A method for high-density optical storage in lithium niobate using femtosecond laser

Abstract

The invention discloses a method for high-density optical storage in lithium niobate using a femtosecond laser, which belongs to the field of optical storage technology and includes: preparation of a sample to be written, writing of data by a femtosecond laser and reading of the data; the method is based on femtosecond laser double-seed induced processing to write nanometer-sized storage points inside a lithium niobate crystal; a double-seed structure is used to replace a single seed in the previous femtosecond laser induced processing technology, and the enhanced local light field formed by the double-seed gap is used to further reduce the size of the induced area, thereby reducing the size of the induced written storage points and effectively improving the storage density; data recognition and extraction of the second harmonic confocal microscope photos of the sample are performed through image recognition, which is a parallel reading method with higher efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical storage, and specifically relates to a method for performing optical storage in a lithium niobate crystal by using a femtosecond laser, and improving storage density by using a local field enhancement effect. Background Art

[0002] Lithium niobate crystal is a functional material with multiple optical / electrical properties such as piezoelectric, thermoelectric, and electro-optical. It has important and extensive applications in high-speed modulators, electro-optical frequency combs, optical storage, on-chip broadband spectrometers, etc. Among them, there are two main technical means of holographic optical storage based on lithium niobate, namely, holographic storage using the photorefractive effect and image / data storage using the "microphase change" caused by femtosecond laser to form structural color.

[0003] The storage principle of holographic storage using the photorefractive effect (photorefractive effect) is as follows: light irradiation is used to cause the electro-optical material to generate a spatial charge field, thereby achieving spatial modulation of the refractive index; this storage technology has problems such as light damage, limited storage capacity, and storage stability.

[0004] The storage principle of using the "microphase change" caused by femtosecond laser to form structural color for image / data storage is as follows: femtosecond laser pulses are decomposed into e pulses and o pulses inside the crystal, and local modification is performed to form a "microphase change" area, and then different colors (i.e. structural colors) are obtained by modulating the length of the microphase change area, thereby realizing image storage or octal data storage; this technology has problems such as insufficient storage density and storage efficiency. Summary of the invention

[0005] In view of the deficiencies in the prior art, the technical problem to be solved by the present invention is: to provide a method for high-density optical storage in lithium niobate using a femtosecond laser, wherein the method is based on femtosecond laser double-seed induced processing to write nanometer-sized storage points inside a lithium niobate crystal; a double-seed structure is used to replace the single seed in the previous femtosecond laser induced processing technology, and the enhanced local light field formed by the double-seed gap or one side of the double seed is used to further reduce the size of the induced area (i.e., the size of the written storage point), thereby achieving high-density storage.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for high-density optical storage in lithium niobate using femtosecond laser, the specific steps are as follows:

[0008] Step 1: Preparation of samples to be written;

[0009] The specific steps are as follows: first, ultrasonically clean the sample to be processed in acetone, ethanol and deionized water respectively, and blow dry it with an ear-cleaning bulb, then confirm the crystal plane of the sample to be processed, and find the -z plane as the surface to be processed;

[0010] Step 2: femtosecond laser writing data;

[0011] The specific steps are as follows: first, the information to be stored is converted into ASCII code, and then converted into binary 1 and 0, which correspond to the inverted ferroelectric domain and the uninverted ferroelectric domain in the sample to be processed, respectively; then, the sample to be processed is fixed on a three-dimensional electric control platform with the -z surface facing up, and the femtosecond laser emitted by the femtosecond laser amplifier in the femtosecond laser writing system is focused on the surface or inside of the sample to be processed, and the surface of the sample to be processed is assumed to be the XOY plane, the Z axis is the sample depth direction, and the X axis is the femtosecond laser scanning direction, and the processing parameters of a single seed are determined; first, a seed point A is generated with a pulse number m, and then the focus of the femtosecond laser is moved along the Y axis by a distance △Y or along the Z axis by △Z, and a seed point B is generated with a pulse number n, and a double seed structure with a nanogap is obtained; then, the focus is moved to one side of the double seed structure, and the laser processing power is adjusted until it exceeds the target. The inversion threshold of the ferroelectric domain in the processed sample is then written with the pulse number x, where x < m, and the size of the first storage point is determined by the local light field excited by the double seed structure, which is consistent with the size of the local light field, and the ferroelectric domain inversion at the first storage point points in the opposite direction of the temperature gradient; for the next storage point "1", it is still written with the pulse number x, and the size of the next storage point is consistent with the previous storage point "1"; the subsequent continuous storage points "1" are written with the previous storage point "1" as the seed, and so on; for the storage point "0", the optical gate is closed during writing, so that the ferroelectric domain here does not reverse; for the continuous storage points "0", the double seed structure is written at the last point to induce the next storage point "1"; finally, all storage points are written one by one according to the converted binary data;

