Polarization-Resolved Speckle Technique for Rapid Non-Destructive Characterization of Macroporous Silica Thin Films

TL;DR

This paper introduces a new non-destructive method using polarization-resolved speckle imaging to study the structure of porous silica films and how they affect light polarization.

Contribution

The novel use of polarization-resolved speckle imaging to rapidly characterize structural and optical anisotropies in macroporous silica thin films.

Findings

Surface porosity significantly influences the degree of polarization and light scattering in macroporous silica films.

Poincaré sphere mapping reveals distinct polarization-conversion pathways and scattering regimes not detectable with traditional methods.

The technique is rapid, cost-effective, and non-destructive, making it suitable for photonic and nanostructured material analysis.

Abstract

Macroporous silica thin films were synthesized via the sol–gel method to elucidate the relationship between pore structure and the degree of polarization of light (DoP). The films were characterized by scanning electron microscopy (SEM) to determine their mean pore size and surface porosity, while polarization-resolved speckle imaging was employed to evaluate the degree of polarization and its distribution on the Poincaré sphere. The results show that surface porosity is a key structural parameter governing the DoP, with increasing values leading to enhanced scattering and a progressive isotropization of the polarization-state distributions. Poincaré sphere mapping further reveals distinct scattering regimes and polarization-conversion pathways, providing insights that are not accessible with conventional optical measurements. Overall, these findings show that speckle imaging is a rapid, cost-effective, and non-destructive approach to probing structural and optical anisotropies in porous materials, with direct relevance to systems where pore accessibility dictates performance, including liquid-crystal devices, photochromic coatings, and other nanostructured photonic platforms.

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1. Introduction

Hierarchically porous materials are inorganic, organic, or hybrid organic–inorganic materials composed of interconnected pores of different sizes: micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) [1]. These materials have attracted significant attention in materials science due to their unique properties, such as high specific surface area, pore interconnectivity, and thermal or mechanical stability. These attributes enable applications in diverse fields, including biomedicine, energy storage, separation, sensing, and catalysis [2,3,4,5,6,7,8]. Various strategies for preparing these materials include templating methods (e.g., emulsions, surfactants, macroporous polymers, colloidal crystals), as well as bioinspired processes, supercritical fluids, freeze-drying, zeolitization, phase separation, and sol–gel processing [9,10,11,12,13,14,15,16,17].

To validate the properties of newly prepared porous materials, accurate and advanced characterization techniques are essential. Gas adsorption and mercury porosimetry are well-established methods for measuring surface area and pore size distribution [1,18], while imaging techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning probe microscopy (SPM) provide insights into the morphology of three-dimensional pore networks [19]. Additionally, properties such as crystallinity or thermal stability are often assessed using X-ray diffraction (XRD) and thermogravimetric analysis (TGA), respectively [20,21]. However, despite their utility, these techniques face significant challenges, particularly for thin films. Limited film thickness often requires destructive sample preparation, which may lead to misinterpretation of adsorption isotherms or theoretical models [22]. Claudia Weidenthaler has highlighted these and other common pitfalls in the characterization of nanoporous materials, providing practical guidance for achieving accurate analysis [23].

Ellipsometric porosimetry (EP) and spectroscopic ellipsometry (SE) have emerged as optical, non-contact and non-destructive alternatives to conventional methods, particularly for the characterization of thin films. EP analyzes changes in refractive index and film thickness when an adsorbate (e.g., water or ethanol) is introduced into the porous structure, enabling the determination of porosity, pore size distribution, and even Young’s modulus of mesoporous thin films on various substrates under ambient conditions [22,24]. SE, a related technique, measures the optical constants and thickness of films and can be adapted for porous materials when coupled with porosimetry setups [25,26]. Its relative scalability in industrial contexts, such as the microelectronics sector for low-k dielectrics, further highlights its practical value [27]. However, their reliance on adsorption–desorption isotherms imposes critical limitations, including the need for equilibrium conditions, sensitivity to environmental variables, and reduced accuracy when dealing with macroporous structures (>50 nm), where capillary condensation mechanisms are ineffective. Additionally, EP requires careful optical modeling, often assuming idealized pore geometries, which can lead to inaccuracies in disordered or hierarchical porous networks typical of sol–gel-derived films [28]. Similar limitations are also encountered in radiation-based techniques like small-angle X-ray scattering (SAXS) and grazing-incidence small-angle X-ray scattering (GISAXS) [29,30].

