Open Aperture Research Articles

Plasmon mediated extra-ordinary optical transmission through an apertureless plasmonic metagrating

Extra-ordinary optical transmission (EOT) through subwavelength plasmonic nanoapertures is possible due to the funneling of light via surface plasmons (SPs) at the resonant wavelengths through the apertures. In this Letter, we experimentally demonstrate EOT through a plasmonic metagrating which does not have any open apertures. The plasmonic metagrating was fabricated by deposition of silver (Ag) on a one-dimensionally patterned flexible and transparent polydimethylsiloxane grating obtained via pattern imprinting and subsequent peeling off a commercially available blue ray disk. For normal incidence of transverse magnetic-polarized light on the top surface of plasmonic metagrating, transmission of light through it was obtained in the visible wavelength range of electromagnetic spectrum. Control experiments on variation of Ag film thickness were performed to attain optimal parameters for maximum transmission, followed by polarization and refractive index (RI) dependent performance of the plasmonic metagrating. Electric fields and Poynting vector profiles were simulated using a finite element method to explain the interaction of light with the plasmonic metagrating and the mechanism of plasmon mediated optical transmission. Such a large optical transmission is possible because the SP modes generated at metal–air interface penetrate through metagrating and couple with those supported by the metal–substrate interface. As a model application, RI sensing using the plasmonic metagrating was demonstrated. The present study shows that optical transmission is possible from apertureless structures and enriches literature with better understanding of EOT. Moreover, it opens avenues for development of flexible, cost-effective plasmonic metagratings for sensors, spectral filters, polarizers, etc.

Nonlinear optical properties of gold nanoparticles containing borate glasses at spectral region of surface plasmon resonance

Herein, we subjected the sodium borate glass activated with gold nanoparticles (NPs) to thermal treatment at 500 °C for different holding time to improve their nonlinear optical (NLO) characteristics in visible spectral region. The linear optical density and microscopic measurements confirmed the presence gold NPs in the glass. The outcomes also divulged the growth of Au NPs with an annealing schedule. The NLO attributes were ascertained through the Z–scan method, in which the samples were pumped at 532 nm utilizing nanosecond laser pulses fired at a rate of 1 kHz. Open aperture Z–scan measurements disclosed the existence of two–photon absorption nonlinearity, whereas closed aperture Z–scan experiments revealed the presence of self–defocusing nonlinear refraction in the studied glasses. The measurements unveiled the enhance trend of two–photon absorption and nonlinear refraction coefficients of glass host with thermal holding time owing to the growth of NPs. It was identified that the titled glass embedded with gold NPs annealed for 7 h at 500 °C owes maximum NLO features. Therefore, NBAu–7 glass examined in the current sturdy is beneficial for bio–imaging, optical limiting and NLO nanophotonic applications.

Anatomical drivers of stomatal conductance in sorghum lines with different leaf widths grown under different temperatures.

Sustaining crop productivity and resilience in water-limited environments and under rising temperatures are matters of concern worldwide. We investigated the leaf anatomical traits that underpin our recently identified link between leaf width (LW) and intrinsic water use efficiency (iWUE), as traits of interest in plant breeding. Ten sorghum lines with varying LW were grown under three temperatures to expand the range of variation of both LW and gas exchange rates. Leaf gas exchange, surface morphology and cross-sectional anatomy were measured and analysed using structural equations modelling. Narrower leaves had lower stomatal conductance (gs ) and higher iWUE across growth temperatures. They also had smaller intercellular airspaces, stomatal size, percentage of open stomatal aperture relative to maximum, hydraulic pathway, mesophyll thickness, and leaf mass per area. Structural modelling revealed a developmental association among leaf anatomical traits that underpinned gs variation in sorghum. Growing temperature and LW both impacted leaf gas exchange rates, but only LW directly impacted leaf anatomy. Wider leaves may be more productive under well-watered conditions, but consume more water for growth and development, which is detrimental under water stress.

