The atomically precise nanoclusters offer precise control of optical properties on the nanoscale, as long as we understand the relation among the size, composition, and structure of nanoclusters and their physical properties. So far, the primary interest has been in the synthesis of nanoclusters with controlled optical properties, in not only the one-photon but also the two-photon regime. Large two-photon absorption cross sections of nanoclusters were reported, but the possibility of modulation of nonlinear optical (NLO) properties by external stimuli has been scarcely explored. We present here the experimental data supported by DFT calculations on two-photon absorption of gold nanoclusters, which are modified by oxidation-reduction processes. Such reactions can be an important regulator of the nonlinear optical properties of various materials, which is well established, e.g., for organometallics, but has not been demonstrated for nanoclusters. The reversible oxidation of [Au25(SR)18]- (where SR = 2-phenylethanethiol) to its neutral form results in distinct changes in the one-photon absorption spectra, but even more pronounced differences in the two-photon absorption. The change in the oxidation state of the Au25 cluster results in the 2-fold enhancement of the two-photon absorption cross sections in the wavelength range of 825-1150 nm and switching between saturable absorption and two-photon absorption below 825 nm. DFT calculations show that the presence of the counterion may contribute to the change, as it decreases the two-photon absorption cross sections of the system. Our results demonstrate that even seemingly minor electronic differences between the anionic and neutral Au25(PET)18 clusters can lead to pronounced variations in their NLO properties.

We report the use of σ-alkynyl d6 electron-rich transition metal complexes as electron-releasing end-groups in octupolar molecules designed for nonlinear optical (NLO) applications, specifically, N,N′,N″-triarylisocyanurates (5,7,8,10,12) and 1,3,5-triarylbenzenes (6,9,11) functionalized by Fe(II) and Ru(II) organometallic moieties, and their NLO properties, as assessed by hyper-Rayleigh scattering (HRS) and Z-scan. The redox properties are briefly investigated through isolation of the corresponding Fe(III) trications 5[PF6]3 and 6[PF6]3. The second-harmonic generation (SHG) or two-photon absorption (2PA) performance of the Fe(II) and Ru(II) parents is compared with the help of TD-DFT calculations performed on models. Comparison with tris-ferrocenyl isocyanurate 4 reveals that the σ-connection of the metallic centers to the π-manifold is superior to the η5-connection for enhancing NLO properties. The positive effect of organometallic end-groups on NLO properties relative to purely organic electron-releasing substituents is established. The mechanism by which NLO enhancement occurs is complex and possibly connected to the polarizable π-electrons in the ligands surrounding the metal alkynyl units, but in most cases, the observed NLO enhancement must arise from the transition metal centers interacting with the central π-manifold.

We present hydrothermal synthesis and comprehensive physicochemical characterization of water-soluble Ag2S quantum dots (QDs) emitting in the second near-infrared biological window (NIR-II), stabilized by chiral ligands: l- and d-penicillamine (Pen). By systematically optimizing reaction temperature, time, and the Ag : S ratio, we significantly improved the photophysical properties of the QDs. The obtained Ag2S/Pen QDs exhibited NIR-II emission (lambda EM. similar to 1040 nm) with a fair quantum yield (QY = 1.1%), high colloidal stability, low cytotoxicity, and fluorescence lifetime reaching 84 ns in aqueous media. The use of enantiomerically pure and racemic variants of Pen allowed us to investigate the influence of stereoisomer configuration on morphology and consequently, linear and nonlinear optical (NLO) properties of the QDs. Spectrally-resolved NLO study using a femtosecond laser Z-scan technique showed the presence of two-photon absorption (2PA) with a peak cross section reaching sigma 2 similar to 511 GM (Goeppert-Mayer units) for Ag2S/d-Pen excited at lambda EXC. = 824 nm and 260 GM (lambda EXC. = 950 nm) for Ag2S/l-Pen, respectively. Biocompatibility studies in THP-1 macrophages and HLMEC endothelial cells revealed favorable tolerance profiles for Ag2S/Pen QDs, particularly in immune cells. Notably, macrophages maintained high viability and even showed enhanced metabolic activity, while endothelial cells exhibited good tolerance at lower concentrations, supporting the potential of these QDs for biomedical applications involving immune and vascular systems. These findings demonstrate that the structure of surface ligands plays a key role in controlling the structural and optical properties of Ag2S QDs, providing insights for the design of NIR fluorescent nanomaterials for biomedical and photonic applications.

The requested changes include providing a more explicit version of Equation (1), the inclusion of 11 additional references, and reordering of the other citations [...]

Optimized synthesis of chiral NIR-emitting Ag 2 S QDs with nonlinear optical characterization and toxicity evaluation.