The speckle method is a promising alternative: it is rapid, cost affordable, non-destructive and capable of capturing light-scattering effects directly related to macroporosity, structural disorder, and optical performance. This advanced optical technique leverages random interference patterns, or “speckles,” created when coherent light (e.g., from a laser) scatters off a rough surface. Speckle properties are highly sensitive to small surface deformations or structural changes, making the technique ideal for measuring strain, surface roughness, or mechanical deformation in thin films [31]. In addition, the technique has real-world relevance for quality control and other industrial applications, as speckle setups can be adapted for in situ or real-time measurements [32,33]. Rather than replacing established structural characterization techniques, polarization-resolved speckle imaging is intended to complement them by providing a rapid, non-destructive optical assessment of macroporous thin films, particularly once correlations between optical response and morphology have been established.

In this work, two sets of macroporous silica thin films with different compositions and pore architectures were prepared by the sol–gel method. The samples were characterized by SEM to determine the mean pore diameter and surface porosity. The data obtained serve as reference values, enabling a correlation to be established between the degree of polarization and the porosity. Polarization-resolved speckle measurements were then performed to determine the degree of polarization and to map the corresponding polarization-state distributions on the Poincaré sphere. Using this combined structural–optical methodology, we systematically investigated how the DoP evolves with surface porosity. The results establish a framework for interpreting the DoP mechanisms in macroporous media and underscore speckle imaging analysis as a practical, morphology sensitive technique for optical assessment.

2. Materials and Methods

2.1. Preparation of Macroporous Films

The sol–gel process is a versatile synthetic route that enables the preparation of a wide range of materials—metal oxides, glasses, ceramics, and hybrid organic–inorganic networks—through kinetically controlled chemical reactions. During hydrolysis, Si-OR groups are converted into reactive Si-OH species, which subsequently undergo condensation to form a growing Si-O-Si network via water or alcohol elimination (see Figure 1). The precursors type, temperature and pH, among other parameters, strongly affect the hydrolysis and condensation rates and therefore the structure and morphology of the final material [34].

Castor oil was chosen as the pore-forming agent because of its high miscibility in the initial sol–gel solution, non-affinity to the formed matrix and low volatility, which are essential requirements for successful phase separation. As the reactions proceed, miscibility between the oil and the silica-condensed species is significantly reduced, leading to phase separation into oil-rich and silica-rich regions. If the sol–gel transition is faster than the phase separation, a closed-pore structure is obtained. On the contrary, when phase separation is faster, microspheres are formed. At intermediate rates between the sol–gel transition and phase separation, materials with different pore structures can be obtained [35].

The phase separation process in binary mixtures can be thermodynamically described using the Flory–Huggins theory. As condensation reactions proceed, the system can enter different regions of the miscibility window. If phase separation occurs in the metastable region of the phase diagram, it evolves via nucleation and growth, leading to the formation of discrete oil droplets within the silica matrix. These can later coalesce to form bigger droplets and eventually form an interconnected network if condensation of the silica matrix is slow. In contrast, when the system reaches the unstable region, phase separation occurs via spinodal decomposition. In this case, when the silica-rich and oil-rich phases are in comparable volume fractions, a three-dimensional bicontinuous network with a narrow domain size distribution is formed [36,37]. The phase separation process is schematically represented in Figure 2.

