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Abstract
Thin-film polarizing beam splitters (PBSs) fulfill a pivotal role in laser beam splitting, modulation, shaping and isolation. In this study, a high-reliability infrared broadband thin-film PBS was developed. To correct for tensile stress in Ge/YbF3 multilayer coatings, ZnSe compensation layers were incorporated in the multilayer design. The effects of different symmetrical periods on the spectral properties of the infrared PBS were systematically discussed. The infrared PBS operated at 45° and in the long-wave infrared (LWIR) band. Using the percent of optical extrema monitoring (POEM) strategy combined with the high-temperature optical constants (HTOC) of Ge film, the infrared PBS was precisely fabricated on ZnSe substrates. Subsequently, the spectral performance and film reliability of the infrared PBS were carefully characterized. Specifically, the transmittance of p-polarization surpassed 96%, while the extinction ratio exceeded 100:1 within the 10.6 ± 0.15 µm band. The infrared PBS demonstrated commendable environmental reliability, in addition to exhibiting excellent spectral characteristics.
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1. Introduction
Infrared lasers have numerous scientific and industrial applications. Some key areas include laser cutting and welding in manufacturing, laser surgery in medicine and driving lasers in extreme ultraviolet lithography [1,2]. These applications require optical components, such as mirrors, lenses and PBSs. Infrared PBSs play a key role in laser beam splitting, modulation, shaping and isolation. They are specifically designed to separate two orthogonal polarization components of light, namely s- and p-polarization, directing them into divergent pathways [3]. Key characteristics of infrared PBSs include wavelength band, extinction ratio and reflectance or transmittance for s- and p-polarization. Generally, PBSs are mostly developed using optical interference coatings or birefringent crystals. The PBSs constructed using birefringent crystals offer notable advantages, namely, a high extinction ratio and a broad spectral range. Nevertheless, their operating wavelengths are predominantly confined to the visible and near-infrared regions. Additionally, the manufacturing process of birefringent crystal-based PBSs is marked by complexity and expense. Thin-film PBSs are based on the principle of light interference within multilayer films. Meanwhile, thin-film PBSs offer greater flexibility and the ability to achieve large aperture sizes. They can be applied in the ultraviolet (UV) and visible up to the LWIR bands.
Significant progress [4–8] has been made in the design and fabrication of PBSs, focusing on optimizing key characteristics such as high p-polarization transmittance, substantial extinction ratio and wide polarizing bandwidth. Li et al. introduced a novel broadband and wide-angle PBS design based on light interference and frustrated total internal reflection (FTIR) [9]. Perla et al. designed a wide-angle, high-extinction-ratio, infrared PBS using FTIR by a centrosymmetric multilayer stack, which was embedded in a high-index prism [10]. Wiley created a Brewster angle thin-film polarizer with 14 layers of BaF2 and ZnSe films, achieving exceptional p-polarized transmittance (>99.77%) and low s-polarized transmittance (<0.491%) at 10.6 µm [11]. On the other hand, the spectral properties of thin-film polarizers are susceptible to manufacturing errors, emphasizing the need for accurate control of film thickness during production. Previous studies [12,13] used mixed monochromatic optical and quartz crystal monitoring for HfO2/SiO2 Brewster angle polarizers. In addition, in a separate study [14], a broadband optical monitoring strategy with error self-compensation capability was used to fabricate ZrO2/SiO2 Brewster angle thin-film polarizers. However, the working angles of thin-film PBSs studied in these reports are mainly focused on the Brewster angle. There has been limited research on the design and preparation of infrared thin-film PBSs with relatively small working angles. For small working angles, infrared thin-film PBSs require a significant increase in the number and thickness of layers to separate s-polarized and p-polarized light effectively. These coatings are notably thicker than their visible band counterparts. In addition, infrared coatings are mostly composed of soft materials with limited firmness and stability. These factors pose significant challenges in the design and preparation of high-performance infrared thin-film PBSs.
In this study, a high-reliability infrared broadband thin-film PBS was developed. To correct for tensile stress in Ge/YbF3 multilayer coatings, ZnSe compensation layers were incorporated in the multilayer design. The effects of different symmetrical periods on the spectral properties of the infrared PBS were systematically discussed. The infrared PBS was accurately prepared on ZnSe substrates by the direct optical monitoring (DOM) method with a single wavelength. The HTOC of Ge film provides a more accurate theoretical monitoring curve, which gives the POEM strategy a more accurate baseline for real-time correction of trigger points, thereby improving monitoring accuracy. The resulting PBS exhibits excellent spectral characteristics and environmental reliability. The incorporation of ZnSe layers in the Ge/YbF3 multilayer provides the ability to produce wide-polarization-band, high-reliability infrared PBS coatings.
