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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. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Nonlinear absorption and OL properties of multi-branched molecules were explored. • NBTT still shows strong RSA properties at the excitation wavelength of 1030 nm. • NBTT has excellent OL performance at 800 nm and 1030 nm. • Malonitrile substitution extends the CT range, improves NLO performance. [ABSTRACT FROM AUTHOR]

Abstract: Four polyacetylenes containing chromophores with different conjugation bridge structure or terminal substituents were designed and prepared by using [Rh(nbd)Cl]2 as catalysts, respectively. Their structures and properties were characterized and evaluated by IR, NMR, UV, TGA, optical limiting and nonlinear optical analyses. All the polymers show high thermal and photo stability and novel optical limiting properties. The functional polyacetylene with stilbene pendant shows better optical limiting property than that with azobenzene chromophore pendant. Their optical limiting mechanisms are mainly originated from reverse saturable absorption of molecules. Their nonlinear optical properties are significantly affected by their molecular structures and the shorter spacer group will be beneficial in increasing the electronic interaction between chromophore pendant and polyacetylene conjugation backbone to result in higher third-order optical nonlinearity. [Copyright &y& Elsevier]

[Display omitted] • Investigate the optical nonlinearities of SQ-640 under variouswavelengthsand pulse widths. • Displayself-focusing at visible wavelength andself-defocusing at NIR wavelengths. • Discusstheoriginof optical nonlinearitiesvia transient absorption and refraction dynamics. • The results suggest SQ-640 is a potential material for NLR based devices. The abundant nonlinear optical responses of a squaraine derivative (2, 4-bis[4-(diethylamino)-2-hydroxyphenyl]squaraine, SQ-640) are explored at visible and near-infrared (NIR) regions. The sign and magnitude of the third-order (intensity depend) nonlinear absorption and refraction for SQ-640 are determined under different pluses with Z-scan method. The results of this work show reverse saturable absorption (RSA) of SQ-640 at visible and NIR region and its potential for optical limiting (OL) application. Moreover, SQ-640 shows self-focusing at visible wavelength while shows self-defocusing at NIR wavelengths. The transient refraction dynamics indicate that positive excited-state refraction causes self-focusing at visible region while negative nonlinear refraction at NIR region mainly originates from bound electron response (Kerr effect) and negative excited state refraction. Besides, the nonlinear refraction index of SQ-640 under 532 nm ranges from ∼ 10-19 m2/W at femtosecond pulses to ∼ 10-16 m2/W at nanosecond pulses and the second order hyperpolarizability is about 4 × 10-28 esu at nanosecond case. The Kerr refraction coefficient is about −6.54 × 10-19 m2/W at 700 nm under femtosecond laser pulses. The obtained OL thresholds at 532 nm are ∼ 26.8 mJ/cm2 for picosecond pulses and ∼ 85.6 mJ/cm2 for nanosecond pulses. The enriching information extracted from NLR in SQ-640 may offer an optional perspective to understand the structure–property relationships in these compounds. The results of optical nonlinearities in SQ-640 gained from this study may be of assistance to its better applications in laser protection and optical switching. [ABSTRACT FROM AUTHOR]

Highlights • Composite thin films of naphthalocyanines and TiO 2 nanoparticles were prepared. • Deposition of high quality thin films by layer-by-layer assembly method. • The films exhibit strong optical limiting response and good thermal stability. • The interaction between naphthalocyanines and TiO 2 causes the nonlinear enhancement. Abstract In this work, an ultrathin nanocomposite films containing tetracarboxylic Nickel (Ⅱ) naphthalocyanine (NiNc) and titania nanoparticles are fabricated through electrostatic self-assembled layer-by-layer (LBL) technique. UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) are utilized to characterize the film. The optical limiting properties of the composite films are investigated through intensity-dependent transmittance measurements at 600 nm with 18 ps laser pulses. The results show that the composite films have notable optical limiting response (β ∼ 5.4 × 10−7 m/W), which is strongly enhanced compared to both TiO 2 and NiNc-only samples. Our results suggest that the nanocomposite films possess the potential for optical limiting applications in the phototherapeutic window. [ABSTRACT FROM AUTHOR]

