Method for turning barium titanate crystal surface micro-domain all-light

Abstract

The invention provides a method for turning over micro-domains on the surface of a barium titanate crystal in an all-optical mode. According to the method, the overturning of the ferroelectric domain of the barium titanate crystal is realized by utilizing the interaction of a space electric field generated by irradiating the barium titanate crystal by focused laser and an additional metal wire (an iron wire, a zinc wire, a tungsten steel probe and the like). The size of the micro-domain on the surface of the barium titanate crystal can be effectively controlled by adjusting the laser irradiation time and the light intensity and replacing different metal wires, and in addition, patterning of the micro-domain on the surface of the barium titanate crystal is successfully achieved by adjusting the shape of a laser spot. The method is of great significance in promoting development of the fields of optical communication, nonlinear optics, quantum optics, integrated optics and the like.

Description

Technical Field

[0001] The present invention relates to a technique for all-optical flipping of surface microdomains of barium titanate crystals. Specifically, it is a technique that utilizes the interaction between the spatial electric field generated by laser irradiation of barium titanate crystals and a metal wire to achieve the regulation of the size of surface microdomains of barium titanate crystals and the patterning of microdomains. Background Art

[0002] Ferroelectric domain engineering is an important research direction in ferroelectric materials science. The core goal is to optimize the properties of materials and develop new functional devices by precisely regulating the domain structure in ferroelectric crystals (including domain walls, domain orientations, domain sizes, and distributions, etc.). And making ferroelectric domains flip is one of the most important techniques in domain engineering. The rise of this technique has enabled ferroelectric crystal materials to have extensive application potential in fields such as optics and acoustics. Ferroelectric crystals have the property of spontaneous polarization within a certain temperature range. Under the action of an external electric field, the polarization direction can be reversed, that is, domain polarization flipping. The external electric field required to make the domains flip is called the coercive field of the crystal.

[0003] Currently, the methods based on ferroelectric domain flipping basically make the crystal polarize and then the domains flip by applying a high electric field exceeding the coercive field of the crystal. However, it is very difficult to prepare domain structures of arbitrary shapes by this external electric field polarization method. Moreover, a relatively high external electric field is very likely to break down the crystal. In recent years, through continuous exploration by researchers, a brand-new domain flipping method has been discovered. At present, it seems quite reasonable, which is the photo-induced domain flipping method. Based on this method, this patent proposes a method for all-optical flipping of surface microdomains of barium titanate crystals. Firstly, all-optical flipping does not require the application of complex external electrodes. It only needs to reach the coercive field required for crystal domain flipping under the combined action of the photovoltaic electric field of the crystal itself and an external conductive material (such as a metal wire or a conductive graphene film), thus avoiding the risk of breakdown of the crystal by a relatively high electric field. Secondly, the experimental device is relatively simple, and all laser irradiations use the visible light band, with higher safety. This domain flipping technique plays an important role in the future development of domain engineering.

[0004] In 1995, Myers et al. successfully flipped ferroelectric domains using the external electric field polarization method. Although a liquid electrode was used instead of a metal electrode, saving the trouble of photolithography. However, this external electric field polarization method is easily restricted by many factors, making it very difficult to prepare domain structures of arbitrary shapes and sub-micron-sized domain structures. At the same time, due to the relatively high external electric field, it is also easy to break down the crystal.

[0005] In 2017, Ge X et al. prepared domain structures using the high-voltage tip polarization method. This method has good flexibility and can be locally processed, making up for the defect that the external electric field polarization method cannot manipulate crystals at the microscale. Therefore, domain structures with small size and high spatial resolution can be prepared. However, this method has high requirements for equipment, resulting in high costs. At the same time, the preparation efficiency is low, and the uniformity of the domain structures prepared by it is poor. Summary of the Invention

[0006] The domain inversion methods of ferroelectric crystals reported so far are still restricted in many aspects. For example: the external electric field polarization method requires adding a complex electrode outside the crystal or lithographing an electrode on the crystal surface. The operation is cumbersome and the electric field is relatively high, which is easy to break down the crystal. The high-voltage tip polarization method, although this method has good flexibility and can be locally processed to prepare domain structures at the microscale. However, this method has high requirements for equipment and the preparation efficiency is also low. To address the above problems, the present invention provides a simple and stable method for realizing ferroelectric domain inversion. This method uses the photovoltaic electric field generated by laser irradiation of barium titanate crystals and the conductive metal wire to jointly act to reach the coercive field required for crystal polarization inversion, thereby causing ferroelectric domain inversion.

