A method and device for light deflection based on strontium barium niobate crystal with gradient components

Abstract

This invention relates to the field of optoelectronic functional crystal materials technology, specifically to a method and device for optical deflection of barium strontium niobate (SBN) crystals with a compositional gradient along the [001] direction. First, an SBN crystal with a compositional gradient along the [001] direction is grown using the Czochralski method. A cuboid optical wafer is then cut according to the crystal orientation. On the polished (001) surface, a double-layer metal electrode with ohmic contact is fabricated using magnetron sputtering. The crystal temperature is controlled at an effective Curie temperature of 3-5°C. Light is incident along the [100] or [001] direction of the crystal, and an electric field is applied along the direction of the compositional gradient. Optical deflection can be achieved at the emission end, with a maximum deflection angle of 9.6 mrad at a voltage of 800 V / mm. Compared with traditional deflectors based on the linear electro-optic effect, this invention achieves a larger deflection angle and significantly reduces device size and structural complexity, making it suitable for high-speed random scanning, displays, laser processing, and space optical communication.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photoelectric functional crystal materials, and particularly relates to a light deflection method and device based on a barium strontium niobate crystal with gradient components. BACKGROUND

[0002] A light beam deflector, also known as a light beam scanner, is a device that changes the spatial propagation direction of a light beam according to a certain rule. Light beam deflection technology is widely used in the fields of display, sensing, optical storage, laser printing, microscopic imaging, laser processing, laser radar, and space optical communication.

[0003] Current light beam deflection technologies can be divided into mechanical and non-mechanical types. Traditional mechanical deflection usually uses rotating mirrors (CN104749565B), galvanometer mirrors (CN111289955B), or micro-electromechanical system (MEMS) scanning mirrors (CN109085600B). The inertia of mechanical components results in low scanning frequency and stability. Non-mechanical light beam deflection technologies usually use acousto-optic (AOD) (CN107850796B) or electro-optic deflection devices (EOD) (CN110543035B), which have the advantages of small device size and easy implementation of high-frequency scanning. In particular, electro-optic deflection has more advantages in deflection angle and response speed than acousto-optic deflection, and can better meet the application requirements of high-speed random scanning.

[0004] An electro-optic deflector changes the spatial propagation direction of a light beam using the electro-optic effect, i.e., by applying an external voltage to change the refractive index of the crystal under the action of the electro-optic effect, thereby changing the propagation direction of the light. Early electro-optic deflection devices were prism-shaped (Lee T, Zook J. Light beam deflection with electrooptic prisms [J]. IEEE Journal of Quantum Electronics, 1968, 4(7): 442-454.). The advantage of this prism-shaped electro-optic deflector is that it can achieve fast response and continuous change of deflection. However, since deflection only occurs at the interface of two materials, a larger deflection angle requires the use of multiple prisms to form an array, resulting in a large device size, complex structure, and the need for a high driving voltage, which limits its application.

[0005] Another electro-optic deflection is the deflection of the light beam caused by the refractive index gradient perpendicular to the direction of light beam propagation, which usually uses special-shaped electrodes to create a gradient electric field inside the crystal, such as the deflector with a four-electrode structure (Zhao Z, Chen M, Liu Z, et al. Four-electrode octagonal structure electro-optic deflector [J]. Laser and Optoelectronics Progress, 2020, 57(15): 199-204.), which forms a gradient refractive index distribution perpendicular to the direction of light beam propagation. The light beam will deflect in the direction of increasing refractive index gradient during propagation, and the deflection angle can be controlled by the applied voltage. The advantage of gradient refractive index deflection is that the light beam deflection does not occur only on the surface of the material, but accumulates during the propagation of the medium. Compared with the deflection device with a prism structure, a larger deflection angle can be obtained with a shorter optical path and a smaller device size. However, this deflector has a complex component shape, which brings some difficulties to processing, and is limited by the electro-optic coefficient of the electro-optic material, and the deflection efficiency is low. For example, the deflection efficiency of the four-electrode structure deflector proposed by Zhao Z et al. is only 0.312 mu rad / V.

[0006] Both the above-mentioned deflector based on the prism structure and the four-electrode structure (gradient electric field) rely on the refractive index change caused by the electro-optic effect. The electro-optic effect can be divided into linear electro-optic effect and quadratic electro-optic effect according to the relationship between refractive index change and applied electric field. Since the quadratic electro-optic effect of general crystals is much smaller than the linear electro-optic effect, the current commercial electro-optic deflection device is based on the linear electro-optic effect, and the quadratic electro-optic effect is rarely used. SUMMARY

[0007] In view of the above problems of the prior art, the present application prepares a strontium barium niobate crystal with a gradient composition, and proposes an optical deflection method and device based on the quadratic electro-optic effect of the tetragonal strontium barium niobate crystal, which solves the problems of complex structure, difficult processing and low deflection efficiency of traditional electro-optic deflection devices.