Pure inorganic (TMOS) and hybrid organic–inorganic (TEOS/EtTES) macroporous silica thin films were prepared. For inorganic samples, tetramethyl orthosilicate (TMOS) was mixed with ethanol, water and hydroxiacetone as a catalyst. The mixture was allowed to react for 1 day at 40 °C under vigorous stirring to initiate hydrolysis and condensation reactions. Subsequently, different amounts of castor oil were added to the sol prior deposition. The films were obtained by spin coating deposition at 2000 rpm onto transparent thin glasses (0.18 mm) and were dried in the oven at 80 °C for 2 h. Lastly, the resulting films were thoroughly rinsed with ethanol and acetone to remove the oil and access their intrinsic porosity. Samples are denoted according to the precursor type and castor oil content (% w/w) as TMOS-13.8, TMOS-24.2, TMOS-32.4 and TMOS-39.1.

For the hybrid organic–inorganic samples, tetraethyl orthosilicate (TEOS) and ethyltriethoxysilane (EtTES) were mixed in ethanol, followed by the addition of water and nitric acid. The sol was left to react for 1 h at room temperature and castor oil was added in different amounts prior to deposition. The films were prepared by spin coating deposition at 2000 rpm onto transparent thin glasses and dried for 1 day at ambient conditions. As with the TMOS series, the samples were rinsed with ethanol and acetone to remove the oil and access to their intrinsic porosity. Samples are denoted as TEOS/EtTES-1.5, TEOS/EtTES-2.9, TEOS/EtTES-7, TEOS/EtTES-8.2, and TEOS/EtTES-9.5.

The sol compositions used in the present work suggest that phase separation mainly occurs in the metastable region, which is consistent with the non-interconnected pore morphology observed by SEM. In contrast, previous experiments performed with higher castor oil contents (TMOS-70.5 and TEOS/EtTES-13) revealed three-dimensional bicontinuous morphologies that are compatible with both spinodal decomposition and bicontinuous coarsening, as shown in Figure 3.

2.2. SEM Image Analysis

The surface morphology and porous structure of the macroporous thin films was visualized by SEM (FE-SEM, FEI Nova NanoSEM). The resulting SEM images were processed in MATLAB R2024b (MathWorks, Natick, MA, USA) to obtain surface porosity (ϕ2D) and average pore size of the samples [38]. Porosity is defined as the ratio between the pore volume and the total volume of the film. In principle, if the pores are unbranched and exhibit uniform depth, the volume ratio can be approximated by the area ratio and surface porosity (ϕ2D) can be described as

$$[eqn]$$

In our context, given that the synthesis conditions remain constant for each preparation of samples, it seems reasonable to conjecture that an increase in surface porosity correlates with an increase in the volumetric capacity of the porous network to host guest molecules. While absolute pore volumes may not be directly accessible from surface imaging alone, a larger projected pore area likely reflects larger or deeper pore structures. Furthermore, if we approximate each pore as a semi-sphere, which is justified considering the emulsion-based sol–gel synthesis route, where castor oil droplets act as colloidal templates, the relationship between surface area and volume becomes clearer. Larger surface pores, resulting from larger droplet templates, would correspond to greater internal volumes, supporting the qualitative correlation between surface porosity and total pore accessible volume [39,40].

The porosity ϕ2D was calculated automatically using MATLAB by analyzing pictures of each sample. The program also detects the diameter of each pore and displays its statistical distribution using a histogram. From this, the average size and dispersion can be determined. Although the pores in all the samples used have mostly circular geometries, we have also considered the possibility of any other geometry in the program, even though in our case it is not relevant. Then, the calculation considers that pores may not be perfectly circular. Further information about the program can be found in Appendix B.