[0007] A method for all-optical inversion of microdomains on the surface of barium titanate crystals, characterized in that, using barium titanate crystals as the substrate, and using deionized water, alcohol, insulating oil, air, etc., as the environmental medium, attaching the conductive metal wire to the -z plane of the substrate, and irradiating the contact area between the tip of the metal wire and the crystal surface with a laser, ferroelectric domain inversion can be realized at the tip of the metal wire. The size of the domain will increase with the increase of the irradiation time.

[0008] According to the method for all-optical inversion of microdomains on the surface of barium titanate crystals described in claim 1, it is characterized in that: by changing the laser irradiation time, the laser intensity, the different types of metal wires, and the types of surrounding environmental media, the size of the microdomains on the surface of barium titanate crystals can be regulated.

[0009] According to the method for all-optical inversion of microdomains on the surface of barium titanate crystals described in claim 1, it is characterized in that: by adjusting the shape of the laser spot, patterning of the microdomains on the surface of barium titanate crystals can be successfully realized.

[0010] Compared with the prior art, the advantages of the present invention are as follows: First, using barium titanate crystals, a ferroelectric crystal, as the substrate, the ferroelectric domain inversion is realized by the combined action of the photovoltaic electric field generated by laser irradiation of barium titanate crystals and the conductive metal wire, overcoming the disadvantage that the traditional inversion method must use an external electrode. And patterning of the domains can be realized at any position on the crystal surface by changing the spot shape. Brief Description of the Drawings

[0011] Figure 1This is a schematic diagram of the overall structure of the device for realizing the surface microdomain inversion of barium titanate crystals by irradiating barium titanate crystals in contact with metal wires with focused laser light according to the present invention.

[0012] Figure 2 This is a process diagram of an embodiment (Embodiment 1) of the present invention for realizing the surface microdomain inversion of barium titanate crystals by irradiating barium titanate crystals in contact with iron wires with focused laser light.

[0013] Figure 3 This is a process diagram of an embodiment (Embodiment 2) of the present invention for realizing the surface microdomain inversion of barium titanate crystals by irradiating barium titanate crystals in contact with zinc wires with focused laser light.

[0014] Figure 4 This is a process diagram of an embodiment (Embodiment 3) of the present invention for realizing the surface microdomain inversion of barium titanate crystals by irradiating barium titanate crystals in contact with tungsten steel probes with focused laser light.

[0015] Figure 5 This is a process diagram of an embodiment (Embodiment 4) of the present invention for realizing the patterned inversion of microdomains on the surface of barium titanate crystals by applying a mask to change the shape of the light spot. Detailed implementation manners

[0016] The present invention will be further described below in conjunction with embodiments and the accompanying drawings. The present invention discloses a method for all-optical inversion of surface microdomains of barium titanate crystals. The device includes: a laser 1, an electronic shutter 2, a diaphragm 3, an eyepiece mirror 4, a laser mirror 5, a focusing objective lens 6, a micro test bench 7, a background light source 8, a filter 9, and a JVC camera 10.

[0017] The present invention discloses a method for all-optical inversion of surface microdomains of barium titanate crystals. The operating steps of this method are: smoothly attaching a conductive metal wire to the -z plane of the barium titanate crystal; adjusting the position of the objective lens using a three-dimensional platform so that the incident laser is precisely focused on the surface of the barium titanate crystal and the contact point of the metal wire tip, and adjusting the CCD camera so that a clear image can be captured; turning on the laser, adjusting it to an appropriate light intensity, turning on the electronic shutter, and the laser irradiates the surface of the barium titanate crystal to generate a space charge field, and the generated charges will be transferred and exchanged between the crystal and the metal wire. When the charges accumulate to a certain extent to reach the coercive field of the crystal, domain inversion occurs.

[0018] Considering the above experiments and cost factors, the wavelength range of the laser 1 is 400 - 500 nm, the background light source is a halogen lamp, the magnification of the focusing objective lens is selected as 20 times, and in order to ensure the collimated incidence of the laser and accurate power measurement, each optical element is fixed on a rigid connecting frame.