2. Experimental methods
2.1 Design principle
Maxwell's equations dictate that electric and magnetic fields follow tangential continuity at multilayer film interfaces under oblique incidence. The behavior of optical films at these interfaces yields diverse polarization-dependent responses, leading to distinctive changes in optical admittance. The optical admittances are given by
where ηs and ηp are the optical admittances of s- and p-polarization, respectively, n is refractive index, and θ is the incident angle.The thin-film PBS design relies on the unequal optical admittances of s- and p-polarization. High transmittance of p-polarization is achieved through the interference effect. The thin-film PBS structure is similar to a longwave-pass filter. The width of the high-reflectance region of a quarter-wave stack depends on the admittance ratio of the two materials used, which varies with the angle of incidence and is different for s- and p-polarization. The high-reflectance region for p-polarization is consistently narrower compared to that for s-polarization. In the region beyond the high-reflectance region for p-polarization but within the high-reflectance region for s-polarization, transmittance is low for s-polarization, but conversely, high for p-polarization. As a result, the component effectively functions as a polarizer [15]. The width of the high-reflectance region for p-polarization is
Therefore, the separation width for the s- and p-polarization high-reflectance region is
To enhance the polarization region width, it is necessary to maximize the refractive index of high-index film layer and minimize that of low-index film layer. In this study, the infrared PBS designs were performed using commercial software (FilmWizard, Scientifc Computing International, USA).
2.2 Sample preparation
The infrared PBS and single-layer film samples were prepared using a vacuum coater equipped with an optical monitoring system (OMS5100) (LAB900, Leybold Optics, Germany). The Infrared PBS was deposited on ZnSe substrates with a dimension of Φ30mm × 3 mm. The substrates are polished, and the root-mean-square (RMS) roughness of surface is better than 1λ/50 (λ=632.8 nm). Ge, ZnSe and YbF3 were selected as film materials. To obtain accurate optical constants, single Ge and YbF3 layers were deposited on ZnSe substrates, while a single ZnSe layer was deposited on Ge substrates. To investigate the mechanical properties of thin film materials, single-layer films of these three materials were deposited on ZnSe substrates. To improve the film-substrate bonding, the substrates were bombarded for 5 minutes with ions using an RF ion source before coating. Ge and YbF3 films were deposited by electron beam evaporation, while ZnSe films were deposited by thermal evaporation with a molybdenum boat. The deposition rates for Ge, ZnSe and YbF3 films were 1.0 nm/s, 3.0 nm/s and 1.2 nm/s, respectively, which were controlled by a 4-probe quartz crystal oscillator. The temperature measured by a thermocouple placed in front of the substrate carrier was maintained at 200 ± 1°C during deposition.
The film thickness was monitored by the DOM method with a single wavelength, and the POEM strategy was used as a trigger point algorithm for optical thickness monitoring. The POEM strategy [16] involves terminating the current layer deposition at a specified percentage of the transmittance excursion between the previous two extrema. The schematic diagram of this monitoring strategy is presented in Fig. 1. The equation for the actual transmittance at trigger point is as follows:
2.3 Sample characterization
The transmittance spectra of samples were measured using a Fourier transform infrared (FTIR) spectrometer (VERTEX 80, Bruker Co., Germany), and the measurement error was within 0.08%. The measured transmittance spectra of Ge, ZnSe and YbF3 single-layer samples ranging from 2.0 to 12.0 µm were fitted by the envelope method and the dispersion models to obtain the optical constants and thicknesses. The transmittance spectra of Ge film at high temperatures were measured by the FTIR spectrometer with a high-temperature accessory. Six temperatures were set, which were room temperature (RT, 25°C) and from 100°C to 300°C with 50 °C interval. The calefactive speed was about 5°C/min. After the temperature reached the set value, the temperature was maintained for 10 minutes before measurement. From transmittance spectra, the refractive indices (n) and extinction coefficients (k) of Ge film at high temperatures were calculated using the Cauchy exponential model. In the dispersion model, the absorption coefficient varies exponentially with frequency. This allows the Cauchy exponential model to fit a large variation of k values with wavelength. The following formulas were used to define the optical constants [17,18]:
where λ is the wavelength in µm, An, Bn, and Cn are the material coefficients of the refractive index, Am is the amplitude, Ex is the exponent factor and Eg is the band edge.A laser interferometer (VERIFIRE QPZ, Zygo Co., USA) was used to measure the curvature radii of ZnSe substrates before and after coating. The residual stress (σ) of infrared films can be obtained by the Stoney equation [19,20]:
To explore the infrared PBS reliability, standard environmental reliability tests were conducted as follows: (1) Adhesion test: a standard polyester tape was firmly attached to the coated surface, and then swiftly pulled in the direction normal to the surface to evaluate the adhesion. (2) Soaking test: the sample underwent an 8-hour immersion in pure water at 45°C. (3) Humidity test: the sample was subjected to 45°C and 95% relative humidity conditions for 24 hours. (4) Temperature alternating test: the sample was cycled between 50°C and −40°C for 1 hour each, completing three cycles. The morphologies of the infrared PBS surface before and after the environmental reliability tests were measured by an optical digital microscope (DSX500, Olympus Co., Japan).
