Anisotropic Strain-Tailoring Nonlinear Optical Response in van der Waals NbOI2.

Two-dimensional (2D) NbOI2 demonstrates significant second-harmonic generation (SHG) with a high conversion efficiency. To unlock its full potential in practical applications, it is desirable to modulate the SHG behavior while utilizing the intrinsic lattice anisotropy. Here, we demonstrate direction-specific modulation of the SHG response in NbOI2 by applying anisotropic strain with respect to the intrinsic lattice orientations, where more than 2-fold enhancement in the SHG intensity is achieved under strain along the polar axis. The strain-driven SHG evolution is attributed to the strengthened built-in piezoelectric field (polar axis) and the enlarged Peierls distortions (nonpolar axis). Moreover, we provide quantifications of the correlation between strain and SHG intensity in terms of the susceptibility tensor. Our results demonstrate the effective coupling of orientation-specific strain to the anisotropic SHG response through the intrinsic polar order in 2D nonlinear optical crystals, opening a new paradigm toward the development of functional devices.

Time-resolved study on signaling pathway of photoactivated adenylate cyclase and its nonlinear optical response.

Photoactivated adenylate cyclases (PACs) are multidomain BLUF proteins that regulate the cellular levels of cAMP in a light-dependent manner. The signaling route and dynamics of PAC from Oscillatoria acuminata (OaPAC), which consists of a light sensor BLUF domain, an adenylate cyclase domain, and a connector helix (α3-helix), were studied by detecting conformational changes in the protein moiety. Although circular dichroism and small-angle X-ray scattering measurements did not show significant changes upon light illumination, the transient grating method successfully detected light-induced changes in the diffusion coefficient (diffusion-sensitive conformational change (DSCC)) of full-length OaPAC and the BLUF domain with the α3-helix. DSCC of full-length OaPAC was observed only when both protomers in a dimer were photoconverted. This light intensity dependence suggests that OaPAC is a cyclase with a nonlinear light intensity response. The enzymatic activity indeed nonlinearly depends on light intensity, that is, OaPAC is activated under strong light conditions. It was also found that both DSCC and enzymatic activity were suppressed by a mutation in the W90 residue, indicating the importance of the highly conserved Trp in many BLUF domains for the function. Based on these findings, a reaction scheme was proposed together with the reaction dynamics.

Nonlocal Optical Response of Particle Plasmons in Single Gold Nanorods.

The investigation of particle plasmons in metal nanoparticles has predominantly relied on local optical response approximations. However, as the nanoparticle size approaches the average distance of electrons to the metal surface, mesoscopic effects such as size-dependent plasmon line width broadening and resonance energy blue shifts are expected to become observable. In this work, we compared the experimental spectral characteristics with simulated values obtained by using a generalized nonlocal optical response theory-based local analogue model. Our results show that the nonlocal plasmon damping effects in single nanoparticles are less pronounced than those observed in plasmon-coupled systems. Furthermore, our research demonstrates that single-particle dark-field spectroscopy is an effective tool for investigating the nonlocal optical response of particle plasmons in single nanoparticles. These results have important implications for the rational design of novel nanophotonic devices.

The role of the local environment on the structural heterogeneity of carotenoid ß-ionone rings.

Our analysis of the X-ray crystal structure of canthaxanthin (CAN) showed that its ketolated ß-ionone rings can adopt two energetically equal, but structurally distinct puckers. Quantum chemistry calculations revealed that the potential energy surface of the ß-ionone ring rotation over the plane of the conjugated π-system in carotenoids depends on the pucker state of the ß-ring. Considering different pucker states and ß-ionone ring rotation, we found six separate local minima on the potential energy surface defining the geometry of the keto-ß-ionone ring-two cis and one trans orientation for each of two pucker states. We observed a small difference in energy and no difference in relative orientation for the cis-minima, but a pronounced difference for the position of trans-minimum in alternative pucker configurations. An energetic advantage of ß-ionone ring rotation from a specific pucker type can reach up to 8 kJ/mol ([Formula: see text]). In addition, we performed the simulation of linear absorption of CAN in hexane and in a unit cell of the CAN crystal. The electronic energies of [Formula: see text] transition were estimated both for the CAN monomer and in the CAN crystal. The difference between them reached [Formula: see text], which roughly corresponds to the energy gap between A and B pucker states predicted by theoretical estimations. Finally, we have discussed the importance of such effects for biological systems whose local environment determines conformational mobility, and optical/functional characteristics of carotenoid.

