Domain wall straightening method for periodically poled ferroelectric crystal

Abstract

Disclosed in the present invention is a domain wall straightening method for a periodically poled ferroelectric crystal. In the present invention, by means of testing a piezoelectric coefficient of a poled sample, the poling situation of the sample can be non-destructively determined, and regions with uniform poling can be selected; moreover, whether to continue poling or terminate poling and restart poling after single-domain formation can be determined on the basis of a duty cycle calculation result; and the straightness of domain walls is determined on the basis of the variation of the piezoelectric coefficient, such that the magnitude and duration of a poling voltage are adjusted in a timely manner, thereby accurately controlling a poling process, and thus obtaining a high-quality periodically poled crystal.

Description

A method for straightening domain walls of a periodically poled ferroelectric crystal TECHNICAL FIELD

[0001] The present application relates to the technical field of nonlinear optical crystal device preparation, and more particularly to a method for straightening domain walls of a periodically poled ferroelectric crystal. BACKGROUND

[0002] Frequency conversion of laser, such as frequency doubling, difference frequency, sum frequency and optical parametric oscillation, can be achieved by birefringence phase matching of nonlinear optical crystals. However, birefringence phase matching can only achieve the above functions at specific angles and limited wavelength ranges, cannot utilize the optimal nonlinear optical coefficient direction of the nonlinear optical crystal, and has a large laser walk-off angle, increasing the difficulty of device adjustment. By periodically polarizing the nonlinear optical crystal with ferroelectric properties, i.e., periodically changing the direction of the nonlinear coefficient, the phase mismatch of laser frequency change in the crystal can be compensated. This method is called quasi-phase matching. Quasi-phase matching can achieve frequency conversion in the entire light transmission range of the crystal, can utilize the maximum nonlinear optical coefficient direction of the crystal, overcomes the spatial walk-off effect, improves the frequency conversion efficiency, and greatly expands the application field of the crystal.

[0003] When the crystal is periodically polarized, the degree of curvature of the domain wall seriously affects the conversion efficiency of the device after polarization. The ideal domain wall should be completely flat. The most common method for observing the domain morphology is etching, i.e., etching the polarized crystal in the corresponding etchant. By utilizing the different etching rates of the positive and negative domains of the crystal, a clear and dark interlaced stripe can be formed on the surface of the crystal, and the domain morphology and distribution can be observed through an optical microscope. The advantage of this method is easy observation. However, this method destroys the electrode structure plated during polarization. If the domain distribution is not good, the sample cannot be polarized continuously, making the sample unusable.

[0004] The distribution of the domain can be indirectly reflected by optical methods, such as second harmonic method, electro-optic effect method, diffraction method, etc. The main principle is to detect the frequency conversion efficiency or light intensity change of laser after passing through the crystal. The advantage of this method is that it does not damage the sample. However, the sample needs to be precisely machined, the best domain distribution area cannot be effectively screened, and the wavelengths of the laser sources needed for different periods are different, increasing the complexity of detection.

[0005] The piezoelectric properties of ferroelectric crystals can also be used to indirectly reflect the domain distribution. This method involves measuring the piezoelectric coefficients of the crystal before and after polarization, and calculating the degree of polarization, i.e., the duty cycle, based on the difference in piezoelectric coefficients. The advantages of this method are that it is non-destructive to the sample, requires no precision machining of the light-transmitting surface, and can screen regions with a consistent duty cycle distribution, which is beneficial for selecting the effective area of ​​the final device. However, a disadvantage is that if the domain walls have an irregular, curved shape (see Figure 1a), although the piezoelectric coefficient distribution is consistent, it cannot accurately reflect the straightness of the domain walls.

