Third-harmonic optical process in Zn:NiO thin films under the influence of femtosecond and continuous wave laser excitation

In this study, we focused on the impact of nonlinear optical properties on Zn-doped NiO thin film, which was analyzed using z scan and THG technique. The z-scan technique was performed using a continuous wave laser. The open aperture shows that all films exhibit the reverse saturable absorption and the mechanism responsible for two-photon absorption, excited state absorption, and free carrier absorption. The closed aperture results in the negative nonlinear refraction caused by the thermal effects. The enhancement in the third-order susceptibility from 5.37 × 10−3 to 13.24 × 10−3 esu with Zn doping is due to the presence and increase in the concentration of defect levels in the films. The THG studies were performed using femtosecond and nanosecond lasers and revealed that the enhancement in the signal with the rise in Zn doping concentration was attributed to the enhancement of photoexcitation and relaxation processes within the sample. These results suggest that Zn-doped NiO films have significant potential for applications in the realm of optoelectronic applications.


Introduction
In the dynamic world of material science, constant efforts are directed toward improving the performance of photonics and optoelectronics applications.A key area of focus lies in enhancing the NLO properties of materials, as it holds the potential to revolutionize the efficiency and functionality of diverse technological applications.Transition metal oxides have emerged as promising contenders for such advancements, owing to their exceptional transparency and chemical stability.Among these, Nickel oxide, a notable p-type semiconductor, shines brightly with its remarkable stability, durability, and band gap value range from 3.6 to 4 eV.These exceptional characteristics have led to its utilization in a plethora of cutting-edge applications, including photodetectors [1], electrochromic applications [2], optoelectronic [3], and sensor applications [4].
Central to this pursuit of maximizing NLO performance lies the successful fabrication of nickel oxide films.In this regard, a myriad of deposition methods, encompassing both physical and chemical approaches, have been extensively explored.Commonly employed physical deposition techniques include RF sputtering [5], DC sputtering [6], and thermal evaporation methods [7].Additionally, chemical methods such as spray pyrolysis [8], spin coating [9], and sol-gel methods [10] have been investigated for their effectiveness in depositing nickel oxide films.The deposition parameters play a crucial role in achieving a uniform and high-quality thin film.Spray pyrolysis stands out as a preferred method due to its user-friendly nature, cost-effectiveness, ability to cover large areas, and the absence of any vacuum requirements during the deposition process, setting it apart from other deposition methods.
In the pursuit of enhancing the properties of NiO, numerous elements, including base metals, transition metals [11], and rare earth metals have been doped into the material.Among these dopants, incorporating Zn 2+ ions into NiO films are particularly promising, as it is expected to boost optoelectronic applications performance significantly.Zinc, a n-type semiconductor with five electrons in its valence shell, holds special interest as a substituted element in NiO due to its closely matched ionic radius to that of Ni [12,13].This characteristic makes Zn doping an intriguing avenue for modifying the features of NiO films.
This study aims to explore the impact of Zn doping, at varying concentrations (1%, 5%, and 10 wt%), affects the structural, morphological, optical, and nonlinear optical attributes of NiO films.The evaluation involves utilizing XRD, AFM, UV-vis spectrophotometry, and PL spectroscopy.A continuous wave laser with a wavelength of 632.8 nm was used in a z-scan configuration to explore the nonlinear optical characteristics.Moreover, Third Harmonic Generation (THG) measurements were conducted using an induced laser.Specifically, a femtosecond Yb: YV4 laser operated at 1045 nm, exhibiting a pulse duration of 190 fs, while simultaneously, a Nd:YAG laser with a pulse duration of 8 nanoseconds as the primary radiation source.To minimize the influence of external undesirable light scattering, the entire measurement setup was enclosed within a protective box.

Experimental details
2.1.Preparation of the films NiO films were prepared using nickel acetate tetrahydrate as the host precursor solution.Zinc acetate dihydrate used as the dopant precursor solution, and it was prepared at concentrations of 1, 5, and 10 wt%, respectively, for the synthesis of Zn-doped NiO films.The dopant precursor was mixed with the host precursor and stirred for an additional 30 min to create a uniform solution.Concurrently, the glass substrate underwent a cleansing process involving sequential immersions in soap water, acetone, and isopropyl alcohol (IPA), each lasting 10 min within an ultrasonic cleaner.Subsequently, the samples were dried using nitrogen gas.To obtain homogeneous films using spray pyrolysis, the substrate temperature was maintained at 450 °C and flow rate of 2 ml min −1 .The resultant films displayed a distinct brown coloration, with a measured thickness of 250 nm.

