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The effect of Fe defects on carrier recombination and two-photon induced ultrafast exciton dynamics in GaN crystals were investigated using femtosecond transient absorption spectroscopy. The absorption kinetics exhibited completely different characteristics under different nonequilibrium carrier concentrations and distributions. The carrier recombination mechanisms under different excitation conditions can be interpreted by the model of Fe defect-related energy level. The absorption under one-photon excitation originates from the intraband transition of free holes, and the capture of holes by Fe2+ leads to the decay of the absorption. The ultrafast excitonic state [Fe2+,hVB] formation time under two-photon excitation results in a rapid absorption decay (<20 ps), while the long tail of the absorption response could be attributed to long-lived excitonic state (>100 ns). These findings are crucial to the applications of GaN for ultrafast optoelectronics and integrated nonlinear optics. [ABSTRACT FROM AUTHOR]

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]

Using the Z-scan technique with 532 nm 4 ns laser pulses at various energy levels and repetition rates, we investigated the optical nonlinearities of tetra-tert-butylphthalocyaninatogallium iodide (t-Bu)4PcGaI. As a result, we found that the soluble peripherally tert-butyl-substituted phthalocyanine bound with the core of gallium iodide exhibited a giant two-photon absorption coefficient and Kerr refractive index. This suggests the potential applications of this material in numerous fields of nonlinear optics. However, when tackling the large nonlinearly induced loss and phase distortion analytically, we failed to fit the corresponding experimental results with the existent series sum formulas, which retain only the first few terms in the expansion. We performed the fitting by retrieving the high-order terms and justified this approach by dealing with the light-matter interaction and wave propagation. [ABSTRACT FROM AUTHOR]

A simple and high-sensitivity technique is presented to investigate nonlinear refraction. In this technique, a combination of aperture and obscuration disk is introduced in the measurement system. Compared to the top-hat Z-scan, the curves of modified top-hat Z-scan for the nonlinear refraction show a single peak. Furthermore, the sensitivity of this new technique can be more than two orders of magnitude enhanced. Nonlinear refraction of Nd-doped phosphate glass is investigated using this technique with 19 ps pulses at wavelength of 532 nm. [ABSTRACT FROM AUTHOR]

Abstract: A reversal 4f coherent imaging system with phase objects is presented to measure nonlinear refraction of the materials. The modified system can increase the sensitivity compared with the conventional nonlinear-imaging technique with phase objects. The sensitivity enhancement of the modified system is about two times greater than the conventional technique within 0≤ Δφ NL ≤1. CS2 is used to demonstrate the measurement using the reversal 4f coherent imaging system with phase objects. [Copyright &y& Elsevier]

The time-resolved nondegenerate pump-probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532 nm laser beam with 21 ps duration is used as the excitation and the laser beams of 600 and 680 nm with 10 ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump-probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined. [ABSTRACT FROM AUTHOR]

Abstract: A one-laser-shot measurement technique, which can characterize the nonlinear optical properties of the film prepared on the opaque substrates conveniently and exactly, is presented. This method is based on the reflection 4f coherent imaging system with phase objects. The proposed technique is examined in the case of a novel ultrathin film [CuPc(COONa)4]/PDDA coated on an opaque silicon substrate with 532nm picosecond laser pulses. By measuring the reflected intensity, but not the transmitted one, we verify both nonlinear absorption and refraction of the coated thin film. This progress obviously ensures that nonlinearities caused by reflection of opaque surfaces are measurable. [Copyright &y& Elsevier]

Abstract: The nonlinear optical (NLO) properties of a novel cluster Polymer {WS4Cu4I2(bpe)3} n solution are studied by using Z-scan technique with laser pulses of 4.5ns pulse-width at a wavelength of 532nm. The results show that the cluster solution possesses strong nonlinear absorption and refraction. Nonlinear refraction of the cluster is composed of third-order nonlinear refraction and transient thermal effect. The thermal effect is mainly due to the strong nonlinear absorption. Numerical simulations obtained by solving simultaneously photo-acoustic and electromagnetic wave equations, agrees basically with experimental results. [Copyright &y& Elsevier]

Two benzothiazole derivatives, neutral benzothiazole derivative and the corresponding benzothiazolium salt, were designed and synthesized. Broadband reverse saturable absorption (RSA) were observed for both compounds in different solvents under femtosecond Z-scan, and the range of RSA of benzothiazolium salt showed larger redshift compared with that of the neutral benzothiazole. However the variations of absorption properties of these samples in femtosecond and picosecond time regimes were different. The transient absorption spectrum reveal the conversion process from the initial local excited (LE) state to the charge transfer (CT) state and the nonlinear optical response of two compounds in different time regimes were analysed. • The benzothiazole derivatives containing pyrene have large nonlinear responses. • Substitution of benzothiazole by benzothiazolium results in large redshift in linear absorption and two-photon absorption. • Conversion from the local excited state to the charge transfer state was present in both samples. • The benzothiazolium is more affected by different time regimes and solvents than the neutral benzothiazole. [ABSTRACT FROM AUTHOR]

