Z-scan Experiments Research Articles

Upconversion Enabled Optical Limiting Behaviour in Y2O3: Yb, Er Nanophosphors under 532 nm and 1064 nm Laser Excitation

Y2O3: Yb, Er nanophosphors were synthesized by sol-gel approach and preliminary characterization confirms the existence of lanthanide dopants and the host material in the appropriate ratio with nanosphere-like morphology. Linear absorption displays visible and NIR absorption regions due to the sub-bandgap states involved in f-f transitions of Er- Yb ions. PL study shows more intense red emissions than blue and green emissions due to the combination of energy transfer and cross-relaxation process in Er ions. Wavelength-dependent nonlinear optical response of Y2O3: Yb, Er was examined by adapting the intensity-dependent Z-scan technique (open aperture) using nano pulsed Nd: YAG laser. Remarkably Y2O3: Yb, Er nanophosphors show reverse saturable absorption ascribed due to the two-photon absorption and two/three-photon absorption at 532 nm and 1064 nm respectively. The nonlinear absorption coefficient reliant on the intensity of the laser unambiguously demonstrates the presence of a sequential multi-photon absorption process. The results from the Z-scan experiment demonstrate the influence of the sub-bandgap energy states of the Y2O3 matrix due to the Yb and Er dopants in the excited state absorption behaviour. Upconversion integrated optical limiting of Y2O3: Yb, Er nanophosphors provide a potential origin for designing high-performance broadband solid-state optical limiters for laser protection devices.

Tunable nonlinear optical properties in polyaniline-multiwalled carbon nanotube (PANI-MWCNT) system probed under pulsed Nd:YAG laser

The investigation of the nonlinear optical (NLO) properties of polyaniline (PANI) and polyaniline doped with multi-walled carbon nanotube (PANI-MWCNT) in both solution and film forms was conducted using the open-aperture z-scan approach. A Q-switched resonant Nd: YAG laser with a wavelength of 532nm and two different fluences was utilized in this study. Based on the z-scan experiments, it was observed that the NLO behaviour of PANI and PANI-MWCNT composites exhibited a transition from reverse saturable absorption (RSA) to saturable absorption (SA) when the samples changed from a liquid to a film state. Two photon absorption (TPA) and thermally induced nonlinear scattering are the main mechanisms behind the RSA behaviour of the materials in solution form. The reason for SA behaviour of these materials in film form is attributed as the ground-state free electron bleaching in the conduction band due to the increased laser intensity. Due to increased structural disorder and the defect states or trap levels created by the dopants in the PANI system, the NLO properties of the PANI-MWCNT composite in both solution and film forms have significantly improved compared to those of pure PANI. Urbach tail analysis and carbon cluster determination revealed the presence of defect states in the composites system. The composite between PANI and MWCNT was verified using Fourier transform infrared (FTIR) spectroscopy. The functional elements present in the composites are verified by X-ray photoelectron spectroscopy. The NLO parameters of the samples, viz., the nonlinear absorption coefficient (β), the imaginary part of the nonlinear, and susceptibility χi(3) the figure of merit (FOM) of PANI-MWCNT, exhibited a notable enhancement in their values over PANI. Moreover, the PANI-MWCNT film demonstrated superior SA performance and the PANI-MWCNT solution demonstrated better OL performance compared to that of the PANI film and solution respectively. Also the effects of dopant induced structural modifications and lattice disorder on the NLO behaviour of these materials are correlated with the obtained NLO parameters. The tunable NLO properties accomplished by controlling the structural phase and dopant-induced defects enhance the possibility of these nanostructures for applications in optical limiting, optical switching, optical modulation, optical pulse compression and laser pulse narrowing.

A Porphyrin-Based MOF Thin Film with Oriented Nanosheet Arrays for Optimizing a Nonlinear Optical Response.