Characterization of the films
The structural studies were analyzed using a Rigaku SmartLab x-ray diffractometer with a Cu-Kα source.The samples underwent scanning from 30 to 90 degrees at a scan speed of 1 degree per minute.For optical studies, a Shimadzu 1900 UV-vis spectrophotometer was utilized, with a scan range of 190 to 1100 nm.At room temperature, defects in the films were probed through photoluminescence investigations using a JASCO FP-8500 fluorescence spectrometer and a 300 nm excitation wavelength.Additionally, the Raman spectra were investigated using the LabRAM HR (UV) system to confirm the structure and assess vibrational modes of films.Morphology and film roughness were examined using a Bruker Icon Atomic force microscope (AFM) in tapping mode configuration.For studying the nonlinear optical parameters, a z-scan setup with a continuous wave laser operating at a wavelength of 632.8 nm and an input intensity of 20 mW was employed.The investigation aimed to evaluate the third harmonic signal efficiency of the films by employing a Nd:YAG laser with a wavelength of 1064 nm.Furthermore, a femtosecond (fs) Yb:YV4 laser fs was utilized as the inducing laser for Third Harmonic Generation (THG).

Structural analysis
The structural studies of pure and Zn-doped NiO films were conducted using glancing angle x-ray diffraction (GAXRD) at room temperature.The scans were performed from 30°to 90°; the results are presented in figure 1.
The XRD graph validated the polycrystalline nature of all prepared films.The peaks observed were at (111), (200), ( 220), (311), and (222), indicating the NiO films have a FCC structure with the space group Fm-3m, matching JCPDS file number 01-073-1519.Importantly, the graph had no impure peaks, confirming that the Zn dopant was effectively integrated into the NiO lattice.
The most intense peak was observed at the (200) plane, but upon reaching a 10% Zn doping concentration, it shifted to the (220) plane.The change in orientation is a result of lattice imperfections in the film induced by Zn doping [14].With an increase in Zn doping content, there is a noticeable trend of broadening in the film and an accompanying increase in intensity.This could be ascribed to the existence of micro strain, defects within the crystal structure, and an uneven composition in the films [15].
Table 1 displays the determined structural parameters obtained from XRD analysis of the prepared films.The crystallite size (D) of the films is determined using the Scherrer equation, which is provided as, The shape factor (k) accounts for the shape of the crystallites, the wavelength of x-rays, denoted by λ, β in radians describes the broadening of x-ray diffraction peaks, and Bragg's angle (θ) is the angle at which x-rays interact with the crystalline lattice.
The crystallite size of NiO is measured at 8.6 nm, and it diminishes upon the introduction of Zn into the NiO lattice.The reduction in the size of crystallites can be attributed to the larger ionic radius of Zn 2+ ions (0.74 Å) in contrast to Ni 2+ ions (0.69 Å).This difference in ionic radii induces lattice imperfections and strain in the film, thereby impeding the growth of NiO crystals [16].Moreover, the presence of Zn 2+ ions may act as obstructions at the grain boundaries, limiting the growth of grains within the film [17].The similar kind of result is obtained by Sharma g et al [18], Dewanet al [19], Manouchehri et al [20].
Additionally, the method of size strain plot (SSP) was also employed to evaluate the films' crystallite size.In this method, the Gaussian function handles the strain profile while the Lorentzian function describes the crystallite size profile, plotted along Here, d hkl represents the interplanar spacing.Figure 2 displays the size-strain plot for Zn doped NiO films.Table 1 presents the average size of crystallites as determined by the size-strain plot method and Debye-Scherrer equation.
In figure 3 closer examination reveals a diffraction angle shift with Zn doping concentration.There is peak shift associated with 5% and 10% Zn dopant to right and left angle of the peak of pure NiO films.
At 5% doping, there is a shift towards higher angles in the XRD, which could be according to the generation of compressive strain in the NiO lattice, resulting in lattice distortion and the formation of defects in the film [24].An observed gradual shift in the peak position towards lower diffraction angles is observed with a 10% Zn doping.This shift signifies the presence of tensile strain within the NiO lattice, attributed to the larger ionic radii of Zn when substituting into the Ni position.As a result, the lattice undergoes tensile strain, causing the lattice spacing to increase and the interplanar distance to expand, as shown by the calculated lattice parameters in table 1 [25].The dislocation density and lattice strain were calculated using the formulas shown below:   The film's strain and dislocation density increased after Zn doping, indicating the presence of disorder and the formation of defect states.