[0008] To achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0009] A strontium barium niobate crystal with a gradient composition is grown by the Czochralski method, and an optical deflection method and device based on the quadratic electro-optic effect of the tetragonal strontium barium niobate crystal are proposed. x Ba 1-x Nb2O6 crystal (SBN crystal), the initial raw material ratio should deviate from the solid-liquid same composition point (i.e. Sr content x = 0.61, at this time the Sr / Ba content in the raw material and the crystal is the same), in order to ensure that the crystal has a large composition gradient, the initial raw material molar ratio range should be: 0.65:0.35 < SrCO3:BaCO3 < 0.79:0.21, the composition gradient (composition change per unit length) of the crystal obtained in the growth direction is Δx / l > 10 -3 / mm.

[0010] The SBN crystal with the component gradient is processed into a cuboid wafer, each surface corresponds to the (100), (010) and (001) surface of the crystal, the optical surface and the electrode-coated surface are polished to the optical grade, the metal electrode is coated on the two surfaces perpendicular to the growth direction (i.e. the (001) surface), the light is transmitted along the

[100] or

[010] direction (i.e. perpendicular to the (100) surface or the (010) surface), the linearly polarized light polarized along the

[001] direction is obtained by using a linear polarizer before the light beam enters the crystal, and the electric field is applied to the crystal along the

[001] direction, so that the light beam deflection is obtained at the exit end of the crystal.

[0011] Further, since the crystal itself has a component gradient, i.e. the components at different positions in the crystal are different, correspondingly, the Curie temperatures at different positions of the crystal are different, and the temperature corresponding to the maximum dielectric constant can be read by using a dielectric temperature spectrum measuring instrument, which is called "effective Curie temperature" (denoted as T c When the SBN crystal is used as an electro-optic deflection element, it is usually required to control the temperature of the crystal to be within 3-5 ℃ below the effective Curie temperature. The temperature control precision is ±0.1 ℃, and the temperature control method adopts the TEC semiconductor temperature control technology.

[0012] Further, since the SBN crystal is usually grown in a growth furnace with a nitrogen atmosphere, a large number of oxygen vacancies are formed in the crystal during the high-temperature growth process due to the lack of oxygen, so that the crystal appears deep blue. In order to improve the transmittance and make it more suitable for optical applications, it is required to perform annealing in a high-temperature oxygen atmosphere to reduce the oxygen vacancy defects, so that the crystal becomes light yellow or nearly transparent. The annealing temperature is 1200-1300 ℃, the constant temperature time is 12-24 hours, and the temperature rising and falling speed is 100-200 ℃ / h.

[0013] Further, since the strontium barium niobate crystal is in the ferroelectric phase below the Curie point, theoretically, the electro-optic effect of the crystal at this time should exhibit linear electro-optic effect (the quadratic electro-optic effect is relatively weak), but through experimental research, it is found that the tetragonal phase SBN crystal exhibits quadratic electro-optic effect before polarization treatment, which is because although each ferroelectric domain in the SBN crystal has spontaneous polarization and exhibits linear electro-optic effect, the linear electro-optic effects of different ferroelectric domains cancel each other out due to different orientations, and exhibit quadratic electro-optic effect to the outside. Theoretical and experimental researches show that in the SBN crystal with a gradient component, the electro-optic deflection angle based on the quadratic electro-optic effect is much larger than the deflection based on the linear electro-optic effect, and therefore, in order to obtain a larger deflection angle, the tetragonal phase SBN crystal cannot be subjected to polarization treatment when used as an electro-optic deflection element (i.e. cannot be changed into a single-domain crystal by high-voltage polarization).

[0014] Furthermore, when unpolarized SBN crystals are used as electro-optic deflection devices, they are easily polarized into single-domain crystals under the action of an applied voltage, which causes them to change from a secondary electro-optic effect to a linear electro-optic effect and causes the electro-optic deflection angle to decay. To avoid the deflection angle decay caused by the polarization effect of high voltage, the maximum deflection voltage of electro-optic deflection should be less than 800V / mm (where the length is the thickness of the crystal in the electric field direction).

[0015] Furthermore, to reduce energy loss at the electrode contact surface and improve device performance and operational stability, it is typically necessary to form an ohmic contact between the SBN crystal and the metal electrode. Therefore, we chose a process of depositing a double-layer electrode on the SBN crystal surface. According to the relevant theory of work function, as an n-type semiconductor material, for SBN crystal to form an ohmic contact electrode, the work function of the selected metal electrode must be lower than the work function of the SBN crystal. That is, choosing a metal with a lower work function makes it easier to form an ohmic contact on the SBN crystal surface. After theoretical and experimental screening, the inner electrode material in contact with the crystal is one of Al, Ti, or Ag, and the outer electrode material in contact with air is one of Au or Pt. The outer electrode serves as a protective electrode, preventing oxidation and corrosion of the inner electrode, avoiding thermal and physical damage, ensuring the stability of electrical performance, and extending device lifespan.