3. Results and discussion
3.1 Single layer investigation
The infrared PBS design is inherently linked to the optical constants and mechanical properties of film materials. In this study, Ge, ZnSe and YbF3 were selected as film materials. Since Ge and ZnSe are transparent in the 2.0∼12.0 µm wavelength range, the dispersion function for the Ge and ZnSe films was expressed using the Cauchy dispersion model [21]:
where λ is the wavelength in µm, An, Bn, and Cn are the material coefficients of the refractive index, Ak, Bk, and Ck are the material coefficients of the extinction coefficient. The classical Lorentz oscillator model was used to determine the optical constants of the YbF3 film from 2.0 to 12.0 µm. The real part (ɛr) and the imaginary part (ɛi) of the complex dielectric function (ɛ) in the Lorentz oscillator model are described as follows [22]:The measured transmittance results were fitted by minimizing the root mean squared error (RMSE) value from the measured and calculated transmittance values T, and the RMSE is defined as
Fig. 2. (a) Experimental (points) and fitted (lines) transmittance spectra of (a) Ge, (b) ZnSe and (c) YbF3 films; refractive indices and extinction coefficients of (d) Ge, (e) ZnSe and (f) YbF3 films.
Table 1. Cauchy model parameters of the Ge and ZnSe films
Table 2. Lorentz Oscillator model parameters of the YbF3 film
The mechanical properties of optical thin films mainly include stress, adhesion, hardness and related mechanical parameters. Residual stress is a crucial mechanical property of optical thin films, and large residual stress may lead to problems such as wrinkling, cracking and delamination of thin-film devices [27]. Film hardness refers to its resistance to deformation and fracture, and the elastic modulus is an important parameter characterizing the ability of the film to undergo elastic deformation. These mechanical properties are crucial for the stability and reliability of thin films. Figure 3 shows the surface shapes of Ge, ZnSe and YbF3 film samples, respectively. Table 3 presents film thicknesses, substrate deflections and residual stresses of Ge, ZnSe and YbF3 film samples. In the table, a positive stress value indicates that the film is subjected to tensile stress, the substrate deflection is positive, and the coated surface is concave. On the contrary, a negative stress value means that the film is subjected to compressive stress, the substrate deflection is negative, and the coated surface is convex. The table indicates that both Ge and YbF3 films exhibit tensile stress, especially the YbF3 film with a stress value reaching 144.4 MPa, while the ZnSe film shows compressive stress. Figure 4 illustrates the nanohardness and elastic modulus of Ge, ZnSe and YbF3 films. Obviously, YbF3 film has higher nanohardness and elastic modulus, followed by ZnSe film, and Ge film has lower nanohardness and elastic modulus.