Developing two-dimensional (2D) hybrid nanosheet arrays integrating inorganic and organic components is highly significant for third-order nonlinear optical (NLO) applications. Herein, an oriented 2D porphyrin-based MOF (ZnTPyP(Co)) thin film composed of vertically stacked ultrathin nanosheets was fabricated via the liquid-phase epitaxial (LPE) layer-by-layer (LBL) method. The prepared ZnTPyP(Co) thin film exhibits an outstanding third-order NLO response with a high third-order nonlinear susceptibility of ∼2.63 × 10-7 esu, which is ascribed to the hybrid nanosheet array structure. Additionally, experimental Z-scan measurement and theoretical calculations also demonstrate that the substitution of Co metal ions in the porphyrinic core can increase the level of delocalization of the porphyrinic group and contribute to the material's enhanced NLO properties. These findings not only provide new film candidates for NLO application but also highlight the potential of 2D MOF nanosheets in advanced optical devices.

Effect of morphology on the optical limiting of BiVO4 films

Precise control over the morphology and crystal structure of semiconductor-based optical materials is essential for optimizing their nonlinear optical properties. Morphological factors significantly affect the material’s interaction with light, influencing nonlinear absorption, harmonic generation, and optical confinement. In this study, the nonlinear optical (NLO) properties and optical limiting (OL) performance of bismuth vanadate (BiVO4) films depending on precursor solution pH condition, powder morphology and weight percentage of the BiVO4 powders in PMMA (10–30 wt.%) were investigated. The BiVO4 powders were synthesized via the hydrothermal method and the effect of precursor solution pH (pH of 1, 7.5 and 12) was investigated on the size and morphology of the BiVO4 powders. BiVO4 films were prepared using the spin coating method utilizing the BiVO4 powders and PMMA. The bandgap values of films were found to change from 3.39 to 3.56 eV depending on the BiVO4 growth solution pH. The nonlinear absorption coefficients (βeff), saturation intensity thresholds (ISAT) and optical limiting (OL) threshold were obtained using the open aperture (OA) Z-scan experiment. It has been observed that the nonlinear absorption and optical limitation performance of the BiVO4 films can be altered by changing the powder morphology and BiVO4 weight concentration. BiVO4 thin films fabricated using a precursor solution pH 12 and weight concentration of 30 wt.% showed the best performance among all fabricated films with the highest nonlinear absorption coefficient (βeff), saturation density threshold (ISAT) and lowest OL threshold value.

Push-Pull Carbazole-Based Dyes: Synthesis, Strong Ultrafast Nonlinear Optical Response, and Effective Photoinitiation for Multiphoton Lithography.

The present work reports on the ultrafast nonlinear optical (NLO) properties of a series of D-π-Α and D-A push-pull carbazole-based dyes and establishes a correlation between these properties and their efficiency for potential photonic and optoelectronic applications such as multiphoton lithography (MPL). The ultrafast NLO properties of the studied dyes are determined by two distinct experimental techniques, Z-scan and pump-probe optical Kerr effect (OKE), employing 246 fs laser pulses at 515 nm. The results indicate that chemical functionalization of the carbazole moiety with various strong electron-donating and/or electron-withdrawing groups, such as benzene, styrene, 4-bromostyrene, nitrobenzene, trimethyl isocyanurate, methyl, and indane-1,3-dione, can result in a controlled and significant enhancement of the NLO absorptive and refractive responses. In the context of potential applications, the efficiency of carbazole-based organic materials as photoinitiators (PIs) for MPL applications is demonstrated. The fabricated woodpile microstructure using chemically functionalized carbazole as a PI demonstrates improvements in both feature size and MPL efficiency compared to that using unfunctionalized carbazole as a PI. This is attributed to the efficient charge transfer resulting from chemical functionalization, which leads to a substantial increase (approximately 1 order of magnitude) in the values of the imaginary part of the second-order hyperpolarizability (Imγ) and the two-photon absorption cross section (σ). The achieved feature size of 280 nm is comparable to that obtained with other widely used PIs in MPL applications. Additionally, owing to the strong NLO properties of the studied functionalized carbazole, they could also be promising candidates for further applications in photonics and optoelectronics.

