Preparation method and application of ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing

Abstract

The invention belongs to the technical field of optics, and particularly relates to a preparation method and application of a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing. The first substrate and the second substrate are ultrasonically cleaned, and ozone cleaning is performed after drying; the edges of the first substrate and the second substrate which are cleaned are coated with spacers, and the two substrates are bonded and solidified in a staggered mode to form a liquid crystal box; filling a ferroelectric liquid crystal material into the liquid crystal box to form a liquid crystal layer; and directly writing a pattern of the liquid crystal layer through a femtosecond laser direct writing device so as to control the molecular orientation direction of the liquid crystal layer, thereby obtaining the ferroelectric liquid crystal material grating. According to the femtosecond laser direct writing device, patterns of a direct writing liquid crystal layer comprise any one of a strip-shaped pattern, a radial line pattern and a circular ring pattern. According to the invention, the control on the orientation direction of the polar liquid crystal molecules is realized at room temperature through the femtosecond laser direct writing technology, and the nonlinear optical response is successfully realized.

Description

Technical Field

[0001] The invention belongs to the field of optical technology, and in particular relates to a preparation method and application of a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing. Background Art

[0002] Ferroelectric materials have the characteristics of spontaneous electric polarization, and the polarization direction can be switched by an external electric field. They are widely used in the fields of electronics, electronic optics, and micro-electromechanical integration. Among them, materials such as LiNbO3 that have been widely used in the field of optics belong to inorganic metal ferroelectric crystals. In order to realize their multi-field applications in modern scientific and technological research, people have proposed a variety of 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 adjustability, non-toxicity, good biocompatibility, low cost and low power consumption. They have great application potential in the fields of biomedicine, optoelectronic devices, etc. The use of various domain engineering technologies in organic ferroelectrics is still in the research boom.

[0003] Liquid crystal is a well-known organic material that has both the fluidity of liquid and the anisotropy of crystal. It is sensitive to external stimuli and has unique anisotropic optical properties, self-assembly ability, and dynamic adjustability. A new type of liquid crystal discovered in recent years, ferroelectric nematic liquid crystal, breaks the head-to-tail symmetry compared to traditional liquid crystals, has spontaneous polarization characteristics, large dielectric constant and strong nonlinear optical response, and is very suitable for programmable and dynamic structured light field manipulation in the field of nonlinear optics. In the past few years, many important achievements have been made in the study of polarity order control in this organic ferroelectric material. However, the existing ferroelectric nematic liquid crystals such as RM734 and DIO are limited by the influence of temperature. When the temperature control is unstable, the material is easily crystallized and shows a disordered and defective polycrystalline phase at room temperature. It is generally believed that such a crystalline state is difficult to realize application. Therefore, it is necessary to explore a method that can manipulate and utilize ferroelectric liquid crystals at room temperature to open up new possibilities for its flexible application in the fields of new optoelectronic and advanced photonic device design. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for controlling the molecular orientation direction of ferroelectric liquid crystal materials at room temperature by using femtosecond laser direct writing technology, thereby obtaining a fine patterned structure, and utilizing the characteristics of liquid crystals that respond to external stimuli to realize applications in the field of nonlinear photonics. The femtosecond laser direct writing technology realizes the manipulation of the liquid crystal structure at room temperature, thereby solving the problem that the temperature of the liquid crystal element and the heating and cooling rate need to be controlled in the current application of ferroelectric nematic liquid crystals to prevent defects caused by material crystallization and the liquid crystal structure from becoming chaotic and disordered.

[0005] In order to solve the above technical problems, the present invention discloses a method for preparing a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing, and the specific steps are: using ultrasound to clean a first substrate and a second substrate, drying and then ozone cleaning; applying a spacer to the edges of the cleaned first substrate and the second substrate, and forming a liquid crystal box after the two substrates are staggered and bonded and cured; pouring a ferroelectric liquid crystal material into the liquid crystal box to form a liquid crystal layer; using a femtosecond laser direct writing device to directly write a pattern of the liquid crystal layer, thereby controlling the molecular orientation direction of the liquid crystal layer, and obtaining the ferroelectric liquid crystal material grating;

[0006] Wherein, the pattern of the direct writing liquid crystal layer of the femtosecond laser direct writing device includes any one of a stripe pattern, a radial line pattern and a circular ring pattern.

