A method of erasing ferroelectric domain structures in ferroelectric crystals and applications thereof

Abstract

The application relates to the field of laser processing, in particular to a method for erasing a ferroelectric domain structure of a ferroelectric crystal and application. The method for erasing the ferroelectric domain structure of the ferroelectric crystal comprises the following steps: (a) marking the position of a ferroelectric domain inversion structure to be erased on the ferroelectric crystal; (b) focusing a multi-pulse femtosecond laser inside the ferroelectric crystal to induce a thermal electric field at the position close to the ferroelectric domain inversion structure to be erased, the thermal electric field drives the polarization direction of the ferroelectric domain inversion structure to be reversed, and the ferroelectric domain inversion structure is erased. The method for erasing the ferroelectric domain structure of the ferroelectric crystal has wide application range, high precision, high erasing efficiency, can quickly erase the ferroelectric domain inversion structure, and greatly improves the processing efficiency of the ferroelectric domain structure.

Description

Technical Field

[0001] This invention relates to the field of laser processing, and more specifically, to a method and application for erasing the ferroelectric domain structure of a ferroelectric crystal. Background Technology

[0002] Ferroelectrics are a general term for materials with spontaneous polarization. This spontaneous polarization originates from the ferroelectric domains formed within the ferroelectric material. These domains can have different polarization directions, and are separated by domain walls. Physical quantities of ferroelectric materials, such as their nonlinear coefficients and photoelectric coefficients, are closely related to the polarization direction of their domains. Domain engineering refers to customizing domain structures within ferroelectrics and spatially modulating their nonlinear coefficients and other physical quantities, thereby enabling applications in nonlinear optics, optical communication, optical sensing, and non-volatile memory. Methods for customizing domain structures include writing ferroelectric domains and erasing ferroelectric domains.

[0003] Ferroelectric domain writing technology can be used to fabricate ferroelectric domain devices such as nonlinear optical devices and optical communication devices. Methods for writing ferroelectric domains for inversion mainly include electric field polarization, optically assisted electric field polarization, all-optical polarization, and tip polarization. Among these, all-optical polarization is simple to process and can fabricate three-dimensional domain inversion structures, giving it a significant advantage in further improving the performance of lithium niobate ferroelectric domain devices.

[0004] Ferroelectric domain erasure technology is also widely used in the fabrication of ferroelectric domain devices. In the fabrication of non-volatile memories, frequent erasure of ferroelectric domains is necessary to achieve state transitions; in the multiplexing of nonlinear optical devices, such as multiplexing a frequency converter corresponding to one wavelength to a frequency converter corresponding to another wavelength, all previous ferroelectric domain inversion structures need to be erased; similarly, when writing incorrect or redundant ferroelectric domains, the unwanted ferroelectric domains also need to be erased.

[0005] Currently, the main ferroelectric domain erasure technology is femtosecond laser scanning erasure, proposed by Zhang Yong et al. from Nanjing University. They use a femtosecond laser to scan and erase part of the domain inversion structure obtained by direct writing inside the crystal, leaving nanoscale domain inversion structures, thus realizing the fabrication of nanodomains. However, the femtosecond laser scanning erasure method requires scanning along the domain structure one by one, which is slow and has low erasure efficiency, which undoubtedly limits the industrial production of ferroelectric domain devices.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] To address the shortcomings of existing femtosecond laser scanning methods for erasing ferroelectric domain structures, such as slow processing speed and low erasure efficiency, this invention proposes a method and application for erasing ferroelectric domain structures in ferroelectric crystals. This erasure method has high erasure efficiency and can quickly erase ferroelectric domain inversion structures, greatly improving the processing efficiency of ferroelectric domain structures.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0009] One aspect of the present invention relates to a method for erasing the ferroelectric domain structure of a ferroelectric crystal, comprising the following steps:

[0010] (a) Mark the location of the ferroelectric domain inversion structure to be erased on the ferroelectric crystal;

[0011] (b) A multi-pulse femtosecond laser is focused inside the ferroelectric crystal to induce a thermoelectric field near the ferroelectric domain inversion structure to be erased. The thermoelectric field drives the polarization direction of the ferroelectric domain inversion structure to reverse, thereby erasing the ferroelectric domain inversion structure.

[0012] The method for erasing the ferroelectric domain structure of ferroelectric crystals has a wide range of applications, high precision, and high erasing efficiency. It can quickly erase the ferroelectric domain inversion structure and greatly improve the processing efficiency of ferroelectric domain structures.

