A method for aligning domain walls of periodically polarized ferroelectric crystals

Abstract

The invention discloses a domain wall straightening method for a periodically poled ferroelectric crystal. The invention can non-destructively judge the polarization condition of a sample by testing the piezoelectric coefficient of the sample after polarization, and can screen out a region with uniform polarization. At the same time, it can judge whether to continue polarization or terminate polarization or re-polarize after single domainization according to the duty cycle calculation result. The straightness of the domain wall is judged by the change in the piezoelectric coefficient, the polarization voltage and duration are adjusted in time, the polarization process is accurately controlled, and a high-quality periodically poled crystal is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of nonlinear optical crystal devices, and more particularly to a domain wall alignment method for periodically polarized ferroelectric crystals. Background Art

[0002] The frequency conversion of laser can be realized by using the birefringence phase matching of nonlinear optical crystals, such as frequency doubling, difference frequency, sum frequency, optical parametric oscillation, etc. However, birefringence phase matching can only realize the above functions at specific angles and within a limited wavelength range, and cannot utilize the optimal nonlinear optical coefficient direction of nonlinear optical crystals. In addition, the laser walk-off angle is large, and the device adjustment is more difficult. By periodically polarizing the nonlinear optical crystal with ferroelectric properties, that is, periodically changing the direction of its nonlinear coefficient, the phase mismatch of the laser when the frequency changes in the crystal can be compensated. This method is called quasi-phase matching. Quasi-phase matching can realize frequency changes within the entire light transmission range of the crystal, and can utilize the direction of the maximum nonlinear optical coefficient of the crystal, overcoming the spatial walk-off effect, improving the frequency conversion efficiency, and greatly expanding the application field of the crystal.

[0003] When the crystal is periodically polarized, the curvature of the domain wall seriously affects the conversion efficiency of the device after polarization. The ideal domain wall should be completely straight. The most common method for observing domain morphology is to use the corrosion method, that is, to corrode the polarized crystal in the corresponding corrosive agent. By taking advantage of the different corrosion rates of the positive and negative domains of the crystal, light and dark stripes can be formed on the surface of the crystal. The domain morphology and distribution can be observed through an optical microscope. The advantage of this method is that it is easy to observe. However, this method destroys the electrode structure plated during polarization. If the domain distribution is not good, the sample cannot continue to be polarized, making the sample unusable.

[0004] The distribution of domains can be indirectly reflected by optical methods, such as the second harmonic method, the electro-optical effect method, the diffraction method, etc. The main principle is to detect the frequency conversion efficiency or the change of light intensity after the laser passes through the crystal. The advantage of this method is that it does not damage the sample, but the disadvantage is that it requires precision processing of the light-transmitting end face of the sample, and cannot effectively screen out the best domain distribution area. In addition, different wavelengths of the laser source required for different periods increase the complexity of detection.

[0005] The piezoelectric properties of ferroelectric crystals can also indirectly reflect the distribution of domains. The principle of this method is to measure the piezoelectric coefficient of the crystal before and after polarization. The degree of polarization, that is, the duty cycle, can be calculated based on the difference in the piezoelectric coefficient. The advantages of this method are that it does not damage the sample, does not require precision machining of the light-transmitting surface of the sample, and can screen out areas with consistent duty cycle distribution, which is conducive to the selection of the effective area of ​​the final device. The disadvantage is that if the domain wall is irregularly curved, see attached Figure 1a. Although the piezoelectric coefficient distribution is consistent, it cannot truly reflect the flatness of the domain wall. Summary of the invention

[0006] The purpose of the present invention is to provide a method for characterizing domain inversion of periodically poled ferroelectric crystals and a method for domain wall straightening to overcome the shortcomings of the prior art. Specifically, the domain inversion situation is characterized based on a piezoelectric performance method combined with the change in the piezoelectric coefficient, and a domain wall straightening method is provided to obtain high-quality periodic poling results.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A method for aligning domain walls of a periodically polarized ferroelectric crystal comprises the following steps:

[0009] Step 1: Using the quasi-static test method, place the sample to be polarized with the negative polarity facing upwards. Its piezoelectric coefficient is d s At this time, d s >0, and then the sample is subjected to periodic polarization;

[0010] Place one side of the polarized sample on an insulating plate and measure its piezoelectric coefficient d c Then the sample is placed on an insulating plate in reverse and its piezoelectric coefficient d is tested. r ;

[0011] When c and d r The absolute value of s / 2, continue to polarize the sample;

[0012] When the absolute value of the tested piezoelectric coefficient is less than d s / 2, if d c With d r If the absolute value difference is less than 20%, it is judged that the polarization reversal domain runs through the entire crystal, and the domain runs through the crystal, which is reserved;