2.3. Polarization-Resolved Measurements

The laser speckle patterns from the samples were recorded using the optical setup illustrated in Figure 4. The laser beam was spatially filtered by a diaphragm to ensure a uniform beam profile prior to interacting with the sample. The laser spot was around 5 millimeters wide. A collimating lens was placed after the sample to collect the scattered light and maintain consistent imaging conditions. A linear polarizer and a quarter-wave plate were employed to analyze the polarization components of the scattered light.

The resulting speckle images were captured using a CCD camera (Andor iXon 888, 1024 × 1024 pixels). Two laser sources were used for the measurements: a blue Ar-ion laser (λ = 488 nm) and an orange He-Ne laser (λ = 594 nm).

The degree of polarization (DoP) is defined as the fraction of the intensity attributable to polarized light states and can be described mathematically in terms of the Stokes vector elements as

$$[eqn]$$

where S0 denotes the total intensity; S1 and S2 represent the linear polarization components along the horizontal/vertical and diagonal axes, respectively; and S3 corresponds to the circular polarization component [41]. Since the Stokes vector cannot be measured directly, several measurements must be made and combined to infer the Stokes parameters. In this work, four speckle images were recorded under distinct polarization configurations: I0, I1, I2 and I3. To obtain I0, I1 and I2, a linear polarizer was oriented at 0°, 45°, and 90°, respectively. For I3, the polarizer was set at 45° and combined with a quarter-wave plate to access circular polarization component. The quarter-wave plate was only used in this measurement. Under these conditions, the Stokes parameters may be written as

$$[eqn]$$

Each sample was measured in five different not-overlapping regions to ensure representative and consistent results. The resulting speckle images were processed in MATLAB and the average DoP was obtained for each sample. Full polarization-state distributions (DoP is computed for every pixel) were also obtained and projected onto the Poincaré sphere. This 3D representation provides a powerful tool for both visualizing and quantitatively analyzing polarization-state variations, enabling the detection of anisotropies, depolarization effects, and internal structural differences that may not be captured by absolute values alone [42,43].

3. Results

The porous structure and surface morphology of the sol–gel coatings were examined by scanning electron microscopy (SEM). The addition of castor oil ensured pore formation within the coatings, as illustrated in Figure 5 and Figure 6. These figures also include the corresponding pore size distributions, while the average pore diameters and their standard deviations are summarized in Table 1.

For the inorganic samples, increasing the castor oil content from 13.8 to 39.1% w/w led to a significant increase in average pore size and a broadened pore size distribution. This can be attributed to the combined effects of the sol composition and the dynamics of film deposition. For these samples, the prepared sol was highly diluted in ethanol and was rapidly evaporated at spin coating deposition, therefore forcing a fast condensation of the silica network, trapping the castor oil droplets almost immediately. At low castor oil concentrations, this rapid immobilization prevents significant droplet coalescence, yielding a homogeneous pore structure with smaller, uniformly distributed pores. At higher oil concentrations, the slower condensation due to bonding interactions between oil molecules and silica hydroxyl groups, hindering the matrix formation, allows droplet coalescence and Ostwald ripening prior to immobilization, producing fewer but larger pores and broadening the pore size distribution. Smaller uncoalesced droplets are also trapped, contributing to a heterogeneous macroporous structure. A complementary study of the samples was carried out using near-field scanning optical microscopy (NSOM). Figure A1 (see Appendix A) reveals the 3D topography of the TMOS samples, where the pore morphology and even their depth are shown in greater detail and no interconnections between pores are observed.

For the hybrid organic–inorganic samples, the average pore diameters are smaller compared to those observed in the TMOS series, and their evolution with castor oil content (2.9–9.5% w/w) does not follow a clear trend. At intermediate castor oil content (TEOS/EtTES-7.0), the largest mean pore diameter is obtained (173 ± 57 nm), whereas further increases in castor oil content led to smaller average pore sizes (147 ± 80 nm for TEOS/EtTES-9.5) but larger standard deviations. The broadening of the pore size distributions at higher castor oil contents indicates the coexistence of pores with markedly different sizes, as shown in Figure 6d for the TEOS/EtTES-9.5 sample. This behavior arises from competing effects associated with the hybrid sol chemistry, in which the presence of the organosilane component alters condensation kinetics and viscosity, influencing both droplet nucleation and coalescence during film formation. As a result, variations in castor oil content primarily affect pore density and size dispersion rather than inducing a systematic change in the average pore diameter.