Optical characterization and thermal analysis of rare earth-doped PbO-GeO2-Ga2O3 glasses embedded with gold nanoparticles

This study investigated the optical properties of PbO-GeO2-Ga2O3 glasses doped with Er3+ and Yb3+ rare earth ions, deposited with gold nanoparticles and annealed at different times (1 h, 12 h, 28 h, and 52 h). By varying the annealing times, we can understand how these thermal processes influence the position of the energy bands, which in turn affects the material’s optical properties such as the energy gap and Urbach energy. The study found that the sample annealed for 52 h had the lowest Eg value and the highest Eu value, indicating significant structural changes due to prolonged thermal treatment. The study aimed to determine the energy gap (Eg) and Urbach energy (EU), Judd-Ofelt parameters (Ω2, Ω4, Ω6), oscillator strength (fexp and fcal), and root mean square (RMS) deviation. Additionally, the study reports on the linear absorption and heat transfer results, which were used to evaluate temperature changes during Z-scan experiments. The data obtained indicate that the sample subjected to 52-hour annealing has the smallest Eg value, suggesting that intense thermal processes and crystallisation affect the positions of the material’s energy bands, increasing its ability to absorb light. Consequently, the EU value for this sample is the highest at 0.312 eV, decreasing to 0.203 eV for the sample annealed for 1 h. This indicates that longer annealing times result in more significant structural changes in the glass matrix, thereby modifying its optical properties. The analysis of the obtained results, leads to the conclusion that the lowest values of both experimental (fexp) and calculated (fcal) oscillator strength occur at 546 nm and 811 nm, indicating the potential energy properties of the 4S3∕2 and 4I9∕2 levels in the atomic structure. It was observed that the J-O parameters did not exhibit significant changes with increased annealing time, suggesting that short-term thermal processes may be sufficient to achieve a stable energy configuration. This finding is of significant practical importance, as it indicates that the material can maintain its optical properties even with shorter annealing times, thereby enhancing the efficiency of the production process.

Enhanced nonlinear optical properties and optical limiting performance of graphene modified polyaniline composite under pulsed laser regime

Polyaniline (PANI) and its graphene-doped composite (PANI-GNP) were subjected to open-aperture (OA) z-scan studies to investigate their nonlinear optical (NLO) properties and optical limiting (OL) behavior. The z-scan experiments were carried out using a nanosecond pulsed Nd:YAG laser at a wavelength of 532 nm. The z-scan studies revealed that PANI and PANI-GNP composites exhibited a reverse saturable absorption (RSA), and graphene doping modified the NLO behaviour and improved the OL property. The nonlinear absorption coefficient (β), OL threshold, and imaginary part of nonlinear susceptibility ( χi(3) ) of PANI and PANI-GNP were calculated for three different laser input pulse energies and found to be in the order of 10−9 cm W−1, 106 W cm−2, and 10−10 esu, respectively. These results qualify the PANI-GNP composite as having potential applications in OL and optoelectronic devices. The successful incorporation of graphene into PANI in PANI-GNP composite was confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). Linear optical properties were analysed using UV–visible absorption spectroscopy. To the best of our knowledge, the present work is the first attempt to explore the intensity-dependent NLO properties, OL behaviour and the calculation of their related NLO parameters of PANI-GNP composites.

Thermal nonlinear optical responses of native and oxidized low-density lipoprotein solutions at visible and infra-red wavelengths: complementary approaches

Single beam Z-scan (ZS) experiments at 532 nm (visible) and 979 nm [infra-red (IR)] wavelengths were used to determine photothermal responses of native and oxidized aqueous suspensions of human low-density lipoproteins (LDLs). The wavelengths employed in the measurements were chosen according to the optical absorption solute (LDL particles) and solvent (water) of the suspension. At 532 nm, water presents negligible absorbance, and the LDL is responsible for the light absorption. On the other hand, at 979 nm, the water is the main light absorber. In the visible light case, the particles absorb the laser light and, by conduction, transfer heat to water to form the thermal lens. In the IR experiments, water is the main absorber and transfers the heat to the particles to form the thermal lens. We show that with the IR light it is possible to investigate high degrees of oxidation of LDL, not possible with the usual visible light experiments. Differently from the usual ZS experiments with LDL at visible light, the magnitude of the thermal lens formed in the IR experiments was shown to be bigger in oxidized samples with respect to that of the native samples. For both wavelengths, all samples whose response was measured presented negative nonlinearity (self-defocusing behavior). It was also observed, in experiments with IR light, that the formation time of the thermal lens tends to decrease with the increase in the degree of oxidation of the sample.

