Z-scan Research Articles

Exploring nonlinear optical absorption in cobalt-doped Nickel–Zinc nanomaterials

This study emphasises on synthesis of ferrite nano powders via hydrothermal route within the compositional range of Ni0 65Zn0.35CoxFe(2-2x/3) O4 (x = 0.00 to 0.05 in steps of 0.01 wt%). Structural analysis via XRD and FTIR unveiled the cubic spinal structure, confirmed by signature absorption bands existence in the infrared spectra. The FESEM analysis revealed the morphological characteristics of ferrite crystals, while characterizations indicated a diminution in crystallite sizes from 53 to 37 nm. The magnetic behaviour displayed the existed correlation between Ms and Y–K angles along with finite size effects of the samples. Notably, low coercively values (0.13 Oe to 1.09 Oe) and magneto crystalline anisotropy pointed towards the magnetic quality of the ferrites with soft nature. Exhibited dielectric loss factor near 10−1, and ac-conductivity (σac) within the range of 10−5S/m to 10−4S/m of the samples indicates their improved dielectric properties. Further analysis from UV–vis. Diffuse reflectance (UV-DRS), PL spectroscopy, and Raman studies exhibits slight variations in optical band gaps, and vibrational modes due to varying cobalt doping concentrations. The evaluation of optical limiting characteristics using the Z-scan technique revealed absorption coefficients ranging from 1.40 to 4.80 × (10−10 m/W) and onset limiting values between 1.93 W/m2 and 1.11 W/m2. These findings position the Ni–Zn–Co ferrites as promising contenders for optical limiting applications owing to their optical nonlinearity and absorption characteristics.

Effect of WO3 in the third-order optical nonlinearities of tungsten lead pyrophosphate glasses

The binary glassy system Pb2P2O7-WO3 demonstrates promising photonics applications owing to its high polarizability and chemical stability. The domain of glass's chemical state and linear and nonlinear optical properties have been investigated using X-ray photoelectron spectroscopy (XPS), UV–visible spectroscopy, and the Z-scan technique, respectively. The optical absorption spectra revealed that the absorption edge was redshifted with increased WO3 content, suggesting a decrease in bandgap energy. Nonlinear refractive index increased with the WO3 content from an average value of 0.70 x 10−19 m2 W−1 to 0.98 x 10−19 m2 W−1. Such an increase is due to the rise of non-bridging oxygens and the polarizability of W6+ and W5+ species. XPS studies found that a high concentration of WO3 leads to the transformation of the oxidation state of W6+ and W5+ related to the presence of oxygen vacancies. Therefore, our high nonlinear refractive index and polarizability results indicate that the glass system studied here is a potential candidate for photonic devices.

Synthesis, structural elucidation and characterization of a new organic single crystal Guanidinium adipate monohydrate (GuAM) for nonlinear optical applications

A slow evaporation solution growth process was employed to grow a new organic single crystal Guanidinium adipate monohydrate (GuAM) at room temperature. The structure refinement and cell specifications were elucidated using the X-ray diffraction method. Single crystal XRD study revealed that the grown crystal crystallized in the monoclinic system with unit cell dimensions a = 4.948 (9) Å; b = 16.770 (2) Å; c = 13.554 (2) Å; α and γ = 90.00⁰; β = 95.302⁰ (3), V = 1120.1(3) Å3 and Z = 4. The structure of the crystal was solved by direct method and refined by full matrix least-square method using SHELXL-2018/3 program. The crystalline nature of the sample was evaluated by powder X-ray diffraction (PXRD) study. Fourier transform infrared (FTIR) spectral study was employed to recognize the diverse functional groups present in the synthesized crystal. Optical attributes such as absorbance and emission nature of the GuAM were explored using UV–visible-NIR absorbance study and photoluminescence study respectively. The laser damage threshold value of the formed crystal has been determined by employing an Nd-YAG laser with a wavelength of 1064 nm. The mechanical nature of the GuAM was examined utilizing Vicker's microhardness test and other related parameters were also calculated. TGA-DTA analysis was utilized to estimate the title crystal's thermal stability. The Z-scan technique has been employed to gain insight into the third order nonlinear (NLO) parameters of the GuAM. These findings imply that guanidinium adipate monohydrate (GuAM), a grown single crystal, may be a viable choice for NLO applications.