[0013] Preferably, the electric field direction of the thermoelectric field is opposite to the polarization direction of the ferroelectric domain inversion structure to be erased, and the electric field strength of the thermoelectric field is greater than that of the coercive field.

[0014] Preferably, the length of the erased ferroelectric domain inversion structure is controlled by adjusting the relative position of the thermoelectric field and the ferroelectric domain inversion structure, as well as the intensity of the thermoelectric field.

[0015] Preferably, the intensity of the thermoelectric field is controlled by adjusting the parameters of the multi-pulse femtosecond laser.

[0016] Preferably, the parameters of the multi-pulse femtosecond laser include at least one of the following: wavelength, average power, repetition frequency, spot size, pulse width, number of pulses, and single pulse energy.

[0017] Preferably, the relative position of the thermoelectric field and the ferroelectric domain inversion structure is controlled by adjusting the focusing position of the multi-pulse femtosecond laser inside the ferroelectric crystal.

[0018] Preferably, the multi-pulse femtosecond laser is incident from the +z surface of the ferroelectric crystal.

[0019] Preferably, the positional relationship between the ferroelectric domain inversion structure and the thermoelectric field is as follows: the ferroelectric domain inversion structure is regarded as a cylinder with a radius of less than 100 micrometers and a height of 40 to 100 micrometers, and the center of the thermoelectric field is located in the region from the upper surface of the cylinder to 20 to 50 micrometers directly below the upper surface.

[0020] Preferably, a femtosecond laser is used to mark the position of the ferroelectric domain inversion structure to be erased on the +z surface of the ferroelectric crystal.

[0021] Another aspect of the present invention relates to a method for fabricating a ferroelectric domain device, comprising the method for erasing the ferroelectric domain structure of a ferroelectric crystal.

[0022] The method for fabricating the ferroelectric domain device described above has high fabrication efficiency and fast fabrication speed.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) The method for erasing ferroelectric domain structures in ferroelectric crystals provided by the present invention uses a femtosecond laser to mark the position of the ferroelectric domain inversion structure to be erased on the ferroelectric crystal, which can improve the erasure accuracy. By focusing the femtosecond laser inside the crystal to generate a thermoelectric field, the thermoelectric field is used to drive the ferroelectric domain inversion structure to invert again, which can erase multiple domain inversion structures at the same time, and realize the rapid erasure of ferroelectric domain structures. Compared with the laser scanning method for erasing domain inversion structures, the erasure efficiency can be improved by up to two orders of magnitude. It is a new, long-distance, large-scale, and high-efficiency laser erasure technology for ferroelectric domains, and it is applicable to most ferroelectric materials.

[0025] (2) The method for preparing ferroelectric domain devices provided by the present invention has high preparation efficiency and fast preparation speed. Attached Figure Description

[0026] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1 A schematic diagram of a method for efficiently erasing the ferroelectric domain inversion structure of a ferroelectric crystal using a femtosecond laser.

[0028] Figure 2 This is a characterization result of the ferroelectric domain inversion structure before erasure of ferroelectric domains in this invention;

[0029] Figure 3 The figure shows the characterization results of the ferroelectric domain structure after erasing the ferroelectric domains using multi-pulse femtosecond lasers with different single-pulse energies according to the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0031] One aspect of the present invention relates to a method for erasing the ferroelectric domain structure of a ferroelectric crystal, comprising the following steps:

[0032] (a) Mark the location of the ferroelectric domain inversion structure to be erased on the ferroelectric crystal;

[0033] (b) A multi-pulse femtosecond laser is focused inside the ferroelectric crystal to induce a thermoelectric field near the ferroelectric domain inversion structure to be erased. The thermoelectric field drives the polarization direction of the ferroelectric domain inversion structure to reverse, thereby erasing the ferroelectric domain inversion structure.

[0034] Existing femtosecond laser scanning erasure uses a femtosecond laser to scan and erase part of the domain inversion structure previously obtained by direct writing within the crystal, leaving nanoscale domain inversion structures, thus realizing the fabrication of nanodomains. However, the femtosecond laser scanning erasure method for ferroelectric domain structures requires scanning along the domain structure one by one, resulting in slow processing speed and low erasure efficiency, which undoubtedly limits the industrial production of ferroelectric domain devices.