In situ formation of silver nanoparticles within labradorite mineral crystals exhibiting third-order nonlinearity

Silver nanoparticles are cost-effective plasmonic materials for the nonlinear optics, but the applications suffer incompatibility with the matrices. This study explores the in situ formation of silver nanoparticles within the crystalline matrix of labradorite minerals. The methodology involves a high-temperature diffusion process, enabling the nucleation and growth of silver nanoparticles directly within the labradorite crystal lattice. Characterization through spectroscopic and microscopic techniques reveals the successful formation of silver nanoparticles with a diameter of 8.40 ± 4.32 nm, exhibiting notable surface plasmonic resonances absorption at 414 nm. Additionally, the nonlinear optical property of the resulting material is investigated through Z-scan experiments. The third-order nonlinear optical susceptibility β at 400 nm was estimated to be 5.0 × 10−12 m/W. This research not only advances our understanding of nanoscale phenomena within mineral crystals but also underscores their potential applications in nonlinear optical devices and related fields.

Influence of Bi3+ ions on structural, morphology, elemental, vibrational, optical, magnetic, NLO and optical limiting properties of lanthanum ferrites nanoparticles

Nanoparticles of LaFe1-xBixO3 were successfully synthesized through the microwave combustion process (MCP) employing l-alanine as the fuel source. XRD and FT-IR analyses conclusively confirmed the orthorhombic structure of lanthanum ferrite, with the mean crystallite size varying between 56.32 and 25.43 nm. The optical energy gap value, calculated from UV–vis spectra, fell within the range of 1.84 to 2.49 eV. EDX analyses verified the presence of Bi, La, Fe, and O elements. Furthermore, FE-SEM images exhibited the nanoparticles' distinctive aggregated spherical morphology. Notably, VSM analysis showcased the robust diamagnetic behavior of LaFe1-xBixO3 nanoparticles. In photoluminescence's spectra, the samples underwent excitation with a wavelength of 315 nm. The values of Hc (coercivity) and Mr (remanence) fell within the range of 121.98 to 332.39 Oe and 0.1592 to 0.0907 emu/g for LaFe1-xBixO3 respectively. Z-scan experiments yielded values for the nonlinear susceptibility, index of nonlinear refraction, and coefficient of nonlinear absorption. The findings from optical limiting studies serve as additional verification, underscoring the suitability of these materials as promising options for integration into devices, including optical switches and optical power limiters.

Hirshfeld surface analysis, experimental and theoretical evaluation of a new monohydrated organic salt of 2-amino-4‑hydroxy-6-methylpyrimidine (AHMP) and 4-acetamidobenzoic acid (AMBA) for nonlinear optical applications

The organic crystal (AHMP+.AMBA−) of the AHMP and AMBA organic compounds was successfully synthesized and grown by the slow evaporation solution process. The present work focuses on the crystal structure, Hirshfeld surface, spectroscopic and DFT analyses of the title salt. The crystal geometry of AHMP+.AMBA− monohydrated salt was characterized by the single-crystal XRD technique. The synthesized salt crystallizes in the triclinic crystal system and P-1 space group. The identification of the functional groups of the compound has been done by FTIR spectroscopy and DFT/B3LYP-D3/6–311++G(d,p) level of theory. Further, Hirshfeld surface mapping and its associated two-dimensional fingerprint plots predict that the O⋅⋅⋅H and H⋅⋅⋅H interactions are dominant for stabilizing the crystal structure. TG-DTA reveals the crystal is thermally stable up to 168 ºC. The optical studies have been performed by the time-dependent DFT (TD-DFT) and UV–Vis spectroscopic methods. Moreover, the Z-scan experiment has been done to obtain third-order NLO properties. Additionally, the HOMO-LUMO energy gap, MEP maps and theoretical NLO parameters have also been calculated.