Table 3. Film thicknesses, substrate deflections and residual stresses of Ge, ZnSe and YbF3 film samples
3.2 Infrared PBS comprising layers of Ge and YbF3 materials
The infrared PBS design focuses on achieving high p-polarization transmittance and extinction ratio within the operating band. This requires eliminating passband ripple oscillation for p-polarization and ensuring sufficient width of s-polarization high-reflectance region. In this study, the infrared PBS needs to meet specific optical performance requirements that the transmittance of p-polarization (Tp) within the 10.6 ± 0.15 µm wavelength band exceeds 96%, and the extinction ratio (Tp/Ts) surpasses 100 at 45°, where Ts is the transmittance of s-polarization. To achieve a wider polarization bandwidth and reduced film thickness, an infrared PBS was designed using a Ge/YbF3 material combination with a refractive index ratio of 2.90 at 10.6 µm. This design utilized a longwave-pass filter and matching layers design. It exhibits a polarizing region with a width of 468 nm, where the transmittance of p-polarization exceeds 96% and the extinction ratio is greater than 100, as illustrated in Fig. 5(a). And it has a total thickness of 14.5 µm with 17 layers, as shown in Fig. 5(b). However, the weak bonding between Ge and YbF3 layers and the large tensile stress make the thin-film device vulnerable to delamination at higher total thickness, as illustrated in Fig. 5(c). Although the Ge/YbF3 material combination has been reported in the literature for the development of beamsplitters [28], it operates in the near-infrared and mid-infrared spectral ranges, with film layers being relatively thin, measuring only a few microns. Film stress is the force between film layers, and as stress is generated between each adjacent film layer, the destructive force caused by film stress increases with the film thickness. Since both Ge and YbF3 films exhibit tensile stress, tensile stress accumulates as the total film thickness of the PBS increases. When the film thickness reaches a certain level, the destructive force exceeds the bonding force between film molecules, leading to device failure. Meanwhile, good adhesion is generally observed between soft films and soft films, and between hard films and hard films, but adhesion between hard films and soft films is usually poor [29]. The significant difference in nanohardness and elastic modulus between Ge and YbF3 films is also a crucial factor contributing to their poor adhesion.
Fig. 5. (a) Theoretical transmittance spectrum, (b) layer thickness profile and (c) sample image of the Ge/YbF3-based PBS.
3.3 Infrared PBS design based on Ge, ZnSe and YbF3 materials
To improve environmental adaptability, ZnSe film layers were inserted between Ge and YbF3 layers as compensation layers. As the stress properties of ZnSe layers are opposite to those of Ge and YbF3 layers, this allows the tensile and compressive stresses between the film layers to cancel each other out, thereby reducing the overall residual destructive force in the layers. Figure 6(a) shows the equivalent admittances of the symmetrical period [aN 2(0.5−a)L aN] at vertical incidence. Here, the reference wavelength is 8.0 µm; N and L denote the quarter-wave optical thicknesses of ZnSe and YbF3 layers, respectively; the values before N and L refer to the optical thickness coefficient; a is a parameter ranging from 0∼0.5. At the reference wavelength, when a takes different values, the symmetrical period can obtain different equivalent admittances, which can be any value between the refractive indices of ZnSe and YbF3. Meanwhile, as a increases, the equivalent admittance gradually increases. Figure 6(b) shows the equivalent admittances of the symmetrical period [0.5 H aN 2(0.5−a)L aN 0.5 H] at vertical incidence. H denotes the quarter-wave optical thickness of Ge layer. In the figure, the green and orange diagonal striped boxes represent the width of the high-reflectance band of the symmetrical film stack when a is 0 and 0.5, respectively. When a is between 0 and 0.5, the width of the high-reflectance band falls between these two values. And as a increases, the width of the high-reflectance band gradually decreases. This is because as a increases, the equivalent admittance of the symmetrical period [aN 2(0.5−a)L aN] gradually increases, resulting in a decrease in the admittance ratio between H and the symmetrical period [aN 2(0.5−a)L aN].
Fig. 6. (a) Equivalent admittances for symmetrical period [aN 2(0.5−a)L aN]; (b) equivalent admittances for symmetrical period [0.5 H aN 2(0.5−a)L aN 0.5 H]; a is a parameter with values of 0, 0.1, 0.2, 0.3, 0.4 and 0.5.
