Modification of the Structure and Linear/Nonlinear Optical Characteristics of PVA/Chitosan Blend through CuO Doping for Eco-Friendly Applications

The solution casting technique is utilized to fabricate blank and CuO-doped polyvinyl alcohol/chitosan (PVA/CS) blends for eco-friendly applications. The structure and surface morphologies of prepared samples were explored by Fourier transform infrared (FT-IR) spectrophotometry and scanning electron microscopy (SEM), respectively. FT-IR analysis reveals the incorporation of CuO particles within the PVA/CS structure. SEM analysis exposes the well-dispersion of CuO particles in the host medium. The linear/nonlinear optical characteristics were found on the basis of UV-visible-NIR measurements. The transmittance of the PVA/CS decreases upon CuO increasing to 20.0 wt%. The optical bandgap (Eg dir./Eg ind.) decreases from 5.38/4.67 eV (blank PVA/CS) to 3.72/3.12 eV (20.0 wt% CuO-PVA/CS). An obvious improvement in the optical constants of the PVA/CS blend is achieved by CuO doping. The Wemple-DiDomenico (WDD) and Sellmeier oscillator models were utilized to examine the CuO role dispersion behavior of the PVA/CS blend. The optical analysis shows clear enrichment of the optical parameters of the PVA/CS host. The novel findings in the current study nominate CuO-doped PVA/CS films for applications in linear/nonlinear optical devices.


Introduction
Polymeric composites have received great attention because of their effective role in various applications, including the industrial, biological, medical, shielding and entertainment fields [1][2][3][4][5]. Polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), chitosan (CS), carboxymethyl cellulose (CMC) and polyethylene glycol (PEG) possess many attractive features over the rest of polymers, such as non-toxicity, water-solubility, bio-compatibility, eco-friendly and degradability [6][7][8]. Polymeric composites (PCs) are mainly produced by doping small amounts of fillers in a host polymeric matrix for such an application while blending two polymers or more is another scientific trend to yield new polymeric hosts with specific characteristics for updated applications. Particularly, PVA and CS polymers could be blended to produce a novel polymeric host for a lot of applications. PVA possesses high transmittance Vis/NIR regions and a broad bandgap (5.40 eV). Moreover, the hydroxyl groups (-OH) attached to its carbon-chain backbone perform as a hydrogen bonding source that enhances the complexation process [9], while CS, as chitin's derivative, is the most available polymer that exists in nature [10]. CS could play a dominant role in medical issues because of its unique biocompatibility, antifungal and antimicrobial activities [11]. Mixing PVA and CS produces a PVA/CS polymeric blend to serve as a novel host for various kinds of dopants.
Lots of former works related to PCs are found in the literature. For example, the Heiba research group made great progress using CdS/Mg nanostructures (NPs) on the enhancement of optical characteristics of PVA/CMC. The al-Harthi group proved that the

Methods and Materials
Solution casting technique presented in the literature was carried out to fabricate different contents (0.5, 1.0, 5.0, 10.0 and 20.0 wt%) of CuO-doped PVA/CS polymeric blends. To perform the process, starting sources of PVA (M. W.: 85,000 g·mol −1 ), chitosan (CS) in powder form (≥75% deacetylated) and copper oxide (CuO; purity > 99.0%) were obtained from Sigma-Aldrich Co. (St. Louis, MO, USA). First, at 70 • C, 7.5 PVA grams were dissolved in 250 mL of double distilled water (DDW) for 4 h. In parallel, 2.5 CS grams were dissolved in acetic acid/DDW/(1:9) at 25 • C for 24 h. Both solutions were mixed for 4 h until a homogenous PVA/CS blend (3:1) was achieved. Next, certain amounts of CuO powder were blended to prepare CuO-doped PVA/CS blends. Afterward, the different CuO-PVA/CS blends were cast in Petri dishes for one day at 55 • C. Next, the samples were peeled out and marked by C 0 (blank blend) to C 20.0 (20.0 wt% of CuO-PVA/CS blend). A digital micrometer was used to measure films' thickness and found 0.18 ± 0.01 mm.
