are changed by applying an electric field, a magnetic field or changing the temperature to realize the adjustable band gap of the photonic crystal, it will inevitably produce a series of new effects and new phenomena. materials have wider applications.

Theoretically, tunable photonic crystals can be produced by controlling one or more of the factors such as the dielectric constant difference of the medium, the filling ratio, and the crystal lattice structure. Currently, the materials used to prepare tunable photonic crystals include liquid crystals, ferroelectrics, and hydrogels.

Photonic crystals based on ferroelectric materials and traditional liquid crystal photonic crystals generally adjust the dielectric constant of the material by applying an external electric field and magnetic field to achieve a change in the photonic band gap. Hydrogel-based photonic crystals generally change the two-dimensional deformation of photonic crystals by adding temperature to adjust the filling ratio of the photonic crystal surface structure to achieve changes in the photonic band gap. The above structures often require additional complex modulation equipment, and it takes a long time for the structure to change after external excitation, and the deformation of the photonic crystal is limited to a two-dimensional range. The high-performance photodeformable liquid crystal polymer tunable photonic crystal designed and produced by the present invention uses clean energy such as ultraviolet light and visible light as modulation excitation, and completes the change and recovery of the photonic crystal structure in three-dimensional space in a short time. Make the photonic crystal achieve better narrow-band filtering effect and optical switching characteristics.

At present, liquid crystal-based tunable photonic crystals are prepared by self-assembly to form crystal structures of colloidal particles such as SiO ₂ , PS (polystyrene, polystyrene), PMMA, etc., and then fill the gaps of the colloidal particles with liquid crystal materials. However, the preparation of photonic crystals by the self-assembly method requires many preparation steps and high technical difficulty, and it is difficult to introduce defects such as resonant cavities during the production process, which cannot make good use of the characteristics of photonic crystals with resonant cavities. For this reason, the present invention proposes a method for making liquid crystal three-dimensional photonic crystals directly by using technologies such as fluid casting and nanoimprinting. Then the lattice constant of the liquid crystal photonic crystal can be adjusted by changing the intensity of the modulated light source, incident angle, exposure time, external temperature, etc., so as to change the photonic bandgap characteristics. The method has the advantages of simple preparation process, short production period and low cost.

Contents of the invention

The purpose of the present invention is to provide a method for forming a three-dimensional photonic crystal with simple process, fast and effective and capable of simply tuning the photonic band gap, and provide a three-dimensional tunable photonic crystal of photodeformable liquid crystal polymer.

In the invention, a three-dimensional photonic crystal pattern structure in visible light is firstly designed, a SiO ₂ template is produced, and then the photonic crystal structure on the SiO ₂ is transferred to the liquid crystal by nano-imprinting and other technologies. The liquid crystal material can be selected from polymers of azobenzene, 1,2-stilbene and fulgid anhydride. The specific steps and devices are shown in Figure 1 and Figure 2:

1. The SiO ₂ template is produced by using integrated circuit technology, as shown in Figure 1(a), and the produced template is shown in Figure 3. Such as: (a) honeycomb structure, (b) regular hexagonal structure, (c) square structure, etc.

2. In a dark room, place the silicon-based template on a hot stage above the clearing point temperature of the reaction precursor mixture at 90° C., and place the mixture on the silicon-based template, as shown in FIG. 1( b ).

3. After the mixture melts into a liquid state, place the prepared glass slide on the mixture, as shown in FIG. 1(c). After cooling down to the polymerization temperature of 60° C., turn on the mercury lamp to carry out photopolymerization with light with a wavelength of 545 nm, and the polymerization time is 2.5-3.5 hours.

4. After the photopolymerization is finished, take out the template, and use a sharp and clean blade to peel off the film from the silicon-based film plate to obtain a cross-linked liquid crystal polymer film, as shown in Figure 1(d). Its top view is shown in Figure 4.

In the present invention, the liquid crystal photonic crystal is placed on the glass substrate, and the liquid crystal is converted from the nematic phase to the isotropic phase by controlling the intensity of the externally modulated light, the incident angle, the exposure time, and the external temperature, thereby changing the three-dimensional structural parameters of the photonic crystal, That is, the shape, size, hole spacing, hole depth, etc. of the pattern are periodically arranged to shift the position of the photon forbidden band.

The liquid crystal tunable photonic crystal based on the nanoimprint technology of the present invention has wide applications, not only can design polarizers, all-optical optical switches, all-optical adjustable optical attenuators, but also can improve the detection accuracy of microfluidic and biological monitoring sensors .

Description of drawings

Fig. 1 is a schematic diagram of the process for preparing liquid crystal photonic crystals. Wherein: (a) making a template; (b) placing the mixture on the silicon-based template; (c) placing the prepared glass slide on the mixture; (d) peeling off the thin film from the silicon-based membrane plate.

Figure 2. Schematic diagram of the setup for photopolymerization.