An Insight into nd10 Metal Cyanide-based Coordination Polymers Through ab-initio Calculations: Electronic Properties and Optical Response.

Semiconductors are essential for modern life since they are the basis of many current technologies that are related to better living standards. Some of them, characterized by the periodic assembling of metal cyanides with filled d-shell (nd10 ) constitute an interesting series of cyanide-based coordination polymers with physical properties such like anomalous anisotropic thermal expansion and quantum confinement effects related to the polymer's width that can be exploited for technological applications. Herein, the electronic structure of nd10 metal cyanide-based systems were studied both experimentally and through Density Functional Theory. The band gap found for one-dimensional (1D) -M-C≡N- (M=Cu, Ag, Au) and tetrahedral M-(C≡N)2 (M=Zn, Cd, Hg) systems can be attributed to Laporte-allowed π → ${\to }$ π* (Metal to Ligand Charge Transfer mechanism) combined with metal center (d → ${\to }$ s,p) electronic transitions. Aurophilic bonding was found on the AuCN structure, and a new forbidden electronic transition associated to its band gap is reported. Computed effective and reduced masses from carriers revealed that carrier mobility and quantum confinement effects are greater in 1D systems.

Optical response of plasmonic silver nanoparticles after treatment by a warm microwave plasma jet.

This work investigates the effect of plasma treatment on the morphology and composition of 15 × 15 mm2silver nanoparticle (70-80 nm) thin films. The silver nanoparticles are deposited onto thermal silica (SiO2/Si) substrates by spin-coating, then they are treated by an open-to-air microwave argon plasma jet characterized by a neutral gas temperature of 2200 ± 200 K. Scanning electron microscopy analysis reveals that the number of isolated nanoparticles in the film sample decreases after exposure to multiple jet passes, and that polygonal structures with sharp corners and edges are produced. Similar structures with much rounder edges are obtained after conventional thermal annealing at temperatures up to 1300 K. Based on localized surface plasmon resonance analysis in the range of 350-800 nm, the main extinction band of silver nanoparticles experiences a redshift after treatment with the plasma jet or with thermal annealing. Moreover, both treatments induce surface oxidation of the nanoparticles, as evidenced by x-ray photoelectron spectroscopy. However, only the plasma-exposed samples exhibit a significant rise in the surface-enhanced Raman scattering (SERS) signal of oxidized silver at 960 cm-1. 29×29µm2mappings of hyperspectral Raman IMAging (RIMA) and multivariate curve resolution analysis by log-likelihood maximization demonstrate that the SERS signal is controlled by large-scale micrometer domains that exhibit sharp corners and edges.

Exploiting Dynamic Nonlinearity in Upconversion Nanoparticles for Super-Resolution Imaging.

Single-beam super-resolution microscopy, also known as superlinear microscopy, exploits the nonlinear response of fluorescent probes in confocal microscopy. The technique requires no complex purpose-built system, light field modulation, or beam shaping. Here, we present a strategy to enhance this technique's spatial resolution by modulating excitation intensity during image acquisition. This modulation induces dynamic optical nonlinearity in upconversion nanoparticles (UCNPs), resulting in variations of nonlinear fluorescence response in the obtained images. The higher orders of fluorescence response can be extracted with a proposed weighted finite difference imaging algorithm from raw fluorescence images to generate an image with higher resolution than superlinear microscopy images. We apply this approach to resolve single nanoparticles in a large area, improving the resolution to 132 nm. This work suggests a new scope for the development of dynamic nonlinear fluorescent probes in super-resolution nanoscopy.

Disentangling Many-Body Effects in the Coherent Optical Response of 2D Semiconductors.

In single-layer (1L) transition metal dichalcogenides, the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS2. In order to disentangle many-body effects, we perform exciton line-shape analysis, and we study its temporal dynamics as a function of the excitation photon energy and fluence. We find that resonant photoexcitation produces a blue shift of the A exciton, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the nonlinear absorption of the material and its dependence on excitation conditions.

Geometric, Electronic, and Optoelectronic Properties of Carbon-Based Polynuclear C3O[C(CN)2]2M3 (where M = Li, Na, and K) Clusters: A DFT Study.

