Enhancement of Non-Linear Optical laser Power-Depend Properties of ZnO Nanoparticle Synthesized by Green Method

INTRODUCTION
The field of nonlinear optical (NLO) materials has garnered significant attention due to advancements in powerful ultrafast lasers [1]. These materials are integral to a variety of applications, including optical limiting, optical computing, and optical communication [2], as well as nonlinear optical spectroscopy [3, 4] and biomedical fields [5]. As we look to the future, the expected capabilities of optoelectronic devices include outstanding performance, high operational speeds, and compact designs. Among the diverse materials employed in these technologies, metal/metal oxide nanoparticles and nanocomposites are particularly notable. Celebrated for their unique surface effects and quantum confinement properties, which are absent in their bulk counterparts, these materials enhance the functionality of optical devices [6].
Among these innovative materials, Transparent Conducting Oxide (TCO) nanomaterials stand out due to their exceptional optical and electrical properties. These materials, which include wide bandgap semiconductors like zinc oxide (ZnO), are essential in many optoelectronic devices. They combine semiconductor properties with a transparency that is crucial for applications requiring the manipulation of visible light and sufficient carrier density to ensure electrical conductivity [7, 8]. Zinc oxide nanoparticles, in particular, are renowned not only for their applications in fields such as UV light emitters and piezoelectric transducers but also for their biocompatibility, making them suitable for biomedical applications [9].
The synthesis of ZnO nanoparticles through physical [10], chemical [11], and, biological methods [12], presents varied environmental and economic challenges. Physical methods often demand high pressures and temperatures, making them costly and environmentally burdensome [13]. Chemical methods, while effective, pose significant environmental and safety risks due to hazardous chemicals used as reducing agents [14]. Conversely, biological synthesis using natural agents such as plant extracts, fungi, and bacteria provides a greener, safer alternative [15, 16]. This study explores the innovative synthesis of ZnO nanoparticles using mint plant extracts, a method that not only supports sustainable material development but also potentially enhances the nanoparticles’ optical properties due to the unique interactions of plant-derived phytochemicals.
Beyond their sustainable synthesis, ZnO nanoparticles exhibit exceptional physical properties that justify their widespread use in advanced applications [17]. Recent studies have highlighted their capabilities in high-speed carrier dynamics and significant nonlinear refraction and absorption, crucial for the development of broadband optical networks where high-speed optical switching devices are essential [18, 19]. These devices require materials with minimal optical losses and rapid response times, achievable through materials like ZnO with a band gap at least twice the photon energy employed [20-22].
The Z-scan technique, extensively used to assess the intensity-dependent nonlinear susceptibilities of materials, including their nonlinear refractive index and absorption characteristics, plays a pivotal role in our study [23, 24]. This highly sensitive method, developed by Sheik-Bahae et al. [23], is crucial for measuring how materials respond to varying light intensities, a critical factor in the development of optical limiting devices that operate based on nonlinear absorption or refraction.
In this study, we synthesize zinc oxide (ZnO) nanoparticles using a novel green synthesis method with mint plant extracts, analyzing their nonlinear optical properties through both closed and open aperture Z-Scan techniques at varying laser powers and wavelengths. This approach not only highlights the sustainability of using phytochemicals in nanoparticle fabrication but also suggests that these compounds enhance the nonlinear refractive index and absorption characteristics of ZnO NPs. Our findings underscore the potential of integrating environmentally sustainable methods with advanced optical applications, paving the way for high-performance materials that leverage biological synthesis to optimize material properties uniquely. This synthesis method aligns with environmental goals and opens new avenues for material functionality that traditional methods cannot achieve, making it a significant contribution to the field of material science.

MATERIALS AND METHODS
Chemicals and Materials 
In this study, double-distilled water was used for both the growth and treatment processes. All chemical materials, such as zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and sodium hydroxide (NaOH), were sourced from Sigma-Aldrich and used as received, without undergoing additional purification. This standardization of reagents ensures reliability and consistency in the experimental setup, facilitating reproducible results across various research trials.

Plant collection
Fresh and healthy mint leaves were collected from Rawanduz city, located at coordinates 36.1901° N, 43.9930° E, in the Erbil Governorate of the Kurdistan Region, Iraq. This specific sourcing of mint leaves ensures the consistency and ecological relevance of the plant materials used in the study.

