Method for preparing low-drift lithium niobate material by means of magnetoelectric-field coupling process

Abstract

Disclosed in the present invention is a method for preparing a low-drift lithium niobate material by means of a magnetoelectric-field coupling process. The method comprises: simultaneously applying an electric field and a magnetic field to a lithium niobate material, and heating the lithium niobate material, such that non-collinear magnetic moments in lithium niobate are excited by means of strong thermal fluctuations and a magnetoelectric coupling mechanism, so as to form dense metastable magnetic structures, i.e., magnetoelectric-coupled skyrmions, thereby eliminating direct-current drift. The method in the present invention can suppress the intrinsic direct-current drift characteristics of lithium niobate materials without modifying any device design.

Description

A method for preparing weakly drifting lithium niobate materials via magnetoelectric field coupling process Technical Field

[0001] This invention relates to the fields of condensed matter physics and photonic integrated circuits (PIC), specifically to a method for preparing lithium niobate, a ferroelectric material with weak electrical DC drift characteristics. Background Technology

[0002] Lithium niobate (LiNbO3) is a ferroelectric material with excellent light transmission window and high linear electro-optic coefficient. It can be used to prepare a variety of high-speed, low-energy devices, such as electro-optic modulators, lasers, and fiber optic gyroscopes. Therefore, it has significant application prospects and commercial value in optical communication, quantum computing, and various sensing and monitoring fields.

[0003] In these system applications, the most critical aspect is achieving stable voltage modulation of lithium niobate to enable stable and high-speed conversion of various signals into optical signals. However, lithium niobate material exhibits an intrinsic defect (DC drift), which manifests as a change in refractive index after applying a voltage. This change in refractive index cannot remain stable but weakens over a long period until it returns to its initial state. The physical mechanism of DC drift is not yet fully understood, but the mainstream view in the academic community is that this drift phenomenon originates from the charge accumulated at the interfaces between the lithium niobate layer and various cladding layers in the voltage modulation device. By removing the silicon oxide cladding on the device and treating the surface of lithium niobate, researchers have confirmed that DC drift of lithium niobate can be reduced, but this approach has two drawbacks:

[0004] 1. To ensure the stable operation of lithium niobate-related optical devices, it is necessary to add a cladding layer to the optical waveguide for protection. Therefore, removing the cladding to reduce DC drift exposes the optical path, making it highly susceptible to contamination by dust and various organic substances in the air. As a supplement, additional hermetic protection schemes would also significantly increase the cost of the device.

[0005] 2. After removing the cladding, the DC drift of lithium niobate devices can only be partially alleviated. Whether it is short-term drift or long-term drift, it is still quite obvious, which makes the devices still unable to be directly used as products in applications.

[0006] Therefore, as of this invention, there is still no theoretically and experimentally acceptable solution to the DC drift phenomenon of lithium niobate. Summary of the Invention

[0007] To address the aforementioned technical problems in the existing technology, this invention provides a method for preparing weakly drifting lithium niobate materials through a magnetoelectric field coupling process. This method is a solution to eliminate the DC drift characteristics of lithium niobate. By utilizing the magnetoelectric coupling characteristics of lithium niobate materials, a multiferroic skyrmion structure is excited through thermal fluctuations in a ferroelectric polarization and magnetic field environment, and the electrical drift is suppressed by relying on the rapid response between the skyrmions.

[0008] A method for eliminating electrical DC drift in lithium niobate materials through magnetic treatment includes the following steps:

[0009] 1.1): An electric field was applied to lithium niobate material to obtain polarized lithium niobate intermediate 1;

[0010] 1.2): Polarized lithium niobate material is subjected to high temperature to obtain polarized lithium niobate intermediate 2;

[0011] 1.3): Apply a magnetic field to the lithium niobate intermediate 2 obtained in step 1.2) to obtain a metastable lithium niobate state with a collinear spin magnetic structure (skyrmion);

[0012] 1.4): The lithium niobate obtained in step 1.3) is cooled in a metastable state to obtain weakly drifting lithium niobate with a stable non-collinear spin magnetic structure;

[0013] In step 1.1), the electric field strength range is 2V / μm to 80V / μm.

[0014] In step 1.2), the heating temperature range is 200℃~700℃.

[0015] In step 1.3), the magnetic field strength range is 0.5T to 4T.

[0016] The material used in this invention is lithium ferroelectric niobate (LiNbO3), including lithium niobate materials in single-crystal thin films and single-crystal blocks.

