Comparative study of NIR-MIR beamsplitters based on ZnS/YbF3 and Ge/YbF3

Two beamsplitters operating across the near-infrared (770-1050 nm) and midinfrared (4-8 μm) spectral ranges are developed. For the first time, the beamsplitters based on thin-film materials combinations of ZnS/YbF3 and Ge/YbF3 are investigated. The multilayers operate at the Brewster angle of ZnSe substrate. There are no special temperature conditions. The dichroic coatings are designed, produced, and carefully characterized. Potentials of the ZnS/YbF3 and Ge/YbF3 thin-film material combinations are discussed based on analytical estimations, as well as on optical and non-optical characterization results. The ZnS/YbF3 pair provides high reflectance in the near-infrared spectral range. The Ge/YbF3 solutions exhibit very broadband reflection zones. The Ge/YbF3 coatings are thinner and comprise fewer layers than ZnS/YbF3 multilayers. Ge/YbF3 pair has high potential for design and production of NIRMIR coatings. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
In the recent years, the mid-infrared (MIR) spectral range from 3 to 15 µm attracts more and more interest of chemists and biologists since most organic molecules exhibit fundamental vibrational and rotational modes in this range [1,2]. Several research groups investigate organic compounds in the gaseous or liquid phase with the help of powerful laser systems (see, for example [3,4],). Harnessing MIR radiation in the laser systems demands high quality multilayer optical elements operating in the near-infrared (NIR) and in the MIR spectral ranges, so called NIR-MIR coatings. Typically, such coatings are used to split or combine the laser beams into more powerful near-infrared and less powerful mid-infrared components. The coatings exhibit desired reflectance in a specified NIR range and high transmittance in a required MIR region. The acceptable levels of reflectance and transmittance as well as the angle of incidence (AOI) are dependent on the laser setup and the goals of the laser experiments. In addition, target specifications arising from the application area may contain not only desired spectral performance but also non-optical properties (good adhesion, low mechanical stress, thermal stability). For laser applications, the low mechanical stress is crucial in order not to deform the laser beam wave front of the entire laser system. These specifications affects the choice of thinfilm materials and design structures since the coatings operating in the MIR range may contain thick layers, and adhesion or/and stress problems may arise at some combinations of substrate/thin-film materials.
In the present work, two NIR-MIR beamsplitters (BS) on ZnSe substrates with sophisticated target spectral characteristics are reported. The multilayer elements operate at the Brewster angle of the ZnSe substrate (67.9°). The beamsplitters are supposed to exhibit reflectance (p-polarized light) exceeding 75% in the range from 860 to 1050 nm, high reflectance (s-polarized light) exceeding 95% in the range from 770 to 830 nm, and high transmittance close to 100% in the MIR range from 4 to 8 µm. The reflectance curves in the high reflection (HR) zones should cover the entire spectral range of interest, i.e., they have to exhibit minimal deviations from the average reflectance value for both polarizations. The optical elements should exhibit low mechanical stress. The beamsplitters can be used in pump-probe spectroscopy studying ultrashort dynamics of biological molecules. Three possible pairs of three thin-film available materials, namely ZnS, Ge, and YbF 3 , were used for design of the BS. Two BS comprising ZnS/YbF 3 and Ge/YbF 3 layers were successfully produced using e-beam evaporation. After the deposition, beamsplitters were optically characterized and their adhesion was tested with the help of a tape. Mechanical stresses of the designs were estimated based on stresses in single films of Ge, YbF3, and ZnS using a wellknown formula [5,6]. Although Ge layers are absorbing in the HR zones, Ge/YbF 3 multilayers demonstrate a few benefits in comparison with the non-absorbing pair ZnS/YbF 3, namely, very broadband high-reflection zones, smaller number of layers and smaller total thickness. Optical and non-optical properties of the manufactured multilayer elements were compared. The third pair Ge/ZnS did not provide the desired reflectance values already at the design step.
According to our knowledge, the thin-film materials combinations ZnS/YbF 3 and Ge/YbF 3 have been exploited for the production of a sophisticated NIR-MIR multilayer beamsplitters for the first time. There exist only several publications reporting on production of single YbF 3 thin films (see, for example [7][8][9],). The YbF 3 thin-film material was chosen based on its optical and mechanical properties reported in the literature sources [10][11][12][13]. Having a low refractive index 1.5, ≈ YbF 3 replaces highly toxic ThF 4 which was used earlier as a low-index material in MIR coatings [14]. Also, the refractive index contrast of YbF 3 with Ge is very high ( 2.7, ≈ and with ZnS is relatively high ( 1.5 ≈ ). YbF 3 is highly suitable for ebeam evaporation [11][12][13]. YbF 3 films are transparent from 200 nm up to 12 um [11,12,15], they exhibit low tensile stress lower than at a number of other fluoride materials [11], and good environmental resistance [11,13]. In addition to this, the deposition/characterization experience with YbF 3 was already accumulated. This experience was in a full agreement with reference data [10][11][12][13].

