Nonlinear optical properties of some nanosized ammonium dihydrogen phosphate crystals incorporated in aluminum oxide nanopores

Interest in synthesizing new crystals for nonlinear optics continues for a wide range of materials.^1–3 Interestingly, many second- and third-order effects have been observed in both single crystals and fine-grained ceramics¹ and metamaterials⁴ in the infrared spectral range.³ These materials can be used for the nonlinear mixing of waves and excitation by external laser light.² The physical properties of composite materials can be controlled and optimized over a wide range of values by changing the composition, structure, and size of their constituent elements. Of particular interest is the preparation of composite materials from dielectric materials, such as alumina, silica, and polymers that contain a high density of homogeneous pores with diameters ranging from several nanometers to several micrometers and lengths of up to hundreds of micrometers, known as nanoporous matrices or membranes. Using liquid-phase processes, these pores can easily be filled with metals, semiconductors, dielectrics, or liquid crystals.^5–7 Thus, the created composite can combine the special properties of both the nanoporous matrix and the embedded nanosized material and could adjust them.

It should be noted that in addition to highly efficient single-crystal nonlinear optical materials,^8,9 a new type can be created simply by filling the pores in a porous matrix with nonlinear optical materials. In this case, the composite can be designed to provide phase matching, whereas the pore-filling material contributes to the efficiency of second- and third-harmonic generation.¹⁰ Therefore, the efficiency of the nanocomposite can be increased by filling it with nonlinear optical nanocrystals in a certain direction of crystal orientation, causing anisotropy in the entire crystalline nanocomposite. The creation of crystalline nanocomposites with engineered anisotropy, which enables the directed crystallization of nanocrystals in matrix pores, was studied in Refs. 11–13. Studies^14,15 have shown that KH₂PO₄ (KDP) crystals with embedded aluminum oxyhydroxide nanoparticles exhibit high optical quality and uniformity. An increase in the nonlinear refractive index and inversion of its sign was observed, as well as an increase in the efficiency of second-harmonic generation compared to that of nominally pure KDP crystals upon excitation by pico- or nano-second laser pulses. Here, we report the incorporation of a well-known and widely used nonlinear optical material, ADP (NH₄H₂PO₄), into aluminum oxide nanopores.¹³ When substances with active physical properties are introduced into a porous matrix, the functionality of the introduced components is significantly expanded, and the practical significance of such structures increases significantly.¹⁶ The wetting properties of the saturated solutions of the selected salts are particularly important when filling nanoporous plates with crystalline materials. Al₂O₃ nanoporous plates exhibit good wettability in saturated aqueous solutions, such as KDP, ADP, and KB₅ (KB₅O₈).¹⁷ 

One of the most important piezoelectric materials is ammonium dihydrogen phosphate–NH₄H₂PO₄ (ADP), which is of great interest for practical applications in laser physics, quantum radiophysics, nonlinearity, and acousto-optics.^18,19 It crystallizes in a tetragonal geometry and belongs to space group I-42d. ADP is commonly employed as a second, third, and fourth harmonic generator in Nd:YAG and Nd:YLF lasers.

The process of preparing saturated aqueous solutions for the growth of ADP nanocrystals in the nanopores of Al₂O₃ matrices consists of recrystallization of the purchased raw material NH₄H₂PO₄ and the subsequent preparation of a saturated aqueous solution of these salts at the growth temperature. ADP crystals obtained by recrystallization (purification of raw materials) were poured into a glass container at a certain ratio with deionized water until a saturated solution was obtained at the appropriate temperature (depending on the growth temperature). Subsequently, the container with the solution was placed on a magnetic stirrer with controlled heating at T = 50–60 °C until the crystals were completely dissolved.^20,21 To avoid contamination by the nanoporous Al₂O₃ matrices, the plates were washed with isopropyl alcohol in an ultrasonic bath and dried in a drying cabinet at 50 °C, followed by annealing at 180–200 °C for 3 h. Porous Al₂O₃ matrices were immersed in the obtained saturated aqueous NH₄H₂PO₄ solution in separate containers and preheated to a temperature of the solution, −56 °C. ADP nanocrystals were grown in Al₂O₃ matrices in a saturated aqueous solution of NH₄H₂PO₄ using the temperature reduction method from 56 to 50 °C for 1.5 h. The samples were then removed, and the solution residues were removed from the surface, kept at room temperature for 30 min, and dried at 55 °C, where excess water from the pores of the matrix slowly evaporated. The microcrystals that appeared on the surface of the samples outside the pores were removed by perfectly polishing the surface. Subsequently, these samples were annealed for 3 h at temperatures of ∼80–100 °C to lower the content of structured water and reduce its influence on the response of the structures under study.

Second-harmonic generation (SHG) and third-harmonic generation (THG) of nanocomposite samples based on nanoporous silica (SiO₂) or alumina (Al₂O₃) host matrices and nanosized ADP or LiNbO₃ crystals were measured using the SHG/THG setup shown in Fig. 1.