Spindle-type molecules containing twisted acenes ( PyBTA-1 & PyBTA-2 ) are designed, synthesized characterized. Picosecond Z-scan experiments under 532 nm show reverse saturable absorption and negative nonlinear refraction , indicating large third-order optical nonlinearity in PyBTA-1 . The mechanism of the optical nonlinearity is investigated and the results show that the nonlinear absorption and refraction in PyBTA-1 originates from a charge transfer (CT) state. Furthermore, relatively long lifetime and absorptive cross section of the CT state are measured. Based on the excited state absorption in PyBTA-1 , strong optical limiting with ∼0.3 J/cm 2 thresholds are obtained when excited by picoseconds and nanoseconds pulses. The findings on nonlinear optics suggest PyBTA-1 a promising material of all optical modulation and laser protection, which enrich the potential applications of these spindle-type molecules. Comparing to the previously reported spindle-type molecules with analogous structures, the introduction of ICT in PyBTA-1 & PyBTA-2 dramatically decreases the two-photon absorption while enhances the nonlinear refraction. The results could be used to selectively tailor the optical nonlinearity in such kind of compounds. [ABSTRACT FROM AUTHOR]

• Al:CdO thin films with different doping concentrations were designed and synthesized. • The structure and properties of the film were studied. • The change of Al concentration can effectively modulate the nonlinear optical properties of Al:CdO films, and the maximum nonlinear absorption coefficient is 2 × 10−6 m/W. • The results of the study on the carrier dynamics characteristics of the films show that the relaxation lifetime of the carriers, the absorption cross-section of the carriers and the refraction volume of the carriers in the high-aluminum-concentration films are all increased. Al:CdO thin films with different aluminum doping concentrations were prepared via magnetron sputtering technique with various sputtering power. It is found that the band gap E g of Al:CdO films gradually increases with the increase of Al doping concentration, while the crystallinity became poor at high doping concentration. The nonlinear absorption properties of the film were characterized by nanosecond Z-scan measurements. The Z-scan results show that the nonlinear absorption coefficient β of Al:CdO thin films could be effectively modulated by Al concentration, and the maximum of β (2 × 10−6 m/W) is achieved around 20 W sputtering power. The carrier dynamics of Al:CdO thin films were studied by femtosecond pump–probe measurements. The results show that the carrier relaxation lifetime, carriers absorption cross section and carriers refraction volume are larger in the high Al concentration film. The optical limiting properties of Al: CdO films were also studied. These results demonstrate that the Al:CdO film is a promising candidate for applications in all optical switches and laser devices. [ABSTRACT FROM AUTHOR]

• Investigate the modulation of optical nonlinearities by ICT and planar π conjugation in hydrazone derivatives. • The broadband (600–900 nm) RSA resulting from TPA-induced ESA. • The intensity-dependent nonlinear absorption under multiple pulse widths (fs, ps, ns) was explored. • Transient absorption spectroscopy revealed the ultrafast dynamics processes and discussed in detail. Two novel hydrazone derivatives (BTH-1 and BTH-2) were designed and synthesized to investigate the modulation of intramolecular charge transfer (ICT) and planar π conjugation on optical nonlinearities. Femtosecond Z-scan measurements show that trithiophene-substituted BTH-2 exhibits stronger broadband (600–900 nm) reverse saturable absorption (RSA) resulting from two-photon absorption (TPA)-induced excited state absorption (ESA). Combined with quantum chemical calculations, the results indicate that the local excitation and spatial delocalization enhanced by planarized trithiophene are more conducive to promoting the nonlinear optical (NLO) response of hydrazone molecules than the ICT introduced by twisted triphenylamine. In addition, the intensity-dependent transition from saturable absorption to RSA of BTH-2 at 532 nm under multiple pulse widths (fs, ps, ns) was explored. Transient absorption spectroscopy provided information on the excited state dynamics of the compounds. This work extends NLO research on hydrazone derivatives and provides a reference for the design of hydrazone molecules with high nonlinearity. The results also suggest that BTH-2 is a promising candidate for optical limiting applications. [ABSTRACT FROM AUTHOR]

The ultrafast optical nonlinear properties of an indolium squarine 1,3-bis[(3,3-dimethylindolin-2-ylidene)methyl]squaraine (ISQ) are systematically investigated via multiple pulse-width Z-scan technique and femtosecond transient absorption spectrum. The results indicate that the sample exhibits superior excited-state absorption which mainly originates from the singlet excited-state. The relatively long lifetime measured in ISQ provides efficient excitons accumulation on excited-state, making it suitable for optical limiting (OL) under various pulse duration. The nonlinear absorption coefficient of ISQ ranges from ∼10−8 m/W with nanosecond (ns) pulse to ∼10−12 m/W with femtosecond (fs) pulse. ISQ film is also prepared via doping ISQ into polymethyl methacrylate, the OL thresholds of ISQ solution and ISQ film are measured with 532 nm ns pulse. The OL threshold of ISQ /toluene is 25.5 mJ/cm2, and that of the film is 30.6 mJ/cm2, which basically retain the OL performance of solution. Our results indicate that indolium squaraine could be a potential candidate for future application in laser protection. [Display omitted] • Investigate the optical nonlinearities of ISQ solution/film. • The superior RSA originates from excited-state absorption under multiple pulse widths. • Low OL threshold in ns pulse suggests promising application in laser protection. [ABSTRACT FROM AUTHOR]

