Multilayer film with Faraday rotation and anti-reflection functions and preparation method thereof

Abstract

The invention discloses a multilayer film with Faraday rotation and anti-reflection functions and a preparation method thereof, and belongs to the technical field of magneto-optical devices. Tantalum pentoxide (Ta2O5) and cerium trifluoride (CeF3) are adopted as high and low refractive index film layer materials of the multilayer film, an eight-layer film structure is formed by alternate deposition on a silicon dioxide glass substrate, the structure design is (0.25 H < 0.25 > L < 0.25 > H < 0.25 > L < 0.25 > H < 0.25 > L < 0.25 > H < 0.25 > L), the thickness of each layer is lambda / 4n, and lambda is reference wavelength 1064nm; the multilayer film is prepared through a physical vapor deposition process by selecting an electron beam evaporator and combining specific parameter combination, firstly, SiO2 glass is cleaned with absolute ethyl alcohol and pure water, and then pre-melted CeF3 and Ta2O5 film materials and the SiO2 glass are placed in the electron beam evaporator to be subjected to multilayer film deposition to obtain the multilayer film.

Description

Technical Field

[0001] This invention belongs to the field of magneto-optical device technology, specifically relating to a multilayer film with both Faraday rotation and anti-reflection functions and its preparation method. Background Technology

[0002] With the rapid development of high-power laser systems, high-speed fiber optic communication, and precision optical sensing technologies, the performance requirements for core passive optical devices, including optical isolators, are becoming increasingly stringent. The core function of an optical isolator is to allow unidirectional transmission of optical signals while effectively suppressing interference and damage caused by reverse light. Its performance directly depends on the comprehensive performance of its internal core component, the magneto-optical material. An ideal magneto-optical material should simultaneously possess the following key characteristics: a high Faraday rotation angle, extremely low optical insertion loss, a high laser damage threshold, excellent environmental stability, and low manufacturing cost.

[0003] Currently, technological development in this industry mainly revolves around the exploration and optimization of novel magneto-optical materials, primarily falling into two categories. One category comprises bulk single-crystal materials, such as terbium gallium garnet (TGG) and terbium scandium aluminum garnet (TSAG). These materials, due to their high Wilderness constant and excellent optical quality, have long been considered standard components for high-performance optical isolators. Through meticulous design of the crystal composition, their Wilderness constant can be increased to over 200 rad·T⁻¹·m⁻¹, while maintaining optical losses below 0.1 dB / cm and laser damage resistance thresholds above 15 GW / cm², essentially meeting the stringent requirements of current high-power lasers and 5G optical communication systems. For example, Chinese patent CN114318537A, filed on December 31, 2021, discloses a terbium gallium garnet crystal and its growth method. The method involves placing a mixture in a high-temperature furnace and sintering it four times in sequence to fully synthesize polycrystalline materials, reduce the volatilization of gallium oxide during subsequent crystal growth, control the temperature gradient during crystal growth to ensure that no twisting occurs during crystal growth, effectively avoid the occurrence of crystal growth spirals, and obtain a large-size terbium gallium garnet crystal.

[0004] To adapt to the trend of device miniaturization and integration, researchers have developed another class of materials, such as bismuth (Bi)-doped rare-earth iron garnet (RIG) heterojunction films. These thin film materials are prepared by epitaxial growth techniques, can achieve high Faraday effects on specific substrates, and show potential for integration with planar optical waveguide circuits (PLCs). For example, Chinese patent CN114318536A, filed on December 30, 2021, discloses a bismuth-doped rare-earth iron garnet single-crystal thin film, its preparation method, and optical devices. During the deposition of single-crystal components, the crucible and single-crystal substrate are controlled to rotate in opposite directions, and their rotation directions are switched according to a set period. This can better homogenize the melt. In addition, the temperature difference of the melt at different positions inside the crucible is strictly controlled so that the temperature conditions are similar when the substrate grows at different heights in the melt. This provides sufficient conditions for layered growth and helps to eliminate the concentration changes caused by the consumption of single-crystal components in the melt near the single-crystal substrate during the growth process. This results in the growth of a single-crystal thin film with more uniform magneto-optical and optical properties, a large Faraday coefficient, uniform film quality, few surface defects, and less susceptibility to cracking.

