WO2024155532A1 - Multilayer optical thin films with continuously varying refractive index - Google Patents

Abstract

A system and a method fabricating thinner and lighter optical polymers of high image quality using vapor flow with ratio of a high refractive material RH and a low refractive index material RL controlled in real time to form a film comprising a stack of layers with very precisely, individually controlled refractive indexes and thicknesses. Examples of the RH materials are poly(4-vinylpyridine) (P4VP) and Divinylbenzene (DVB) monomers, and examples of two RL materials are (a) RL1, which is poly(1H,1H,2H,2H-perfluorodecylacrylate) (pPFDA) and (b) RL2, which is poly(1H,1H,6H,6H-perfluorohexyl diacrylate) (pPFHDA).

Description

Multilayer Optical Thin Films with Continuously Varying Refractive Index

REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to and incorporates by reference the entirety of U.S. Provisional Patent Application Ser. No. 63/439,457 filed January 17, 2023.

FIELD

[0001] This patent specification pertains to thin films designed to improve optical and other properties of components of optical systems.

BACKGROUND

[0002] The numbers in square brackets in the description below refer to publications fully identified at the end of the written description, each of which is hereby incorporated by reference.

[0003] Optical devices can be improved significantly by thinner and lighter optical materials that do not compromise image quality. This has garnered interest in gradient or graded refractive index materials that can be used for compact optical design. The graded-index material can have a continuous index gradient, or they can comprise discrete layers with different refractive indexes. Stacks of nanolayer polymer films can be used in Plano-convex Graded Refractive Index (GRIN) lenses [1] (Hsieh et al. 2019), [2] (Jin et al. 2007), antireflection coatings [3] (Wu et al. 2006), waveguides [4] (Brown et al. 2016), and optical fibers [5] (Anuszkiewics et al. 2018).

[0004] Graded-index materials can enhance focusing power, correct aberrations, and reduce the number of components needed for an effective optical system. To date, only a few flexible and all-dry processes are believed to available that can fabricate copolymers with large overall refractive index changes and control the thickness of the deposited films [14] (Jiang et al. 2004).

[0005] It is particularly challenging to create optical coatings with spatially varying optical thickness in a low thickness range (less than wavelength of the light). While using some complex processing techniques of polymer multilayers may potentially produce graded-index structures, it is challenging to control the differences in the thickness or refractive index of each layer. Alternatively, some efforts have been made to control the optical thickness, but these methods are believed to require advanced equipment and wet chemistry techniques to fabricate multilayer thin films [23] (Szczesny et al. 2020).

[0006] Plasma-enhanced chemical vapor deposition (PECVD) is a well- established technique and is widely used for deposition of a wide range of materials, including optical coatings [13] (Jiang et al. 2008), [10, 11] (Hiller et al. 2002). Despite its several advantages, PECVD films can lack the regular repeat units found in conventional polymers. This may be due to the complexity of plasma: the creation of a plasma in the deposition chamber can not only activate and decompose the precursor gases, but also a highly energetic species in plasma can result in partial excitation, dissociation and ionization process that can lead to multitude of reactions on the substrate [8] (Cools et al. 2019). Plasma copolymerization is a highly intricate process that involves not only polymerization but also other reactions like cross-linking, rearrangement, fragmentation, elimination, and ablation. Due to the strong competition among these reactions, the resulting films are a complex blend of oligomers and polymer fragments. [13] (Jiang et al. 2008).

SUMMARY

[0007] The summary below reflects the initially claimed subject matter, which can evolve in prosecution of this patent application. [0008] The disclosure below relates to innovative polymerization approaches to control optical properties of nanoscale multilayer films. Ultrathin layers of copolymer films with independent control over the refractive index can manipulate the interaction of light with the surface. The new technology described herein enables fabrication of graded-index material in the form of (a) continuous index gradient films or (b) multilayer thin films that comprise discrete layers with different refractive indices.

[0009] As described in greater detail below, nanoscale polymer thin films are synthesized with exquisite thickness control, outstanding conformality, and high spatial uniformity. By controlling the composition of monomers in the precursor mixture, desirable optical properties of the films can be manipulated, and thin films can be fabricated with a large, yet controllable, range of refractive indexes. In addition to fabricating single-layer copolymer and homopolymer thin films in a wide range of refractive indexes, the new technology can form multilayer thin films with refractive index gradients that vary continuously or in discrete steps. The optical constant and the thickness profile of multilayer thin films can be monitored with in-situ ellipsometry, and the results validated with ex-situ ellipsometry.

[0010] According to some embodiments, a system for fabricating a nanoscale multilayer optical film with real-time control over refractive indexes and layer thickness in the multilayer film comprises: a deposition chamber (10), a stage (22) comprising a substrate, a vapor flow source (20) for selectively supplying to said stage a vapor flow comprising a high refractive index component RH and a low refractive index component RL in a controlled ratio RH/RL for initiated chemical vapor deposition over the substrate forming a polymer and thus fabricating on the substrate a multilayer film (30, 72, 74) in which the layers are in a stack, each layer has a respective refractive index, and the refractive indexes differ between at least two of the layers; an in-situ measurement system comprising a source (26) of a beam impinging on said film as the layers thereof are being fabricated and a detector (28) receiving the beam after modulation thereof by interaction with said layers; and a control operatively coupled with the vapor flow source and with said measurement system and configured to control said ratio RH/RL and thicknesses of said layers in real time during fabrication of said multilayer film in response to outputs of said measurement system.

[0011] According to some embodiments, the system can further comprise one or more of the following: (a) the vapor flow source can be configured to supply a vapor flow in which the component RH is at least one of 4-vinylpyridine (4 VP) and Divinylbenzene (DVB), and the component RL is at least one of (i) RLI, which is lH,lH,2H,2H-perfluorodecylacrylate (PFDA) and (ii) RL2, which is lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA); (b) the component RH can be only 4 VP; (c) the component RH can be only DVB; (d) the component RL can be only RLI; (e) the component RL can be only RL2; (f) the thickness of the stack of layers or of at least one of the layers of the the fabricated thin multilayer film stack of layers can be less than 100 nm; (g) the thickness of at least one of the layers in the stack can vary by less than 10% over an area 8 cm in diameter; (h) the layers in said stack of layers can be in an ordered sequence of low to high refractive index values in a selected direction; (i) the layers in said stack of layers can be in an arbitrary sequence of refractive index values; (j) the stack of layers can have a thickness that is uniform such that local thicknesses differ by no more than 0.2 nm in a film that is at least 8 cm in diameter; (k) the refractive indexes of the layers change in steps from one layer to another; (1) the refractive index of the stack of layers changes essentially continuously from one layer to another so that the stack is essentially a continuum of infinitesimally thin layers; (m) at least one of the layers in the stack can be a homopolymer; (n) at least one the layers in the stack can be a co-polymer; (o) at least one of the layers in the stack is a homopolymer; and (p) at least one of the layers is a co-polymer.

[0012] According to some embodiments, a nanoscale multilayer optical film comprises: a stack (30, 71, 74) of layers having respective refractive indexes; wherein each layer comprises a polymer of a high refractive index component RH and/or a low refractive index component RL in a controlled ratio RH/RL such that the refractive index in the stack changes with thickness of the stack; wherein the component RH is at least one of 4-vinylpyridine (4 VP) and Divinylbenzene (DVB), and the component RL is at least one of (i) RLI, which is 1H,1H,2H,2H- perfluorodecylacrylate (PFDA) and (ii) RL2, which is lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA).

[0013] According to some embodiments, the multilayer optical film described in the immediately preceding paragraphs can further comprise one or more of the following: (a) the component RH can be only 4 VP; (b) the component RH can be only DVB; (c) the component RL can be only R I; (d) the component RL can be only RL2; (e) the stack of layers can be less than 100 nm thick.; (f) at least one of the layers can be less than 100 nm thick and the thickness thereof can vary by less than 10% of an average thickness of the layer over an area 8 cm in diameter; (g) the layers in said stack of layers can be in an ordered sequence of low to high refractive index values in a selected direction; (h) the layers in said stack of layers can be in an arbitrary sequence of refractive index values; (i) the stack of layers can have a thickness that is uniform such that local thicknesses differ by no more than 0.2 nm over a film that is at least 8 cm in diameter; (j) the refractive indexes of the layers can change in steps from one layer to another; (k) the refractive index of the stack of layers can change essentially continuously from one layer to another so that the stack is essentially a continuum of infinitesimally thin layers; (1) at least one the layers can be a homopolymer; and (m) at least one of the layers can be a co-polymer.

