Synthesis, Photophysical, and Optical Limiting Properties of Twistacene-Functionalized Arenes.

To realize the relationship of structure and property, four novel twistacene-functionalized arenes, namely, 1,4-bis(2,7-di-tert-butyl-9,14-bis(4-(tert-butyl)phenyl)dibenzo[de,qr]tetracen-11-yl)buta-1,3-diyne (4), 1,4-bis(3,5,10,12-tetra-tert-butyltribenzo[a,d,g]coronen-16-yl)buta-1,3-diyne (7), 1,4-bis(2,7-di-tert-butyl-9,14-bis(4-(tert-butyl)phenyl)dibenzo[de,qr]tetracen-10-yl)buta-1,3-diyne (10), 1,4-bis(3,5,10,12-tetra-tert-butyltribenzo[a,d,g]coronen-15-yl)buta-1,3-diyne (13), linked with butadiyne as π bridges have been strategically synthesized and characterized. The nonlinear optical properties are detailly examined in solution through the open-aperture Z-scan method in a comparative manner, indicating that molecules 4 and 7 exhibit better nonlinear optical responses than 10 and 13. Among them, 4 and 7 exhibit excellent optical limiting responses with limiting thresholds of 0.17 and 0.19 J/cm2, respectively, being superior to the state-of-the-art material C60. The ultrafast transient absorption test and DFT calculations suggest that the nonlinear absorption mechanisms belong to TPA-induced ESA. In addition, the effective percentage calculated from TD-DFT can provide a brief glance to evaluate the optical limiting performance.

Polyhedral {Ag12} and {Ag16} Clusters: Synthesis, Structural Characterization and Third-Order Nonlinear Optical Properties.

Two polyhedral silver-thiolate clusters, [S@Ag16(Tab)10(MeCN)8](PF6)14 (Ag16) and [Ag12(Tab)6(DMF)12](PF6)12 (Ag12), were synthesized by using electroneutral Tab species as protective ligands (Tab=4-(trimethylammonio)benzenethiolate, DMF=N,N-dimethylformamide, MeCN=acetonitrile). Ag16 has a decahedral shape composed of eight pentagon {Ag5} units and two square {Ag4} units. The structure of Ag12 is a cuboctahedron, a classical Archimedean structure composed of six triangular faces and eight square faces. The former configuration is discovered in silver-thiolate cluster for the first time, possibly benefited from the more flexible coordination between the Tab ligand and Ag+ facilitated by the electropositive -N(CH3)3 + substituent group. Third-order nonlinear optical studies show that both clusters in DMF exhibit reverse saturate absorption response under the irradiation of 532 nm laser.

Study of two-photon absorption and excited-state dynamics of coumarin derivatives: the effect of monomeric and dimeric structures.

Intramolecular charge transfer (ICT) and π-electron delocalization are two key factors affecting the nonlinear optical absorption of organic molecules. To clarify the different influences of ICT and π-electron delocalization on two-photon absorption (TPA) and excited-state absorption (ESA), monomeric coumarin C1 and dimeric coumarin C2 are synthesized and studied. Transient absorption spectroscopy (TAS) analysis of these coumarin derivatives in solvents of varying polarities describes the polarity-dependent excited-state dynamics and reveals the ESA signals of the charge transfer state (CTS) and local excited state (LES) with different spectral features. Femtosecond broadband Z-scan experiments indicate that dimeric coumarin C2 has a more significant TPA response than monomeric coumarin C1 in the near-infrared region. Natural transition orbital (NTO) analysis further theoretically characterizes the electron transition feature induced by TPA. Our results reveal that the TPA of these coumarin derivatives can be significantly enhanced by expanding π-electron delocalization, but their ESA is mainly dominated by ICT performance. This study indicates that coumarin derivatives will exhibit extremely broad application prospects in the field of ultrafast optical limiting (OL) through reasonable molecular design.