[0012] Step 3: Reading out the data;

[0013] The specific steps are as follows: Use the second harmonic confocal microscope system to scan and image the processed sample, and the ferroelectric domain inversion point (storage point "1") is displayed as a bright spot, and the non-inversion point (storage point "0") is displayed as a dark spot; identify the bright spot in the image as a storage point "1"; define and measure the image recognition unit size corresponding to a single bright spot, and identify the dark spot of the same size as a storage point "0" in the image; then use image recognition technology to perform parallel recognition and extraction of data from the taken photos, and the stored data can be fully read out.

[0014] Furthermore, in step one, the sample to be processed is a transparent lithium niobate crystal sheet with a thickness of 0.3-0.5 mm.

[0015] Furthermore, in step 2, the femtosecond laser writing system includes a femtosecond laser, a first concave lens L 1 , second convex lens L 2 , the third convex lens L 3 , the first half-reflecting half-mirror M 1 , the second half-reflecting half-mirror M 2 , image sensor CCD, objective lens O and three-dimensional electronic control platform; the femtosecond laser emitted by the femtosecond laser passes through the first concave lens L in sequence 1 and the second convex lens L 2 Then the light beam passes through the first half-reflecting half-mirror M 1 Arrives at the objective lens O; In addition, the mercury lamp light source passes through the second half-reflecting half-mirror lens M 2 , the first half-reflecting half-mirror M 1 The objective lens O and the focusing ring illuminate the sample on the platform, and the reflected light finally passes through the second half-reflecting half-mirror M 2 After reflection, it passes through the third convex lens L 3 Arriving at the image sensor CCD to form an image.

[0016] Furthermore, in step 2, the three-dimensional electric control platform is a three-axis electric control platform of X-axis, Y-axis and Z-axis, wherein the movement range of X-axis and Y-axis is 0-5cm, the accuracy is 1-100nm, the movement range of Z-axis is 0-5mm, and the movement accuracy is 1-100nm.

[0017] Furthermore, in step 2, the central wavelength of the femtosecond laser of the femtosecond laser direct writing processing system is 343-1030nm; the laser repetition frequency is 90-500kHz, and the pulse width is 100fs-10ps; the objective lens used is a 5×-40× lens with a numerical aperture of 0.1-0.6; the single pulse energy of laser ablation is 0.8nJ-0.8μJ; the laser scanning speed is 0.1-300mm / s; the line spacing of the processing data is 200-1000nm, and the layer spacing is 100-500nm.

[0018] Furthermore, in step 2, the processing parameters of a single seed are determined by adjusting the femtosecond laser parameters, observing the morphology of the processed structure using a microscope, and measuring the refractive index and transmittance. When the processed structure is detected to have birefringence and the optical transmittance is greater than 97%, it is the desired seed structure.

[0019] Furthermore, in step 2, in the double-seed structure, when seed point A and seed point B have the same size, the local light field is confined to the nanogap of the double-seed structure, and the size of the nanogap and the local light field are both 1-300 nm.

[0020] Furthermore, in step 2, in the dual-seed structure, when the sizes of seed point A and seed point B are different, the dual seeds are densely packed without gaps, the local light field is confined to the outside of the smaller seed point, the local light field size is 1-300nm, and the dual seed diameter difference is 100-900nm.

[0021] Furthermore, in step 2, the size of the storage point induced by the dual seed structure is 1-300nm, the number of pulses m, n, x for generating a single seed and writing the storage point is 1-300; and the storage point spacing is 100-1000nm.

[0022] Furthermore, when writing the first storage point, the focus of the femtosecond laser is moved to one side of the local light field excited by the double-seed structure, with a distance from the local light field of 100-600nm.