[0014] According to some embodiments, a method of fabricating a nanoscale optical device comprises; supplying a deposition chamber (10) with vapor flow (20) comprising a high refractive index component RH and a low refractive index component RL to fabricate a stack of layers of a polymer on a substrate by vapor deposition; measuring in-situ and in real time refractive and physical properties of the stack as the stack is being fabricated; and controlling in real time the ratio RH/RL of said vapor flow and a time for said vapor deposition as a function of said measuring as the stack is being fabricated to thereby control the respective refractive index of each layer and the respective thickness of each layer and to cause the layer refractive indexes to differ between at least two of the layers.

[0015] The method can further comprise one or more of the following: (a) the component RH can be at least one of 4-vinylpyridine (4 VP) and Divinylbenzene (DVB), (b) the component RH can be only 4 VP; (c) the component RH can be only DVB; (d) the component RL can be at least one of RLI, which is lH,lH,2H,2H-perfluorodecylacrylate (PFDA) and RL2, which is 1H,1H,6H,6H- perfluorohexyl diacrylate (PFHDA); (e) the component RL can be only R I; (f) the component RL can be only R 2: (g) the stack of layers can be less than 100 nm thick; (h) the controlling step can arrange the layers in an ordered sequence of low to high refractive index values in a selected direction; (i) the controlling step can arrange the stack of layers in an arbitrary sequence of refractive index values; and (j) the controlling step can result in a stack thickness that is uniform such that local thickness varies by no more than 0.2 nm in an area at least 8 cm in diameter, or in a stack in which at least one of the layers is less than 100 nm thick and has local thickness that varies by to more than 10% over an area at least 8 cm in diameter.. [0016] According to some embodiments, a nanoscale, multilayer optical film comprises: a stack (30, 71, 74) of layers having respective refractive indexes, wherein at least two of the layers have refractive indexes that differ from each other; each of at least some of the layers comprises a polymer of a high refractive index component RH and/or a low refractive index component RL in a controlled ratio RH/RL such that the refractive index in the stack changes with thickness of the stack; and the thickness of at least one of the layers is precisely controlled such that the layer thickness is less than 100 nm and varies by less than 10% in an area at least 8 cm in diameter.

[0017] According to some embodiments, the film described in the immediately preceding paragraph can additionally include one or more of: (a) at least one of the layers is a homopolymer; and (b) at least one of the layers is a copolymer.

BRIEFS DESCRIPTION OF THE DRAWINGS

[0018] To further clarify the above and other advantages and features of the subject matter of this patent specification, specific examples of embodiments thereof are illustrated in the appended drawings. It should be appreciated that these drawings depict only illustrative embodiments and are therefore not to be considered limiting of the scope of this patent specification or the appended claims. The subject matter hereof will be described and explained with additional specificity and detail using the accompanying drawings in which:

[0019] Fig. 1 shows an example of a high refraction index material RH (4VP) and low refraction materials RL, namely, R I (PFDA) and RL2 (PFHDA), useful in fabricating polymer thin films with continuously varying refractive index gradients, according to some embodiments. [0020] Fig. 2 schematically illustrates an iCVD (initiated Chemical Vapor Deposition) system, in-situ ellipsometry, a fabricated multilayer thin film, and an indication of changes in a feed ratio of two components RH and RL, according to some embodiments.

[0021] Fig. 3 illustrates in graphical and table forms ellipsometry results for single-layer thin films fabricated with different ratios of RLI (pPFDA) and RH (p4VP), according to some embodiments.

[0022] Fig. 4 is otherwise like Fig. 3a but shows results for single-layer thin films fabricated for with RL2 (pPFHDA) and RH (p4VP), according to some embodiments.

[0023] Fig. 5 shows in graphical and table form ellipsometry results for single-layer co-polymer and homopolymer thin films with continuously varying refractive index, acquired by in-situ ellipsometry measurements validated with ex situ ellipsometry, including the refractive index of a single-layer film as a function of the ratio RH/ R 2 (poly (4-vinylpyridine (p4VP)) /((poly(lH,lH,6H,6H- perfluorohexyl diacrylate) (pPFHDA)), the thickness uniformity of the layer, and the optical transparency of the layer across the visible and NIR (near infrared) regions, according to some embodiments.

[0024] Fig. 6 illustrates how the refractive index of a multilayer film changes as a function of distance from the substrate in nm for three different arrangements of high refractive index and low refractive index layers of a multilayer film and the optical constant depth profile of the films, according to some embodiments.

[0025] Fig. 7 illustrates in graphical form in-situ measured thickness profiles of multilayer thin films, according to some embodiments.

[0026] Fig. 8 illustrates in graphical form refractive index measurements obtained by ellipsometry as a function of DVB monomer composition in (%) in panel a, and dispersion of refractive index versus wavelength for different composition of the high index and low index monomer in panel b, according to some embodiments.

[0027] Fig. 9 illustrates in graphical form FTIR spectra in panel a, XPS survey scan spectra in panel b, Cis high resolution scan spectra of homopolymerized PFHDA (top) and DVB (bottom) films, according to some embodiments. Spectra in between bottom to top are from copolymer films using a decreasing DVB/PFHDA feed ratio. The spectra are normalized based on their thickness for clarity. Panel d illustrates linear dependence of the refractive index on the F/C atomic composition ratio of each film.

[0028] Fig. 10 illustrates in graphical form thickness evolution in panel a, and refractive index evolution of poly(DVB-co-PFHDA) in different temperature profiles from 25°C to 300°C for a prolong time of 12 hours, according to some embodiments.

[0029] Fig. 11 illustrates in graphical form in situ monitoring thickness and refractive index variations of multilayer thin films with ellipsometry, according to some embodiments. The first layer is DVB homopolymer, the second and the third layers are DVB-co-PFHDA and PHDA homopolymer, respectively.

DETAILED DESCRIPTION

[0030] A detailed description of examples of preferred embodiments is provided below. While several embodiments are described, the new subject matter described in this patent specification is not limited to any one embodiment or combination of embodiments described herein, but instead encompasses numerous alternatives, modifications, and equivalents. In addition, while numerous specific details are set forth in the following description to provide a thorough understanding, some embodiments can be practiced without some or all these details. Moreover, for the purpose of clarity, certain technical material that is known in the related art has not been described in detail to avoid unnecessarily obscuring the new subject matter described herein. Individual features of one or several of the specific embodiments described herein can be used in combination with features of other described embodiments or with other features. Further, like reference numbers and designations in the various drawings indicate like elements. [0031] Initiated Chemical Vapor Deposition (iCVD) can produce thin films with higher purity due to the absence of plasma. [24] (Tenhaeff and Gleason 2008), [28] (Schroder, et al. 2020). iCVD can involve introducing a monomer and initiator into a vacuum chamber that contains resistively heated filaments. As the initiator breaks down into radicals, a free-radical polymerization of the monomer is initiated at the substrate surface. This technique allows for the replication of solution-phase free-radical polymerization in the vapor phase, making it possible to deposit a wide range of thin polymer films. iCVD is not only an efficient technique for fabricating homopolymers, but it can also be used to fabricate a wide range of copolymers. Moreover, iCVD copolymerization generally does not involve monomer fragmentation as in the process of plasma copolymerization, resulting in a well-designed polymer structure with regular repeating units and pendant functional groups. This also influences the uniformity of the deposition, and better thickness control and over all higher quality films.

[0032] This patent specification describes several examples of fabricating nanolayer polymer films with accurately controlled thicknesses and refractive indexes using different combinations of high and low refractive monomers in an iCVD process and a carrier gas such as Argon infusing monomer liquids and transporting monomer vapors into the reaction chamber.

[0033] In accordance with some embodiments, several polymer thin films were fabricated, with a wide range of refractive indexes and excellent optical transparency in the visible range. Two sets of monomers were used in examples described herein: (a) a high refractive index monomer, and (b) a low refractive index monomer.