Ultrafast Dynamics, Optical Nonlinearities, and Chemical Sensing Application of Free-Standing Porous Silicon-Based Optical Microcavities.

The present work demonstrates the ultrafast carrier dynamics and third-order nonlinear optical properties of electrochemically fabricated free-standing porous silicon (FS-PSi)-based optical microcavities via femtosecond transient absorption spectroscopy (TAS) and single-beam Z-scan techniques, respectively. The TAS (pump: 400 nm, probe: 430-780 nm, ∼70 fs, 1 kHz) decay dynamics are dominated by the photoinduced absorption (PIA, lifetime range: 4.7-156 ps) as well as photoinduced bleaching (PIB, 4.3-324 ps) for the cavity mode (λc) and the band edges. A fascinating switching behavior from the PIB (-ve) to the PIA (+ve) has been observed in the cavity mode, which shows the potential in ultrafast switching applications. The third-order optical nonlinearities revealed an enhanced two-photon absorption coefficient (β) in the order of 10-10 mW-1 along with the nonlinear refractive index (n2) in the range of 10-17 m2 W-1. Furthermore, a real-time sensing application of such FS-PSi microcavities has been demonstrated for detecting organic solvents by simultaneously monitoring the kinetics in reflection and transmission mode.

Charge transfer induced linear and nonlinear optical properties in polypyrrole/ZnO@MoS2 ternary composites

Polymer-based ternary nanocomposites with tunable optical properties are the key components for optoelectronics applications. Here, we demonstrate the linear and nonlinear optical properties of polypyrrole (PPy) and its ternary composites with different concentrations of the MoS2–ZnO core–shell (ZnO@MoS2) heterostructure. The interesting observation of multiple excitons and their transportation through the interface of PPy and ZnO@MoS2 by using photoluminescence and fluorescence lifetime spectroscopy was studied. Furthermore, the third-order nonlinear optical properties of all samples were also measured by employing the z-scan technique at an excitation wavelength of 532 nm. The reverse saturable absorption of pure PPy was switched to saturable absorption after the addition of ZnO@MoS2 at the uniform linear transmittance. These ternary composites with good nonlinear responses provide an option for the development of high-performance nonlinear optical devices and open a new path for the future development of ternary systems in the optical fields.

Optical Limiting Response of Porous Carbon Dispersions.

With the wide application of intense lasers, the protection of human eyes and detectors from laser damage is becoming more and more strict. In this paper, we study the nonlinear optical limiting (OL) properties of porous carbon with a super large specific surface area (2.9 × 103 m2/g) using the nanosecond Z-scan technique. Compared to the traditional OL material C60, the porous carbon material shows an excellent broadband limiting effect, and the limiting thresholds correspond to 0.11 J/cm2 for 532 nm and 0.25 J/cm2 for 1064 nm pulses, respectively. The nonlinear scattering experiments showed that the OL behavior was mainly attributed to the nonlinear scattering effect, which is caused by the rapid growth and expansion of bubbles in the dispersion induced by laser irradiation, and the scattered light distribution is consistent with the results of Mie's scattering. These results suggest that porous carbon materials are expected to be applied to the field of laser protection in the future to further protect the human eye and precision optical instruments.

