A method for preparing a ferroelectric liquid crystal material grating based on femtosecond laser direct writing at room temperature and application thereof

Abstract

The application belongs to the technical field of optics, and particularly relates to a preparation method and application of a ferroelectric liquid crystal material grating based on femtosecond laser direct writing. The first substrate and the second substrate are cleaned by ultrasonic cleaning, and then are dried and cleaned by ozone. After the cleaning, the edges of the first substrate and the second substrate are smeared with spacers, and the two substrates are bonded and solidified to form a liquid crystal cell. The ferroelectric liquid crystal material is filled into the liquid crystal cell to form a liquid crystal layer. The pattern of the liquid crystal layer is directly written by a femtosecond laser direct writing device, so that the orientation direction of the liquid crystal layer molecules is controlled, and the ferroelectric liquid crystal material grating is obtained. The pattern of the liquid crystal layer directly written by the femtosecond laser direct writing device includes any one of a strip pattern, a radial line pattern and a circular ring pattern. The application realizes the control of the orientation direction of polar liquid crystal molecules at room temperature by femtosecond laser direct writing technology, and successfully realizes nonlinear optical response.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optics, and particularly relates to a preparation method and application of a room-temperature ferroelectric liquid crystal material grating based on femtosecond laser direct writing. BACKGROUND

[0002] Ferroelectric materials have the characteristics of spontaneous polarization, and the polarization direction can be switched by an external electric field, and have a wide range of applications in the fields of electronics, electron optics and micro-electro-mechanical integration. The materials such as LiNbO3 which have been widely used in the field of optics belong to inorganic metal ferroelectric crystals. In order to realize multi-field applications in modern scientific research, people have proposed various domain engineering technologies such as electric polarization and femtosecond laser direct writing to regulate the spontaneous polarization of ferroelectric materials. In addition, organic ferroelectric materials have the advantages of light weight, good flexibility, strong tunability, non-toxicity, good biocompatibility, low cost and low power consumption, and have great application potential in the fields of biomedicine and optoelectronic devices. The utilization of various domain engineering technologies in organic ferroelectric materials is still in the research boom.

[0003] Liquid crystals are a kind of well-known organic materials, which have the flowability of liquid and the anisotropy of crystal, and are sensitive to external stimuli, have unique anisotropic optical properties, self-assembly ability and dynamic tunability. A new type of liquid crystal, ferroelectric nematic liquid crystal, which is found in recent years, breaks the head-tail symmetry of traditional liquid crystals, has the characteristics of spontaneous polarization, large dielectric constant and strong nonlinear optical response, and is very suitable for realizing programmable and dynamic structural light field manipulation in the field of nonlinear optics. In the past few years, research on the control of polarity order in organic ferroelectric materials has made many important achievements. However, the existing ferroelectric nematic liquid crystals such as RM734 and DIO are limited by temperature, and are easy to cause material crystallization under unstable temperature control, and show disordered and defective polycrystalline phase at room temperature. It is generally believed that such a crystalline state is difficult to realize application, so it is necessary to explore a method for manipulating and utilizing ferroelectric liquid crystals at room temperature, and to open up new possibilities for flexible application in the fields of new optoelectronic and advanced photonic device design. SUMMARY

[0004] The technical problem to be solved by the present application is to solve the deficiencies of the prior art, to provide a method for regulating the molecular orientation direction of ferroelectric liquid crystal material at room temperature by using femtosecond laser direct writing technology, to obtain a fine patterned structure, and to realize application in the field of nonlinear photonics by utilizing the characteristics of liquid crystal response to external stimuli. The femtosecond laser direct writing technology realizes the manipulation of liquid crystal structure at room temperature, solves the problems of temperature control of liquid crystal element and temperature rising and falling rate in the application of current ferroelectric nematic liquid crystals, so as to prevent material crystallization from causing defects, and the liquid crystal structure becomes disordered and chaotic.