This technique uses a mode-locked Nd:YAG laser to excite a sample. The laser generated picosecond pulses at a wavelength of 1064 nm with a duration of 30 ps and operated at a repetition rate of 10 Hz. To synchronize the acquisition, two beam splitters (BSs) extract part of the incident beam to reach the first photodiode (Ph1). Precise adjustments to the incident polarization on the sample are necessary because the polarization direction significantly influences the nonlinear properties. To achieve accurate control of the intensity and polarization, Glan–Taylor polarizers (P) and half-wave plates (λ/2) were incorporated.

A converging lens (L) with a focal length of 250 mm was used to focus the beam onto the sample, and its rotation axis was positioned near the focal point of the lens. A motorized rotation stage (RS) was coupled with a manual translation stage to optimize the position of the rotation axis of the sample with respect to the incident laser beam. A second polarizer positioned after the sample enabled a change in the detection polarization direction between S and P, thereby facilitating the study of various polarization configurations.

The wave was then passed through a filter assembly that included a KG3 filter to remove the fundamental and selective interference and another filter at 532 nm (FL532) to isolate only the SHG signal. The doubled signal was detected by a photomultiplier tube (PMT) connected to a digital oscilloscope that was synchronized with the laser, and a computer was used for signal recording. LabVIEW software was used to program the control and acquisition procedures, allowing for plotting of the second harmonic signal against the incident angle. To prevent saturation, Neutral Density (ND) filters were placed consistently before the PMT. A 0.5 mm thick quartz crystal slide was used as the SHG reference material.

For third harmonic generation (THG), the experimental setup mirrors that of second-harmonic generation. The excitation source was a Nd:YAG laser with a wavelength of 1064 nm, duration of 30 ps, and repetition rate of 10 Hz. The distinction lies in the selective interference filter positioned after the second polarizer and KG3 filter, which removes the fundamental beam at 1064 nm. A filter (FL355) was used to retain the third harmonic signal at 355 nm. The reference material used to calibrate the experimental setup for this technique (THG) was fused silica with the chemical formula SiO₂. It is widely used in optical applications, and, in most cases, it is the primary material used in the manufacturing process.

Several theoretical models employing various approximations have been described to ascertain the value of $χelec(3)$ from the shape of experimental curves obtained using the THG technique.^25–27 Two selected models were employed to explain the experimental results: the comparative model and the Reintjes model. The theoretical formulas for these models are given below.

The x-ray structural analysis of the prepared Al₂O₃:ADP samples confirmed the presence of ADP crystallites in the Al₂O₃ matrix. Studies conducted using an FEI electron microscope (resolution of 0.8 nm) visually confirmed the filling of the nanopores of the Al₂O₃ matrix with ADP nanocrystals, as shown in Fig. 2. The characteristics of nanocrystals grown in the pores are listed in Table I.

The measurements of SHG and THG responses in Al₂O₃:ADP samples were performed by the rotational maker fringe technique for s- and p-polarized fundamental laser beams, with a laser energy of 100 μJ, which, in our experimental configuration, corresponds to a light intensity at a focal point of 70 GW/cm². This high intensity is essential for inducing the nonlinear optical processes that are necessary for third-harmonic generation. No SHG was observed when the SHG/THG method was used. THG was successfully observed in three samples including ADP. The angular dependence of the THG intensity of the Al₂O₃:ADP sample is shown in Fig. 3. No significant differences were observed between the THG measurements conducted under the s-s and p-p polarization configurations. This suggests that the polarization state of the incident laser beam did not significantly affect THG in the Al₂O₃:ADP samples. The absence of a second-harmonic generation (SHG) signal can be attributed to several factors. One significant reason for this is the phase mismatch. In SHG, the generation of the second-harmonic signal relies on phase-matching conditions between the fundamental and second-harmonic waves. If these conditions are not met, for example, owing to material properties or if the thickness of the Al₂O₃:ADP samples exceeds the coherent length, the SHG signal may not be observed. The presence of nanopores and their sizes can affect the refractive index distribution within the Al₂O₃ matrix, potentially altering the phase-matching conditions. The optimal pore sizes must be determined to balance confinement, scattering, and phase matching to maximize the SHG signal.

Another crucial factor is the material properties. Certain materials may exhibit low SHG efficiencies owing to their crystal structures and electronic properties. For instance, materials with centrosymmetric structures often exhibit negligible SHG responses because their symmetry cancels out the nonlinear polarization required for SHG. In addition, surface effects play a crucial role. Surface roughness or defects in the sample can significantly affect the efficiency of SHG generation. Such imperfections can introduce scattering or absorption mechanisms that reduce the SHG signal, thereby making it difficult to detect. These factors collectively influence the presence and strength of the SHG signal; therefore, understanding these factors is crucial for optimizing the SHG experiments and material selection.

As presented in the double-logarithmic plot of the pump power vs the blue light output power in Fig. 4, the output power clearly scales nonlinearly with the incident infrared (IR) pump power and shows a slope close to three, which proves the frequency-tripling nature of the obtained blue light signals according to the third-order nonlinear optical (NLO) process. This confirms the pure THG process and the absence of damage in the studied guest-host films at the investigated laser intensities.

