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The use of two-photon absorption (TPA) for such applications as microscopy, imaging, and photodynamic therapy (PDT) offers several advantages over the usual one-photon excitation. This creates a need for photosensitizers that exhibit both strong two-photon absorption and the highly efficient generation of reactive oxygen species (ROS), as well as, ideally, bright luminescence. This review focuses on different strategies utilized to improve the TPA properties of various multi-photon absorbing species that have the required photophysical properties. Along with well-known families of photosensitizers, including porphyrins, we also describe other promising organic and organometallic structures and more complex systems involving organic and inorganic nanoparticles. We concentrate on the published studies that provide two-photon absorption cross-section values and the singlet oxygen (or other ROS) and luminescence quantum yields, which are crucial for potential use within PDT and diagnostics. We hope that this review will aid in the design and modification of novel TPA photosensitizers, which can help in exploiting the features of nonlinear absorption processes.

As an effective and noninvasive treatment of various diseases, photodynamic therapy (PTD) relies on the combination of light, a photosensitizer, and oxygen to generate cytotoxic reactive oxygen species that can damage malignant tissue. Much attention has been paid to covalent modifications of the photosensitizers to improve their photophysical properties and to optimize the pathway of the photosensitizers interacting with cells within the target tissue. Herein we report the design and synthesis of a supramolecular heterometallic Ru–Pt metallacycle via coordination-driven self-assembly. While inheriting the excellent photostability and two-photon absorption characteristics of the Ru(II) polypyridyl precursor, the metallacycle also exhibits red-shifted luminescence to the near-infrared region, a larger two-photon absorption cross-section, and higher singlet oxygen generation efficiency, making it an excellent candidate as a photosensitizer for PTD. Cellular studies reveal that the metallacycle selectively accumulates in mitochondria and nuclei upon internalization. As a result, singlet oxygen generated by photoexcitation of the metallacycle can efficiently trigger cell death via the simultaneous damage to mitochondrial function and intranuclear DNA. In vivo studies on tumor-bearing mice show that the metallacycle can efficiently inhibit tumor growth under a low light dose with minimal side effects. The supramolecular approach presented in this work provides a paradigm for the development of PDT agents with high efficacy.

The nonlinear optical properties of aza‐borondipyrromethene (aza‐BODIPY) derivatives modified with 4‐methoxyphenyl, 2,4‐dimethoxyphenyl, and 4‐N,N‐diphenylaminophenyl groups at the 1 and 7 positions of the core structure are evaluated. The effects of substituents and solvent polarity (tetrahydrofuran (THF) and chloroform (CHCl3)) on the linear absorption and fluorescence properties are systematically investigated. The target aza‐BODIPYs exhibit higher absorption in CHCl3 compared to THF. While solvent polarity exerted a minimal effect on the absorption and fluorescence intensity, the fluorescence in BOD2 and BOD3 is markedly quenched as a result of the electron‐donating methoxy groups and the pronounced intramolecular charge transfer (ICT) characteristics of the diphenylamine unit. Moreover, all compounds shows strong near‐infrared two‐photon absorption (TPA) behavior. The TPA cross‐sections of BOD1 and BOD3 are measured as 61 and 269 GM at 1000 nm, respectively, and BOD3 showed the highest value due to the enhanced ICT efficiency. Density functional theory calculations are also performed to study the thermodynamic and photophysical properties in different media. The findings indicated an enhancement in molecular stability and a reduction in HOMO–LUMO gaps, particularly in THF. Among the compounds, BOD3 stood out with its low energy gap and high polarizability, and its theoretical NLO parameters are in strong agreement with the experimental TPA findings. Aza‐BODIPY compounds is designed with 4‐methoxyphenyl, 2,4‐dimethoxyphenyl, and 4‐N,N‐diphenylaminophenyl groups at the 1 and 7 positions. The effects of substituents and solvent polarity (tetrahydrofuran and chloroform) on the linear absorption, fluorescence behaviors as well as nonlinear absorption properties are investigated. Nonlinear absorption properties and charge transfer dynamics are analyzed by open aperture Z‐scan experiments and ultrafast pump‐probe spectroscopy measurements. The thermodynamic and photophysical properties in both solvents are also determined by Density functional theory calculations.