In the case of the TEOS/EtTES-1.5 sample, the pores are extremely small and fall below the resolution required for reliable image analysis, as observed in Figure 7. Consequently, pore size distributions could not be obtained, and its surface porosity was assumed to be approximately zero.

Surface porosity (ϕ2D) was estimated from SEM micrographs by MATLAB-based image analysis, and the resulting values are summarized in Table 1. The dependence of ϕ2D on castor oil content is shown in Figure 8. In the inorganic series (from 13.8 to 39.1% w/w), surface porosity generally increases with increasing castor oil concentration, reflecting the higher fraction of oil-rich domains introduced during sol preparation, which translate into a more open porous structure after condensation and oil removal. However, the increase is not strictly linear, and a tendency towards saturation is observed at higher oil contents, consistent with the formation of closely packed and interconnected pores rather than a continuous increase in projected pore area. In contrast, the hybrid organic–inorganic coatings (from 1.5 to 9.5% w/w) exhibit a more complex behavior. While an overall increase in surface porosity is observed when comparing low and high castor oil contents, intermediate compositions (e.g., TEOS/EtTES-7.0) show a reduced ϕ2D despite a higher oil loading. This indicates that surface porosity is not solely governed by the amount of castor oil added, but also by the interplay between droplet nucleation, coalescence, and the modified condensation kinetics imposed by the hybrid sol chemistry [44]. As a result, variations in castor oil content can lead to different pore packing and connectivity scenarios, producing non-linear changes in the projected pore area at the surface.

Although ϕ2D represents a two-dimensional projection of the porous network, it provides a reliable descriptor of the relative openness and accessibility of the coatings. Inorganic samples exhibit comparatively higher surface porosity values, associated with their densely packed pore structures observed by SEM, whereas hybrid coatings display lower and more dispersed ϕ2D values, consistent with a reduced and less uniformly distributed pore population. Since surface porosity directly reflects the structural features responsible for light scattering and variation in the degree of polarization (DoP), it constitutes a more meaningful parameter for correlating microstructure with optical response than the nominal castor oil content, which acts only as an indirect synthesis variable.

Figure 9 shows the degree of polarization (DoP) as a function of surface porosity for both inorganic and hybrid samples measured with two incident laser wavelengths (488 nm and 594 nm). A general decrease in polarization with increasing surface porosity is observed for all samples, reflecting the fact that a higher density of pores increases the number of scattering events experienced by the transmitted light. In the inorganic TMOS coatings, this trend becomes wavelength dependent. For λ = 488 nm, a greater variation in the degree of polarization is observed with increasing surface porosity compared to the 595 nm wavelength. In contrast, the hybrid TEOS/EtTES coatings exhibit a small dependence across almost the entire analyzed range, with a noticeable variation only observed at the end, for the sample with the highest porosity. For this type of sample, the use of one wavelength or another is of little relevance. The reason for this may lie in the relationship between the pore size of the samples and the wavelength. Note that the average sizes for these samples are on the order of a fraction of λ, whereas in the TMOS samples they are on the order of λ.

The depolarization mechanism occurs in a complex way. It depends on different factors such as the relationship between wavelength and pore size, but also on the number of pores, the statistical distribution of their diameters, and the intrinsic optical anisotropy of the hybrid matrix. Local refractive-index variations and nanoscale birefringence may also affect the polarization state in the light–matter interaction process.