Films' surface morphology was investigated using a scanning electron microscope (JSE-6390LA, JEOL Ltd., Tokyo, Japan). Absorption bands and structures' changes were explored at room temperature (RT) using FT-IR (Shimadzu, IRAffinity-1S, Kyoto, Japan) spectrophotometer with the KBr pellets technique. UV-visible-NIR measurements were recorded at RT using a spectrophotometer (JASCO V670, Jasco Corp., Easton, MD, USA). Tauc's technique was applied to investigate both direct/indirect bandgap (E g ) values as follows [6,18]: where α(= 1 d ln 1 T [6]), and d are optical absorption coefficient and films' thickness, B is constant and m is a parameter that may take 2 and 1/2 values for allowed direct/indirect electronic transitions [18,19]. Localized states and created defects' role in host's bandgap as a result of CuO doping is investigated via the determination of the Urbach energy (E u ) as follows [13]: where α 0 is a constant. The refractive index (n), extinction coefficient (K), and optical conductivity (σ opt. ) in UV/Vis/NIR regions were calculated as where C is light speed, R is reflectance, and λ is photons/wavelength. The dielectric permittivity constants (real ε r , imaginary ε i ) and surface/volume energy loss functions (SELF/VELF) were also determined from [20,21]: Moreover, Wemple-DiDomenico (WDD) model was followed to examine n dispersion [22], whereas Sellmeier oscillator relations were applied to investigate the rest of the optical parameters as infinite refractive index (n ∞ ), average oscillator strength (S 0 ), average inter-band oscillator wavelength (λ 0 ), infinite dielectric parameter (ε ∞ ), lattice dielectric parameter (ε L ) and free carrier concentration/effective mass (N/m*) as [23,24]: where e is free electron charge, and ε 0 is space dielectric constant. The linear first-order susceptibility (χ (1) ), nonlinear third-order susceptibility (χ (3) ) and nonlinear refractive index (n 2 ) were investigated as [25,26]

Morphological Analysis
Films' surface morphologies were captured by a scanning electron microscope (SEM). Figure 1a-f illustrates SEM micrograms of the blank and different (0.5 to 20 wt%) CuO-PVA/CS films, respectively. The SEM microgram of the blank film is spot-free with a smooth surface (Figure 1a), whereas distinguishable bright spots related to the CuO granules are clearly noticed in SEM micrographs of 0.5 and 1.0 wt% of CuO-PVA/CS films. These spots become denser, closer and more compact as the CuO concentration is increased to 20 wt%.   These changes confirm the interactions between the CuO molecules with the structure of the host PVA/CS matrix. This interaction mainly takes place by replacing the OH groups in the host structure with that of the CuO ones [27]. Relative to FT-IR spectra of blank PVA/CS film and pure CuO material, the main absorption bands and vibrations are recorded (Table 1). Similar FT-IR performance is noticed in the CuO-PVA/CS films with clear intensity variations and slight location shifts with broadening in the absorption bands. These changes are pronounced in the regions 3900-3600 cm −1 and 1300-400 cm −1 as background shadows in Figure 1a, whereas the absorption bands correspond to the Cu-O bonds may overlap with those of the host matrix at the 1300-400 cm −1 region, as shown in Figure 1b. Our findings reveal the complete incorporation of CuO and the host medium. The same trends are reported in the literature [27][28][29].

FT-IR Analysis
Polymers 2023, 15, x FOR PEER REVIEW 5 of 20 Figure 2a,b depicts FT-IR transmittance spectra of blank and different CuO-PVA/CS films in the 400 to 4000 cm −1 range, as demonstrated by plots, clear variations in intensity and sites of dominant absorption bands of doped samples with respect to the blank one. These changes confirm the interactions between the CuO molecules with the structure of the host PVA/CS matrix. This interaction mainly takes place by replacing the OH groups in the host structure with that of the CuO ones [27]. Relative to FT-IR spectra of blank PVA/CS film and pure CuO material, the main absorption bands and vibrations are recorded (Table 1). Similar FT-IR performance is noticed in the CuO-PVA/CS films with clear intensity variations and slight location shifts with broadening in the absorption bands. These changes are pronounced in the regions 3900-3600 cm −1 and 1300-400 cm −1 as background shadows in Figure 1a, whereas the absorption bands correspond to the Cu-O bonds may overlap with those of the host matrix at the 1300-400 cm −1 region, as shown in Figure 1b. Our findings reveal the complete incorporation of CuO and the host medium. The same trends are reported in the literature [27][28][29].