Effect of water salinity on the focal tuning of an injected liquid lens used to modify the z-scan method

A water-injected liquid lens is fabricated to tune its focal length using the change in water salinity. It is found that when the salinity of water is changed from zero to 34.25%, the focal length can be changed by about 12.6 mm from 73.7 mm to 86.3 mm. A focal length resolution of approximately 0.75 × 10−2 mm and high temporal stability over a long period have been achieved for the lens foci. This lens is then used to modify the z-scan technique where the lens and the sample both remain fixed without displacement. The performance of the fabricated lens is evaluated by nonlinear refractive index measurement of a sample containing 10.82-pH-synthesized Silver nanoparticles suspended in water with 15 mM of concentration. For verification of the results, a nonlinear refractive index of (−10.6 ± 1.0) × 10−7 cm2 W−1 is firstly measured for the sample using a classical z-scan benefiting from a conventional focal-fixed lens. Interestingly, we found out that when the fabricated lens is replaced in the modified z-scan, the nonlinear refractive index of about (−8.1 ± 0.2) × 10−7 cm2 W−1 can be measured, indicating a similarity in the order and small difference in the coefficient compared to the classical z-scan. This outcome highlights the potential capability and simplicity of the fabricated lens in the modification of the classical z-scan technique.

Nano-structuration influence on the polyimide-based spatial light modulator’s parameters

The classical and the nano-structured spatial light modulators (SLMs) especially based on the polyimide photosensitive layers, as the key element of the optoelectronic devices, display, and telecommunications schemes are considered. The main emphasis is placed on the device with a polyimide photo-layer due to its high sensitivity and acceptable performance. A modulator’s basic characteristics are studied taken into account the comparison with the different types of the photo-layers, such as: ZnSe, ZnS, a-Si:H. Liquid crystal (LC) media is considered as the modulation system. It is indicated that the different methods and approaches are applied for investigation of the basic SLM parameters, such as: Z-scanning technique, third harmonic generation, four-wave mixing set-up, etc. In the current paper the laser holographic technique is used to investigate the resolution, sensitivity, and speed of the LC-SLM devices. The influence of the fullerene doping on the organic photo-layers based on the polyimide materials is presented. This influence of this nano-structuration process on the modulator’s basic parameters is discussed.

Nonlinear optical response of strain-mediated gallium arsenide microwire in the near-infrared region

Abstract Gallium arsenide (GaAs) semiconductor wires have emerged as potent candidates for nonlinear optical devices, necessitating bandgap engineering for an expanded operational wavelength range. We report the successful growth of strain-mediated GaAs microwires (MWs) with an average diameter of 1.1 μm. The axial tensile strain in these wires, as measured by X-ray diffraction and Raman scattering, ranges from 1.61 % to 1.95 % and from 1.44 % to 2.03 %, respectively. This strain condition significantly reduces the bandgap of GaAs MWs compared to bulk GaAs, enabling a response wavelength extension up to 1.1 μm. Open aperture Z-scan measurements reveal a nonlinear absorption coefficient of −15.9 cm/MW and a third-order magnetic susceptibility of −2.8 × 10−8 esu at 800 nm for these MWs. I-scan measurements further show that the GaAs saturable absorber has a modulation depth of 7.9 % and a nonsaturation loss of 3.3 % at 1050 nm. In laser applications, GaAs MWs have been effectively used as saturable absorbers for achieving Q-switched and dual-wavelength synchronous mode-locking operations in Yb-bulk lasers. These results not only offer new insights into the use of large diameter semiconductor wires but also expand the potential for applications requiring bandgap tuning.

Novel structure and excellent nonlinear optical performance nanocomposite derived from flower-like MOF grown on MoS2