Towards high-performance nonlinear optical materials through embedding a D-A system into ß-ketoenamine-linked COFs.

Two covalent organic framework (COF) films supported by a glass substrate were obtained by solvothermal reaction of an electron donor with electron acceptor 1,3,5-triformylbenzene (TF) or 2,4,6-triformylphloroglucinol (TFP), respectively. The TFP-BD film exhibits a nonlinear absorption coefficient of -3.01 × 105 cm GW-1. The TFP-BD film can aggregate electrons around the connected monomer through the D-A effect due to its highly polar and electronegative carbonyl oxygen atoms, thereby modulating the electronic structure of the COFs. This work provides a novel approach for the structural modulation of optical materials with strong nonlinearity.

Broadband optical nonlinearity and all-optical switching features in low-defect GaN.

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.

Investigation of Broadband Optical Nonlinear Absorption and Transient Dynamics in Orange IV Containing Azobenzene.

Broadband reverse saturable absorption is systematically investigated via Z-scan, transient absorption spectrum (TAS). The excited state absorption and negative refraction of Orange IV are observed in the Z-scan experiment at 532 nm. Meanwhile, two-photon-induced excited state absorption and pure two-photon absorption are observed at 600 nm and 700 nm with the pulse width of 190 fs, respectively. An ultrafast broadband absorption in the visible wavelength region is observed via TAS. The different nonlinear absorption mechanisms at multiple wavelengths are discussed and interpreted from the results of TAS. In addition, the ultrafast dynamics of negative refraction in the excited state of Orange IV are investigated via a degenerate phase object pump-probe, from which the weak long-lived excited state is extracted. All studies indicate that Orange IV has the potential to be further optimized into a superior broadband reverse saturable absorption material and also has certain reference significance for the study of optical nonlinearity in organic molecules containing azobenzene groups.

Modulating the nonlinear absorption response of SnOx thin films via phase engineering.

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO2 and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (ßeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.

Enhanced Ultrafast Broadband Reverse Saturable Absorption in Twistacenes with Enlarged π-Conjugated Central Bridge.

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.

Ultrafast nonlinear absorption with multiple transformations and transient dynamics of gold nanobipyramids.

The process and condition of saturable absorption (SA) and reverse saturable absorption (RSA) of ultrafast nonlinear optics in metal nanoparticles are essential for applications including light generation, amplification, modulation, and switching. Here, we first discover and explore the multiple transformations (SA-RSA-SA) of ultrafast nonlinear absorption behavior of metal nanoparticles in femtosecond pulses. Correspondingly, the energy level model and fitting formula of multiple transformations are established to illustrate the process of optical response. The femtosecond transient absorption spectra provide information about their ultrafast dynamics process and vibrational mode, which further reveals the multiple transformation mechanisms of nonlinear absorption in gold nanobipyramids (Au-NBPs). Furthermore, Au-NBPs exhibit a significantly higher SA modulation depth up to 42% in the femtosecond, which is much higher than the reported values of other nanomaterials. Our results indicate that Au-NBPs can be used as broadband ultrafast Q-switching and mode-locking, and the conversion offers new opportunities for metal nanostructures in applications of optical switching.

Improved Ultrafast Carrier Relaxation and Charge Transfer Dynamics in CuI Films and Their Heterojunctions via Sn Doping.

CuI is one of the promising hole transport materials for perovskite solar cells. However, its tendency to form defects is currently limiting its use for device applications. Here, we report the successful improvement of CuI through Sn doping and the direct measurement of the carrier relaxation and interfacial charge-transfer processes in Sn-doped CuI films and their heterostructures. Femtosecond-transient absorption (fs-TA) measurements reveal that Sn doping effectively passivates the trap states within the bandgap of CuI. The I-V characteristics of heterostructures demonstrate drastic improvement in transport characteristics upon Sn doping. Fs-TA measurements further confirm that the CuSnI/ZnO heterojunction has a type-II configuration with ultrafast charge transfer (<280 fs). The charge transfer time of a CuI/ZnO heterostructure is ∼2.8 times slower than that of the CuSnI/ZnO heterostructure, indicating that Sn doping suppresses the interfacial states that retard the charge transfer. These results elucidate the effect of Sn doping on the performance of CuI-based heterostructures.

