To improve two-photon absorption (TPA) performance in the first near-infrared (NIR-I) window, we constructed a D-A-D type BODIPY derivative (BDP-750) through simultaneous functionalization at the 2, 3, 5, and 6 positions. Rather than relying on conventional limited-site substitution, this multi-position strategy allows the conjugation framework to extend more effectively across the molecular backbone. The incorporation of dimethylaminophenyl and thiophene fragments, linked through C = C and C C bridges, generates a rigid and highly delocalized It-system. Such structural reinforcement strengthens charge migration from the electron-rich dimethylaminophenyl units toward the BODIPY core, a tendency consistently supported by spectroscopy analysis and time-dependent density functional theory (TD-DFT) calculations. Under femtosecond excitation, BDP-750 exhibits a TPA cross-section of 3766 GM at 900 nm, placing it among the more efficient BODIPY-based systems reported to date. Simultaneously, pronounced negative nonlinear refraction is observed across 850 similar to 900 nm. The compound also displays efficient optical limiting behavior, particularly in media with strong refractive backgrounds. Limiting thresholds of 0.00393 J.cm(-2) in toluene and 0.00219 J.cm(-2) in carbon disulfide were recorded. In CS2, both the limiting threshold and switching dynamics show further improvement, suggesting that solvent-assisted refractive contributions play a non-negligible role. To clarify this effect, we established an equivalent optical limiting model integrating the finite element method (FEM) and beam envelope method (BEM). The simulation enables quantitative evaluation of how nonlinear refraction cooperates with absorption processes in thick samples, thereby isolating its contribution to the overall limiting response. Taken together, these results indicate that multi-site conjugation engineering offers a viable pathway toward high-performance ultrafast laser protection and all-optical switching materials.

Organic photovoltaic materials and nonlinear optical materials share inherent commonalities in molecular characteristics—such as strong light absorption, high charge carrier mobility, and tunable energy levels. Therefore, this study selects a bithiophene-fused ring system with photovoltaic application potential as the research subject. Using TTTTB6-2CHO (TB1) and IDTTB6-2CHO (TB2) as comparative molecules, their nonlinear optical properties in the near-infrared region were systematically investigated. Transient absorption spectroscopy results demonstrate that TB1 exhibits strong and persistent excited-state absorption within the spectral range of 650–900 nm, endowing it with excellent two-photon absorption performance (a cross-section of up to 5591 GM at 650 nm) and an ultralow optical limiting threshold (0.00147 J/cm2 under 800 nm femtosecond laser irradiation). The findings of this study not only confirm the feasibility of developing nonlinear optical materials from photovoltaic candidate molecules but also highlight the effectiveness of the “thiophene-for-benzene substitution” strategy in significantly enhancing optical nonlinearity. These results provide valuable design principles for the development of multifunctional organic optoelectronic materials, particularly for application scenarios such as laser protection.

Designing functionalized molecular structures to modulate intramolecular charge transfer (ICT) characteristics plays a pivotal role in significantly enhancing the optical nonlinearity of pyrene-based chalcone derivatives. Building upon this foundation, we designed three molecular systems (X4, X2, and X1) featuring diethylaminophenyl groups via one-dimensional and two-dimensional (1D and 2D) extension of the chalcone backbone, simultaneously regulating pyrenyl charge density and pi-pi* transition intensity. Experimental assessment of 1D and 2D extended structures revealed distinct variations in optical nonlinearity. Broadband femtosecond Zscan measurements (532-900 nm) and non-degenerate phase-object pump-probe (POPP) experiments demonstrated significantly enhanced nonlinear optical responses in X4 and X2 relative to X1, while key parameters including two-photon absorption cross-sections, excited-state absorption coefficients, and nonlinear refractive indices exhibited remarkable proximity between X4 and X2. From a charge transfer perspective, these observations can be rationalized by the saturation behavior of charge density on the central pyrene core with increasing numbers of electron-donating arms. This phenomenon is analogous to the finite volume of a container, wherein the saturated pyrene unit suppresses charge transfer, thereby limiting further enhancement of optical nonlinearity. These findings establish that while the 2D extended molecule system (X4) achieves greater absolute nonlinear enhancement, the 1D extended molecule system (X2) delivers a superior enhancement ratio per structural unit. This validates 1D extension as a highly efficient design strategy, providing valuable principles for optimizing pyrene-chalcone derivatives and related nonlinear optical materials.