Two benzothiazolium salts of D-π-A structure, (E)-3-ethyl-2-(2-(pyren-1-yl) vinyl) benzo[d]thiazol-3-ium iodide (EPBI) and (E)-3-ethyl-2-(2-(9-ethyl- 9H -carbazol-3-yl) vinyl) benzo[d]thiazol-3-ium iodide (ECBI) were synthesized and spectroscopically characterized. Z-scan measurements were performed at 532 nm with 190 femtosecond and 15 picosecond laser beam respectively. EPBI displays the efficient third-order nonlinear optical susceptibility χ ( 3 ) of 1.5 × 10 −11  esu and molecular second-order hyperpolarizability γ of 2.8 × 10 −29  esu which are one order larger than those of ECBI, and EPBI exhibits reverse saturable absorption and self-defocusing refraction while ECBI exhibits saturable absorption and self-focusing refraction in 15 ps time mechanisms. In 190 fs time mechanisms, both of them exhibits saturable absorption. Further dynamics analysis for these transition in different time regimes and determination of key parameters were performed via DFT calculation and transient absorption experiment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Study of the optical nonlinearity of two thiophene derivatives with different donors. • Discussion of photodynamic evolution in different solvents by transient absorption. • Z-scan experiment shows that 3-TH have strong excited state absorption. • The nonlinear absorption coefficients are about 10-9m/W (4 ns) and 10-11m/W (21 ps). 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. [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]

• The surface topography and crystallization degree of CdO/Ag/CdO films are successfully regulated by the insertion of Ag layers. • The insertion of the Ag layers enhances the electrical transport characteristics of CdO/Ag/CdO sandwich-type films. • An enhanced thermal electron transfer mechanism emerges between the CdO layer and the Ag layer. • CdO/Ag/CdO sandwich-type films have large nonlinear absorption and nonlinear refraction. 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 semi-continuous 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 (β = 1.605 × 10-8m/W) and nonlinear refractive index (n 2 = -6.70 × 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. [ABSTRACT FROM AUTHOR]

The ultrafast nonlinear optical (NLO) properties of novel planar π-conjugated chalcone derivatives, (E)-3-(4-nitrophenyl)-1-(3-bromo-4-fluorophenyl) prop-2-en-1-one (1503) and (E)-3-(4-nitrophenyl)-1-(phenothiazin-2-yl) prop-2-en-1-one (1526) are investigated and compared. The effect of intramolecular charge transfer on NLO absorption and refraction is discussed without changing the planar structural properties of the chalcone molecules. Transient absorption experiments are used to observe the evolution of 1526 from local excited (LE) state to the intramolecular charge transfer (ICT) state. In the Z-scan experiments, 1526 exhibits respectively different nonlinear absorption (NLA) mechanisms (reverse saturable absorption and two-photon induced excited state absorption) at 532 nm (4 ns, 22ps, 190 fs) and 700–900 nm, 190 fs. The NLA and nonlinear refraction (NLR) phenomena of 1526 are observed and discussed in the POPP experiments. The experimental results indicate that the chalcone derivative can become the potential materials for laser protection. [Display omitted] • Two planar chalcone derivatives (1503 and 1526) are designed and synthesized. • Large enhancement of nonlinear optical absorption and refraction by terminal substitution. • The chalcone derivative 1526 can become the potential materials for laser protection. [ABSTRACT FROM AUTHOR]

[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 • Two π-conjugated pyrene derivatives were synthesized successfully. • The benzene peripheral substituent enhance the third-order optical nonlinearity. • The difference phenomenon for excited-state lifetime between both compounds. • Detailing description on transient absorption to interpret the dynamic process. Abstract In this work, two pyrene derivatives with delocalized π-electron system 1-(pyren-1-yl)−3-(5-Chlorothiophene-2-yl)acrylic ketone (1612) and 1-(pyren-1-yl)−3-(5-Phenylthiophene-2-yl) acrylic ketone (1613) were synthesized. The third-order nonlinear optical (NLO) response of these compounds dissolved in DMSO were investigated using Z-scan technique with 190 fs laser pulses at 532 nm. Large two-photon absorption was found for the two compounds, while the NLO response of 1613 was larger than 1612. The femtosecond transient absorption (TA) results demonstrate that both compounds have broad TA absorption band over the visible regime. Compared to 1612 , the TA band of 1613 exhibit a 15-nm blue shift of its peak while its UV–Vis absorption peak have 50 nm red-shift. Meanwhile, the TA decay curve also show that the excited-state lifetime of 1613 is longer, indicating the substituent group functionalization also has a significant impact on the excited-state relaxation processes. Our results show that both the NLO response and photo-physical dynamics of pyrene chromophores are readily tunable via peripheral substituent group, which is essential for the development of novel NLO materials based on this conjugated molecules. [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]