Figure 7 illustrates the design results of the infrared PBS based on different symmetrical periods after incorporating ZnSe compensation layers. All designs utilized a longwave-pass filter and matching layers design. And an antireflection coating was designed on the backside to eliminate the effects of reflection from substrate backside. The first PBS design is based on the symmetrical period (0.5 H 0.1N 0.8 L 0.1N 0.5 H) with 33 alternating layers and a total film thickness of 15.3 µm. This design exhibits a polarizing region with a width of 370 nm, where the transmittance of p-polarization exceeds 96% and the extinction ratio is greater than 100, as depicted in Fig. 7(a). The second design is based on the symmetrical period (0.5 H 0.2N 0.6 L 0.2N 0.5 H) and consists of 33 layers with a thickness of 14.5 µm. This design can theoretically achieve a bandwidth of 367 nm, as presented in Fig. 7(b). The third design is based on the symmetrical period (0.5 H 0.3N 0.4 L 0.3N 0.5 H). This design has a total thickness of 15.6 µm with 37 layers, showing a polarizing region with a width of 307 nm, as illustrated in Fig. 7(c). The fourth design is based on the symmetrical period (0.5 H 0.4N 0.2 L 0.4N 0.5 H) with a thickness of 18.4 µm and 45 layers. The width of the theoretical polarizing region is 226 nm, as shown in Fig. 7(d). The characteristics of the infrared PBS with different designs are summarized in Table 4. As the thickness of the inserted ZnSe layers increases, the total thickness of the YbF3 layers gradually decreases, indicating that the tensile stress in the layers also gradually decreases. At the same time, the bandwidth of the polarizing region gradually decreases as a increases, which is due to the decrease in the admittance ratio between H and the symmetrical period [aN 2(0.5−a)L aN]. Obviously, if the thickness of the inserted ZnSe layers is too large, the advantage of the Ge/YbF3 combination with a high-refractive-index contrast will be lost, resulting in a narrower polarizing region. However, if the inserted ZnSe thickness is too thin, it may not effectively compensate for tensile stress. Therefore, the design based on the symmetrical period (0.5 H 0.2N 0.6 L 0.2N 0.5 H) is considered a more desirable design. This design structure is Sub | 0.967 H 0.494N 0.6 L 0.342N (0.5 H 0.2N 0.6 L 0.2N 0.5 H)^6 0.558N 0.599 L 0.5N 0.52 H | Air, where Sub refers to the ZnSe substrate, and the values before H, N and L refer to the optical thickness coefficient; the reference wavelength is 8.33 µm. By inserting the ZnSe layers of appropriate thickness, this design can reduce the large tensile stress in the layers, while also maintaining the advantage of the Ge/YbF3 combination, i.e. a relatively wide polarizing region. Subsequently, this design was subjected to sample preparation and performance characterization.
Fig. 7. Theoretical transmittance spectra of the infrared PBS based on different symmetrical periods: (a) 0.5 H 0.1N 0.8 L 0.1N 0.5 H; (b) 0.5 H 0.2N 0.6 L 0.2N 0.5 H; (c) 0.5 H 0.3N 0.4 L 0.3N 0.5 H; (d) 0.5 H 0.4N 0.2 L 0.4N 0.5 H.
Table 4. Characteristics of the infrared PBS with different designs
3.4 POEM strategy combined with high-temperature optical constants of Ge film
Although the POEM strategy can correct the trigger points of the monitoring signal in real time when the measured transmittance deviates from the theoretical value, it still generates a large monitoring error when the actual measured value deviates significantly from the theoretical value, and the larger the deviation, the larger the error. In the POEM strategy, the theoretical transmittance monitoring curve is calculated based on the given optical constants of film materials, which are usually measured and fitted at room temperature. When the optical constants of film materials do not change much between room temperature and deposition temperature, the POEM strategy has a good application. However, when the optical constants of film materials differ greatly between room temperature and deposition temperature, it is necessary to give the exact optical constants at the deposition temperature to reduce the monitoring error. The optical properties of Ge film are temperature-dependent, particularly the extinction coefficient increases at higher temperatures [30,31]. Due to the limitations of the transparent region of Ge film material and the InGaAs detector in the OMS5100, the monitoring wavelength is preferably chosen between 1.8 and 2.5 µm for the infrared PBS preparation. Figure 8(a) and (b) describe the refractive indices and extinction coefficients of Ge film at different temperatures in the 1.8∼5.0 µm wavelength range, respectively. Table 5 states the Cauchy exponential model parameters of the Ge film at different temperatures, which could be used to describe the variation of optical constants. The refractive index of Ge film increases gradually with increasing temperature, especially the extinction coefficient increases rapidly in the range of 1.8∼2.5 µm. At 2.0 µm, the refractive index increases from 4.16 at room temperature to 4.28 at 200°C, and the extinction coefficient increases from 5.8 × 10−3 at room temperature to 1.2 × 10−2 at 200°C. The increase in optical absorption of Ge film at high temperature is mainly due to the increased number of thermally generated free carrier holes [32]. Since the bandgaps of ZnSe and YbF3 are much wider than that of Ge [33–35], the effect of high temperature on their optical constants is negligible. Figure 8(c) illustrates the theoretical monitoring curves of a single Ge layer calculated using the room-temperature optical constants (RTOC) and HTOC of Ge film, respectively. As can be seen, the theoretical transmittance monitoring curves calculated using the HTOC of Ge film differ greatly from those calculated using the RTOC, and the higher the deposition temperature, the greater the deviation, which directly affects the precise judgement of trigger points. Therefore, the POEM strategy combined with the HTOC of Ge film is expected to improve the accuracy of monitoring the thickness of the PBS films.