[0013] If d c With d r If the absolute value difference is greater than 20%, it is judged that the inversion domain does not penetrate the entire crystal, and the polarization is stopped, the sample is single-domained, and periodic polarization is performed again;

[0014] Step 2: Using the quasi-static test method, the domain-penetrating crystal in step 1 is placed with the original negative polarity facing upwards, not placed on an insulating plate, and its piezoelectric coefficient is tested as d p ;

[0015] Step 3, by the formula D = (d p +d s ) / 2d s Calculate the duty cycle of the test area. If dp >0, the duty cycle of the inversion domain is 1-D, indicating that the area of ​​the inversion domain is smaller than that of the non-inversion domain. p <0, then the duty cycle of the inversion domain is D, indicating that the area of ​​the inversion domain is larger than that of the non-inversion domain;

[0016] Step 4, selecting multiple points on the polarized sample to perform piezoelectric coefficient testing, and calculating the duty cycle distribution of different areas of the polarized sample according to step 3;

[0017] Step 5: Based on the result obtained in step 3, if the duty cycle D is greater than 20% of the designed duty cycle, the sample is monodomainized and periodically polarized again; if the duty cycle D is greater than 0 but less than 20% of the designed duty cycle, the sample is polarized again and the piezoelectric coefficient d is calculated. p Change Δd p Adjust the polarization voltage;

[0018] Step 6: Test the equally spaced regions on the same axis. If any region has 2% d s ≥Δd p ≥0.5%d s , it means that the inversion lateral growth rate is within a reasonable range, indicating that the domain wall tends to be straight. According to step 5, it is determined whether to continue polarization, and finally the duty cycle D and the designed duty cycle reach an error range of ±0.5%;

[0019] If any region Δd p >2%d s , then reduce the voltage, take out the sample after polarization is completed to test the piezoelectric coefficient, and repeat the above calculation to determine whether to continue polarization until the duty cycle reaches 50%;

[0020] If any region Δd p <0.5%d s , then increase the voltage; after polarization is completed, take out the sample for piezoelectric coefficient test, and repeat the above calculation to determine whether to continue polarization until the duty cycle reaches 50%.

[0021] Preferably, the sample in step 1 is potassium titanyl phosphate crystal with a thickness of ≥0.5 mm and an inversion domain period of ≥1 μm.

[0022] Preferably, a frame-type periodic electrode is plated on the -Z surface of the sample, the electrode width is ≤1 / 2Λμm, Λ is the inversion domain period, the electrode duty cycle is ≤50%, and a complete electrode is plated on the +Z surface.

[0023] Taking a 46μm period as an example, the theoretical width of the inversion domain is 23μm. In order to reserve space for expansion, the electrode width is generally smaller than the theoretical width of the inversion domain.

[0024] Preferably, the pulse voltage of the periodic polarization condition is 3 kV / mm, the pulse width is 46 ms, and the number of pulses is ≥1.

[0025] Polarization can occur when the voltage is greater than the coercive field (the theoretical coercive field of KTP is 2.1 kV / mm). The specific value is determined according to the period and actual conditions; the pulse width can directly correspond to the period.

[0026] It can be seen from the above technical solution that compared with the prior art, the present invention has the following beneficial effects:

[0027] By testing the piezoelectric coefficient of the sample after polarization, the polarization condition of the sample can be determined non-destructively, and the area with uniform polarization can be screened out. At the same time, based on the duty cycle calculation results, it can be determined whether to continue polarization or terminate polarization, and then re-polarize after single domainization. The flatness of the domain wall can be determined by the change in the piezoelectric coefficient, and the polarization voltage and duration can be adjusted in time to accurately control the polarization process and obtain high-quality periodically polarized crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0029] Figure 1 is the domain wall image of the polarized sample;

[0030] Among them, a is the curved domain wall image of the actual polarized sample, b is the straight domain wall image of the actual polarized sample, and c is the unmerged domain wall image at the center of the actual polarized sample;

[0031] Figure 2 is a schematic diagram of periodic polarization;

[0032] Among them, 1 is a periodic electrode, 2 is a schematic diagram of the longitudinal growth direction, and 3 is a schematic diagram of the lateral growth direction;

[0033] Figure 3 Schematic diagram of the piezoelectric coefficient of the test sample;

[0034] Among them, 4 is a ferroelectric crystal;

[0035] Figure 4 This is a schematic diagram of the piezoelectric coefficient when one side of the test sample is placed on an insulating plate;

[0036] Among them, 4 is a ferroelectric crystal, and 5 is an insulating plate. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] Example 1

[0039] Figure 2 The electrode structure and polarization schematic diagram for polarizing a ferroelectric crystal are shown in the figure. Figure 1 In the electrode 1 shown, the polarization voltage is applied to the crystal in the vertical direction where the electrode 1 is located.