Two asymmetric pyrene derivatives (E)-1-(4-(dimethylamino)-3-(pyren-1-yl)prop-2-en-1-one) ( DAPP-1 ) and (E)-3-(4-(diemthylamino)phenyl)-1-(pyren-1-yl)prop-2-en-1-one ( DAPP-2 ) were synthesized to access lossless broadband nonlinear refraction. Femtoseconds Z-scan from 600 nm to 1030 nm showed positive third-order nonlinear refractive index with high linear transmission (>90%). Both compounds had ≥2.5 figure of merit (F) at 1030 nm, while DAPP-2 held >1 F value in a wide spectral region. Mechanism of ultrafast refraction under off & on-resonant regime were investigated via degenerate & non-degenerate pump-probe with phase object, respectively. Results indicate pure response of bound electrons under off-resonance and excited-state enhanced nonlinear refraction under on-resonance. All the study suggested that optical nonlinearity in pyrene derivatives has been underestimated and molecules like DAPP-2 possess the potential to be further optimized into superior broadband nonlinear refractive materials. [ABSTRACT FROM AUTHOR]

Two novel compounds of the alternating donor (D)/acceptor (A) system were designed and synthesized. In BSB (A-π-D-π-A) and SBS (D-π-A-π-D), thiophene, 2,1,3-benzothiadiazole (BTD) and ethylene act as donor, acceptor groups and π-bridge moiety, respectively. Z-scan experiments at 532 nm with various pulse durations show a transition from saturable absorption to reverse saturable absorption and self-defocusing effects in both compounds. The different alternating modes of D-A units lead to an order of magnitude increment in the second-order hyperpolarizability γ of SBS (4.4 × 10−28 esu) compared to BSB. Ultrafast transient absorption spectroscopy was carried out to analyze the enhancement of second-order hyperpolarizability. Results indicate that this enhancement originates from excited state absorption of the long-lived intramolecular charge transfer state. Using quantum chemical calculations, the stronger symmetrical intramolecular charge transfer in SBS can be demonstrated by the difference between the ground- and excited-state dipole moments Δ μ ge. It is shown intuitively in the S 0 –S 1 hole-electron analysis. All of the experimental and theoretical results suggest that changing the D-A alternation in organic molecules can effectively modulate the third-order nonlinear optical behavior, which may serve as a guide for future molecular design. [Display omitted] • Two novel ethyne-linked donor-acceptor chromophores are designed and synthesized. • The third-order nonlinear optical behavior of two compounds is discussed by Z-scan and TAS. • The second-order hyperpolarizability γ of SBS (4.4 × 10−28 esu) is significantly enhanced by modulating the alternating mode of D-A groups. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two novel twistacenes isomers containing pyrene are designed and synthesized. • The ultrafast nonlinear absorption of both compounds is discussed by Z-scan and TAS. • The TPA of end-substituted planar molecular (C1) is significantly enhanced. • The ESA in twistacenes is modulated by differences in spatial structure. 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 π-conjugated 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 optimization of nonlinear functional isomeric materials. [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]

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]

Abstract: Signs change of nonlinear refractive index in ZnSe is observed by employing a modified double 4f imaging system at the wavelength of 800nm using picosecond pulses with different pulse energies. This process results from the competition of the bound electronic nonlinear refraction and the free carrier refraction. At low intensity, positive nonlinear refraction is obtained, which is attributed to bound electrons. As the increase of laser beam intensity, the nonlinear refractive index become small, and changes to negative. This is ascribed to free carriers generated by two-photon absorption. Additionally, the nonlinear refractive index of bound electron and the refractive index change of free carrier are determined unambiguously by a simple method. [Copyright &y& Elsevier]

