Preparation method of kbbf family nonlinear optical crystal optical coupling device and device

Abstract

The application discloses a preparation method of an optical coupling device of a KBBF family nonlinear optical crystal, the coupling device comprising a first prism, a second prism and a KBBF family nonlinear optical crystal therebetween, the method comprising a first step and a second step, the first step comprising fixing the first prism by using a first clamp in a vacuum bonding device, positioning the crystal, and performing surface activation treatment on each bonding surface under vacuum conditions, and then performing normal-temperature in-situ pressure bonding on the activated bonding surfaces; the second step comprising positioning the second prism in the vacuum bonding device, performing surface activation treatment on each bonding surface under vacuum conditions, and then performing normal-temperature in-situ pressure bonding on the activated bonding surfaces, so that the second prism is bonded to the crystal, and the optical coupling device is obtained. The preparation method can completely eliminate microscopic surface defects such as interface bubbles, improve the temperature stability of the device, and increase the conversion efficiency by 2-4 times.

Description

Technical Field

[0001] This invention relates to the field of optical crystal coupling device fabrication technology. More specifically, it relates to a method for fabricating an optical coupling device using a KBBF family nonlinear optical crystal and the optical coupling device fabricated based on this method. Background Technology

[0002] Frequency doubling and sum-frequency conversion of fundamental frequency lasers using nonlinear optical crystals is currently a highly efficient mainstream technology for generating ultraviolet and deep ultraviolet lasers. To achieve phase matching and optimal conversion efficiency, the nonlinear optical crystal needs to be oriented and fabricated into a functional device along the phase-matching direction. However, some deep ultraviolet nonlinear optical crystals face significant challenges in oriented phase-matching cutting and fabrication, with KBBF family crystals being the most typical example, primarily including KBe₂BO₃F₂ (KBBF) and RbBe₂BO₃F₂ (RBBF). These crystals exhibit extremely strong layered growth characteristics, with a typical c-axis growth thickness of only 2-3 mm, and the c-plane is easily dissociated, making it impossible to cut and fabricate the device along the required phase-matching direction. To circumvent the crystal orientation cutting challenge, existing technologies employ oblique incidence of the fundamental frequency light, allowing the light to be refracted by the crystal and propagated within the crystal along the phase-matching direction, thereby achieving phase matching. However, this method has inherent wavelength threshold limitations: the shortest wavelength of the fundamental frequency light for phase matching by oblique incidence of KBBF crystal is 235 nm. Fundamental frequency light with wavelengths less than 235 nm cannot achieve phase matching through oblique incidence, while the shortest matching wavelength for RBBF crystal is 255 nm. The core application advantage of KBBF family crystals is deep ultraviolet laser frequency conversion. To achieve deep ultraviolet laser output with wavelengths less than 235 nm from thin-film KBBF family crystals, Chen Chuangtian et al. proposed prism coupling technology (see CN1381930A), which effectively broke through the bottleneck of phase matching technology in this deep ultraviolet band. The core points of this technology are: optically adhesive-fixed prism structures on two large faces of the thin-film KBBF family crystal, the prisms are made of fused silica or calcium fluoride crystal materials with refractive indices matching those of the KBBF family crystal, and the prism apex is custom-cut according to the crystal phase matching direction. This coupling device achieves effective phase matching without oblique cutting of the KBBF family crystal, enabling various deep ultraviolet laser outputs, including a neodymium ion laser at frequency six times (177.3 nm) and a Ti:sapphire laser at frequency four times (175-235 nm) with wide tuning. Furthermore, Chinese patent document CN102768450A discloses another KBBF family crystal prism coupling optical device and its fabrication method. This fabrication process mainly includes five core steps: ultra-smooth precision optical polishing, surface coating, mechanical finishing, pressure baking, and deep photoresist application.

[0003] However, the existing prism coupling fabrication techniques still have significant limitations. KBBF crystals are soft crystals with a Mohs hardness of less than 4. Their ultra-smooth precision polishing requires stringent processing parameters, including a root mean square roughness Ra < 0.5 nm and a surface accuracy better than λ / 8 (where λ is the incident light wavelength), making processing extremely difficult. Furthermore, the surface of KBBF crystals naturally contains micro-pits, making it difficult to achieve ultra-high surface finish. During the photoresist bonding process between the crystal and the prism, air is easily trapped within these micro-pits, causing bubble defects at the photoresist interface. When such devices are applied in laser frequency conversion scenarios, these interface bubbles absorb laser irradiation energy, resulting in a temperature rise, which is particularly pronounced under high-power laser operating conditions. The thermal expansion of these bubbles continuously damages the photoresist interface, potentially leading to device failure. Moreover, the shorter the laser wavelength, the more significant the device degradation caused by these interface defects becomes, severely limiting the service performance and lifespan of KBBF deep ultraviolet laser devices.