[0019] Working principle of the solution of the present invention: When a laser irradiates the surface of a barium titanate crystal, due to the bulk photovoltaic effect, carriers are generated. These carriers generate a photovoltaic current under diffusion and drift motions. The photovoltaic charges accumulate on the +z plane and -z plane of the barium titanate crystal. Negative charges accumulate on the +z plane and positive charges accumulate on the -z plane, forming a photovoltaic electric field parallel to the polarization direction in the same direction. When the crystal contacts a conductive medium (such as a metal wire), the shielding charges (positive and negative ions) in the medium form a shielding layer, continuously neutralizing the surface photovoltaic charges. There are excessive negative ions and positive ions on the -z plane and +z plane respectively, forming a shielding electric field parallel to the polarization direction in the opposite direction. The internal photovoltaic electric field and the external shielding electric field are in opposite directions, and the two are superimposed to form a net electric field, whose direction is opposite to the spontaneous polarization direction. With the continuous irradiation of the laser, the shielding charges and photovoltaic charges continuously accumulate, and the net electric field gradually increases, eventually exceeding the crystal coercive field, triggering the nucleation and flipping of ferroelectric domains. By changing the contact position between the metal wire and the crystal surface, domain flipping can be achieved controllably at a fixed point. By changing the surrounding solution medium, the light intensity, and the illumination time, the size of the domain can be controlled. By changing the shape of the laser spot, patterning of the domain can be achieved.

[0020] The following gives specific embodiments of the present invention for realizing microdomain flipping on the surface of a barium titanate crystal using a focused laser. The specific embodiments are only used to illustrate the present invention in detail and do not limit the protection scope of the claims of this application.

[0021] Embodiment 1 A 488 nm laser is used, the laser power is 10 mw, the background light source is a halogen lamp, the magnification of the focusing objective lens is 20 times. A 100 μm iron wire is attached to the surface of the barium titanate crystal. The micro experimental platform is placed in a square tank filled with deionized water. The laser is turned on and the laser spot is moved to the contact position between the tip of the tungsten wire and the crystal surface, and irradiation starts. As time goes by, microdomain flipping and expansion on the surface of the barium titanate crystal are achieved.

[0022] Embodiment 2 A 488 nm laser is used, the laser power is 10 mw, the background light source is a halogen lamp, the magnification of the focusing objective lens is 20 times. A 0.5 mm zinc wire is attached to the surface of the barium titanate crystal. The micro experimental platform is placed in a square tank filled with deionized water. The laser is turned on and the laser spot is moved to the contact position between the tip of the zinc wire and the crystal surface, and irradiation starts. As time goes by, microdomain flipping and expansion on the surface of the barium titanate crystal are achieved.

[0023] Embodiment 3 Using a 488 nm laser with a laser power of 10 mW, a halogen lamp was selected as the background light source, and the magnification of the focusing objective lens was 20 times. A 20-μm tungsten steel probe was attached to the surface of the barium titanate crystal. The micro experimental platform was placed in a square tank filled with deionized water. The laser was turned on and the laser spot was moved to the contact position between the tip of the tungsten steel probe and the crystal surface, and irradiation was started. As time passed, the inversion and expansion of microdomains on the surface of the barium titanate crystal were achieved.

[0024] Example 4 Using a 488 nm laser with a laser power of 10 mW, a halogen lamp was selected as the background light source, and the magnification of the focusing objective lens was 20 times. The shape of the light spot was modulated by a mask, changing from a laser spot to a long-strip light spot. In addition, the conductive medium metal wire was replaced with a conductive graphene film, and the surrounding environmental medium was air. The laser was turned on and the laser spot was moved to any position on the crystal surface, and irradiation was started. As time passed, patterning of microdomains on the surface of the barium titanate crystal was achieved.

[0025] The above specific examples have further elaborated on the technical solutions and implementation methods of the present invention. It should be understood that the above examples are not only applicable to the present invention. Any equivalent modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be within the protection scope of the present invention.

Claims

1. A method for all-optical flipping of surface microdomains of barium titanate crystals, characterized in that: Using a barium titanate crystal as a substrate and air and deionized water as environmental media, one end of a metal wire is brought into contact with the -z plane of the barium titanate crystal. By irradiating the contact point between the tip of the metal wire and the barium titanate crystal with a laser, the interaction between the spatially generated electric field after irradiation and the metal wire realizes the flipping of surface microdomains.

2. A method for all-optical flipping of surface microdomains of barium titanate crystals according to claim 1, characterized in that: By changing the laser irradiation time, the laser intensity, different types of metal wires, etc., the regulation of the surface microdomain size of the barium titanate crystal can be achieved.

3. A method for all-optical flipping of surface microdomains of barium titanate crystals according to claim 1, characterized in that: By adjusting the shape of the laser spot, patterning of surface microdomains on the barium titanate crystal can be successfully realized.

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