Raman spectra studies
To investigate the vibrational modes of molecules within the films and identify traces of secondary phases in Zndoped NiO films, room-temperature Raman spectra were analyzed.The Raman spectra were deconvoluted into six Gaussian curves to analyze peak positions, and the observed Raman modes are summarized and presented in table 2. Figure 4 illustrates the changes in Raman spectra for Zn-doped NiO films, recorded in the range of 200 to 2000 cm-1 using a 532 nm laser.
The pure NiO and Zn doped NiO films exhibit the characteristics peaks of first-order transverse optical (1TO) mode, first-order longitudinal optical mode (1LO), LO + TO mode, second-order longitudinal optical mode (2LO) and two magnon mode (2 M) mode at positions 378 ± 2, 492 ± 1, 952 ± 25, 1065 ± 22, and 1466 ± 33 cm −1 respectively [26].The NiO film displayed a conspicuous and intense peak in the 1LO mode, indicating the presence of a defect, a feature typically absents in stoichiometric NiO films.This anomaly suggests nonstoichiometric, with induced defects arising from both nickel and oxygen deficiencies in the film.Importantly, the 1LO mode indicates that the Ni-O lattice bonds are stretching [27], providing valuable insights into the structural composition and integrity of the film.The peaks of the 2LO mode which induced due to C-O stretching vibrations [28].The intensity of other peaks is very low when compared to 1LO mode, which is owing to the lower size of the NiO crystallite.The intensity enhancement of the TO and LO modes of the film increases as the Zn concentration rises, and this phenomena could be explained by of parity-breaking defects in the NiO lattice [29].Remarkably, these observed conditions align well with the structural disorder and lattice defects identified in the Zn-doped NiO structure, demonstrating good agreement with the XRD findings.The peak at 200 cm −1 aligns with the zone-boundary phonon mode [30].A peak located at 1466 corresponds to the 2 M  mode, representing a magnon excitation in NiO nanostructures.This excitation arises from antiferromagnetic nature of the NiO films, and it arises from the superexchange interactions between adjacent Ni ions along Ni 2+ -O 2− -Ni 2+ chain.chain [31].As Zn doping increases, there is a noticeable fall in the 2 M mode's intensity.This effect may be ascribed to the reduced size of crystallite and disorder introduced by defects in the film [32].
The small size of crystallites in NiO films is the underlying reason for the wider width of the 2LO Raman peaks, and it correlates with the XRD analysis findings [4].Raman analysis of prepared film revealed the absence of any secondary or impurity peaks associated with the Zn dopant concentration.Figure 5 illustrates that the 2LO mode of films undergoes a peak shift towards the smaller wavenumber with an increase Zn doping.The shift in peak at 1086 may be due to the stress induced in NiO matrix after introducing Zn dopant.Ravikumar et al demonstrated that as the size of the crystallites decreased, the 2 P LO mode experienced a shift towards lower wavenumbers, confirming the occurrence of a redshift in the Raman frequency.The size-induced phonon confinement effect, which is further impacted by the intensity of the two-phonon coupling and surface relaxation, is responsible for this spectral shift.Furthermore, a red shift in phonon vibrations can also be caused by defects and surface effects [29].

Morphological analysis
AFM measurements utilizing tapping mode were employed to examine the topographical images of the prepared samples.Figure 6 illustrates 2D and 3D images of Zn doped NiO films, obtained from a film scan area of .5 μm × .5 μm.
The prepared films exhibit a uniform and smooth surface, covered throughout with grain-like structures [18,33].From AFM images, it is evident that the zinc doping has an impact on the morphology of NiO films.As the concentration of doping increases from 1% to 10%, the depicted figure illustrates a proportional enlargement in the size of the grain-like structures.This growth of the grains can be ascribed to the integration of Zn 2+ into the NiO lattice.The 1% Zn doping has smaller grain like structure compared to pure NiO films, whereas 5% and 10% Zn doping shows well defined grain growth.At a 10% Zn doping level, the film appears to be densely packed, exhibiting agglomeration of the grains [34].These changes may be due to the structural defect arising in the films at higher Zn incorporation.Table 3 presents the surface roughness measurements for the film, which exhibit variations based on the Zn dopant concentration.The increase in grain size within Zn-doped NiO films is responsible for an increased roughness in the film.This observation aligns well with the data obtained from XRD.And the same result is reported in [3,18].