[0005] Currently, magneto-optical devices mainly rely on high-cost single-crystal materials or noble metal doping. However, the growth of high-performance TGG and TSAG single crystals requires extremely high raw material purity, precise temperature field control, and a long growth cycle, resulting in extremely high raw material costs and energy consumption, leading to expensive finished products. Similarly, high-quality magneto-optical thin films typically require complex processes such as pulsed laser deposition (PLD) or metal-organic chemical vapor deposition (MOCVD). These methods place stringent demands on equipment investment and process control, limiting the large-scale application of these high-performance magneto-optical materials in cost-sensitive consumer optoelectronic products.

[0006] Secondly, for bulk crystals, their optical surfaces require precise polishing and anti-reflective coating before they can be used. This additional coating process not only increases complexity and cost but also introduces potential laser damage risks at the interface. Under high-power laser irradiation, the interface can become the source of heat accumulation and damage. Furthermore, the absorption of light by the bulk material itself, although small in absolute value, can still induce a significant thermal lensing effect at extremely high power, leading to beam quality degradation and affecting system performance.

[0007] Furthermore, traditional magneto-optical devices typically employ discrete component assembly, separating the magneto-optical crystal from anti-reflective coatings, polarizers, and other components. This discrete structure hinders the miniaturization and stability improvement of optical systems. While thin-film technology offers the possibility of integration, existing magneto-optical thin films often focus on achieving a single Faraday rotation function. Their inherent surface reflection introduces insertion loss, and to achieve low loss, additional anti-reflective structures are still required, which further increases the complexity of design and fabrication.

[0008] In summary, there is an urgent need in this field to research more novel magneto-optical materials that can significantly reduce manufacturing costs, simplify device structures, and improve fabrication efficiency and reliability, while endowing them with strong Faraday rotation effects and inherent broadband anti-reflection functions, so as to better adapt to the development trend of optical system integration. Summary of the Invention

[0009] To address the problems existing in the prior art, this invention provides a multilayer film with both Faraday rotation and anti-reflection functions, and its preparation method. By combining magneto-optical materials CeF3 and Ta2O5, high transmittance is achieved while significantly improving the Wilder constant and effectively suppressing the thermal lensing effect. It has good application prospects in high-power laser systems and optical communication devices.

[0010] The technical solution of the present invention is as follows: One of the objectives of this invention is to provide a multilayer film that combines Faraday rotation and anti-reflection functions, the multilayer film comprising a substrate and a multilayer film system deposited on the substrate; The multilayer film system consists of alternating layers of high-refractive-index material and low-refractive-index material, with a total of 8 layers. The high-refractive-index material is tantalum pentoxide (Ta₂O₅), and the low-refractive-index material is cerium trifluoride (CeF₃). The structural design is (0.25H|0.25L|0.25H|0.25L|0.25H|0.25L|0.25H|0.25L), where 0.25H represents the thickness of Ta₂O₅ as λ / 4n(Ta₂O₅), 0.25L represents the thickness of CeF₃ as λ / 4n(CeF₃), n(Ta₂O₅) and n(CeF₃) are the refractive indices of the materials, which are 1.55 and 1.98, respectively, and λ is the reference wavelength of 1064 nm.

[0011] Furthermore, the substrate material is silicon dioxide (SiO2) glass. Furthermore, the SiO2 glass is a double-sided polished disc with a thickness of 1-5 mm and a diameter of 5-15 mm. Furthermore, the tantalum pentoxide (Ta2O5) is selected as solid particles with a purity of 99.99%. Furthermore, the cerium trifluoride (CeF3) is selected from solid particles with a purity of 99.99% and a size of 1.5-3 mm. Furthermore, the transmittance of the multilayer film is greater than 99% at a working wavelength of 1064 nm, and the Wilder constant is 0.0045 rad / (μm·T); the transmittance of the CeF3 film and the Ta2O5 film deposited on the SiO2 glass surface are 94.0% and 76.6%, respectively. The second objective of this invention is to provide a method for preparing a multilayer film that combines Faraday rotation and anti-reflection functions. The method involves placing pre-melted CeF3 and Ta2O5 film materials, along with cleaned SiO2 glass substrate material, into an electron beam evaporation machine for multilayer film deposition. After deposition, the substrate is removed to obtain the multilayer film.