[0023] The principle of the method of the present invention for high-density optical storage in lithium niobate using femtosecond laser is as follows:

[0024] (1) Data writing principle: The ferroelectric domains inside the lithium niobate crystal are reversed by inducing a femtosecond laser pulse, which means that the ferroelectric domains (electric dipoles) point in the opposite direction of the temperature gradient at the focus, while the ferroelectric domains in the uninduced area still point in the direction of spontaneous polarization. The reversed ferroelectric domains and the unreversed ferroelectric domains correspond to the binary data "1" and "0" respectively. The data to be written is converted into ASCII code, and the femtosecond laser pulse is controlled to scan inside the lithium niobate to complete the data writing.

[0025] (2) Principle of femtosecond laser induced processing: First, a small number of pulses m are deposited at the starting point of the processing to form a seed point, and then the focus of the femtosecond laser is moved to the next processing point and a pulse number n (n < m) is deposited. At this time, the induced point size is determined by the interference light field formed by the incident light and the scattered light of the seed point, which is consistent with the seed.

[0026] (3) Principle of dual-seed localized light field confinement: A dual seed with a nanometer-sized gap is generated by a femtosecond laser pulse. When the femtosecond laser is incident, the excited light field is confined in the nanometer gap of the dual seed. When there is only a single seed, the light field excited by the incident femtosecond laser is distributed in the entire seed area. Therefore, replacing a single seed with a dual seed can obtain a localized light field with higher intensity and smaller size. According to principle (2), it will induce the generation of smaller storage points, thereby improving the storage density.

[0027] (4) Data readout principle: The lithium niobate crystal after data is written is scanned and imaged using a second harmonic confocal microscope. The area corresponding to the data point "1" is imaged as a bright spot because the ferroelectric domain is reversed, which changes the nonlinear optical properties of the lithium niobate here. The data point "0" is the area where the ferroelectric domain is not reversed, and is imaged as a dark spot. Image recognition technology is then used to identify the microscope photo and extract data, so that fast parallel data readout can be achieved.

[0028] Compared with the prior art, the advantages of the present invention are as follows:

[0029] (1) A method of the present invention for high-density optical storage in lithium niobate using a femtosecond laser can obtain an enhanced local optical field by introducing a dual-seed structure, thereby reducing the size of the storage point induced to be written, and effectively improving the storage density;

[0030] (2) The inverted and uninverted states of the ferroelectric domains in the lithium niobate crystal correspond to 0 and 1 in binary data, respectively. When writing data, it only needs to open / close the optical shutter, which is simpler and easier to control;

[0031] (3) Data recognition and extraction of the second harmonic generation confocal microscope photos of the sample are performed through image recognition, which is a parallel reading method with higher efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0033] Figure 1 A schematic diagram of the writing principle of high-density optical storage in lithium niobate crystal using femtosecond laser according to the present invention;

[0034] Among them, (a) is the prior art using a single seed to induce writing; (b) is the local light field induced writing formed by using a double seed structure of the same size provided by the present invention;

[0035] Figure 2 A schematic diagram of a localized light field induced writing storage point formed by a dual seed structure of different sizes according to the present invention;

[0036] Figure 3 This is a writing optical path diagram of high-density optical storage in lithium niobate crystal using femtosecond laser of the present invention;

[0037] Figure 4A schematic diagram of the storage principle of writing binary "0" and "1" in a lithium niobate crystal using a femtosecond laser according to the present invention;

[0038] Among them, (a) is a schematic diagram of writing binary "0"; (b) is a schematic diagram of writing binary "1";

[0039] Figure 5 The present invention is a schematic diagram of parallel data readout using a second harmonic generation confocal microscope. DETAILED DESCRIPTION

[0040] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0041] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0042] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0043] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0044] Embodiment 1: This embodiment utilizes femtosecond laser induced processing technology and introduces a dual seed structure to provide a high-intensity, nanometer-sized localized light field, so that the induced written data points are also limited to nanometer size, achieving an increase in storage density. For the dual seed structure, when seed point A and seed point B are the same size, the localized light field is confined to the nanometer gap of the dual seed structure. The dual seed structure combined with femtosecond laser induced writing technology can realize the writing of nanometer-level storage points.

[0045] Step 1: Preparation of samples to be written;

[0046] First, a z-cut lithium niobate sample with a length of 1 cm, a width of 1 cm, and a thickness of 500 μm was ultrasonically cleaned in acetone, ethanol, and deionized water for 10 min, and then dried with an ear-cleaning bulb. Then, the crystal plane of the lithium niobate sample was confirmed, and the -z plane was found as the surface to be processed. The sample preparation was completed.