Optical limiting behavior of metal (Mn, W) oxides decorated nitrogen-doped reduced graphene oxide nanocomposites stimulated by two-photon absorption

Nanopulsed Nd: YAG green laser-induced nonlinear absorption and optical limiting properties of rGO/MnO2, NrGO/MnO2 and NrGO/MnO2/WO3 nanostructures were studied for the first time utilizing single beam Z-scan experiment. Initially, rGO was prepared by solar exfoliation whereas MnO2, WO3, rGO/MnO2, NrGO/MnO2 and NrGO/MnO2/WO3 nanostructures were synthesized by hydrothermal method. XRD was used to determine the structure and crystallite size of the materials. Carbon allotropes and functional groups of the prepared materials were studied using Raman and FT-IR spectroscopy. SEM images showed layered rGO, as well as rod-like MnO2 and flake-like WO3 nanostructures adorned on the top of NrGO layers. EDS analysis validated the presence of C, N, Mn, W, and O. Linear optical study manifested absorption in the UV–Visible region between 200 nm and 800 nm. The occurrence of reverse saturable absorption (RSA) was confirmed by an open aperture Z-scan technique. Compared to its pure analogs and derivatives of graphene, MnO2 decorated NrGO had a higher two-photon absorption coefficient (8.92 × 10−11 m/W) and the lowest optical limiting (OL) threshold (3.29 × 1012 W/m2). Extensive conjugation for charge transfer in NrGO and rod-like shape, homogeneous decorating, and strong visible absorption in MnO2 combine to provide enhanced nonlinear optical (NLO) response and has the potential use in energy stabilizers and laser safety devices in the 532 nm, 9ns laser domain.

Fabrication, Characterization of Basic Blue 7 Dye-Doped PVA Films and Their Third-Order Nonlinear Optical Properties.

In this work, we report on the fabrication, characterization and the third-order nonlinear optical (NLO) properties of basic blue 7 (BB 7) dye‒doped polyvinyl alcohol (PVA) polymer thin films. The absorption spectra of BB 7 dye‒PVA films were measured by UV‒visible absorption studies. The functional groups of BB 7‒PVA films have been identified by FT‒IR spectroscopic analysis. The surface morphology was examined by AFM and SEM analysis which shows that the BB 7 dye‒doped PVA films have a homogenous and smooth surface. The nonlinear absorption and refraction of BB 7‒PVA films were respectively explored by open aperture and closed aperture Z‒scan technique using a 5 mW semiconductor diode laser of 635nm wavelengths. The BB 7‒PVA films exhibit the reverse saturable absorption (RSA) and self-defocusing effect and the measured third‒order nonlinear susceptibility χ(3) values of these films were found to be of the order of 10- 5 esu. The present experimental results show that BB 7‒PVA films may have potential applications in future photonic and NLO devices.

Bioinspired Drag Reduction of Cavities Induced by Janus Nanostructure Interfaces on Hydrophobic Spheres Plunging into Water with a Low Speed

AbstractForm drag of the blunt object occurs from the pressure difference due to the physical dimensions of the object obstructing and altering the flow of the fluid. The innovative approach of form drag reduction is realized by changing the shape of the blunt object to a streamlined body by introducing a cavity. The construction of superhydrophobic interfaces is widely considered as the best solution for introducing a cavity, because water is prevented from penetrating the space between the nanoscale structures, leading to a Cassie–Baxter wetting regime and then water splashing outward. Here, by introducing Janus nanostructure coating on hydrophobic spheres, it is found that the position (impacting direction) and coverage ratio (r) of the nanostructures, accurately manipulate the performance of water film on the sphere surface. When the nanostructure faces upward, the detached thin film develops to a splash crown only if r increases to 1/3 or even above. Correspondingly, the remaining open aperture aids in increasing the subsequent air entrainment, and an air‐entrainment cavity is trapped on the sphere surface. However, if the nanostructure faces downward, only a tiny region of the nanostructures (r ≤ 1/8) can help a thin liquid film directly develop into a splash crown and cavity.

Growth origin of large-scale fiberform nanostructures in He–W co-deposition environment

When tungsten (W) is deposited with helium (He) plasma (He–W co-deposition) on W surface, enhanced growth of fiberform nanostructure (fuzz) occurs, and sometimes it grows into large-scale fuzzy nanostructures (LFNs) thicker than 0.1 mm. In this study, different numbers of mesh opening (apertures) and W plates with nanotendril bundles (NTBs), which are tens of micrometers high nanofiber bundles, were used to investigate the condition for the origin of the LFN growth. It was found that the larger the mesh opening, the larger the area where LFNs are formed and the faster the formation tends to be. On NTB samples, it was found that NTBs grew significantly when exposed to He plasma with W deposition, especially when the size of the NTB reached sim 0.1 mm. The concentration of the He flux due to the distortion of the shape of the ion sheath is proposed as one of the reasons to explain the experimental results.