Strong nonlinear optical limiting of resin composed by carbon nanotubes

The development of solid-state nonlinear optical limiting (NOL) materials is crucial for advancing the practicality in the field of optical limiting. In this paper, we innovatively prepare a new solid NOL material which is spiral carbon nanotubes doped epoxy resin (SCNTs-doped ER, SER) with a simple physical mixing method, and achieve an excellent nonlinear optical limiting performance. We experimentally measured optical limiting of SER with different SCNTs concentrations (0.14, 0.28, and 0.43 mg/mL) and obtained the nonlinear absorption coefficient, nonlinear refractive index, and third-order nonlinear susceptibility at the wavelength 1064 nm. Z-scan experiment results show that the SER exhibits a large nonlinear absorption coefficient (5.07 ± 0.38) × 10−9 m/W. We also measure the transmittance of the SER to evaluate its nonlinear optical limiting performance. For the SER with 0.43 mg/mL concentration, the linear transmittance and minimum transmittance with NOL effects at 1064 nm are 54.8% and 26.2%, respectively. In addition, the SER also has prominent features such as a high damage threshold and easy fabrication, indicating that the SER is a promising solid material for nonlinear optical limiting.

Heat transfer in Z-scan experiments

In this study, we present the results of theoretical calculations concerning heat exchange in materials subjected to laser irradiation, with particular emphasis on Z-scan experiments. Using explicit difference methods, we model the temperature distribution in three-dimensional samples of various sizes, such as [4 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 4 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 2] mm, [5 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 5 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 2] mm, and [6 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 6 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 2] mm. The simulations encompass two different types of pulsed lasers with durations of 10 (ns) and 100 (ps), adjusting laser parameters to achieve a peak irradiance intensity of 5 GW/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${^2}$$\\end{document}. The temperature distribution in samples, denoted as T(x, y, z, t), is computed using a novel laser heat source model specifically designed for this problem. The computational program Z-lambda enables not only the simulation of thermal processes during the Z-scan experiment but also post-experiment analysis. The development of this software was one of the main objectives of this study. The results indicate that the thermal analysis of Z-scan experiments requires consideration of factors such as laser pulse duration, laser repetition time, sample geometry, and heat exchange between the studied material and the surroundings. Especially in the Z-scan technique, controlling thermal parameters before the experiment is crucial for obtaining precise and reproducible results. It is noteworthy that sample heating during the experiment can significantly influence charge carrier density, subsequently affecting the parameters of nonlinear optics (NLO). Tools like the Z-lambda program allow for the optimization of laser parameters, minimizing sample heating, and reducing the thermal impact on NLO parameters.

Investigation of the third-order optical nonlinear enhancement properties of Ag doped chalcogenide glass films

Chalcogenide glass is widely recognized as an excellent nonlinear optical material. Enhancing its nonlinear optical properties through metal doping has emerged as a prominent research avenue in the field of chalcogenide glass. In this paper, silver (Ag) doped Ge28Sb12Se60 (GSS) chalcogenide glass films with different concentrations were prepared by co-evaporation method. The nonlinear absorption coefficients (β) of different samples were investigated by open-aperture top-hat Z-scan experiments and pump-probe experiments, and the nonlinear refractive index (γ) values were determined using the Tichy-Ticha relation. The results showed that the nonlinear absorption of GSS thin films can be improved by Ag doping. The nonlinear absorption coefficient β exhibits the maximum enhancement of about 2.5 times at 8.3% concentration, with the maximum β reaching 1.83×10−6 m/W. The nonlinear absorption mechanism observed in the thin films can be attributed to free carrier absorption, and the free carrier lifetime is positively correlated with the Ag doping concentration. Within the selected Ag concentration range, the nonlinear refractive index (γ) of Ag doped chalcogenide glass films continues to increase, reaching about 3 times at 30.7% concentration, with the maximum γ being 4.643×10−17 m2/W. Additionally, the third-order nonlinear polarizability (χ(3)) values can be increased from 1.014×10−11 esu to 3.314×10−11 esu with increasing Ag concentration. The research results in present paper are of great significance for further exploration of the optical nonlinear enhancement properties and carrier recombination dynamics in chalcogenide glass.