[0047] Step 2: femtosecond laser writing data;

[0048] The specific steps are as follows: first, the information to be stored is converted into ASCII code, and then into binary 1 and 0, corresponding to the inverted ferroelectric domain and the uninverted ferroelectric domain in the lithium niobate sample respectively; then, the -z surface of the sample to be processed is fixed on a three-dimensional electric control platform with its face upward, and the femtosecond laser emitted by the femtosecond laser amplifier is focused on the surface of the sample to be processed, and the sample surface is set as the XOY plane, the Z axis is the sample depth direction, and the X axis is the femtosecond laser scanning direction; then, the femtosecond laser parameters are adjusted, the morphology of the processed structure is observed using a microscope, and its refractive index and transmittance are measured to find out the processing window parameters of the required single seed. The specific method is as follows: the wavelength of the femtosecond laser is selected as 1030 nm, the pulse width range is 100fs-10ps, the repetition frequency range is 90-500 kHz, the numerical aperture range of the objective lens is 0.1-0.6, the pulse energy is 0.8nJ-0.8μJ, and the number of pulses is 1-300. The obtained structures are classified according to the morphological characteristics and refractive index to find the processing parameter window of the seed structure; the processed structure with detectable birefringence and an optical transmittance greater than 97% is the desired seed structure:

[0049] In this embodiment, the femtosecond laser wavelength is selected to be 1030 nm, the pulse width is 300 fs, the repetition frequency is 200 kHz, the single pulse energy is 0.45 μJ, the number of pulses is 80, the objective lens numerical aperture is 0.25, and the scanning speed of the translation stage is 3 mm / s. These parameters are used to induce a double seed structure on the surface of the lithium niobate crystal. First, seed point A is generated with pulse number m=80, and then the femtosecond laser focus is moved along the Y axis by a distance △Y=1000nm, and seed point B is generated with pulse number n=80, so as to obtain a double seed structure with a nanogap, and the nanogap width is 100nm; at this time, since the seed point A and the seed point B in the double seed structure have the same size, the local light field is confined to the nanogap of the double seed structure; then, the laser power emitted by the laser is adjusted so that the energy of the single laser pulse after passing through the objective lens is reduced to 0.8nJ (at this time, the femtosecond laser processing power exceeds the lithium niobate domain inversion threshold), and the focus is moved along the X axis to one side of the nanogap of the double seed structure, with a distance of 500nm from the nanogap (the midpoint of the double seed line), and the first A storage point "1" is created, and its size is determined by the local light field excited by the double-seed structure, which is consistent with the size of the local light field, namely 100nm; the ferroelectric domain at this storage point points in the opposite direction of the temperature gradient; for the next storage point "1", it is still written with the number of pulses x, and its size is determined by the previous storage point "1" and remains consistent with it; the writing of consecutive storage points "1" takes the previous storage point "1" as a seed, and so on; for the storage point "0", the optical gate is closed during writing, and the ferroelectric domain here does not reverse and points to the direction of spontaneous polarization; for the consecutive storage points "0", a double-seed structure needs to be written at the last point to induce the next storage point "1"; finally, all storage points are written one by one according to the converted binary data.

[0050] like Figure 1 From (a), we can see that according to the principle of femtosecond laser induced processing, a seed point is first generated in the lithium niobate crystal using a femtosecond laser. When processing the next point, the number of pulses needs to be appropriately reduced. The femtosecond laser incident on the seed point will excite the local light field. At this time, the seed point can be regarded as a dipole, and the excited light field is localized in the seed point (dipole field). Furthermore, the local light field interferes with the incident light and induces the generation of the next processing point. Studies have shown that the size of the point is determined by the local light field of the seed point, that is, the induced point is the same size as the seed point. Based on this, Figure 1(b) shows that a double seed structure with a nanogap (width of 100nm) is used to replace a single seed point. When a femtosecond laser is incident on the double seed structure of the same size, the excitation light fields of the two seed points will be coupled, thereby forming a local light field with a size of 100nm that is smaller than a single seed in the gap. According to the above-mentioned induced processing principle, when the local field interferes with the incident light, the size of the induced processing point will be consistent with the nanogap, which is 100nm. The minimum storage point that can be generated by a single seed induction in the prior art is 500nm. It can be seen that the double seed structure provided in this embodiment can significantly reduce the size of the induced processing point, and femtosecond laser writing storage is performed on this basis, which can significantly improve the storage density.