Hybrid nanosystems, such as those combining plasmonic, dielectric, and quantum-confined nanostructures, have long been of interest for enhancing and tailoring diverse light-matter interactions. Here, we present a series of hybrid nanomaterials exhibiting strongly enhanced nonlinear optical (NLO) properties, fabricated by combining silver sulfide quantum dots (Ag2S QDs) with silica and gold nanostructures. We studied their NLO properties (two-photon absorption and saturable absorption) in colloidal solutions over a wide spectral range (500-1600 nm) using the femtosecond Z-scan technique. Embedding Ag2S QDs into silica nanospheres gives rise to remarkable enhancement of two-photon absorption (up to a factor of 16 increase in the merit factor σ2/M compared to bare QDs), whereas covering such QD-doped silica nanospheres with gold nanoparticles or attaching the QDs to the surface of gold nanoshells (NSs) leads to even further enhancement (up to 73-fold increase in σ2/M), accompanied by a competing effect of saturable absorption. Furthermore, in the case of QD-doped silica spheres covered with a continuous gold layer, we observe a previously unreported saturation of extinction in the near-infrared region that follows an unusual intensity dependence, suggesting the involvement of two-photon absorption as the pumping mechanism. In addition to the experimental studies, we have performed numerical simulations, revealing the plasmonic origin of the observed spectral dependences of the NLO properties, with the underlying enhancement mechanisms involving local field enhancement and, possibly, also coupling between plasmon modes and QD excitons, giving rise to a double peak in the σ2 spectrum. Our findings demonstrate the unique potential of hybrid NLO nanomaterials combining quantum-confined, plasmonic, and dielectric components.

This study reports on the linear and nonlinear optical (NLO) properties of water-dispersed carbon nanodots (CNDs) fabricated via a rapid one-step hydrothermal microwave-assisted technique. The CNDs exhibit two-photon excited luminescence, which was characterized with spectrally tunable femtosecond laser pulses as involving the two-photon absorption (TPA) cross sections (σ2) as large as 1.4 × 103 Goeppert-Mayer (GM) at the excitation wavelength of 720 nm and the quantum yield (QYs) of 28%. By analyzing the σ2 spectra, specific wavelength ranges optimal for excitation via the two-photon process were identified. In addition, the potential of the CNDs as sensors for the selective and sensitive detection of Fe3+ ions through one- and two-photon induced fluorescence quenching was investigated. To gain deeper insights into the mechanism underlying the observed decrease in fluorescence intensity upon addition of Fe3+ ions, potentially involving dynamic quenching, fluorescence quenching experiments across various temperatures were conducted, being the first such study in both one-photon and two-photon excitation regimes for this sensor. The possibility of energy transfer between CNDs and Fe3+ ions was investigated by analyzing the luminescence kinetics using time-correlated single-photon counting (TCSPC) and streak camera techniques, in one- and two-photon regime, respectively. The pronounced nonlinear optical response of the CNDs highlights their potential as active optoelectronic materials for optical sensors operating in the near-infrared (NIR) region. Cytotoxicity studies of the water-dispersed CNDs revealed no observable toxicity, even at high concentrations, making them suitable for biorelated applications.

The internal structure (oxygenous groups and conjugated aromatic domains) and the hydrogen-bonding network with solvent molecules are two ways to tune the fluorescence of carbon nanodots upon one- and two-photon excitation.

While circular dichroism (CD) and optical activity (OA) are well established as optical effects used in characterization of chiral media, harmonic generation and multiphoton circular dichroism are increasingly seen as new, convenient ways of exploring chirality thanks to their operation in the near-infrared range of wavelengths. However, quantitative data about two-photon circular dichroism (2PCD) of organic and inorganic materials are scarce, and even less can be found about three-photon circular dichroism (3PCD). Here, we show that both 2PCD and 3PCD can be readily detected in chiral atomically precise gold nanoclusters via polarimetric Z-scan technique. We provide quantitative data on 2PCD and 3PCD of both enantiomers of Au38(PET)24 nanoclusters, which exhibit extraordinary chiroptical properties arising from the interplay of several levels of molecular organization. Interestingly, the corresponding two- and three-photon dissymmetry factors of Au38(PET)24 enantiomers are significantly enhanced in comparison to the one-photon CD by factors of 178 and 217, respectively.

Spectrally-resolved third-order nonlinear optical properties of water-dispersed sulfur quantum dots were used as combined one- and two-photon active sensor for heavy metal ions detection.