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

[0008] Specifically, the width of the stripe pattern increases with the increase of the power of the femtosecond laser 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 reflector and a microscope objective, a liquid crystal sample and a high-precision translation stage.

[0010] Preferably, the laser wavelength generated by the femtosecond laser is 800 nanometers, the duration is 75 femtoseconds, and the repetition frequency is 80 MHz.

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

[0012] Furthermore, the room temperature ferroelectric liquid crystal material structure control method described in the present invention can adjust the size and pattern of the resulting liquid crystal ferroelectric domain structure by setting the parameters of the femtosecond laser direct writing system (laser power, direct writing speed, focus depth, etc.) and the movement trajectory of the laser spot, thereby hopefully expanding the application of liquid crystal elements obtained by direct writing to more fields.

[0013] Specifically, the liquid crystal layer material uses ferroelectric liquid crystal material to achieve nonlinear optical response sensitive to external stimuli. Ferroelectric liquid crystal material has spontaneous polarization characteristics and can regulate the incident light field. The nonlinear liquid crystal element of the present invention uses 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-degradable material, and a photoinduced cis-trans isomerization material;

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

[0016] Among them, the photo-controlled orientation material molecules will be arranged in a direction perpendicular to the linear polarization direction under the irradiation of linear polarization light, and through the interaction with the liquid crystal molecules, the liquid crystal molecules will form an orderly orientation, thereby realizing the manipulation of the liquid crystal molecular orientation; the rubbing orientation agent will be arranged along the rubbing direction under the action of friction, and through the interaction with the liquid crystal molecules, it will induce the alignment along the alignment direction of the orientation agent molecules. In actual operation, it is not limited to these two materials;

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

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

[0019] Specifically, the drying conditions are as follows: 120 degrees Celsius and a drying time of 40 minutes.

[0020] Specifically, the spacer is a mixture of microspheres and frame-sealing adhesive; wherein 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 injecting the material into the liquid crystal box through a capillary glass tube.

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

[0023] Preferably, the spin coating has the following specific conditions: the first step is to spin coat at 800 rpm for 8 seconds; the second step is to spin coat at 3000 rpm for 40 seconds.

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

[0025] Specifically, in the preparation process of the ferroelectric liquid crystal material grating containing the first orientation layer and the second orientation layer, it is necessary to pre-orient the first orientation layer and the second orientation layer before pouring the ferroelectric liquid crystal material after forming the liquid crystal box. The specific operation is: the first orientation layer and the second orientation layer are uniformly pre-oriented by linearly polarized light with a wavelength of 405 nanometers and a power of 1W to 2W.

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

[0027] Furthermore, the application of the above-mentioned room temperature 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 invention;

[0028] Specifically, in some embodiments of the present invention, a grating structure is obtained by setting the power of a femtosecond laser to 350 mW and direct writing with a period of 20 μm. The wavelength of the incident light is set to 800 nm, and grating linear and nonlinear diffraction spots are obtained respectively. The nonlinear diffraction of the grating conforms to the nonlinear Raman-Ness diffraction, and there are symmetrically distributed multi-order diffraction lights, and the intensity of the 0th order spot is the strongest. The first-order diffraction angle of linear diffraction is twice that of 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 invention realizes the control of the polarization direction of polar liquid crystal molecules at any angle in the plane at room temperature through femtosecond laser direct writing technology. The femtosecond laser focus induces a thermoelectric field to drive the liquid crystal molecules to align, without being limited by the traditional orientation methods (such as light-controlled orientation, friction orientation, etc.) that rely on orientation agents to induce the orientation of liquid crystal molecules, and avoids the problem of splay alignment caused by the flexoelectric effect of the ferroelectric nematic phase liquid crystal system, and avoids the problem that the traditional orientation method may be affected by temperature changes and cause structural defects. Ferroelectric liquid crystal material gratings were prepared at room temperature using femtosecond laser direct writing technology and nonlinear optical beam steering control was successfully achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more clear.