[0004] In addition, existing coupling devices mostly employ traditional bonding processes such as photoresist bonding, organic adhesive layers, dielectric transition layers, or high-temperature bonding to achieve structural bonding, which generally suffer from poor process stability and insufficient reliability. Among these, the thermal resistance characteristics of photoresist interfaces, organic adhesive layers, and dielectric transition layers are discrete and uncontrollable, making them prone to aging and degeneration under conditions such as thermal cycling and high-power laser irradiation, which can induce interface cracking and void defects, resulting in device performance degradation and a significant reduction in lifespan. High-temperature bonding processes require high-temperature annealing, which can easily generate residual thermal stress that is difficult to eliminate within the crystal-prism coupling structure, causing problems such as device surface distortion and deterioration of refractive index uniformity, severely degrading optical uniformity and laser frequency conversion output performance. Summary of the Invention

[0005] To address the aforementioned problems, one aspect of the present invention provides a method for fabricating a KBBF-family nonlinear optical crystal optical coupling device, wherein the KBBF-family nonlinear crystal optical coupling device includes a first prism, a second prism, and a KBBF-family nonlinear optical crystal therebetween, and the method includes a first bonding step and a second bonding step. The first bonding step includes The first prism and the crystal are placed in a vacuum bonding apparatus. The first prism is fixed to the first bonding rod of the vacuum bonding apparatus using a first clamp, and the crystal is positioned on a second bonding rod. The bonding surfaces of the first prism and the crystal are opposite each other. Under vacuum conditions, Plasma surface activation treatment was performed on the bonding surfaces of the first prism and the crystal, respectively. The activated first prism surface to be bonded and the crystal surface to be bonded are subjected to room temperature in-situ pressure bonding, so that the crystal is bonded to the first prism and the other surface of the crystal to be bonded is exposed. The second bonding step includes The second prism is placed in the vacuum bonding apparatus, and the second prism is positioned on the second bonding rod using a third clamp, with the bonding surface of the second prism facing the other bonding surface of the crystal. Under vacuum conditions, Plasma surface activation treatment was performed on the bonding surface of the second prism and the other bonding surface of the crystal, respectively. The activated second prism surface to be bonded and the other surface to be bonded of the crystal are subjected to room temperature in-situ pressure bonding, so that the second prism is bonded to the crystal, thereby obtaining the crystal optical coupling device.

[0006] Optionally, the first prism and the second prism are CaF2 prisms or SiO2 prisms, respectively; or The KBBF family crystal is a KBBF crystal, and the first prism and the second prism are BaF2 crystal prisms, respectively.

[0007] Optionally, the method further includes Before the first bonding step, the crystal, the first prism and the second prism are irradiated with plasma to obtain the etching rate of various surfaces to be bonded, and the surface activation treatment parameters of each surface to be bonded are determined based on the etching rate.

[0008] Optionally, each surface to be bonded is subjected to surface activation treatment to remove 0.1 ~ 10 nm of surface protrusions from the surface to be bonded, thereby obtaining a surface to be bonded with dangling bonds.

[0009] Optionally, the surface finish of the crystal to be bonded is better than 40 / 20, and the root mean square roughness Ra ≤ 10 nm.

[0010] Optionally, the surface activation treatment parameters for each surface to be bonded are: voltage of 100~5000 V, current of 1~100 mA, and irradiation time of 0.1~30 min.

[0011] Optionally, the ambient temperature in-situ pressure bonding includes applying a bonding pressure of 50–1000 N using the mechanical transmission module in the vacuum bonding equipment and maintaining it for more than 30 minutes.

[0012] Optionally, the in-situ pressure is applied perpendicular to the two surfaces of the crystal to be bonded.

[0013] Optionally, the first prism and the second prism are characterized by being right-angled prisms, equilateral triangular prisms, or scalene triangular prisms. The method further includes using clamps to fix the first prism and the second prism respectively, so as to expose the bonding surfaces of each prism.

[0014] Another aspect of the present invention provides an optical coupling device made using a KBBF family nonlinear optical crystal prepared by the method described above. The coupling device consists of a first prism, a KBBF family nonlinear optical crystal, and a second prism bonded together in sequence, with the c-plane of the KBBF family optical crystal being the bonding surface.

[0015] Beneficial effects of the present invention This invention provides a method for fabricating a KBBF family nonlinear crystal optical coupling device based on the principle of surface activation. The method involves two bonding steps: plasma activation treatment of the KBBF family thin-film crystal and the bonding surfaces of the prism to be bonded, followed by in-situ room temperature pressure bonding, to obtain an optical coupling device comprising two room temperature pressure bonded heterojunction interfaces.

[0016] This invention provides a method for fabricating optical coupling devices based on the principle of surface activation bonding, which effectively solves many defects of existing technologies. In a vacuum environment at room temperature, this invention utilizes plasma to activate the bonding surfaces of a KBBF group crystal and a prism, eliminating the need for surface transfer and high-temperature annealing processes. In-situ pressure bonding is then directly applied to the activated surfaces. This process completely eliminates microscopic surface defects such as interface bubbles, significantly improving the temperature stability of the device under laser frequency conversion conditions. No oxide interlayers are formed at the bonding interface, effectively suppressing light scattering and absorption losses caused by these interlayers. Furthermore, the room-temperature activation and bonding process avoids the problems of interfacial thermal strain and insufficient device strength caused by the difference in thermal conductivity between the KBBF group crystal and the prism material during high and low temperature cycling processes. The optical coupling devices fabricated using this invention can improve the frequency doubling conversion efficiency from approximately 1‰ of traditional photoresist devices to 4‰, representing a 2-4 fold increase in conversion efficiency.

[0017] Meanwhile, this invention reduces the stringent requirements of traditional photoresist processes for ultra-precision polishing of KBBF crystals through plasma surface activation modification, thereby reducing the difficulty of device fabrication and improving the yield and mass production stability of optical coupling devices. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart illustrating the preparation method of an embodiment of the present invention is shown; Figure 2A-2C A schematic diagram of the bonding process according to an embodiment of the present invention is shown; Figure 3 A schematic diagram of an optical coupling device according to an embodiment of the present invention is shown.