[0035] To overcome the shortcomings of existing technologies, the present invention provides a method for erasing ferroelectric domain structures in ferroelectric crystals. This method uses a femtosecond laser to mark the positions of the ferroelectric domain inversion structures to be erased on the ferroelectric crystal, thereby improving the erasure accuracy. By focusing the femtosecond laser inside the crystal to generate a thermoelectric field, the thermoelectric field drives the ferroelectric domain inversion structures to invert again, allowing multiple domain inversion structures to be erased simultaneously. This achieves rapid erasure of ferroelectric domain structures. Compared with laser scanning methods for erasing domain inversion structures, the erasure efficiency can be improved by up to two orders of magnitude. This is a new, long-distance, large-scale, and highly efficient laser erasure technology for ferroelectric domains.

[0036] The method for erasing the ferroelectric domain structure of a ferroelectric crystal is applicable to most ferroelectric crystals.

[0037] Ferroelectric domains are regions in a ferroelectric material that exhibit the same spontaneous polarization direction. They are tiny regions in a ferroelectric material with the same spontaneous polarization direction. A crystalline medium that exhibits spontaneous polarization, and whose spontaneous polarization direction can change with the direction of an external electric field, is simply called a ferroelectric material.

[0038] In some specific embodiments, the electric field direction of the thermoelectric field is opposite to the polarization direction of the ferroelectric domain inversion structure to be erased, and the electric field strength of the thermoelectric field is greater than that of the coercive field.

[0039] The coercive field refers to the strength of an electric or magnetic field that causes spontaneous polarization or magnetization of a material to disappear in the presence of such a field. It describes the properties of ferromagnetic materials, representing the minimum magnetic field strength required for a material to resist a change in its magnetization direction. It is a fundamental parameter in solid-state magnetism and one of the important indicators for comparing the performance of magnetic materials in materials engineering.

[0040] In some specific implementations, the length of the erased ferroelectric domain inversion structure is controlled by adjusting the relative position of the thermoelectric field and the ferroelectric domain inversion structure, as well as the intensity of the thermoelectric field.

[0041] In some specific implementations, the intensity of the thermoelectric field is controlled by adjusting the parameters of the multi-pulse femtosecond laser.

[0042] In some specific embodiments, the parameters of the multipulse femtosecond laser include at least one of the following: wavelength, average power, repetition frequency, spot size, pulse width, number of pulses, and single pulse energy.

[0043] The wavelength of a femtosecond laser is transparent to ferroelectric crystals, allowing it to be focused into the interior of the ferroelectric crystal.

[0044] In some specific embodiments, the wavelength of the multi-pulse femtosecond exciter is 1026 nm, the pulse width is 190 fs, the repetition frequency is 1000 kHz, the focusing objective is a 50× microscope objective (NA = 0.42), the focusing spot diameter is 1 to 2 μm, the single pulse energy is 1000 nJ to 1300 nJ, and the number of pulses is 500,000.

[0045] In some specific embodiments, the relative position of the thermoelectric field and the ferroelectric domain inversion structure is controlled by adjusting the position where the multi-pulse femtosecond laser is focused inside the ferroelectric crystal.

[0046] In some specific embodiments, the multipulse femtosecond laser is incident from the +z surface of the ferroelectric crystal.

[0047] In some specific embodiments, the positional relationship between the ferroelectric domain inversion structure and the thermoelectric field is as follows: the ferroelectric domain inversion structure can be regarded as a cylinder with a radius of less than 100 micrometers and a height of 40 to 100 micrometers. The best erasing effect is achieved when the center of the thermoelectric field is located in the region from the upper surface of the cylinder to 20 to 50 micrometers directly below the upper surface and on the central axis of the cylinder.

[0048] In some specific embodiments, a femtosecond laser is used to mark the location of the ferroelectric domain inversion structure to be erased on the +z surface of the ferroelectric crystal.

[0049] In some specific embodiments, the femtosecond laser has a wavelength of 1026 nm, a pulse width of 190 fs, a repetition frequency of 1000 kHz, a single pulse energy of 300 nJ, a focusing objective of 50× microscope objective (NA = 0.42), and a focused spot diameter of 1 to 2 μm.

[0050] In some specific embodiments, the method for erasing the ferroelectric domain structure of a ferroelectric crystal includes the following steps:

[0051] Step 1: Characterize the sample using a Cherenkov-type second harmonic confocal microscope, observe the location of the ferroelectric domain structure to be erased, and mark the location of the ferroelectric domain structure to facilitate precise positioning when erasing the ferroelectric domains.