[0031] Figure 1 is a structural diagram of a liquid crystal element prepared in an embodiment of the present invention;

[0032] Figure 2 The texture pattern and brightness information changes of the uniformly pre-oriented liquid crystal element prepared in the embodiment of the present invention under a polarizing microscope at room temperature;

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

[0034] Figure 4 A schematic diagram of the orientation change of a femtosecond laser direct-writing ferroelectric liquid crystal element prepared in an embodiment of the present invention;

[0035] Figure 5 The texture diagram of the stripe structure of the ferroelectric liquid crystal material prepared in the embodiment of the present invention at room temperature under a polarizing microscope;

[0036] Figure 6 The texture image of the radial line and circular ring pattern directly written by femtosecond laser prepared in the embodiment of the present invention under a polarizing microscope;

[0037] Figure 7 The texture pattern of the grating prepared in the embodiment of the present invention under a polarizing microscope and its corresponding linear and nonlinear diffraction patterns. DETAILED DESCRIPTION

[0038] To make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the embodiments of the present invention will be described clearly and completely in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them.

[0039] Unless otherwise defined, technical or scientific terms used herein shall have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs.

[0040] The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0041] Embodiment 1:

[0042] The present invention proposes a method for preparing a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing and its application. Figure 1 This is the structure diagram of the liquid crystal element prepared in this embodiment, which specifically includes a first substrate and a second substrate, a first alignment layer and a second alignment layer are provided inside the first substrate and the second substrate, microspheres are provided at the edges of the first substrate and the second substrate, and a liquid crystal box is formed between the first substrate and the second substrate after the first substrate and the second substrate are bonded together, and a liquid crystal layer is formed after the liquid crystal material is poured into the liquid crystal box. The steps for preparing the liquid crystal grating are as follows:

[0043] Step 1: Both the first substrate and the second substrate are ITO glass substrates. First, use ITO cleaning solution to ultrasonically clean the first substrate and the second substrate for 30 minutes. Then use ultrapure water to ultrasonically clean twice, each time for 10 minutes. Put the cleaned substrate into a drying oven, adjust the temperature of the drying oven to 120 degrees Celsius, and dry it for 40 minutes. Finally, clean the glass substrate with ultraviolet ozone for 30 minutes;

[0044] Step 2: Spin-coat the photo-alignment agent SD1 on the first substrate and the second substrate. The specific spin-coating method is: the first step, 800 rpm spin-coating for 8 seconds; the second step, 3000 rpm spin-coating for 40 seconds. The thickness of the photo-alignment agent layer finally formed is about 30 nanometers. After the spin-coating of the alignment agent, the glass substrate spin-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 3: Use a mixture of microspheres and UV sealant as spacers, where the microspheres are 10-micron silica microspheres or polystyrene microspheres, mix them evenly and apply them on the edges of the first and second substrates. Stagger the upper and lower substrates and bond them under ultraviolet light until the sealant is cured to form a liquid crystal box. The thickness of the liquid crystal box is controlled by changing the diameter of the microspheres. The thickness of the liquid crystal box in this embodiment is measured by interference method to be about 9.6 microns.

[0046] Step 4: uniformly aligning the first and second alignment layers by linearly polarized light with a wavelength of 405 nanometers, using a 405 nanometer LED light source with a power of 1W to 2W, fully irradiating the liquid crystal box after passing through a linear polarizer, and directly uniformly aligning the first and second alignment layers in-plane;

[0047] Step 5: Pour liquid crystal material between the first substrate and the second substrate, heat the ferroelectric nematic liquid crystal material RM734 to 188 degrees Celsius, and pour it into the liquid crystal box through a capillary glass tube to arrange the liquid crystal molecules according to the orientation directions of the first orientation layer and the second orientation layer. After the pouring is completed, cool the liquid crystal box to room temperature.

[0048] The liquid crystal molecular orientation of the liquid crystal element obtained in step 5 is studied. Figure 2The texture pattern and brightness information change of a uniformly oriented liquid crystal element under an orthogonal polarizing microscope after cooling to room temperature. It can be seen from the figure that the liquid crystal texture is chaotic and disordered at room temperature, and the uniform orientation effect is destroyed. Under an orthogonal polarizing microscope, the sample is rotated and it is found that the texture brightness has no extinction and the brightness change is less than 10%, which shows that the optical anisotropy induced by orientation is very weak at this time. The texture pattern and its brightness change law here prove that the traditional liquid crystal orientation methods such as the intermolecular force of the oriented molecules in the orientation layer and the surface structure induction do not achieve the expected liquid crystal polarization direction control.