Fully understanding and modulating the nonlinear absorption in GaN are crucial for designing ultrafast photonic devices. In this work, both the ultra-broadband transient absorption spectra and carrier recombination time in GaN were found to be significantly altered by carbon defects. An energy band model for carbon defect dynamics was established based on transient absorption and photoluminescence spectroscopy. Our model discernibly reveals that CN and tri-carbon in GaN intricately modulate both the absorption spectrum and carrier capture process: The rapid capture of holes by the CN defect significantly reduces the hole recombination time to hundreds of femtoseconds in the near-infrared band. Conversely, the tri-carbon defect exhibited a higher absorption cross section by an order of magnitude than that of free carrier in the visible region with a long carrier recombination time. This work clarifies the modulation mechanisms of complex carbon defects in GaN's nonlinear absorption and provides scientific guidance for designing broadband and integrated ultrafast optical nonlinear devices.

The robust Cr-(III) metal-organic frameworks (MOFs) have attracted considerable interest in water adsorptions. However, the synthesis of highly crystalline Cr-MOFs is complicated by the kinetically inert nature of Cr-(III). The main method for the preparation of Cr-MOFs now is postsynthetic metathesis, which is all consuming. Here, we report the de novo synthesis of a series of multi-module, single-crystalline Cr-MOFs at the micro scale (up to 6 μm). A dual-modulator (HF-pyridine) strategy is developed to modulate the multiple competitive coordination bonding in dual-ligand-based Cr-MOFs, with the secondary modulator of pyridine having the functional group of the secondary module of tripyridyl ligands, which thus acts as an additional inhibitor to nucleation. The high crystallinity has enabled the structure determination of a novel Cr-MOF (Cr-bpdc-tph) at atomic resolution by the continuous rotation electron diffraction (cRED) technique. Cr-bpdc-tph integrates both high porosity with a BET surface area up to 3191 m2/g and exceptional chemical resistance in extreme acidic and basic solutions (pH < 0 and pH > 14). The water adsorption results show that Cr-bpdc-tph has achieved an effective working efficiency of 76% within the relative humidity range of 45-65%.

The nonlinear optical properties of fused-ring electron acceptors are enhanced by extending their conjugation length. And IEICO-4F with a narrow bandgap exhibits remarkable laser protection performance in the near-infrared region.

Inspired by the promising nonlinear optical (NLO) properties of acene-based and thiophene-containing materials, we investigate the enhancement achieved by incorporating a benzene ring into cyclopentadiene-thiophene fused cores, aligning with non-fullerene acceptors in solar cells. We examine two non-fullerene films, IHIC and ITIC-DM, using femtosecond transient absorption and Z-scan experiments to evaluate their ultrafast NLO properties. Our results demonstrate that ITIC-DM exhibits superior nonlinear absorption (NLA) and nonlinear refraction (NLR), attributed to enhanced π–π* transitions. ITIC-DM also exhibits a significant improvement in NLO properties compared to other high-performance organic thin films. This study provides a new strategy for designing high-performance nonlinear optical organic materials.

In this research, we studied the ultrafast dynamics and phase transition of VO2 films with different thicknesses. Using transient absorption spectroscopy, the two VO2 films with thicknesses of 59.5 and 148 nm showed broadband excited state absorption from 450-750 nm, and the maximum absorption peak was at similar to 450 nm. Under low excitation fluences, the VO2 films exhibit three sequential dynamic processes with different time scales from femtoseconds (fs) to nanoseconds (ns). Depending on the film thickness and excitation fluence, a phase transition is observed in the 148 nm VO2 film at a time scale of similar to 21 ps when the excitation fluence reaches 1.27 mJ cm(-2). However, the 59.5 nm VO2 film did not undergo a phase transition in this fluence. The femtosecond nonlinear optical properties of the two VO2 films were also investigated via a Z-scan technique. The nonlinear absorption coefficients and nonlinear refractive coefficients of the VO2 films at a wavelength of 515 nm are beta(eff)(148nm) = 1220 +/- 100 cm GW(-1), beta(eff)(59.5nm) = 1020 +/- 90 cm GW(-1), gamma(eff)(148nm) = -4.3 +/- 0.7 x 10(-3) cm(2) GW(-1), and gamma(eff)(59.5nm) = -2.1 +/- 0.5 x 10(-3) cm(2) GW(-1), respectively. Compared with the reported values in the near-infrared wavelengths, the nonlinear absorption coefficients exhibit an order of magnitude enhancement.

We report the wavelength dependencies of the two-photon absorption coefficients β as well as the nonlinear refractive index n2 of undoped β-Ga2O3 single crystal in the spectral range 350–515 nm (0.51 < Ephoto/Eg < 0.75). Femtosecond Z-scan measurements show that n2 decreases monotonically toward the shortwave side, while the maximum value of n2 is 5.0 × 10−19 m2/W near half of the bandgap, Eg. Interestingly, a second upward trend in β was observed when the incident photon energy is larger than 0.6Eg, indicating the impact of additional inter-band transitions with higher energy. A higher-energy band-to-band transition around 6.0 eV was determined by both femtosecond nondegenerate two-photon absorption spectroscopy and ab initio calculations. Our results could provide guidance for designing nonlinear β-Ga2O3 photonic devices in the UV-visible spectral range.