[0040] Figure 3 and Figure 4 The schematic diagram of the piezoelectric coefficient test of the ferroelectric crystal is shown in FIG.

[0041] This embodiment provides a method for aligning domain walls of a periodically poled ferroelectric crystal, comprising the following steps:

[0042] Step 1: Using the quasi-static test method, place the sample to be polarized with the negative polarity facing upwards and the piezoelectric coefficient d s At this time, d s >0, and then the sample is subjected to periodic polarization;

[0043] In this embodiment, the sample used for periodic poling is potassium titanyl phosphate crystal, and has a thickness of 1 mm. Its length and width can be cut into any size according to requirements, and the inversion domain period is 46 μm;

[0044] A frame-type periodic electrode is plated on the -Z surface of the potassium titanyl phosphate crystal, with an electrode width of 15 μm and an electrode duty cycle of 32.6%, and a complete electrode is plated on the +Z surface;

[0045] The pulse voltage of periodic polarization condition is 3kV / mm, the pulse width is 46ms, and the number of pulses is ≥1;

[0046] When the number of pulses reaches 3, the polarization voltage is turned off and the potassium titanyl phosphate crystal is taken out. Using the quasi-static test method, the potassium titanyl phosphate crystal is placed on an insulating plate to test its piezoelectric coefficient d c Then the sample is placed on an insulating plate in reverse and its piezoelectric coefficient d is tested. r When d c and d r The absolute value of s / 2, continue to polarize the sample; when the absolute value of the tested piezoelectric coefficient is less than d s / 2, if d c With d r If the absolute value difference is less than 20%, it is judged that the polarization reversal domain runs through the entire crystal; if d c With d r If the absolute value difference is greater than 20%, it is judged that the inversion domain does not penetrate the entire crystal, and the polarization is stopped, the sample is single-domained, and periodic polarization is performed again.

[0047] Step 2: Using a quasi-static test method, the domain-penetrating crystal is placed with its original negative polarity facing upwards, not on an insulating plate, and its piezoelectric coefficient is tested as d p .

[0048] Step 3: Select multiple points on the polarized sample to test the piezoelectric coefficient. p +d s ) / 2d s The duty cycle of the test area is calculated, and the duty cycle distribution of different areas of the polarized sample is statistically analyzed. When calculating, d p The sign of the test value. p >0, the duty cycle of the inversion domain is 1-D, indicating that the area of ​​the inversion domain is smaller than that of the non-inversion domain. p <0, then the duty cycle of the inversion domain is D, indicating that the area of ​​the inversion domain is larger than that of the non-inversion domain.

[0049] Step 4: Based on the result obtained in step 3, if the duty cycle D is greater than 20% of the designed duty cycle, the sample is monodomainized and periodically polarized again; if the duty cycle D is greater than 0 but less than 20% of the designed duty cycle, the sample is polarized again and the piezoelectric coefficient d is calculated. p Change Δd p Adjust the polarization voltage and duration.

[0050] The inversion domain after periodic poling is as follows Figure 1 As shown, Figure 1 a is the domain wall bending diagram, Figure 1 b is the domain wall flattening diagram, Figure 1 c is the unmerged diagram of the domain wall center. When the crystal is periodically polarized, the vertical growth rate of the domain is much greater than the horizontal growth rate, and the speed ratio is about 100 to 1000:1, see the attached Figure 2 Schematic diagram of periodic poling. If an electric field with a constant pulse width is applied, the higher the electric field, the greater the ratio of the longitudinal growth rate to the lateral growth rate of the domain. The faster the lateral growth rate of the inversion domain, the easier it is for the domain wall to bend; conversely, if the voltage is too low, the lateral growth rate of the inversion domain is too slow, and the center is very likely to be unmerged, and the piezoelectric coefficient d p Change Δd p It can indirectly reflect the lateral growth rate of the reverse domain.