Influence of arch shaped notch angle, length and opening on the failure mechanism of rock like material and acoustic emission properties: Experimental test and numerical simulation

The uniaxial compressive failure mechanism of rock like specimens with arched shaped notches was studied experimentally and numerically considering the effects of cracks’ properties on the mechanical behavior of the rocks. The rock like material specimens were provided in laboratory in form of cubes with dimensions of 150 mm × 150 mm × 50 mm containing one arch shaped notch at the center of the specimen where, the notch diameter was changed as 20 mm, 40 mm, and 60 mm. the notch apperture was also varied as 2 mm. 4 mm and 6 mm. The axisymetric axis of the arched notch made angles changed from 0° to 90° by increasing the 15°. In this research 63 different models of the compression tests were made by the two-dimensional particle flow code (PFC2D). The experimental tests and their numerical simulations performed simultaneously to compare them for the arch shaped notch tests under compression. The results of these analyses revealed that the geometric characteristics of the arch shape notch such as the notch diameter, angle and its opening (aperture) governe the failure process of the models and tests under compressive loading conditions. It is also shown that the failure mechanisms of the arch shaped notches and their fracture modes are associated with the compressive strengths of the testing samples. The changes in notch diameter, notch angle and notch opening may make some significant changes in AE hits during the failure process of the specimens. The AE hits grow rapidly after the loading process and before the peak load. Any stress drop may be detected by changes in AE hits during the test. It is concluded that the fracture pattern and failure of the experimental tests and their corresponding numerical simulations are similar to one another which verify the validity and accuracy of the proposed approaches in this research.

A scanning focus nuclear microscope with multi-pinhole collimation

Microscopic nuclear imaging down to spatial resolutions of a few hundred microns can already be achieved using low-energy gamma emitters (e.g. 125I, ∼30 keV) and a basic single micro-pinhole gamma camera. This has been applied to in vivo mouse thyroid imaging, for example. For clinically used radionuclides such as 99mTc, this approach fails due to penetration of the higher-energy gamma photons through the pinhole edges. To overcome these resolution degradation effects, we propose a new imaging approach: scanning focus nuclear microscopy (SFNM). We assess SFNM using Monte Carlo simulations for clinically used isotopes. SFNM is based on the use of a 2D scanning stage with a focused multi-pinhole collimator containing 42 pinholes with narrow pinhole aperture opening angles to reduce photon penetration. All projections of different positions are used to iteratively reconstruct a three-dimensional image from which synthetic planar images are generated. SFNM imaging was tested using a digital Derenzo resolution phantom and a mouse ankle joint phantom containing 99mTc (140 keV). The planar images were compared with those obtained using a single-pinhole collimator, either with matched pinhole diameter or with matched sensitivity. The simulation results showed an achievable 99mTc image resolution of 0.04 mm and detailed 99mTc bone images of a mouse ankle with SFNM. SFNM has strong advantages over single-pinhole imaging in terms of spatial resolution.

Investigation on the performance of an active noise control window system mounted on the top-hung window of a residential room

Urban noise mitigation experiment through active noise control window is not commonly carried out in in situ urban setting. This paper outlines our experimental settings of performing active noise control across the open aperture of the top-hung window frame in a dormitory room. A multi-channel adaptive filtered-x least mean square method is implemented in real-time. Experiments and analyses are carried out to investigate how the noise reduction performance of this active noise control window system is affected by the number of reference microphones, the distance between reference microphones and secondary sources, and the distributions of error microphones under practical restrictions of the real system implementation. Variation of the secondary path estimation is another important consideration in active noise control system design. Real-time experiments are carried out to investigate the sensitivity of the ANC window system over the secondary path variation caused by varying window opening status, and whether a generalised secondary path measured in one window opening setting can be applied to other window opening conditions.