[0034] For a first series of samples, 4-vinylpyridine (4 VP) was used as the high refractive index monomer (RH) and it copolymerized with 1H,1H,2H,2H- perfluorodecylacrylate (PFDA), which was used as the low refractive index monomers (RLi).

[0035] For a second series of samples, RH was copolymerized with lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA), which was used as the RL2. [0036] For a third series of samples, Divinylbenzene (DVB) monomers were used as the high refractive index monomer (RH) and copolymerized with lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA), which was used as the RL2. [0037] Work utilizing two monomers for the first and second series of samples resulted in thin films with a large refractive index difference when individually polymerized with iCVD. By sequentially depositing each monomer in a specific order, one layer at a time, until the stack reaches a thickness of a quarter wavelength (X/4), a multilayer stack was created. This resulted in the formation of a stop band, also known as a reflection notch, [17] (Kumar et al., 2020), [14] (Jiang et al., 2004) at the design wavelength. The stop band refers to a range of wavelengths where the multilayer stack strongly reflects light while allowing other wavelengths to pass through. This can be used in the design of optical filters and mirrors [15] (Kedawat et al., 2013). Simple alternating layer filters can suffer from issues that can lower the overall transmission of any filter design [14] (Jiang et al., 2004). To overcome with these limitations, researchers have developed more complex structures to prepare slowly varying refractive index modulation in films. To prepare such complex structures, it is important to control the optical thickness, and a key to achieving this control is by preparing multilayer graded-index material [11] (A. Hiller et al., 2002). Such optical coatings are useful for many other applications, especially in polymeric nanophotonic devices [22] (Purayil et al., 2021), and design of the waveguide for augmented reality (AR) glasses [6] (Boo et al., 2022).

[0038] Adding RH to fluorinated acrylate backbone (RL) reduced the C-F bond density in the co-polymer and thereby increased the refractive index. Finely tuning the feed gas ratio (RH/RL) enabled fabrication of thin films that were highly transparent in the visible and near-infrared regions in a wide range of refractive indices (e.g., 1.35 to 1.58). The optical performance of the single-layer coatings was characterized by in-situ spectroscopic ellipsometer. In-situ ellipsometry results were validated by ex-situ ellipsometry with multiple angles of incidence to ensure high accuracy of the measurements. Ellipsometry mapping showed high uniformity of the copolymer thin film. The thickness ranged from 20.9 to 21.1 nm in a 50.2 cm² scanned area (8 cm diameter), showing that the film thickness varies by only 0.2 nm between the extremums, indicating excellent uniformity of the coating. Preferably, at least one of the layers in the stack of layers is less than 100 nm thick and its local thickness varies by less than 10% over an area at least 8 cm in diameter. This variation can be relative an average thickness over such area.

[0039] In addition to successfully developing uniform, transparent, and highly reproducible single-layer polymeric thin films, multilayer thin films were developed with an arbitrary stack structure. To confinn the arbitrary layout stack structure, three distinct types of multilayers were prepared. Each stack comprised four discrete layers, having a different refractive index profile. A first stack structure (high index to low index) comprised a first layer of high refractive index homopolymer, a second layer of a copolymer having a second refractive index (which is slightly lower than the first layer), a third layer that also was a copolymer having a third refractive index which is slightly lower than the layers beneath it, and finally, a last layer that was the low refractive index homopolymer. The second and third stack structures were essentially the same materials but in different orders, namely, an arbitrary order and a low index to high index order. As the refractive index spatially varies through the thickness of multilayer thin films, in- situ ellipsometry was carried out during layer fabrication to measure the optical constant and thickness profile of the multilayer thin films. When comparing the in- situ ellipsometry results with multiangle ex-situ ellipsometry, the results were almost identical, showing high accuracy of the in-situ measurements. Additionally, the obtained results showed excellent reproducibility, as sample-to-sample variation in the refractive index for a given composition was negligible. Overall, these unique copolymer compositions of thin films with varying refractive indices are expected to have wide applications in developing GRIN materials in the nanometer range with well-controlled optical and other properties.

[0040] Fig. 1 shows examples of materials useful in fabricating thin films with continuously variable refractive index gradient, according to some embodiments. An example of a High Index (RH) material is (4-vinylpyridine) (4 VP). Two examples of Low Index (RL) materials are: (a) a first Low Index (RLI) Material (lH,lH,2H,2H-perfluorodecylacrylate) (PFDA), and (b) a second Low Index (RL ) Material (IH,lH,6H,6H-perfluorohexyl diacrylate) (PFHDA). Fig. 1 shows the compositions and structures of these examples of suitable materials. [0041] Fig. 2 schematically illustrates an iCVD (initiated Chemical Vapor Deposition) system, in-situ ellipsometry, a fabricated multilayer thin film, and an indication of changes in a feed ratio of two components RH and RL, according to some embodiments. As iCVD is a known process, details of conventional equipment are not described in detail and the focus is on the features unique to the new process of this patent specification. As illustrated, a vapor flow source 20 of a mixture of High Index RH and Low Index RL vapors at a controlled ratio of 4 VP to PFHDA is fed into a deposition chamber 10 such that surface polymerization takes place over a substrate with vapor activation of an initiator 23. The deposition chamber possesses two optical windows 24 mounted inside vacuum flanges (not shown in drawing) through which a beam, such as a polarized light, from a source 26 enters and is modulated by interaction with (reflection from) the film being formed on the substrate before reaching a detector 28. In one example, the beam has an incident angle of 70 degrees with respect to the normal of the substrate on which the film is formed. The process fabricates a multilayered film 30 that in this example comprises a bottom layer of High Index material, a top layer of Low Index material and intermediate layers of material each with intermediate index of refraction. As illustrated by the arrow at right in Fig. 1, at the start of the process the PFHDA/4VP ratio (RL/RH) of the feed vapor is adjusted such that the feed vapor from source 20 is only, or essentially only 4VP, the high index material, carried in Argon gas, to thereby form a bottom layer of High Index material. The RL/RH ratio of feed vapor 20 is then increased gradually or in steps to form the intermediate layers. Finally, the RL/RH ratio is adjusted such that vapor flow from source 20 is essentially only PFHDA (the low index material) in carrier gas. During layer fabrication, the ratio can be controlled as a function of the measurements from detector 28. The arrow to the right of stack 30 illustrates how the ratio RL/RH changes with height in the stack of layers. A control 21 is operatively coupled with chamber 10 and detector 28 to receive measurements of the layers as they are fabricated and to control the ratio RL/RH and the deposition time DT to thereby direct the process to fabricate a thin film in the form of a stack of layers that have the desired refractive index and height (thickness). In situ spectroscopic ellipsometry (SE) as used here is a nondestructive optical technique for studying various deposition and etching processes [16] (Kovalgin et al., 2017). It uses polarized light and measures the polarized state of the beam reflected from the sample as function of wavelength. This technique makes it possible to optimize optical properties of deposited films, control film growth with few nanometers’ sensitivity, and monitor growth kinetics. To interpret the measured data, a reliable optical model is used. In this work, the optical models comprise of a silicon (Si) substrate with a single or multiple-layer Cauchy layer on top. For transparent films, which is the case for the samples discussed here, the extinction coefficient, k, is zero over the wavelength range 400-1690 nm. To avoid modeling complexities associated with abortion, a Cauchy model was used to calculate optical properties of thin films.

[0042] Fig. 3 illustrates, in graphical and table forms, ellipsometry results for single-layer thin films fabricated with changing ratios of PFDA and 4 VP, according to some embodiments. The two graphs and the table illustrate how the refractive index of the single-layer thin film changes at different ratios RH/RLI of the High Index material RH (4 VP) to the Low Index material RLI (1H,1H,2H,2H- perfluorodecylacrylate) (PFDA)), using the system illustrated in Fig. 2. The table at the lower part of Fig. 3 lists the parameters for successively formed single layer homopolymer and co-polymer thin films. The first line of the table informs that a first film was formed with vapor flow from source 20 of 50 seem (standard cubic centimeters per minute) R I (V PFDA) and 0 seem RH (v_4VP), the deposition time t dep to form the first layer was 34 minutes, the thickness h of first layer was 227 nm, the deposition rate DR for the first layer was 6.7 nm/min, and the refractive index RI was 1.355 at 632.8 nm light wavelength. The lowest curve in the graph at upper left in Fig. 3 shows how the refractive index of the first film changes with wavelength of light and that it is 1.355 at 632.8 nm wavelength. The second line in the table lists the parameters of the film at the stage after a second sample has been fabricated. The table shows that the second film was fabricated with vapor flow of 50 seem RLI and 1 seem RH, the deposition time t dep to form the second layer was 14 minutes, the thickness h of the second layer was 217 nm, the deposition rate DR to form the second layer was 15.5 nm/min, and the refractive index RI of the second film at this time was 1.398 at 632.8 nm wavelength. The table similarly states these parameters for the subsequently formed films. The graph at upper right in Fig. 3 shows how the refractive index of the films changes as a function of the RH/R I ratio, and that refractive index of the last film is 1.558 (at the same wavelength of 632.8 nm).

