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.

Corneal deposits and nerve alterations in Bietti Corneoretinal Crystalline Dystrophy imaged using in vivo confocal microscopy.

PURPOSE: To evaluate the imaging features of corneal deposits and nerve alterations in Chinese patients with Bietti Corneoretinal Crystalline Dystrophy (BCD) using in vivo confocal microscopy (IVCM). METHODS: Twenty patients with BCD and 20 age- and sex-matched healthy controls were enrolled in this retrospective, observational study. Corneal deposits and sub-basal nerve plexus (SNP) were observed by IVCM. Parameters of SNP including total nerve density/number, main nerve trunk density/number, and branch nerve density/number were analyzed by Neuron J. RESULTS: Corneal deposits were observed in both eyes of all patients by IVCM. These crystals appeared as dot-shaped, needle-shaped, and rod-shaped hyperreflective bodies and were located not only in the sub-epithelium and stroma of cornea, but in endothelium which were not reported before. There was a decrease of total nerve density (P < 0.001), main nerve trunk density (P = 0.007), and branch nerve density (P = 0.001), in BCD compared to controls. The number of total nerves/frame (P = 0.001), main nerve trunks/frame (P = 0.005), and branch nerves/frame (P = 0.006) in BCD were lower than controls. CONCLUSION: New findings in locations of corneal crystals by IVCM expand the phenotype spectrum of BCD. Corneal deposits may be useful for diagnosis of BCD, especially ones without retinal deposits. Corneal nerve parameters were reduced in BCD, which may provide new insights to be further explored to contribute to our understanding of BCD. IVCM is a promising tool to evaluate corneal deposits and nerve alterations in BCD.

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.

Effect of intramolecular charge transfer on nonlinear optical properties of chalcone derivatives: a visual description of the charge transfer process.

Intramolecular charge transfer (ICT) is an important factor in the nonlinear optical (NLO) properties of organic molecules. In order to study the effect of ICT on two-photon absorption (TPA) and excited-state absorption (ESA), three chalcone derivatives (1, 2 and 3) with different electron push-pull systems were designed and synthesized. The ICT performance of these chalcone derivatives depends on the electron push-pull systems and mainly includes ultrafast ICT in the femtosecond time domain and long-lived charge transfer state (CTS) in the picosecond time domain, which dominate the performance of molecular TPA and ESA respectively. Hole-electron analysis and femtosecond Z-scan experiment indicate that the TPA cross section of these chalcone derivatives can be effectively enhanced by introducing stronger ultra-fast ICT in the case of little difference in ground-state absorption and expanding the molecular π-conjugated structure. Transient absorption spectrum (TAS) experiments of these compounds in solvents of varying polarities were conducted to visualize the establishment of CTS. The local excited state (LES) and charge transfer state (CTS)-based ESA of these chalcone derivatives are extremely dependent on the strength of ICT. Our experimental results show that the superposition of LES and CTS by enhancing ICT performance can effectively improve the ESA, which offers us a practical method to improve the long-impulse response of organic materials.

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.

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.

Aggregation-induced emission on benzothiadiazole dyads with large third-order optical nonlinearity.

Two kinds of D-A molecular of (4-(4-(9H-carbazol-9-yl)phenyl))-7-nitrobenzothiadiazole (BSC) and 4-((4-(9H-carbazol-9-yl)phenyl)ethynyl)-7-nitrobenzothiadiazole (BEC) containing carbazole moieties as the donor were synthesized. X-ray crystal data elucidated the multiple intermolecular interactions. They exhibit distinctly different self-assembly behaviours. The nonlinear optical properties were studied using the top-hat Z-scan technique at 532 nm with a 21 ps pulse. The results indicate that they exhibit large third-order nonlinear absorption effects. The nonlinear absorption coefficients α2 fitting the experimental data are 6.3 × 10(-12) m W(-1) for BSC and 3.6 × 10(-11) m W(-1) for BEC. The time-resolved pump-probe results show that both nonlinear absorption and nonlinear refraction of BEC in CH2Cl2 solution have rapid optical responses, which indicate the nonlinear absorption and nonlinear refraction mechanism are excited-state nonlinear. Moreover, both of these two compounds are observed to be aggregation-induced emission (AIE) active. The aggregates of the well-formed one-dimensional microrods of BEC and BSC endow the material with potential applications in the field of optical devices.

Lactate Injection by Electric Currents for Bioremediation of Tetrachloroethylene in Clay.