Design approach
The NIR-MIR beamsplitters were produced using Syruspro710 deposition plant (Bühler Leybold Optics, Germany) based on the electron-beam evaporation. The nominal refractive indices of Ge, ZnS, and YbF 3 thin films in the broadband spectral range from 400 nm to 8000 nm were accurately determined based on optical characterization of single-layer samples and test multilayers containing a small number of layers. The ZnSe substrate was optically characterized in the range 400-8000 nm. Optical constants of Ge thin layers were found using a non-parametric approach [16,17] that allows one to solve the most complicated characterization problems, where the optical constants cannot be described by well-known few-parametric models such as Cauchy or Sellmeier. Also, this approach is applied when optical constants in a very broad spectral range are to be determined. Refractive indices of YbF 3 , ZnS, and ZnSe were determined using Cauchy model and cross-checked using the nonparametric approach. The detailed description of the sophisticated characterization process of the MIR films in a broadband spectral range lies out of the scope of the current manuscript and will be published separately. In Fig. 1(a), the nominal refractive indices of ZnS, Ge, YbF 3 layers, and ZnSe substrate are plotted; Fig. 1(b) shows the extinction coefficients of Ge layers and ZnSe substrate.
Design of the BS was performed by the needle optimization algorithm incorporated into OptiLayer Thin Film software [18,19]. The layer thicknesses were searched for based on the minimization of a merit function MF evaluating the closeness of the actual spectral characteristics to the target ones: , ,

Beamsplitter comprising layers of YbF 3 and ZnS materials
First, a thin-film materials pair ZnS/YbF 3 was utilized for design. In the course of the design process, (1) different starting designs were used and (2) the tolerances (Eq. (1)) were varied. As the result, the optimal solutions were found to be in a vicinity of a pivotal 14-layer BS design (further BS-ZnS/YbF 3 ) with the profile shown in Fig. 3(a). It shows that the pattern of the profile resembles the structure of a quarter-wave mirror (QWM). In Fig. 2(a) and 2(c), reflectance in p-and s-polarization cases and transmittance (p-polarization) of the design are shown. For the sake of convenience, reflectance of the 14-layer QWM with the central wavelength of 920 nm and matching AOI of 67.9°is shown in Fig. 2 Actually, smaller values of , T R σ indicate flatter spectral curves in the corresponding HR/HT spectral ranges. It is seen from Fig. 2(a) that the p-polarized reflectance in the range 860-1050 nm does not reach high values and deviates significantly from the average reflectance value in this range. Achieving high reflectance values (p-polarization) in the entire HR spectral range of interest at the large AOI of 67.9° is a critical point of the considered design problem. In other words, av R should be as large as possible and R σ (Eq. (2)) is to be as small as possible. The corresponding av R and R σ values are presented in Table 1. It is well-known that spectral performance of the multilayer can be improved on the expense of increase of the total thickness and the number of layers [20,21]. Increasing the thickness and correspondingly the number of layers with the help of the gradual evolution algorithm [20], one can obtain solutions with higher av R values. The structure of all design solutions resembles the quarterwave stacks as well. For example, in Fig. 2(b), spectral characteristics of a 20-layer beamsplitter (BS-ZnS/YbF 3 -20layers) are presented. It is seen that although the p-polarized reflectance increases, it still cannot cover the entire spectral range 860-1050 nm (corresponding av R and R σ values are shown in Table 1). In addition to this, p-polarized transmittance in the MIR range 4-8 µm deviates significantly from the constant value. The structure of the 20-layer solution resembling a quarter-wave stack is shown in Fig. 3(b). For the sake of convenience, in Fig. 2 can be estimated using equations from [22], p. 74 and p. 77 rewritten for the oblique incidence case: In Eqs.  Table 1.
One can observe a remarkable agreement between these estimations and the corresponding values of the BS-ZnS/YbF 3 and BS-ZnS/YbF 3 -20layers design solutions. It follows from Eqs.
(3), (4) that further increasing the number of layers and total coating thickness will lead only to an insignificant growth of the maximum p-polarized reflectance ( ) ( ) 0 p R λ and as a consequence to an insignificant increase of av R . The width of the HR zone does not depend on the number of layers and the coating thickness. It means that for the considered pair of thin-film materials, it is not possible to cover entirely a HR range broader than 160 nm. It should also be noted that further increase of the number of layers may lead to accumulation of deposition errors and therefore to worsening of the produced sample performance. In addition to this, thicker coatings may exhibit larger stresses and cause adhesion problems. This issue is considered in detail in Section 3. In this case, a compromise between target specifications and feasibility demands has to be found since exploitation of ZnS/YbF 3 pair has limitations originated from physics. It should be noted that the spectral characteristics of the BS-ZnS/YbF 3 design vary quite insignificantly in the angular range from 65 to 69°. This positive feature can be helpful in the course of implementation in laser setups. Taking into account all considerations listed above, a trade-off solution BS-ZnS/YbF 3 was chosen for the deposition.