[0005] In order to solve the above technical problems, the application discloses a preparation method of a ferroelectric liquid crystal material grating based on femtosecond laser direct writing, and the specific steps are as follows: the first substrate and the second substrate are cleaned by ultrasonic cleaning, and then ozone cleaning is performed after drying; after the first substrate and the second substrate are cleaned, a spacer is applied to the edge of the first substrate and the second substrate, and the two substrates are bonded and cured to form a liquid crystal cell; the ferroelectric liquid crystal material is filled into the liquid crystal cell to form a liquid crystal layer; the pattern of the liquid crystal layer is directly written by a femtosecond laser direct writing device, so that the molecular orientation direction of the liquid crystal layer is controlled, and the ferroelectric liquid crystal material grating is obtained.

[0006] The pattern of the liquid crystal layer directly written by the femtosecond laser direct writing device includes any one of a strip pattern, a radial line pattern and a circular ring pattern.

[0007] Optionally, the inner sides of the first substrate and the second substrate are respectively provided with a first alignment layer and a second alignment layer.

[0008] Specifically, the width of the strip pattern increases with the increase of the femtosecond laser power in the femtosecond laser direct writing device.

[0009] Specifically, the femtosecond laser direct writing device is composed of a femtosecond laser, a shutter, a half-wave plate, a polarization beam splitter, a polarizer, a lens, an aperture, a CCD camera, a reflecting mirror, a microscope objective, a liquid crystal sample and a high-precision displacement stage.

[0010] Preferably, the laser generated by the femtosecond laser has a wavelength of 800 nanometers, a duration of 75 femtoseconds and a repetition frequency of 80 megahertz.

[0011] Preferably, the microscope objective is a microscope objective with a numerical aperture of 0.7, a magnification of 50 times and a numerical aperture of 0.3, a magnification of 10 times.

[0012] Further, the structure control method of the ferroelectric liquid crystal material at room temperature can adjust the size and pattern of the obtained liquid crystal ferroelectric domain structure by setting the parameters (laser power, direct writing speed, focusing depth, etc.) of the femtosecond laser direct writing system and the moving track of the laser spot, so as to expand the application of the directly written liquid crystal element to more fields.

[0013] Specifically, the liquid crystal layer material realizes the nonlinear optical response sensitive to external stimulation by using ferroelectric liquid crystal material. The ferroelectric liquid crystal material has the characteristics of spontaneous polarization, and can control the incident light field. The nonlinear liquid crystal element disclosed by the application selects ferroelectric nematic liquid crystal RM734, but is not limited to this material.

[0014] Specifically, the alignment agent used in the first alignment layer and the second alignment layer includes any one of a surfactant, a rubbing alignment agent, a photo-crosslinking material, a photo-degradation material and a photo-isomerization material;

[0015] Preferably, the alignment agent is an azo photo-alignment material SD1 or a rubbing alignment agent polyimide (PI).

[0016] The photo-alignment material molecules are arranged along a direction perpendicular to the direction of linearly polarized light under the irradiation of linearly polarized light, and form ordered alignment of liquid crystal molecules through interaction between the photo-alignment material molecules and the liquid crystal molecules, so as to realize the manipulation of the alignment of the liquid crystal molecules; the rubbing alignment agent is arranged along the rubbing direction under the action of rubbing force, and is arranged along the arrangement direction of the rubbing alignment agent molecules through interaction between the rubbing alignment agent and the liquid crystal molecules. In actual operation, it is not limited to these two materials.

[0017] More preferably, the alignment agent is an azo photo-alignment material SD1.

[0018] Specifically, the ultrasonic cleaning includes ITO cleaning solution ultrasonic cleaning and ultrapure water ultrasonic cleaning; the ITO cleaning solution ultrasonic cleaning lasts for 30 minutes, and the ultrapure water ultrasonic cleaning is divided into two times, each of which lasts for 10 minutes.

[0019] Specifically, the drying has the following specific conditions: 120 degrees Celsius, and a drying time of 40 minutes.

