Nonlinear Optical Properties of Discotic Hexylthiotruxene Derivatives.

The search for efficient and transparent nonlinear optical (NLO) media has led to the investigation and development of alternative organic optical materials. In this context, a series of new hexylthiotruxene derivatives have been synthesized, and their linear and NLO properties are explored. These truxene derivatives show large NLO absorption due to their C3 symmetry, presence of large hyperpolarizability, and extended π-conjugation. Herein, we show that two-photon absorption and three-photon absorption processes are the main cause of nonlinear absorption in these materials under 5 ns and 100 fs excitations at 532 and 800 nm excitations, respectively. The nonlinear absorption coefficients have high values of 2 to 7.9 × 10-10 m/W in the nanosecond domain and 2.2 to 7.4 × 10-21 m3/W2 in the femtosecond domain. The corresponding nonlinear absorption cross-section (δ) values and the nonlinear susceptibilities were also calculated from the numerically obtained nonlinear absorption coefficient values. Tailored truxene derivative showed an excellent optical limiting threshold of 4.5 J/cm2 and is comparable to or better than most recently reported and benchmark optical limiting materials. Longer alkyl members of the series showed the largest nonlinear absorption in both excitation domains and could be a potential optical limiter.

Acousto-optic modulator-based improvement in imaging through scattering media.

Reduced visibility is a common problem when light traverses through a scattering medium, and it becomes difficult to identify an object in such scenarios. What we believe to be a novel proof-of-principle technique for improving image visibility based on the quadrature lock-in discrimination algorithm in which the demodulation is performed using an acousto-optic modulator is presented here. A significant improvement in image visibility is achieved using a series of frames. We have also performed systematic imaging by varying the camera parameters, such as exposure time, frame rate, and series length, to investigate their effect on enhancing image visibility.

Synthesis, crystal structure and Hirshfeld surface analysis of the ortho-rhom-bic polymorph of 4-bromo-N-(4-bromo-benzyl-idene)aniline.

The crystal structure of the title compound, C13H9Br2N [systematic name: (E)-N,1-bis-(4-bromo-phen-yl)methanimine], is a second polymorph (Form II) crystallizing in the ortho-rhom-bic space group Pccn. The first polymorph (Form I) crystallizes in the monoclinic space group P21/c [Bernstein & Izak (1975 ▸). J. Cryst. Mol. Struct. 5, 257-266; Marin et al. (2013 ▸). J. Mol. Struct. 1049, 377-385]. The mol-ecule is disordered about an inversion center situated in the middle of the C=N bond, similar to the situation in the monoclinic polymorph: the C=N bond length is 1.243 (7) Å. In the crystal, mol-ecules stack along the b-axis direction and are linked by C-H⋯π inter-actions. The inter-atomic contacts in the crystal for both polymorphs were studied by Hirshfeld surface analysis and have notable differences. The solid-state fluorescence spectrum of Form II shows an emission peak at ca 469 nm. The two-photon absorption coefficient measured from the open aperture Z-scan technique is 1.3 × 10 -11 m W-1, hence, Form II shows optical limiting behaviour.

Two Photon Absorption Properties of CBHB and DEABHB Single Crystals for Optical Limiting Applications.

Novel materials of (E)-N'-(4-chlorobenzylidene)-4-hydroxybenzohydrazide (CBHB) and (E)-N'-(4-(diethylamino) benzylidene)-4-hydroxybenzohydrazide (DEABHB) were synthesized by condensation reaction process and solvent evaporation method was employed to grow CBHB and DEABHB single crystals at room temperature. Lattice parameters of CBHB and DEABHB compounds were recorded using single crystal X-ray diffraction method. The presence of functional groups of the synthesized CBHB and DEABHB compounds were confirmed by Fourier transform infrared and Fourier transform Raman spectral analyses. Various intermolecular interactions were studied using Hirshfeld surface analysis. Thermal stability of the hydrazone Schiff base compounds CBHB and DEABHB were studied by thermogravimetric and differential thermal analyses. Third order nonlinear optical properties of CBHB and DEABHB were measured using open aperature Z scan technique. Two photon absorption coefficient and optical limiting properties of the crystals were reported from the Z scan studies.