[0052] Step 2: Place the sample on a displacement platform and focus the laser onto the crystal surface using images captured in real time by an industrial camera; move the displacement platform and focus the multi-pulse femtosecond laser onto the crystal near the ferroelectric domain inversion structure prepared by the two-step laser polarization method, according to the previously marked position, to generate a thermoelectric field. This causes the direction of the thermoelectric field near the domain inversion structure to be opposite to the polarization direction of the domain inversion structure, thus reversing the polarization direction of the domain inversion structure again and erasing the domain inversion structure.

[0053] In step two, the multi-pulse femtosecond laser focused inside the crystal generates heat accumulation and forms a thermoelectric field near the domain inversion structure with a direction opposite to the polarization direction of the domain inversion structure and an intensity greater than the coercive field through the thermoelectric effect. Furthermore, by changing the laser parameters, including energy, wavelength, repetition frequency, and pulse width, thermoelectric fields of different intensities can be induced. By changing the laser focusing position, thermoelectric fields at different positions can be induced. By changing the relative position of the thermoelectric field and the domain inversion structure, as well as the intensity of the thermoelectric field, the length of the erased domain inversion structure can be changed.

[0054] In some specific implementations, in step one, the laser wavelength used for observation is 900nm, and the focusing objective is a 25× microscope objective.

[0055] This invention utilizes the thermoelectric field generated inside a ferroelectric crystal by focusing a femtosecond laser to simultaneously erase multiple domain inversion structures. Compared to laser scanning methods for erasing domain inversion structures, the erasing efficiency can be improved by up to two orders of magnitude. This is a new, long-distance, large-scale, and highly efficient laser erasing technology for ferroelectric domains.

[0056] Another aspect of the present invention relates to a method for fabricating a ferroelectric domain device, comprising the method for erasing the ferroelectric domain structure of a ferroelectric crystal.

[0057] The method for fabricating the ferroelectric domain device described above has high fabrication efficiency and fast fabrication speed.

[0058] The implementation schemes of the present invention will now be described in detail with reference to specific embodiments.

[0059] Lithium niobate crystal is one of the most widely used ferroelectric materials. This invention uses lithium niobate crystal as a specific example. It should also be noted that the method for erasing the ferroelectric domain structure of ferroelectric crystals provided by this invention can be applied to other ferroelectric materials.

[0060] Example 1

[0061] The method for erasing the ferroelectric domain structure of a ferroelectric crystal provided in this embodiment is as follows: Figure 1 As shown, it includes the following steps:

[0062] Step 1: Characterize the sample using a Cherenkov-type second harmonic confocal microscope, observe the location of the ferroelectric domain structures to be erased, and mark the locations of the ferroelectric domain structures to facilitate precise positioning when erasing the ferroelectric domains.

[0063] For step one, this embodiment can be implemented in the following way:

[0064] The location of the ferroelectric domain structure was observed using a Cherenkov-type second-harmonic confocal microscope with a laser wavelength of 900 nm and a 25× microscope objective. Figure 2 This is the characterization result of the ferroelectric domain structure before erasure. The positions of the ferroelectric domain structure were marked on the +z surface of the crystal using a femtosecond laser. The wavelength of the femtosecond laser was 1026 nm, the pulse width was 190 fs, the repetition frequency was 1000 kHz, the single pulse energy was 300 nJ, the focusing objective was a 50× microscope objective (NA = 0.42), and the focused spot diameter was 1 to 2 μm.

[0065] Step 2: Place the sample on a displacement platform and focus the laser onto the crystal surface using real-time images captured by an industrial camera. Move the displacement platform to focus the multi-pulse femtosecond laser onto the crystal interior near the ferroelectric domain inversion structure prepared by the two-step laser polarization method, according to the previously marked position. This generates a thermoelectric field, causing the direction of the thermoelectric field near the domain inversion structure to be opposite to the polarization direction of the domain inversion structure, thus reversing the polarization direction of the domain inversion structure again and erasing the domain inversion structure.

[0066] For step two, this embodiment can be implemented in the following way:

[0067] The sample was a z-cut lithium niobate crystal doped with 5% magnesium oxide, with a thickness of 1 mm. The sample was mounted on a high-precision triaxial XYZ displacement platform (the laser focus remained stationary during actual processing, while the platform moved relative to it). Adjustable parameters of the femtosecond laser included wavelength, average power, repetition rate, pulse width, and pulse number. Adjustable focusing parameters included focus position and focused spot size. During processing, we used a femtosecond laser with a wavelength of 1026 nm, a pulse width of 190 fs, a repetition rate of 1000 kHz, a 50× microscope objective (NA = 0.42), and a focused spot diameter of 1 to 2 μm. During erasure, multi-pulse lasers with single-pulse energy ranging from 1000 nJ to 1300 nJ and a pulse number of 500,000 were incident from the +z surface of the lithium niobate to a position 10 μm below the crystal surface near the domain inversion structure to induce a thermoelectric field. Figure 3 This is a characterization result of the ferroelectric domain structure after erasing the domain inversion structure.