[0016] Furthermore, both layers of metal electrodes are coated on the surface using magnetron sputtering. For the inner electrode, if the film is too thin, it is difficult to ensure ohmic contact, and if the film is too thick, it is easy to introduce large impedance at high frequencies. Typically, the thickness of the inner electrode is 50-150 nm. For the outer electrode, if the film is too thin, its protective effect will be weakened, and if the film is too thick, it will easily lead to the waste of precious metal materials. Typically, the thickness of the outer electrode is 100-200 nm.

[0017] Furthermore, after electrode sputtering, in order to further promote the interfacial reaction between the electrode and the crystal, reduce the contact barrier, and lower the contact resistance, the crystal needs to be subjected to rapid annealing again (secondary annealing). The reason for using rapid annealing instead of conventional annealing is that rapid annealing can effectively limit the excessive diffusion of metal at high temperatures. The annealing temperature is 300-500℃, the holding time is 5-10 minutes, the heating time is usually 5-30 seconds, and the annealing atmosphere is nitrogen or argon.

[0018] The beneficial effects of this invention are as follows:

[0019] (1) By using a small crucible to grow crystals, the non-uniform melting characteristics of SBN crystals when they deviate from the same composition point are utilized to prepare SBN crystals with a specific composition gradient along the growth direction and achieve beam deflection. Compared with electro-optic deflectors with cascaded prism structures or quadrupole structures, the structure is simpler and the device size is smaller.

[0020] (2) A significant secondary electro-optic effect was found in the unpolarized SBN crystal. Theoretical and experimental studies show that the deflection angle based on the composition gradient and the secondary electro-optic effect of the crystal is much greater than that of the linear electro-optic effect. The maximum deflection angle is increased from the urad level of the linear electro-optic deflection device to the mrad level. The proposed scheme provides a new implementation scheme for the design of electro-optic deflection devices. The improvement of the deflection angle also greatly expands the application range of electro-optic deflection devices. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 To illustrate the compositional changes of the melt and crystal during the growth process using a binary phase diagram of SBN crystals;

[0023] Figure 2 The transmittance of SBN crystal before and after the first high-temperature annealing;

[0024] Figure 3 This refers to SBN crystals with gradient composition grown using the Czochralski method.

[0025] Figure 4 The compositional uniformity of SBN crystals in the (001) and (100) directions;

[0026] Figure 5 The IV characteristic curves before and after rapid annealing when using Ag+Au electrodes;

[0027] Figure 6 The IV characteristic curves before and after rapid annealing when using Ti+Au electrodes;

[0028] Figure 7 The IV characteristic curves before and after rapid annealing when using Al+Pt electrodes;

[0029] Figure 8 This is a schematic diagram of an SBN crystal electro-optic deflection system;

[0030] Figure 9 Dielectric temperature spectrum of SBN crystal with composition gradient;

[0031] Figure 10 The positions of the deflection spot of the SBN electro-optic deflector under different voltages;

[0032] Figure 11The deflection angle of the SBN electro-optic deflector under different voltages;

[0033] Figure 12 The Mach-Zehnder interferometer system used to measure the second electro-optic coefficient of SBN;

[0034] Figure 13 A comparison of the waveforms of the interference light intensity signal and the applied voltage signal measured for the Mach-Zehnder interferometer system;

[0035] Figure 14 A schematic diagram of the electro-optic deflection principle based on gradient components. Detailed Implementation

[0036] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0037] Example 1: Growth of SBN crystals with a composition gradient (initial raw material molar ratio of SrCO3:BaCO3 = 0.73:0.27)

[0038] Figure 1 The binary phase diagram of SBN crystals is shown. To obtain SBN crystals with a compositional gradient, we set the initial ratio of the raw materials to the left of the solid-liquid isotope point. Figure 1 At point A, due to the non-continuous melting characteristic of crystal growth (i.e., the melt and crystal compositions are different), during crystal growth, the melt composition moves from point A to point B, and the corresponding crystal composition moves from a to b, thus obtaining a crystal with a compositional gradient along the crystal growth direction. To obtain crystals with a larger compositional gradient, the "small crucible crystal growth" method is usually used. In all embodiments of this invention, the crucible size used is an iridium crucible with a diameter of φ50mm × 50mm, and the grown crystal size is not less than φ20mm × 20mm.

[0039] The crystals were grown using the Czochralski method. The experimental raw materials were SrCO3, BaCO3, and Nb2O5 powders with a purity of 4N, prepared in a ratio of SrCO3:BaCO3:Nb2O5 = 0.73:0.27:1. The powders were mixed in a mixer for 48 hours to ensure thorough and uniform mixing. To further enhance the formation of polycrystalline material, two pre-sintering processes were performed in a muffle furnace. The first pre-sintering temperature was 900℃ for 36 hours. The pre-sintered polycrystalline material was then placed in an agate crucible. The powder was ground into powder and then pre-sintered at 1300℃ for 36 hours. After pre-sintering, the powder was placed in an iridium crucible and then grown in a growth furnace under a nitrogen atmosphere. The temperature was lowered from 1465℃, the pulling speed was 1 mm / h, and the rotation speed was 3 rpm. After more than 70 hours, a transparent blue-black SBN crystal was obtained. Then, it was annealed in a muffle furnace at 1200℃ for 24 hours. Finally, a light yellow or nearly colorless transparent SBN crystal was obtained.