Fluorescence and Nonlinear Optical Response of Graphene Quantum Dots Produced by Pulsed Laser Irradiation in Toluene.

Graphene quantum dots (GQDs), the zero dimensional (0D) single nanostructures, have many exciting technological applications in diversified fields such as sensors, light emitting devices, bio imaging probes, solar cells, etc. They are emerging as a functional tool to modulate light by means of molecular engineering due to its merits, including relatively low extend of loss, large outstretch of spatial confinement and control via doping, size and shape. In this article, we present a one pot, facile and ecofriendly synthesis approach for fabricating GQDs via pulsed laser irradiation of an organic solvent (toluene) without any catalyst. It is a promising synthesis choice to prepare GQDs due to its fast production, lack of byproducts and further purification, as well as the control over the product by accurate tuning of laser parameters. In this work, the second (532 nm) and third harmonic (355 nm) wavelengths of a pulsed nanosecond Nd:YAG laser have been employed for the synthesis. It has been found that the obtained GQDs display fluorescence and is expected to have potential applications in optoelectronics and light-harvesting devices. In addition, nonlinear optical absorption of the prepared GQDs was measured using the open aperture z-scan technique (in the nanosecond regime). These GQDs exhibit excellent optical limiting properties, especially those synthesized at 532 nm wavelength.

Fabrication of Silver-Decorated Graphene Oxide Nanohybrids via Pulsed Laser Ablation with Excellent Antimicrobial and Optical Limiting Performance.

The demand for metallic nanoparticle ornamented nanohybrid materials of graphene oxide (GO) finds copious recognition by virtue of its advanced high-tech applications. Far apart from the long-established synthesis protocols, a novel laser-induced generation of silver nanoparticles (Ag NPs) that are anchored onto the GO layers by a single-step green method named pulsed laser ablation has been exemplified in this work. The second and third harmonic wavelengths (532 nm and 355 nm) of an Nd:YAG pulsed laser is used for the production of Ag NPs from a bulk solid silver target ablated in an aqueous solution of GO to fabricate colloidal Ag-GO nanohybrid materials. UV-Vis absorption spectroscopy, Raman spectroscopy, and TEM validate the optical, structural, and morphological features of the hybrid nanomaterials. The results revealed that the laser-assisted in-situ deposition of Ag NPs on the few-layered GO surface improved its antibacterial properties, in which the hybrid nanostructure synthesized at a longer wavelength exhibited higher antibacterial action resistance to Escherichia coli (E. coli) than Staphylococcus aureus (S. aureus) bacteria. Moreover, nonlinear optical absorption (NLA) of Ag-GO nanohybrid was measured using the open aperture Z-scan technique. The Z-scan results signify the NLA properties of the Ag-GO hybrid material and have a large decline in transmittance of more than 60%, which can be employed as a promising optical limiting (OL) material.

Mechanistic insights into the optical limiting performance of carbonaceous nanomaterials embedded with core-shell type graphite encapsulated Co nanoparticles.

Globular amorphous carbonaceous materials embedded with graphite encapsulated metallic Co-nanoparticles with a high degree of crystallinity are synthesized by pyrolysis and demonstrated as excellent candidates for optical limiters. The amount of metal precursor (Co-acetylacetonate) used with toluene for pyrolysis is chosen as a strategy to control the degree of graphitization of graphene-like shells around the embedded Co-nanoparticles and also the crystallinity of these Co nanoparticles in the samples. The graphitic shell with an optimum amount of defects tunes the electronic properties of these nanomaterials, providing the electronic states required for the enhancement of nonlinear optical absorption (NLA) through an excited state absorption (ESA) process. Simultaneously, the increase in the crystallinity of the Co nanoparticle enhances its metallic nature, which helps in increasing NLA performance through the free carrier absorption (FCA) process. The importance of highly metallic Co is to involve both the Co nanoparticle and its graphitic encapsulation in facilitating the FCA process, which substantially enhances NLA. In comparison with many similar samples (e.g., Fe3C@C at 100 µJ of laser energy), our present samples show superior NLA performance even at the much lower laser pulse energy of ∼15 µJ. This performance is much better than many of the present-day NLA materials too. The simple, low-cost and one-step pyrolysis synthesis process makes our materials even more attractive.