Nonlinear Optical Properties of Pyrene Derivatives Based on a Donor-Acceptor Structure and Its Polyurethane Composites.

Two pyrenyl Schiff base derivatives with π conjugated structures (B2 and B3) were designed and synthesized. Then, B2 and B3 were added into polyurethane to obtain doped and bonded polyurethane nonlinear optical materials (B2/PU and B3/PU), respectively. The synthesized B2, B3, and polyurethane nonlinear optical materials were tested by a nanosecond (ns) and picosecond (ps) pulse Z-scan at a 532 nm wavelength. Due to the two-photon absorption-induced excited state absorption (TPA-ESA), B2, B3, and polyurethane nonlinear optical materials show reverse saturable absorption (RSA). From a quantum chemistry calculation, it can be concluded that the RSA of B2 and B3 comes from the large π conjugated system and intramolecular charge transfer. Furthermore, B2, B3, and the polyurethane nonlinear optical materials show good optical limiting. B2/PU and B3/PU not only have excellent nonlinear optical properties but also have good transmittance, thermal stability, and processability of polyurethane materials. The combination of pyrenyl Schiff base derivatives and polyurethane materials greatly improves the application of nonlinear small molecules in the field of optical limiting and all-optical switching.

Nonlinear optical properties and optimization strategies of D-π-A type phenylamine derivatives in the near-infrared region.

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.

High-sensitive temperature sensing based on thermal-enhanced emission and non-thermally coupled energy levels of white upconversion luminescence system.

Na3Y(VO4)2:Nd3+,Yb3+,Ho3+,Tm3+ phosphors present significantly improved sensitivity of optical temperature sensing based on thermal-enhanced upconversion luminescence and non-thermally coupled energy levels. Under 808 nm excitation, white upconversion luminescence is successfully achieved in Nd3+-sensitized system. In addition, the emissions intensities originated from 4G5/2→4I9/2 transition of Nd3+ and 3F2,3→3H6 transition of Tm3+ gradually increase with elevating temperature owning to the thermal population effects, as opposed to the blue (1G4→3H6 transition of Tm3+), green (5S2,5F4→5I8 transition of Ho3+) and red (5F5→5I8 transition of Ho3+) emissions intensities show continuous decreasing trend. The temperature sensing behaviors are investigated by employing multiple non-thermally coupled energy levels. Based on non-thermally coupled energy levels of 4G5/2 (Nd3+)/1G4 (Tm3+), the absolute and relative sensitivities are obtained to be 0.433 K-1 and 2.81 % K-1. These results demonstrate that the Na3Y(VO4)2:Nd3+,Yb3+,Ho3+,Tm3+ phosphors with superior optical thermometry performance and white luminescence within a relatively wide temperature range can achieve optical applications in many fields.

Synthesis and Ultrafast Broadband Optical Limiting Properties of a Two-Branched Twistacene.

A novel two-branched twistacene (PyDN) has been designed and synthesized for application on ultrafast optical limiting. This twistacene exhibits excellent two photon absorption and two photon absorption-induced excited singlet state absorption, which was systematically investigated with a femtosecond Z-scan experiment, transient absorption spectrum, and two-photon excited fluorescence experiments. The admirable two photon absorption is attributed to the high degree of π electron delocalization in twistacene which is caused by introduction of two strong donors. The excited singlet state absorption cooperates with two-photon absorption to provide an excellent ultrafast optical limiting behavior with high linear transmittance, where the thresholds are 2.3-5.3 mJ/cm2 in the spectral region of 532-800 nm of femtosecond laser and 133 mJ/cm2 for picosecond pulse at 532 nm. These thresholds are lower than that of most of the optical limiters reported previously, which indicates PyDN is a promising candidate for ultrafast optical limiting.