Linear optical characterization
The transmittance spectra of pure and Zn doped NiO film are recorded and are depicted in figure 7.
In the visible range, all the films prepared displayed excellent transparency.The pure NiO film showed slightly higher transparency compared to the Zn doped NiO films.It is ascribed to the lower micro-strain induced in the crystal structure of the pure NiO film [35].The decrease in transmittance observed in Zn-doped films is due to photon scattering effects caused by the formation of crystal defects.Also the drop in transmittance may be influenced by the enhancement in the metal-to-oxide ratio and surface roughness, as supported by AFM images [36].
The Tauc's relation was employed to calculate the bandgap of the pure and Zn doped NiO films, as expressed by the equation, Where hn represents for photon energy in eV, a denotes linear absorption coefficient, E g stands for energy band gap of the films, and A is the proportionality constant.Figure 8 shows the film bandgap, and values are given in table 4.  The bandgap is reduced from pure to 5% doped NiO films and then increases to 10% Zn doped NiO films.The decrease in bandgap could be attributed to the formation of additional energy levels near the VB edge, results in a decrease in energy needed for transitioning from the VB to the CB [20].

Photoluminescence studies
Photoluminescence studies were conducted at room temperature to analyze the defects present in the Zn doped NiO films, using an excitation wavelength of 300 nm.Generally, NiO exhibits defect centers of interstitial and vacancy, as well as antistites defects.Gaussian deconvolution fitting is employed to analyze the PL spectra, aiming to identify the defect centers and radiative transition in the films, which is given in figure 9.
PL spectra of NiO consists of mainly UV emission and Visible emission parts.The peak position ∼3.1 eV corresponds to the UV emission, which is induced due to NBE emission originating from the recombination of exciton-exciton [37].The NBE emission intensity decreases in the order of 10%, 5%, and 1% with respect to pure NiO films, and this reduction upon Zn doping is attributed to a decrease in crystallites within the NiO lattice and an increase in the formation of defect states [15].Mochizuki et al proposed that transitions occurring under 3 eV in the lattice of NiO originate from nickel-induced vacancies.These vacancies arise from the electron  transfer of Ni 2+ to Ni 3+ [38].In this process, a hole is created by the missing electron in Ni 2+ , and it can be filled when a photon-generated electron combines with it [38].Visible emission in polycrystalline NiO can be due to the structural defect, interstitials, and vacancy defect states at grain-to-grain contacts [39].
The violet emission was observed at ∼2.9 eV, which corresponds to deep-level emissions and occurs due to the transition of trapped electrons at the nickel interstitial to the holes in the VB [40].The transition of electrons from doubly ionized nickel vacancies to the VB was observed at around ∼2.8 eV, marking the presence of blue emission centers.The observation of a red shift in blue emissions after doping validates the existence of strain in the films resulting from the incorporation of Zn into the lattice, a confirmation supported by XRD analysis.A peak is present at ∼2.64 eV luminescent, which implies the transition related to nickel vacancies and oxygen defects present within the films.The peak at ∼2.7 eV and ∼2.0 eV confirm the existence of oxygen vacancy and oxygen interstitial defects, respectively.
The introduction of Zn dopant into the lattice is prominently visible in PL spectra.The green emission center enhances drastically compared to UV emission when Zn is added to the lattice.This implies that oxygen defects increase in the films upon doping.Especially 1% Zn: NiO films showing high green emissions compared to others.When there are low levels of doping, it can cause additional phases to appear at the boundaries  between grains.This creates defect levels, potentially altering in the structural properties and the rate at which emissions recombine in the films [41].The blue emission was observed to increase with an increase in Zn concentration, confirming that the addition of Zn ions to the films increased the amount of nickel vacancies [40].
Table 5 provides information on the proposed contributing bands for the PL emission, along with the respective origins of the defects.