Observation of ion acceleration in nanosecond laser generated plasma on a nickel thin film under rear ablation geometry.

In this article we report acceleration observed for the ions produced in a 50-nm-thick nickel film coated on a quartz substrate, under nanosecond laser ablation, in the rear ablation geometry. A detailed study with varying background pressure and laser energy is done. Spectroscopic study including spectroscopic time of flight (STOF) measurements of ionic and other neutral transitions from the plasma has been undertaken. The STOF spectra recorded for ionic transition clearly show an enhancement in the velocity of the slow component as the background pressure increases. In addition, a large asymmetric spectral broadening in the 712.22-nm neutral line is observed, which increases with background pressure. While these observations have similarity to some of the reported studies on the acceleration of ionic species through double-layer formation, the electric fields calculated from the measured acceleration appear to be anomalously higher, and a double-layer concept seems to be inadequate. Moreover, the large asymmetry observed in the neutral line profile is indicative of microelectric fields present inside the laser produced plasma plume, which may play a role in the continuous acceleration of the ions. Interestingly, this asymmetry in spectral broadening exhibits temporal and spatial dependence, which indicates that significant electric field is present in the plasma plume even for longer duration and larger distance from the target. These spectroscopic observations of acceleration have also been complemented by triple Langmuir probe measurements. To the best of our information, such observations regarding large ion acceleration for the rather low laser intensities as used in this experiment have not been reported in literature so far.

Molecular structure, NLO properties and vibrational analysis of l-Histidine tetra fluro borate by experimental and computational spectroscopic techniques.

l-histidine tetra fluroborate (L-HTFB) is a semi-organic NLO material. Ab-initio computations were performed at CAM-B3LYP/ 6-311++g(d,p) level of theory to arrive at the structures, energies, and vibrational wave numbers. The experimental FT-IR and FT-Raman spectra of L-HTFB have been recorded and analyzed. It is compared with the simulated spectra. The scaled wave numbers obtained are in good agreement with the experimental values. Hirshfeld surface analysis represented in the 2D fingerprint plot reveals the interaction within the compound. Optimized geometry reveals that the complexes l -histidine (L-H) and tetrafluoroborate (TFB) are linked by H-F bond which provides inter and intra molecular hydrogen bonded interactions such as N10-H16⋯F25, N11-H17⋯F22, N11-H14⋯F24, N11-H14⋯N10 and N11-H15⋯O12. The inter-molecular distances H17⋯F22, H14⋯F24 and H16⋯F25 are in between 1.82 Šand 2.53 Å. The intra molecular distances H14⋯N10 and H15⋯O12 are found to be at 2.17 Šand 1.71 Šwhich stabilizes lone pair electron of N10 and O12 with energies 11.89 and 59.91 KJ/mol respectively. The intra and inter molecular hyperconjugations responsible for the stability of the molecule are well identified theoretically using the NBO analysis. Third order optical nonlinearity is measured by means of the open aperture Z-scan technique which reveals that the crystal has optical limiting property. Photoluminescence results clearly indicate the use of L-HTFB as a new violet-light emitting material.

Effect of Mannitol on Intraocular Pressure in Vitrectomized and Nonvitrectomized Eyes: A Prospective Comparative Study.

PURPOSE: To compare the effect of mannitol in reducing intraocular pressure (IOP) in vitrectomized and nonvitrectomized eyes. MATERIALS AND METHODS: Prospective comparative case study. Eyes with IOP≥40 mm Hg were included. Eyes which are vitrectomized and silicon oil filled were classified as group 1, and nonvitrectomized open-angle eyes were classified as group 2. Mannitol (20%, 1 g/kg) was administered intravenously over 30 minutes, and IOP was recorded at 30 minutes interval till 2 hours and at the third and fourth hour from the start of mannitol. RESULTS: Thirty eyes (patients) were recruited in each group. Mean (SD) IOP reduced from 48.5±5.2 to 43.7±8 at 30 minutes, 40.7±8.4 at 60 minutes, 37.3±9.6 at 90 minutes, 35.6±10.4 at 2 hours, 34±10.7 at 3 hours, and 33±11.2 mm Hg at 4 hours in group 1, and from 48.9±6.5 to 43.2±8.6 at 30 minutes, 40.2±7.8 at 60 minutes, 36.7±7.3 at 90 minutes, 35.1±7.7 at 2 hours, 34.2±8.8 at 3 hours, and 35.7±9.4 mm Hg at 4 hours in group 2. There was a significant reduction in IOP at each time point compared with baseline in both the groups (P<0.001). There was no significant difference in IOP between the 2 groups at each time point. CONCLUSION: Mannitol reduces IOP significantly in both vitrectomized and nonvitrectomized eyes.

