A method for high-efficiency ferroelectric domain engineering induced by femtosecond laser

Abstract

This invention discloses a method for high-efficiency domain engineering of femtosecond laser-polarized ferroelectric materials. The method consists of two steps: first, marking points are fabricated on the surface or inside the ferroelectric crystal using a femtosecond laser; then, a three-dimensional thermoelectric field is induced inside the crystal near the marking points using a multi-pulse femtosecond laser. This thermoelectric field drives domain inversion at the marking points. This method significantly improves the quality and processing efficiency of femtosecond laser domain engineering and is applicable to most ferroelectric materials, showing great application potential.

Description

Technical Field

[0001] This invention relates to the field of laser processing, and in particular to a method for high-efficiency polarization of ferroelectric domains using femtosecond lasers. Background Technology

[0002] Ferroelectrics are media that exhibit macroscopic polarization below the Curie temperature. Macroscopic polarization originates from ferroelectric domains formed in ferroelectric materials. These domains can have different polarization directions and coexist in a medium separated by domain walls. The signs and magnitudes of physical quantities such as the macroscopic nonlinear coefficient, photoelectric coefficient, and piezoelectric coefficient of ferroelectric materials are closely related to the polarization direction and degree of these domains.

[0003] Domain engineering refers to the creation of domain structures in ferroelectrics and the spatial modulation of physical quantities such as their nonlinear coefficients, thereby enabling applications in nonlinear optics (e.g., frequency conversion, nonlinear volume holography), optical measurement, optical communication, optical sensing, nonvolatile memory, and photovoltaics. Methods for inducing domain formation and the use of domain structures to achieve precise multi-dimensional control of optical fields are at the forefront of physical science research. While my country's basic research in domain engineering has consistently been at the international forefront, industrialization still lags behind, primarily due to limitations in polarization processes. Current main methods for realizing domain formation in bulk ferroelectrics include electric field polarization, optically assisted electric field polarization, all-optical polarization, electron beam irradiation polarization, and direct writing at the tip of an atomic force microscope. Among these methods, femtosecond lasers offer significant advantages in fabricating domain structures in transparent ferroelectrics, such as high spatial fraction, high degrees of freedom, and true three-dimensional processing. However, their disadvantages include slow processing speed, the need for sequential multi-pulse scanning when processing multi-domain structures, low processing efficiency, and the presence of severe crystal defects in the induced structures, all of which limit industrialization. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a novel all-optical polarization technique. This method, through a focused femtosecond laser performing pre-marking followed by collectively induced domain inversion in a ferroelectric crystal, can significantly improve the efficiency of femtosecond laser ferroelectric domain engineering.

[0005] Through extensive prior experiments, the inventors observed that ferroelectric materials, after local modification, more readily induce ferroelectric domains. Therefore, they proposed and implemented a two-step laser polarization method: marking followed by induction. This method utilizes a laser-induced three-dimensional thermoelectric field to efficiently and accurately induce the reversal of initially marked point groups, effectively solving the aforementioned problems of low resolution and low efficiency. This technology enables rapid fabrication of large-area, high-quality, multi-dimensional domain engineering in lithium niobate crystals. Furthermore, this technology can be applied to most other ferroelectric materials, providing support for the practical application of advanced and diverse nonlinear photonic devices.

[0006] The technical solution proposed in this invention is a method for high-efficiency polarization of ferroelectrics using femtosecond lasers. This method includes two processes: femtosecond laser marking and femtosecond laser-induced three-dimensional polarization electric field. Domain inversion is induced at the marked point by the three-dimensional polarization electric field, as detailed below:

[0007] Step 1: Use a femtosecond laser to create marking points on or inside the ferroelectric crystal;

[0008] Step 2: A multi-pulse femtosecond laser is used to radiate inside the crystal and near the marked point to induce a three-dimensional thermoelectric field. The thermoelectric field drives domain inversion at the marked point.

[0009] Furthermore, the focal energy of the femtosecond laser is greater than the crystal damage threshold at the focal position, which can generate a modified region on or inside the crystal, that is, form a marker point.

[0010] Furthermore, the marked point serves as a domain inversion nucleation point.

[0011] Furthermore, the adjustment parameters of the femtosecond laser source include average power, repetition frequency, pulse width, wavelength, focal distance, and size, and different sizes of markers can be processed by adjusting these parameters.

[0012] Furthermore, the marking points are processed using a single-pulse laser.

[0013] Furthermore, femtosecond laser processing methods include single-pulse radiation, multi-pulse radiation, and pulse sequence radiation.

[0014] Furthermore, ferroelectric crystals are infrared transparent materials, including LiNbO3, LiTaO3, and KTiOPO4, etc.

[0015] Furthermore, the coercive field of the polar axis where the marker point is located is reduced by processing the marker point.

[0016] Furthermore, in step two, the heat generated by the multi-pulse laser accumulates at the marked point, inducing a thermoelectric field larger than the coercive field.

[0017] Furthermore, in step two, a three-dimensional thermoelectric field is generated in different regions by the relative displacement of the laser and the sample.