[0051] Depend on Figure 3 It can be seen that the femtosecond laser emitted by the laser passes through the first concave lens L 1 and the second convex lens L 2 Then the light beam passes through the first half-reflecting half-mirror M 1 Arrives at the objective lens O; In addition, the mercury lamp light source passes through the second half-reflecting half-mirror lens M 2 , the first half-reflecting half-mirror M 1 The objective lens O and the focusing ring illuminate the sample on the platform, and the reflected light finally passes through the second half-reflecting half-mirror M 2 After reflection, it passes through the third convex lens L 3 Arriving at the image sensor CCD to form an image.

[0052] Embodiment 2: This embodiment uses femtosecond laser induced processing technology and introduces a dual seed structure to provide a high-intensity, nanometer-sized localized light field, so that the induced written data points are also limited to nanometer size, achieving an increase in storage density. For the dual seed structure, when the seed point A and the seed point B are of different sizes, it also has a "focusing" effect on the light field, and the formed localized light field is confined to the smaller seed side of the dual seed structure and is nanometer-sized. The use of this localized light field can also achieve the writing of nanometer-sized storage points.

[0053] Step 1: Preparation of the sample to be written; same as Example 1.

[0054] Step 2: femtosecond laser writing data;

[0055] Specific steps: binary conversion of stored information, construction of a femtosecond laser direct writing processing system, placement of samples, determination of seed structure window parameters, etc., are all the same as in Example 1. First, a seed point A is generated with a pulse number m=100, and then the focus of the femtosecond laser is moved along the Z axis by a distance △Z=800nm, and a seed point B is generated with a pulse number n=50, so as to obtain a large and a small double seed structure densely packed along the Z axis; at this time, since the sizes of A and B in the double seed structure are different, the femtosecond laser will focus the excited light field along the Z axis direction on the outside of the smaller seed B after being incident, forming a local light field with a size of about 200nm; then, the laser power emitted by the laser is adjusted so that the energy of the single laser pulse after passing through the objective lens is reduced to 0.8nJ (at this time, the femtosecond laser processing power exceeds the lithium niobate domain inversion threshold), and the focus is moved along the X axis to one side of the above-mentioned local light field, 500nm away from the local light field (i.e., the outer vertex of the small seed B), and the first storage point "1" is written with a pulse number x=30, and its size is determined by the excited local light field of the double seed structure, which is consistent with the size of the local light field, that is, 200nm; the writing of subsequent storage points is the same as in Example 1.

[0056] determining seed pulse parameters and write pulse parameters;

[0057] Depend on Figure 2 It can be seen that when the two seeds in the two double-seed structures are of different sizes, the research results show that the coupling of the excitation light fields of the two seeds will also lead to the focusing of the light field (called the "delay effect"), localizing the light field. At this time, the focused local light field is located outside the smaller seed; according to the above-mentioned femtosecond laser induced processing principle, the local light field is induced by interference with the incident light, and storage points with a size much smaller than a single seed point can also be generated, thereby realizing high-density optical storage in lithium niobate crystals.

[0058] Depend on Figure 4 It can be seen that there are a large number of ferroelectric domains distributed in the lithium niobate crystal. Under normal room temperature and no external field environment, these ferroelectric domains will spontaneously align along the Z-axis direction due to the dipole interaction between each other, pointing inward perpendicular to the surface to be processed, such as Figure 4 (a) is shown in Figure 1. When a femtosecond laser is focused on a lithium niobate crystal, the high-energy femtosecond laser pulse will cause the temperature at the focus to rise rapidly, thereby forming a temperature field near the focus. Furthermore, this temperature field will cause the corresponding area in the crystal to polarize, thereby generating a depolarization field. When the power of the femtosecond laser used exceeds the inversion threshold of the ferroelectric domains in lithium niobate, the depolarization field is sufficient to reverse all the ferroelectric domains near the focus, as shown in Figure 1. Figure 4As shown in (b), the direction is opposite to the temperature gradient. Since the inversion of ferroelectric domains will cause the nonlinear optical properties of the corresponding area in lithium niobate to change, the inverted ferroelectric domains can be imaged using a second harmonic confocal microscope. Based on this, the uninverted state and inverted state of the ferroelectric domains in lithium niobate can be respectively corresponded to "0" and "1" in binary storage, and written processing can be performed using femtosecond laser induced technology, that is, the written storage point is confined by the local light field excited by the double seed structure, so that ultra-small storage points smaller than the size of a single seed can be written to achieve high-density optical storage. In addition, when it is necessary to erase the written data, it is only necessary to apply an electric field in the same direction as the temperature gradient when writing the storage point in the area where the storage point "1" is written, so that the ferroelectric domain in this area can be reversed again; the intensity of the erase electric field applied needs to exceed the inversion threshold of the lithium niobate ferroelectric domain, which can be achieved by directly applying an electric field; it can also be erased using a femtosecond laser, that is, after the femtosecond laser is incident, a temperature gradient opposite to that when writing the storage point is generated, and at the same time, the femtosecond laser power is adjusted to exceed the inversion threshold of the lithium niobate ferroelectric domain.