[0020] in 100 - Vacuum bonding cavity; 101 - Upper bonding rod; 102 - Lower bonding rod; 200 - Optical coupling device; 210 - First prism; 220 - KBBF crystal; 230 - Second prism; 212 - First bonding interface; 223 - Second bonding interface; 301 - First clamp; 302 - Second clamp; 303 - Third clamp. Detailed Implementation

[0021] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further explains the invention. Similar components in the drawings are indicated by the same reference numerals. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0022] Embodiments of the present invention provide a method for fabricating a KBBF family crystal optical coupling device, wherein the fabricated KBBF family crystal optical coupling device is as follows: Figure 3 As shown, the optical coupling device 200 includes a first prism 210, a KBBF family nonlinear optical crystal 220, and a second prism 230 arranged sequentially along the optical path.

[0023] In this invention, the KBBF family of nonlinear optical crystals mainly includes KBe₂BO₃F₂ (KBBF) and RbBe₂BO₃F₂ (RBBF). The KBBF family crystal is in the form of a thin sheet, with parallel surfaces designated as the c-plane, serving as the first and second bonding surfaces. The shape of the KBBF family crystal can be rectangular, square, triangular, trapezoidal, etc., and this invention does not impose any limitations.

[0024] The preparation method of the present invention and the resulting optical coupler are described below using KBBF crystal as an example.

[0025] In this invention, the first prism and the second prism are selected from optical crystal prisms and optical glass prisms, respectively, and each prism provides a bonding surface. For example, if the nonlinear optical crystal is a KBBF crystal, the first prism and the second prism can be selected from CaF2 crystal prisms and SiO2 prisms, respectively; or if the nonlinear optical crystal is a KBBF crystal, both the first prism and the second prism are BaF2 crystal prisms. That is, the optical coupling device of this invention includes combinations such as KBBF group crystals with calcium fluoride prisms, RBBF group crystals with calcium fluoride prisms, KBBF group crystals with quartz / calcium fluoride prisms, and KBBF crystals with BaF2 prisms. The optical coupling device includes two pressure bonding interfaces between the nonlinear optical crystal and the prism located on both sides of the nonlinear optical crystal.

[0026] In this invention, both the first prism and the second prism are right-angle prisms or triangular prisms. The prism angles are designed according to the requirements of the matching angle of nonlinear optical crystals.

[0027] like Figure 1 As shown, the fabrication method of the KBBF family nonlinear optical crystal optical coupling device in this embodiment of the invention includes a first bonding step S100 and a second bonding step S200.

[0028] The first bonding step S100 includes, S110, the first prism and the crystal are placed in a vacuum bonding apparatus. The first prism is fixed using a first clamp, and the crystal is positioned so that the bonding surfaces of the first prism and the crystal are opposite each other. The vacuum bonding apparatus is evacuated, and the c-face of the crystal is the bonding surface. S120, plasma surface activation treatment is performed on the bonding surface of the first prism and the bonding surface of the crystal, respectively.

[0029] S130, the activated first prism surface to be bonded and the crystal surface to be bonded are subjected to room temperature in-situ pressure bonding, so that the crystal is bonded to the first prism and the other surface to be bonded of the crystal is exposed.

[0030] The second bonding step S200 includes, S210, Place the second prism in the vacuum bonding apparatus, position the second prism so that the surface of the second prism to be bonded is opposite to the other surface of the crystal to be bonded, and evacuate the vacuum bonding apparatus. S220, perform surface activation treatment on the bonding surface of the second prism and the other bonding surface of the crystal, respectively; S230, the activated second prism surface to be bonded and the other surface to be bonded of the crystal are subjected to room temperature in-situ pressure bonding, so that the second prism is bonded to the crystal, thereby obtaining the crystal optical coupling device.

[0031] Before placing the KBBF family crystal and prism into the vacuum equipment, the crystal and prism need to undergo pretreatment such as polishing and cleaning.

[0032] The polishing pretreatment of this invention refers to the surface processing of the KBBF crystal and prism surfaces to be bonded, achieving a surface finish better than 40 / 20, a root mean square roughness Ra ≤ 20 nm, preferably Ra ≤ 10 nm, and free from scratches and pitting. Conventional optical coupling device fabrication processes require ultra-fine surface polishing of the KBBF crystal to achieve a surface finish better than 20 / 10 and a root mean square roughness Ra < 0.5 nm. This method not only demands high surface finish but also results in a crystal scrap rate exceeding 50% in order to meet these requirements. In the fabrication method of this invention, the pretreatment polishing is optical polishing, which significantly reduces the difficulty of pretreatment of the KBBF crystal surfaces to be bonded, significantly shortens the pretreatment polishing time, improves the pretreatment yield, and reduces the processing cost of the KBBF crystal and the manufacturing cost of the KBBF crystal optical coupling device.

[0033] In step S210, the first prism is fixed to the upper bonding rod 101 of the vacuum bonding cavity 100 of the vacuum bonding equipment using the first clamp, and the crystal is positioned on the lower bonding rod 102, with the bonding surface of the first prism and one bonding surface of the crystal facing each other.