UV/Vis/NIR Investigations
The effect of CuO concentration on the optical parameters of the PVA/CS blend has been explored on the basis of the UV/Vis/NIR measurements. The wavelength dependence of the transmittance (T) and absorbance (A) of blank and different CuO contents filled PVA/CS blends are presented in Figure 3a,b, respectively. It is noticed that at any certain λ, T decreases in visible-NIR regions as the CuO content is increased from 0 to 20 wt%. For example, the T of the blank PVA/CS film is more than 80% in the visible region, while it decreases to about 3% for 20 wt% of CuO-PVA/CS film in the same region. Moreover, as the CuO content is increased from 0 to 20 wt%, the UV cut-off edge is red-shifted to longer wavelengths from 225 nm to 358 nm. This valuable result nominates the possible role of CuO-PVA/CS films in UV-shielding applications. In contrast, the absorption increases due to the increase of CuO contents. In addition, clear redshifts in the absorption edges are noticed. Furthermore, two absorption peaks at 211 nm and 258 nm are detected in all absorption spectra that correspond to the PVA electronic π → π * transitions [42], whereas the absorption edge detected at 324 nm refers to the electronic n → π * transitions [43]. The decrease in the optical transmittance and hence increment in the absorption amounts due to CuO doping is attributed to the increase in defects (shown below), which leads to a decrease in the optical band gap of the PVA/CS blend, as discussed in Figure 4.
Based on Tauc's equation (Equation (1)), direct/indirect optical bandgap (E g dir. /E g ind. ) of blank and CuO-PVA/CS films has been obtained from (αhν) 2 and (αhν) 1/2 curves vs. hν, respectively, as depicted ( Figure 4). The x-axis intercepts of extrapolated linear parts of these curves to hν = 0 equal E g values as listed in Table 2. The obtained E g dir. /E g ind. values of blank PVA/CS film are 5.38 eV and 4.67 eV. These values are well-consistent with the reported ones [34,44]. The E g dir. /E g ind. values of CuO-PVA/CS films decrease to 3.72 eV and 3.12 eV as CuO concentration is upraised to 20 wt%. Moreover, it is clear that both 0.5 wt% and 1.0 wt% CuO-PVA/CS films possess second bandgap values of 4.79 eV and 4.57 eV, respectively, as illustrated in Figure 4a. This finding reveals that the absorption happens as a result of charge transfer between two different energy levels. The first transition occurs between the molecular orbits of the host matrix, while the other electronic transition takes place between the created energy state due to CuO particles and those of the host matrix. Similar findings were recorded in previous works [21,45]. So, the E g narrowing mainly results due to localized states and defects created between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the PVA/CS blend due to CuO doping [42,46]. Similarly, Heiba et al. concluded that 4 wt% of Cd 0.9 Mg 0.1 S nanofillers led to a reduction in the E g of PVA/CMC blend from 5.4 eV to 5.02 eV [44]. Additionally, the E g of the PVA/CMC/GO blend was reduced to 3.34 eV using 1.0 wt% of Fe 3 O 4 doping [13]. Formerly, we modified the optical bandgap of PVA/Gr from 5.38 eV to 4.78 eV by Fe 2 O 3 doping [47].