Nonlinear optical studies
In our investigation of the NLO properties of films, we have employed two distinct methods: Z-scan and the THG technique.Through Z-scan studies, our focus is on unveiling the nonlinear absorption (NLA) and nonlinear refraction (NLR) features of the Zn-doped NiO films.Alternatively, THG explores a fascinating world where we convert the light frequency from ω to 3ω through a process known as the third-order susceptibility.This ultrafast electronic process in photonics has caught our attention, and we're excited to uncover its unique dynamics and potential applications.

Nonlinear absorption
Nonlinear absorption coefficients were examined using a single-beam laser with z-scan experimentation employing an input intensity of 20 mW.When a high-intensity laser is passed through a material, its optical properties change nonlinearly.Then the nonlinear absorption and refractions are no longer independent of intensity, it is given as [42], Where , eff 0 a b are linear absorption coefficient, and nonlinear absorptioncoefficient respectively.Similarly, n , 0 n 2 are linear refractive index and nonlinear refractive indices and I represent the intensity in W/ m 2 .Figure 10 represents the nonlinear absorption traces of pure and Zn doped NiO films obtained from open aperture (OA) z-scan curves.All prepared films exhibit a dip at focus, which representing the reverse saturable absorption (RSA).The display of RSA behavior is evidenced by the transmittance value reaching a minimum at focus (z = 0) with increasing intensity.As the Zn doping concentration rises, the valley becomes significantly deeper, indicating an enhancement in nonlinear absorption.
There are many mechanisms that cause the existence of RSA process in films, such as two-photon absorption (TPA), RSA, excited state absorption (ESA), transient absorption, free carrier absorption (FCA), nonlinear scattering, and the combination of these processes [43].In semiconductors, TPA occurs only if the energy of the laser source is less than the energy bandgap of the material and should be greater than half of the energy gap.Then, the necessity of the requirement is fulfilled.But, if only TPA is the reason of nonlinearity, the nonlinear absorption eff b should independent of I [44], but in the studied films, the effective eff b undergoes variations with respect to the intensity.However, the observed nonlinear absorption is substantial, indicating that the observed RSA is mainly due to the phenomenon arising from the interaction of excited-state absorption and FCA present within in the films.After Zn doping, an increase in defect states is observed in the NiO lattice, which is confirmed by PL studies.Upon exposure to a laser, electrons residing in the lower energy state undergo excitation to higher energy states.Subsequently, these electrons, after absorbing photon energy, undergo deexcitation to the ground state within nanoseconds.Additionally, these excited electrons can transition to defect states, serving as intermediate levels within the bandgap.Electrons in the defect state can further undergo excitation to higher levels, contributing to the ESA mechanism, thereby amplifying the RSA in the films [45].Furthermore, the increased nonlinear absorption observed in Zn-doped NiO films, in contrast to pure NiO films, could be explained by Zn ions within the NiO matrices, leading to FCA [46].The following formula was used to determine the OA transmittance [42], Where m denotes the multiphoton absorption, and m is equal to 1 is implies the TPA, and And effective length is defined as, L, are input intensity, Rayleigh length and thickness of the films respectively.Figure 10 is plotted using the equation (8).The obtained the nonlinear absorption coefficient of pure and Zn: NiO films are reported in table 6.