Molybdenum disulfide (MoS2) is a nonlinear optical material with graphene-like structure. In order to further improve the nonlinear optical performance of MoS2 and expand the application fields of MoS2, we report a simple in-situ self-growth method to functionalize MoS2 nanosheets with metal-organic framework (MOF) for the first time to pursuit excellent reverse saturable absorption performance and optical limiting performance. Firstly, defective MoS2 nanosheets was obtained by chemical etching with H2O2 and cysteamine was inserted into the sulfur vacancies under ultrasound to obtain MoS2–NH2. Then 5,10,15,20-Tetra (4-carbomethoxyphenyl) porphyrin (TCPP) as MOF active growth sites were grafted on MoS2–NH2 by amido linkage to obtain MoS2-TCPP in mild conditions. Finally, flower-like porphyrin-based CuMOF was grown on MoS2-TCPP by solvothermal method to obtain MoS2-CuMOF with stable structure. The third order nonlinear optical performance of MoS2-CuMOF were studied by Z-scan technique. It was found that the third order nonlinear absorption coefficient of MoS2-CuMOF was significantly improved compared with MoS2. Under the same energy of laser irradiation, the third order nonlinear absorption coefficient of MoS2-CuMOF nanocomposite in solution was 6 × 10−10 m/W about 2.3 times of the pristine MoS2 (2.6 × 10−10 m/W). After blending the above materials with polymethyl methacrylate (PMMA) to prepare a solid device, the third order nonlinear absorption coefficient of MoS2-CuMOF/PMMA was 18 × 10−10 m/W about 3.6 times of the MoS2/PMMA (5 × 10−10 m/W). And MoS2-CuMOF/PMMA had better optical limiting ability than MoS2/PMMA. The significant improvement in the performance of the composites can be attributed to the energy transfer between MoS2 and CuMOF, and porphyrin aggregation was inhibited by PMMA after the preparation of devices. Energy transfer can be proved by fluorescence spectroscopy. This work not only provided an example for the covalent functionalization of MoS2, but also offered a new method for designing materials with excellent nonlinear optical performance.

Single crystal of barium bis para-nitrophenolate para-nitrophenol tetrahydrate for NLO applications: crystal growth and DFT analysis

Abstract Single crystals of barium bis para-Nitrophenolate para-Nitrophenol tetrahydrate (BBPNT) were grown via the slow evaporation technique. The crystal dimensions were measured to be 15 mm × 10 mm × 6 mm. The BBPNT crystal was analyzed using X-ray diffraction, FTIR spectrum, UV–vis spectrum, dielectric studies, thermo-gravimetric (TG) differential thermal analyses (DTA), Vickers microhardness test, second-harmonic generation efficiency, Z-scan technique, PL spectra, and laser damage threshold studies. These findings were previously published in our paper. In the current situation, the theoretical methods include analyzing several aspects, such as the molecular structure determined using X-ray experimentation at its lowest energy state. This analysis is then contrasted by means of density functional theory (DFT) with a suitable set of basic functions. The first-order hyperpolarizability may also be determined via the use of Density Functional Theory (DFT) methods. The molecule’s stability, resulting from hyper conjugative interactions, was examined using the natural bond orbital approach to analyses its nonlinear optical activity and charge delocalization. The magnitude of the HOMO–LUMO energy gap indicates the potential for charge transfer inside the molecule. The investigation of donor-acceptor interactions was conducted using NBO analysis based on optimized ground state geometries.

Nonlinear Optical Refraction and Absorption Features of Methyl Orange Dye in Polar Solvents.

Herein, we report the nonlinear optical (NLO) refraction and absorption features of azo dye namely, methyl orange (MO) dissolved in ethanol, methanol, acetone, 1-propanol, DMF and DMSO. The UV-Visible absorption study reveals that the maximum absorption spectrum of MO dye appeared towards longer wavelength by increasing the solvent polarizability is the result of red shift or bathochromic shift. The Z-scan method is utilized to measure the third-order NLO features of MO dye in different polar solvents. A continuous wave laser with 5-mW power and an excitation wavelength of 405nm is employed in the Z-scan technique. The NLO features including nonlinear index of refraction (n2), nonlinear coefficient of absorption (β) and third-order NLO susceptibility (χ3) are calculated to be the order of 10-7 cm2/W, 10-2cm/W and 10-7 esu, respectively. The NLO index of refraction shows peak-valley transmittance is the result of self-defocusing and NLO absorption coefficient exhibits both positive and negative nonlinearity owing to saturable absorption (SA) and reverse saturable absorption (RSA). The effect of solvent polarizability and dipole moment on third-order NLO susceptibility of MO dye is discussed. Based on the experimental results, an azo dye MO appears to be a promising option for NLO applications in the future.