Fig. 8. (a) Refractive indices and (b) extinction coefficients of Ge film at different temperatures; (c) comparison of theoretical transmittance monitor curves of a single Ge layer calculated using the RTOC and HTOC of Ge film.
Table 5. Cauchy exponential model parameters of the Ge film at different temperatures
3.5 Spectral performance and environmental reliability
Figure 9 presents the theoretical and experimental transmittance spectra along with extinction ratios of the infrared PBS. Obviously, the transmittance spectrum and extinction ratio of the PBS prepared by the POEM strategy combined with the HTOC of Ge film are closer to the design curves, while those of the PBS prepared using the RTOC of Ge film deviate greatly from the design curves. The transmittance of p-polarization surpasses 96%, while the extinction ratio exceeds 100:1 within the 10.6 ± 0.15 µm band. The infrared PBS exhibits outstanding spectral characteristics, which is attributed to the DOM method and the POEM strategy combined with the HTOC of Ge film. Primarily, the DOM method significantly reduces error propagation stemming from tooling factor and deposition process conditions, compared to alternative methods. Secondly, the HTOC of Ge film provides a more accurate theoretical transmittance monitoring curve, which gives the POEM strategy a more accurate baseline for real-time correction of trigger points, thus improving monitoring accuracy.
Fig. 9. Comparison of theoretical and experimental (a) transmittance spectra and (b) extinction ratios for the infrared PBS.
In addition to spectral properties, the environmental reliability characteristics of the infrared PBS are critical. Figure 10(a) and (b) illustrate the morphologies of the infrared PBS surface before and after the environmental reliability tests, respectively. No noticeable scratches, cracks or any other undesirable effects appear on the surface of the infrared PBS sample. The infrared PBS demonstrates excellent environmental stability. This is primarily because the incorporation of ZnSe layers allows the mutual cancellation of tensile and compressive stresses between the PBS film layers. This reduces the overall residual stress in the layers and greatly enhances the stability of thin-film device. Simultaneously, the nanohardness and elastic modulus of ZnSe film are between those of Ge and YbF3 films, allowing the ZnSe layers to act as a transition and buffer, contributing to the environmental reliability of the infrared PBS.
Fig. 10. The surface images of the infrared PBS sample (a) before and (b) after the environmental reliability tests.
4. Conclusion
Thin-film PBSs play a critical role in laser beam splitting, modulation, shaping and isolation. In this study, a high-reliability infrared broadband thin-film PBS was developed. Despite the remarkable refractive index contrast of the Ge/YbF3 material combination, the weak bonding between Ge and YbF3 layers and the large tensile stress predispose the device to failures such as cracking or delamination of the films. To enhance the environmental adaptability of the Ge/YbF3-based PBS, we inserted ZnSe film layers between the Ge and YbF3 layers. As the stress properties of ZnSe layers are opposite to those of Ge and YbF3 layers, this allows the tensile and compressive stresses between the film layers to cancel each other out, thereby reducing the residual stress of the entire coatings. Meanwhile, the nanohardness and elastic modulus of ZnSe film are between those of Ge and YbF3 films, allowing the ZnSe layers to play a transitional and buffering role, contributing to the reliability of the infrared PBS. The effects of various symmetrical periods on the spectral properties were discussed. The infrared PBS was accurately prepared on ZnSe substrates by the DOM method. The HTOC of Ge film provides a more accurate theoretical transmittance monitoring curve, which gives the POEM strategy a more accurate baseline for real-time correction of trigger points, thus improving monitoring accuracy. The resulting PBS exhibits outstanding spectral characteristics, with p-polarization transmittance exceeding 96% and an extinction ratio surpassing 100:1 within the 10.6 ± 0.15 µm band. Additionally, comprehensive tests verify its commendable environmental reliability. The incorporation of ZnSe layers in a Ge/YbF3 multilayer provides the ability to produce wide-polarization-band, high-reliability infrared PBS coatings.
Funding
National Natural Science Foundation of China (11674062, 61775042, 62275053, 62275256); National Key Research and Development Program of China (2021YFB3701500); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2019241, 2023248); Fudan University-CIOMP Joint Fund (FC2017-003); Eastern Talent Plan Youth Project 2022.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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