Intermediate-term outcome of Aurolab aqueous drainage implant.

PURPOSE: To report the intermediate-term safety and efficacy of Aurolab aqueous drainage implant (AADI) in patients with glaucoma. METHODS: Retrospective review of patients who underwent AADI between January 2013 and December 2016. Patients aged >16 years and with a minimum follow-up of 6 months were included. Success was defined as complete when the intraocular pressure was ≥6 and ≤21 mmHg without antiglaucoma medication and as qualified if those requiring additional antiglaucoma medications were included. RESULTS: The study included 55 patients (55 eyes) with a mean age ± standard deviation (SD) of 47.3 ± 18.1 years with a mean follow-up of 16.7 ± 11.4 months. Mean intraocular pressure reduced from 30.8 ± 11.1 mmHg to 13.1 ± 4.7, 14.1 ± 4.8, 15.7 ± 2.5 (P < 0.001) mmHg at 6 months, 1 year, and 2 years, respectively. The mean number of antiglaucoma medications reduced from 3.4 ± 1 to 0.8 ± 1.2, 0.7 ± 1.1, 0.8 ± 1 (P < 0.001) at 6 months, 1 year, and 2 years, respectively. The cumulative probability of complete and qualified success was 62% and 100% at 6 months, 54% and 92% at 1 year, and 43% and 88% at 2 years, respectively. Four patients failed during the follow-up period. Postoperative complication occurred in 28 eyes (51%), of which 17 eyes (31%) required intervention. CONCLUSION: AADI is a safe and effective treatment for the control of intraocular pressure in patients with glaucoma.

Enhanced Ultrafast Nonlinear Optical Response in Ferrite Core/Shell Nanostructures with Excellent Optical Limiting Performance.

Nonlinear optical nanostructured materials are gaining increased interest as optical limiters for various applications, although many of them suffer from reduced efficiencies at high-light fluences due to photoinduced deterioration. The nonlinear optical properties of ferrite core/shell nanoparticles showing their robustness for ultrafast optical limiting applications are reported. At 100 fs ultrashort laser pulses the effective two-photon absorption (2PA) coefficient shows a nonmonotonic dependence on the shell thickness, with a maximum value obtained for thin shells. In view of the local electric field confinement, this indicates that core/shell is an advantageous morphology to improve the nonlinear optical parameters, exhibiting excellent optical limiting performance with effective 2PA coefficients in the range of 10-12 cm W-1 (100 fs excitation), and optical limiting threshold fluences in the range of 1.7 J cm-2 . These values are comparable to or better than most of the recently reported optical limiting materials. The quality of the open aperture Z-scan data recorded from repeat measurements at intensities as high as 35 TW cm-2 , indicates their considerably high optical damage thresholds in a toluene dispersion, ensuring their robustness in practical applications. Thus, the high photostability combined with the remarkable nonlinear optical properties makes these nanoparticles excellent candidates for ultrafast optical limiting applications.

Optical nonlinearity and charge transfer analysis of pyrene adsorbed on silver: Computational and experimental investigations.

The interaction of pyrene on silver has been investigated using both experimental and computational methods. Hyperpolarizabilities computed theoretically together with experimental nonlinear absorption from open aperture Z-scan measurements, point towards a possible use of pyrene adsorbed on silver in the rational design of NLO devices. Presence of a red shift in both simulated and experimental UV-Vis spectra confirms the adsorption on silver, which is due to the electrostatic interaction between silver and pyrene, inducing variations in the structural parameters of pyrene. Fukui calculations along with MEP plot predict the electrophilic nature of the silver cluster in the presence of pyrene, with NBO analysis revealing that the adsorption causes charge redistribution from the first three rings of pyrene towards the fourth ring, from where the 2p orbitals of carbon interact with the valence 5s orbitals of the cluster. This is further confirmed by the downshifting of ring breathing modes in both the experimental and theoretical Raman spectra.