Influence of Polar Solvents on Third-Order Nonlinear Optical Features of Methyl Red Dye.

This study emphasis the solvent effect on third-order nonlinear optical (NLO) features of methyl red (MR) dye dissolved in polar solvents including ethanol, methanol, acetone, 1-propanol, DMF and DMSO using low power diode laser. Z-scan technique operating at 405nm wavelength, is used to estimate the third-order NLO features of MR dye in various solvents. The dye discloses self-defocusing nonlinear index of refraction (n2), which is determined to be the order of 10-7 cm2/W. The nonlinear coefficient of absorption (β) of MR dye displays both negative and positive value owing to saturable absorption (SA) and reverse saturable absorption (RSA), respectively. The real and imaginary components of the third-order NLO susceptibility of MR dye in polar solvents are measured to be the order of 10-6 esu and 10-7 esu, respectively. The dye exhibits a large NLO susceptibility in DMSO, which is estimated to be 1.21 × 10-6 esu. The effect of solvent spectral features on MR dye is determined by applying a multi-parameter scale called Kamlet-Abboud-Taft. The experiment results indicate that MR dye is a promising NLO material that may find applications in photonics and optoelectronics.

Exploring nonlinear and linear optical properties of N, N’-bis(salicylidene)-o-phenylenediamine (salophen) through Z-scan technique and TD-DFT computational analysis

This investigation delves into the comprehensive analysis of the linear and nonlinear optical characteristics exhibited by N, N’-bis(salicylidene)-o-phenylenediamine (salophen) using a combination of Z-scan methodology and quantum chemistry calculations. The Z-scan technique facilitated meticulous computations of the third-order susceptibility, nonlinear absorption coefficient, and nonlinear refractive index of the specimen. Specifically, in the solvent DMSO, the assessed values for the nonlinear refractive index, nonlinear absorption coefficient, and third-order susceptibility were determined to be 0.035 × 10–10 cm2 W −1 , −0.024 × 10–5 cm W −1 , and 0.596 × 10–5 esu, respectively. Furthermore, quantum mechanical analyses were employed to meticulously calculate the molecular hyperpolarizabilities (β and γ), dipole moment (μ), and dipole polarizability (α) of salophen. This thorough exploration highlighted a notable congruence between the outcomes derived from experimental observations and those obtained through quantum mechanical simulations. The collective findings from both theoretical computations and experimental assessments distinctly showcase the robust nonlinear potential inherent in salophen. These insights suggest its promising suitability and potential as a viable candidate for applications in optical devices. The alignment between theoretical predictions and experimental results underlines the reliability and potential practicality of salophen in the realm of optical technology, emphasizing its significance as a potential material for advancing optical device functionalities.

Visible-light optical limiting of vanadia–polyvinylpyrrolidone nanofibers

In this work, vanadium pentoxide (V2O5) nanoparticles-filled electrospun polyvinylpyrrolidone (PVP) nanofibers were investigated systematically at various nanofiller weight percentages (8 and 10 wt%) and input intensities to reveal the effective optical limiting feature in the visible spectrum. XRD analysis demonstrated the purity of the produced V2O5 nanoparticles. According to SEM findings, V2O5 nanoparticles were effectively integrated into the PVP nanofibers. Two distinct absorption bands were observed at around 400 and 217 nm. These bands were related to PVP and V2O5 nanoparticles in linear absorption measurements, respectively. Moreover, an increased Urbach energy value was obtained with an increase in V2O5 nanofiller content within PVP. Open-aperture Z-scan measurements were taken at 532 nm considering the band gap energy of the V2O5 nanofillers in PVP composite nanofibers. In 8 wt% V2O5 nanofilled PVP nanofibers, one-photon absorption (OPA) was the main nonlinear absorption (NA) mechanism, and the defect states of the V2O5 nanoparticles had no contribution to NA. On the other hand, sequential two-photon absorption was the main NA mechanism, and the defect states of the nanoparticles caused more efficient NA behavior in 10 wt% V2O5 nanofilled PVP nanofibers. The effective optical limiting behavior was obtained for this composite nanofiber with lower limiting threshold as 1.49 × 10–5 J/cm2. The V2O5 nanofilled PVP nanofibers presented strong potential optical limiters in the visible wavelength region. This was attributed to their high linear transmittance at low input intensities and their robust NA behavior at higher input intensities.