[0034] The vacuum bonding apparatus of the present invention includes a vacuum chamber 100, a vacuum device (not shown), a plasma device (not shown), upper and lower bonding rods 101 and 102, and alignment and leveling modules (not shown) for adjusting the prisms and crystals on the upper and lower bonding rods, respectively, as well as a mechanical transmission mechanism (not shown). The vacuum conditions of the present invention refer to the vacuum chamber used for activation and bonding having a vacuum level lower than 1 × 10⁻⁶. -5 Vacuum degree in Pa.

[0035] The upper and lower bonding rods within the vacuum chamber have alignment and leveling functions. For example, by combining the alignment and leveling modules of the bonding equipment, the bonding rods can be adjusted to ensure that the KBBF crystal and the bonding surfaces of the prism are parallel and edge-aligned. As a specific example, laser collimators are used to align the bonding surfaces of the first prism and KBBF, the other bonding surface of the KBBF, and the bonding surface of the second prism.

[0036] The plasma surface activation treatment of this invention mainly employs rapid atomic bombardment. The atomic source used in rapid atomic bombardment is, for example, an inert gas, such as argon (Ar) or xenon (Xe). Electrons in a high-voltage current bombard the inert gas, generating gas ions, which are then focused and accelerated to form a high-energy ion beam. The charge carried by the ion beam is first neutralized, becoming a high-speed, neutral atomic beam, which then propels at high speed to collide with the surfaces to be bonded to KBBF and prisms, causing physical or chemical processes such as atomic-atom or atomic-molecular reactions, thereby altering the binding energies of corresponding elements such as Ca, F, K, Be, and B on the surfaces to be bonded to KBBF and prisms. A highly active atomically level surface is obtained by removing the passivation layer on the KBBF and calcium fluoride surfaces at the nanoscale.

[0037] Before the first and second bonding steps, plasma is used to irradiate the surfaces of the crystal material and the prism material to be bonded for different durations to obtain the etching rate of each surface. Based on the obtained etching rates, surface activation treatment parameters for each surface are determined. Specifically, the surface activation treatment parameters of this invention are: voltage of 100~5000 V, preferably 500~5000 V, 800~5000 V, 1000~5000 V, 1000~4000 V, 1000~3000 V, 800~3000 V; current of 1~100 mA, preferably 3~80 mA, 3~70 mA, 3~60 mA; and irradiation time of 0.1~30 min, preferably 0.5~25 min, 1~25 min; to remove 0.1~10 nm of surface protrusions from the surfaces to be bonded, while simultaneously obtaining a surface with dangling bonds.

[0038] The room-temperature in-situ pressure bonding of the present invention includes placing a KBBF crystal and a prism in a vacuum bonding chamber, and using, for example, clamps to align the bonding surfaces of a first prism, a KBBF crystal, and a second prism. Each clamp has a shape adapted to the shape of the first prism, the KBBF crystal, and the second prism. Specifically, the first clamp 301 has a shape corresponding to the first prism 210 and is used to align the bonding surface of the first prism with the bonding surface of the KBBF crystal. In the first bonding step, the first clamp 301 is used to fix the first prism 210 to the upper bonding rod 101 of the vacuum bonding device. The KBBF crystal can be placed on the second bonding rod using the second clamp 302 or directly without clamps, as long as the bonding surface of the KBBF crystal and the bonding surface of the first prism can be aligned and leveled through adjustment. Based on this, under vacuum conditions, for example using a mechanical transmission module, the activated KBBF is brought into contact with the bonding surface of the prism, and pressure is applied perpendicularly to the bonding surface via a bonding rod. The pressure range is controlled between 50 and 1000 N, preferably 50 to 800 N, 50 to 700 N, 50 to 600 N, or 50 to 500 N. This pressure is maintained at room temperature for at least 30 minutes, preferably at least 100 minutes, 200 minutes, or 300 minutes. The KBBF is then bonded to the first prism through the first bonding interface 212. By using a mechanical rotation module, one bonding rod is disengaged from the other, exposing the other bonding surface of the KBBF, thus completing the first bonding step.

[0039] The third clamp 303 has a shape corresponding to the second prism 230, used to align the bonding surface of the second prism with another exposed bonding surface of the KBBF crystal. In the second bonding step, the second prism is positioned on the lower bonding rod 102 of the vacuum bonding cavity, so that the bonding surface of the second prism is aligned with another exposed bonding surface of the KBBF, and alignment and leveling are achieved through adjustment. Based on this, under vacuum conditions, for example using a mechanical transmission module, the exposed bonding surface of the activated KBBF is brought into contact with the bonding surface of the second prism, and pressure is applied perpendicularly to the bonding surface. The pressure range is controlled between 80 and 300 N, and the pressure is maintained for more than 300 minutes at room temperature. The second prism is bonded to the KBBF through the second bonding interface 223 to obtain the KBBF optical coupling device. By using a mechanical rotation module, one bonding rod is disengaged from the other bonding rod. The first prism, KBBF, and second prism bonded together are separated from the first clamp to obtain the KBBF nonlinear optical crystal optical coupling device of the present invention.

[0040] This invention relates to a method for fabricating KBBF crystal optical couplers based on surface activation treatment. Plasma surface activation treatment is performed under vacuum conditions before bonding to remove a thickness of 0.1-10 nm from the surface to be bonded. This process can obtain a surface to be bonded that forms dangling bonds and improve the surface smoothness and root mean square roughness of the surface to be bonded. It also effectively reduces the number of bubbles in the formed bonding interface and improves the optical performance and high-temperature optical stability of the obtained optical coupler.