Syntheses, Spectroscopic, Electrochemical, and Third-Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2-phenylpyridine)}iridium Complex.

The synthesis of fac-[Ir{N,C1'-(2,2'-NC5H4C6H3-5'-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡CH)}3] (10), which bears pendant ethynyl groups, and its reaction with [RuCl(dppe)2]PF6 to afford the heterobimetallic complex fac-[Ir{N,C1'-(2,2'-NC5H4C6H3-5'-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡C-trans-[RuCl(dppe)2])}3] (11) is described. Complex 10 is available from the two-step formation of iodo-functionalized fac-tris[2-(4-iodophenyl)pyridine]iridium(III) (6), followed by ligand-centered palladium-catalyzed coupling and desilylation reactions. Structural studies of tetrakis[2-(4-iodophenyl)pyridine-N,C1'](µ-dichloro)diiridium 5, 6, fac-[Ir{N,C1'-(2,2'-NC5H4C6H3-5'-C≡C-1-C6H2-3,5-Et2-4-C≡CH)}3] (8), and 10 confirm ligand-centered derivatization of the tris(2-phenylpyridine)iridium unit. Electrochemical studies reveal two (5) or one (6­10) Ir-centered oxidations for which the potential is sensitive to functionalization at the phenylpyridine groups but relatively insensitive to more remote derivatization. Compound 11 undergoes sequential Ru-centered and Ir-centered oxidation, with the potential of the latter significantly more positive than that of Ir(N,C'-NC5H4-2-C6H4-2)3. Ligand-centered π­π* transitions characteristic of the Ir(N,C'-NC5H4-2-C6H4-2)3 unit red-shift and gain in intensity following the iodo and alkynyl incorporation. Spectroelectrochemical studies of 6, 7, 9, and 11 reveal the appearance in each case of new low-energy LMCT bands following formal IrIII/IV oxidation preceded, in the case of 11, by the appearance of a low-energy LMCT band associated with the formal RuII/III oxidation process. Emission maxima of 6­10 reveal a red-shift upon alkynyl group introduction and arylalkynyl π-system lengthening; this process is quenched upon incorporation of the ligated ruthenium moiety on proceeding to 11. Third-order nonlinear optical studies of 11 were undertaken at the benchmark wavelengths of 800 nm (fs pulses) and 532 nm (ns pulses), the results from the former suggesting a dominant contribution from two-photon absorption, and results from the latter being consistent with primarily excited-state absorption.

Ensemble effect of intrinsic defects and Mn ions on the enhancement of third order nonlinearity of ZnO nanoparticles.

Mn doped ZnO nanoparticles were synthesized by the chemical co-precipitation method and XRD confirms the hexagonal structure of nanoparticles with good crystallinity. From TEM analysis, the average crystallite size of the nanoparticles is evaluated as 7-13 nm. All the prepared nanoparticles exhibit reverse saturable absorption in open aperture Z-scan measurements. The experimental data best fits for two-photon absorption along with saturable absorption and the obtained ß values are found to be in the range of 0.3-0.5 × 10(-10) m W(-1). The enhancement of ß upon increasing Mn concentration is attributed to the increase of absorption due to defect states created by Mn doping. EPR and PL measurements provide evidence of the presence of zinc vacancy defects. The self-defocusing nonlinearity exhibited in Mn doped ZnO nanoparticles at 532 nm indicates the reverse saturable absorption (RSA) based optical limiting behavior.

Defect engineering in ZnO nanocones for visible photoconductivity and nonlinear absorption.

Nanostructured ZnO is a promising material for optoelectronic and nonlinear optical applications because of the flexibility of band gap engineering by means of various defect states present in it. Employing the time-correlated single photon counting photoluminescence technique, the correlation between defect levels and optoelectronic and nonlinear optical properties of ZnO is explored in this work. By a facile solution method, ZnO nanocones with a dominating preferential orientation along energetically less favorable, oxygen terminated (10̄11) facets were synthesized using a passivating capping agent. Photoluminescence spectra demonstrate that the as-grown samples have both oxygen and zinc vacancies, and after calcination in air oxygen vacancies vanish, but zinc vacancies are enhanced. Photoconductivity of the samples reduces significantly upon calcination, confirming the reduction in oxygen vacancies. However, the samples exhibit a significant enhancement in the nonlinear optical absorption coefficient upon calcination, indicating that the effective two-photon absorption causing the nonlinear optical behaviour originates from zinc vacancies. These results illustrate the vast possibilities of band gap engineering in intrinsic ZnO for future optoelectronic applications.