We demonstrate how two-photon excitation with quantum light can influence elementary photochemical events. The azobenzene trans → cis isomerization following entangled two-photon excitation is simulated using quantum nuclear wave packet dynamics. Photon entanglement modulates the nuclear wave packets by coherently controlling the transition pathways. The photochemical transition state during passage of the reactive conical intersection in azobenzene photoisomerization is strongly affected with a noticeable alteration of the product yield. Quantum entanglement thus provides a novel control knob for photochemical reactions. The distribution of the vibronic coherences during the conical intersection passage strongly depends on the shape of the initial wave packet created upon quantum light excitation. X-ray signals that can experimentally monitor this coherence are simulated.

Third-order nonlinear optical (NLO) properties of indium tin oxide (ITO) thin film were studied using high repetition rate (80 MHz), femtosecond (100 fs), and near-infrared (NIR) (750–820 nm) laser pulses. An ITO thin film was prepared using an RF magnetron sputtering system. The film thickness was determined using a scanning electron microscope (SEM), while the linear optical properties of the thin film were measured using a UV–Vis spectrophotometer. Nonlinear absorption (NLA) studies of ITO thin film using an open aperture Z-scan revealed a combination of nonlinear phenomena: reverse saturable absorption (RSA) and saturable absorption (SA). A saturation model has been used to explain the observed saturation of two-photon absorption (2PA) at high incident powers. The ITO film's NLA properties were found to be excitation power and wavelength dependent. The NLA coefficient was shown to be inversely proportional to the excitation power and wavelength. Furthermore, NLA measurements of ITO thin films show that the transition from RSA to RSA-SA-RSA switching behavior was influenced by ITO thickness. Because of their high third-order nonlinear optical responses, these ITO thin films are ideal candidates for photonic applications.

Organic phosphorescence materials have received wide attention in bioimaging for bio‐low toxicity and large Stokes. Herein, a design strategy to achieve near‐infrared (NIR) excitation and emission of organic room‐temperature phosphorescence through two‐stage confinement supramolecular assembly is presented. Via supramolecular macrocyclic confinement, the host–guest complexes exhibit phosphorescence with two‐photon absorption (excitation wavelength up to 890 nm) and NIR emission (emission wavelength up to 800 nm) in aqueous solution, and further nano‐confinement assembly significantly strengthens phosphorescence. Moreover, the nano‐assemblies possess color‐tunable luminescence spanning from the visible to NIR regions under different excitation wavelengths. Intriguingly, the prepared water‐soluble assemblies maintain two‐photon absorption and multicolor luminescence in cells or vivo. A multi‐stage assembly strategy involving macrocyclic confinement and nano‐confinement is reported, which induces and enhances guests’ phosphorescence with two‐photon absorption and NIR emission. Moreover, the assemblies possess excitation‐dependent emission properties, in which luminescence covers the visible and NIR regions under different excitation wavelengths.

Organic–inorganic hybrid perovskites carry unique semiconducting properties and advanced flexible crystal structures. These characteristics of organic–inorganic hybrid perovskites create a promising candidacy for circularly polarized light (CPL) detection. However, CPL detections based on chiral perovskites are limited to UV and visible wavelengths. The natural quantum well structures of layered hybrid perovskites generate strong light–matter interactions. This makes it possible to achieve near‐infrared (NIR) CPL detection via two‐photon absorption in the sub‐wavelength region. In this study, cooperative strategies of dimension increase and mixed spacer cations are used to obtain a pair of chiral multilayered perovskites (R‐β‐MPA)EA2Pb2Br7 and (S‐β‐MPA)EA2Pb2Br7 (MPA = methylphenethylammonium and EA = ethylammonium). The distinctive bi‐cations interlayer and multilayered inorganic skeletons provide enhanced photoconduction. Moreover, superior photoconduction leads to the prominent NIR CPL response with a responsivity up to 8.1 × 10−5 A W−1. It is anticipated that this work can serve as a benchmark for the fabrication and optimization of efficient NIR CPL detection by simple chemical design. Two chiral multilayered perovskites with alternating cations in the interlayer are unprecedentedly obtained by dimension increase and mixed spacer cations. The distinctive structure gives rise to an enhancement of photoconduction, leading to the high near‐infrared (NIR) circularly polarized light (CPL) responsivity (8.1 × 10−5 A W−1). This work highlights the design flexibility of hybrid perovskites for high‐performance NIR CPL detection.