Synthesis, physicochemical and quantum chemical studies of 7-azaindolinium-3,5-dinitrobenzoate single crystal for nonlinear optical applications

Abstract A novel hydrogen bonded single crystal of 7-azaindolinium-3,5-dinitrobenzoate (7AIDNB) has been grown, and the structure of the crystal was determined by single-crystal XRD analysis. The results show that the grown crystal belongs to a triclinic crystal system with centrosymmetric space group P-1 and the unit cell parameters: a = 6.998(9) Å, b = 8.498(12) Å, c = 13.103(17) Å, V = 710.78(17) Å3, Z = 2. The presence of expected discrete functional groups in 7AIDNB has been identified by an FT-IR spectroscopic study. The thermal stability and decomposition of the title crystal have been studied by TG-DTA analysis. The optical property of the grown crystal has been analyzed by UV-Vis-NIR spectroscopy analysis. The DFT with 6-311++G(d, p) basis set has been used to examine optimization of molecular geometry, Mulliken charge, and HOMO-LUMO. As the frequency of the grown crystal increases, the dielectric loss and dielectric constant decrease. The third-order NLO parameters of 7AIDNB have been examined by a Z-scan technique.

Advanced spectroscopic investigation of colour centres in LiF crystals irradiated with monochromatic hard x-rays

Nominally-pure lithium fluoride (LiF) crystals were irradiated with monochromatic hard x-rays of energy 5, 7, 9 and 12 keV at the METROLOGIE beamline of the SOLEIL synchrotron facility, in order to understand the role of the selected x-ray energy on their visible photoluminescence (PL) response, which is used for high spatial resolution 2D x-ray imaging detectors characterized by a wide dynamic range. At the energies of 7 and 12 keV the irradiations were performed at five different doses corresponding to five uniformly irradiated areas, while at 5 and 9 keV only two irradiations at two different doses were carried out. The doses were planned in a range between 4 and 1.4 × 103 Gy (10.5 mJ cm−3 to 3.7 J cm−3), depending on the x-ray energy. After irradiation at the energies of 7 and 12 keV, the spectrally-integrated visible PL intensity of the F2 and F3 + colour centres (CCs) generated in the LiF crystals, carefully measured by fluorescence microscopy under blue excitation, exhibits a linear dependence on the irradiation dose in the investigated dose range. This linear behaviour was confirmed by the optical absorption spectra of the irradiated spots, which shows a similar linear behaviour for both the F2 and F3 + CCs, as derived from their overlapping absorption band at around 450 nm. At the highest x-ray energy, the average concentrations of the radiation-induced F, F2 and F3 + CCs were also estimated. The volume distributions of F2 defects in the crystals irradiated with 5 and 9 keV x-rays were reconstructed in 3D by measuring their PL signal using a confocal laser scanning microscope operating in fluorescence mode. On-going investigations are focusing on the results obtained through this z-scanning technique to explore the potential impact of absorption effects at the excitation laser wavelength.