[0018] Furthermore, in step two, three-dimensional electric fields of different intensities and shapes are induced by changing the size, shape, and depth of the laser focus, as well as the laser parameters.

[0019] Furthermore, in step two, the electric field of the three-dimensional thermoelectric field near the distribution marker point is greater than the coercive field at that point, and the direction of the electric field is opposite to the direction of spontaneous polarization.

[0020] Furthermore, by changing the size and shape of the marker points, the intensity and distribution of the three-dimensional thermoelectric field, and the relative positions of the marker points and the thermoelectric field, different domain structures are induced at different marker point positions and polar axes.

[0021] The beneficial effects of the present invention are as follows:

[0022] This invention induces domain inversion at marked points by a three-dimensional thermoelectric field formed inside the crystal through multi-pulse femtosecond laser radiation. It is a novel femtosecond laser polarization method that can induce ferroelectric domain inversion over long distances, over a wide area, with high quality and in multiple dimensions. It can form an ultra-high efficiency all-optical polarization technology with great application potential. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the two-step femtosecond laser method for inducing domain engineering in lithium niobate crystals according to the present invention.

[0024] Figure 2 These are the marking points processed by femtosecond laser on the +z surface of lithium niobate crystal according to the present invention.

[0025] Figure 3 This is a schematic diagram of the results of the present invention, which uses multi-pulse lasers of different energies to induce domain inversion at the marked location. Detailed Implementation

[0026] To facilitate understanding and application of the present invention by those skilled in the art, the invention will be further described below with reference to the accompanying drawings. Obviously, the described embodiments are merely a part of the invention, and not all of it. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0027] Lithium niobate crystal is one of the most widely used ferroelectric materials, possessing superior nonlinear, electro-optic, piezoelectric, and acousto-optic properties, as well as stable chemical properties. It is a key material used in integrated optical devices and is known in the industry as "optical silicon." This invention uses lithium niobate crystal as a specific example of the implementation of this technology, while also noting that this technology can be applied to other ferroelectric materials.

[0028] Example

[0029] This embodiment proposes a method for femtosecond laser-polarized lithium niobate ferroelectric domains by first labeling and then inducing them, such as... Figure 1 As shown, the implementation process is as follows:

[0030] Step 1: Focus the femtosecond laser onto the surface of the ferroelectric crystal and process marking points on the crystal surface.

[0031] For step one, this embodiment implements it in the following way:

[0032] The sample was a Z-cut 5% magnesium oxide-doped lithium niobate crystal with a thickness of 500 μm. The sample was mounted on a high-precision displacement bracket (Aerotech) equipped with a PC-driven 3-axis XYZ cross roller bearing positioning system.

[0033] For femtosecond laser sources, adjustable parameters include average power, repetition frequency, pulse width, and wavelength; for focusing parameters, adjustable parameters include focal distance, focal shape, and size; for processing methods, there are single-pulse radiation, multi-pulse radiation, and pulse sequence radiation, all of which can affect the effect of femtosecond laser marking.

[0034] A general-purpose femtosecond laser manufacturing system was used for laser marking and inducing three-dimensional thermoelectric fields. The light source was a regeneratively amplified Yb:KGW-based laser system (Pharos, Light Conversion) with a wavelength of 1026 nm, a repetition rate of 1000 kHz, and a pulse width of 170 fs. The pulse energy could be continuously adjusted between 0 and 400 μJ using an attenuator consisting of a half-wave plate and a polarizer. The laser was focused using a 50x microscope objective (NA = 0.42) to produce a focal spot with a diameter of approximately 1-2 micrometers. During processing, the light was polarized along the Y-direction of the crystal. Figure 2 As shown, the marking points are processed on the lithium niobate+z surface using a single femtosecond pulse with an energy of 120 nJ.

[0035] Step 2: Focus the femtosecond laser into the interior of the crystal and near the marked point to generate heat accumulation. The three-dimensional electric field formed by the thermoelectric effect induces domain inversion at the marked point.

[0036] For step two, this embodiment adopts the following method:

[0037] For femtosecond laser sources, adjustable parameters include average power, repetition frequency, pulse width, and wavelength; for focusing parameters, adjustable parameters include focal distance, focal shape, and size; for processing methods, there are single-pulse radiation, multi-pulse radiation, and pulse sequence radiation, all of which can affect the effect of femtosecond lasers inducing a three-dimensional thermoelectric field in lithium niobate.

[0038] The femtosecond laser manufacturing system used in this step is the same as in step one. For example... Figure 3 As shown, a three-dimensional thermoelectric field is induced at a location 20 μm below the surface of lithium niobate+z by incident multi-pulse laser light from the surface of the crystal, thereby inducing domain inversion at the marked point. In the example, the first step of laser marking is denoted by LM, and the second step of laser induction is denoted by LI.