[0041] The method of this invention utilizes two bonding steps. In the first step, the vacuum equipment is opened, and clamps are used to fix the first prism and position the KBBF crystal. In the second step, only the second prism is positioned, thus providing an optical coupler with two heterogeneous bonding interfaces. In each bonding step, in-situ pressure is applied to bond two highly reactive atomic-level surfaces together, resulting in a high-strength heterogeneous interface. In this method, the other KBBF surface to be bonded is exposed at the end of the first bonding step. Opening the vacuum equipment to set up the second prism at this point does not affect the heterogeneous interface formed in the first bonding step. In the second bonding step, after the equipment is shut down, a new vacuum condition is provided for surface activation treatment and in-situ pressure bonding to form the second heterogeneous bonding interface. The KBBF nonlinear optical crystal optical coupler of this invention has no interlayers at either of the two heterogeneous bonding interfaces.

[0042] The two bonding steps of this invention, which combine surface activation treatment and in-situ pressure bonding to form the bonding interface between KBBF and the prism, effectively overcome the problem of poor mechanical properties of KBBF crystals, improve the mechanical strength of the bonding interface between KBBF and the prism, and enhance the mechanical performance of the crystal optical coupler. Compared with photoresist KBBF optical couplers, the pressure-bonded KBBF optical coupler based on surface activation treatment of this invention can operate stably for extended periods without the need for precision vibration damping and temperature control components.

[0043] The optical coupling device prepared by the method of this invention has no interlayer, no air gap, and few bubbles at the heterogeneous pressure bonding interface between the two prisms and the KBBF crystal. This overcomes the problems of existing adhesive coupling devices, such as the presence of adhesive layers or air gaps, which lead to easy aging of the adhesive layer, generation of bubbles, and debonding under long-term deep ultraviolet high-power laser irradiation, resulting in wavefront distortion, scattering loss, and reflection loss, and thus low overall transmittance and low frequency doubling conversion efficiency. The frequency doubling conversion efficiency of the KBBF optical coupling device of this invention is improved by 2-4 times.

[0044] This invention provides a method for fabricating KBBF nonlinear optical crystal optical coupling devices that is industrially feasible, simple in process, and has a high yield by utilizing two bonding steps and an in-situ pressure bonding process based on surface activation treatment in each bonding step.

[0045] The fabrication method of the optical coupling device of the present invention will be specifically described below with reference to specific embodiments.

[0046] Example 1 Example 1 provides a method for preparing a SiO2-KBBF-SiO2 prism coupling device at room temperature.

[0047] like Figure 3 As shown, the SiO2-KBBF-SiO2 prism optical coupling device 200 consists of a KBBF sheet crystal 220 and two right-angled triangular pyramidal fused silica SiO2 210 and 230. The KBBF crystal has dimensions, for example, 16×6×1.5mm, with the two 16×6mm c-faces serving as the bonding surfaces. The two SiO2 crystals have dimensions of 20mm×10mm×8mm and a vertex angle of 68.6°. The beveled surfaces of the SiO2 triangular pyramids are optically polished to serve as the bonding surfaces. The surface finish of each bonding surface is better than 40 / 20, the root mean square roughness Ra ≤ 10nm, the surface shape accuracy is better than λ / 6, and the surface is free of scratches and pits.

[0048] Perform the first bonding step. Turn on the vacuum bonding equipment. Using the first clamp 301, place the SiO2 first prism 210 into the high vacuum chamber and fix it to the upper bonding rod 101. Using the second clamp 302, or without clamps, place the KBBF crystal onto the lower bonding rod 102 within the vacuum chamber 100. Adjust the surface parallelism and edge alignment of the KBBF crystal and the first prism to be bonded. See [link to relevant documentation]. Figure 2A .

[0049] The vacuum level inside the vacuum chamber is 8×10 -3 Under Pa conditions, the surfaces to be bonded, including the first prism and KBBF, are subjected to plasma irradiation for surface activation. For example, an Ar particle beam with a voltage of 2300 V and a current of 20 mA is used, and the irradiation time for the SiO2 first prism is 5 min; the irradiation time for KBBF is 3 min. Subsequently, a bonding pressure of 120 N is applied in situ and maintained for 900 min. The pressure application direction is perpendicular to the c-plane of the crystal, i.e., perpendicular to the surfaces to be bonded. In situ pressure bonding is performed on the first prism and KBBF crystal, and the two activated surfaces to be bonded are bonded together to form the SiO2-KBBF first bonding interface 212, while simultaneously exposing the second surface to be bonded of the KBBF crystal.

[0050] Proceed to the second bonding step. Turn on the vacuum bonding equipment. Using the third clamp 303, place the SiO2 second prism 230 into the high vacuum chamber 100. Align the bonding surface of the second prism with the exposed bonding surface of the KBBF crystal, ensuring surface parallelism and edge alignment. (See...) Figure 2B .

[0051] The vacuum level inside the vacuum chamber is 8×10-3 Under Pa conditions, the exposed bonding surfaces of the KBBF crystal and the second prism are subjected to plasma irradiation for surface activation treatment. For example, using the same conditions as the surface activation treatment in the first bonding step, an Ar particle beam is formed with a voltage of 1700 V and a current of 10 mA. The irradiation time for the SiO2 second prism is 5 min, and the irradiation time for the KBBF is 3 min. Subsequently, a bonding pressure of 120 N is applied perpendicularly to the bonding surfaces and maintained for 900 min. The second prism and the exposed bonding surfaces of the KBBF crystal are then pressure bonded in situ, and the two activated bonding surfaces are bonded together to form a KBBF-SiO2 second bonding interface 223. This yields a SiO2-KBBF-SiO2 prism optical coupling device 200, see [reference needed]. Figure 2C .