[0043] Fig. 4 is otherwise like Fig. 3 but shows results of fabricating a single layer thin films using a different Low Index material RL2 (1H,1H,6H,6H- perfluorohexyl diacrylate) (PFHDA)) and the same High Index material RH. The table in Fig. 4 shows the composition of the vapor flow from source 20 for the different layers, i.e., the flow rate of R 2 (V PFHDA), of RH (v_4VP), the deposition times t dep to form the film, the thickness h of the layer, the deposition rates DR to form the layer, and the changes of refractive index RI of the single layer films. As in Fig. 3, Fig. 4 shows at upper left how the refractive index of the single layer films that is being fabricated changes with wavelength of incident light and shows at upper right how the refractive index of the fabricated single-layer films change with the altering the RH/RL2 ratio.

[0044] Fig. 5 illustrates in graphical fonn ellipsometry results for single-layer copolymer and homopolymer thin films that have continuously varying refractive index, according to some embodiments. The results are from in-situ ellipsometry measurements as described for the system of Fig. 2, validated with ex-situ ellipsometry, including the refractive index of a single-layer film as a function of the ratio RH/ R 2 (lH,lH,6H,6H-perfluorohexyl diacrylate) (PFHDA)), the thickness uniformity of the layer, and the optical transparency of the layer across the visible and NIR (near infrared) regions. The graph at upper left shows how the reflective index of the layer changes as a function of the ratio RH/ R 2 and that the curve from in-situ ellipsometry conforms to that from ex-situ ellipsometry, indicating high accuracy of the in-situ measurements. The plot in the center of Fig.5 is ellipsometry mapping that shows high uniformity of the thickness of the single-layer film. The film sample was over 50 square cm (8 cm in diameter). The scale on the right of the thickness uniformity plot shows the thickness range in nm. The graph at right in Fig. 5 illustrates optical transmittance in % as a function of wavelength across the visible and NIR (near infrared) regions for each of several films fabricated with respective different ratios RH/ R 2 compared with the transmittance of an uncoated substrate. The ratios of RH/ R 2 to which the respective curves correspond are indicated in the insert at right. The shown result indicated that all samples (homopolymer and co-polymer) exhibit excellent optical transparency across the visible and NIR regions. Reflection loss was not excluded in the shown results. The results are for an uncoated substrate and for the same substrate covered with the fabricated film.

[0045] Fig. 6 illustrates how the refractive index of multilayer films changes as a function of distance from the substrate in nm for three different arrangements of ordering layers in a stack to fabricate a multilayer film and the optical constant profile of the films, according to some embodiments. The panel at left in Fig. 6 shows an arrangement in which the bottom layer 1 is a high refractive index material, the top layer 4 is a low index material, the intermediate layers 2 and 3 have indexes between those of bottom and top layers 1 and 4, with the index of layer 2 being greater than that of layer 3. The graph in the middle of the left panel shows the respective refractions indexes of layers 1-4 as a function of their distance in from a substrate, and the refractive index of a native oxide. The table at bottom in the left panel of Fig. 6 identifies the layers and for each layer lists the flow rate in seem of the RH component in vapor flow 20, the flow rate of RL2 in seem in vapor flow 20, the layer thickness in nm, and the refractive index RI at 632.8 nm wavelength light. The optical constant depth profile was at 634.5 nm. The middle panel of Fig. 6 is otherwise like the left panel but shows results for a multilayer film with a different, arbitrary arrangement of layers - the bottom layer 1 is an intermediate refractive index layer, layer 2 is a low index material, layer 3 is a high index material and the top layer 4 is an intermediate index material. The graph in the middle of the middle panel of Fig. 6 and the table in the middle panel are structured the same way as in the left panel. The right panel in Fig. 6 is otherwise like the left panel but for a multilayer film in which the order of the layers is reversed - the low refractive index layer 1 is at the bottom of the stack of layers, the top layer 4 layer is the high index material, and layers 2 and 3 have indices intermediate those of layers 1 and 4, with the index of layer 3 greater than that of layer 2. The results shown in Fig. 6 indicate that the fabrication this patent specification describes achieves excellent control over thickness and refractive index for multilayer thin films.

[0046] Fig. 7 illustrates in-situ measured thickness profiles of multilayer thin films with different thicknesses, according to some embodiments. The left panel of Fig. 7 shows a stack 72 of layers 1-4 that form a multilayer film over a Silicon substrate covered with a 2.6 nm thick native oxide. The graph in the left panel of Fig. 7 shows the change in thickness in nm of the multilayer film with time in minutes as the film is being fabricated as described above. Real time monitoring, using a system as described in connection with Fig. 2 shows exquisite control of the thickness growth during the film deposition process. The deposition rates in the examples of Fig. 7 were kept intentionally low to facilitate in-situ characterization. The deposition rates can be much higher in a scaled-up process. The panel at right in Fig, 7 is otherwise like the panel at left but is for a stack 74 forming a multilayer film of layers 1-4 that have the different indicated thicknesses.

[0047] The innovative copolymerization approach described herein effectively controls the optical properties of multilayer films. Ultrathin layers of copolymer films with independent control over the refractive index manipulate the interaction of light with the surface. iCVD thin film processing enables fabricating graded-index material in the form of either: a continuous index gradient or multilayer thin films which comprise discrete layers with different refractive indices. Controlling the composition of monomers enabled manipulating the optical properties of the films and fabricating thin films with a large, yet controllable, range of refractive indices (An >0.01). In addition to fabricating single-layer copolymer and homopolymer thin films in a wide range of refractive indices, multilayer thin films with continuously varying refractive index gradients were fabricated. Finally, monitoring the optical constant and the thickness profile of multilayer thin films with in-situ ellipsometry and validating the results with ex- situ ellipsometry led to the desirable film properties. Employing iCVD which is solvent-free vapor deposition technique enabled synthesizing nanoscale polymer thin films with exquisite thickness control, outstanding conformality, and high spatial uniformity in a relatively short deposition time.

[0048] For the third series of samples, iCVD was used to deposit the single layer and multilayer polymeric thin film, using equipment such as illustrated in Fig. 2. Vapor flow source 20 comprises a bubbler system introducing argon as a carrier gas infusing through monomer liquids and transporting RH and RL monomer vapor into the reaction chamber. The benefits of using the bubbler system have been explained in previous work [27] (Zhao et al., 2021). Argon was used as a carrier gas for both lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA) and Divinylbenzene (DVB) monomers to carry monomer vapor inside the iCVD chamber. Additionally, argon was used as diluent gas which mixes earner gas- monomer flow and initiator flow before entering the reaction chamber. Diluting gas enables a good blend of all gaseous species, also it controls the deposition rate and uniformity of the coating. Argon can be likewise used for the first and second series of samples.

[0049] To start the deposition, a flow of Initiator enters the reaction chamber. An initiator has relatively high vapor pressure, therefore, a carrier gas should be used to establish a flow of initiator in the reaction chamber. The initiator is dissociated when it is in the vicinity of the heated filament array in the reaction chamber, and it forms primary radicals. The polymerization is believed to start by the collision of such radicals with monomer molecules and their adsorption on the substrate surface. The films growth continues until the flux of radicals stops.

[0050] To fabricate the third series of single layer polymer thin films with a wide range of refractive indices, two sets of monomers were used: high refractive index and low refractive index monomers. DVB was used as the high refractive index monomer (RH) and it copolymerized with PFHDA which was used as the low refractive index monomers (R ). The addition of RH to fluorinated acrylate backbone (RL) is believed to reduce the C-F bond density in the copolymer and thereby increases the refractive index.