The Si-based photonic crystal template (top view) designed and fabricated in Fig. 3 . Among them: (a) honeycomb structure; (b) regular hexagonal structure; (c) square structure.

Figure 4: Liquid crystal photonic crystal (top view) obtained by embossing a Si-based template. Among them: (a) honeycomb structure; (b) regular hexagonal structure; (c) square structure.

Fig. 5 Schematic diagram of light modulation principle.

Detailed ways

Liquid crystal is a thermodynamically stable state between the crystalline state and the liquid state. It has both the characteristics of "liquid" and "crystal", that is, the flow characteristics of liquid materials and the ordered arrangement of anisotropy of crystalline materials. So some people call it "flowable ordered structure". According to the different liquid crystal phases, it can be divided into nematic liquid crystal, smectic liquid crystal and cholesteric liquid crystal. According to the molecular weight of liquid crystal materials, they can be divided into small molecule liquid crystals (such as liquid crystal materials in liquid crystal displays) and polymer liquid crystals (such as body armor materials). Compared with small-molecule liquid crystals, polymer liquid crystals have better thermal stability and mechanical strength. Compared with other polymers, it has a molecular order structure unique to liquid crystal phases. Therefore, it has high molecular weight and orderly structure. Liquid crystal polymers with dual characteristics have been widely used.

The high-performance photodeformable liquid crystal polymer material selected for the production of tunable photonic crystals in the present invention has photosensitivity and temperature sensitivity, and can produce deformation in three-dimensional space with changes in incident light intensity, incident light angle, and temperature, thereby changing the photonic crystal. The size of the photon gap shifts. The liquid crystal material used in the invention is a crosslinked liquid crystal polymer CLCPs (Crosslinked Liquid-crystalline Polymers) film.

The preparation process is as follows:

1) Preparation of template

Through computer simulation, photonic crystals with honeycomb, regular hexagonal, and square structures are designed, with an aperture size of 3 to 5 microns. First, Si-based photonic crystal structures are produced as shown in a, b, and c in Figure 3.

2) Preparation of materials

First prepare the reaction precursor mixture, and completely dissolve the mixture of oxyazobenzene and benzoyloxymethylbenzene in a molar ratio of 4:6 with dichloromethane solvent, then add the photoinitiator in the darkroom, fully Mix to dissolve completely, then evaporate the solvent to dryness, wrap it in aluminum foil and dry it in a vacuum desiccator at 45--50°C for 20-24 hours before use.

3) Preparation of liquid crystal photonic crystal

The process and specific devices for preparing liquid crystal polymer tunable photonic crystals according to the present invention are shown in Fig. 1 and Fig. 2 .

a) In a dark room, the silicon-based template is placed on a hot stage above the clearing point temperature of the mixture at 90° C., and the mixture is placed on the silicon-based template, as shown in FIG. 1( b ).

b) After the mixture melts into a liquid state, place the prepared glass slide on the mixture, as shown in Fig. 1(c). After cooling down to the polymerization temperature of 60° C., the mercury lamp was turned on to carry out photopolymerization with light with a wavelength of 545 nm, and the polymerization time was 3 hours.

c) After the photopolymerization, the template was taken out, and the film was peeled off from the silicon-based film plate with a sharp and clean blade to obtain the CLCPs film, as shown in Figure 1(d). Its top view is shown in Figure 4.

When ultraviolet light is applied, the crystal orientation of the surface part of the liquid crystal polymer photonic crystal changes from "nematic phase" to "isotropic phase", and the liquid crystal azophenyl element undergoes cis-isomerization and photoinduced changes, resulting in a three-dimensional space. deformation. When visible light is applied, the liquid crystal azophenyl element undergoes reverse isomerization photochange and returns to the original shape before ultraviolet light irradiation, as shown in FIG. 5 . Since the lattice constant of the surface structure of the photonic crystal changes, the position of the photonic forbidden band of the photonic crystal shifts.

Claims (1)

1. A method for preparing a three-dimensional tunable photonic crystal of a photodeformable liquid crystal polymer, which is characterized in that: first design a three-dimensional photonic crystal pattern structure in visible light, and make _{a SiO} template; The photonic crystal structure on the template is transferred to the liquid crystal; the liquid crystal material is a polymer of azobenzene, 1,2-stilbene, and fulginic anhydride; the operation steps are as follows: (1), using integrated circuit technology to produce SiO ₂ templates, the three-dimensional photonic crystal graphic structure is honeycomb structure, regular hexagonal structure, square structure; (2) In a dark room, place the template on a hot stage above the clearing point temperature of the reaction precursor mixture at 90° C., and place the mixture on the template; (3) After the mixture melts into a liquid state, place the prepared glass slide on the mixture, cool down to the polymerization temperature of 60° C., turn on the mercury lamp and perform photopolymerization with light with a wavelength of 545 nm. The polymerization time is 2.5-3.5 hours; (4) After the photopolymerization is completed, the template is taken out, and the film is peeled off from the template to obtain the required cross-linked liquid crystal polymer film.

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