Tetradentate organic ligand 1,2,4,5-tetrakis (4-pyridyl)benzene (tpb) is firstly developed to build cluster-containing metal-organic frameworks (C -MOFs). Two unusual W/S/Cu C -MOFs {[WS 4 Cu 4 I 2 (TPB)]·DMF} n (1) and {[WS 4 Cu 3 Cl(TPB)]·2DMF} n (2) have been synthesized and structurally characterized. 1 is fabricated by 8-connected pentanuclear planar open-shaped cluster [WS 4 Cu 4 ]2+, 4-connected tetrakis-pyridine TPB unit and I− bridge to form a three-dimensional (3-D) pillar-layered framework. Its (4,8)-connected topology firstly appears in W(Mo)/S/Cu C -MOFs. 2 is constructed by 4-connected T-shaped cluster [WS 4 Cu 3 ]+, 4-connected TPB unit and terminal Cl− ligand, and possesses a 2-D wave-like (4,4) network. Z- scan experiment reveals that 1 – 2 have third-order nonlinear optical (NLO) properties. Particularly, 1 exhibits significantly enhanced NLO performance. Moreover, 2 displays strong luminescence, highly sensitive and selective sensing of 2,4,6-trinitrophenol (TNP) in aqueous medium. The efficient emission quenching mechanism of 2 towards TNP is studied as well. W/S/Cu cluster and tetrakis (4-pyridyl)benzene-based metal-organic frameworks exhibit enhanced nonlinear optics and efficient TNP sensing properties. [Display omitted] • Tetrakis(4-pyridyl)benzene is firstly developed to build cluster-containing metal-organic frameworks. • The (4,8)-connected topology firstly appears in W(Mo)/S/Cu C -MOFs. • 1 – 2 exhibit enhanced NLO performance. • 2 displays efficient luminescence sensing of 2,4,6-trinitrophenol in aqueous medium. [ABSTRACT FROM AUTHOR]

• Four isomeric molecules with various torsion angles were investigated their NLO performance. • All molecules exhibited configuration-dependent RSA at 600 and 700 nm. • The electron spatial delocalization can be improved by modifying the π- conjugated system configuration. • Transient absorption revealed the ultrafast dynamics processes and discussed in detail. The nonlinearities involves the interaction between multiple sources, to minimize variable factors, four trithiophene chalcone isomeric molecules were designed and synthesized to investigate the modulation of torsion angle changes (6°, 25°, 37°, 60°) on optical nonlinearities. The electronic excitation properties and relevant parameters were calculated through quantum chemical methods. Femtosecond Z-scan measurements showed that the four molecules exhibited configuration-dependent reverse saturation absorption (RSA) at 600 and 700 nm and had ≥1 figure of merit (F) at 700 nm. Combined with the orbital delocalization index, the results indicated that the spatial delocalization of electrons is gradually suppressed with system twisting. Transient absorption spectra revealed a transition from the locally excited (LE) state to the charge transfer (CT) state, and the planarized structure promotes the relaxation of excited particles. This study suggest that these novel trithiophene derivatives possess potential in nonlinear optical applications and provide an approach for the optimization of nonlinear functional isomeric materials. [ABSTRACT FROM AUTHOR]

• The impact of solvent polarity on the NLO properties of a nickel-dithiolene complex (1) is investigated. • NLO absorption is studied via Z-scan and femtosecond transient absorption experiment. • The mechanism of selectively tuning optical nonlinearity in metal dithiolene complexes is discussed. The ultrafast optical nonlinear of a dithiolene complex of nickel (bis(1,2-diphenyl-1,2-ethenedithiolato-S,S')-nickel) (1) has been systematically investigated. Results of Z-scan with picosecond and nanosecond pulses show that compound 1 in N,N'-dimethylformamide (DMF) has a selective reverse saturable absorption (RSA), which could be observed with picosecond pulses and would be dramatically suppressed under ns pulses. The pulse-selective response vanishes in dichloromethane (DCM) and toluene solvents, indicating a connection with the decrease of solvent polarity. The ultrafast decay processes of excited state in different solvents are further investigated via femtosecond transient absorption (TA) experiment. The results are studied with global & target analysis and show that the excited state lifetime of 1 is remarkably shortened with the increasing polarity of the solvents, which suppresses the accumulation of excitons on excited-state and result in the substantially decrease of RSA in DMF solvent under nanosecond pulses. Our results are expected to shed light on the mechanism of selectively tuning optical nonlinearity in metal dithiolene complexes on an ultrafast time scale. [ABSTRACT FROM AUTHOR]

• Two bis-chalcone derivatives C1 & C2 are synthesized. • Third-order optical nonlinearity is investigated via Z-scan and TAS. • An ultrafast transition from SA to RSA is studied. In order to further exploit the optical nonlinearities in chalcone derivatives, two bis-chalcone molecules (C1 & C2) are synthesized. The third-order optical nonlinearities of both molecules are investigated via Z-scan. Enhanced nonlinear absorption and refraction are measured in C1 due to its superior planar π-conjugation over C2. Furthermore, an ultrafast sign change in both nonlinear absorption and refraction are revealed in C1 under femtoseconds and picoseconds lasers. A sub-picosecond ultrafast transfer is illustrated via femtoseconds transient absorption spectrum and considered as the mechanism behind the sign change. The results of this research may further extend the potential of chalcone derivatives in nonlinear optical applications and provide another method to tune the optical nonlinearity in such materials. [ABSTRACT FROM AUTHOR]