We present the use of lanthanide doped NaYF4 based core@multiple-shell nanoparticles as optical materials showing merged possibilities of engineered photon management processes. We have successfully synthesized and characterized series of NaYF4 core@shell@shell@shell nanomaterials in which the core parts were doped with NIR absorbing/emitting lanthanides (i.e., Nd3+, Yb3+, and Tm3+) to benefit from the complete separation of peristatic quenching processes connected to the interaction with solvent and/or ligand molecules. Followed by covering with an inert shell, Pr3+ ions were doped for VIS-to-UVC up-conversion in the next layer , and finally covered with yet another un-doped protecting shell. The obtained materials were characterized by the means of morphology, crystal structure and spectroscopic properties, with the emphasis put on the measurements in the UVC and NIR spectral regions. The obtained results showed the interesting possibility of merging within designed nanoparticles architectures both UVC light generation for future therapeutic purposes and NIR emission for deep tissue optical imaging diagnostics.

Atomically-precise gold nanoclusters covered with thiol ligands are attractive candidates for biophotonics applications. We show that their covalent linking may offer a pathway toward atomically-precise materials with enhanced nonlinear optical properties. Clusters oligomerization leads to moderate modification of the linear optical properties, but has significant impact on their two-photon absorption performance.

Progress in syntheses and understanding of the intriguing properties of chiral noble metal nanoclusters sparks interest to extend investigations of their chiroptical response to the nonlinear optics regime. We present a quantitative determination of two-photon circular dichroism of chiral gold nanoclusters with ATT and L- or D-Arg ligands (ATT = 6-aza-2-thiotymine and Arg = arginine). Introduction of arginine ligands enables the formation of two enantiomers of the nanoclusters, with strong chiroptical effects in both linear and nonlinear regime. We present two-photon absorption and luminescent properties measured in a wide range of wavelengths, with the two-photon absorption cross section reaching 1743 GM and two-photon brightness ∼1102 GM at 825 nm. We report strong, 245-fold enhancement of the two-photon circular dichroism of nanoclusters with respect to the one-photon absorption counterpart - the dissymmetry factor. The presence of multiple advantages of nanoclusters: high fluorescence quantum yield, strong nonlinear optical properties and well-controlled chirality is a powerful combination for applications of such clusters in multiphoton microscopy.

We present an enhancement of optically triggered anti-microbial treatment based on visible to ultraviolet up-conversion emission in lanthanide-doped yttrium silicates. A series of Pr3+/Tm3+/Yb3+ co-doped Y2Si2O7 powders was synthesized and characterized, including determination of their crystal structure, morphology, and spectroscopic properties. The emphasis was put on the examination of the influence of lanthanide ion doping on the luminescence characteristics in the ultraviolet region of light, since the emission from this part of the spectra can be effectively used for decontamination purposes. Due to the additional Tm3+ doping into the Pr3+ containing Y2Si2O7 powders, we observed up-conversion emission lines in both UVA and UVC regions of spectra, while the solely Pr3+ doped materials gave only the emission in the UVC region. The synthesized luminescent powders were further used for decontamination experiments with three different types of pathogenic microorganisms (i.e., A. baumannii, S. aureus, and C. albicans) formed biofilms. The microbial cell viability studies demonstrated the higher deactivation efficiency for all investigated microbial species with the use of combined UVA and UVC irradiation generated by Pr3+ and Tm3+ co-doped phosphors. At the same time, the incorporation of Yb3+ ions could additionally offer an alternative NIR-to-UVA excitation pathway via a 980 nm laser diode, which is beneficial for deeper light penetration experiments.

Wide spectral wavelength range (500-1600 nm) measurements of nonlinear optical properties of silver sulfide (Ag2S, with 2- or 3-mercaptopropionic acid, 2 or 3MPA ligands) quantum dots (QDs) in aqueous colloidal solutions were performed using the Z-scan technique with tunable similar to 55 fs laser pulses at 1 kHz. We have identified regions of the occurrence of various NLO effects including two-photon absorption, nonlinear refraction, as well as saturation of one-photon absorption. At the same time, we evaluated the relationship between the properties of the QDs and the variation of the material that covers their surface. The peak two-photon absorption cross section (sigma(2)) values were determined to be 632 +/- 271 GM (at 850 nm) for Ag2S-2MPA QDs and 772 +/- 100 GM (at 875 nm) for Ag2S-3MPA QDs. The physicochemical factors influencing the three-dimensional self-organization of Ag2S QDs in water as well as their impact on spectroscopic properties were also investigated.

Non-invasive imaging of morphological changes in biologically relevant lipidic mesophases is essential for the understanding of membrane-mediated processes. However, its methodological aspects need to be further explored, with particular attention paid to the design of new excellent fluorescent probes. Here, we have demonstrated that bright and biocompatible folic acid-derived carbon nanodots (FA CNDs) may be successfully applied as fluorescent markers in one- and two-photon imaging of bioinspired myelin figures (MFs). Structural and optical properties of these new FA CNDs were first extensively characterized; they revealed remarkable fluorescence performance in linear and non-linear excitation regimes, justifying further applications. Then, confocal fluorescence microscopy and two-photon excited fluorescence microscopy were used to investigate a three-dimensional distribution of FA CNDs within the phospholipid-based MFs. Our results showed that FA CNDs are effective markers for imaging various forms and parts of multilamellar microstructures.