The third-order NLO susceptibilities, which characterize the efficiency of the THG process, were calculated based on experimental data and are presented in Table II. These values were determined using two theoretical models: the comparative and Reintjes models.

The Reintjes theoretical model appears to be the most suitable one for explaining the THG experimental values of the Al₂O₃:ADP samples. This model considers the majority of the parameters that affect the value of $χelec(3)$⁠, which demonstrates a comparable trend, with sample 84 exhibiting the highest $χelec(3)$ value compared to samples 130 and 131, which exhibit similar $χelec(3)$ values. The observed differences can be attributed to the varying nanopore diameters of Al₂O₃. Al₂O₃ matrices with nanopores exhibit significant light scattering, and this tendency increases when the nanopore diameter decreases, particularly when the nanopore size approaches or becomes smaller than the wavelength of incident light. This is because smaller pores create more interfaces and discontinuities within the material, which can enhance scattering owing to the interaction of light with the numerous boundaries. Increased scattering can lead to a reduction in the effective light intensity reaching ADP crystals, which can diminish the THG signal. Furthermore, scattering can disrupt the phase-matching conditions necessary for efficient THG, thereby further reducing the signal. The nanopore size can influence the local electric field inside pores. Smaller pores can more tightly confine light, potentially enhancing the local field and increasing the nonlinear optical response. This enhancement could lead to stronger THG signals. However, if the pores are too small, they might restrict the growth or proper alignment of ADP crystals, which could negatively affect the THG efficiency. The number of ADP crystals that could fit into the pores depended on the pore size. Larger pores can accommodate more ADP material, which might lead to a stronger THG signal owing to the larger interaction volume of the nonlinear process. Conversely, smaller pores might limit the amount of ADP, reducing THG efficiency.

A nanocomposite nonlinear optical material, NH₄H₂PO₄, incorporated into aluminum oxide nanopores was studied. X-ray and electron microscopy studies confirmed the preparation of ADP nanocrystals in Al₂O₃ matrices from a saturated aqueous solution at low temperatures. This study demonstrates the significant potential of Al₂O₃ matrices with embedded ADP nanocrystals for nonlinear optical applications, particularly third-harmonic generation (THG). Structural analysis confirmed the successful integration of ADP crystals into the nanopores of the Al₂O₃ matrix, and optical absorption measurements highlighted the influence of pore size on the light scattering and transmission properties. The experimental results revealed a strong third-order nonlinear response, while second harmonic generation (SHG) was notably absent, likely owing to phase mismatch and material properties such as centrosymmetry and surface roughness. Theoretical modeling using the comparative model and the Reintjes model provided insights into the third-order nonlinear optical susceptibilities (⁠$χelec(3)$⁠) of the samples. It is important to note that this study identified the pivotal role of nanopore size in influencing the efficiency of third-order harmonic generation (THG). The presence of smaller nanopores has been observed to enhance light scattering, which can potentially disrupt phase matching, thereby reducing THG signal strength. Conversely, the use of appropriate pore sizes has been demonstrated to enhance local electric fields, which can lead to enhanced nonlinear optical responses. These findings emphasize the importance of optimizing the nanopore dimensions and material properties to maximize the efficiency of nonlinear optical processes in nanocomposites. The findings of this study can inform the design and development of advanced photonic devices that can leverage the unique properties of nanoporous matrices and embedded nanocrystals to enhance optical performance.

The results of this investigation are part of a project that has received funding from the European Union’s Horizon 2020 Research and Innovation Program under Marie Sklodowska–Curie grant agreement No. 778156 and from the Ministry of Education and Science of Ukraine in the frames of projects “Nanoarchitektonika” (Grant No. 0124U000826) and “Nanoelectronics” (Grant No. 0123U101695).

The authors have no conflicts to disclose.

All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

D.G. and I.G. contributed to the investigation of the research, analysis of the results, visualization of data, and writing of the manuscript. W.A., V.A., N.A., I.T., and D.S. analyzed the results and reviewed and edited the manuscript. B.S. and A.A. supervised the project and reviewed and edited the manuscript.

D. Guichaoua: Data curation (equal); Formal analysis (equal); Investigation (equal); Software (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). W. Alnusirat: Formal analysis (equal); Investigation (equal); Writing – review & editing (equal). V. Adamiv: Formal analysis (equal); Methodology (equal); Validation (equal); Writing – review & editing (equal). N. Andrushchak: Conceptualization (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). I. Teslyuk: Investigation (equal); Methodology (equal); Writing – review & editing (equal). D. Shulha: Formal analysis (equal); Investigation (equal); Writing – review & editing (equal). A. Andrushchak: Project administration (equal); Resources (equal); Supervision (equal). I. Gnilitskyi: Methodology (equal); Supervision (equal); Writing – review & editing (equal). B. Sahraoui: Project administration (equal); Resources (equal); Supervision (equal); Validation (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.