UV/Vis/NIR Investigations
The effect of CuO concentration on the optical parameters of the PVA/CS blend has been explored on the basis of the UV/Vis/NIR measurements. The wavelength dependence of the transmittance (T) and absorbance (A) of blank and different CuO contents filled PVA/CS blends are presented in Figure 3a,b, respectively. It is noticed that at any certain λ, T decreases in visible-NIR regions as the CuO content is increased from 0 to 20 wt%. For example, the T of the blank PVA/CS film is more than 80% in the visible region, while it decreases to about 3% for 20 wt% of CuO-PVA/CS film in the same region. Moreover, as the CuO content is increased from 0 to 20 wt%, the UV cut-off edge is red-shifted to longer wavelengths from 225 nm to 358 nm. This valuable result nominates the possible role of CuO-PVA/CS films in UV-shielding applications. In contrast, the absorption increases due to the increase of CuO contents. In addition, clear redshifts in the absorption edges are noticed. Furthermore, two absorption peaks at 211 nm and 258 nm are detected in all absorption spectra that correspond to the PVA electronic π → π * transitions [42], whereas the absorption edge detected at 324 nm refers to the electronic n → π * transitions [43]. The decrease in the optical transmittance and hence increment in the absorption amounts due to CuO doping is attributed to the increase in defects (shown below), which leads to a decrease in the optical band gap of the PVA/CS blend, as discussed in Figure 4.  Figure 4a. This finding reveals that the absorption happens as a result of charge transfer between two different energy levels. The first transition occurs between the molecular orbits of the host matrix, while the other electronic transition takes place between the created energy state due to CuO particles and those of the host matrix. Similar findings were recorded in previous works [21,45]. So, the Eg narrowing mainly results due to localized states and defects created between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the PVA/CS blend due to CuO doping [42,46]. Similarly, Heiba et al. concluded that 4 wt% of Cd0.9Mg0.1S nanofillers led to a reduction in the Eg of PVA/CMC blend from 5.4 eV to 5.02 eV [44]. Additionally, the Eg of the PVA/CMC/GO blend was reduced to 3.34 eV using 1.0 wt% of Fe3O4 doping [13]. Formerly, we modified the optical bandgap of PVA/Gr from 5.38 eV to 4.78 eV by Fe2O3 doping [47].
The defects and localized energy states created in CuO-PVA/CS films could be proved by investigating the Urbach energy (Eu) (Equation (2)). It shows the exponential dependence of the absorption coefficient and photons energy (hν). Eu is estimated ( Table  2) by plotting lnα vs. hν, as illustrated in Figure 5. It was noticed that Eu grows from 0.48 eV (blank PVA/CS) to 1.79 eV (20 wt% CuO-PVA/CS). The increase in Eu indicates the growth of localized states and defects that works as trapping centers in the forbidden region of the PVA/CS host [48]. Similar evidence is reported in the literature [13,25,49]. As an original result, the optical bandgap of PVA/CS is tailored by CuO doping for a lot of optical and environmental applications.  The defects and localized energy states created in CuO-PVA/CS films could be proved by investigating the Urbach energy (E u ) (Equation (2)). It shows the exponential dependence of the absorption coefficient and photons energy (hν). E u is estimated (Table 2) by plotting lnα vs. hν, as illustrated in Figure 5. It was noticed that E u grows from 0.48 eV (blank PVA/CS) to 1.79 eV (20 wt% CuO-PVA/CS). The increase in E u indicates the growth of localized states and defects that works as trapping centers in the forbidden region of the PVA/CS host [48]. Similar evidence is reported in the literature [13,25,49]. As an original result, the optical bandgap of PVA/CS is tailored by CuO doping for a lot of optical and environmental applications.