[0051] For real-time monitoring of thin film processing, an ellipsometer was mounted on the iCVD chamber at an incident angle of 70° with respect to the substrate normal. In situ spectroscopic ellipsometry (SE) is a nondestructive optical technique that has been used for studying various deposition and etching processes [16] (Kovalgin et al., 2017). It uses polarized light and measures the polarized state of the beam reflected from the sample as function of wavelength. This technique makes it possible to optimize optical properties of deposited films, control film growth with few nanometers’ sensitivity, and monitor growth kinetics. To interpret the measured data, a reliable optical model is used. In this work, the optical models comprise a silicon (Si) substrate with a single or multiple-layer Cauchy layer on top. For transparent films, which is the case for all samples discussed in this patent specification, the extinction coefficient, k, is zero over the wavelength range 400- 1690 nm. To avoid modeling complexities, a Cauchy model was used to calculate optical properties of thin films.

[0052] Nanoscale single layer iCVD polymer films were fabricated on Si wafers. The deposition parameters were optimized to develop transparent thin films with high spatial uniformity. The deposition parameters for seven single layer thin films are provided in the table below, where v_DVB and v PFHDA are flow rates of the high index RH (DVB) and low index RL (pFHDA) monomers with Ar as a carrier gas; d is the thickness of the coatings; Rd is deposition rate calculated based on thickness values divided by deposition time; and RI is refractive index measured with ellipsometry: v DVB ^a) v PFHDA d R_d

DVB (%) DVB/PFHDA Rl@632.8

[seem] [seem] [nm] [nm min ^-1]

0 0 10 0 217 11.7 1.427

5 9 50 0.1 364 4.7 1.467

5 11.1 40 0.125 322 2.9 1.478

5 20 20 0.25 207 1.2 1.502

5 33.3 10 0.5 224 2.6 1.532

5 50 5 1 278 2.7 1.561

6 100 0 436 7 1.583

[0053] The homopolymer and copolymer thin films for the third series of samples were fabricated using different feed compositions as detailed in the table above. The thickness and refractive index of the single layer copolymer and homopolymers were collected by in situ ellipsometry. For all samples, chamber pressure (1500 mTorr), stage temperature (30 °C), monomer temperature (25 °C), initiator current (0.7 amps), and initiator flow rate (2sccm) were kept constant. [0054] When compared to homopolymers, the deposition rate of iCVD copolymer films is reduced. This can be attributed to the iCVD process, where the deposition rate of polymers depends on the concentration of the adsorbed monomers on the surface. In copolymerization, at a fixed substrate temperature, the surface concentrations of monomers can vary depending on their partial vapor pressure in the gas phase and their saturation pressures. [7] (Cihanog, 2022), [26] (Yilmaz et al., 2020). PFHDA and DVB have different vapor pressures and they have different surface energy. This can affect the deposition kinetics, which in turn can result in a slower polymerization rate and, therefore, a lower deposition rate for copolymers.

[0055] Ellipsometry was used to characterize the optical properties of all homo- and copolymerized films. Fig. 8 illustrates, in panel a, the refractive index values as a functions of DVB monomer percentage. The refractive index of the coating increased progressively with the increase in the percentage of DVB. When DVB (%) = 0 and DVB (%) = 100, the deposited samples are homopolymers, namely pPFHDA-homopolymer and DVB -homopolymer, respectively. The rest of the samples are poly(DVB-co-pFHDA) copolymers. As indicated in Fig. 2a, the copolymerization process results in films with a continuously varying refractive index. The relationship between feed ratio and refractive index allows for the deposition of films with predictable refractive index profiles.

[0056] Fig. 8 illustrates, in panel b, dispersion of refractive index versus wavelength for different compositions of the high refractive index and low refractive index monomers in the third series of samples. The dispersion of the refractive index is presented as a function of wavelength (400 nm - 1690 nm) for all homo- and copolymerized films of the third series of samples. All samples exhibit normal dispersion, meaning the index of refraction decreases with an increase in wavelength. Furthermore, these plots are nonlinear; they are often steeper in the blue region of light and smoother in the red region of light. This can be used to create anti-reflection coatings, where a gradual decrease in refractive index with wavelength can reduce reflection form the surface of the material. [0057] In addition to refractive index, extinction coefficient k also was measured for all samples, and was essentially zero. As such, it can be concluded that samples exhibit transparency over a wide range of wavelengths, as k directly relates to wave attenuation or light absorption [27] (Zhao et al., 2021).

[0058] Overall, the fact that optical coatings with normal dispersion can be designed to have low reflectivity over a broad range of wavelengths, makes them useful for broadband antireflection coatings which is a goal of this research. Additionally, normal dispersion coatings can minimize chromatic aberration [25] (Wang et al., 2016).

[0059] In the fabrication of optical coatings, ensuring refractive index and thickness uniformity are important as the homogeneity of each layer determines the consistency of optical performance across the entire surface of the optics.

Variations in thickness can result in deviations in optical interference, while defects caused by composition inhomogeneity can cause scattering or haze. Therefore, high compositional and thickness uniformity, low roughness as well as negligible refractive index variations, are desired in coating materials to ensure that the targeted performance of the optical coating can be achieved.

[0060] To determine thickness and refractive index uniformity of the coating ellipsometry mapping was performed on pFHDA-co-DVB -coated sample 10 mm diameter. Thickness ranged from 206 to 209 nm in a 78.5 mm2 scanned area, showing that the film thickness varies by only 2 nm between the extremums, indicating an excellent uniformity of the coating. Additionally, the refractive index of the coating varied ±0.002, indicating a uniform coating composition over the scanned area. The average roughness of the scanned area was below 5 nm, indicating smoothness of the coating.

[0061] An estimator, such as Mean Squared Error (MSE) was used to quantify the difference between curves. The lower the MSE indicates a better fit and better optical model. The mapping results showed a very low MSE value which is an indication of reliable fitting.

[0062] The changes in chemical composition of the iCVD films with the feed ratio were substantiated by Fourier-transformed infrared (FTIR) spectroscopy, as shown in Figure 9, panel a. The bottom curve is the FTIR spectrum of DVB homopolymer film and the top one is that of the pFHDA homopolymer film. Between these two are the FTIR spectra of the copolymer films, with a progressive decrease in the DVB/pPFHDA feed ratio (1, 0.25, 0.1, respectively), reading from bottom to the top. The DVB feed flow was kept constant at 5 seem for all copolymers and pFHDA feed flow increased from 5 to 50 seem.

[0063] In the FTIR spectrum of the PFHDA homopolymer film, a comparatively broad band in the range about 1000 to 1450 cm’¹ is believed due to the C-F bond. (Jiang et al., 2008). Strong stretching modes of C=O at 1758 cm’¹ were observed for PFHDA homopolymer film. The C-O-C stretching modes are also present between 1331 and 1230 cm’¹. Notably, no peak was observed at 1637 cm ¹ for PFHDA film. This peak was previously observed after FTIR measurements of PFHDA monomer. The absence of monomer spectrum in the polymer coatings confinns the polymerization of the monomer [27] (Zhao et al., 2021). [0064] In the FTIR spectrum of DVB homopolymer film (Figure 9, panel a, bottom spectra), the low intensity band at 903 cm’¹ may result from CH2 deformation in unreacted vinyl groups, and the appearance of it after polymerization may be due to the presence of pendant vinyl bonds [21] (Petruczok et al., 2013). The -CH2- stretching bands at 2871 cm’¹, confirm the formation of the backbone chain. Successful polymerization of DVB homopolymer is confirmed by the emergence of the methylene peak at 2930 cm ¹ [19] (Moni et al., 2018). The peaks in the 3000 - 3100 cm-1 region such as 3057, 3081, and 3096 cm’¹ correspond to aromatic -CH- stretching. Furthermore, the bands between 700 and 1000 cm-1 are characteristics of substituted phenyl group [18] (Liu et al., 2014). [0065] The FTIR spectra of copolymers present the characteristic bands associated with the component, confirming the successful copolymerization and retention of chemical functionality from both reactants. The trend is observable in FTIR spectra of copolymers from bottom to the top as the pFHDA content in the copolymer increases, the intensity of the peaks due to C-F stretching increase. Subsequently, the intensity of the bands are attributed to DVB contribution decreases.