[0020] Specifically, the spacer is a mixture of microspheres and frame glue; the microspheres are silica microspheres or polystyrene microspheres, and the particle size is 10 microns.

[0021] Specifically, the liquid crystal layer is formed by heating the ferroelectric nematic liquid crystal material RM734 to 188 degrees Celsius and filling the liquid crystal cell through a capillary glass tube.

[0022] Specifically, the first alignment layer and the second alignment layer are formed by spin coating the alignment agent on the inner side of the first substrate and the second substrate and performing annealing.

[0023] Preferably, the spin coating has the following specific conditions: in the first step, 800 revolutions per minute for 8 seconds; in the second step, 3000 revolutions per minute for 40 seconds.

[0024] Preferably, the annealing has the following specific conditions: a temperature of 100 degrees Celsius, and a time of 10 minutes.

[0025] Specifically, in the preparation process of the ferroelectric liquid crystal material grating containing the first alignment layer and the second alignment layer, the first alignment layer and the second alignment layer need to be pre-aligned before the ferroelectric liquid crystal material is filled into the liquid crystal cell after the liquid crystal cell is formed, and the specific operation is as follows: the first alignment layer and the second alignment layer are uniformly pre-aligned by linearly polarized light with a wavelength of 405 nm and a power of 1 W to 2 W.

[0026] Further, the ferroelectric liquid crystal material grating based on femtosecond laser direct writing prepared by the preparation method is also within the protection scope of the present application.

[0027] Further, the application of the ferroelectric liquid crystal material grating based on femtosecond laser direct writing in realizing linear and / or nonlinear grating diffraction is also within the protection scope of the present application.

[0028] Specifically, in some embodiments of the present application, the grating structure is directly written by setting the femtosecond laser power to 350 mW and taking 20 μm as the period. The grating linear and nonlinear diffraction light spots are obtained by setting the incident light wavelength to 800 nm. The nonlinear Raman-Nath diffraction exists in the nonlinear diffraction of the grating, and there are symmetrically distributed multi-order diffraction lights, and the 0-order light spot has the strongest light intensity. The first-order diffraction angle of the linear diffraction is twice that of the nonlinear diffraction, which proves the application prospect of the above-mentioned liquid crystal grating in realizing linear and / or nonlinear grating diffraction.

[0029] Beneficial effects: The present application realizes the in-plane arbitrary angle control of the polarization direction of the polar liquid crystal molecules at room temperature by using the femtosecond laser direct writing technology. The thermal electric field at the focal point of the femtosecond laser drives the liquid crystal molecules to arrange, and is not limited to the traditional alignment mode (such as photo-alignment, rubbing alignment, etc.) which depends on the alignment agent to induce the liquid crystal molecule alignment, and avoids the problem of flexoelectric effect of the ferroelectric nematic phase liquid crystal system. The structure defect problem caused by the temperature change in the traditional alignment mode is avoided. The ferroelectric liquid crystal material grating is prepared at room temperature by using the femtosecond laser direct writing technology, and the nonlinear optical beam steering control is successfully realized. BRIEF DESCRIPTION OF DRAWINGS

[0030] The above and / or other aspects of the present application will become more apparent by describing in detail the preferred embodiments thereof with reference to the attached drawings.

[0031] Figure 1 The structure diagram of the liquid crystal element prepared in the embodiments of the present application is shown in the following figure;

[0032] Figure 2 The texture diagram and the brightness information change of the liquid crystal element prepared in the embodiments of the present application under the polarizing microscope at room temperature after uniform pre-alignment are shown in the following figure;

[0033] Figure 3A schematic diagram of a femtosecond laser direct writing device used in the present application;

[0034] Figure 4 A schematic diagram of orientation change of a ferroelectric liquid crystal element prepared in an embodiment of the present application;

[0035] Figure 5 A texture map of a bar-shaped structure of a ferroelectric liquid crystal material prepared in an embodiment of the present application under a polarizing microscope at room temperature;

[0036] Figure 6 A texture map of a radial line and a circular ring pattern prepared in an embodiment of the present application under a polarizing microscope;

[0037] Figure 7 A texture map of a grating prepared in an embodiment of the present application under a polarizing microscope and its corresponding linear and nonlinear diffraction patterns. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme of the embodiments of the present application will be described clearly and completely below in conjunction with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application.