[0040] We measured the transmittance of SBN crystals before and after annealing at different wavelengths, and the results are as follows: Figure 2 As shown, the transmittance of the SBN crystal after annealing (dashed line) is significantly higher than that before annealing (solid line). This indicates that annealing effectively improves the optical properties of the crystal, especially in the wavelength range of 400-500 nm. The transmittance after annealing rises rapidly from a lower value and is significantly higher than before annealing. This is mainly because the presence of oxygen vacancies before annealing gives the crystal a bluish-black color, resulting in strong absorption in the 400-500 nm wavelength band. Oxygen annealing effectively reduces oxygen vacancy defects, thus the improvement in transmittance in the 400-500 nm range is more significant.

[0041] Figure 3 (a) is the SBN crystal with a compositional gradient that we grew, with a length of 21 mm along the growth direction and a maximum diameter of 25 mm. (a) From the inside of the crystal along... Figure 3 (b) Cut a 10mm × 10mm × 1mm wafer in the direction shown in the diagram. Figure 3 As shown in (c), the compositional distribution of the surface was measured using an EPMA-8050G electron probe microanalysis device, yielding Sr... x Ba 1-x The compositional distribution of Nb₂O₆ crystals along the (100) and (001) directions ( Figure 4 The crystal composition does not change much in the (100) direction, but there is a compositional gradient in the growth direction (001). The Sr content gradually decreases along the growth direction (001). The calculated compositional gradient within the range of l = 10 mm in the (001) direction is Δx / l = 0.0015 mm. -1 .

[0042] Example 2: Growth of SBN crystals with a composition gradient (initial raw material molar ratio of SrCO3:BaCO3 = 0.65:0.35)

[0043] The crystal was grown using the Czochralski method. The molar ratio of the raw materials was SrCO3:BaCO3:Nb2O5 = 0.65:0.35:1, and the purity of the raw materials was 4N grade. The initial cooling temperature for crystal growth was 1490℃, and the annealing temperature was 1250℃. Other mixing, pre-sintering, crystal growth, and annealing process parameters were basically the same as in Example 1. A 10mm × 10mm × 1mm wafer was cut from the final crystal, and the composition gradient within the (001) direction l = 10mm was measured to be Δx / l = 0.0012mm. -1 .

[0044] Example 3: Growth of SBN crystals with a composition gradient (initial raw material molar ratio of SrCO3:BaCO3 = 0.79:0.21)

[0045] The crystal was grown using the Czochralski method. The molar ratio of the raw materials was SrCO3:BaCO3:Nb2O5 = 0.79:0.21:1, and the purity of the raw materials was 4N grade. The initial cooling temperature for crystal growth was 1460℃, and the annealing temperature was 1300℃. Other mixing, pre-sintering, crystal growth, and annealing process parameters were basically the same as in Example 1. A 10mm × 10mm × 1mm wafer was cut from the final crystal, and the composition gradient within the (001) direction l = 10mm was measured to be Δx / l = 0.001mm. -1 .

[0046] Example 4: Fabrication of SBN crystal ohmic contact electrode (Ag+Au bilayer electrode)

[0047] To reduce energy loss at the electrode contact surface and improve device performance and operational stability, an ohmic contact needs to be formed between the SBN crystal and the metal electrode. We chose a process of depositing a double-layer electrode on the surface of the SBN crystal, in which the inner electrode material in contact with the crystal is Ag, and the outer protective electrode material is Au. Both double-layer electrodes are deposited using magnetron sputtering technology.

[0048] A wafer with dimensions of 3.8 mm × 3 mm × 1 mm was fabricated from the virgin crystal (after annealing) obtained in Example 1. The two planes of the 3.8 mm × 3 mm wafer are the (001) planes. These two planes were first polished, and then electrodes were deposited. Before electrode deposition, the SBN crystal was ultrasonically cleaned and purged with nitrogen. A magnetron sputtering system equipped with dual targets was used for deposition. The crystal temperature was first heated to 70°C, and bias sputtering was turned on to remove impurities from the crystal surface. Then, DC sputtering was used to first sputter a 50 nm thick Ag metal film on the crystal surface, and then a 100 nm thick Au metal film to form a double-layer metal thin film electrode. The equipment parameters during sputtering were set as follows: argon flow rate 18 sccm, DC power supply power 400 W, and cavity pressure 0.9 Pa during sputtering. After sputtering, the sample was naturally cooled to room temperature. The sample was flipped over, and the other side was sputtered using the same process to obtain an SBN optical element with Ag+Au electrodes deposited on both sides.