[0052] This optical device is used to generate a 177nm frequency-doubled output with a frequency-doubled conversion efficiency of 3‰.

[0053] Example 2 Example 2 provides a method for preparing a CaF2-KBBF-CaF2 prism coupling device at room temperature.

[0054] The CaF2-KBBF-CaF2 prism optical coupling device consists of a KBBF sheet crystal and two right-angled triangular pyramidal CaF2 crystal prisms. The KBBF crystal has dimensions, for example, 16×6×1.5mm, with the two 16×6 mm c-facets serving as the bonding surfaces. The two triangular pyramidal prisms have right-angled sides of 26mm and a thickness of 7mm. The beveled surfaces of the CaF2 triangular pyramids are optically polished and cleaned to serve as the bonding surfaces. The KBBF bonding surfaces have a surface finish better than 40 / 20, are free of scratches and pits, have a root mean square roughness Ra ≤ 10nm, and a surface accuracy better than λ / 6. The CaF2 prism bonding surfaces, after optical polishing and cleaning, have a surface finish better than 20 / 10, a root mean square roughness Ra ≤ 10nm, and a surface accuracy better than λ / 8.

[0055] In the first bonding step, the vacuum bonding equipment is turned on, and the first CaF2 prism is placed in the high vacuum chamber using the first clamp and fixed on the upper bonding rod. The KBBF is placed on the lower bonding rod in the vacuum chamber using the second clamp or without clamps. The surfaces of the KBBF crystal and the first prism to be bonded are adjusted for surface parallelism and edge alignment.

[0056] Unlike Example 1, Example 2 was performed at a vacuum degree of 9×10⁻⁶. -3Under Pa conditions, surface activation treatment was performed by Ar particle beam irradiation with a voltage of 2000 V and a current of 20 mA. The irradiation time of the first prism of CaF2 was 6 min, and the irradiation time of KBBF was 4 min. Subsequently, a bonding pressure of 150 N was applied in situ perpendicular to the surfaces to be bonded and maintained for 800 min. The two activated surfaces to be bonded were bonded together to obtain the first bonding interface of CaF2-KBBF, while the second bonding surface of KBBF crystal was exposed.

[0057] Proceed to the second bonding step. Turn on the vacuum bonding equipment and use the third clamp to place the CaF2 second prism inside the high vacuum chamber. Align the surface of the second prism to be bonded with the exposed surface of the KBBF crystal, ensuring surface parallelism and edge alignment. Unlike Example 1, the vacuum level in Example 2 is 9 × 10⁻⁶. -3 Using a voltage of 2000 V and a current of 20 mA, the CaF2 second prism was irradiated for 6 min and the KBBF irradiation time was 4 min. A bonding pressure of 150 N was applied in situ perpendicular to the surfaces to be bonded and maintained for 800 min. The two activated surfaces to be bonded were bonded together to form the KBBF-CaF2 second bonding interface, thus obtaining the CaF2-KBBF-CaF2 prism optical coupling device.

[0058] This optical coupling device is used to generate a 177nm frequency-doubled output with a frequency-doubled conversion efficiency of 3.5‰.

[0059] Example 3 Example 3 provides a method for preparing a SiO2-KBBF-CaF2 prism coupling device at room temperature.

[0060] The SiO2-KBBF-CaF2 prism consists of a KBBF sheet crystal, a right-angled triangular pyramid of SiO2, and a right-angled triangular pyramid of CaF2 crystal. The KBBF crystal has dimensions, for example, 16×6×1.5 mm, with the two 16×6 mm c-facets serving as the bonding surfaces. The right-angled sides of the SiO2 and CaF2 crystals are 24 mm long and 5 mm thick, respectively. The beveled surfaces of the CaF2 and SiO2 triangular pyramids are optically polished and serve as the bonding surfaces. The surface finish of each bonding surface is better than 40 / 20, the root mean square roughness Ra ≤ 10 nm, and the surface is free of scratches and pits. In this embodiment, CaF2 is used as the first prism and SiO2 as the second prism for illustration.

[0061] First bonding step. Turn on the vacuum bonding equipment, use the first clamp to place the CaF2 first prism in the high vacuum chamber and fix it on the upper bonding rod. Use the second clamp or no clamp to place the KBBF on the lower bonding rod in the vacuum chamber, and adjust the surface parallelism and edge alignment of the KBBF crystal and the first prism to be bonded.

[0062] Example 3 at a vacuum degree of 9×10 -3 Under Pa conditions, surface activation treatment was performed by Ar particle beam irradiation with a voltage of 2300 V and a current of 12 mA. The irradiation time of the first prism of CaF2 was 7 min, and the irradiation time of KBBF was 4 min. Subsequently, a bonding pressure of 200 N was applied in situ perpendicular to the surfaces to be bonded and maintained for 600 min. The two activated surfaces to be bonded were bonded together to obtain the first bonding interface of CaF2-KBBF, while the second bonding surface of KBBF crystal was exposed.