[0049] Step 6: Focus the femtosecond laser into the liquid crystal layer through a femtosecond laser direct writing device, and the thermal gradient field formed at the laser focus induces a thermoelectric field. Under the action of the thermoelectric field, the polarization direction of the ferroelectric liquid crystal molecules is regulated and arranged along the electric field direction. As the laser focus moves, the position of the thermoelectric field moves accordingly, driving the polarization direction at the position after the thermoelectric field moves to rearrange and form a ferroelectric domain structure. Multiple equally spaced direct-written strip domain structures can obtain the room temperature ferroelectric liquid crystal material grating based on femtosecond laser direct writing.

[0050] Among them, the position covered by the thermoelectric field after the movement will rewrite the polarization direction of the polarized liquid crystal in the area, even if part of the area has been directly written by the thermoelectric field at the previous moment.

[0051] in, Figure 3 The schematic diagram of the femtosecond laser direct writing device used in the present invention 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 reflector and a microscope objective, a liquid crystal sample and a high-precision translation stage. 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 the laser power is adjusted by a half-wave plate and a polarization beam splitter.

[0052] Specifically, the liquid crystal box is focused through an objective lens group 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, generating an effective spot size of tens of microns, and the sample is moved by a high-precision translation stage to scan the focused light beam on the sample.

[0053] Figure 4 This is a 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 femtosecond laser direct writing. Before direct writing, the orientation direction of the liquid crystal box is in a chaotic and disordered state at room temperature. After direct writing, under the action of different laser pulse energies, the orientation state of the liquid crystal molecules changes and becomes orderly along the scanning direction of the laser spot.

[0054] Through the changes in the polarization direction of the liquid crystal after steps five and six, it can be concluded that the orientation layer in this embodiment actually does not play a role in the prepared liquid crystal grating. Therefore, it can be inferred that the technical solution of the present invention can also be without an orientation layer, and it will not affect the integrity of the technical solution. However, the existence of the orientation layer still makes the sample before direct writing more uniform to a certain extent and has a certain degree of anisotropy (although it is very weak, with a brightness difference of ~10%).

[0055] Figure 5 The texture diagram of the strip structure of the ferroelectric liquid crystal material prepared in this embodiment at room temperature under a polarizing microscope. Specifically, the femtosecond laser power is set to 50 to 150 mW in sequence, and the power of each structure changes by 20 mW. Six strip ferroelectric domain structures are obtained in the directions perpendicular and parallel to the uniform orientation direction of the liquid crystal box. Figure 5 It can be seen that as the femtosecond laser power increases, the stripe domain width increases.

[0056] Figure 6 This is a texture diagram of the radial line and circular ring patterns directly written by a femtosecond laser prepared in the present invention under a polarizing microscope.

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

[0058] Optionally, in this embodiment, strips, radial lines and rings are used as patterns for femtosecond laser direct writing, but are not limited to these patterns and include any other patterns.

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

[0060] The present invention provides a method for preparing a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing and an idea for its application. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A method for preparing a ferroelectric liquid crystal material grating at room temperature based on femtosecond laser direct writing, characterized in that: The specific steps are: using ultrasound to clean the first substrate and the second substrate, drying and then ozone cleaning; applying spacers on the edges of the cleaned first substrate and the second substrate, and bonding the two substrates in an offset manner and curing them 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 the pattern of the liquid crystal layer by a femtosecond laser direct writing device, thereby controlling the orientation direction of the molecules of the liquid crystal layer, thus obtaining the ferroelectric liquid crystal material grating; Wherein, the pattern of the direct writing liquid crystal layer of the femtosecond laser direct writing device includes any one of a stripe pattern, a radial line pattern and a circular ring pattern.

2. The preparation method according to claim 1, characterized in that: A first alignment layer and a second alignment layer are respectively disposed inside the first substrate and the second substrate.

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

4. The preparation method according to claim 3, characterized in that: 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 reflector 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 wavelength generated by the femtosecond laser is 800 nanometers, the duration is 75 femtoseconds, and the repetition frequency is 80 MHz.

6. The preparation method according to claim 4, characterized in that: The microscope objective lens has 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.

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-degradable material, and a photoinduced cis-trans isomerization material.

9. A ferroelectric liquid crystal material grating at room temperature prepared by the preparation method according to any one of claims 1 to 8 based on femtosecond laser direct writing.

10. Application of the room temperature ferroelectric liquid crystal material grating based on femtosecond laser direct writing as described in claim 9 in realizing linear and / or nonlinear grating diffraction.

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