[0039] Experimental steps are as follows Figure 1 As shown, multiple marker points are first fabricated on the surface using a laser, and then a thermoelectric field is induced near these marker points using a laser to achieve large-scale manipulation. Three marker rings, with diameters of 20, 30, and 40 μm respectively, were fabricated on the crystal surface using a laser with a pulse number of 1 and a pulse energy of 150 nJ. The fabricated marker points are imaged under an optical microscope as shown in the figure. Figure 2 As shown. Because the crystal damage threshold must be exceeded, very slight ablation is left on the crystal surface, with the markers approximately 1 μm in size. The number of markers on the inner, middle, and outer rings are 32, 47, and 63 respectively, with a spacing of 2 μm between the markers.

[0040] Then, a thermoelectric field was induced at the center of a ring 20 μm below the crystal surface using Li with 500,000 pulses and pulse energies of 300, 400, 500, 600, 700, 800, and 900 nJ. The experimental results are shown in Figure 3. Before the Li energy reached 400 nJ, the induced electric field was very weak, and ferroelectric domains could only be sporadically reversed at the marked sites on the inner ring. However, when the energy reached 400 nJ, the domain reversal rate on the inner ring increased to ~30%. With increasing energy, more and more marked sites on the inner ring experienced domain reversal. At 900 nJ, the domain reversal rate was 100%. Similar trends were observed at marked sites on the middle and outer rings with increasing Li energy. The difference was that the minimum Li energy required to manipulate multi-domain reversal increased to 600 nJ and 700 nJ, respectively, while the domain reversal rate was not as high as on the inner ring. These results reflect two points: firstly, the randomness of the "quality" of the markers, with each site having a different coercive field. Second, the strength and range of the electric field increase with increasing LI energy. If 100% domain inversion is required on the middle or outer ring, the energy can be continuously increased or the LI can be moved closer to the marker point on the middle or outer ring.

[0041] The average domain length changes with LI energy, decreasing from 80 μm to 10 μm and from 30 μm to 15 μm for the inner and middle circular domains, respectively. The domain length on the outer circle increases from 20 μm to 40 μm. At LI energy of 700 nJ, the domain lengths on the three circles are approximately equal. When the polarization direction of the marked region is reversed by the thermoelectric field, domain growth is driven not only by the thermoelectric field but also by the kinetic and potential energy of the displaced atoms after the domain reversal. This energy is provided by the polarization electric field; when it is greater than the electric field in the opposite direction, the domain can continue to grow, exceeding the depth of LI. For all LI energies, the longest domains are more likely to appear in the outermost layer of the domain group. At lower LI energies, the longest domains appear in the inner ring; as LI energy increases, the longest domains first appear in the middle ring and then on the outer ring. The longest domains are generally about 80 μm, but can reach up to 130 μm. The laser-induced electric field is a bipolar field moving from low to high temperature, meaning that the opposite electric field below the polar electric field inhibits domain growth. Increasing the LI energy increases the thermoelectric field at the marker point, which also increases the electric field strength below the LI focus, in the direction of the crystal's spontaneous polarization. This inhibits domain growth guided by domain inversion dynamics. Relative to a specific marker point, with the LI position unchanged, the domain length below the marker point increases and then decreases at different LI energies. The thermoelectric field distribution formed by LI is non-uniform, and when the marker point is on the crystal surface, the length of the inverted domains produced is inconsistent. The domain length can be limited by adding marker points inside the crystal.

[0042] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for high-efficiency polarized ferroelectric domain engineering using femtosecond lasers, characterized in that, Includes the following steps: Step 1: Use a femtosecond laser to create marking points on or inside the ferroelectric crystal; Step 2: A multi-pulse femtosecond laser is used to radiate inside the crystal and near the marked point to induce a three-dimensional thermoelectric field. The thermoelectric field drives domain inversion at the marked point.

2. The method according to claim 1, characterized in that: The focal energy of the femtosecond laser is greater than the crystal damage threshold at the focal position, which can generate a modified region on or inside the crystal, that is, form a marker point.

3. The method according to claim 1, characterized in that: The adjustment parameters of a femtosecond laser source include average power, repetition rate, pulse width, wavelength, focal distance, and size. By adjusting these parameters, markers of different sizes can be processed.

4. The method according to claim 1, characterized in that: Femtosecond laser processing methods include single-pulse radiation, multi-pulse radiation, and pulse sequence radiation.

5. The method according to claim 1, characterized in that: The ferroelectric crystal is an infrared transparent material, including LiNbO3, LiTaO3 and KTiOPO4.

6. The method according to claim 1, characterized in that: The coercive field of the polar axis where the marker point is located is reduced by processing the marker point.

7. The method according to claim 1, characterized in that: In step two, the heat generated by the multi-pulse laser accumulates at the marked point, inducing a thermoelectric field larger than the coercive field.

8. The method according to claim 1, characterized in that: In step two, a three-dimensional thermoelectric field is generated in different regions by the relative displacement of the laser and the sample.

9. The method according to claim 1, characterized in that: In step two, three-dimensional electric fields of different intensities and shapes are induced by changing the size, shape, and depth of the laser focus, as well as the laser parameters.

10. The method according to claim 1, characterized in that: In step two, the electric field of the three-dimensional thermoelectric field near the distribution marker point is greater than the coercive field at that point, and the direction of the electric field is opposite to the direction of spontaneous polarization.

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