[0063] The second bonding step. Turn on the vacuum bonding equipment. Using the third clamp, place the SiO2 second prism inside the high vacuum chamber. Align the bonding surface of the second prism with the exposed bonding surface of the KBBF crystal, ensuring surface parallelism and edge alignment. The vacuum level inside the chamber is 9 × 10⁻⁶. -3 Under Pa conditions, an Ar particle beam was formed using a voltage of 2300 V and a current of 20 mA, irradiating the SiO2 second prism for 8 min and the KBBF for 4 min. Subsequently, an in-situ pressure bond was performed on the exposed surfaces of the second prism and the KBBF crystal at a bonding pressure of 200 N for 600 min. The two activated surfaces bonded together, forming the KBBF-SiO2 second bonding interface. This yielded a CaF2-KBBF-SiO2 prism optical coupling device.

[0064] This optical coupling device is used to generate a 177nm frequency-doubled output with a frequency-doubled conversion efficiency of 4.0‰.

[0065] Comparative Example 1: Atomic Diffusion Thermal Bonding Control Experiment A comparative experiment on atomic diffusion thermal bonding was conducted using KBBF sheet crystals of the same batch and size as in Example 2, and two right-angled triangular pyramidal CaF2 prisms. All surfaces to be bonded underwent the same optical polishing and cleaning process as in Example 2. The KBBF crystal surface finish was better than 40 / 20, with no scratches or pits, a root mean square roughness Ra ≤ 10 nm, and a surface accuracy better than λ / 6. The CaF2 crystal surface finish was better than 20 / 10, with no scratches or pits, a root mean square roughness Ra ≤ 10 nm, and a surface accuracy better than λ / 8.

[0066] The KBBF crystal and the first CaF2 prism were placed in a high-temperature vacuum bonding furnace. The alignment accuracy of the component edges was adjusted to ensure contact between the KBBF and CaF2 surfaces to be bonded. The vacuum level inside the furnace was then evacuated to 1×10⁻⁶. - The temperature was increased to 400℃ at a rate of 5℃ / min and held for 2 hours without applying any bonding pressure. After the holding period, the temperature was allowed to cool naturally to room temperature, with a total cooling time of approximately 12 hours. Experimental results showed that the thermal conductivity of KBBF crystal along the c-axis was approximately 1.38 W / (m·K), while that of CaF2 crystal was approximately 10 W / (m·K), indicating a significant difference in thermal conductivity (>7 times). A large temperature gradient was generated at the interface during the cooling process. Simultaneously, the coefficient of thermal expansion of KBBF along the c-axis was 5.5 × 10⁻⁶. -6 The coefficient of thermal expansion of CaF2 is 18.9 × 10⁻⁶ K. -6 The thermal parameters of the two are mismatched, resulting in continuous interfacial thermal stress during heating and cooling. Combined with the typical layered crystal structure of KBBF crystal, its c-plane bonding force is weak and it is prone to cleavage cracking. Under thermal stress, the crystal shows multiple cracks and damage along the c-plane, and there is no bonding between the various components of the prism.

[0067] Comparative Example 2: Direct Pressing Control Experiment on Ultra-Smooth Surfaces A direct lamination control experiment on ultra-smooth surfaces was conducted using the same process as in Example 2, except for the surface activation step. Specifically, the same batch and size of KBBF sheet crystals and two right-angled triangular pyramidal CaF2 prisms were used. After the same polishing and cleaning, the KBBF crystals and the first CaF2 prism were placed in the vacuum chamber of the aforementioned vacuum bonding equipment. The edge alignment was adjusted to be consistent with Example 2, and the vacuum level was evacuated to 9 × 10⁻⁶. - ³ Pa, without any surface activation treatment, a bonding pressure of 150 Newtons was applied directly through the mechanical transmission module and maintained at 25℃ for 800 min; after the end, the mechanical transmission was removed from the crystal, no bonding phenomenon occurred, the crystal fell off, and the experiment was terminated.

[0068] Comparative Example 3: Activation Dose Control Experiment A control experiment was conducted using KBBF sheet crystals and right-angled triangular pyramidal CaF2 crystal prisms of the same batch and size as in Example 2. The optical polishing, vacuum degree, surface activation treatment voltage and current parameters, alignment accuracy, bonding pressure, and holding time were all completely consistent with those in Example 2. Only the surface activation irradiation time was adjusted: 10s for KBBF crystal surface irradiation and 30s for CaF2 prism surface irradiation. For fluoride and KBBF family crystals, a 2-5 nm thick layer of adsorbed hydrocarbon contaminants, a hydration layer, and a thin oxide passivation layer naturally exists on the surface. A corresponding sputtering dose is required to completely remove the surface contaminants. In this experimental group, the total particle flux corresponding to the irradiation duration did not reach the activation threshold for KBBF and CaF2. Only the loose physical adsorbates on the outermost layer could be removed, and continuous removal of the passivation layer was not possible. A continuous contaminant barrier layer remained on the surfaces to be bonded. Furthermore, as a layered soft crystal, KBBF has lower surface energy and a more stable passivation layer. 30 seconds of irradiation could not break the stable passivation state on its surface, resulting in both surfaces failing to meet the atomically clean bonding requirements. Subsequently, in-situ pressure bonding was performed on the two surfaces to be bonded after surface activation treatment. After the pressure was released, the sample showed local separation in the vacuum chamber. After the chamber was removed, KBBF and CaF2 completely detached without any effective bonding strength and could be easily separated by hand. Ultrasonic microscopy showed that there was no continuous bonding area at the interface, only a very small number of point contact traces, and no effective heterogeneous bonding interface was formed.