[0017] Lithium niobate materials that eliminate DC drift will allow for long-term stable modulation of electrical signals, enabling high-speed and stable conversion of electrical signals into various physical quantities (intensity, phase, polarization) of photons, thus solving the drawback of lithium niobate active optical devices being unable to operate stably for long periods due to signal distortion.

[0018] This invention can significantly reduce or even eliminate the DC drift of lithium niobate materials, reducing it from more than 30% / minute to less than 1% / hour.

[0019] The generation method includes the following steps:

[0020] 1) High-temperature polarization:

[0021] A predetermined electric field is applied to the ferroelectric single crystal, and the temperature is increased for a period of time. This step can enhance the free energy of bound electrons in the crystal lattice.

[0022] 2) Electro- and magnetization coupling: A magnetic field of a specific intensity is further applied to the ferroelectric single crystal that has already been heated and subjected to an electric field, and this application is sustained for a period of time. This step can induce bound electrons to form mesoscale micromagnetic structures, thereby forming spin states with strong topological protection.

[0023] Preferably, the electric field is 10V / μm or higher. The greater the electric field strength, the smaller the required magnetic field and heating temperature.

[0024] Preferably, the heating temperature is above 400°C. The higher the temperature, the lower the required electric and magnetic field strengths. When the heating temperature is below 200°C, the electron thermal fluctuations in the material are too weak, and the spin structure cannot be formed.

[0025] Preferably, the magnetic field strength used is greater than 0.6T. The greater the magnetic field strength, the smaller the required electric field strength.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. Excellent performance: This invention can achieve DC drift close to 0, which is better than the previously reported state of removing the cladding. At the same time, since this scheme does not require the removal of the cladding on the waveguide, it solves the drawback of the original scheme that may cause optical path pollution.

[0028] 2. Compatibility: This invention can be directly applied to lithium niobate materials and is compatible with all currently supported process solutions. There is no need to find new materials or develop new platforms, which has advantages in terms of R&D costs and time.

[0029] 3. Pioneering in the field: The physical mechanism of DC drift of lithium niobate related to its magnetism was proposed for the first time, and a drift solution route completely different from previous studies was developed. At the same time, the optical, electrical and magnetic properties of lithium niobate were unified. Attached Figure Description

[0030] Figure 1 shows an electro-optic modulator based on the Mach-Zehnder architecture, using a thin-film lithium niobate platform to evaluate the magnitude of the lithium niobate electro-optic coefficient in this embodiment. The figure shows its design structure and a photograph of the modulator used for testing.

[0031] Figure 2 shows the Lorentz transmission electron microscopy (LTEM) measurement results of the waveguide region in the lithium niobate electro-optic modulator without the treatment of the present invention;

[0032] Figure 3 shows the DC drift measurement results of the lithium niobate electro-optic modulator without the treatment of the present invention;

[0033] Figure 4 shows the LTEM observation results of the waveguide region of the lithium niobate electro-optic modulator after each step in Example 1.

[0034] Figure 5 shows the DC drift measurement results of the lithium niobate electro-optic modulator after each step in Example 1;

[0035] Figure 6 shows the DC drift measurement results of the lithium niobate electro-optic modulator after each step in Example 2;

[0036] Figure 7 shows the DC drift measurement results of the lithium niobate electro-optic modulator after each step in Example 3;

[0037] Figure 8 shows the LTM image of the lithium niobate material after each step. Detailed Implementation

[0038] To make the objectives and technical solutions of this invention clearer, the technical solutions in this invention will be clearly described below with reference to the accompanying images. In this embodiment, an electro-optic modulator based on the Mach-Zehnder architecture of a thin-film lithium niobate platform is used to evaluate the strength of the DC drift of the lithium niobate material. Its typical structure and the microscope image of the device used in this study are shown in Figure 1. Please note that this does not limit the types of devices that can be applied to this invention.

[0039] The embodiments of the present invention will be further described below with reference to Figures 2-8, which include the structural design of the lithium niobate modulator, microscopic photographs, and drift test results. In short-term drift tests, we applied instantaneous bias voltage changes (-4V to 4V to 0V) at 0s, 40s, and 80s, and evaluated the intensity of DC drift by observing the corresponding response of the optical signal. Ideally, the optical signal of the device should change in accordance with the sudden change in DC bias voltage and remain stable when the voltage remains constant. In long-term drift tests, we used a (0V to 8V to 0V) format to evaluate the long-term stability of the device.