[0066] To obtain a deeper knowledge on the iCVD homo- and copolymerized films, XPS measurements were performed on each sample of the third series of samples. The XPS survey spectra is represented in Figure 9, panel b, and the atomic composition, determined from survey scans is reparented in the table below. The XPS measurements further confirmed the variations in the chemical structure as indicated by fluorine/carbon (F/C) and oxygen/carbon (O/C) atomic ratio. As the DVB/PFHDA feed ratio increases, the total fluorine amount determined by XPS decreases which leads to reduction in F/C ratio. As expected, no trace of fluorine was detected on DVB homopolymer. O/C ratio also decreases by increasing the DVB/PFHDA feed ratio. DVB/pFHDA Ols Cis FIs F/C O/C

0 16.9 52.8 30.2 0.57 0.32

0.1 15.7 59.2 25.0 0.42 0.26

0.25 11.7 70.6 17.5 0.24 0.16

1 10.1 84.0 5.8 0.06 0.12

8.0 92.0 0 0 0.08

[0067] To reveal more information on the type of function groups incorporated on the iCVD films, high resolution Cis, FIs, and Ols peaks are recorded and analyzed. As can be seen in Figure 9, panel c, the Cis envelope of the DVB homopolymer is composed of two peaks assigning to carbon atoms in different positions: C-C/C-H (285 eV), C-0 (286 eV) [9] (Esbah Tabaei et al., 2020). In case of copolymer films, the Cis peak exhibits additional peak at the higher binding energy side due to the contribution of fluorine. The fluorine containing functionalities displaying the at 288.8 eV and 292.3 eV can be attributed to C-F and CF2 [20] (Pachchigar et al., 2022) The intensity of the peak is significantly higher than the other observed peaks. Given the abundance of these bonds and the overlap between their binding energies, the Cis envelope is not fitted by separate peaks as the resulting fitting may not lead to reliable quantification of the different functionalities. The analysis therefore restricted to a visual comparison between the C l s envelopes of the homo- and copolymerized films. Additionally, a qualitative comparison between the different FIs and Ols envelopes was also carried out. The FIs spectra were very symmetric, which makes the fitting by separate peaks rather arbitrary and misguiding for extraction of meaningful data. However, in case of Ols spectra for DVB homopolymer and poly(DVB-co-PFHDA) 5:5 which are essentially lowest fluorinated films, additional shoulder at the higher binding energy side appeared that can be correlated to formation of C-0 bond. The Ols spectra of other copolymers and PFHDA homopolymer do not have a characteristic peak.

[0068] When comparing refractive index values (measured by ellipsometry), to XPS results a good correlation between the refractive index and the F/C ratio of each film can be found. As can be seen in Figure 9, panel d, the copolymerization process results in films with a continuously varying refractive index. The measured refractive index values follow a linear relationship with F/C for the homo- and copolymerized films. With increasing the fluorine content in the iCVD films, the refractive index gradually decreases. Therefore, it can be inferred that a correlation exists between the feed ratio and refractive index, enabling the deposition of films with anticipated refractive index profiles.

[0069] The environmental stability of single- and multi-layer coatings is a further important aspect to consider. The thermal stability of polymer thin films is significant for their performance, as many polymer-based coatings and films are used in high-temperature applications, or they may undergo high-temperature excursions during further processing [12] (Huo & Tenhaeff, 2022). As such, the samples should remain stable over a wide range of temperatures to ensure their performance is not affected.

[0070] To characterize thermal expansion and to monitor the refractive index variation of the single layer and multi-layer coatings at elevated temperatures, an in situ thermal cell was used in combination with ellipsometry. The thickness of pDVB, pPFHDA, and poly(DVB-co-PFHDA) coatings on Si wafer was measured during thermal ramp from 25 °C to 300 °C in 12 hours. To better monitor the thickness variations, a very specific temperature profile was designed: at first the temperature ramped from 25°C to 100°C in 20 min and then held for 2 hours. This temperature profile was repeated for the second, third, fourth and fifth ramp (all in 20 minutes) to reach 150 °C, 200 °C, 250 °C, and 300 °C, respectively, and then held for 2 hours to further assess thermal stability in each temperature. Finally, in the final step the stage was cooled down to 25 °C in 30 minutes and, as can be seen in Figure 10, panel a, the overall thickness loss was 3.4%. The reason for this thickness loss is because pDVB polymer is less thermally stable than pPFHDA due to the difference in their chemical structure and composition. On the other hand, pPFHDA is a fluorinated polymer. Fluorine is known to provide exceptional thermal stability due to its strong bond strength and low polarizability. The presence of fluorine atoms in the polymer backbone makes it less susceptible to thermal degradation compared to non-fluorinated polymers. Additionally, the polymer's high molecular weight and crosslinking also contribute to its thermal stability. The thermal stability of pPFHDA is remarkable in the prolong time of 10 h at 300 °C [27] (Zhao et al., 2021). Fig. 10 in panel b illustrates changes in refractive index with time.

[0071] After successfully developing high quality single-layer polymeric thin films in the third series of samples, multilayer thin films were developed to demonstrate the capability of iC VD polymerization in fabrication of highly constrained optical designs, of differing refractive index.

[0072] Fig. 11 shows results of in situ monitoring thickness and refractive index variations of multilayer thin films with ellipsometry. As indicated in Fig. 11, the first layer is DVB homopolymer, the second and the third layers are DVB-co- PFHDA and PHDA homopolymer, respectively. Initially, a multilayer stack structure was fabricated with an arbitrary order utilizing PFHDA, P4 VP- homopolymer, and poly(4VP-co-pFHDA) copolymers. To confirm the arbitrary layout stack structure, three distinct types of multilayers were prepared. Each stack comprised four discrete layers, having a different refractive index profile. The first stack structure (high index to low index) has a first layer of high refractive index homopolymer, the second layer is a copolymer having a second refractive index (which is slightly lower than the first layer), the third layer is also a copolymer having a third refractive index which is slightly lower than the layers beneath it, and finally, the last layer is the low refractive index homopolymer. The second and third stack structures are essentially the same materials but in different orders namely: Arbitrary and Low refractive index to high refractive index order. As the refractive index spatially varies through the thickness of multilayer thin films, in- situ ellipsometry was performed to measure the optical constant and thickness profile of the multilayer thin films. When comparing the in-situ ellipsometry results with multi-angle ex-situ ellipsometry, the results are almost identical, showing the high accuracy of the measurements.

[0073] Another important consideration is the environmental stability of multilayer coatings. As multilayer coatings are used to enhance the perfonnance of optical systems, the coatings can be subjected to high temperatures during use, and their thermal stability is highly relevant to maintaining their optical properties. Additionally, when multilayer films are subjected to high temperature it is important that the integrity of each layer is preserved. Moreover, poor thermal stability can lead to degradation of one or two layers and therefore, can comprise the sample transparency. The table below lists deposition parameters for fabricating multilayer thin film and the thickness variations before and after thermal stability tests. Results are fitted with a 3 -layer Cauchy model. sample v PFHDA v DVB Deposition rate Rl@632.8 h(nm) h(nm) (seem) (seem) (nm/min) Before After thermal test Thermal test

Layer#l- DVB 0 5 7.2 1.594 105 100

Layer#2- 10 5 2.6 1.525 104 97

DVB-co-PFHDA

Layer#3- PFHDA 10 0 9.1 1.462 100 100

[0074] The embodiments disclosed herein can be combined in one or more of many ways to provide improved flexible coatings. The disclosed embodiments can be combined with prior methods and apparatus to provide improved coatings. While preferred embodiments have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. Various alternatives to the embodiments of the invention described herein may be employed in practicing the new approach. It is intended that the following claims, as they may be amended in prosecuting this patent application, define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.

References

[1] H .-A. Hsieh, Y.-H. Lin, Y.-J. Wang, Electrically tunable gradient-index lenses via nematic liquid crystals with a method of spatially extended phase distribution, Optics Express, Vol. 27, Issue 22, Pp. 32398-32408. 27 (2019) 32398-32408. http s : //doi . or g,^z 10.1364/OE .27.032398.