ZnO Green Synthesize
Twenty-five grams of fresh, seasonal mint leaves were harvested, washed, chopped into small pieces, and then boiled in 200 mL of double-distilled water at 70 °C for 40 minutes. The resulting aqueous extract was filtered using Whatman No.1 filter paper and refrigerated for later use. For the synthesis of zinc oxide nanoparticles (ZnO NPs), 50 mL of the mint leaf extract was gradually mixed with an equal volume of zinc nitrate [Zn(NO3)2] solution while being continuously stirred magnetically at 80 °C for 45 minutes. The pH of the mixture was adjusted by adding 1 M sodium hydroxide (NaOH) until a color change occurred, indicating the start of ZnO NP nucleation. This color shift, which can be monitored by UV/Vis double beam spectroscopy, is typically accompanied by the formation of white sediment and signifies nanoparticle formation, with a yellowish hue indicating the presence of ZnO NPs. Following this, the sediment was separated by centrifugation at 7000 rpm for 30 minutes. The collected solid powder was then rigorously washed with methanol and distilled water four times to remove any impurities and organic residues. Finally, the sample was partially annealed on a Bunsen burner at 500°C for 1 hour to enhance its properties, as shown in Fig. 1. This method effectively demonstrates the production of nanomaterials, providing visual and spectroscopic evidence of the synthesis process.

Z-Scan System 
Fig. 2 illustrates the setup for the Z-Scan technique. To synthesize the ZnO nanoparticles (NPs), ZnO nanomaterial was dispersed in 70 milliliters of distilled water. This mixture was prepared in different concentrations, labeled as samples C1, C2, and C3. The mixtures were then stirred on a magnetic stirrer for 30 minutes to ensure a homogeneous white solution was achieved. This setup is critical for examining the nonlinear optical properties of the ZnO nanoparticles under varying conditions.
 Therefore, volume fraction (V) was computed using the Eq. 1; 

Where 𝑉𝐿 is the liquid volume, 𝑉𝑠 = 𝑚/𝜌 is the volume of ZnO NPs, 𝜌 is the ZnO density, and 𝑚 is the mass of ZnO particle dispersed in solution, the detail of results is listed in the (Table 1).
Green and red Z-Scan measurements were utilized to examine the nonlinear optical (NLO) response of each sample, employing different laser powers to demonstrate the impact of laser power on NLO characteristics. The results of these measurements are detailed in (Table 2) . This approach allows for a comprehensive analysis of how varying laser power levels affect the NLO properties of the samples, providing valuable insights into their potential applications in optical devices.

Characterization techniques
X-ray diffraction measurements were performed using a PANalytical X’ Pert PRO instrument equipped with CuKα radiation (λ = 1.5406 Å). The scans were conducted at a rate of 1° per minute across a 2θ range of 20° to 80°. The synthesis and presence of ZnO nanoparticles were verified using a UV-Vis double-beam spectrophotometer (Super Aquarius-1000). Additionally, the morphology of the nanoparticles was examined using ionic emission scanning electron microscopy (SEM-Quanta 450). To determine the chemical composition of the synthesized nanostructures, energy-dispersive X-ray spectroscopy (EDS) analyses were performed within the SEM setup. FTIR spectroscopy was conducted using a Perkin Elmer spectrometer with a resolution of 4 cm−1 to investigate the functional groups present in the synthesized ZnO NPs . This comprehensive set of analytical techniques ensures a thorough characterization of the ZnO nanoparticles, confirming their structure, composition, and physical properties.

RESULTS AND DISCUSSION
ZnO Nanoparticles Characterization 
UV-Vis Analysis 
The optical properties of zinc oxide nanoparticles (ZnO NPs) were investigated using ultraviolet-visible (UV-Vis) absorption spectroscopy. This technique is pivotal for confirming the successful synthesis of ZnO NPs, as the oscillations of conducting electrons within specific wavelength ranges provide insight into the surface plasmon resonance (SPR) effect. As shown in Fig. 3, the UV-Vis spectrum of the synthesized ZnO NPs features an absorption band centered at 365 nm, which confirms their presence in the solution. Notably, the absence of any additional absorption peaks in the spectrum indicates the purity of the ZnO NPs, free from other elemental or biomolecular impurities. These findings are consistent with results from previous studies [18-20], reinforcing the reliability of the synthesis and characterization methods used.