[0039] Unless otherwise defined, the technical terms or scientific terms used herein should be understood as the usual meanings understood by those skilled in the art to which the present application belongs.

[0040] In the following examples, the experimental methods are conventional methods unless otherwise specified, and the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0041] Example 1:

[0042] The present application proposes a preparation method and application of a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing, Figure 1 A structure diagram of a liquid crystal element prepared in the present embodiment, specifically comprising a first substrate and a second substrate, the inner side of the first substrate and the second substrate being provided with a first alignment layer and a second alignment layer, the edge of the first substrate and the second substrate being provided with microspheres, the first substrate and the second substrate being bonded to form a liquid crystal cell in the middle of the two, and the liquid crystal cell being filled with liquid crystal material to form a liquid crystal layer. The preparation steps of the liquid crystal grating are as follows:

[0043] Step one: both the first substrate and the second substrate are ITO glass substrates, first, the first substrate and the second substrate are cleaned by ultrasonic cleaning with ITO cleaning solution, the cleaning time is 30 minutes. Then the substrates are cleaned by ultrasonic cleaning with ultrapure water twice, each time for 10 minutes. The cleaned substrates are placed in a drying oven, the temperature of the drying oven is adjusted to 120 degrees Celsius, the drying time is 40 minutes, and finally the glass substrates are cleaned by ultraviolet ozone for 30 minutes.

[0044] Step two: spin-coat photo-alignment agent SD1 on the first substrate and the second substrate, the specific spin-coating method is: first step, 800 rpm for 8 seconds; second step, 3000 rpm for 40 seconds. The thickness of the finally formed photo-alignment agent layer is about 30 nanometers. After spin-coating the alignment agent, the glass substrate coated with the photo-alignment agent SD1 is annealed to form the first alignment layer and the second alignment layer, the annealing temperature is 100 degrees Celsius, and the time is 10 minutes.

[0045] Step three: the mixture of microspheres and UV sealant is used as a spacer, wherein the microspheres are 10 micrometer silica microspheres or polystyrene microspheres, and the mixture is evenly coated on the edges of the first substrate and the second substrate. The upper and lower substrates are bonded with a certain offset and placed under ultraviolet light to cure the sealant, forming a liquid crystal cell. The thickness of the liquid crystal cell is about 9.6 micrometers, which is controlled by changing the diameter of the microspheres and measured by interference method.

[0046] Step four: the first and second alignment layers are uniformly oriented by linearly polarized light with a wavelength of 405 nanometers, and a 405 nanometer LED light source with a power of 1W to 2W is used, which is fully irradiated on the liquid crystal cell after passing through a linear polarizer, directly uniformly in-plane orienting the first and second alignment layers;

[0047] Step five: fill the liquid crystal material between the first substrate and the second substrate, heat the ferroelectric nematic liquid crystal material RM734 to 188 degrees Celsius, fill it into the liquid crystal cell through a capillary glass tube, so that the liquid crystal molecules arrange according to the orientation direction of the first and second alignment layers, and then cool the liquid crystal cell to room temperature after filling is completed.

[0048] The orientation of the liquid crystal molecules of the liquid crystal element obtained in step five is studied, Figure 2For the uniformly oriented liquid crystal cell, the texture map and its brightness information change under the orthogonal polarizing microscope after cooling to room temperature. It can be seen from the figure that the liquid crystal texture at room temperature presents a chaotic and disordered state, and the uniform orientation effect is destroyed. Under the orthogonal polarizing microscope, the sample is rotated, and it is found that the texture brightness has no extinction, and the brightness change is only less than 10%, which can indicate that the optical anisotropy induced by orientation at this time is very weak. The texture map and its brightness change rule here prove that the intermolecular interaction, surface structure induction and other traditional liquid crystal orientation methods through the orientation layer have not obtained the expected polarization direction control of liquid crystal.