The optical performance of such material is mainly established by investigating the refractive index (n* = n-iK) to dictate its applications. The real (n) and imaginary (K) parts describe the dispersion behavior of the electromagnetic wave within the material. Both n and K at the swept wavelength (λ) are calculated using Equations (3) and (4), respectively. The wavelength dependence of n and K of blank and CuO-PVA/CS films are illustrated in Figure 6a,b, respectively. According to Figure 6a, it is noted that n follows the absorbance performance (Figure 3b). In other words, n decreases steeply upon raising λ in the UV region, whereas it remains semi-steady in Vis/NIR regions. Moreover, it is seen that the n of PVA/CS is enhanced as a result of CuO doping, which proposes it for updated applications in optical and optoelectronic devices. For instance, n increases from 1.2 (blank PVA/CS) to 2.25 (20 wt% CuO-PVA/CS) at 650 nm. The improvement in the n value refers to the growth in the films' density and intermolecular bonds due to CuO doping [27,50,51], whereas n remains quasi-steadily in low photons energy due to films' restricted absorbance in this region, whereas, according to Figure 6b, K declines with increasing λ in the UV region, whereas it increases gradually in visible-NIR regions. Furthermore, K increases as the CuO content is increased. These findings could be understood on the basis of the increment of the dispersion as a result of the reflectance increase due to defects' growth [51].     The optical performance of such material is mainly established by investigating the refractive index (n* = n-iK) to dictate its applications. The real (n) and imaginary (K) parts describe the dispersion behavior of the electromagnetic wave within the material. Both n and K at the swept wavelength (λ) are calculated using Equations (3) and (4), respectively. The wavelength dependence of n and K of blank and CuO-PVA/CS films are illustrated in Figure 6a,b, respectively. According to Figure 6a, it is noted that n follows the absorbance performance (Figure 3b). In other words, n decreases steeply upon raising λ in the UV region, whereas it remains semi-steady in Vis/NIR regions. Moreover, it is seen that the n of PVA/CS is enhanced as a result of CuO doping, which proposes it for updated applications in optical and optoelectronic devices. For instance, n increases from 1.2 (blank PVA/CS) to 2.25 (20 wt% CuO-PVA/CS) at 650 nm. The improvement in the n value refers to the growth in the films' density and intermolecular bonds due to CuO doping [27,50,51], whereas n remains quasi-steadily in low photons energy due to films' restricted absorbance in this region, whereas, according to Figure 6b, K declines with increasing λ in the UV region, whereas it increases gradually in visible-NIR regions. Furthermore, K increases as the CuO content is increased. These findings could be understood on the basis of the increment of the dispersion as a result of the reflectance increase due to defects' growth [51]. Based on the absorption coefficient α and n, σ opt. of CuO-PVA/CS samples was determined (Equation (5)) and illustrated in Figure 7. It is observed that σ opt. of the films behaves in a similar way to the optical absorbance ( Figure 3b). As λ is red-shifted to longer values in the UV region, σ opt. decreases steeply, while it behaves steadily in the Vis/NIR regions. In contrast, σ opt. increases upon increasing the CuO content in the PVA/CS host. For example, at 650 nm, σ opt. enhances from 2.56 × 10 10 s −1 (blank PVA/CS) to 9.85 × 10 10 s −1 (20 wt CuO-PVA/CS). The σ opt. enhancement is understood on the basis of the increment in created electrons as a result of the absorption increase of the incident photons [27,52]. The increase in absorption is also reinforced by the growth in the defects, as discussed in E u findings. These findings are very consistent with reported data [21,53,54]. Shamekh et al. proved that σ opt. of PVA was pronouncedly enhanced by MgO doping.