[0069] Comparative Example 4: Activation Dose Control Experiment A control experiment was conducted using KBBF sheet crystals and right-angled triangular pyramidal CaF2 crystal prisms of the same batch and size as in Example 2. Optical polishing, vacuum level, surface activation treatment voltage and current parameters, alignment accuracy, bonding pressure, and holding time were all identical to those in Example 2, with only the surface activation irradiation time adjusted: 5 minutes for the KBBF crystal surface and 9 minutes for the CaF2 prism surface. As the irradiation dose increased, the surface etching depth and lattice damage continued to increase. For CaF2 crystals, 9 minutes of irradiation caused non-uniform surface etching, forming nanoscale pits and protrusions, increasing the root mean square roughness of the surface, directly leading to a significant decrease in the actual contact area at the interface and introducing numerous structural defects. For layered soft crystals of the KBBF family with a Mohs hardness of less than 4, their sputtering resistance is far lower than that of CaF2. 5 minutes of irradiation caused preferential sputtering and stripping of light elements such as F and K on the surface, disrupting the stoichiometry of the crystal surface and inducing interlayer microcracks, fundamentally deteriorating the interfacial bonding performance. In addition, prolonged bombardment can cause sputtered atoms from the inner wall of the cavity to redeposit onto the sample surface, introducing foreign impurities and becoming the nucleation core for interfacial voids.

[0070] Subsequently, in-situ pressure bonding was performed on the two surfaces to be bonded after surface activation treatment. After the pressure was released, the sample exhibited weak adhesion and did not detach on its own. However, after being removed from the cavity, it delaminated under slight lateral force. Ultrasonic microscopy revealed a large number of diffusely distributed microvoids at the interface, with the effective bonding area accounting for less than 25%. Tensile testing showed that the average breaking force was only 0.7 N, far lower than the bonding strength of normally activated samples. Moreover, the fracture surface was entirely located in the KBBF subsurface damage layer, exhibiting typical cohesive failure characteristics, and the bonding interface completely separated.

[0071] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for fabricating an optical coupling device using a KBBF family nonlinear optical crystal, characterized in that, The KBBF family nonlinear crystal optical coupling device includes a first prism, a second prism, and a KBBF family nonlinear optical crystal therebetween; the method includes a first bonding step and a second bonding step. The first bonding step includes The first prism and the crystal are placed in a vacuum bonding apparatus. The first prism is fixed using a first clamp, and the crystal is positioned so that the bonding surfaces of the first prism and one of the bonding surfaces of the crystal are opposite each other. Under vacuum conditions, Plasma surface activation treatment was performed on the bonding surfaces of the first prism and the crystal, respectively. The activated first prism surface to be bonded and the crystal surface to be bonded are subjected to room temperature in-situ pressure bonding, so that the crystal is bonded to the first prism and the other surface of the crystal to be bonded is exposed. The second bonding step includes The second prism is placed in the vacuum bonding apparatus and positioned so that the surface of the second prism to be bonded is opposite to the other surface of the crystal to be bonded. Under vacuum conditions, Plasma surface activation treatment was performed on the bonding surface of the second prism and the other bonding surface of the crystal, respectively. The activated second prism surface to be bonded and the other surface to be bonded of the crystal are subjected to room temperature in-situ pressure bonding, so that the second prism is bonded to the crystal, thereby obtaining the crystal optical coupling device.

2. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 1, characterized in that, The first prism and the second prism are respectively CaF2 prisms or SiO2 prisms; or The KBBF family crystal is a KBBF crystal, and the first prism and the second prism are BaF2 crystal prisms, respectively.

3. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 1, characterized in that, The method further includes Before the first bonding step, the crystal, the first prism and the second prism are irradiated with plasma to obtain the etching rate of various surfaces to be bonded, and the surface activation treatment parameters of each surface to be bonded are determined based on the etching rate.

4. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 2, characterized in that, Each surface to be bonded is subjected to surface activation treatment to remove 0.1 ~ 10 nm of surface protrusions from the surface to be bonded, thereby obtaining a surface to be bonded with dangling bonds.

5. The method for fabricating the KBBF family crystal optical coupling device according to claim 1, characterized in that, The surface finish of the crystal to be bonded is better than 40 / 20, and the root mean square roughness Ra ≤ 10 nm.

6. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 4, characterized in that, The surface activation treatment parameters for each surface to be bonded are: voltage 100~5000 V, current 1~100 mA, and irradiation time 0.1~30 min.

7. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 1, characterized in that, The ambient temperature in-situ pressure bonding involves applying a bonding pressure of 50–1000 N using the mechanical transmission module in the vacuum bonding equipment and maintaining it for at least 30 minutes.

8. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 1, characterized in that, The in-situ pressure is applied perpendicular to the two surfaces of the crystal to be bonded.

9. The method for fabricating the KBBF family nonlinear optical crystal optical coupling device according to claim 1, characterized in that, The first prism and the second prism are right-angle prisms, equilateral triangular prisms, or scalene triangular prisms. The method further includes using clamps to fix the first prism and the second prism respectively, so as to expose the bonding surfaces of each prism.

10. An optical coupling device made of a KBBF family nonlinear optical crystal using the fabrication method according to any one of claims 1-9, characterized in that, The coupling device consists of a first prism, a KBBF family nonlinear optical crystal, and a second prism bonded together in sequence, with the c-plane of the KBBF family optical crystal being the bonding surface.

Images