We investigate the nonlinear optical response of Pb 2+ -doped ZnMoO 4 nanostructures synthesized by chemical precipitation technique. X-ray diffraction (XRD) and Raman spectroscopy results confirm the formation of ZnMoO 4 nanostructures with a triclinic structure. High-Resolution Transmission electron microscope (HRTEM) reveals the deformed spherical shape of nanostructures. Further, the successful doping and molecular bonding of Pb 2+ with ZnMoO 4 was evidenced by FTIR and XPS. UV–Vis absorption spectra reveals the alteration in the electronic band structure of ZnMoO 4 nanostructures upon Pb doping, leading to a considerable increase in bandgap and enhanced charge transfer within ZnMoO 4 : Pb nanostructure. The nonlinear optical absorption and optical limiting characteristics of the nanoparticles is measured via the open aperture Z-scan technique with an Nd: YAG pulsed laser at 532 nm, exhibiting a reverse saturable absorption and a rapid decrease in the commencement of the optical limiting threshold with the increased in Pb concentration. The observed nonlinear absorption can be attributed to the crystal structure, particle size, creation of oxygen vacancies, and defects on the host lattice. These results suggest that Pb 2+ -doped ZnMoO 4 nanoparticles have excellent prospects for optical limiting (OL) applications. This could be useful for applications such as laser protection and optoelectronic device applications. Graphical abstract

Mixed (FAPbI3)0.92(MAPbBr3)0.08 perovskite thin films exhibit strong nonlinear optical responses, rendering them promising candidates for applications in photonics and optical communications. In this work, we present a systematic study on the ultrafast third-order nonlinear optical processes in mixed perovskite nanocrystals (NCs) by exploring the generation of third harmonic radiation and giant two-photon absorption-based photoluminescence (PL) when excited by femtosecond laser pulses of a 1030 nm central wavelength. A comparative analysis of the coherent third harmonic generation in the thin-film-containing perovskite nanocrystals has shown a 40× enhancement of the third harmonic signal compared to the signal generated in the pure quartz substrate. The cubic dependence of the third-nonlinear optical response of the (FAPbI3)0.92(MAPbBr3)0.08 perovskites on the intensity of the driving radiation was identified using broadband 38 femtosecond driving pulses. The positive nonlinear refractive index (γ = +1.4 × 10−12 cm2·W−1) is found to play an important role in improving the phase-matching conditions of the interacting pulses by generating a strong third order harmonic. The giant two-photon absorption (TPA)-assisted PL peak was monitored and a blue shift of the PL was obtained in the higher intensity range of the laser pulses, with the absorption coefficient β estimated to be~+7.0 cm·MW−1 at a 1030 nm laser wavelength.

Broadband nonlinear optical properties from 500 to 1550 nm of ultrasmall black phosphorus quantum dots (BPQDs) have been extensively investigated by using the open-aperture Z-scan technique. Our results show that BPQDs exhibit significant nonlinear absorption in the visible range, but saturable absorption in the near-infrared range under femtosecond excitation. The calculated nonlinear absorption coefficients were found to be (7.49 ± 0.23) × 10−3, (1.68 ± 0.078) × 10−3 and (0.81 ± 0.03) × 10−3 cm/GW for 500, 700 and 900 nm, respectively. Femtosecond pump-probe measurements performed on BPQDs revealed that two-photon absorption is responsible for the observed nonlinear absorption. The saturable absorption behaviors observed at 1050, 1350 and 1550 nm are due to ground-state bleaching induced by photo-excitation. Our results suggest that BPQDs have great potential in applications as broadband optical limiters in the visible range or saturable absorbers in the near-infrared range for ultrafast laser pulses. These ultrasmall BPQDs are potentially useful as broadband optical elements in ultrafast photonics devices.