The dielectric parameters (ε r and ε i ), together with the surface/volume energy loss functions (SELF/VELF) of the blank and CuO-PVA/CS films, have been determined. These constants are investigated to nominate their possible participation in many fields as superconductors and energy storage devices. ε r associates with traveling wave dispersions within such material, while ε i relates to the dissipated energy rate through their propagation [21]. ε r , ε i , SELF and VELF were calculated by Equations (6) to (9) and presented in Figure 8a-d, respectively. According to ε r spectra (Figure 8a), it follows the refractive index n performance. ε r decreases steeply as λ is red-shifted in the UV region, whereas it remains semi-constant in the visible-NIR regions. Furthermore, ε r rises as CuO content is increased to 20 wt%. For example, ε r enhanced from 1.43 (blank PVA/CS) to 5.08 (20 wt% CuO-PVA/CS) at λ = 650 nm. The enhancement in ε r results due to the increase in the dispersion as a result of a defects increase (Urbach energy findings). On the other hand, the imaginary part ε i of the film performs similarly to the extinction coefficient K (Figure 6b). ε i decreases greatly as λ increases in the UV region, while it increases slowly in Vis/NIR region for small CuO contents (≤5 wt%) and increases pronouncedly for the high CuO contents (10 and 20 wt%). Moreover, ε i increases as the CuO content is raised. This behavior refers to polarization and dipole motion fluctuations [26,55,56]. Similar findings are reported in the literature [26,57,58]. Moreover, it is noticed that SELF and VELF spectra perform in a similar way. Both SELF and VELF values increase noticeably as λ is red-shifted to longer wavelengths in the Vis/NIR regions. Moreover, it is noted that at any λ, the VELF value is larger than the SELF value, which indicates that the energy loss by the traveling electrons within the films due to the doped CuO particles is larger than those traveling on their surfaces. In addition, both SELF and VELF increased upon increasing the CuO contents. This increment in SELF and VELF refers to growth in vacant energy levels generated in the host band gap [59]. Similar behavior is noticed El-naggar et al. [26]. They showed SELF and VELF increase of the PVA/PVP upon increasing SnS 2 /Fe concentration. Based on the absorption coefficient α and n, σopt. of CuO-PVA/CS samples was determined (Equation (5)) and illustrated in Figure 7. It is observed that σopt. of the films behaves in a similar way to the optical absorbance ( Figure 3b). As λ is red-shifted to longer values in the UV region, σopt. decreases steeply, while it behaves steadily in the Vis/NIR regions. In contrast, σopt. increases upon increasing the CuO content in the PVA/CS host. For example, at 650 nm, σopt. enhances from 2.56 × 10 10 s −1 (blank PVA/CS) to 9.85 × 10 10 s −1 (20 wt CuO-PVA/CS). The σopt. enhancement is understood on the basis of the increment in created electrons as a result of the absorption increase of the incident photons [27,52]. The increase in absorption is also reinforced by the growth in the defects, as discussed in Eu findings. These findings are very consistent with reported data [21,53,54]. Shamekh et al. proved that σopt. of PVA was pronouncedly enhanced by MgO doping. The dielectric parameters (εr and εi), together with the surface/volume energy loss functions (SELF/VELF) of the blank and CuO-PVA/CS films, have been determined. These constants are investigated to nominate their possible participation in many fields as superconductors and energy storage devices. εr associates with traveling wave dispersions within such material, while εi relates to the dissipated energy rate through their propagation [21]. εr, εi, SELF and VELF were calculated by Equations (6) to (9) and presented in Figure 8a-d, respectively. According to εr spectra (Figure 8a), it follows the refractive index n performance. εr decreases steeply as λ is red-shifted in the UV region, whereas it remains semi-constant in the visible-NIR regions. Furthermore, εr rises as CuO content is increased to 20 wt%. For example, εr enhanced from 1.43 (blank PVA/CS) to 5.08 (20 wt% CuO-PVA/CS) at λ = 650 nm. The enhancement in εr results due to the increase in the dispersion as a result of a defects increase (Urbach energy findings). On the other hand, the imaginary part εi of the film performs similarly to the extinction coefficient K ( Figure  6b). εi decreases greatly as λ increases in the UV