[0006] Summary of the Invention

[0007] The purpose of this invention is to provide a domain inversion characterization method and domain wall straightening method for periodically polarized ferroelectric crystals to overcome the shortcomings of the prior art. Specifically, it is based on the piezoelectric performance method and combined with the piezoelectric coefficient change to characterize the domain inversion situation, and provides a domain wall straightening method to obtain high-quality periodic polarization results.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for straightening domain walls in a periodically polarized ferroelectric crystal includes the following steps:

[0010] Step 1: Using the quasi-static testing method, the sample to be polarized is placed with its negative polarity facing upwards, and its piezoelectric coefficient is d. s At this time, d s >0, then periodically polarize the sample;

[0011] One side of the polarized sample was placed on an insulating plate, and its piezoelectric coefficient d was measured. c Then, the sample was placed upside down on the insulating plate, and its piezoelectric coefficient d was tested. r ;

[0012] When d c and d r The absolute values ​​of all are greater than d s At / 2, continue polarizing the sample;

[0013] When the absolute value of the piezoelectric coefficient being tested is less than d s When / 2, if d c With d r If the absolute difference is less than 20%, it is determined that the polarization reversal domains penetrate the entire crystal, and the domain-penetrating crystal is obtained for future reference.

[0014] If d c With d r If the absolute difference is greater than 20%, it is determined that the reversed domain has not penetrated the entire crystal. Polarization is stopped, the sample is single-domainized, and periodic polarization is performed again.

[0015] Step 2, using quasi-static test method, the domain-penetrating crystal in step 1 is placed with original negative polarity direction upwards, not placed on an insulating board, and the piezoelectric coefficient d p ;

[0016] Step 3, the duty cycle of the test area is calculated by the formula D=(d p +d s ) / 2d s ; if d p >0, the duty cycle of the reversed domain is 1-D, indicating that the area of the reversed domain is smaller than that of the non-reversed domain; if d p <0, the duty cycle of the reversed domain is D, indicating that the area of the reversed domain is larger than that of the non-reversed domain.

[0017] Step 4, multiple points on the polarized sample are selected for piezoelectric coefficient test, and the duty cycle distribution of different regions of the polarized sample is counted according to step 3.

[0018] Step 5, if the duty cycle D is greater than 20% of the designed duty cycle according to the result of step 3, the sample is monodomainized and re-polarized; if the duty cycle D is greater than 0 and less than 20% of the designed duty cycle, the sample is continuously polarized, and the polarization voltage is adjusted according to the piezoelectric coefficient d p variation Δd p .

[0019] Step 6, the equidistant regions on the same coordinate axis are tested; if 2% d s ≥Δd p ≥0.5% d s , it indicates that the reversed lateral growth speed is in a reasonable range, and the domain wall tends to be flat; whether to continue to polarize is determined according to step 5, so that the duty cycle D reaches the error range of ±0.5% of the designed duty cycle.

[0020] If Δd p >2% d s in any region, the voltage is reduced, the sample is taken out for piezoelectric coefficient test after polarization, and whether to continue to polarize is determined by repeating the above calculation until the duty cycle reaches 50%.

[0021] If Δd p <0.5% d s in any region, the voltage is increased; the sample is taken out for piezoelectric coefficient test after polarization, and whether to continue to polarize is determined by repeating the above calculation until the duty cycle reaches 50%.

[0022] Preferably, the sample in step 1 is potassium titanium oxide phosphate crystal with a thickness of ≥0.5 mm and a reversed domain period of ≥1 μm.

[0023] Preferably, frame type periodic electrodes are plated on the sample-Z surface, the electrode width is ≤1 / 2Λμm, Λ is the period of the domain, the electrode duty cycle is ≤50%, and complete electrodes are plated on the +Z surface.

[0024] Taking a period of 46μm as an example, the theoretical width of the domain is 23μm, and the electrode width is generally less than the theoretical width of the domain to reserve the expansion space.

[0025] Preferably, the pulse voltage under the periodic polarization condition is 3kV / mm, the pulse width is 46ms, and the pulse number is ≥1.