Effects of boron doping in InSe single crystals on optical limiting performance in the near-infrared region

Identification of photonic materials with high infrared transmittance and high nonlinear optical coefficients is one of the main emphases in material science as a result of the rapid advancement in infrared photonics. In this study, undoped and B (boron) -doped InSe single crystals were grown by using the modified vertical Bridgman method, and their nonlinear optical properties were investigated to reveal their usability as an optical limiter in the near-infrared region. The decreasing band gap energies and increasing defect states were determined with increasing B concentration in InSe single crystals. The effect of the B concentration on the nonlinear absorption (NA) and optical limiting properties of the InSe single crystals was investigated via open aperture (OA) Z-scan experiments under ultrafast laser excitation at 1200 nm wavelength with 100 femtosecond pulse duration. Two-photon absorption (TPA) was the dominant NA mechanism at 1200 nm excitation wavelength in the femtosecond domain. The results revealed that the NA became stronger with increasing input intensity and increasing amount of B dopant atoms in the InSe single crystal. The observed enhanced NA can be attributed to two possible events (i) increasing input intensity induced more excited electrons which led to more contribution to NA through TPA and (ii) increasing B dopant atoms in InSe single crystal induced more defect states. The NA may be more enhanced with the contribution of these defect states related NA mechanisms. The high transparency and strong NA behavior at the near-infrared region make these single crystals exceptional potential candidates for developing various optoelectronics and filters at the near-infrared spectral region.

Nonlinear Responses and Optical Limitation of Copper Nanoparticles by Z-scan Method

Recently, copper particles in nano dimensions have been gaining attention because they have advanced features and potential applications in various fields. This work studies the nonlinear optical performance of copper nanoparticles. We utilized laser ablation to create copper nanoparticles. In this technique, a piece of copper in distilled water is irradiated using the laser beam. Laser ablation was done for 30 min. The optical nonlinearities are measured by means of the Z-scan technology. The open-aperture Z-scan experiments showed that Cu nanoparticles have strong nonlinear absorption. Through a closed aperture Z-scan experiment, we could analyze the nonlinear refraction of sample. We analyze the optical limiting effect of copper nanoparticles. A theoretical model is presented and shows that at high laser light intensity, nonlinear light scattering occurs. By analyzing the experimental data with the presented model, the values of optical nonlinearities of copper particles were found to be n2=0.8×10-19m2w-1 and γ=1.46×10-13m/W, respectively. Our work confirmed that Cu nanoparticles show outstanding nonlinear optical features, which can cause the development of nonlinear optics. Received: 8 September 2023 | Revised: 2 January 2024 | Accepted: 3 January 2024 Conflicts of InterestThe authors declare that they have no conflicts of interest to this work. Data Availability Statement Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Multivalency-induced structural variations of 2D selenium nanosheets: facile solution-phase synthesis and optical properties.

The structural stability of two-dimensional (2D) phases derived from bulk selenium (Se) is intrinsically rooted in the multivalent nature of the material. The emergence of 2D Se, as its morphology evolves from 1D to 2D, was initially inspired by theoretical predictions of various quasi-stable structural phases of 2D Se. Here, we report a facile liquid-phase synthesis of free-standing few-layer selenium nanosheets (SeNS) employing a simple magnetic stirring of their bulk counterpart in N-methyl pyrrolidone (NMP). The synthesized SeNS possess lateral dimensions ranging from several hundreds of nanometers to a few microns, with a minimum thickness of ∼1 nm. High-resolution transmission electron microscopy reveals the existence of α- and β-selenene. Fourier transform infrared analysis suggests that the inherent surface/edge functionalization of 2D SeNS by NMP enhances their dispersion stability. The UV-vis-NIR absorption spectrum of SeNS exhibits a shoulder peak at 330 nm, attributed to surface/edge functionalization, and multiple peaks across the vis-NIR region, stemming from size quantization effects. The functionalized selenium nanosheets generate photoluminescence that spans the blue-green range, while the size quantization of SeNS leads to green-orange luminescence. The non-linear optical studies following Z-scan experiments with an open aperture revealed reverse saturable absorption (RSA) and strong optical limiting in 2D SeNS under 532 nm, 10 ns laser pulses. Notably, a transition from RSA to saturable absorption (SA) has also been observed in samples stirred over an extended period. In this perspective, the results illustrate the first experimental realization of free-standing multivalent 2D selenium in allotropic forms with unique optical properties.