A series of dyes containing electron-donating (D) and electron-accepting (A) groups connected through a It-conjugated linker were designed and their linear and nonlinear absorption properties were studied. The dyes combined 1,3-oxazol-5(4H)-one core with methoxy, methyl, phenyl, cyano, and nitro group in the para-position of a phenyl ring. Spectrally resolved two-photon absorption cross-section measurements carried out by the femtosecond Z-scan technique showed wide wavelength regions of nonlinear optical absorption. The studied compound containing the phenyl ring exhibits relatively high effective two-photon absorption cross-section equal to 772 +/- 100 GM at 600 nm. The experimental optical characterization is supported by the results of electronic-structure calculations.

A common motif in the design of organic dye compounds is that of an electron donor and an acceptor moiety linked through a conjugated bridge, but more complicated structures can also be built using such motifs as building blocks. Following these principles, we synthesized two styrylpyridinium dyes, a linear one and a branched molecule composed of three units of the mono moiety connected by benzyl group and investigated their photophysical properties in different environments: their linear absorption, one-photon excited fluorescence and two-photon absorption properties. The compound with the symmetrically substituted phenyl core shows a redshift of the absorption and fluorescence bands. Spectrally resolved two-photon absorption cross section measurements carried out by femtosecond Z-scan technique show that the value for the trimer is over twelve times larger than that for the monomer, and not just three times as expected from the molar mass increase. Large two-photon absorption (2PA) values were measured in the near infrared (NIR) region. The prototropic study indicates that the protonation of the amino group blocks a lone pair on the nitrogen atom, making the photoinduced electron transfer process from the amine group to the pyridinium ring ineffective. This results in the reduction of the long-wavelength absorption with a simultaneous increase of the band in the short-wavelength region of the spectrum and a decrease in the fluorescence intensity. The behavior of dyes in the presence of beta-cyclodextrin was also studied based on NMR, UV-Vis, and fluorescence spectroscopy. The performed experiments indicated the formation of 1:1 inclusion complex in aqueous solution. (C) 2022 Published by Elsevier B.V.

The one- and two-photon absorption (1PA and 2PA) properties of three expanded aceneporphyrinoids, 28-thia-, 28-selena- and 28-tellura-2,7-naphthiporphyrin, have been studied. The open-aperture Z-scan technique was used to determine two-photon absorption cross-sections in the near infrared range using an amplified femtosecond laser system. The maximum values of the cross sections were found to be 99, 200 and 650 GM at 900 nm and 1, 13 and 31 GM at 1400 nm for the three investigated compounds, respectively. These results demonstrate enhanced 2PA properties compared with well-known porphyrin photosensitizers, such as Foscan®, showing the potential of porphyrin core modification for optimizing infrared nonlinear absorbers.

The past decade has witnessed the rise of low-dimensional materials, such as graphene, transition metal dichalcogenides, black phosphorus, organic-inorganic hybrid perovskites and MXenes. They have received tremendous attention due to their peculiar prop-erties, as compared to their bulk phase. These unique properties have ushered in a paradigm shift in many applied fields and technologies including, but not limited to optoelectronics, catalysis, biomedical research and quantum information sciences. The fundamental processes determining the performance of devices, utilizing the low-dimensional materials, are photogeneration of charge carriers and carrier transport. A detailed understanding of these processes is therefore indispensable to the design and optimization of advanced high-performance low-dimensional materials-based devices for broadband photodetection, high-speed modulation, high-efficiency solar energy har-vesting, and energy storage, among others. Herein, we critically review recent advances in studies of photoinduced carrier dynamics in low-dimensional semiconductors and semimetals. Transient carrier generation, trapping and recombination dynamics can be controlled by temperature, charge transfer in hybrid structures, electrical and magnetic field, and stress. Revealing the mechanisms and strategies of their tuning should help design and optimize multifunctional materials to enable high-performance devices. In this review, we do not focus exclusively on the photoinduced species (excitons and charge carriers), but also discuss possible strategies to adjust their properties and their impact on device characteristics. To conclude, we summarize current status, describe existing challenges, and provide a subjective opinion on future opportunities to advance this exciting field.We hope that this review will be appealing to a broad materials physics audience and will be helpful in exploring new physics and discovering theory guided novel materials with robust performance.(c) 2022 Published by Elsevier B.V.