[0012] Furthermore, the SiO2 glass substrate was cleaned using anhydrous ethanol and pure water. Furthermore, the pre-melting step of the film material is as follows: The granular film materials CeF3 and Ta2O5 were placed separately in independent crucibles of an electron beam evaporation machine. The cavity door of the evaporation machine was closed, and the pressure inside the cavity was kept below 5.0 × 10⁻⁶. -3 Pa is bombarded with a high-energy electron beam emitted by an electron gun. The pre-melting is completed when the scattered granular film material is completely melted into a block shape similar to the crucible. The pre-melted CeF3 and Ta2O5 film materials are placed in the vapor deposition machine for use.

[0013] Furthermore, the electron beam evaporation machine is set to an internal pressure of 1.0 × 10⁻⁶. -3 -2.0×10 -3 Pa, temperature 190-210°C. Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Based on the principle of destructive optical interference, this invention discloses for the first time a novel eight-layer film system composed of alternating high-refractive-index tantalum pentoxide (Ta₂O₅) and low-refractive-index magneto-optical material cerium trifluoride (CeF₃). CeF₃ provides the Faraday rotation effect, while the alternating stacking of Ta₂O₅ and CeF₃ constitutes a standard λ / 4 optical thickness anti-reflection film stack. This allows the multilayer film to rotate the polarization plane of incident light through the magneto-optical effect and significantly suppress surface reflection through multilayer interference. This film structure effectively disperses and reduces photothermal absorption, thereby significantly suppressing the thermal lensing effect in high-power applications, ensuring the quality of the output beam, and enabling it to meet the demands of more stringent high-power laser applications. This lays a new material foundation for realizing high-performance, high-reliability integrated magneto-optical devices.

[0014] 2. The novel multilayer film designed in this invention possesses ideal optical performance, magneto-optical performance, and structural stability. It achieves a transmittance exceeding 99% at a working wavelength of 1064 nm, while reducing reflection and absorption losses to extremely low levels, effectively ensuring the optical power transmission efficiency in optical communication or laser systems. Furthermore, although it adopts a thin-film morphology, the multilayer film has a Wilder constant of 0.0045 rad / (μm·T), and its Faraday rotation capability per unit length is significantly superior to that of traditional terbium gallium garnet bulk crystals. The required isolation effect can be achieved with a shorter optical path. Moreover, the film system composed of Ta₂O₅ and CeF₃ is structurally stable, and the functionality is integrated within the thin film, avoiding the problem of traditional crystal elements becoming weak points due to thermal damage at the bonding or additional coating interfaces under high-power laser irradiation.

[0015] 3. Electron beam evaporation is a physical vapor deposition technique, and the setting of its process parameters directly affects the quality, structure, and performance of the thin film. This invention defines the parameter combination range of the intracavitary pressure and substrate temperature in the electron beam evaporation machine. Within the substrate temperature range of 190-210°C, the provided energy is not only sufficient to promote the migration of evaporating atoms to the substrate for film formation, alleviating the stress generated by low-temperature deposition and thus reducing film cracking or peeling, but also to remove H2O and other gas molecules adsorbed on the substrate surface, improving film adhesion and enhancing interfacial bonding strength. Based on this, the intracavitary pressure of the electron beam evaporation machine is set to 1.0 × 10⁻⁶. -3 -2.0×10 -3 Pa, with its synergistic effect, avoids collisions between residual gas molecules and evaporating atoms, reducing impurity doping and thus improving film purity. In addition, within this parameter range, it also avoids undesirable plasma caused by excessive pressure, which can lead to electron beam deflection and evaporation rate fluctuations. By extending the mean free path of gas molecules with an appropriate vacuum level, it enables evaporating atoms to fly in a straight line to the substrate, improving deposition efficiency and reducing scattering losses.