Nonlinear absorption and refraction of Cs3Cu2Br5 perovskite

Halide perovskite (Cs3Cu2Br5) is synthesized by a thermal injection method. The X-ray diffraction (XRD) proves that the Cs3Cu2Br5 crystal phase is an intersection phase. The Cs3Cu2Br5 organic glasses are fabricated by dispersing Cs3Cu2Br5 materials in methyl methacrylate (MMA). The nonlinear absorption (NLA) and the nonlinear refraction (NLR) performances of Cs3Cu2Br5/PMMA organic glasses are measured via the open-aperture (OA) and the closed-aperture (CA) Z-scan technique, respectively. The samples show typical reverse saturable absorption (RSA) and self-focusing effect with the change of input energy (40–72 μJ). Meanwhile, the room-temperature photo-luminescence (PL) measurement reveals light emission for Cs3Cu2Br5. The effects on nonlinear optical (NLO) properties are investigated by controlling the input energy and the amount of the sample. The Cs3Cu2Br5/PMMA organic glasses exhibit excellent NLA and NLR properties. The NLA coefficient of (Cs3Cu2Br5)18/PMMA organic glass is much higher than those of lead-containing perovskite such as CsPbCl3 and CsPbBr3 nanosheets. This results in Cs3Cu2Br5/PMMA organic glasses show superior optical limiting (OL) performance. The Cs3Cu2Br5 perovskite materials show great potential in devices such as light-emitting diodes and optical switches.

Nature of Linear Spectral Properties and Fast Relaxations in the Excited States and Two-Photon Absorption Efficiency of 3-Thiazolyl and 3-Phenyltiazolyl Coumarin Derivatives.

Coumarin-based fluorescent agents play an important role in the manifold fundamental scientific and technological areas and need to be carefully studied. In this research, linear photophysics, photochemistry, fast vibronic relaxations, and two-photon absorption (2PA) of the coumarin derivatives, methyl 4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]butanoate (1) and methyl 4-[4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]phenoxy]butanoate (2), were comprehensively analyzed using stationary and time-resolved spectroscopic techniques, along with quantum-chemical calculations. The steady-state one-photon absorption, fluorescence emission, and excitation anisotropy spectra, as well as 3D fluorescence maps of 3-hetarylcoumarins 1 and 2 were obtained at room temperature in solvents of different polarities. The nature of relatively large Stokes shifts (∼4000-6000 cm-1), specific solvatochromic behavior, weak electronic π → π* transitions, and adherence to Kasha's rule were revealed. The photochemical stability of 1 and 2 was explored quantitatively, and values of photodecomposition quantum yields, on the order of ∼10-4, were determined. A femtosecond transient absorption pump-probe technique was used for the investigation of fast vibronic relaxation and excited-state absorption processes in 1 and 2, while the possibility of efficient optical gain was shown for 1 in acetonitrile. The degenerate 2PA spectra of 1 and 2 were measured by an open aperture z-scan method, and the maximum 2PA cross-sections of ∼300 GM were obtained. The electronic nature of the hetaryl coumarins was analyzed by quantum-chemical calculations using DFT/TD-DFT level of theory and was found to be in good agreement with experimental data.

Influence of embedded NiO-nanoparticles on the nonlinear absorption of tungsten disulfide nanolayers

2D transition metal dichalcogenides possess fascinating properties due to their direct bandgap, strong spin-orbit coupling, and promising electronic/mechanical properties. In this work, we synthesized pure tungsten disulfide (WS2) nanolayers and NiO nanoparticles (NPs) decorated in few-layered WS2 and measured the third-order nonlinear optical properties using femtosecond Z-scan measurements. The open aperture Z-scan data illustrated that the inclusion of NiO nanoparticles into the WS2 layers increases the nonlinear absorption at 800 and 400 nm wavelengths. Furthermore, we observed the switchability of the nonlinear absorption from saturable absorption to two-photon absorption or reverse saturable absorption by changing the pump intensity. Thus, the embedded NiO NPs played a crucial role in the variation of intensity-dependent nonlinear absorption mechanism of WS2 nanolayers and thus can be helpful for various optical applications such as laser pulse compression and optical limiting to prevent over-exposure of protective photosensitive sensors by intense ultrashort laser pulses.

Engineering the nonlinear optical properties of barium molybdate by doping Sn4+ ions for optical limiting device applications

Materials exhibiting strong two-photon absorption and low photo-damage threshold are highly desirable owing to their wide application in optoelectronic and photonic devices. Metal molybdates have gained significant attention for optoelectronic applications due to their excellent optical and luminescent properties. Herein, we report the nonlinear properties of the unexplored tin (Sn) doped barium molybdate (BaMoO4) nanostructures for tunable optical limiting applications. XRD and Raman Spectroscopy extensively studied structural properties. The UV–Vis absorption spectra clearly reveal the strong absorption in the blue region and suggest the bandgap reduction with increased Sn concentration in the host lattice. Open aperture Z scan mode, exhibits strong reverse saturable absorption owing to two-photon absorption. In addition, we observed that the nonlinear absorption coefficient and optical limiting threshold depend extensively on the excitation laser energy and can be altered by varying the concentration of Sn ions. The observed nonlinearity confirms that the synthesized Tin (Sn) doped Barium Molybdate (BaMoO4) nanostructures are potential for tunable optical limiting applications.