[0049] To further optimize the contact characteristics between the electrode and the semiconductor material and reduce the contact resistance, we performed a rapid annealing process on the sputtered SBN wafer. Using a Nobadi NBD-HR1200-110TI rapid annealing furnace, the electrode-plated SBN crystal was placed in the furnace, and after evacuation, nitrogen gas was introduced. The heating time was 30 seconds, the annealing temperature was 450℃, and the annealing time was 8 minutes. After annealing, the wafer was allowed to cool naturally to room temperature to form an electro-optical element with ohmic contact electrodes.

[0050] Figure 5 The figures show the IV characteristic curves of SBN crystals before and after secondary annealing. Generally, if the IV curves exhibit linear characteristics, it is considered an ohmic contact; if they exhibit nonlinear characteristics, it is considered a Schottky contact. From... Figure 5 It can be seen that before annealing, when the applied voltage is small, the IV curve is nonlinear, and then gradually becomes approximately linear as the voltage increases. After annealing, the IV curve is linear throughout the entire range. It is evident that rapid annealing significantly improves the ohmic contact characteristics between the SBN crystal and the electrode.

[0051] Example 5: Fabrication of SBN crystal ohmic contact electrode (Ti+Au bilayer electrode)

[0052] First, wafers of the same size and crystal orientation were fabricated using the same method as in Example 4. Then, a 150 nm thick Ti metal film was deposited on the surface of the SBN crystal using magnetron sputtering, followed by a 120 nm thick Au metal film. Other sputtering process parameters were the same as in Example 4. The sputtered SBN wafers were then subjected to rapid annealing at 500°C for 10 minutes, with a heating time of 20 seconds. The annealing atmosphere was argon, and other annealing parameters were the same as in Example 4. The IV characteristic curves before and after annealing were compared, as shown below. Figure 6The IV curve after annealing exhibits linear characteristics, indicating that a relatively reliable ohmic contact electrode is formed after rapid annealing.

[0053] Example 6: Fabrication of SBN crystal ohmic contact electrode (Al+Pt bilayer electrode)

[0054] First, wafers of the same size and crystal orientation were fabricated using the same method as in Example 4. Then, a 100 nm thick Al metal film was deposited on the surface of the SBN crystal using magnetron sputtering, followed by a 200 nm thick Pt metal film. Other sputtering process parameters were the same as in Example 4. The sputtered SBN wafers were then subjected to rapid annealing at 300°C for 5 minutes, with a heating time of 5 seconds. The annealing atmosphere was argon, and other annealing parameters were the same as in Example 4. The IV characteristic curves before and after annealing were compared, as shown below. Figure 7 The IV curve after annealing exhibits linear characteristics, indicating that a relatively reliable ohmic contact electrode is formed after rapid annealing.

[0055] Example 7: Electro-optic deflection based on SBN composition gradient (using a composition gradient of 0.0015 mm) -1 (crystal)

[0056] SBN electro-optic deflection experimental setup, such as Figure 8 As shown in (a), a wafer with dimensions of 3.8 mm × 3 mm × 1 mm was processed from the native crystal (after annealing) obtained in Example 1. Figure 8 b) Its component gradient is 0.0015 mm. -1 Using the method described in Example 4, Ag+Au double-layer electrodes were deposited on two (001) surfaces, and then their dielectric temperature spectra were measured. Figure 9 The effective Curie temperature was found to be Tc = 65℃. A TEC temperature control device was used to maintain the crystal temperature 4 degrees below the Curie point, i.e., 61℃. The incident light was a 632.8nm He-Ne laser, which was polarized by a polarizer into vertically polarized light along the

[001] direction and incident perpendicularly to the crystal (100) plane. The voltage signal generated by the signal generator was amplified by a high-voltage amplifier and applied to the crystal along the z-direction. Changing the voltage applied to the crystal allowed the light spot to move on the receiving screen. A beam quality analyzer (Thorlabs PDA100A2) was used to record the position of the light spot under different voltages. Figure 10 The distance from the crystal's output end to the beam quality analyzer's receiving surface is 50mm. From this, the deflection angle under different voltages can be calculated, such as... Figure 11The midpoint curve represents the measured and calculated results. It can be seen that the electro-optic deflection angle increases approximately parabolically with increasing applied voltage. When the voltage reaches 800 V / mm, the deflection angle reaches saturation (maximum deflection angle is 10 mrad). Further increasing the voltage will actually decrease the deflection angle. This is because unpolarized SBN crystals are easily polarized into single-domain crystals under applied voltage, causing the secondary electro-optic effect to transform into a linear electro-optic effect, leading to a decrease in the electro-optic deflection angle. To avoid deflection angle attenuation caused by high-voltage polarization, the maximum deflection voltage for electro-optic deflection should be less than 800 V / mm.