[0049] Step six: focus the femtosecond laser into the liquid crystal layer through the femtosecond laser direct writing device, and the thermal gradient field formed at the laser focus point induces a thermoelectric field. The polarization direction of the ferroelectric liquid crystal molecules is controlled under the action of the thermoelectric field and is arranged along the direction of the electric field. With the movement of the laser focus point, the position of the thermoelectric field moves correspondingly. After driving the thermoelectric field to move, the polarization direction of the position is rearranged, forming a ferroelectric domain structure. A plurality of equally spaced linear domain structures can obtain the femtosecond laser direct writing based on the room temperature ferroelectric liquid crystal material grating.

[0050] Wherein, the position covered by the moved thermoelectric field will rewrite the polarization direction of the polarization liquid crystal in the region, even if part of the position in the region is directly written by the thermoelectric field at the moment.

[0051] Wherein, Figure 3 The schematic diagram of the femtosecond laser direct writing device used in the application, the device includes a femtosecond laser as a light source, a shutter, a half-wave plate, a polarization beam splitter, a polarizer, a lens, an aperture, a CCD camera, a reflecting mirror and a microscope objective, a liquid crystal sample and a high-precision displacement table. Specifically, the device uses a femtosecond laser with a wavelength of 800 nanometers, a duration of 75 femtoseconds and a repetition frequency of 80 megahertz for direct writing, and adjusts the laser power through the half-wave plate and the polarization beam splitter.

[0052] Specifically, the objective lens group focuses the liquid crystal cell in the liquid crystal cell with a numerical aperture of 0.7 and a magnification of 50 times, and a numerical aperture of 0.3 and a magnification of 10 times, respectively, to generate an effective spot size of tens of microns, and the sample is moved by a high-precision displacement table to scan the focused beam on the sample.

[0053] Figure 4 The schematic diagram of the orientation change of the femtosecond laser direct writing ferroelectric liquid crystal element prepared in this embodiment, which shows the change of the orientation direction of the liquid crystal molecules before and after the direct writing. Before direct writing, the orientation direction of the liquid crystal cell presents a chaotic and disordered state at room temperature, and after direct writing, the orientation state of the liquid crystal molecules changes under the action of different laser pulse energies, and presents an ordered state along the scanning direction of the laser spot.

[0054] The change of liquid crystal polarization direction after step five and step six can show that the alignment layer in this embodiment is actually not effective in the prepared liquid crystal grating, so it can be concluded that the alignment layer can also be omitted in the technical solution of the present application, and the integrity of the technical solution will not be affected, but the existence of the alignment layer still makes the sample before direct writing more uniform and has a certain degree of anisotropy (although very weak, the brightness difference is about 10%).

[0055] Figure 5 The texture map of the bar-shaped structure of the ferroelectric liquid crystal material prepared in this embodiment under a polarizing microscope at room temperature. Specifically, the femtosecond laser power is set to 50 to 150 mW, the power of each structure changes by 20 mW, and 6 bar-shaped ferroelectric domain structures are obtained in the vertical and parallel uniform orientation directions of the liquid crystal cell, respectively, by Figure 5 It can be seen that the width of the bar-shaped domain increases with the increase of the femtosecond laser power.

[0056] Figure 6 The texture map of the radial line and circular ring pattern prepared by the femtosecond laser direct writing in the present application under a polarizing microscope.

[0057] Among them, the radial line is obtained by setting the femtosecond laser power to 260 mW, fixing the starting point, taking the circumference with a diameter of 200 μm as the ending point, and directly writing 1 radial line along the radial direction of the circle every 30 degrees, a total of 12 radial lines; and the circular ring is obtained by directly writing a circular ring with a circle with a diameter of 200 μm as the track.