region, while it increases slowly in Vis/NIR region for small CuO contents (≤5 wt%) and increases pronouncedly for the high CuO contents (10 and 20 wt%). Moreover, εi increases as the CuO content is raised. This behavior refers to polarization and dipole motion fluctuations [26,55,56]. Similar findings are reported in the literature [26,57,58]. Moreover, it is noticed that SELF and VELF spectra perform in a similar way. Both SELF and VELF values increase noticeably as λ is redshifted to longer wavelengths in the Vis/NIR regions. Moreover, it is noted that at any λ, the VELF value is larger than the SELF value, which indicates that the energy loss by the traveling electrons within the films due to the doped CuO particles is larger than those traveling on their surfaces. In addition, both SELF and VELF increased upon increasing Moreover, the dispersion parameters of the blank and CuO-PVA/CS films are examined by a single oscillator model (WDD model; Equation (10)) in the normal dispersion region. Investigating E o and E d are essential parameters to nominate the applications of the prepared films in communication systems and spectra analysis devices [21]. The values of E o and E d are found from (n 2 − 1) −1 plots vs. (hν) 2 as depicted in Figure 9a, where the slopes equal −1/(E 0 E d ) and the intersections equal E 0 /E d . Table 3 includes E o and E d values. Both E o and E d values increase upon increasing the CuO content in the host PVA/CS. This increase in the dispersion energies refers to the increase in the optical transition strength of the system bonds [60]. Table 3. Dispersive parameters of CuO-PVA/CS films. the CuO contents. This increment in SELF and VELF refers to growth in vacant energ levels generated in the host band gap [59]. Similar behavior is noticed El-naggar et al. [26 They showed SELF and VELF increase of the PVA/PVP upon increasing SnS2/Fe concen tration. Moreover, the dispersion parameters of the blank and CuO-PVA/CS films are exam ined by a single oscillator model (WDD model; Equation (10)) in the normal dispersion region. Investigating Eo and Ed are essential parameters to nominate the applications of the prepared films in communication systems and spectra analysis devices [21]. The val ues of Eo and Ed are found from (n 2 − 1) −1 plots vs. (hν) 2 as depicted in Figure 9a, where the slopes equal −1/(E0Ed) and the intersections equal E0/Ed. Table 3  Furthermore, the infinite refractive index (n ∞ ), the infinite dielectric constant (ε ∞ ), and the average oscillator strength (S 0 )) of the blank and CuO-PVA/CS films are determined on the basis of the Sellmeier oscillator relations (Equations (11) to (15)). By plotting (n 2 − 1) −1 vs. λ −2 (Figure 9b) and equating the slopes with 1/S 0 and the intersections with 1/S 0 λ 2 0 , the values of λ 0 , n ∞ , S 0 and ε ∞ are obtained and listed in Table 3. While N/m* and ε L are obtained by plotting n 2 vs. λ 2 (Figure 9c), where the slopes = e 2 4π 2 C 2 ε 0 N m * and intersections (=ε L ) as listed in Table 3. It is obvious that all optical behaviors of the PVA/CS blend are altered with CuO doping. For example, ε ∞ of the blank PVA/CS film is greatly enhanced from 1.35 to 3.94 (20 wt% CuO-PVA/CS film). The enhancement in ε L and ε ∞ refers to the dispersion lattice vibrations as a result of CuO particles [37]. Similar ε L and ε ∞ findings related to polystyrene filled with manganese (III) chloride were found by Al-Muntaser et al. [37], while N/m* of the blank PVA/CS film is duplicated due to 20 wt% of CuO doping. This result is reasonable as a result of the increment of the free carriers due to CuO doping [21]. Our results are compatible with the literature [61,62].

CuO wt%
The nonlinear optical behavior of blank and CuO-PVA/CS samples is explored to recommend their probable applications in nonlinear optical devices. Optical materials with the optical nonlinearity character play an effective role in many applications such as ultrafast lasing switching, frequency converters and telecommunications [21,63]. The nonlinear optical response arises because of the nonlinear polarization that occurs owing to intense electromagnetic wave exposure [21,64,65]. Based on Equations (16) to (18), χ (1) , χ (3) and n 2 are calculated and presented in Figure 10a-c, respectively. It is noted that χ (1) , χ (3) and n 2 behave semi-steadily in the Vis/NIR regions, whereas they rise rapidly upon increasing hν in the UV region. In addition, as the CuO content is increased to 20 