X-Ray Diffraction Analysis for ZnO NPs
The X-ray diffraction (XRD) patterns of ZnO nanoparticles (NPs) synthesized through green methods are depicted in Fig. 4. The identified peaks at 31.90°, 34.57°, 36.41°, 47.70°, 56.76°, 62.97°, 66.58°, 68.07°, 69.21°, 72.75°, 76.98°, and 81.51° correspond to the lattice planes (100), (002), (101), (012), (110), (013), (200), (112), (201), (004), and (202) respectively. These planes are consistent with the hexagonal wurtzite structure of ZnO, as indexed by the Joint Committee on Powder Diffraction Standards (JCPDS-36–1451). The absence of additional peaks and signals confirms the phase purity of the ZnO NPs. At room temperature, the lattice constants of the ZnO sample were determined to be 3.2346 Å for the ‘a’ parameter and 5.2120 Å for the ‘c’ parameter. The presence of narrow and intense diffraction peaks, particularly for the (100), (002), and (101) planes, underscores the high crystalline quality of the ZnO sample. Using the Debye-Scherrer formula, the estimated size of the ZnO NPs was calculated from the XRD data:

Where D is the crystalline size, λ is the X-ray wavelength, equals to 1.5406 Å, and β is the full width half maximum (FWHM) of the dominant peak (101), ~0.003. Therefore, the average crystalline size of the ZnO NPs can be calculated to be in the range of 70–80 nm. The obtained XRD result of ZnO NPs is in good agreement with previous work [25, 26].

SEM and EDS Analysis
SEM analysis of ZnO nanoparticles, synthesized using a green method with mint plant extracts, reveals a morphology characterized by predominantly spherical nanoparticles that tend to cluster into aggregates. This aggregation, observable in the SEM images, is typical of nanoparticles synthesized through environmentally friendly methods, where natural compounds act as stabilizing agents. The detailed SEM image provides insight into the anisotropic nature of these nanoparticles and their surface characteristics at the nanoscale level.
Complementing the SEM analysis, the particle size distribution histogram (Fig. 5) provides a quantitative insight into the size range of the synthesized nanoparticles. The peak of the distribution curve occurs at approximately 80 nm, indicating a modal particle size that is optimal for the intended applications in photonic devices. The controlled particle size distribution not only reflects the efficacy of the green synthesis method but also suggests that the size and uniformity of these nanoparticles are conducive to enhancing their nonlinear optical properties. The precise control of particle size is crucial for applications that rely on the specific interaction of light with matter, such as in optical limiting and high-speed optical switching devices, where the nanoparticles’ ability to respond to light at ultrafast speeds is essential.
To gain deeper insights into the topographical features of ZnO nanoparticles (NPs), Energy-dispersive X-ray (EDS) analysis was conducted on the same region scanned by the SEM, as shown in Fig. 6. The elemental analysis revealed that the ZnO NPs consist of approximately 78.5% zinc (Zn) and 21.5% oxygen (O), demonstrating high levels of purification [27]. Zn and O are the primary components of the ZnO sample and are uniformly dispersed across the surface of the nanoparticles. The presence of gold (Au) detected in the EDS analysis is attributed to the gold coating applied to the samples to enhance the quality of SEM imaging. The combined results from the X-ray diffraction peaks, SEM images, and EDS analysis confirm that the synthesized ZnO NPs are of high quality, free from impurities or secondary phases.

FTIR Analysis
Fourier-transform infrared (FT-IR) spectroscopy was utilized to confirm the purity and characteristics of the green-synthesized ZnO nanoparticles (NPs). Metal oxides, including ZnO, often show absorption bands in the fingerprint region below 1000 cm−1, which are indicative of interatomic vibrations. The FT-IR spectrophotometer was thus employed to assess the composition and structural integrity of the ZnO NPs. Additionally, the FT-IR spectrum offers insights into the types of bonding present within the samples across all phases of matter [28]. 
The FT-IR spectra for the powdered samples of synthesized ZnO NPs, as displayed in Fig. 7, reveal several key features. The broad absorption peak at approximately 3323 cm−1 is attributed to the O–H stretching vibrations of intermolecular hydrogen bonds, possibly due to atmospheric moisture absorption. The peak around 1467 cm−1 is linked to the amine (N–H) groups from proteins or enzymes present in the plant extract used in the synthesis. Another peak at 1062 cm−1 corresponds to the C-O bond, indicative of carbohydrate presence. The peak at 878.31 cm−1 likely stems from polyphenolic groups in the plant extract, which act as capping agents [29]. Finally, the sharp peak observed around 402 cm−1 is attributed to the Zn-O stretching vibrations specific to the ZnO NPs, confirming their presence and purity. These findings are consistent with previous research [30-32], demonstrating the successful synthesis of high-quality ZnO nanoparticles using green methods.