[0016] 4. In terms of fabrication process, this invention avoids the time-consuming and costly methods of traditional crystal growth, employing electron beam evaporation, a physical vapor deposition process. This process offers advantages such as short fabrication cycle, high film purity, strong process controllability, and ease of achieving large-area uniform deposition. Furthermore, by combining specific process parameters such as substrate temperature and intracavity pressure, the fabricated film is guaranteed to possess excellent density, low scattering, and strong adhesion. The fabrication method provided by this invention not only increases production capacity and reduces manufacturing costs but is also suitable for large-scale production, breaking through the bottleneck faced by high-performance magneto-optical materials due to cost limitations. The resulting multilayer film can provide a reference for the miniaturization, integration, and high performance of core materials for next-generation optical isolators, circulators, and other magneto-optical devices. It is particularly suitable for applications with stringent requirements for size, cost, and power stability, such as miniaturized optical modules, integrated photonic chips, and high-power fiber laser systems in 5G / 6G optical communication networks, demonstrating broad market application prospects. Attached Figure Description

[0017] Figure 1 The transmittance curve of the multilayer film described in Example 1 of this invention is shown in the 900-1200 nm range. Figure 2 This is the divergence angle curve of the multilayer film described in Embodiment 1 of the present invention at a beam width of 60-200W. Detailed Implementation

[0018] The present invention will be further described below with reference to preferred embodiments. The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values. These ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed herein. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods, performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions.

[0019] Unless otherwise specified, all materials and reagents used in the following examples are commercially available. Example 1 This embodiment provides a multilayer film that combines Faraday rotation and anti-reflection functions, and its preparation method includes the following steps: S1. Clean a 3mm thick, 10mm diameter double-sided polished SiO2 glass disc with anhydrous ethanol and pure water, and remove residual moisture from the surface. S2. Place 99.99% pure CeF3 solid particles and 2.5mm pure Ta2O5 solid particles into separate crucibles in an electron beam evaporation machine. Close the evaporation machine chamber door and wait until the pressure inside the chamber is less than 5.0 × 10⁻⁶. -3 Pa, using a high-energy electron beam emitted by an electron gun to bombard, until the scattered granular film material is completely melted into a block shape similar to the crucible, the pre-melting ends, and the pre-melted CeF3 and Ta2O5 film material is placed in the vapor deposition machine for use; S3. The pre-melted CeF3 and Ta2O5 film materials, along with the cleaned SiO2 glass substrate, are placed in an electron beam evaporation machine for multilayer thin film deposition, with the pressure inside the chamber set to 1.5 × 10⁻⁶. -3 Pa, temperature 200°C; S4. After the thin film deposition is completed, the SiO2 glass substrate is removed, thus obtaining the multilayer film. Example 2 This embodiment provides a multilayer film that combines Faraday rotation and anti-reflection functions, and its preparation method includes the following steps: S1. Clean a 5mm thick, 15mm diameter double-sided polished SiO2 glass disc with anhydrous ethanol and pure water, and remove residual moisture from the surface. S2. Place 99.99% pure CeF3 solid particles and 2.5mm pure Ta2O5 solid particles into separate crucibles in an electron beam evaporation machine. Close the evaporation machine chamber door and wait until the pressure inside the chamber is less than 5.0 × 10⁻⁶. -3 Pa, using a high-energy electron beam emitted by an electron gun to bombard, until the scattered granular film material is completely melted into a block shape similar to the crucible, the pre-melting ends, and the pre-melted CeF3 and Ta2O5 film material is placed in the vapor deposition machine for use; S3. The pre-melted CeF3 and Ta2O5 film materials, along with the cleaned SiO2 glass substrate, are placed in an electron beam evaporation machine for multilayer thin film deposition, with the pressure inside the chamber set to 1.0 × 10⁻⁶. -3 Pa, temperature 210°C; S4. After the thin film deposition is completed, the SiO2 glass substrate is removed, thus obtaining the multilayer film.