[0059] Example 3: This example uses femtosecond laser induced processing technology combined with the confinement effect of the local light field to achieve an ultra-small storage point that is much smaller than the wavelength of the writing laser. Since the storage points "1" and "0" correspond to the inverted and uninverted states of the ferroelectric domain, respectively, and the inverted state will cause a significant change in the local nonlinear optical properties, the written storage point can be imaged using a second harmonic confocal microscope, and the image recognition technology can be used to quickly read the data from the imaged photo.

[0060] Step 1: Preparation of the sample to be written; same as Example 1.

[0061] Step 2, femtosecond laser writing data; As in Example 1, the size of each written storage point "1" is determined by the local light field formed in the double-seed nanogap, which is 100nm, thereby achieving the writing of ultra-small storage points that are much smaller than the wavelength size;

[0062] Step 3: Reading out the data;

[0063] The specific steps are as follows:

[0064] The sample is scanned and imaged using a second harmonic confocal microscope system. The ferroelectric domain inversion point (storage point "1") is displayed as a bright spot, and the non-inversion point (storage point "0") is displayed as a dark spot. The bright spot in the image is identified as a storage point "1". The image recognition unit size corresponding to a single bright spot is defined and measured, and the dark spot of the same size is identified as a storage point "0" in the image. The image recognition technology is then used to perform parallel recognition and extraction of data from the photographs, and the stored data can be fully read out. The specific method is: Figure 5As shown, the lithium niobate sample with written data is scanned and imaged using a second harmonic confocal microscope, and a microscope photo as shown in the figure can be obtained. Among them, the green bright spot corresponds to the storage point "1", and the dark spot corresponds to the storage point "0". First, based on the storage point "1" written continuously in the first row, the actual size of the storage point "0" is measured. As shown in the figure, any bright spot in the first row and the gap between it and the adjacent bright spot (dark area) are defined as an image recognition unit, and its size (length x width of the dotted box area) is measured, and it is defined that: dark spots with the same size can be identified as a storage point "0"; in other words, dark spots with a size smaller or larger than the image recognition unit will not be identified as storage point "0". Based on this, the second and third rows are randomly written storage points "1" and "0". Image recognition is performed based on the data measurement of the first row, first identify the data row with bright spots, and then identify "1" and "0" in each row; that is, a bright spot in the data row is identified as a storage point "1", and a dark spot with the same size as the image recognition unit is identified as a storage point "0"; in this way, the storage points "1" and "0" can be identified respectively. Figure 5 The second and third rows of data are identified as: 100000111 and 101001101. It can be seen that in the above manner, using image recognition to read data can achieve parallel, efficient and accurate data reading.