[0057] Based on the deflection angle of the SBN crystal at 800V being 9.6mrad, we can calculate its deflection efficiency to be 12μrad / V, which is nearly two orders of magnitude higher than the deflection efficiency of 0.312μrad / V of the four-electrode structure deflector proposed by Zhao Zhuo et al. in the background art.

[0058] Example 8: Electro-optic deflection based on SBN composition gradient (using a composition gradient of 0.0012 mm) -1 (crystal)

[0059] A wafer with dimensions of 5.6 mm × 3 mm × 1.5 mm and a compositional gradient of 0.0012 mm was fabricated from the primary crystal (after annealing) obtained in Example 2. -1 Al+Pt double-layer electrodes were deposited on two (001) surfaces using the method of Example 6. The effective Curie temperature was obtained by measuring its dielectric temperature spectrum as Tc = 69.4℃. The crystal temperature was controlled at 64.4℃, 5 degrees below the Curie point, using a TEC temperature control device. Other test conditions and methods were the same as in Example 7. Based on the measurement results, the maximum deflection angle was calculated to be 6.5mrad (@ maximum deflection voltage of 760V / mm).

[0060] Example 9: Electro-optic deflection based on SBN composition gradient (using a composition gradient of 0.001 mm) -1 (crystal)

[0061] A wafer with dimensions of 6 mm × 3 mm × 2.2 mm and a compositional gradient of 0.001 mm was fabricated from the primary crystal (after annealing) obtained in Example 3. -1 Using the method of Example 6, Ti+Au double-layer electrodes were deposited on two (001) surfaces. Then, by measuring its dielectric temperature spectrum, its effective Curie temperature was found to be Tc = 58.1℃. The crystal temperature was controlled at 3 degrees below the Curie point, i.e., 55.1℃, using a TEC temperature control device. Other test conditions and methods were the same as in Example 7. Based on the measurement results, its maximum deflection angle was calculated to be 2mrad (@ maximum deflection voltage of 720V / mm).

[0062] Example 10: Experimental verification of the electro-optic effect type of unpolarized SBN crystal

[0063] Structurally, tetragonal SBN crystals are ferroelectric phases with a 4mm point group. It is generally believed that the electro-optic effect of tetragonal SBN crystals exhibits a linear electro-optic effect. However, due to the presence of ferroelectric domains, polarization is usually required to obtain crystals with a large linear electro-optic coefficient. High-voltage polarization is also a routine operation for most crystals with ferroelectric domains (such as lithium niobate) before they are used as electro-optic crystals. Since the electro-optic effect of unpolarized ferroelectric crystals is usually not obvious, their electro-optic performance before polarization is generally not a focus. However, we found a relatively obvious electro-optic deflection in unpolarized SBN crystals. To further prove that this effect is a secondary electro-optic effect rather than a nonlinear electro-optic effect, we constructed a Mach-Zehnder interferometry system using the electro-optic modulation characteristics of SBN crystals. Figure 12 By comparing the frequency relationship between the input voltage signal and the output modulation signal, it can be determined whether the deflection is caused by a linear electro-optic effect or a secondary electro-optic effect.

[0064] exist Figure 12 In the experiment, the laser beam is split into two beams after passing through beam splitter 1. One beam experiences a phase delay through the crystal and interferes with the other beam at beam splitter 2. Changing the voltage applied to the SBN crystal causes the interference fringes to change in brightness. The midpoint between the bright and dark fringes is taken as the observation point. In the experiment, a small AC signal voltage of a certain frequency is output by a signal generator, which is then amplified by a high-voltage amplifier and applied to the crystal. The intensity of the interference light obtained after passing through BS2 can be recorded by an oscilloscope along with the signal output from the signal generator after passing through a photodetector. According to the conclusions given by Aillerie M et al. (Aillerie M, Theofanous N, Fontana MD. Measurement of the electro-optic coefficients: description and comparison of the experimental techniques[J]. Applied Physics B, 2000, 70: 317-334), if the crystal exhibits a secondary electro-optic effect, the relationship between the waveform of the output light intensity recorded by the photodetector and the change of the applied voltage (i.e., the voltage applied to the crystal after the signal output from the signal generator is amplified) follows the following formula:

[0065]

[0066] It can be seen that the intensity of the interference light under the action of the secondary electro-optic effect is proportional to the square of the voltage.

[0067] If the crystal exhibits a linear electro-optic effect, then: v out ∝E,(2)

[0068] That is, the intensity of the interference light under the linear electro-optic effect is proportional to the voltage.

[0069] Figure 13 The waveforms of the applied voltage and interference light intensity recorded by the oscilloscope are used to determine the electro-optic effect. According to formulas (1) and (2), if the frequencies of the two waveforms are equal, it should be a linear electro-optic effect; if the waveforms exhibit a harmonic relationship, it should be a quadratic electro-optic effect. Figure 13 It can be seen that the waveform of the output light intensity recorded by the photodetector and the waveform of the applied voltage (i.e., the waveform of the signal generator) exhibit a frequency doubling relationship (i.e., the frequency doubles and the period is halved), proving that the unpolarized SBN crystal used here has a secondary electro-optic effect.