Synthesis and spectroscopic characterization of copper zinc aluminum nanoferrite particles.

Copper doped zinc aluminum ferrites CuxZn1-x.(AlxFe2-x)O4 are synthesized by the solid-state reaction route and characterized by XRD, TEM, EPR and non linear optical spectroscopy techniques. The average particle size is found to be from 35 to 90nm and the unit cell parameter "a" is calculated as from 8.39 to 8.89Å. The cation distributions are estimated from X-ray diffraction intensities of various planes. The XRD studies have verified the quality of the synthesis of compounds and have shown the differences in the positions of the diffraction peaks due to the change in concentration of copper ions. TEM pictures clearly indicating that fundamental unit is composed of octahedral and tetrahedral blocks and joined strongly. The selected area electron diffraction (SAED) of the ferrite system shows best crystallinity is obtained when Cu content is very. Some of the d-plane spacings are exactly coinciding with XRD values. EPR spectra is compositional dependent at lower Al/Cu concentration EPR spectra is due to Fe(3+) and at a higher content of Al/Cu the EPR spectra is due to Cu(2+). Absence of EPR spectra at room temperature indicates that the sample is perfectly ferromagnetic. EPR results at low temperature indicate that the sample is paramagnetic, and that copper is placed in the tetragonal elongation (B) site with magnetically non-equivalent ions in the unit cell having strong exchange coupling between them. This property is useful in industrial applications. Nonlinear optical properties of the samples studied using 5ns laser pulses at 532nm employing the open aperture z-scan technique indicate that these ferrites are potential candidates for optical limiting applications.

The role of defects in the nonlinear optical absorption behavior of carbon and ZnO nanostructures.

In bulk materials, defects are usually considered to be unwanted since deviations from perfect lattices may degrade device performance. Interestingly, the presence of defects throws open new possibilities in the case of nanostructures due to the properties related to their limited size scale. Defects and disorders which alter the electronic structure of nanostructures can significantly influence their electronic, magnetic and nonlinear optical properties. Here, we show that defect engineering is an effective strategy for tailoring the nonlinear optical (NLO) properties of carbon and ZnO nanostructures. The effects of surface states, lattice disorders, polycrystalline interfaces and heterogeneous dopants on the nonlinear absorption behaviour of these nanostructures are discussed in detail. Realistic tunable NLO features achieved by controlling such defects enhance the scope of these nanostructures in device applications such as optical limiting, optical switching, pulse shaping, pulse compression and optical diode action.

Enhanced optical limiting in polystyrene-ZnO nanotop composite films.

In this work we investigate the optical limiting property of polystyrene-zinc-oxide (ZnO) nanotop composite films, using an open aperture Z-scan technique. The nanocomposites are prepared for different loading concentrations of ZnO and are fabricated using spin and dip coating techniques. On exposing the films to a pulsed nanosecond laser at 532 nm, the nonlinear absorption (NLA) coefficient is found to be greater for spin-coated films compared to dip-coated films. The measured NLA coefficient is found to be enhanced with an increase in loading concentration of ZnO in the monomer for both spin- and dip-coated films.

Optical diode action from axially asymmetric nonlinearity in an all-carbon solid-state device.

Nanostructured carbons are posited to offer an alternative to silicon and lead to further miniaturization of photonic and electronic devices. Here, we report the experimental realization of the first all-carbon solid-state optical diode that is based on axially asymmetric nonlinear absorption in a thin saturable absorber (graphene) and a thin reverse saturable absorber (C60) arranged in tandem. This all-optical diode action is polarization independent and has no phase-matching constraints. The nonreciprocity factor of the device can be tuned by varying the number of graphene layers and the concentration or thickness of the C60 coating. This ultracompact graphene/C60 based optical diode is versatile with an inherently large bandwidth, chemical and thermal stability, and is poised for cost-effective large-scale integration with existing fabrication technologies.