To further elucidate the origin of the changes in the DoP trends shown in Figure 9, the measured polarization states were projected onto the Poincaré sphere. This representation provides additional insight into the nature of polarization beyond global DoP values. As shown in Figure 10 for the inorganic TMOS samples, the distribution of polarization states broadens progressively with increasing surface porosity. At low porosity, the polarization clusters remain compact, indicating that most of the scattered light preserves a similar polarization state. As surface porosity increases, the distributions expand over the sphere, reflecting enhanced multiple scattering and a higher degree of polarization randomization. This spreading is more pronounced under blue laser illumination, consistent with the stronger wavelength dependence observed in Figure 9 and with the higher sensitivity of shorter wavelengths to pore-induced scattering. Importantly, the topology of the polarization-state distributions reveals anisotropic features that are not accessible from DoP averages alone, indicating that its value is not purely isotropic but involves preferential polarization-conversion pathways.

In Figure 11, projections along the S1 axis highlight how the scattered light redistributes between horizontal and vertical linear polarization states. TMOS-13.8 and TMOS-24.2 exhibit narrow distributions that remain close to the sphere surface under blue illumination, indicating minimal depolarization. In contrast, TMOS-32.4 and TMOS-39.1 show broader distributions that extend toward the center of the sphere, consistent with the transition to a multiple-scattering regime responsible for the increased depolarization. The spread is anisotropic, with elongation preferentially along the S2 and S3 axes, suggesting that a linear-to-elliptical polarization conversion plays a crucial role. Notably, under orange illumination the change in the DoP remains weaker but more anisotropic, particularly for TMOS-13.8 and TMOS-24.2, where a pronounced elongation along the S2 direction is observed, indicating that phase retardation effects dominate over multiple scattering at longer wavelengths.

Figure 12, showing projections along the S2 axis, further supports this interpretation. Samples with lower surface porosity maintain compact, polarized distributions, close to the sphere surface, whereas higher-porosity coatings exhibit a markedly broader spread, indicating stronger variation in the DoP. This effect is most pronounced under blue illumination, consistent with the sharper decrease in DoP at 488 nm observed in Figure 9 and with the higher sensitivity of shorter wavelengths to pore-induced multiple scattering. In contrast, the orange wavelength shows a more confined redistribution, suggesting that polarization randomization is less efficient at 594 nm over the same porosity range.

For the hybrid TEOS/EtTES samples, a similar behavior is observed in Figure 13, as the distribution of polarization states broaden with increasing surface porosity. However, in contrast to the inorganic TMOS coatings, the distributions remain largely similar for blue and orange illumination, confirming the weak wavelength dependence of depolarization inferred from the DoP values observed in Figure 9.

In Figure 14, samples with lower surface porosity (TEOS/EtTES-1.5 and -2.9) exhibit narrow, surface-confined distributions under both illumination wavelengths, indicating strong preservation of linear polarization states. As surface porosity increases, the distributions progressively broaden and shift inward, consistent with enhanced P due to increased scattering at rougher or more porous interfaces. However, the degree of anisotropy does not scale solely with the absolute surface porosity value. In particular, samples with intermediate castor oil content (TEOS/EtTES-2.9 and TEOS/EtTES-7.0) display more pronounced anisotropic elongation than the nearly nonporous TEOS/EtTES-1.5 coating, despite their non-monotonic ϕ2D values. This anisotropy is likely related to internal anisotropies within the film (variation in refractive index, dispersion of pore diameters, etc.). This behavior indicates that in the hybrid system, the decrease in the DoP is governed not only by the amount of surface porosity but also by the presence and spatial distribution of localized organic–inorganic interfaces, as observed in SEM. The elongation of the distributions along the S3 direction becomes more evident in higher-porosity samples, indicating an increasing contribution of linear-to-elliptical polarization conversion. This suggests that the values of the DoP in the hybrid coatings are not dominated by deep multiple scattering, but rather by localized polarization transformations associated with interfacial scattering, refractive-index fluctuations, and nanoscale birefringence within the hybrid matrix.