[0070] Comparative Example 1: Comparison of deflection angles between electro-optic deflection based on the linear electro-optic effect of SBN crystal and electro-optic deflection based on its secondary electro-optic effect (theory combined with experiments)

[0071] As mentioned in Example 8 above, "when the secondary electro-optic effect of the SBN crystal transforms into a linear electro-optic effect, it leads to a decrease in the electro-optic deflection angle." This is based on our understanding... Figure 11 Based on the experimental results, we will now derive and calculate the deflection angles for these two types of deflection theoretically:

[0072] We know that when a beam of light propagates in a non-uniform medium, it always bends in the direction of increasing refractive index, such as... Figure 14 When a light beam enters the crystal along the x-direction, due to the compositional gradient in the z-direction, there will be a deflection angle θ0 (defined as intrinsic deflection) even without an applied voltage. According to the theory of refractive index gradient by AFDugan et al. (Dugan AF, Doyle WM, Sutton PM. KTN concentration gradient light beam deflector[J]. Applied Optics, 1968, 7(3): 556-1-558.), the relationship between the intrinsic deflection angle θ0 and the refractive index gradient can be expressed as follows:

[0073]

[0074] Where L is the length of the crystal in the direction of beam propagation. Let z be the refractive index gradient of the crystal at any position z.

[0075] When an external voltage is applied to a crystal, the crystal will undergo a change in refractive index under the influence of the electro-optic effect. If the crystal exhibits a second-order electro-optic effect, the change in refractive index can be expressed as follows:

[0076]

[0077] Where, n(z), s ij (z) represents the refractive index and second electro-optic coefficient of the crystal at any position z without voltage applied, respectively, and E is the electric field strength.

[0078] At this time, the beam deflection angle Δθ caused by the applied voltage can be expressed as (e.g.) Figure 13 )

[0079]

[0080] Substituting equation (4) into equation (5), we get E. It means that it has been obtained.

[0081]

[0082] If used separately and Let represent the refractive index gradient and electro-optic coefficient gradient along the growth direction of the crystal. When the polarization direction of the incident light is parallel to the electric field direction, the refractive index and second-order electro-optic coefficient at any position within the crystal can be expressed as , respectively.

[0083] n(z)=n0-n'z (7)

[0084] s ij (z)=s 11 -s'z (8)

[0085] Where n0 is the refractive index of the unusual light at z = 0, and s 11 The second-order electro-optic coefficient is at z = 0.

[0086] Therefore, substituting (7) and (8) into equation (6) yields...

[0087]

[0088] Among them, (n0-n'z) terms and (s 11 The terms -s'z) represent the refractive index and second electro-optic coefficient at different locations inside the crystal when no electric field is applied. When used in the calculation in equation (9), since n'z=n0 and s'z=s 11 Therefore, these two terms can be approximately expressed as (n0-n'z)≈n0, (s 11 -s'z)≈s 11 Therefore, (9) can be written as

[0089]

[0090] According to equation (10), to calculate the angle of electro-optic deflection, we need to know the refractive index n0 and the second electro-optic coefficient s at z = 0. 11 , as well as the refractive index gradient and the second electro-optic coefficient gradient of the crystal along the z-direction.

[0091] For the calculation of the second electro-optic coefficient, we start from... Figure 3 (a) Two 1×1×1mm wafers are cut from the upper and lower parts of the crystal, using... Figure 12 The Mach-Zehnder interferometer shown can be used to measure its second-order electro-optic coefficients, and the gradient value of the second-order electro-optic coefficients can be calculated: s' = 2 × 10⁻⁶. -15 m / v 2 In this process, we approximate the refractive index gradient of the large crystal as the same as that of the small crystal.

[0092] For the calculation of the refractive index gradient, we adopted the method used by Tunyagi A R. et al. (Tunyagi A R. Non-Collinear second harmonic generation in strontium barium niobate[J]. Unpublished doctoral dissertation, Osnabrück, Osnabrück, Germany, 2004., gives the Sellmeier equation for calculating the refractive index.

[0093]

[0094] Among them, the four coefficients A, B, C, and D of the Sellmeier equation are related to the component x of SBN:

[0095]

[0096] The wavelength of the incident light is λ = 632.8 nm, measured using an EPMA-8050G. Figure 8 (b) shows the upper and lower surfaces and the middle position of the crystal. Figure 13 The components at z = 0 are 0.6410, 0.6395, and 0.6403, respectively. The corresponding refractive indices calculated from equation (11) are 2.3036, 2.3034, and 2.3035, respectively. Therefore, the refractive index gradient of the crystal is n' = 1 × 10⁻⁶. -4 / mm, therefore, substituting the above data into equation (10), we can obtain the theoretical curve of the electro-optic deflection angle changing with the applied voltage, such as Figure 11As shown by the solid line at the top, when the voltage is less than 800V, the trend of the curve calculated by theory is basically consistent with the experimental results. Both curves increase parabolically with the increase of voltage. When the voltage is greater than 800V, the theoretical curve deviates from the experimental results. This is due to the polarization effect of high voltage on the internal domain structure of the crystal.