wt%, χ (1) , χ (3) and n 2 increase noticeably. For instance, at 4.0 eV, χ (1) of the blank PVA/CS film is enhanced from 0.17 esu to 1.36 esu via 20 wt% CuO doping, while χ (3) and n 2 of the blank film are enhanced about by three-order of magnitude at the same incident photons energy. These findings are compatible with previous works [25,26,64]. For example, the Ali group found that the nonlinear optical constant of PVA was enhanced pronouncedly by fullerene doping [25]. The obtained nonlinear optical findings of the CuO-PVA/CS films in this study nominate their applications in nonlinear optical devices [65]. Furthermore, the infinite refractive index (nꝏ), the infinite dielectric constant (εꝏ), and the average oscillator strength (S0)) of the blank and CuO-PVA/CS films are determined on the basis of the Sellmeier oscillator relations (Equations (11) to (15)). By plotting (n 2 − 1) −1 vs. λ −2 (Figure 9b) and equating the slopes with 1/S0 and the intersections with 1/ 0 0 2 , the values of λ0, nꝏ, S0 and εꝏ are obtained and listed in Table 3. While N/m* and εL are obtained by plotting n 2 vs. λ 2 (Figure 9c), where the slopes (= 2 4 2 2 0 * ) and intersections (=εL) as listed in Table 3. It is obvious that all optical behaviors of the PVA/CS blend are altered with CuO doping. For example, εꝏ of the blank PVA/CS film is greatly enhanced from 1.35 to 3.94 (20 wt% CuO-PVA/CS film). The enhancement in εL and εꝏ refers to the dispersion lattice vibrations as a result of CuO particles [37]. Similar εL and εꝏ findings related to polystyrene filled with manganese (III) chloride were found by Al-Muntaser et al. [37], while N/m* of the blank PVA/CS film is duplicated due to 20 wt% of CuO doping. This result is reasonable as a result of the increment of the free carriers due to CuO doping [21]. Our results are compatible with the literature [61,62].  and n2 behave semi-steadily in the Vis/NIR regions, whereas they rise rapidly upon increasing hν in the UV region. In addition, as the CuO content is increased to 20 wt%, χ (1) , χ (3) and n2 increase noticeably. For instance, at 4.0 eV, χ (1) of the blank PVA/CS film is enhanced from 0.17 esu to 1.36 esu via 20 wt% CuO doping, while χ (3) and n2 of the blank film are enhanced about by three-order of magnitude at the same incident photons energy. These findings are compatible with previous works [25,26,64]. For example, the Ali group found that the nonlinear optical constant of PVA was enhanced pronouncedly by fullerene doping [25]. The obtained nonlinear optical findings of the CuO-PVA/CS films in this study nominate their applications in nonlinear optical devices [65].

Conclusions
The solution casting method was followed to fabricate blank and CuO doped in polyvinyl alcohol/chitosan (PVA/CS) blends. The effect of CuO concentrations (0, 0.5, 1.0, 5.0, 10.0 and 20.0 wt%) on PVA/CS structure and linear/nonlinear optical characteristics is discussed in detail. Scanning electron microscope examinations disclose obvious changes in the surface morphologies of PVA/CS film owing to CuO doping. FT-IR measurements prove noticeable modifications in PVA/CS structure due to CuO doping. Noticeable modifications in absorption band's locations and intensities of CuO-PVA/CS films as compared with the blank one. The linear/nonlinear optical parameters were discussed. The transmittance of the PVA/CS blend reduces as a result of CuO increasing to 20.0 wt%. The optical bandgap (E g dir. /E g ind. ) decreases from 5.38/4.67 eV (blank PVA/CS) to 3.72/3.12 eV (20.0 wt% CuO-PVA/CS). This decrease in the optical bandgap is interpreted in terms of defects and created states, as verified by Urbach energy investigations. The refractive index, optical conductivity and dielectric constants of PVA/CS are clearly enhanced due to CuO doping, which nominates it for updated applications in optoelectronic devices. The CuO doping role in the dispersion performance of PVA/CS has been investigated using a single oscillator and Sellmeier oscillator relations. For instance, the infinite dielectric constant is greatly enhanced from 1.35 (blank PVA/CS) to 3.94 (20 wt% CuO-PVA/CS film), whereas the concentration of free carriers/effective mass of blank PVA/CS film is duplicated. The nonlinear optical parameters of PVA/CS are also enhanced via CuO doping. χ (3) and n 2 are improved by about three orders-of-magnitude at 4.0 eV incident photons energy. These novel findings nominate CuO-PVA/CS films for updated optical applications.