Nonlinear refraction
The closed aperture (CA) z-scan setup is employed to ascertain both the sign and magnitude of nonlinear refraction (NLR).The trace of the CA for both pure and Zn doped NiO films is presented in figure 11.All films show a self-defocusing effect, occurs when a peak is followed by a valley, leading to a negative value of the nonlinear refractive index.In this scenario, the material's refractive index decreases under the influence of laser light, causing the light to diverge or spread out as it propagates through the medium.
Nonlinear refraction in materials can originate from various physical mechanisms, such as electronic, molecular, electrostrictive, or thermal effects.Our experiment utilized a continuous wave laser, so the nonlinearity is arising due to thermal effect.When a high intensity light fall on the materials microstructure, the  defect states absorb a slight quantity of energy, causing a localized rise in temperature.This uneven temperature distribution induces changes in the refractive index across the material, giving rise to thermal nonlinearity [47].In all our samples, there is an observed peak-valley separation of approximately 2 times the Rayleigh range (2 Z R ).Whenever the peak to valley separation exceeds 1.7 Z R , this strongly suggests thermal nonlinearity, implying the process is third order [48].
The nonlinear refractive index in SI unit is given as [42], is the phase change in on axis, ∆T p v -is the difference in transmittance from peak to valley, S is the linear transmittance regime, and for our case it is 0.7.The normalize transmittance of closed aperture is given as [42], The NLR and NLA are related through the real and imaginary components of the third order nonlinear susceptibility ( 3 c ) as described by the equation [42], where 0 e is denoted as free space permittivity and c is the speed of light in meter per second.
Table 7 represents the third order nonlinear optical parameters of the prepared films.The third-order susceptibility experiences an increase as a result of doping of Zn into the NiO lattice.This improvement can be ascribed to an increase in both the nonlinear absorption coefficient and nonlinear refractive index, possibly induced by ESA, FCA and local heating effect, respectively.Remarkably, NiO film with 1% Zn doping exhibits a higher χ(3) value compared to other doping concentrations.The high third-order susceptibility may be attributed to the greater presence of nickel and oxygen defect states within the films, as confirmed by photoluminescence studies.Additionally, the increased nonlinear susceptibility in the NiO film doped with 1% Zn may result from its crystallite size and reduced surface roughness [49].Samples with higher surface roughness tend to scatter light, leading to a decrease in nonlinear optical response.The interplay of these factors indicates a synergistic effect, underscoring the diverse influences on the material's nonlinear behavior which can apply in optoelectronic application.

Third harmonic generation studies
In the realm of nonlinear optics, the process of third harmonic generation produces a beam at the third harmonic frequency, or 3ω, by interacting a high-intensity laser beam operating at frequency ω with a nonlinear material.The THG technique offers a distinct advantage by selectively probing the ultrafast electronic response of materials.This specificity ensures that contributions from vibrational, orientational, and thermal effects, which might otherwise influence the overall nonlinear response of the material, are effectively excluded from consideration.This precision in isolating the ultrafast electronic dynamics enhances the technique's capability to elucidate and characterize the intrinsic nonlinear optical properties of the material under investigation.Studies on the generation of the third harmonic were presented in a measurement setup (see figure 12), which consists of two experimental paths.The first path includes chopper (C) and a femtosecond pumping laser, which was used to cause an induce changes in the dipole moments of the thin film under study.The chopper, installed in the path of the femtosecond laser, allowed for modulation of the light beam, reducing the number of photons hitting the sample at a repetition frequency of the laser at 63 MHz.This is crucial to avoid thermal damage to the material during prolonged exposures.In the second track, the probing beam of the nanosecond laser (1064 nm) passed through a Glan polarizer (P) and was directed through a partially transmissive mirror (M) both to the detector (Si PD) that synchronizes the measurement system, and onto the sample surface, where changes in optical dipole moments were previously induced using the femtosecond pumping laser.The third harmonic generation signal was recorded using a photomultiplier (PM) connected to an oscilloscope (OSC).In the photomultiplier track, an interference filter with a wavelength of 355 nm was placed before the photomultiplier for THG signal reading.By changing the angle of the polarizer rotation (every 5 degrees), we manipulated the energy density of the 1064 nm probing laser reaching the sample.This mechanism allowed the detection of the THG signal at the minimum energy density of the nanosecond laser.
The dependence of the third harmonic generation signal intensity on the energy density of the Nd:YAG probing nanosecond laser at 1064 nm was presented in figure 13.A femtosecond laser with a wavelength of 1045 nm served as the source inducing optical changes in the dipole moments in Zn-doped NiO thin films.Knowing the energy of the nanosecond laser, the diameter of the generated beam, and the pulse duration, we determined the energy density of a single pulse hitting the sample.The third harmonic signal is exhibited for Zn doped NiO films which is given in figure 13.The third harmonic signal experiences a gradual rise as the concentration of Zn dopant increases from 1% to 10%.Notably, the highest THG signal is observed in the presence of 10% Zn-doped NiO.This occurrence could be attributed to enhancement photoexcitation and relaxation processes within the sample [50], potentially facilitated by local defect levels that increased by doping concentration.Furthermore, the increase in THG signal is also can be influenced by multiphoton excitation [51].
In summary, the observed disparities between nonlinearity obtained from the z-scan technique and THG measurement in Zn-doped NiO films can be rationalized by considering the intricate interplay of various excitation of the laser and material characteristics.In the z-scan experiment, we directed a continuous wave of laser light into the material, resulting in enhanced heating effects within the material.This led to a change in the refractive index and, thus, the emergence of nonlinearity in the material [52].On the other hand, harmonic generation using a femtosecond laser involves an electronic contribution to nonlinearity.In the THG technique, the response primarily reflects the ultrafast electronic effects, excluding contributions from vibrational, orientational, and thermal phenomena, which can also influence the overall nonlinear response of the material [53].In this case, the ultra-short pulses of the femtosecond laser interact with the material on timescales comparable to or shorter than the electron-phonon relaxation time, leading to electronic excitations and a different nonlinear optical response compared to the thermal effects seen with CW lasers.
In the context of Z-scan measurements, the heightened third harmonic susceptibility observed for 1% Zn doped NiO may be attributed to the utilization a continuous wave laser.The continuous wave laser induces thermal excitation within the material, thereby revealing the NLO properties.The increased thermal effects, combined with the 1% Zn doping, create conditions that amplify the nonlinear response.Additionally, the higher third-order susceptibility obtained by Z-scan for the 1% Zn doped NiO film can be linked to a smaller crystallite size, a more uniform surface, and elevated oxygen defects.These factors collectively influence the comprehensive nonlinear response of the material, underscoring the complex interplay of various factors contributing to the observed nonlinear response.
Conversely, the heightened THG value observed for 10% Zn doped NiO is linked to pulsed excitation from the primary source, the Nd:YAG laser, and the induced laser from Yb: YV4laser.Specifically, the Nd:YAG laser involves nanosecond excitation, while the Yb: YV4 laser employs femtosecond excitation.The interplay between these nanosecond and femtosecond laser sources induces changes in the films, creating conditions that efficiently generate higher-order harmonics and ultimately contribute to the observed elevated THG value.Moreover, the heightened THG signal in the 10% Zn doped NiO sample is associated with the presence of dopant-induced defects, localized electronic states, and increased surface roughness.Dopant aggregation at this concentration likely leads to the formation of regions with favorable conditions for efficient THG processes.The findings underscore the need for a comprehensive understanding of material properties and nonlinear mechanisms to interpret and optimize the performance of doped NiO films for potential applications in nonlinear optics.