[0097] To further determine the effect of high-voltage polarization on the electro-optic effect of SBN, we... Figure 8 The crystal in (b) underwent polarization treatment. Using a single-channel polarimeter (BALAB SPD10KV), the SBN crystal was polarized for 30 min at a DC electric field of 9KV / cm at a temperature 3°C above the Curie temperature, followed by slow cooling to room temperature, thus preparing a single-domain SBN crystal. Figure 11 The Mach-Zehnder experimental setup shown illustrates how the change in interference light intensity with applied voltage reveals the disappearance of the frequency doubling phenomenon. Both exhibit the same periodic variation pattern, proving that this is a linear electro-optic effect. That is, during the process of the applied voltage causing the tetragonal SBN crystal to transform from a disordered domain structure orientation state to a single-domain state, the crystal also transitions from a secondary electro-optic effect to a linear electro-optic effect.

[0098] We then theoretically derived the expression for the electro-optic deflection angle based on the linear electro-optic effect (the method is similar to that of formula (10)):

[0099]

[0100] Where, γ 33 γ represents the linear electro-optic coefficient of the crystal at z = 0, and γ' represents the gradient of the linear electro-optic coefficient. 33 Typical values ​​for γ and γ' are taken, referring to the linear electro-optic coefficient values ​​of different components that have been measured in relevant literature. 33 =520pm / v,γ'=9×10 -12 Substituting / v into equation (13), we obtain that when the applied voltage is 800V, the electro-optic deflection angle caused by the linear electro-optic effect is 0.018mrad. This value is much smaller than the deflection angle (9.6mrad) caused by the secondary electro-optic effect in Example 1. This proves that the deflection of the unpolarized SBN crystal at low voltage is mainly due to the secondary electro-optic effect of the crystal, and also proves that the decrease in the deflection angle at high voltage is caused by the crystal changing from the secondary electro-optic effect to the linear electro-optic effect.

Claims

1. A method for optical deflection based on barium strontium niobate crystals with gradient compositions, characterized in that, The method includes the following steps: (1) Sr with a composition gradient was grown using the Czochralski method. x Ba 1-x The initial molar ratio of Nb₂O₆ crystals is in the range of 0.65:0.35 < SrCO₃:BaCO₃ <0.79:0.

21. The composition gradient along the growth direction ([001]) should be greater than 10. -3 / mm; (2) The SBN crystal with composition gradient is processed into a cuboid wafer and optically polished. Each surface corresponds to the (100), (010) and (001) planes of the crystal, respectively. A double-layer metal electrode is deposited on the two (001) planes. The electrode material in contact with the crystal is one of Al, Ti or Ag, and the electrode material in contact with the air is one of Au or Pt. The electrode and the crystal should form an ohmic contact. (3) Light is incident perpendicular to the (100) or (010) plane of the crystal. Before the light beam enters the crystal, a polarizer is used to convert the incident light into linearly polarized light polarized along the [001] direction. An electric field is applied to the SBN crystal along the [001] direction, and the light beam can be deflected at the crystal exit end.

2. The optical deflection method based on barium strontium niobate crystals with gradient composition according to claim 1, characterized in that: When the SBN crystal is used as an electro-optic deflection element, its temperature is controlled within a range of 3 to 5°C below the effective Curie temperature.

3. The optical deflection method based on barium strontium niobate crystals with gradient composition according to claim 1, characterized in that: SBN crystals grown by the Czochralski method need to be annealed in a high-temperature oxygen atmosphere. The annealing temperature is 1200℃-1300℃, the holding time is 12-24 hours, and the heating and cooling rate is 100-200℃ / h.

4. The optical deflection method based on barium strontium niobate crystals with gradient composition according to claim 1, characterized in that: The SBN crystal is tetragonal and cannot be subjected to high-voltage polarization when used for electro-optic deflection.

5. The optical deflection method based on barium strontium niobate crystals with gradient composition according to claim 1, characterized in that: The maximum deflection voltage applied to the SBN crystal should be less than 800V / mm.

6. The optical deflection method based on barium strontium niobate crystals with gradient composition according to claim 1, characterized in that: Both double-layer metal electrodes are deposited on the surface using magnetron sputtering. The thickness of the inner electrode is 50-150 nm, and the thickness of the outer electrode is 100-200 nm.

7. The optical deflection method based on barium strontium niobate crystals with gradient composition according to claim 1, characterized in that: After the metal electrode is sputtered, it needs to be rapidly annealed. The annealing temperature is 300-500℃, the holding time is 5-10 minutes, the heating time is 5-30 seconds, and the annealing atmosphere is nitrogen or argon.

8. An electro-optic deflection device using the optical deflection method as described in any one of claims 1-7.

Images