[0026] Polarization can occur when the voltage is greater than the coercive field (the theoretical coercive field of KTP is 2.1kv / mm), and the specific value is determined according to the period and the actual situation; the pulse width can be directly corresponding to the period.

[0027] It can be known from the technical solution that, compared with the prior art, the present application has the following beneficial effects:

[0028] By testing the piezoelectric coefficient of the polarized sample, the polarization condition of the sample can be judged without damage, and the region with uniform polarization can be screened out, and whether to continue polarization or terminate polarization or re-polarize after single-domain can be judged according to the duty cycle calculation result; the flatness of the domain wall is judged by the piezoelectric coefficient change, and the polarization voltage size and time are adjusted in time to accurately control the polarization process and obtain high-quality periodic polarization crystals. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0030] Fig. 1 is a domain wall diagram of a polarized sample;

[0031] Among them, a is the curved domain wall diagram of the actual polarized sample, b is the straight domain wall diagram of the actual polarized sample, and c is the non-merged domain wall diagram of the center of the actual polarized sample;

[0032] Fig. 2 is a periodic polarization schematic diagram;

[0033] Among them, 1 is a periodic electrode, 2 is a longitudinal growth direction schematic diagram, and 3 is a transverse growth direction schematic diagram;

[0034] Fig. 3 is a piezoelectric coefficient test schematic diagram of a sample;

[0035] Among them, 4 is a ferroelectric crystal;

[0036] Fig. 4 is a schematic diagram of piezoelectric coefficient of a test sample placed on an insulating plate;

[0037] In the figure, 4 is a ferroelectric crystal, and 5 is an insulating plate. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0039] Embodiment 1

[0040] Fig. 2 is a schematic diagram of an electrode structure for polarizing a ferroelectric crystal and polarization. As shown in the figure, the periodic electrode structure of the present application is electrode 1 shown in Fig. 1, and a polarization voltage is applied to the crystal in the vertical direction of electrode 1.

[0041] Figs. 3 and 4 are schematic diagrams of piezoelectric coefficient test of a ferroelectric crystal. As shown in the figures, the ferroelectric crystal of the present application is 4 shown in the figures, and the insulating plate is 5 shown in the figures.

[0042] The embodiment provides a domain wall straightening method of a periodically polarized ferroelectric crystal, which comprises the following steps:

[0043] Step 1: using a quasi-static test method, a sample to be polarized is placed with a negative polarity face upward, and the piezoelectric coefficient d s At this time, d s > 0, and then the sample is periodically polarized;

[0044] In the embodiment, the sample to be periodically polarized is a potassium titanium oxypolyphosphate crystal, and has a thickness of 1 mm. The length and width dimensions can be cut into any size according to requirements, and the reverse domain period is 46 μm.

[0045] A frame-shaped periodic electrode is plated on the -Z face of the potassium titanium oxypolyphosphate crystal, the electrode width is 15 μm, the electrode duty cycle is 32.6%, and a complete electrode is plated on the +Z face.

[0046] The pulse voltage of the periodic polarization condition is 3 kV / mm, the pulse width is 46 ms, and the pulse number is ≥1.

[0047] When the pulse number is 3, the polarization voltage is turned off, and the potassium titanium oxypolyphosphate crystal is taken out. Using a quasi-static test method, the potassium titanium oxypolyphosphate crystal is placed on an insulating plate, and the piezoelectric coefficient d c Then the sample is placed reversely on the insulating plate, and the piezoelectric coefficient d r When d cand d r The absolute value of d s When the absolute value of the piezoelectric coefficient of the sample is less than d s / 2, the polarization is continued; when the absolute value of the piezoelectric coefficient of the sample is less than d c If the absolute value of d r differs by less than 20% from d c , it is judged that the polarization inversion domain penetrates the whole crystal; if the absolute value of d r differs by more than 20% from d p , it is judged that the inversion domain does not penetrate the whole crystal, and the polarization is stopped, the sample is monodomainized, and the periodic polarization is re-performed.