Biological transformation of tetrachloroethylene (PCE) in silty clay samples by ionic injection of lactate under electric fields is evaluated. To prepare contaminated samples, a silty clay slurry was mixed with PCE, inoculated with KB-1(®) dechlorinators and was consolidated in a 40 cm long cell. A current density between 5.3 and 13.3 A m(-2) was applied across treated soil samples while circulating electrolytes containing 10 mg L(-1) lactate concentration between the anode and cathode compartments to maintain neutral pH and chemically reducing boundary conditions. The total adsorbed and aqueous PCE was degraded in the soil to trichloroethylene (TCE), cis-1,2-dichloroethene (cis-DCE), vinyl chloride (VC) and ethene in 120 d, which is about double the time expected for transformation. Lactate was delivered into the soil by a reactive transport rate of 3.7 cm(2) d(-1) V(-1). PCE degradation in the clay samples followed zero order transformation rates ranging from 1.5 to 5 mg L(-1) d(-1) without any significant formation of TCE. cis-DCE transformation followed first order transformation rates of 0.06 to 0.10 per day. A control experiment conducted with KB-1 and lactate, but without electricity did not show any significant lactate buildup or cis-DCE transformation because the soil was practically impermeable (hydraulic conductivity of 2×10(-7) cm s(-1)). It is concluded that ionic migration will deliver organic additives and induce biological activity and complete PCE transformation in clay, even though the transformation occurs under slower rates compared to ideal conditions.

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.

D-π-A type conjugated indandione derivatives: ultrafast broadband nonlinear absorption responses and transient dynamics.

The ultrafast nonlinear optical response of two 1,3-indandione derivatives (INB3 and INT3) was systematically investigated by the femtosecond Z-scan and pump-probe technique at multiple visible and near infrared wavelengths. Both compounds show strong broadband nonlinear absorption (NLA) and different wavelength-dependent two-photon absorption (TPA) characteristics in the range of 650-1100 nm. The TPA cross section of trithiophene-based compound INT3 was found to be larger than that of triphenylamine-based compound INB3 in the red region (650-800 nm), which is attributed to its longer π-conjugated structure and better molecular planarity. Interestingly, the effective NLA of INB3 was found to be larger than INT3 in the NIR region (800-1100 nm), which is related to the excited state absorption (ESA) induced by TPA. The ultrafast dynamics of both compounds were investigated by femtosecond transient absorption spectroscopy, which revealed a broadband ESA including several relaxation processes. This work extends nonlinear optical research on indandione derivatives, and the results suggest that these derivatives are promising candidates for optical limiting applications.

Correction: D-π-A type conjugated indandione derivatives: ultrafast broadband nonlinear absorption responses and transient dynamics.

[This corrects the article DOI: 10.1039/D2RA00349J.].

An investigation of broadband optical nonlinear absorption and transient nonlinear refraction in a fluorenone-based compound.

A novel fluorenone derivative, FO52, is designed and synthesized. The fluorenone group is introduced to provide the central π-conjugated system in the molecule and triphenylamine is substituted at both sides. Intramolecular Charge Transfer (ICT) from the terminal groups to the molecular center is confirmed via DFT calculations. Ultrafast optical nonlinearities are investigated via Z-scan and transient absorption spectroscopy (TAS) studies with a 190 fs laser. Reverse saturable absorption, two-photon induced excited-state absorption, and pure two-photon absorption are observed at 532 nm, 650 nm, and 800 nm, respectively. The different mechanisms at these wavelengths are discussed and interpreted with assistance from the results from TAS. Furthermore, strong excited-state refraction and ultrafast negative refraction from the bound electron response are resolved and discussed in phase object pump probe (POPP) experiments. The results suggest that the ICT-enhanced optical nonlinearities provide FO52 with strong optical limiting capabilities at visible wavelengths and ultrafast refraction with tiny attenuation in the near infrared region. The combination of these properties in one compound could be attractive for applications like laser protection and low-loss all-optical switching.

Ultrafast broadband nonlinear optical properties and excited-state dynamics of two bis-chalcone derivatives.

The development of organic nonlinear optical (NLO) chromophores is vital for various fields such as two-photon biomedical imaging, optical limiting, etc. In this work, two bis-chalcone molecules 1,4-bis[3-(2,4-dimethoxyphenyl)-2-acryloyl]benzene (C1) and 4,4'-bis[3-(2,4-bimethoxy phenyl)-2-acryloyl]biphenyl (C2) were synthesized and characterized. The excited-state dynamics of these two chromophores were studied using femtosecond transient absorption (TA) measurements. And their broadband nonlinear absorption properties and optical limiting (OL) response were investigated by femtosecond open-aperture Z-scan and intensity-dependent transmittance measurements in the wavelength range from 515 nm to 800 nm, respectively. The TA results demonstrate that C2 has strong excited-state absorption behavior and longer lifetime. In addition, the nonlinear absorption response of C2 was found to be superior to that of C1 in the visible range after 500 nm, which is attributed to a two-photon-absorption induced excited-state absorption mechanism. These results indicate that the nonlinear optical response and excited-state dynamics in bis-chalcone compounds could be enhanced via intramolecular charge-transfer.

Anthracene derivatives as broadband nonlinear optical materials: nonlinear absorption and excited-state dynamics analysis.