Figure 15 further confirms the previous results. With increasing surface porosity, the polarization-state distributions evolve from compact and surface-localized clusters to more diffuse, yet still anisotropic patterns. Even at the highest porosity values, the distributions retain preferential elongation along specific axes, in contrast to the TMOS series, which exhibits a stronger tendency toward isotropic depolarization at high porosity.

This distinction highlights the different mechanisms operating in the two systems leading to changes in the DoP: while the more interconnected pore networks of the TMOS coatings promote multiple scattering and polarization randomization, the lower and less interconnected surface porosity of the TEOS/EtTES samples leads to depolarization dominated by localized interface scattering and intrinsic optical anisotropy.

From these results, it can be inferred that surface porosity is a key parameter governing the values of the DoP in macroporous sol–gel coatings, although its optical response strongly depends on the material system. The combined analysis of DoP measurements and Poincaré sphere representations provides a robust framework to distinguish between different depolarization mechanisms. This approach is particularly relevant for applications in which pore accessibility governs functionality, such as liquid-crystal devices, photochromic coatings, and other nanostructured optical systems.

4. Discussion

The results presented here show that optical DoP in macroporous sol–gel coatings is strongly modulated by surface porosity, whereas the underlying depolarization pathways depend critically on the specific material system and the topology of its pore network. By combining morphological characterization with polarization-resolved optical measurements, we identify distinct DoP regimes for purely inorganic and hybrid organic–inorganic coatings.

For inorganic TMOS derived films, increasing surface porosity promotes a transition from single to multiple scattering, yielding pronounced, wavelength-dependent DoP in line with classical scattering models for porous silica. In contrast, hybrid TEOS/EtTES coatings display a markedly different response: despite comparable pore geometry and measurable surface porosity, the weak wavelength dependence and the persistent anisotropy of the polarization-state distributions indicate that strong multiple scattering is not the dominant mechanism. Instead, the change in the DoP appears to be governed primarily by anisotropic polarization transformations, likely driven by local refractive-index fluctuations, nanoscale birefringence, and interfacial effects intrinsic to the hybrid matrix. Overall, these findings emphasize that surface porosity alone cannot fully account for the optical response of hybrid systems, and that pore distribution and connectivity must also be considered.

Accordingly, the purpose of the study is not to determine highly accurate absolute values of pore size or surface porosity, but rather to gain insight into the relationship between structural parameters and the DoP of light, and to establish polarization-resolved speckle imaging as a complementary optical tool for the characterization of macroporous materials. Finally, several limitations of the present study should be acknowledged. Surface porosity was estimated from two-dimensional SEM micrographs; complementary three-dimensional characterization would enable a more complete description of pore connectivity and tortuosity and could further strengthen structure property correlations.

5. Conclusions

This study establishes a direct relationship between pore structure and optical DoP in macroporous sol–gel silica thin films by combining morphological characterization with polarization-resolved speckle measurements. Through the correlation of SEM-estimated surface porosity with degree of polarization data and Poincaré sphere representations, surface porosity is identified as the key parameter governing depolarization, while the dominant mechanisms responsible for changes in the DoP are shown to depend strongly on the material system. In inorganic TMOS-based coatings, increasing surface porosity drives a transition from single to multiple scattering, resulting in pronounced and wavelength-dependent polarization randomization. In contrast, hybrid TEOS/EtTES coatings exhibit weaker wavelength dependence and anisotropic polarization, indicating that the value of p is not dominated by strong multiple scattering. Overall, these results demonstrate that polarization-resolved speckle imaging is a fast, non-destructive, and cost-effective approach to discriminate polarization regimes and to link optical response with structural parameters, such as pore accessibility and connectivity. Importantly, once such correlations are established, speckle analysis can provide indirect but reliable information on morphological features and their evolution for coatings prepared under the same conditions, reducing the need for systematic SEM analysis and reinforcing the relevance of this technique for quality control and practical implementation.