ZIF-8 is a widely studied metal-organic framework, but questions remain about its crystallization mechanism, especially at the initial stages. In this study, we used in-situ nonlinear optical (NLO) light scattering techniques to probe nucleation and crystal growth of ZIF-8 nanocrystals. The NLO measurements provided unique insights into the crystallization mechanism of ZIF-8. The formation of crystalline ZIF-8 nanoparticles was probed through depolarization measurements. By comparing second and third harmonic scattering the formation of defects was revealed in the earliest stages. These measurements highlight the potential of NLO techniques to probe nucleation and crystal growth in situ, providing information missed by traditional X ray based techniques.
Optical interrogation of cellular electrical activity is a crucial tool for understanding how cells function and communicate in complex networks. Scientists often use voltage-sensitive dyes to measure the excitability of cells, but these dyes can interfere with cellular function. Label-free techniques offer a way to measure electrical activity without using external probes. In this study, researchers found that second-harmonic generation from live cells is highly sensitive to changes in transmembrane potential by electrode control, making it a promising label-free approach to measure electrical activity in more complex cellular networks. This research provides a promising framework for a non-invasive label-free tool to measure electrical activity in cells.
The chiral nematic phase of cellulose nanocrystals (CNC) in the suspension liquid state is commonly examined using polarized optical microscopy (POM) as it conveniently reveals the cholesteric pitch through fingerprint textures. However, POM has certain limitations, one of which is the requirement for a perpendicular alignment of the chiral axis with the optical axis to achieve accurate measurements. We propose employing SHG microscopy as an alternative technique, offering high-contrast imaging of the chiral nematic phase with inherent 2-photon confocal effect. An IR pulsed femtosecond laser is raster scanned through the microscope, and the cumulative SHG response of the aligned CNC recreates the fingerprint textures. As is shown in this present work, the tight focussing effect of SHG microscope allowed z-scanning to render high contrast 3D models of the structures. Morphological observations and the tracking of defects in 3-dimensional space was made possible.
Present-day methods for determining the performance of third-order nonlinear optical materials include Z-scan, degenerate four-wave mixing and third-harmonic generation (THG). All these techniques possess severe drawbacks; for example, in THG, since all media (air and glass walls of the cell) present a third-order effect, eliminating these contributions requires careful, complex analysis or use of vacuum chambers. We have developed nonlinear scattering as a sensitive, straightforward technique for determining the second hyperpolarizability of samples in solution. Herein, we will for the first time show the applicability of the technique to measure organometallic Ru-complexes, optimized for high nonlinear responses. The investigated compounds showed a significant second hyperpolarizability |γ|, ranging from 1.1 for the least efficient to 2.8 ∙ 10-33 esu for the most efficient molecule, and comparable to fullerene C60 in thin films. It was deemed infeasible to extract hyperpolarizabilities using a high-frequency femtosecond laser source by a modified z-scan setup, which, in contrast to nonlinear scattering, could not account for the high degree of thermal lensing present in the investigated compounds.
Three-dimensionally (3D) ordered macroporous materials combine interesting structural and optical properties. Accessible and economic fabrication is essential to fully explore the unique possibilities these materials present. A common method to fabricate 3D ordered macroporous materials is by self-assembling colloids, resulting in so-called opals. A templating strategy is then often used to introduce additional functionality inside the porous structure, giving rise to inverse opals. In this work, we developed an easy and versatile method to fabricate highly uniform polymer inverse opals without overlayers. Briefly, our approach consists of sandwiching a resin melt between two opal templates, forcing all material inside or between the macroporous structures. The opal voids are fully filled and the superfluous melt material is extruded before curing the resin. Finally, the opal templates are removed by chemical etching. The resulting structures are freestanding 3D macroporous films with large-area uniformity, displaying strong photonic properties due to their structural order. Additionally, many applications require specific optical functionalities. The versatility of our templating method is uniquely suited for this purpose as it allows doping of the melt before infiltration. Therefore, we can incorporate a large variety of optical functions in the inverse opals using a single approach We believe this method will help the systematic investigation and improvement of existing effects in these structures, while providing a platform for the discovery and demonstration of novel effects. As this method combines 3D ordered macroporous materials with linear and nonlinear optical materials, it is even possible to tune optical interactions, which could be technologically relevant for OLEDs, solar cells, lasers, electro-optical modulators and optical switches.
Previous studies on the radiation effects upon polymer and polymer-based photonic materials suggest that the radiation resistance of the material is heavily dependent on the choice of polymer-host and guest-chromophore. To date, the best results have been achieved with electro optic polymeric materials based on CLD1 doped in APC, which has resulted in improved performance at the device level upon gamma-ray irradiation at moderate doses. However, the physical mechanisms are yet not fully understood. In this paper, we introduce an all-optical (linear and nonlinear) characterization protocol that is aimed to elucidate the mechanisms of the radiation damage/enhancement of electro-optic polymeric materials. This protocol is used to quantify the damage/enhancement effects upon irradiation in terms of the relevant physical parameters on a collection of electro-optic polymeric thin film samples.
In the quest for organic second-order nonlinear materials, self-assembled structures based on octupolar molecules have
shown to be promising candidates. The presented work focuses on an octupolar substituted 1,3,5-trisalkynylbenzene that
forms crystals that show significant second-harmonic generation (SHG). Continuous polarization SHG measurements
were performed to elucidate the tensorial nature of this response. Unfortunately the model at hand was unable to fit the
polarization patterns due to depolarization of the second harmonic beam caused by multiple scattering in the sample.
Although the chromophore is clearly achiral, these patterns revealed a different response for left and right circularly
polarized light, also known as SHG circular difference. This implies that these molecules spontaneously assemble in
chiral superstructures. These surprising results were confirmed by circular dichroism spectroscopy, revealing a
monosignate CD band that must be attributed to a supramolecular chiral organization.
The effect of extending the conjugation length, the deployment of various substituents and configurational locking of the
polyene backbone on the second-order nonlinear optical (NLO) response of a series of indoline based compounds has
been investigated. The compounds were examined using Hyper-Rayleigh scattering (HRS) and relative second-harmonic
generation (SHG) techniques with a femtosecond operating system with a 1300 nm fundamental wavelength. All of the
compounds were found to have high molecular hyperpolarizabilities with β values of up to 1230 × 10-30 esu. At the
macroscopic level-for poled polymer thin films-a strong second-order NLO signal has been detected and d33 values of
up to 217 pm/V are found-a response of some ten times greater than that found for the well known azo dye Disperse
Red 1.
In general, second-harmonic generation (SHG) microscopy is used to image highly ordered structures in biological
samples, like starch, collagen, myosin and tubulin. In an effort to expand the possible targets for SHG microscopy, a
number of new fluorescent probes with high performance in SHG imaging were designed and synthesized. The design is
based on an electron-rich carbazole template, functionalized with pyridinium-like acceptors, resulting in cyanine-like
dyes. In this paper, we report on the linear and nonlinear optical characterization of one of these dyes and its applicability
in microscopy using two-photon excited fluorescence (2PEF) and SHG to visualize the specificity of the dyes in HeLa
cells.
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