A simple and effective single-shot pump-probe technique is reported for studying the ultrafast dynamic processes in various materials. Using only two commercial gratings, a large time window of ~ 95.58 ps is spatially encoded in a single probe pulse, and single-shot time-resolved measurements are implemented. This time window exceeds the maximum reported values for single-shot pump-probe techniques using the echelon or angle beam encoding strategy. The phase difference problem in the echelon encoding strategies is also eliminated and a customized echelon is not needed in this technique. The ultrafast dynamic processes of ZnSe and indolium squaraine at a wavelength of 650 nm were investigated using this technique.

The substitution of heteroatoms and the expansion of pi-conjugated units have significant effects on the photoelectric properties of polycyclic aromatic hydrocarbons (PAHs). In this study, based on the experimental molecule PBC, 10 acene derivatives containing the pyrrole group were designed by three strategies: (1) changing the connection position of the pyrrole group, (2) exchanging the position of the N atom, and (3) increasing the length of the pi-linker. Density functional theory (DFT) was used to optimize the molecular geometric structure. Time-dependent density functional theory (TD-DFT) was used to calculate the relevant parameters of the excited states. The results show that both of the pyrrole group and the N atom in the para-position, and the addition of the pi-linker can reduce the energy gap, cause redshift of the linear absorption peak, and increase the two photon absorption (TPA) cross-section. The analyses of the charge density difference (CDD) and the charge-transfer matrix (CTM) proved that the electron transfer is mainly concentrated in the pi-linker. Moreover, the participation of the multi-ring skeleton on both sides decreases gradually with the increase of the length of the pi-linker. The length of the pi-linker changes the dominant transition channel and intramolecular charge-transfer (ICT) characteristics of TPA, thus affecting the transition dipole moments and nonlinear absorption properties. The second hyperpolarizability of the designed molecule PBI5-p5 is also significantly superior to that of similar materials reported. It is expected that the above molecular design strategies and comprehensive analysis of nonlinear optical (NLO) properties could provide theoretical support for the research on acene derivatives.

Squaraines (SQs) are a significant family of near-infrared dyes with applications in fluorescence bioimaging, photodynamic therapy, etc. Most efforts have been devoted to modulating their nonlinear absorption while their nonlinear refraction has been neglected. Here, near-infrared broadband nonlinear absorption and refraction in two squaraine derivatives (SQ-640 and SQ-670) are investigated with Z-scan (720-1300 nm) and time-resolved pump-probe method. With essentially similar conjugation and planarity in the two molecules, the effect of alkyl cyclization on the nonlinear optical response is discussed. A larger transition dipole moment is reached via alkyl cyclization and results in a wider range of nonlinear optical responses for SQ-670. In addition to conventional transient absorption measurement, transient nonlinear refraction based on phase contrast pump-probe method are used for the study of ultrafast dynamics. Kinetic details from both measurements are compared and analyzed for further details of ultrafast photophysical mechanisms in these molecules. Besides, an intensity-dependent transition from saturable absorption to reverse saturable absorption in SQ-670 is observed at 720 nm which is mainly due to the combined contributions of one-photon absorption and TPA. Both samples display good nonlinear absorption (NLA) at the near-infrared region (750-1300 nm) which originates from two-photon absorption (TPA) and TPA induced excited state absorption. Bound electron response and negative excited state refraction dominate the nonlinear refraction of both molecules. Our work gains a deeper understanding of the NLO properties of squaraine derivatives from an alternative perspective and enriches the experimental research on squaraine derivatives. It may provide a useful reference for the future design of organic materials with excellent nonlinear optical responses.

A thorough investigation was conducted into the impact of pi-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/cm(2) (at 800 nm) and 0.0033 J/cm(2) (at 1030 nm). Quantum chemistry calculations utilizing density functional theory (DFT) show that pi 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.

Sandwich-type materials with tunable optical response and enhanced optical nonlinearity are of great importance for future nanophotonic devices. Herein, we investigate the third-order nonlinear optical response and transient dynamics of a series of CdO/Ag/CdO sandwich-type films with different Ag sputtering time. SEM images demonstrate that the Ag layer in CdO/Ag/CdO sandwich-type films change from a nanoisland to semicontinuous film with increase of Ag sputtering time. Z-scan measurements indicate that the CdO/Ag/CdO sandwich-type films exhibit huge nonlinear optical response on both femtosecond and picosecond time regime. The maximum nonlinear absorption coefficient (& beta; = 1.605 x 10-8m/W) and nonlinear refractive index (n2 = -6.70 x 10-15m2/W) of the CdO/Ag/CdO film with 4 min Ag sputtering time are obtained in picosecond Z-scan, which are about 17 and 10 times greater than that of the single-layer CdO, respectively. Transient absorption spectroscopy results confirm the effect of increased CdO/Ag/CdO interface binding on the thermal electron transfer mechanism. A model is proposed to describe the ultrafast dynamic processes in the CdO/Ag/CdO sandwich-type films. Our results indicate the CdO/Ag/CdO sandwich-type films with tailored nonlinear optical properties are highly promising for applications of optical switches devices.