[0065] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0066] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0067] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for high-density optical storage in lithium niobate using femtosecond laser, characterized in that: The specific steps are as follows: Step 1: Preparation of samples to be written; The specific steps are as follows: first, ultrasonically clean the sample to be processed in acetone, ethanol and deionized water respectively, and blow dry it with an ear-cleaning bulb, then confirm the crystal plane of the sample to be processed, and find the -z plane as the surface to be processed; Step 2: femtosecond laser writing data; The specific steps are as follows: first, the information to be stored is converted into ASCII code, and then converted into binary 1 and 0, which correspond to the inverted ferroelectric domain and the uninverted ferroelectric domain in the sample to be processed, respectively; then, the sample to be processed is fixed on a three-dimensional electric control platform with the -z surface facing up, and the femtosecond laser emitted by the femtosecond laser amplifier in the femtosecond laser writing system is focused on the surface or inside of the sample to be processed, and the surface of the sample to be processed is assumed to be the XOY plane, the Z axis is the sample depth direction, and the X axis is the femtosecond laser scanning direction, and the processing parameters of a single seed are determined; first, a seed point A is generated with a pulse number m, and then the focus of the femtosecond laser is moved along the Y axis by a distance △Y or along the Z axis by △Z, and a seed point B is generated with a pulse number n, and a double seed structure with a nanogap is obtained; then, the focus is moved to one side of the double seed structure, and the laser processing power is adjusted until it exceeds the target. The inversion threshold of the ferroelectric domain in the processed sample is then written with the pulse number x, where x < m. The size of the first storage point is determined by the local light field excited by the double-seed structure and is consistent with the size of the local light field. The ferroelectric domain inversion at the first storage point points in the opposite direction of the temperature gradient. For the next storage point "1", it is still written with the pulse number x, and the size of the next storage point is consistent with the previous storage point "1". The subsequent continuous storage points "1" are written with the previous storage point "1" as the seed, and so on. For the storage point "0", the optical gate is closed during writing, so that the ferroelectric domain here does not reverse. For the continuous storage points "0", the double-seed structure is written at the last point to induce the next storage point "1". Finally, all storage points are written one by one according to the converted binary data. Step 3: Reading out the data; The specific steps are as follows: use the second harmonic confocal microscope system to scan and image the processed sample, and the ferroelectric domain inversion area is displayed as a bright spot, and the non-inversion area is displayed as a dark spot; the bright spot in the image is identified as a storage point "1"; define and measure the image recognition unit size corresponding to a single bright spot, and identify the dark spot of the same size as a storage point "0" in the image; then use image recognition technology to perform parallel recognition and extraction of data from the taken photos, and the stored data can be fully read out.

2. A method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 1, the sample to be processed is a transparent lithium niobate crystal sheet with a thickness of 0.3-0.5 mm.

3. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 2, the femtosecond laser writing system includes a femtosecond laser, a first concave lens, a second convex lens, a third convex lens, a first semi-reflective half-mirror, a second semi-reflective half-mirror, an image sensor, an objective lens and a three-dimensional electronically controlled platform; the femtosecond laser emitted by the femtosecond laser is expanded by the first concave lens and the second convex lens in sequence; then the light beam reaches the objective lens through the first semi-reflective half-mirror; in addition, the mercury lamp light source illuminates the sample on the platform through the second semi-reflective half-mirror, the first semi-reflective half-mirror and the focusing ring objective lens in sequence, and the reflected light is finally reflected by the second semi-reflective half-mirror and then reaches the image sensor through the third convex lens for imaging.

4. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 2, the three-dimensional electric control platform is a three-axis electric control platform of X-axis, Y-axis and Z-axis, wherein the moving range of X-axis and Y-axis is 0-5cm, the accuracy is 1-100nm, the moving range of Z-axis is 0-5mm, and the moving accuracy is 1-100nm.

5. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 2, the central wavelength of the femtosecond laser of the femtosecond laser direct writing processing system is 343-1030 nm; the laser repetition frequency is 90-500kHz, and the pulse width is 100fs-10ps; the objective lens used is 5×-40× lens with a numerical aperture of 0.1-0.6; the single pulse energy of laser ablation is 0.8nJ-0.8μJ; the laser scanning speed is 0.1-300mm / s; the line spacing of the processing data is 200-1000nm, and the layer spacing is 100-500nm.

6. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 2, the processing parameters of a single seed are determined by adjusting the femtosecond laser parameters, observing the morphology of the processed structure using a microscope, and measuring the refractive index and transmittance. When the processed structure is detected to have birefringence and the optical transmittance is greater than 97%, it is the desired seed structure.

7. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 2, in the double seed structure, when the seed point A and the seed point B have the same size, the local light field is confined to the nanogap of the double seed structure, and the size of the nanogap and the local light field are both 1-300 nm.

8. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: In step 2, in the dual-seed structure, when the sizes of seed point A and seed point B are different, the dual seeds are densely packed without gaps, the local light field is confined to the outside of the smaller seed point, the local light field size is 1-300nm, and the dual seed diameter difference is 100-900nm.

9. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: When writing the first storage point, the focus of the femtosecond laser is moved to one side of the local light field excited by the double-seed structure, with a distance of 100-600nm from the local light field.

10. The method for high-density optical storage in lithium niobate using femtosecond laser as claimed in claim 1, characterized in that: The size of the storage point induced by the double seed structure is 1-300nm, the pulse numbers m, n, x for generating a single seed and writing a storage point are 1-300; the storage point spacing is 100-1000nm; and the diameter of a single seed is 100-1000nm.

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