Controlling the emission linewidths of alloy quantum dots with asymmetric strain.

The emission linewidth of quantum dots (QDs) is one of the important optical properties, which is essential for the applications of QD lasers, high-quality displays, and biological imaging. However, we know less about controlling emission linewidth and its underlying mechanisms. Here we introduce a wurtzite ZnSe shell onto a wurtzite CdSe core to produce asymmetric strain due to their large, anisotropic lattice mismatch. Such asymmetric pressure induces significant splitting (ΔAB) between heavy-hole (hh) and light-hole (lh) in valence band (VB). We show that the emission intensity from the lh state (Elh) is significantly suppressed with the increasing ΔAB caused by the strong asymmetric strain. We demonstrate that the exciton-phonon coupling (EPC) is greatly inhibited under the anisotropic lattice strain. The alloying process between the core and shell occurs under the strong lattice strain and raises the longitudinal-optical (LO) phonon energy (ELO). Higher LO phonon energy declines LO phonon occupation numbers (NLO) and synergistically reduces the EPC. The asymmetrically strained alloy QDs ensemble exhibits highly bright emission with ultra-narrow linewidths of 13.8 nm (∼520 nm) and 15.8 nm (∼620 nm). This concept of band structure regulation via asymmetric strain can provide a new platform for high-quality QDs beyond the currently achieved.

Enhancement of Two-Photon Absorption in Boron-Dipyrromethene (BODIPY) Derivatives.

The linear and nonlinear optical properties of two BODIPY derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)-boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenylvinyl)phenyl)-boron-diuoride-azadipyrromethene (ZL-22), were comprehensively investigated based on experimental and theoretical studies. It was found that both compounds show a strong two-photon absorption response in the near-infrared regime, and the two-photon-absorption cross-section values of ZL-61 and ZL-22 were determined to be 8321 GM and 1864 GM at 800 nm, respectively. The improvement of the two-photon absorption cross section in ZL-61 was attributed to the enhancement of the donor group, which was confirmed by transient absorption measurements and DFT calculation. Our results indicate that these BODIPY derivatives are a promising candidate for optical limiting and two-photon imaging applications.

All-optical demonstration of a scalable super-resolved magnetic vortex core.

We first present the all-optical realization of a scalable super-resolved magnetic vortex core (MVC) by tightly focusing two modulated counter-propagating radially polarized doughnut Gaussian beams based on the vectoial diffraction theory and the inverse Faraday effect. It is shown that by imposing spiral phase plates (SPPs) on the incident vectorial beams, single three-dimensional (3D) super-resolved (λ3/22) MVC can be achieved in the 4π focusing setup, which is radically different from that produced with a single lens focusing. Furthermore, the light-induced MVC texture turns to be richer and more complex when the radially polarized beams are tailored by the SPPs and judiciously designed multi-ring filters all together. In this case, we are able to garner not only transverse super-resolved (0.447λ) MVC needle with an uniformly extended area (40λ) in the single lens focusing system, but also the multiple uniform 3D super-resolved (λ3/24) chain-like MVC cells in the 4π focusing system, thus giving rise to the tunable and scalable super-resolved MVC extension. The related physical mechanisms to trigger such peculiar magnetization polarization topologies are unraveled as well. These resultant achievements would pave the way for the integrated transfer and storage of optomagnetic information, atomic trapping, and beyond.

Ultrafast changes in effective permittivity of hyperbolic metamaterials and related multi-resonance-induced ultrafast process excited by femtosecond pulses.