[0048] Step 2: The piezoelectric coefficient of the domain-penetrating crystal is tested by using the quasi-static test method, and the crystal is placed with the original negative polarity direction upwards without being placed on an insulating board, and the piezoelectric coefficient is d p .

[0049] Step 3: The piezoelectric coefficient of the sample after polarization is tested at multiple points, and the duty cycle of the test area is calculated by the formula D=(d s +d s ) / 2d p , and the duty cycle distribution of different areas of the polarization sample is counted. When calculating, the positive and negative signs of d p test value are kept. If d p >0, the duty cycle of the inversion domain is 1-D, indicating that the area of the inversion domain is less than that of the non-inversion domain; if d p <0, the duty cycle of the inversion domain is D, indicating that the area of the inversion domain is greater than that of the non-inversion domain.

[0050] Step 4: If the duty cycle D is greater than 20% than the designed duty cycle according to the result obtained in Step 3, the sample is monodomainized, and the periodic polarization is re-performed; if the duty cycle D is greater than 0 and less than 20% than the designed duty cycle, the polarization of the sample is continued, and the polarization voltage size and time length are adjusted in combination with the piezoelectric coefficient d p variation Δd p .

[0051] The inversion domain after periodic polarization is shown in FIG. 1, FIG. 1a is a domain wall bending diagram, FIG. 1b is a domain wall straight diagram, and FIG. 1c is a domain wall center non-merging diagram. When the crystal is periodically polarized, the longitudinal growth speed of the domain is much greater than the transverse growth speed, and the ratio of the speeds is about 100-1000:1, as shown in the periodic polarization schematic diagram in FIG. 2. If a constant pulse width electric field is applied, the higher the electric field, the greater the ratio of the longitudinal growth speed of the domain to the transverse growth speed. The faster the transverse growth speed of the inversion domain, the easier the domain wall bends; otherwise, if the voltage is too low, the transverse growth speed of the inversion domain is too slow, and the center is prone to non-merging, and the piezoelectric coefficient d pThe reaction can be indirectly reversed to control the domain lateral growth speed.

[0052] Step 5, piezoelectric coefficient d p Test, if any one area d p The change amount relative to the last time is Δd p Satisfies 2% d s ≥ Δd p ≥ 0.5% d s , which indicates that the reverse lateral growth speed is in a reasonable range, and the domain wall tends to be flat, and whether to continue to perform polarization is judged according to the duty cycle calculation result, so that the duty cycle D and the designed duty cycle reach the error range of ±0.5%;

[0053] If any one area Δd p > 2% d s , the polarization pulse voltage is reduced to 2.8 kV / mm, the pulse width is 4.6 ms, the pulse number is 1, after polarization is completed, the potassium titanium phosphate oxide crystal is taken out to perform piezoelectric coefficient test, and whether to continue to perform polarization is judged by repeating the previous calculation, until the duty cycle reaches 50%.

[0054] If any one area Δd p < 0.5% d s , the voltage is increased to 3.2 kV / mm, the pulse width is 4.6 ms, the pulse number is 1, after polarization is completed, the potassium titanium phosphate oxide crystal is taken out to perform piezoelectric coefficient test, and whether to continue to perform polarization is judged by repeating the previous calculation, until the duty cycle reaches 50%.

[0055] According to the embodiment of the present application, the problem of domain wall not being flat during the periodic polarization process of the ferroelectric crystal can be solved, so as to improve the quality of the reverse domain, make it present periodic arrangement, and the duty cycle is close to 50%, and the light conversion efficiency is effectively improved.

[0056] In the specification, each embodiment is described in a progressive manner, and each embodiment mainly explains the difference from other embodiments, and the same or similar parts between each embodiment 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 related parts can be referred to the method part.

[0057] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and, while certain modifications are discussed, it is desired to be protected in accordance with the spirit and scope of the application. Therefore, the application is not limited to the specific embodiments shown and described, but only by the scope of the appended claims, unless otherwise specified.