Conclusion
In conclusion, the spray pyrolysis technique proved effective in fabricating Zn doped NiO films.XRD analysis confirmed the polycrystalline nature of all samples, further affirming the face-centered cubic structure of the NiO film.Raman spectra provided insights into the vibrational modes of the films, highlighting increased intensity attributed to nickel and oxygen defects.AFM studies demonstrated the uniform and smooth distribution of the prepared films, with a notable increase in surface roughness upon Zn doping.Additionally, photoluminescence spectra unveiled the presence of nickel vacancies, interstitial nickel defects, and oxygen defects, with visible emission enhancement observed upon Zn doping.OA z-scan experiments revealed RSA across all films, indicating an increase in excited state absorption due to localized defect levels.CA z-scan experiments indicated a negative value of refractive index in all films.Remarkably, THG studies demonstrated an enhancement in third harmonic signals with increasing zinc doping.This promising finding suggests the potential applicability of the material for tripling frequency applications, further underscoring the versatility and utility of the prepared films.

Figure 5 .
Figure 5.An enlarged view of the Raman spectra for Zn doped NiO films.

Figure 10 .
Figure 10.Open aperture trace of Zn doped NiO films.

Figure 12 .
Figure 12.System for measuring the third harmonic of light.

Figure 13 .
Figure 13.The third harmonic signal of Zn doped NiO films.

Table 1 .
Structural parameters of Zn doped NiO films.

Table 2 .
Summary of deconvoluted Raman mode positions for the film.

Table 3 .
The surface roughness value of Zn doped NiO film.

Table 4 .
Energy bandgap values obtained from tauc's relation for Zn-doped NiO films.

Table 5 .
PL peak position of Zn doped NiO films.

Table 6 .
The NLA coefficient of Zn doped NiO films.

Table 7 .
third order nonlinear optical parameters of Zn doped NiO films.