Mechanism of Green Synthesis ZnO Nanoparticles 
Plant extracts are rich in a diverse array of compounds, including flavonoids, terpenoids, and polyphenols, which facilitate the bio-reduction of metal salt solutions during nanoparticle synthesis. These natural substances serve as reduction, capping, and stabilization agents—roles that are crucial in the synthesis of nanoparticles. Traditionally, chemical agents have been used for these purposes; however, they pose significant environmental and health risks due to their potential for degradation and toxicity. Consequently, there is a pressing need for environmentally friendly alternatives that can fulfill these roles in nanostructure preparation.
Phytochemicals present in various parts of plants—such as stems, fruits, roots, peels, seeds, flowers, and leaves—offer a promising alternative to chemical agents. These phytochemicals are believed to be responsible for the reduction of metal ions to zero-valent metal nanoparticles [33]. This reduction process is often indicated by color changes in the reaction solution, which can be monitored using UV-Vis spectroscopy [34]. Research has shown that mint extract, in particular, contains flavonoids and phenolic acids that significantly aid in the reduction of zinc ions and stabilize the growth of ZnO nanoparticles [35].
Illustrated in Fig. 8, the mechanism of nanoparticle growth highlights the potential bio-molecules in the plant extract responsible for bio-reducing metal ions, initiating nucleation, and subsequently promoting the growth of ZnO nanoparticles in various sizes and shapes. According to this mechanism, and given the presence of hydroxyl (OH) groups in phytochemicals, zinc ions (Zn2+) undergo reduction to zero-valent zinc (Zn0). During the annealing process, due to high oxidation potential of Zn0, it readily reacts with oxygen, leading to the spontaneous formation of ZnO nanoparticles [36]. This synthesis approach not only reduces environmental impact but also enhances the biocompatibility of the nanoparticles, making them suitable for various applications.

Non-Linear Optical Properties 
Fig. 1 depicts the experimental setup for the Z-scan experiment, modeled after the setup used in the work of Zhang et al. [37]. It should be noted that the experiment was conducted at room temperature utilizing continuous wave (CW) green and red laser diodes, operating at wavelengths of 532 nm and 635 nm, respectively. A 10 cm focal length convex lens was employed to focus the laser beam to a small spot, and a micro-stepping controller was used to precisely adjust the sample’s position along the Z-axis. The nonlinear refractive index was measured by moving a glass optical cell (L=2 cm), containing the sample solution, through the beam’s focus along the Z-axis. The nonlinear absorption (NLA) coefficient was then determined based on the Z-scan experiments conducted using both closed aperture (CA) and open aperture (OA) settings.
Measurements on ZnO nanocolloids were performed using the Z-scan technique on three different concentrations of ZnO NPs, irradiated at excitation wavelengths of 532 nm and 635 nm with varying incident peak intensities ranging from 110.14 MW/cm² to 31831 MW/cm² for the 532 nm wavelength and 248.67 MW/cm² for the 635 nm wavelength. The OA Z-scan traces of the ZnO NPs colloids, illustrated in Fig. 9 (a-d), consistently show a valley, aligning with previous findings [38]. This characteristic can be attributed to reverse saturable absorption (RSA) behavior, likely caused by the absorption of free carriers and excitation effects [39]. The nonlinear absorption coefficient was calculated using the Eq. 3:

Where ΔT is the single valley value at the Z-scan curve’s, I0 intensity of laser and Leff is effective sample length given by; 