Nonlinear study of indolamines: A hidden property that might have possible implications in neurodegeneration

Indolamines (e.g., serotonin and melatonin) are tryptophan-derived class of neurotransmitters and neuromodulators that play crucial roles in mood regulation, sleep-wake cycles, and gastrointestinal functions. These biogenic amines exert their effects by binding to specific receptors in the central nervous system, influencing neuronal activity and signalling cascades. Indolamines are vital in maintaining homeostasis, and imbalances in their levels have been implicated in various neurological and psychiatric disorders. Hence, in the present study, we have investigated the nonlinear properties of indolamines under a continuous wave (CW) and pulsed laser excitation using the closed-aperture (CA) Z-scan technique. The CA Z-scan is a cost-effective and sensitive analytical tool for investigating nonlinear properties. It is observed that indolamines show negative refractive and positive absorptive nonlinearity under in vitro physiological conditions. The origin of nonlinearity is ascribed to the thermo-optical effect governed by the saturated atomic absorption and molecular orientation mechanisms under CW and pulsed laser excitation, respectively. The strength of nonlinearity is found to vary linearly with the concentration of indolamines. Overall, serotonin possesses stronger nonlinearity than melatonin. The maximum nonlinearity (refractive index (n2) & absorption coefficient (β)) for melatonin under CW and pulsed laser excitations are (-1.266 × 10−12 m2W−1 and -1.883 × 10−17 m2W−1) & (8.046 × 10−8 mW−1 and 1.516 × 10−13 mW−1), respectively. Meanwhile, the maximum n2 and β under pulsed laser excitation for serotonin are obtained as -3.195 × 10−17 m2W−1 and 6.149 × 10−12 mW−1, respectively. The outcome of the results may be utilized in understanding processes mediated by indolamines and designing therapeutic interventions.

Introducing Optical Nonlinearity in PDMS Using Organic Solvent Swelling

The feasibility of introducing optical nonlinearity in poly-dimethyl siloxane (PDMS) using organic solvent swelling was investigated. The third-order nonlinear refraction and absorption properties of the individual materials, as well as the PDMS/solvent compounds after swelling were characterized. The well-established Z-scan technique served as characterization method for the nonlinear properties under picosecond pulsed laser excitation at a 532 nm wavelength. These experiments included investigations on the organic solvents nitrobenzene, 2,6-lutidine, and toluene, which showed inherent optical nonlinearity. We showed that nitrobenzene, one of the most well-known nonlinear optical materials, has proven suboptimal in this context due to its limited swelling effect in PDMS and comparatively high (non)linear absorption, resulting in undesirable thermal effects and potential photo-induced damage in the composite material. Toluene and 2,6-lutidine not only exhibited lower absorption compared to nitrobenzene but also show a more pronounced swelling effect in PDMS. The incorporation of toluene caused a weight change of up to 116% of PDMS, resulting in substantial nonlinear optical effects, reflected in the nonlinear refractive index of the PDMS/toluene composite n2=3.1×10−15 cm2/W.

Three-photon and four-photon absorption in lithium niobate measured by the Z-scan technique

Open-aperture Z-scan measurements have been carried out to investigate the three-photon (3 PA) and four-photon absorption (4 PA) coefficients at 800 nm and 1030 nm wavelengths, respectively in congruent and stoichiometric lithium niobate (cLN, sLN) with different concentrations of Mg doping. The laser pulse duration at the two wavelengths were 40 and 190 fs. The peak intensity inside the crystals varied between approximately 110 and 550 GW/cm2. The 3 PA and 4 PA coefficients were evaluated using a theoretical model and the results suggest that their minima are at or around the Mg doping level corresponding to the threshold for suppressing photo-refraction for both cLN and sLN. This result can be attributed to the contribution of crystal defects to the 3 PA and 4 PA processes. Furthermore, the 4 PA at 1030 nm exhibited greater nonlinear absorption than the 3 PA at 800 nm under the same intensity level. Possible reasons for this unexpected behavior are discussed. Overall, comparing the 3 PA and 4 PA values of these crystals will enable for selection of the optimum composition of LN crystal for efficient THz generation and for other nonlinear optical processes requiring high pump intensities.