[2] Y. Jin, H. Tai, A. Hiltner, E. Baer, J.S. Shirk, New class of bioinspired lenses with a gradient refractive index, J Appl Polym Sci. 103 (2007) 1834-1841. https://d01.0rg/l 0.1002/APP.25404.

[3] Z. Wu, J. Walish, A. Nolte, L. Zhai, R.E. Cohen, M.F. Rubner, Deformable antireflection coatings from polymer and nanoparticle multilayers, Advanced Materials. 18 (2006) 2699-2702. htps://doi.org/10.1002/ADMA.200601438.

[4] C.T.A. Brown, EH. White, J. Chen, N. Bamiedakis, P.P. Vasil’ev, R. v. Penty, T.J. Edwards, High-Bandwidth and Large Coupling Tolerance Graded-Index Multimode Polymer Waveguides for On-Board High-Speed Optical Interconnects, Journal of Lightwave Technology, Vol. 34, Issue 12, Pp. 2934-2940. 34 (2016) 2934-2940. https://opg.optica.org/abstract.cfm ?uri=jlt-34-l 2-2934 (accessed August 29, 2022).

[5] A. Anuszkiewicz, R. Kasztelanic, A. Filipkowski, G. Stepniewski, T. Stefaniuk, B. Siwicki, D. Pysz, M.Klimczak, R. Buczynski, Fused silica optical fibers with graded index nanostructured core, Scientific Reports 2018 8:1. 8 (2018) 1-13. https://doi.org/10.1038/s41598-018-30284-l.

[6] Boo, H., Lee, Y. S., Yang, H., Matthews, B., Lee, T. G., & Wong, C. W. (2022). Metasurface wavefront control for high-performance user-natural augmented reality waveguide glasses. Scientific Reports 2022 12:1, 12(1), 1-12. https://doi .org/10.1038/s41598-022-09680- 1

[7] Cihanog, G. (2633). CVD-Deposited Oxygen- Selective Fluorinated Siloxane Copolymers as Gas Diffusion Layers. Cite This: Ind. Eng. Chem. Res, 2022. https://doi.org/10.1021/acs.iecr.lc04244

[8] Cools, P., Astoreca, L., Esbah Tabaei, P. S., Thukkaram, M., De Smet, H., Morent, R., & De Geyter, N. (2019). Surface treatment of polymers by plasma. In Surface Modification of Polymers: Methods and Applications. https://doi.org/10.1002/9783527819249.ch2

[9] Esbah Tabaei, P. S., Ghobeira, R., Cools, P., Rezaei, F., Nikiforov, A., Morent, R., & De Geyter, N. (2020). Comparative study between in-plasma and postplasma chemical processes occurring at the surface of UHMWPE subjected to medium pressure Ar and N2 plasma activation. Polymer, 122383. https://doi.Org/10.1016/j.polymer.2020.122383

[10] Hiller, A., Mendelsohn, J. D., & Rubner, M. F. (2002). Reversibly erasable nanoporous anti-reflection coatings from polyelectrolyte multilayers . https://doi.org/10.1038/nmat719

[11] Hiller, J., Mendelsohn, J. D., & Rubner, M. F. (2002). Reversibly erasable nanoporous anti-reflection coatings from poly electrolyte multilayers. Nature Materials 2002 1:1, 7(1), 59-63. https://doi.org/10.1038/nmat719

[12] Huo, N., & Tenhaeff, W. E. (2022). High Refractive Index Polymer Thin Films by Charge-Transfer Complexation. Macromolecules . https://doi.org/10.1021/ACS.MACROMOL.2C02532/ASSET/IMAGES/LARGE/ MA2C02532 0008.JPEG

[13] Jiang, H., Eyink, K., Grant, J. T., Enlow, J., Tullis, S., & Bunning, T. J. (2008). PECVD siloxane and fluorine-based copolymer thin films. Chemical Vapor Deposition, 77(9-10), 286-291. https://doi.org/10.1002/CVDE.200806684

[14] Jiang, H., O’Neill, K., Grant, J. T., Tullis, S., Eyink, K., Johnson, W. E., Fleitz, P., & Bunning, T. J. (2004). Variable Refractive Index Polymer Thin Films Prepared by Plasma Copolymerization. Chemistry of Materials, 16fl 1292-1297. https://doi.org/10.1021/CM035226Q/ASSET/IMAGES/L7KRGE/CM035226QF000 04. JPEG

[15] Kedawat, G., Srivastava, S., Jain, V. K., Kumar, P., Kataria, V., Agrawal, Y., Gupta, B. K., & Vijay, Y. K. (2013). Fabrication of artificially stacked ultrathin ZnS/MgF2 multilayer dielectric optical filters. ACS Applied Materials and Interfaces, 5(11), 4872-4877. https://d0i.0rg/l 0.1021/AM400612Q/SUPPL FILE/AM400612Q SI 001.PDF

[16] Kovalgin, A. Y., Yang, M., Banerjee, S., Apaydin, R. O., Aamink, A. A. I., Kinge, S., & Wolters, R. A. M. (2017). Hot-Wire Assisted ALD: A Study Powered by In Situ Spectroscopic Ellipsometry. Advanced Materials Interfaces, 4(18). https://doi.org/! 0.1002/ADMI.201700058

[17] Kumar, M., Kumari, N., Sharma, A. L., & Kumar, V. S. P. (2020). Design and development of an optical reflective notch filter using the ion assisted deposition technique with stepwise modulated thickness for avionics applications. Applied Optics, Vol. 59, Issue 2, Pp. 564-571, 59(2), 564-571. https://doi.org/10.1364/AO.382437

[18] Liu, A., Goktekin, E., & Gleason, K. K. (2014). Cross-linking and ultrathin grafted gradation of fluorinated polymers synthesized via initiated chemical vapor deposition to prevent surface reconstruction. Langmuir, 30( 7), 14189-14194. https://d0i.0rg/l 0.1021/LA503343X/ASSET/IMAGES/LARGE/LA-2014- 03343X 0008.JPEG

[19] Moni, P., Mohr, A. C., & Gleason, K. K. (2018). Growth Rate and Cross- Linking Kinetics of Poly(divinyl benzene) Thin Films Formed via Initiated Chemical Vapor Deposition. Langmuir, 34(23), 6687-6696. https://doi .org/10.1021/ACS. LANGMUIR.8B00624/ASSET/IMAGES/LARGE/L A-2018-006243 0007.JPEG

[20] Pachchigar, V., Gaur, U. K., Amrutha, T. V., Sooraj, K. P., Hans, S., Srivastava, S. K., & Ranjan, M. (2022). Hydrophobic to superhydrophobic and hydrophilic transitions of Ar plasma-nano structured PTFE surfaces. Plasma Processes and Polymers, 19(9), 2200037. https://doi.Org/10.1002/PPAP.202200037 [21] Petruczok, C. D., Yang, R., & Gleason, K. K. (2013). Controllable crosslinking of vapor-deposited polymer thin films and impact on material properties. Macromolecules, 46(5), 1832-1840. https://doi.org/10.1021/MA302566R/SUPPL_FILE/MA302566R_SI_001.PDF

[22] Purayil, N. P., Kakekochi, V., Dalimba, U. K., & Keloth, C. (2021). All- Optical Diode Action through Enhanced Nonlinear Response from Polymeric Photonic Crystal Microcavity, https://doi.org/10.1021/acsaelm.lc00896

[23] Szczesny, R., Scigala, A., Derkowska-Zielinska, B., Skowronski, L., Cassagne, C., Boudebs, G., Viter, R., & Szlyk, E. (2020). materials Synthesis, Optical, and Morphological Studies of ZnO Powders and Thin Films Fabricated by Wet Chemical Methods. Materials, https://doi.org/10.3390/mal3112559

[24] Tenhaeff, W. E., & Gleason, K. K. (2008). Initiated and Oxidative Chemical Vapor Deposition of Polymeric Thin Films: iCVD and oCVD. Advanced Functional Materials, 18(7), 979-992. https://doi.org/10.1002/ADFM.200701479

[25] Wang, P., Mohammad, N., & Menon, R. (2016). Chromatic-aberration- corrected diffractive lenses for ultra-broadband focusing. Scientific Reports 2016 6:1, 6(1), 1-7. https://doi.org/10.1038/srep21545

[26] Yilmaz, K., Hiiseyin, |, ₍akalak, S., Mehmet Giirsoy, |, & Karaman, | Mustafa. (2020). Vapor deposition of stable copolymer thin films in a batch iCVD reactor. https://doi.org/10.1002/app.50119

[27] Zhao, Y., Huo, N., Ye, S., Boromand, A., Ouderkirk, A. J., & Tenhaeff, W. E. (2021). Stretchable, Transparent, Permeation Barrier Layer for Flexible Optics. Advanced Optical Materials, 9(12). https://d0i.0rg/l 0, 1002/ADQM.202100334.