Beamsplitter comprising layers of YbF 3 and Ge materials
Similar to the case of ZnS and YbF 3 materials, multiple design attempts with another combination of thin-film materials, Ge and YbF 3 , were undertaken. These attempts lead to quasi-periodic structures (QPS) of the type ( ) The widths of the HR zones can be estimated using formulas from [23] rewritten for the oblique incidence case: where ( , ) respectively.
Similarity to the case of the quarter-wave mirrors (Subsection 2.1), the width of the HR zone of the QPS depends on the ratios of high and low refractive indices only. It is seen in Fig. 5(a) that the reflectance covers its HR zone even when 8 m = . Due to the high ratio of the Ge and YbF 3 refractive indices, the HR zone is very broad even for p-polarized light at Brewster AOI. On the contrary to the ZnS/YbF 3 designs considered in Subsection 2.1, the bottle neck of the Ge/YbF 3 designs is the maximal achievable reflectance in both p-and spolarization cases. The reason is that Ge films are absorbing up to 1200 nm and less absorbing in the range 1200-1900 nm. In addition, the optical constants of Ge films exhibit sophisticated patterns, see Fig. 1. Estimations performed with formulas published in [23] show that in the case of neglecting absorption in Ge layers, the reflectance in the HR zone could achieve a high level of 99.9% even for 8-layer solution. In the reality, presence of absorption in Ge layers reduces the achievable reflectance values down to the level of 75-80%. A rough analytical estimation of the decreased reflectance ( , ) s p abs R can be obtained using expressions for the reflectance of a quarter-wave mirror in the absorbing case [22], p. 82: where H k is the extinction coefficient of Ge film at the central wavelength 0 λ .

Beamsplitter comprising layers of ZnS and Ge materials
Synthesis process based on the well established thin-film materials pair Ge/ZnS provides designs which are beneficial in comparison with neither Ge/YbF 3 nor ZnS/YbF 3 solutions found above. Multiple design attempts lead to QPS of the type ( )  Fig. 6(a). In Fig. 6(b), the structure of the BS-Ge/ZnS design is depicted. Increasing the number of design layers and coating thickness do not lead to improvement of the spectral performance. As Ge layers are absorbing, the average reflectance values in the HR zones do not exceed 91.5% and 69.8% in the case of s-and ppolarization, respectively. As the refractive index ratio of Ge and ZnS is not big in comparison to the pair Ge/YbF 3 , the HR zones of BS-Ge/ZnS are narrower. These considerations enable one to conclude that the Ge/ZnS pair is not suitable for the considered design problem. It should be noted that both beamsplitters chosen for the deposition.