Two anthracene derivatives, AN-1 and AN-2, with different π-bridge lengths were designed and synthesized to investigate their optical nonlinearities. The nonlinear absorption (NLA) properties of both derivatives were measured via the femtosecond Z-scan technique with the wavelength range from 532 nm to 800 nm. The reverse saturable absorption (RSA) of both compounds results from two-photon absorption induced excited-state absorption (TPA-ESA). At all wavelengths, the reverse saturable absorption of AN-2 is superior to that of AN-1 due to a better molecular planarity for AN-2. Compared with the results of AN-1, the two-photon absorption coefficient of AN-2 can be increased by nearly 8 times (from 0.182 × 10-2 cm GW-1 for AN-1 to 1.42 × 10-2 cm GW-1 for AN-2) at 600 nm by extending the π-bridge. The evolution of femtosecond transient absorption (TA) spectra reveals the relaxation process from the singlet local excited-state (LES) to charge transfer state (CTS) for both compounds. The results imply that anthracene derivatives may be potential candidates for applications in future laser photonics.

Effect of silver doping on ultrafast broadband nonlinear optical responses in polycrystalline Ag-doped InSe nanofilms at near-infrared.

There is great interest in transition metal-doped InSe because of its high nonlinearity and ultrafast response time at higher light fluence. Herein, Ag-doped InSe nanofilms were precisely manufactured using a direct current-radio frequency sputtering method, and their ultrafast broadband nonlinear optical responses in near-infrared were systematically researched. Ag-doped InSe nanofilm exhibited a broadband nonlinear optical response (800-1100 nm) and ultrafast carrier absorption (<1 ps), and can act as a potential semiconducting material for all-optical devices. Through precise control of the sputtering process parameters, Ag-doped InSe nanofilms were successfully prepared that were smooth, uniform, and exhibited no cracks. Nonlinear optical studies (femtosecond transient absorption spectroscopy and Z-scan measurement) indicated that nonlinear absorption behavior in Ag-doped InSe nanofilm withstands a transformation from saturation absorption to reverse saturation absorption arising from ground state bleaching, free-carrier absorption (FCA), and two-photon absorption (TPA). Additionally, nonlinear refraction behavior in Ag-doped InSe nanofilm was successfully detected near the intrinsic absorption edge, which arose from Kerr refraction and free-carrier refraction. More importantly, the broadband nonlinear response, ultrafast carrier absorption, and carrier recovery time of Ag-doped InSe nanofilm has the ability to controllably tune via Ag doping. Furthermore, Ag-doped InSe nanofilm possesses the nonlinear figure of merit (FOM) of 2.02, which indicates that Ag-doped InSe nanofilm is a promising semiconducting material for all-optical switching devices in near-infrared.

Tuning Nonlinear Optical Behavior by Incorporation of the Chalcogenophene into Twistacenes.

Four chalcogenophene-fused acenes containing O, S, Se, or Te, respectively, were presented, and the chalcogenophene effect on linear and nonlinear optics was systematically investigated. Their excited-state absorption performance and two-photon absorption (TPA) capacity could be modulated by the incorporating chalcogen atoms. The experimental results showed that the heavy chalcogen facilitated the intersystem crossing resulting in the presence of the triplet state absorption for PyPSe and PyPTe, and the TPA capacity gradually increased with the atomic size of the chalcogens. The theoretical calculation inferred that their nonlinear optical performance was closely related to the contributions of the chalcogenophene component to heterotwistacenes in the final excited state. In addition, PyPTe was able to work under various laser pulses from femtoseconds to nanoseconds.

Template-Free Growth of Well-Ordered Silver Nano Forest/Ceramic Metamaterial Films with Tunable Optical Responses.

Currently, the limitations of conventional methods for fabricating metamaterials composed of well-aligned nanoscale inclusions either lack the necessary freedom to tune the structural geometry or are difficult for large-area synthesis. In this Communication, the authors propose a fabrication route to create well-ordered silver nano forest/ceramic composite single-layer or multi-layer vertically stacked structures, as a distinctive approach to make large-area nanoscale metamaterials. To take advantage of direct growth, the authors fabricate single-layer nanocomposite films with a well-defined sub-5 nm interwire gap and an average nanowire diameter of ≈3 nm. Further, artificially constructed multilayer metamaterial films are easily fabricated by vertical integration of different single-layer metamaterial films. Based upon the thermodynamics as well as thin film growth dynamics theory, the growth mechanism is presented to elucidate the formation of such structure. Intriguing steady and transient optical properties in these assemblies are demonstrated, owing to their nanoscale structural anisotropy. The studies suggest that the self-organized nanocomposites provide an extensible material platform to manipulate optical response in the region of sub-5 nm scale.

Ultrafast single-shot measurement of optical Kerr effect based on supercontinuum pulse.

We present an ultrafast single-shot measurement method for the optical Kerr effect based on a polarization gating technique. The advantages of this single-shot technique are demonstrated via a 26-ps chirped supercontinuum pulse used to measure the optical Kerr effect for three transparent organic liquids. The single-shot measurement results agree well with those of the time-resolved optical Kerr gate method, as regards both time and intensity. This method facilitates real-time observation of ultrafast optical Kerr responses of samples and simultaneous high-time-resolution data acquisition at ∼260 fs. We demonstrate that the single-shot measurement method is potentially a powerful tool for investigating the optical Kerr effects of unstable samples, and for application to high-power laser systems.