[28] Schroder, S., Polonskyi, O., Strunskuis, T., Faupel, F., (2020). Nanoscale gradient copolymer films via single-step deposition from the vapor phase. Materials Today, Vol. 37, July/ August 2020, 3542. http://d0i.0rg/l 0.1016/j .mattod.2020.02.004.

Claims

CLAIMS What is claimed is:

1. A system for fabricating a nanoscale, multilayer optical film with real-time control over refractive indexes and layer thickness , comprising: a deposition chamber (10), a stage (22) comprising a substrate, a vapor flow source (20) for selectively supplying to said stage a vapor flow comprising a high refractive index component RH and a low refractive index component RL in a controlled ratio RH/RL for initiated chemical vapor deposition over the substrate forming a polymer and thus fabricating on the substrate a multilayer film (30, 72, 74) in which the layers are in a stack, each layer has a respective refractive index, and the refractive indexes differ between at least two of the layers, and; an in-situ measurement system comprising a source (26) of a beam impinging on said film as the layers thereof are being fabricated and a detector (28) receiving the beam after modulation thereof by interaction with said layers; and a control operatively coupled with the vapor flow source and with said measurement system and configured to control said ratio RH/RL and thicknesses of said layers in real time during fabrication of said multilayer film in response to outputs of said in situ measurement system.

2. The system of claim 1, in which said vapor flow source is configured to supply a vapor flow in which the component RH is at least one of 4- vinylpyridine (4VP) and Divinylbenzene (DVB), and the component RL is at least one of (a) R_LI, which is lH,lH,2H,2H-perfluorodecylacrylate (PFDA) and (b) R_L2, which is lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA).

3. The system of claim 2, in which said component RL is only R_LI.

4. The system of claim 2, in which said component RL is only R_L2-

5. The system of claim 2, in which said component RH is only 4 VP.

6. The system of claim 2, in which said component RH is only DVB.

7. The system of claim 1, in which the thickness of the fabricated thin multilayer film stack of layers is less than 100 nm.

8. The system of claim 1, in which each of at least some of the layers is less than 100 nm thick.

9. The system of claim 8, in which the local thickness of at least some of the layers varies by less than 10% over an area 8 cm in diameter.

10. The system of claim 1, in which the layers in said stack of layers are in an ordered sequence of low to high refractive index values in a selected direction.

11.The system of claim 1, in which the layers in said stack of layers are in an arbitrary sequence of refractive index values.

12. The system of claim 1, in which the stack of layers has a thickness that is uniform such that local thicknesses differ by less than 0.2 nm in a film that is at least 8 cm in diameter.

13. The system of claim 1, in which the refractive indexes of the layers change in steps from one layer to another.

14. The system of claim 1, in which the refractive index in the stack of layers changes essentially continuously from one layer to another so that the stack is essentially a continuum of infinitesimally thin layers that have respective refractive indexes.

15. The system of claim 1, in which at least one of the layers in the stack is a homopolymer.

16. The system of claim 1, in which at least one of the layers in the stack is a copolymer.

17. A nanoscale multilayer optical film comprising: a stack (30, 71, 74) of layers having respective refractive indexes; wherein each of at least some of the layers comprises a polymer of a high refractive index component RH and/or a low refractive index component RL in a controlled ratio RH/RL such that the refractive index in the stack changes with thickness of the stack; wherein the component RH is at least one of 4-vinylpyridine (4 VP) and Divinylbenzene (DVB), and the component RL is at least one of (a) R_LI, which is lH,lH,2H,2H-perfluorodecylacrylate (PFDA) and

(b) R_L2, which is lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA).

18. The thin multilayer optical film of claim 17, in which said component RH is only 4 VP.

19. The thin multilayer optical film of claim 17, in which said component RH is only DVB.

20. The thin multilayer optical film of claim 17, in which the component RL is only RLI.

21. The thin multilayer optical film of claim 17, in which the component RL is only R_L2.

22. The thin multilayer optical film of claim 17, in which the stack of layers is less than 100 nm thick.

23. The multilayer optical film of claim 17, in which at least one of the layers in the stack is less than 100 nm thick and the local thickness thereof varies by less than 10% of an average thickness of the layer over an area 8 cm in diameter.

24. The thin multilayer optical film of claim 17, in which the layers in said stack of layers are in an ordered sequence of low to high refractive index values in a selected direction.

25. The thin multilayer optical film of claim 17, in which the layers in said stack of layers are in an arbitrary sequence of refractive index values.

26. The thin multilayer optical film of claim 17, in which at least one of the layers in the stack has a thickness that is uniform such that local thicknesses differ by less than 0.2 nm over a film that is at least 8 cm in diameter.

27. The thin multilayer optical film of claim 17, in which the refractive indexes of the layers change in steps from one layer to another.

28. The thin multilayer optical film of claim 17, in which the refractive index in the stack of layers changes essentially continuously from one layer to another so that the stack is essentially a continuum of infinitesimally thin layers that have respective refractive indexes.

29. The thin multilayer optical film of claim 17, in which at least one of the layers in the stack is a homopolymer.

30. The thin film multilayer optical fil of claim 17, in which at least one of the layers in the stack is a co-polymer.

31. A method of fabricating a nanoscale optical device, comprising; supplying a deposition chamber (10) with vapor flow (20) comprising a high refractive index component RH and a low refractive index component RL to thereby fabricate a stack of layers of a polymer on a substrate in said deposition chamber by vapor deposition; measuring in-situ and in real time refractive and physical properties of the stack as the stack is being fabricated; controlling in real time the ratio RH/RL of said vapor flow and a time for said vapor deposition as a function of said measuring as the stack is being fabricated to thereby control the respective refractive index of each layer and the respective thickness of each layer and to cause the layer refractve indexes to differ between at least two of the layers.

32. The method of claim 31, in which the component RH is at least one of 4- vinylpyridine (4VP) ) and Divinylbenzene (DVB), and the component RL is at least one of (a) R I, which is lH,lH,2H,2H-perfluorodecylacrylate (PFDA) and (b) RL2, which is lH,lH,6H,6H-perfluorohexyl diacrylate (PFHDA).

33. The method of claim 31, in which said component RH is only 4 VP.

34. The method of claim 31, in which said component RH is only DVB.

35. The method of claim 31, in which the component RL is only R I .

36. The method of claim 31, in which the component RL is only RL2.

37. The method of claim 31, in which the stack of layers is less than 100 nm thick.

38. The method of claim 31, in which the controlling step arranges the layers in an ordered sequence of low to high refractive index values in a selected direction.

39. The method of claim 31, in which the controlling step arranges said stack of layers in an arbitrary sequence of refractive index values.

40. The method of claim 31, in which the controlling step results in a stack in which the stack thickness is uniform such that local thicknesses differ by less than 0.2 nm in an area at least 8 cm in diameter.

41. The method of claim 31, in which the controlling step results in a stack in which at least one of the layers is less than 100 nm thick and the thickness thereof varies by less than 10% over an area at least 8 cm in diameter.

42. A nanoscale, multilayer optical film comprising: a stack (30, 71, 74) of layers having respective refractive indexes, wherein at least two of the layers have refractive indexes that differ from each other; each of at least some of the layers comprises a polymer of a high refractive index component RH and/or a low refractive index component RL in a controlled ratio RH/RL such that the refractive index in the stack changes with thickness of the stack; the thickness of each of at least one of the layers is precisely controlled such that the layer thickness is less than 100 nm and varies by less than 10% in an area at least 8 cm in diameter.

43. A multilayer optical film as in claim 42, in which at least one of the layers in the stack is a homopolymer.

44. A multilayer optical film as in claim 42, in which at least one of the layers in the stack is a co-polymer.