Production and characterization
Two experimental samples, named BS-ZnS/YbF 3 and BS-Ge/YbF 3 , were produced at the SyrusPro 710 high vacuum system (Leybold Optics GmbH, Alzenau, Germany). The samples are based on the corresponding designs BS-ZnS/YbF 3 and BS-Ge/YbF 3 discussed in Section 2.
The coatings were deposited on ZnSe substrates of 1 mm thickness. The substrate temperature during the deposition was 120°. The vacuum system was pumped down to 6 10 − mbar before the process. The deposition rates for Ge, ZnS, and YbF 3 , were 0.6 nm/sec, 1 nm/sec, and 0.3 nm/sec, respectively. The evaporation materials Ge and YbF 3 were initially in granules of 0.7-3.5 mm, purity 99.999% and 1-3 mm, purity 99.99%, respectively. The materials were preconditioned in order to obtain solid discs. ZnS was in granules 1-5 mm, purity 99.99%. After the deposition, optical and mechanical properties of the samples were investigated.
• Adhesion testing is very important to evaluate the quality of optical coatings and their principal applicability in desired applications. A comprehensive review of different adhesion measurement techniques can be found in [24]. In the present work, a simplest type of adhesion test, a tape test, was used. According to [15], tape tests are addressed as test of "go-no-go" nature. A fresh piece of a kapton tape (1x1 cm) was carefully glued on the coating and then the tape was removed steadily in the direction normal to the coated surface. The tests were performed three time within half a year, every two months. Both samples exhibit excellent adhesion.
• Among a variety of environmental factors, which should be tested [15], the two that are important for potential applications of the produced filters in the laser setups, namely humidity and temperature influence. The samples were stored at room temperature without any special precautions. Their spectral characteristics, reflectance and transmittance, in the visible-near-infrared and mid-infrared ranges were recorded every two months. No shifts of the spectral curves were observed. The shift could be an evidence of increasing content of water in the samples. The thermal stability is important for laser related coatings [25]. The produced coatings can definitely survive at the temperatures up to 120-150°. Data on the laser damage threshold and thermal lensing lies out of the scope of the present study. The coatings will be implemented into laser setups and will not suffer from such environmental factors as corrosive fluids, rain, fog, dust storms, and vibrations.
• Normal incidence transmittance in the visible-near-infrared spectral range from 400 to 2600 nm was measured with the help of Lambda 950 spectrophotometer (Perkin Elmer) with the wavelength step of 2 nm. Excellent agreement between experimental and theoretical data can be observed in Fig. 7(a) and Fig. 9(a).
• Normal incidence transmittance data in the MIR range Fig. 8 It should be noted that the films produced by e-beam evaporation do not exhibit ideal dense structure, porosity in the films is inevitable. After taking out to the atmosphere, the porous are filled with water that is clearly observed in Fig. 8. An absorption peak around 4.2 µm is presented in the measurement of the uncoated ZnSe substrate as well and can be attributed to water vapor in the air. The outstanding feature of the produced samples is that they do not absorb additional O-H after a lapse period, see Fig. 8. • Quasi-normal incidence reflectance as well as oblique incidence reflectance in s-and ppolarization cases at AOI = 45° was measured at the Universal Reflectance Accessory of Lambda 950 in the spectral range of 400-2200 nm, Fig. 7(c) and Fig.  9(c).
• Reflectance at the Brewster angle of 67.9° was measured using a Universal Measurement Accessory of the Cary 7000 spectrophotometer in a spectral range of 400-2500 nm. In Fig. 7(d) and Fig. 9(d), an excellent correspondence between experimental and theoretical spectral performances can be observed.
• Mechanical stresses of the samples were estimated using the formula from [5,6]: where E and ν are Young's modulus and Poisson's ratio of the substrate, respectively, s Σ and f Σ are thicknesses of the substrate and a single thin film, 1 2 , R R are radius of curvature before and after deposition, respectively. First, the radius of curvature 1 R of several 1-mm glass substrates were measured using Dektak 150 Stylus Profiler (Veeco). Then, thick layers of Ge, ZnS, and YbF 3 were deposited on these pre-measured substrates, and the radius 2 R were measured at the Profiler. Stress values of Ge, ZnS, and YbF 3 were calculated using Eq. (8) equal to (−50) MPa, (−400) MPa, and 140 MPa, respectively. It means that Ge and ZnS layers exhibit compressive stress, and YbF 3 films have tensile stress [14]. Geometrical thicknesses of high-and low-index layers and stresses in the designed coatings BS-ZnS/YbF 3 , BS-ZnS/YbF 3 -20layers, BS-Ge/YbF 3 , and BS-Ge/YbF 3 -14layers are collected in Table 3. It is seen that the total stresses are not big since the stresses in high-and low-index materials compensate each other. It can be also observed in Table 3 that absolute values of stresses in ZnS/YbF 3 and Ge/YbF 3 coatings are comparable. In the last row of Table  3, expected values of the stress in the beamsplitter comprising ZnS and Ge layers (Section 2.3) is presented. One can see that these values are higher than the stresses in the ZnS/YbF 3 and Ge/YbF 3 beamsplitters.

Discussion
The study carried out in the present work demonstrates two pairs of materials suitable for the production of sophisticated broadband coatings operating in the near-infrared and midinfrared spectral ranges. Two beamsplitters consisting of ZnS/YbF 3 and Ge/YbF 3 layers were designed, produced and characterized. The beamsplitters operate at the Brewster angle of the ZnSe substrate. Excellent agreement between experimental and theoretical curves of the produced samples confirms the reliability of the dispersion curves of all three thin film materials and ZnSe substrate. Measurements of mechanical stresses show that due to opposite signs of the deflections caused by high-and low-index layers, the total stresses of the produced multilayers are at a quite low level. In Table 4, pro et contra of ZnS/YbF 3 and Ge/YbF 3 combinations are collected.

Conclusions
The estimations presented in Section 2 of the present work demonstrate that the only one disadvantage of the Ge/YbF 3 multilayers in comparison with ZnS/YbF 3 coatings is that Ge layers are increasingly absorbing in the considered spectral range from 770 to 1050 nm since this is the range of the dominance of electronic transitions [27]. However, due to a big refractive index ratio of Ge and YbF 3 thin-film materials, even this absorption is not essential limiting factor. Evidently, moving to the longer wavelengths, absorptance in Ge layers will decrease rapidly and therefore, Ge/YbF 3 pair has a very large potential for design and production of NIR-MIR coatings.