Study of two-photon absorption and excited-state dynamics of coumarin derivatives: the effect of monomeric and dimeric structures.

Intramolecular charge transfer (ICT) and π-electron delocalization are two key factors affecting the nonlinear optical absorption of organic molecules. To clarify the different influences of ICT and π-electron delocalization on two-photon absorption (TPA) and excited-state absorption (ESA), monomeric coumarin C1 and dimeric coumarin C2 are synthesized and studied. Transient absorption spectroscopy (TAS) analysis of these coumarin derivatives in solvents of varying polarities describes the polarity-dependent excited-state dynamics and reveals the ESA signals of the charge transfer state (CTS) and local excited state (LES) with different spectral features. Femtosecond broadband Z-scan experiments indicate that dimeric coumarin C2 has a more significant TPA response than monomeric coumarin C1 in the near-infrared region. Natural transition orbital (NTO) analysis further theoretically characterizes the electron transition feature induced by TPA. Our results reveal that the TPA of these coumarin derivatives can be significantly enhanced by expanding π-electron delocalization, but their ESA is mainly dominated by ICT performance. This study indicates that coumarin derivatives will exhibit extremely broad application prospects in the field of ultrafast optical limiting (OL) through reasonable molecular design.

Study of excited-state dynamics of D-π-A thiophene derivatives: Enhanced excited-state absorption and refraction

Two D-π-A-type thiophene derivatives Br-TH and 3-TH with different donor units are designed and synthesized to investigate the effect of intra-molecular charge transfer (ICT) on their excited state absorption (ESA) and excited state refraction (ESR). The photodynamic evolution of these compounds in different polar solvents is analyzed by transient absorption spectra (TAS), and the electronic relaxation process from local excited-state (LES) to charge transfer state (CTS) is observed. Z-scan experiments indicate that ESA and ESR of these molecules can be significantly enhanced by extending the π-conjugated length. Phase object pump–probe (POPP) experiments further describe the photophysical mechanisms of CTS-based ESA and ESR of 3-TH. ESA cross section and ESR volume are obtained by establishing the equivalent five-energy level with corresponding rate equations. Results indicate that ICT performance can be effectively modulated by extending the π-conjugated length of these thiophene derivatives, and further improve their ESA and ESR properties. Furthermore, nanosecond optical limiting (OL) experiments indicate that compound 3-TH has excellent OL performance, which has potential application prospects in protecting sensitive devices or human eyes.

Nonlinear absorption and optical limiting performance of broad-band multi-branched aniline derivatives in the near-infrared region

A thorough investigation was conducted into the impact of π-linker and acceptor group modifications on the nonlinear absorption coefficient and optical limiting (OL) capabilities of multi-branched aniline derivatives. In the experiment, the femtosecond Z-scan technology and the OL experiment were used to detect the nonlinear optical (NLO) response of the samples dissolved in DMF solution at different wavelengths and energies. The femtosecond Z-scan experiments show that all samples exhibit reverse saturable absorption (RSA) caused by excited state absorption (ESA) induced by two-photon absorption (2PA) at specific wavelengths. NBTT still exhibits strong RSA properties at the near-infrared wavelengths, and the 2PA cross-section can reach 293 GM and 289 GM under the incident laser at the wavelength of 900 nm and 1030 nm, while maintaining strong two-photon-induced ESA. Furthermore, the femtosecond OL experiments reveal the commendable OL property of NBTT, and the limiting thresholds reach 0.0026 J/cm2 (at 800 nm) and 0.0033 J/cm2 (at 1030 nm). Quantum chemistry calculations utilizing density functional theory (DFT) show thatπ electrons contribute differently to the bond order on different branches of multi-branched molecules, and the value of Branch 1 is greater than or equal to that of the other two branches at the same position. This leads to the charge transfer (CT) of the molecules NBC and NBTC predominantly concentrated on Branch 1 during the 2PA process, while the addition of conjugate units and cyano-groups in the acceptor groups makes the CT of NBTT not only concentrated on Branch 1, but also extended to Branches 2 and 3. This research seeks to develop the structural design and practical application of multi-branched aniline derivatives to achieve high nonlinear absorption capacity, while maintain a low OL threshold in the near-infrared spectral range.