Growth and characterization of L - Methionine Barium Bromide (LMBB) semi-organic crystal for optical limiting applications

The semi-organic nonlinear optical single crystal was grown from an aqueous solution by a slow evaporation solution technique (SEST) at room temperature. The characterization of the crystal was made using single crystal x-ray diffraction analysis and it was found to be L-Methionine Barium Bromide (LMBB) crystallized in a monoclinic system with space group C2/c and the cell parameter values are a= 10.74Å, b= 7.39Å, c= 8.69Å, α = γ = 90°, β = 113.53°. The UV-Vis-NIR spectroscopic study revealed that the crystal has good optical transparency and a lower cut-off wavelength was found to be 246 nm and also evaluated linear optical properties such as extinction coefficient and reflectance. The luminescence behavior of LMBB has been analyzed by fluorescence (FL) spectral study. The presence of functional groups in the LMBB is confirmed by FT-IR and FT-Raman vibrational patterns. The hardness (Hv), Meyer’s index (n), elastic stiffness constant (C11), yield strength (σy), knoop hardness (Hk), fracture toughness (Kc), and brittleness index (Bi) were analyzed using Vickers microhardness tester. Dielectric behavior as a function of frequency was examined. The laser-induced damage threshold value was found to be higher than that of KDP. The surface morphology of the as-grown crystal was determined using the SEM technique. Energy dispersive X-ray analysis was used to determine the chemical composition of the LMBB crystal. The nonlinearity coefficient (β) is measured as 1.7 × 10−11 m/W using the open aperture Z-scan technique (Nd: YAG, 532 nm, 9 ns), which results from an effective two-photon absorption phenomenon. The resulting optical limiting behavior has a fluence threshold of 8.26 × 1012 W/m2 making the LMBB a potential candidate for optical limiting devices.

Effect of Molar Concentration and Solvent Type on Linear and NLO Properties of Aurintricarboxylic (ATA) Organic Dye for Image Sensor and Optical Limiter Applications

Aurintricarboxylic (ATA) organic dye is a promising chemical compound candidate for several optical and electronic applications. However, this study aims to investigate the impact of molar concentrations and solvent type on the linear/nonlinear optical (NLO) properties of 5,5[Formula: see text]-[(3-carboxy-4-oxocyclohexa-2,5-dien-1-ylidene) methylene] bis (2-hydroxybenzoic acid) organic laser dyes. Three molar concentrations of organic dyes ([Formula: see text], [Formula: see text], and [Formula: see text]) M were respectively dissolved in distilled water and dimethylsulfoxide (DMSO) at room temperature. The samples were investigated using1H-NMR and UV/Visible spectrums. The bandgap ([Formula: see text]) of ([Formula: see text] M) ATA/water is 4.75[Formula: see text]eV, but Eg of ATA/DMSO was 2.93[Formula: see text]eV and 2.65[Formula: see text]eV. The measurements were performed by employing a diode-pumped solid-state laser operating at a 457 nm wavelength and 112[Formula: see text]mW of power. The [Formula: see text]-scan technique was applied to characterize open and close apertures, nonlinear refractive index ([Formula: see text]), and nonlinear absorption coefficient ([Formula: see text]). The obtained results predicted that both the [Formula: see text] and [Formula: see text] coefficients decreased with the reduction of molar concentrations. Additionally, DMSO exhibited better linear/NLO properties than water. The results revealed promising laser dye samples for image sensor and optical limiter applications.

Cocoons, Fungi, Nematodes, Rhizoids, Annelids, Eggs, Arthropoda in Gale Crater, Mars?