[0058] Optionally, this embodiment takes bar-shaped, radial line and circular ring as the pattern of femtosecond laser direct writing, but is not limited to these patterns, and includes any other pattern.

[0059] Figure 7 The texture map of the grating prepared in this embodiment under a polarizing microscope and its corresponding linear and nonlinear diffraction patterns. The grating is formed by periodic arrangement of bar-shaped patterns, specifically by setting the femtosecond laser power to 350 mW, and directly writing the grating structure with a period of 20 μm in the parallel and vertical uniform orientation directions of the liquid crystal cell, respectively. The wavelength of the incident light is set to 800 nm, and the linear and nonlinear diffraction spots of the grating are obtained, respectively. The nonlinear diffraction of the grating conforms to the nonlinear Raman-Nath diffraction, and there are symmetrically distributed multiple diffraction lights, and the 0th light spot has the strongest light intensity. The first-order diffraction angle of linear diffraction is twice that of nonlinear diffraction.

[0060] The application provides a preparation method of a ferroelectric liquid crystal material grating based on femtosecond laser direct writing and an application idea of the ferroelectric liquid crystal material grating, and there are many methods and approaches to realize the technical scheme, and the above description is only the preferred embodiment of the application, and it should be pointed out that, for ordinary skilled in the art, some improvements and refinements can be made without departing from the principle of the application, and the improvements and refinements should also be regarded as the protection scope of the application. The components not explicitly described in the embodiment can be realized by using the prior art.

Claims

1. A method for fabricating a room-temperature ferroelectric liquid crystal material grating based on femtosecond laser direct writing, characterized in that, The specific steps are: cleaning the first substrate and the second substrate by ultrasonic cleaning, and then drying and ozone cleaning; applying a spacer to the edge of the cleaned first substrate and the cleaned second substrate, and then bonding and curing the two substrates to form a liquid crystal box; Filling the ferroelectric liquid crystal material into the liquid crystal box to form a liquid crystal layer; directly writing a pattern of the liquid crystal layer by using a femtosecond laser direct writing device, thereby controlling the molecular orientation direction of the liquid crystal layer, and obtaining the ferroelectric liquid crystal material grating. The femtosecond laser direct writing device directly writes a pattern of the liquid crystal layer, and the pattern includes any one of a strip pattern, a radial line pattern and a circular ring pattern.

2. The production method according to claim 1, characterized by, The first substrate and the second substrate are respectively provided with a first alignment layer and a second alignment layer on the inner side.

3. The preparation method according to claim 1, characterized in that The width of the strip pattern increases with the increase of the femtosecond laser power of the femtosecond laser direct writing device.

4. The production method according to claim 3, characterized by, The femtosecond laser direct writing device is composed of a femtosecond laser, a shutter, a half-wave plate, a polarization beam splitter, a polarizer, a lens, an aperture, a CCD camera, a mirror and a microscope objective, a liquid crystal sample and a high-precision displacement stage.

5. The preparation method according to claim 4, characterized in that The laser generated by the femtosecond laser has a wavelength of 800 nanometers, a duration of 75 femtoseconds and a repetition frequency of 80 megahertz.

6. The preparation method according to claim 4, characterized in that The microscope objective has a numerical aperture of 0.7, a magnification of 50 times, a numerical aperture of 0.3 and a magnification of 10 times.

7. The preparation method according to claim 1, characterized in that The ferroelectric liquid crystal material is ferroelectric nematic liquid crystal RM734.

8. The preparation method according to claim 2, characterized in that The alignment agent used in the first alignment layer and the second alignment layer includes any one of a surfactant, a rubbing alignment agent, a photo-crosslinking material, a photo-degradation material and a photo-induced cis-trans isomerization material.

9. The ferroelectric liquid crystal material grating based on femtosecond laser direct writing prepared by the preparation method of any one of claims 1-8.

10. The application of the ferroelectric liquid crystal material grating based on femtosecond laser direct writing in realizing linear and / or nonlinear grating diffraction.

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