Where, α0 is linear absorption, and L is sample length.
In the open aperture scan results, as detailed in (Table 3), there is a clear increase in the nonlinear absorption coefficient as the concentration of nanoparticles in the solution increases. Specifically, when the concentration of ZnO NPs colloids is increased from C1 to C2 g/70mL, a greater number of nanoparticles are present, which enhances the occurrence of three-photon absorption (3PA). The obtained 3PA coefficient from these measurements is three times larger than that observed in thin films and eight times larger than in bulk zinc. These results align with those reported in previous studies [40], confirming the enhanced nonlinear optical properties of ZnO NPs in colloidal form compared to more traditional physical states.
The nonlinear absorption coefficient, observed at a constant laser wavelength, varies from 0.5 cm/GW to 187 cm/GW, aligning with findings from previous research [18]. Notably, there is an increase in the difference in transmittance between peak and valley values that correlates directly with the emission power of the laser. This relationship suggests that higher laser powers enhance the material’s nonlinear response. Fig. 10 visually represents this behavior by showing the exponential decay of the nonlinear absorption coefficient, plotted against the varying incident laser intensities [41]. This graph effectively illustrates how changes in laser intensity impact the optical properties of the material, providing a clear understanding of the dynamic responses under different conditions.
To calculate the NLO refractive index n2, using close aperture Z- Scan method for three different concentration (C1, C2, C3) 200 mW, 635nm. The transmittance peak to valley ∆Tpv given by the subsequent equation [42];

Where ∆∅ is non -linear phase shift and equal to ∆∅=(2π/λ) n2 Leff  I0.The measured value of non-linear refractive index is ranged between 2.51×10-13 and 5.81×10-13 cm2/W are agreed with previous study [43, 44]. Focusing on the nonlinear optical (NLO) characteristics of ZnO nanoparticles, Fig. 11 illustrates the impact of nanoparticle colloid concentration on the single photon absorption coefficient and nonlinear refractive index, using a 200 mW excitation power at a 635 nm wavelength. The data shows that both the nonlinear refractive index and nonlinear absorption coefficient decrease with the reduction in ZnO NP concentration. This trend suggests that smaller nanoparticle fractions, which occupy less volume, lead to reduced nonlinear optical coefficients. This relationship highlights that the NLO properties of nanoparticles are significantly influenced not only by their concentration but also by factors such as size, shape, and manufacturing process, along with the parameters of the excitation laser. These findings underscore the complex interdependencies that define the optical performance of nanoparticle-based systems.
Additionally, it is evident from the Z-scan trace that fluctuations in the concentration of ZnO nanoparticles lead to changes in the nonlinear phase shift, which in turn affects the nonlinear refractive index (n2). As a result, as shown in Fig. 12, the magnitude of n2 is dependent on the concentrations of ZnO nanoparticles in the nanocomposites [43]. This dependence highlights the sensitivity of the nonlinear optical properties to variations in nanoparticle concentration, underscoring the need for precise control over nanoparticle distribution within the composite to achieve desired optical characteristics.

CONCLUSION
This study has successfully demonstrated the synthesis of ZnO NPs using a green synthesis approach with mint plant extracts, tailored specifically for enhancing their efficacy in photonic applications. The characterization of these nanoparticles through SEM and XRD confirmed their nanoscale size and high crystallinity. Notably, the nanoparticles predominantly displayed a spherical and anisotropic morphology with an average size of about 80 nm, consistent with optimal properties for photonic device applications. The SEM and XRD analyses verified that the nanoparticles synthesized via this green method not only adhere to the desired physical attributes but also exhibit a significant improvement in structural and compositional integrity compared to nanoparticles synthesized through traditional methods. This enhancement is attributed to the effective roles of the phytochemicals present in the mint extract, which act as reducing, capping, and stabilizing agents, thus facilitating a more controlled and environmentally sustainable synthesis process. Further exploration of the nonlinear optical properties of these nanoparticles using both closed and open aperture Z-Scan techniques at varying laser powers and wavelengths demonstrated superior nonlinear absorption coefficients and refractive indices, marking a significant enhancement over the properties reported in previous studies. These findings not only underscore the potential of ZnO NPs for advanced optical limiting applications but also highlight the advantages of integrating environmentally sustainable methods with nanoparticle technology, suggesting avenues for future research in refining synthesis processes and expanding applications into biomedical imaging and targeted drug delivery. This improvement in using ZnO nanoparticles shows how green synthesis methods can greatly benefit material science and photonic technologies, merging technological progress with environmental care. 

ACKNOWLEDGEMENT
We would like to express our sincere gratitude to the Scientific Research Center of Soran University for their support and assistance throughout the course of this research project. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.