Claims

1. A method of domain wall straightening of a periodically poled ferroelectric crystal, characterized by, The method comprises the following steps: Step 1, using the quasi-static test method, the sample to be polarized is placed with the negative polarity facing upward, and its piezoelectric coefficient is d s At this time, d s > 0, and then the sample is periodically polarized; One side of the poled sample is placed on an insulating plate and its piezoelectric coefficient d is tested c The sample is then placed upside down on the insulating plate and its piezoelectric coefficient d is tested r ; When d c and the absolute value of d r are both greater than d s / 2, continue to polarize the sample; When the absolute value of the piezoelectric coefficient of the test is less than d s / 2, if the absolute value of d c differs from d r by less than 20%, it is determined that the polarization reversed domains penetrate the entire crystal, and a domain-penetrated crystal is obtained, which is ready for use. If d c If d r If the absolute value difference is greater than 20%, it is determined that the reversed domain does not penetrate the entire crystal, the polarization is stopped, the sample is monodomainized, and the periodic polarization is re-performed. Step 2, the piezoelectric coefficient d33 of the domain-penetrating crystal in Step 1 was tested by using the quasi-static test method, and the domain-penetrating crystal was placed with the original negative polarity direction facing up, without being placed on an insulating board p ; Step 3, the duty cycle of the test area is calculated by the formula D=(d p +d s ) / 2d s If d p >0, the duty cycle of the reversed domain is 1-D, indicating that the area of the reversed domain is smaller than that of the non-reversed domain. If d p If D < 0, the duty cycle of the reversed domain is D, indicating that the area of the reversed domain region is larger than that of the non-reversed domain region. Step 4: selecting multiple points on the polarized sample to test the piezoelectric coefficient, and according to step 3, the duty cycle distribution of different regions of the polarized sample is counted; Step 5: if the duty cycle D is greater than 20% than the designed duty cycle according to the result of step 3, the sample is monodomainized, and the periodic polarization is performed again; If the duty cycle D is greater than 0 and less than 20% of the designed duty cycle, continue to polarize the sample and combine the piezoelectric coefficient d p The variation Δd p Adjust the polarization voltage size; Step 6, test the equidistance regions on the same coordinate axis, if any one region 2% d s ≥ Δd p ≥ 0.5% d s , it means that the reverse lateral growth rate is in a reasonable range, indicating that the domain wall tends to be flat, According to step 5, it is judged whether to continue to polarize, so that the duty cycle D and the designed duty cycle reach the error range of ±0.5%. If any one of the regions Δd p > 2% d s The voltage is then reduced, and after polarization is complete, the sample is removed for piezoelectric coefficient testing. The above calculations are repeated to determine whether to continue polarization until the duty cycle reaches 50%. If any one of the regions Δd p <0.5% d s If so, the voltage is raised; after polarization is completed, the sample is removed for piezoelectric coefficient testing, and the above calculation is repeated to determine whether to continue polarization until the duty cycle reaches 50%.

2. The method of domain wall straightening of a periodically poled ferroelectric crystal according to claim 1, wherein The sample in step 1 is a potassium titanyl phosphate crystal, the thickness is greater than or equal to 0.5 mm, and the reverse domain period is greater than or equal to 1 μm.

3. The method of domain wall straightening of a periodically poled ferroelectric crystal according to claim 1, wherein A frame type periodic electrode is plated on the -Z surface of the sample, the electrode width is less than or equal to 1 / 2Λ μm, Λ is the reverse domain period, and the electrode duty cycle is less than or equal to 50%; a complete electrode is plated on the +Z surface.

4. The method of domain wall straightening of a periodically poled ferroelectric crystal according to claim 1, wherein The pulse voltage of the periodic polarization condition is 3 kV / mm, the pulse width is 46 ms, and the pulse number is greater than or equal to 1.

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