[0020] Example 3 This embodiment provides a method for preparing a multilayer film that combines Faraday rotation and anti-reflection functions, including the following steps: S1. Clean a double-sided polished SiO2 glass disc with a thickness of 1mm and a diameter of 5mm using anhydrous ethanol and pure water, and remove residual moisture from the surface. S2. Place 99.99% pure CeF3 solid particles and 2.5mm pure Ta2O5 solid particles into separate crucibles in an electron beam evaporation machine. Close the evaporation machine chamber door and wait until the pressure inside the chamber is less than 5.0 × 10⁻⁶. -3 Pa, using a high-energy electron beam emitted by an electron gun to bombard, until the scattered granular film material is completely melted into a block shape similar to the crucible, the pre-melting ends, and the pre-melted CeF3 and Ta2O5 film material is placed in the vapor deposition machine for use; S3. The pre-melted CeF3 and Ta2O5 film materials, along with the cleaned SiO2 glass substrate, are placed in an electron beam evaporation machine for multilayer thin film deposition, with the pressure inside the chamber set to 2.0 × 10⁻⁶. -3 Pa, temperature 190°C; S4. After the thin film deposition is completed, the SiO2 glass substrate is removed, thus obtaining the multilayer film.

[0021] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A multilayer film having both Faraday rotation and antireflection functions, characterized by comprising a plurality of layers of a material having a Faraday rotation function and a plurality of layers of a material having an antireflection function. The multilayer film comprises a substrate and a multilayer film system deposited on the substrate; the multilayer film system is formed by alternately stacking high-refractive-index material layers and low-refractive-index material layers, the total number of layers is 8, the high-refractive-index material is tantalum pentoxide Ta2O5, and the low-refractive-index material is cerium trifluoride CeF3; the structure design is (0.25H|0.25L|0.25H|0.25L|0.25H|0.25L|0.25H|0.25L), 0.25H represents that the thickness of Ta2O5 is λ / 4n(Ta2O5), 0.25L represents that the thickness of CeF3 is λ / 4n(CeF3), n(Ta2O5) and n(CeF3) are the refractive indexes of the materials, and are 1.55 and 1.98 respectively, and λ is a reference wavelength 1064 nm.

2. The multilayer film having Faraday rotation and antireflection functions according to claim 1, wherein The material of the substrate is silica glass SiO2.

3. The multilayer film according to claim 2, wherein the multilayer film has a Faraday rotation function and an antireflection function. The SiO2 glass is a double-side polished wafer, the thickness is 1-5 mm, and the diameter is 5-15 mm.

4. The multilayer film according to claim 1, wherein the multilayer film has a Faraday rotation function and an antireflection function. The tantalum pentoxide Ta2O5 is selected from solid particles with a purity of 99.99%.

5. The multilayer film according to claim 1, wherein the multilayer film has a Faraday rotation function and an antireflection function. The cerium trifluoride CeF3 is selected from solid particles with a purity of 99.99% and a size of 1.5-3 mm.

6. The multilayer film according to claim 1, wherein the multilayer film has a Faraday rotation function and an antireflection function. The transmittance of the multilayer film is greater than 99% at a working wavelength of 1064 nm, and the Verdet constant is 0.0045 rad / (μm·T); the transmittances of the CeF3 film and the Ta2O5 film deposited on the surface of the SiO2 glass are 94.0% and 76.6% respectively.

7. A method for preparing the multilayer film with Faraday rotation and antireflection functions according to any one of claims 1 to 6, characterized in that, The pre-melted CeF3 and Ta2O5 film materials and the cleaned substrate material SiO2 glass are placed in an electron beam evaporation machine to deposit a multilayer film, the substrate is taken out after deposition, and the multilayer film is obtained.

8. The method of claim 7, wherein the method further comprises the step of: The pre-melting step of the film material is as follows: ​ The granular film material CeF3 and Ta2O5 are respectively placed in the independent crucible of the electron beam evaporation machine, the chamber door is closed and the pressure in the chamber is less than 5.0*10 -3 Pa, the high-energy electron beam emitted by the electron gun is used for bombardment, and the pre-melting is ended after the scattered granular film material is completely melted into a block shape same as the crucible. The pre-melted CeF3 and Ta2O5 film materials are placed in the evaporation machine for use.

9. The method for preparing a multilayer film with both Faraday rotation and anti-reflection functions according to claim 7, characterized in that, The substrate material SiO2 glass is cleaned with anhydrous ethanol and pure water.

10. The method for preparing a multilayer film with both Faraday rotation and anti-reflection functions according to claim 7, characterized in that, The electron beam evaporator is set to a pressure in the chamber of 1.0 x 10 -3 -2.0 x 10 -3 Pa, and a temperature of 190-210°C.

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