White and dark gray oblong-ovoid cocoon-egg-like forms (“cocoon-eggs”), less than a mm in diameter, many with a single hole in one end, and some with unidentifiable specimens protruding from (exiting or entering) these holes, were photographed on mudstone in Gale Crater, Mars, on Sol 1302. If biological some of these cocoon-eggs may be frozen, fossilized or have already “hatched” or undergone metamorphosis. Specimens resembling segmented and tubular worms and rhizoids and those with multiple appendages (Arthropoda) were also photographed on these and other substrates on Mars, including on Sols 130, 132, 302, 551, 553, 808, 809, 868, 906, 1280, 1921, in Gale Crater. Many Sol 1302 “cocoon-eggs” are embedded in substrate that appears to consist entirely of a coral-stromatolite-like mass of worm- and larvae-like forms; and similar worm and segmented larvae forms appear in the interior of three semi-transparent ovoid structures. Embedded in the soil surrounding these “cocoon-eggs” are numerous concave patches reminiscent of dried “egg-yolk,” cocoon remnants or fungus, and that may represent the final stage of “cocoon” collapse and disintegration; or they are fungal in origin. Some of the segmented and tubular worm-like forms may include rhizoids and tubular fungi. Ovoids with open apertures may be evidence of fungal sporing. The possibility that fungus and diverse species and their “cocoons” share the same substrate is supported by what appears to be cyanobacteria, worms, insecta (arthropoda), and stalks topped by ovoids or ovoid-shaped microbialite-like sediment. Fungi, lichens, cyanobacteria, stromatolites and arthropoda have been previously reported in other areas of Gale Crater, whereas fungal “puffballs” and specimens attached to rocks with stalks and topped with mushroom-like caps have been observed in Meridiani Planum. The possibility these specimens are abiogenic cannot be ruled out. The evidence, in our view, arguably supports the hypotheses that various organisms colonized Mars in the ancient or recent past.

Au–WS2 Nanohybrids with Enhanced Optical Nonlinearity for Optical Limiting Applications

Utilizing plasmonic effects of metal nanostructures on tailoring the properties of semiconducting materials has gained substantial research attention in the past 10 years. Here, we report the plasmon resonance-induced enhanced optical nonlinearity of Au–WS2 nanohybrids prepared via the blending of liquid phase exfoliated few-layer WS2 nanosheets with chemically synthesized spherical gold nanoparticles (NPs). Nonlinear optical properties were studied using the open aperture Z-scan technique with nanosecond laser pulses of wavelength 532 nm, which lie in the plasmonic band of Au NPs (480–550 nm) and B-excitonic transition range of WS2 nanosheets (520–535 nm). Au–WS2 nanohybrids exhibited an enhanced nonlinear absorption and optical limiting activity with an effective nonlinear absorption coefficient of 182 cm/GW at an on-axis input intensity of 0.14 GW/cm2, which is 3.87 times higher than that of bare WS2 nanosheets. The improved NLO response of nanohybrids is due to the plasmon resonance-induced local field effects, which significantly enhance B-excitonic transitions in WS2 nanosheets. The simulated field intensity enhancement profile also confirmed the effect of formation, where a strong interaction of plasmonic fields with WS2 nanosheets is clearly visible. The enhancement observed in simulated absorption density profiles indicated improved B-excitonic transitions in WS2 nanosheets due to the resonant plasmonic field. Au–WS2 nanohybrids exhibited a low limiting threshold of 0.55 J/cm2, which is better than that of many eminent nanohybrids reported so far. The strategy followed in this work for attaining superior nonlinear optical responses in Au–WS2 nanohybrids will be useful to develop novel nonlinear optical materials for photonic device applications.

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.

Third-order optical nonlinearity and passively Q-switching operation with BiOCl nanosheets at 1.3 µm.

In this contribution, we measured the third-order nonlinear optical response of bismuth oxychloride (BiOCl) nanosheets with the open-aperture (OA) and the closed-aperture (CA) Z-scan techniques with a variable excitation intensity at 1.34 µm. The effective nonlinear absorption coefficient βeff and the nonlinear refractive index n2 of the prepared BiOCl nanosheets with abundant oxygen vacancies were obtained under the excitation intensity. The third-order nonlinear optical susceptibility |χ(3)| was 1.64 × 10-9 esu. The nonlinear optical features of BiOCl enabled it as a superb saturable absorber for pulse laser generation. As a consequence, we demonstrated the first passively Q-switched Nd:GdVO4 laser with the BiOCl saturable absorber, producing a shortest pulse duration of 543 ns and a highest repetition rate of 227 kHz, leading to a maximum pulse energy of 74 nJ. Our findings show that BiOCl nanosheets with oxygen vacancies have large nonlinear optical sensitivities and can be exploited to generate optical pulses.