[0051] Step 5: Calculate the piezoelectric coefficient d for the equally spaced regions on the same coordinate axis. p Test, if any area d p The change relative to the previous time Δd p Meet 2%d s ≥Δd p ≥0.5%d s , it means that the inversion lateral growth rate is within a reasonable range, indicating that the domain wall tends to be straight. Then, according to the duty cycle calculation result, it is determined whether to continue polarization, and finally the duty cycle D and the designed duty cycle reach an error range of ±0.5%;

[0052] If any region Δd p >2%d s , then reduce the polarization pulse voltage to 2.8kV / mm, the pulse width to 4.6ms, and the number of pulses to 1. After the polarization is completed, take out the potassium titanyl phosphate crystal for piezoelectric coefficient test, and repeat the previous calculation to determine whether to continue polarization until the duty cycle reaches 50%;

[0053] If any region Δd p <0.5%d s , then increase the voltage to 3.2kV / mm, the pulse width is 4.6ms, the number of pulses is 1, and after the polarization is completed, take out the potassium titanyl phosphate crystal for piezoelectric coefficient test, and repeat the previous calculation to determine whether to continue polarization until the duty cycle reaches 50%.

[0054] The embodiment of the present invention can solve the problem of uneven domain walls in ferroelectric crystals during periodic poling, thereby improving the quality of inversion domains, making them present a periodic arrangement, and having a duty cycle close to 50%, effectively improving the light conversion efficiency.

[0055] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0056] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for aligning domain walls of a periodically polarized ferroelectric crystal, characterized in that: The following steps are involved: Step 1: Using the quasi-static test method, place the sample to be polarized with the negative polarity facing upwards. Its piezoelectric coefficient is d s At this time, d s >0, and then the sample is subjected to periodic polarization; Place one side of the polarized sample on an insulating plate and measure its piezoelectric coefficient d c Then the sample is placed on an insulating plate in reverse and its piezoelectric coefficient d is tested. r ; When c and d r The absolute value of s / 2, continue to polarize the sample; When the absolute value of the tested piezoelectric coefficient is less than d s / 2, if d c With d r If the absolute value difference is less than 20%, it is judged that the polarization reversal domain runs through the entire crystal, and the domain runs through the crystal, which is reserved; If d c With d r If the absolute value difference is greater than 20%, it is judged that the inversion domain does not penetrate the entire crystal, and the polarization is stopped, the sample is single-domained, and periodic polarization is performed again; Step 2: Using the quasi-static test method, the domain-penetrating crystal in step 1 is placed with the original negative polarity facing upwards, not placed on an insulating plate, and its piezoelectric coefficient is tested as d p ; Step 3, by the formula D = (d p +d s ) / 2d s Calculate the duty cycle of the test area. If d p >0, the duty cycle of the inversion domain is 1-D, indicating that the area of ​​the inversion domain is smaller than that of the non-inversion domain; If d p <0, then the duty cycle of the inversion domain is D, indicating that the area of ​​the inversion domain is larger than that of the non-inversion domain; Step 4, selecting multiple points on the polarized sample to perform piezoelectric coefficient testing, and calculating the duty cycle distribution of different areas of the polarized sample according to step 3; Step 5, if the duty cycle D is greater than the designed duty cycle by 20% according to the result obtained in step 3, the sample is subjected to single domain and periodic poling is performed again; If the duty cycle D is greater than 0 but less than 20% of the designed duty cycle, the sample is polarized and the piezoelectric coefficient d p Change Δd p Adjust the polarization voltage; Step 6: Test the equally spaced regions on the same axis. If any region has 2% d s ≥Δd p ≥0.5%d s , it means that the inversion lateral growth rate is within a reasonable range, indicating that the domain wall tends to be straight. According to step 5, it is determined whether to continue polarization, and finally the duty cycle D and the designed duty cycle reach an error range of ±0.5%; If any region Δd p >2%d s , then reduce the voltage, take out the sample after polarization is completed to test the piezoelectric coefficient, and repeat the above calculation to determine whether to continue polarization until the duty cycle reaches 50%; If any region Δd p <0.5%d s , then increase the voltage; after polarization is completed, take out the sample for piezoelectric coefficient test, and repeat the above calculation to determine whether to continue polarization until the duty cycle reaches 50%.

2. The method for domain wall alignment of a periodically poled ferroelectric crystal according to claim 1, characterized in that: The sample in step 1 is potassium titanyl phosphate crystal with a thickness of ≥0.5 mm and an inversion domain period of ≥1 μm.

3. The method for aligning domain walls of a periodically poled ferroelectric crystal according to claim 1, characterized in that: A frame-type periodic electrode is plated on the -Z surface of the sample, the electrode width is ≤1 / 2Λμm, Λ is the inversion domain period, the electrode duty cycle is ≤50%, and a complete electrode is plated on the +Z surface.

4. The method for domain wall alignment of a periodically poled ferroelectric crystal according to claim 1, characterized in that: The pulse voltage of the periodic polarization condition is 3 kV / mm, the pulse width is 46 ms, and the number of pulses is ≥1.

Images