Hyperbolic metamaterials (HMMs) exhibit rich optical nonlinear responses for the epsilon-near-zero (ENZ) and anisotropy. In this study, we extract the time-dependent change in the effective permittivity of an Ag nanorod array under femtosecond pulses pumping around its ENZ wavelength. The transmittance and transient absorption spectra measured by s- and p-polarizations are used in the extraction process. We experimentally confirm the existence of an ultrafast recovery process with a relaxation time of 0.24 ps in the transient absorption spectra. The calculation based on the extracted nonlinear effective permittivity indicates that the ultrafast signal originates from the superposition of two slower recovery processes, with relaxation times of 0.74 ps and 1.19 ps, respectively. The results indicate that when the responses of two nonlinear processes have different signs and recovery speeds, their superposition may cause faster signal recovery in the combined process than in the two individual processes.

Study on Excited State Kinetics and Optical Limiting Performance of Triphenylamine-Based Chalcone Derivatives: Effect of the Molecular π-Conjugated Structure.

Optical limiting (OL) is an important application of nonlinear optics. Summarizing the structure-property relationship of organic materials is an effective means to develop superior optical limiters. In this work, two triphenylamine-based chalcone derivatives T1 and T2 with different peripheral substituent groups were synthesized to study their transient kinetics and nonlinear optical (NLO) absorption performance. The transient absorption spectrum (TAS) of compounds T1 and T2 in solvents of varying polarities visualizes the intramolecular charge transfer (ICT) processes between the local excited state (LES) and the charge transfer state (CTS). Nanosecond Z-scan experiment and hole-electron analysis indicate that all compounds have excellent reverse saturated absorption (RSA) performance at 532 nm and T1 exhibits stronger RSA than T2 due to the stronger ICT performance of T1 caused by the halogen effect. Degenerate pump-probe experiment shows that the ESA of T2 at 532 nm is significantly enhanced by expanding the molecular π-conjugation. Under the premise of consistent linear transmittance (78%), compound T2 shows better OL performance than compound T1 at 532 nm in the nanosecond time domain. The OL thresholds of T1 and T2 are 3.72 and 0.72 J cm-2, respectively, which are better than those of the most reported OL materials. Our research shows that simple and common chalcone derivatives exhibit amazing NLO performance through a reasonable design.

Construction of cluster-based supramolecular wire and rectangle.

Reactions of [Et4N][Tp*WS3(CuCl)3] (1) (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate) with 2 equiv. of AgOTf (OTf- = trifluoromethanesulfonate) and 1 equiv. of several bidentate pyridine ligands including 2,5-bis(pyridine-4-yl)thiazolo[5,4-d]thiazole (L1), 2,7-di(pyridin-4-yl)-9H-fluorene (L2), 2,7-di(pyridin-4-yl)-9H-carbazole (L3), and 2,7-di(pyridin-4-yl)-9H-fluoren-9-one (L4) afforded four W/Cu/S cluster-based supramolecular compounds [(Tp*WS3Cu2Cl)2(L1)] (2), {[(Tp*WS3Cu3)2(µ-Cl)2(µ4-Cl)]2(L2)2}(OTf)2 (3), {[(Tp*WS3Cu3)2(µ-Cl)2(µ4-Cl)]2(L3)2}(OTf)2 (4) and {[(Tp*WS3Cu3)2(µ-Cl)2(µ4-Cl)]2(L4)2}(OTf)2 (5). Compounds 2-5 were characterized by elemental analysis, IR, UV-vis, 1H NMR, and single-crystal X-ray diffraction analysis. The neutral cluster 2 behaves as a supramolecular wire constructed by L1 bridging two butterfly-shaped [Tp*WS3Cu2Cl] cores. The cluster cations of 3-5 contain two [(Tp*WS3Cu3)2(µ-Cl)2(µ4-Cl)]+ cores linked by two L2, L3, or L4 ligands, which finally formed a cationic supramolecular rectangle. The third-order nonlinear-optical (NLO) properties of 3-5 in DMF were also investigated by Z-scan techniques and their NLO responses were enhanced compared to those of their precursor 1.