There is immense interest in the design of organic nonlinear optical (NLO) chromophores for optical limiting and bioimaging applications. In this work, two near-infrared hemicyanine compounds which are named 2-(2-(3-Hydroxy-8,9-dihydro-7H-pyrido[1,2-a]quinazolin-10-yl)vinyl)-1,1-dimethyl-3-propy l-1H-benzo[e]indol-3-ium iodide (Hemicyanine1) and 1,1-Dimethyl-2-(2-(3-methyl-8,9-dihydro-7Hpyrido[1,2-a]quinazolin-10-yl)vinyl)-3-propyl-1H-benzo[e]indol-3-ium iodide (Hemicyanine2) were synthesized. The difference between Hemicyanine1 and Hemicyanine2 is that one of the auxochrome is hydroxyl and the other is methyl. The third-order nonlinear optical absorption properties of Hemicyanine1 and Hemicyanine2 were investigated using broadband Z-scan measurements at nanosecond time regime. The Z-scan results demonstrate that both chromophores have reverse-saturable absorption (RSA), while the NLO response of Hemicyanine2 is significantly stronger than Hemicyanine1. The corresponding physical mechanism were studied via femtosecond transient absorption (TA) spectroscopy. The results of femtosecond TA measurement demonstrate that the lifetime of excited state of Hemicyanine2 is considerably longer than that of Hemicyanine1. Our study indicated that the auxochrome is critically important in optimizing the NLO response and excited-state lifetime of hemicyanine chromophores for photonic applications. • Two novel near-infrared hemicyanine dyes Hemicyanine1 and Hemicyanine2 which have different auxochromes Hydroxyl and methyl were synthesized to investigate the third-order nonlinear optical properties and ultrafast dynamics. • Both compounds showed reverse saturable absorption and obvious differences under the excitation of nanosecond time regime. • The optical limiting performance and possible damage threshold were investigated on Hemicyanine2. • Femtosecond transient absorption spectra revealed the ultrafast dynamics. • By changing the auxochrome, the lifetimes of excited states can be adjusted to improve the reverse-saturable absorption of Hemicyanine2 , which is significantly enhanced compared to Hemicyanine1. [ABSTRACT FROM AUTHOR]

The ultrafast optical nonlinearities of two novel naphthanthryl chalcone derivatives (NAn-1 and NAn-2) with different push-pull effects were investigated and compared. Combined with femtosecond Z-scan and quantum chemical calculations, the results indicate that both compounds exhibited broadband (532–800 nm) two-photon-induced excited-state absorption, and NAn-2 exhibited wider spatial electron delocalization. The push electronic effects of the peripheral substituent increased the transition dipole moment, originating from the main nonlinear group retained stronger local excitation and ultimately enhanced the nonlinear absorption of NAn-2. Transient absorption spectroscopy revealed a transition from the locally excited to charge transfer state and showed that the dynamic processes of excited states were readily tunable via peripheral push-pull substituents. The results suggest that NAn-2 possess the potential in optical limiting applications and provide guidance for designing polycyclic aromatic hydrocarbon (PAH)-based nonlinear optical materials. • Two PAHs chalcone molecules were designed and synthesized to study their NLO performance. • Both compounds exhibited broadband (532–800 nm) RSA resulting from TPA-ESA. • Peripheral push electronic effect causes the strong local excitation and ultimately enhances transition dipole moment and nonlinearity. • Transient absorption spectroscopy revealed dynamic process of excited state. [ABSTRACT FROM AUTHOR]

Abstract: A series of organic/inorganic molecular hybrid nonlinear optical materials were prepared by incorporating azobenzene nonlinear optical chromophores into nanosized, inorganic, polyhedral oligomeric silsesquioxane. Each of the hybrids was soluble in common solvents such as CHCl3, THF, toluene and C2H4Cl2, and exhibited good film-forming properties. Their structures and properties were characterized using FTIR, 1H NMR, 13C NMR, 29Si NMR, UV, TGA, DSC and nonlinear optical analyses, respectively. All hybrid compounds displayed good optical limiting properties, large nonlinear optical response and high thermal stability. The optical limiting mechanism was investigated. [Copyright &y& Elsevier]