[0040] In addition, we also present images of the treated lithium niobate film observed via LTEM to characterize the material modification of lithium niobate itself under DC drift. Under the LTEM field of view, the magnetic structure of the skyrmion can be described by contrast maps at three focal lengths, characterized by: no contrast in the positive focus case, and opposite contrast in the underfocus and overfocus cases, with the contrast signals exhibiting a ring-like pattern. Simultaneously, the ferroelectric domains exhibit bright and dark contrast under the positive focus case.

[0041] Example 1:

[0042] The electro-optic modulator fabricated using CMOS technology has a modulation region length of 4 mm. Figure 2 shows the LTEM image of the waveguide region without any processing, confirming the absence of any significant magnetic structure within the LN. Figure 3 shows the DC drift test results, confirming that after applying a momentary bias voltage, the signal immediately returns to its initial value after a brief change. This indicates that the device exhibits extremely strong short-term drift characteristics and cannot be modulated by DC bias voltage at all.

[0043] Step 1) Apply an electric field of 15 V / μm to the modulator at room temperature.

[0044] Step 2) Heat the modulator to 500℃.

[0045] Step 3) After the temperature reaches 500℃, continue to apply a magnetic field of 0.9T, maintaining the electric field, magnetic field, and temperature field simultaneously for 60 minutes. After this, rapidly cool to room temperature and remove the electric and magnetic fields.

[0046] After this step, the LTEM results for the modulator waveguide region are shown in Figure 4. The figure shows that nanoscale magnetic vortex structures appeared in the lithium niobate layer, confirming the generation of skyrmions.

[0047] After this step, the DC drift measurement results of the modulator are shown in Figure 5. It can be seen that the optical signal of the device also produced a significant response after applying an instantaneous bias voltage, and remained stable for the duration of the fixed bias voltage. This indicates that the DC drift of the lithium niobate modulator treated by this invention has been significantly suppressed (Figure 5, left). We also evaluated the long-term stability of the treated modulator. It can be seen that the optical signal remained stable one hour after applying the instantaneous bias voltage (Figure 5, right), and the drift was less than the test disturbance (below 0.1 dB), reaching the signal-to-noise level. This proves that the DC drift response of the lithium niobate material obtained after the treatment by this invention has essentially disappeared.

[0048] Example 2

[0049] The process is essentially the same as in Example 1, except that the electric field strength in step 1) is reduced to 5V / μm. The DC drift of the device after the three-step treatment is shown in Figure 6. The figure still confirms a reduction in DC drift, but the reduction is smaller than that in Example 1.

[0050] Example 3

[0051] The method is basically the same as in Example 1, except that the magnetic field strength used in step 1) is reduced to 0.3T. The DC drift of the device after the two-step processing is shown in Figure 7. The figure still confirms the reduction in DC drift, but the reduction is smaller than that in Example 1.

[0052] Figure 8 shows the LTEM results of the lithium niobate film after each step, confirming the presence of significant and dense magnetic skyrmions in the lithium niobate after the drift is reduced. Under positive focusing conditions, the magnetic vortex centers of these skyrmions exhibit significant ferroelectric domain structures, indicating that they possess magnetoelectric coupling properties.

[0053] While the present invention has been described through preferred embodiments, it is not limited to the embodiments described herein, and various changes and modifications can be made without departing from the scope of the invention.

Claims

1. A method for preparing weakly drifting lithium niobate materials via a magnetoelectric field coupling process, characterized in that, Includes the following steps: 1.1): An electric field was applied to lithium niobate material to obtain polarized lithium niobate intermediate 1; 1.2): The polarized lithium niobate intermediate 1 is heated to obtain polarized lithium niobate intermediate 2; 1.3): Apply a magnetic field to the polarized lithium niobate intermediate 2 obtained in step 1.2) to obtain a metastable lithium niobate with a collinear spin magnetic structure; 1.4): The metastable lithium niobate obtained in step 1.3) is cooled down, and all external electric and magnetic fields are removed after cooling to obtain a room temperature stable lithium niobate material with a non-collinear spin magnetic structure and weak DC drift.

2. The method according to claim 1, characterized in that, In step 1.1), the electric field strength range is 2V / μm to 80V / μm.

3. The method according to claim 1, characterized in that, In step 1.2), the heating temperature range is 200℃~700℃.

4. The method according to claim 1, characterized in that, In step 1.3), the strength range of the magnetic field is 0.5T to 4T.

5. The method according to claim 1, characterized in that, After steps 1.1), 1.2), and 1.3), a significant magnetoelectric coupling skyrmion will be generated in the lithium niobate material.

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