GaN is a one of promising materials for nonlinear optical applications. In this work, the broadband nonlinear optical response and potential applications for all-optical switching (AOS) are evaluated in low-defect GaN. In the pump-probe experiments, the ultrafast optical switching times are consistent with pulse widths accompanied with relative weak free-carrier absorption response, and the modulation contrast can reach ∼60% by varying the polarization orientations between the pump and probe lights. In the visible region, the broadband two-photon absorption effect exhibits excellent values for the imaginary part of figure of merit (FOM), providing the possibility of AOS based on nonlinear absorption (magnitude). While in the near-infrared region and under the presence of three-photon absorption, not only the real part of FOM based on Kerr effect is evaluated, but also the maximum light intensity for the usage of AOS based on nonlinear refraction (phase) is determined. The broadband nonlinear optical and AOS features in low-defect GaN will be highly favorable for the applications in the field of integrated nonlinear photonics and photonic circuits.

Optical nonlinearities of two all-carbon twistacenes, DPyA and DPyN, with the different π-conjugated central bridges were investigated. The nonlinear absorption properties of these compounds were measured using the femtosecond Z-scan with wavelengths between 650 and 900 nm. It has been found that the nonlinear absorption originated from two-photon absorption (TPA) and TPA-induced excited state absorption (ESA), wherein DPyA demonstrates higher performance than DPyN. The TPA cross section of DPyA (4300 GM) is nearly 4.3 times larger than that of DPyN at 650 nm. Moreover, the different central structures modulate the intensity of ESA at 532 nm, and DPyA exhibits an excellent ESA at 532 nm with multi-pulse excitation. Meanwhile, the result of data fitting and quantum chemistry calculation shows that the enhancement of nonlinear absorption in DPyA is due to the extended π- conjugated bridge and improved delocalization of π-electrons. These all-carbon twistacenes could yield potential applications in optical power limiting (OPL) technology.

End-substituted planar and twisted isomers (C1 and C2) of pyrene-containing twistacenes were designed and synthesized to investigate the modulation of ectopic substitution on nonlinear optical (NLO) properties. Results of ultrafast transient absorption spectra reveal that C1 exhibits broad-band excited state absorption (ESA), meanwhile C2 displays an additional electronic delocalized state due to charge transfer. The twisted structure caused by large steric hindrance in C2 is thought to be the cause of enhancement according to the density functional theory calculation. Moreover, the evolution of ESA regarding the transition from local excited (LE) to intramolecular charge transfer (ICT) state is observed in the transient spectra. The performance of reverse saturable absorption at 480 nm, 5 ns, is improved by ESA originating from ICT in C2. The extension of It -con-jugated system results in a significant enhancement of the two-photon absorption (TPA) cross-section of C1/ toluene from 235 to 737 GM at 532 nm. With different substitution positions of terminal group in isomers, TPA and ESA of these chromophores can be dramatically modulated. This work may provide an idea for the opti-mization of nonlinear functional isomeric materials.

Modifying simple molecular structures to significantly improve nonlinear optical (NLO) performance is a primary prerequisite for scientific research. Based on the four phenylamine derivatives reported in previous studies, we designed four organic nonlinear molecules by changing the acceptor group and π-linker. (Time-dependent) density functional theory (DFT/TD-DFT) was performed on molecular geometry optimization, the contribution of π electrons to the bond order, linear and two-photon absorption (TPA) spectra, the intra-molecular charge transfer matrix (CTM), and NLO coefficients. These aspects were considered to analyze in detail how the structural modification of acceptors and π-linkers affects NLO characteristics. The three modification methods were: adding a carbonyl group at the junction of the π-linker and the acceptor group, adding a carbonyl group and a nitrogen atom to the acceptor group, and replacing the quinolinone with a pyrenyl group as the π-linker. The latter two methods can significantly reduce the excitation energy and enhance the intensity of intra-molecular charge transfer during the two-photon transition. The maximum TPA cross-sections and wavelengths of the designed molecules are DPPM (84722.6 GM, 815.7 nm) and DDPM (21600.6 GM, 781.3 nm). These two molecules have large TPA cross-sections in the near-infrared region, which renders them as possible NLO materials with broad application prospects.