[0068] from Figure 2 and Figure 3 As can be seen, the number of reverse domains erased increases with the increase of single-pulse energy. When the single-pulse energy is greater than or equal to 1200 nanojoules and less than the ablation threshold of lithium niobate crystal (the ablation threshold is about 1800 nanojoules), the reverse domain length decreases to 0, that is, the reverse domains are completely erased.

[0069] In summary, the present invention has the following advantages:

[0070] The method for erasing ferroelectric domain structures in ferroelectric crystals provided by this invention uses a femtosecond laser to mark the positions of the ferroelectric domain inversion structures to be erased on the ferroelectric crystal, which can improve the accuracy of erasure. By focusing a multi-pulse femtosecond laser inside the crystal to generate a thermoelectric field, the thermoelectric field is used to drive the ferroelectric domain inversion structures to invert again, which can simultaneously erase multiple domain inversion structures and achieve rapid erasure of ferroelectric domain structures.

[0071] The method for erasing ferroelectric domain structures in ferroelectric crystals provided by this invention can improve erasure efficiency by up to two orders of magnitude compared to existing laser scanning methods for erasing domain inversion structures. It is a new, long-distance, wide-range, and highly efficient laser erasure technology for ferroelectric domains and is applicable to most ferroelectric materials.

[0072] The method for fabricating ferroelectric domain devices provided by this invention has high fabrication efficiency and fast fabrication speed.

[0073] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A method for erasing the ferroelectric domain structure of a ferroelectric crystal, characterized in that, Includes the following steps: (a) Mark the location of the ferroelectric domain inversion structure to be erased on the ferroelectric crystal; (b) A multi-pulse femtosecond laser is focused inside the ferroelectric crystal to induce a thermoelectric field near the ferroelectric domain inversion structure to be erased. The thermoelectric field drives the polarization direction of the ferroelectric domain inversion structure to reverse, thereby erasing the ferroelectric domain inversion structure. The electric field direction of the thermoelectric field is opposite to the polarization direction of the ferroelectric domain inversion structure to be erased, and the electric field strength of the thermoelectric field is greater than that of the coercive field. The length of the erased ferroelectric domain inversion structure can be controlled by adjusting the relative position of the thermoelectric field and the ferroelectric domain inversion structure, as well as the intensity of the thermoelectric field. The positional relationship between the ferroelectric domain inversion structure and the thermoelectric field is as follows: the ferroelectric domain inversion structure is regarded as a cylinder with a radius of less than 100 micrometers and a height of 40 to 100 micrometers, and the center of the thermoelectric field is located in the region from the upper surface of the cylinder to 20 to 50 micrometers directly below the upper surface.

2. The method for erasing the ferroelectric domain structure of a ferroelectric crystal according to claim 1, characterized in that, The intensity of the thermoelectric field is controlled by adjusting the parameters of the multi-pulse femtosecond laser.

3. The method for erasing the ferroelectric domain structure of a ferroelectric crystal according to claim 2, characterized in that, The parameters of the multipulse femtosecond laser include at least one of the following: wavelength, average power, repetition frequency, spot size, pulse width, number of pulses, and single pulse energy.

4. The method for erasing the ferroelectric domain structure of a ferroelectric crystal according to claim 1, characterized in that, By adjusting the focusing position of the multi-pulse femtosecond laser inside the ferroelectric crystal, the relative position of the thermoelectric field and the ferroelectric domain inversion structure can be controlled.

5. The method for erasing the ferroelectric domain structure of a ferroelectric crystal according to claim 1, characterized in that, The multi-pulse femtosecond laser is incident from the +z surface of the ferroelectric crystal.

6. The method for erasing the ferroelectric domain structure of a ferroelectric crystal according to claim 1, characterized in that, The location of the ferroelectric domain inversion structure to be erased is marked on the +z surface of the ferroelectric crystal using a femtosecond laser.

7. A method for fabricating a ferroelectric domain device, characterized in that, The method for erasing the ferroelectric domain structure of a ferroelectric crystal as described in any one of claims 1 to 6.

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