Carbon-based polynuclear clusters are designed and investigated for geometric, electronic, and nonlinear optical (NLO) properties at the CAM-B3LYP/6-311++G(d,p) level of theory. Significant binding energies per atom (ranging from -162.4 to -160.0 kcal mol-1) indicate excellent thermodynamic stabilities of these polynuclear clusters. The frontier molecular orbital (FMOs) analysis indicates excess electron nature of the clusters with low ionization potential, suggesting that they are alkali-like. The decreased energy gaps (EH-L) with increased alkali metals size revael the improved electrical conductivity (σ). The total density of state (TDOS) study reveals the alkali metals' size-dependent electronic and conductive properties. The significant first and second hyperpolarizabilities are observed up to 5.78 × 103 and 5.55 × 106 au, respectively. The ßo response shows dependence on the size of alkali metals. Furthermore, the absorption study shows transparency of these clusters in the deep-UV, and absorptions are observed at longer wavelengths (redshifted). The optical gaps from TD-DFT are considerably smaller than those of HOMO-LUMO gaps. The significant scattering hyperpolarizability (ßHRS) value (1.62 × 104) is calculated for the C3 cluster, where octupolar contribution to ßHRS is 92%. The dynamic first hyperpolarizability ß(ω) is more pronounced for the EOPE effect at 532 nm, whereas SHG has notable values for second hyperpolarizability γ(ω).

An Optimized Schwarz Method for the Optical Response Model Discretized by HDG Method.

An optimized Schwarz domain decomposition method (DDM) for solving the local optical response model (LORM) is proposed in this paper. We introduce a hybridizable discontinuous Galerkin (HDG) scheme for the discretization of such a model problem based on a triangular mesh of the computational domain. The discretized linear system of the HDG method on each subdomain is solved by a sparse direct solver. The solution of the interface linear system in the domain decomposition framework is accelerated by a Krylov subspace method. We study the spectral radius of the iteration matrix of the Schwarz method for the LORM problems, and thus propose an optimized parameter for the transmission condition, which is different from that for the classical electromagnetic problems. The numerical results show that the proposed method is effective.

Excitation wavelength- and intensity-dependent stepwise two-photon-induced photochromic reaction.

The photochromic molecules showing wavelength-selective or light intensity-dependent photoresponse are receiving increased attention in recent years. Although a photoswitch with a single chromophore can control the ON and OFF states of a function, that consisting of multi-chromophores would be useful for the specific control in complex systems. Herein, we designed stepwise two-photon induced photochromic molecules (PABI-PIC and PABI-PIC2) consisting of two different photochromic units (PABI and PIC). One-photon absorption reaction in the UV light region of PABI-PIC generates the short-lived transient biradical (BR) that absorbs an additional photon in the visible and UV light region in a stepwise manner to produce the two-photon photochemical product, the quinoidal species (Quinoid). The photochromic properties of these transient species are completely different in color and fading speed. In addition, PABI-PIC also shows the excitation wavelength-dependent photochromism because the excited states of the PABI and PIC units are electronically orthogonal. Therefore, the stepwise photochromic properties of PABI-PIC are easily controlled depending on the excitation light intensity and wavelength. These molecular designs are important for the development of advanced photoresponsive materials.

Ultra-Small 2D PbS Nanoplatelets: Liquid-Phase Exfoliation and Emerging Applications for Photo-Electrochemical Photodetectors.

2D PbS nanoplatelets (NPLs) form an emerging class of photoactive materials and have been proposed as robust materials for high-performance optoelectronic devices. However, the main drawback of PbS NPLs is the large lateral size, which inhibits their further investigations and practical applications. In this work, ultra-small 2D PbS NPLs with uniform lateral size (11.2 ± 1.7 nm) and thickness (3.7 ± 0.9 nm, ≈6 layers) have been successfully fabricated by a facile liquid-phase exfoliation approach. Their transient optical response and photo-response behavior are evaluated by femtosecond-resolved transient absorption and photo-electrochemical (PEC) measurements. It is shown that the NPLs-based photodetectors (PDs) exhibit excellent photo-response performance from UV to the visible range, showing extremely high photo-responsivity (27.81 mA W-1 ) and remarkable detectivity (3.96 × 1010 Jones), which are figures of merit outperforming currently reported PEC-type PDs. The outstanding properties are further analyzed based on the results of first-principle calculations, including electronic band structure and free energies for the oxygen evolution reaction process. This work highlights promising applications of ultra-small 2D PbS NPLs with the potential for breakthrough developments also in other fields of optoelectronic devices.

Theoretical and experimental investigations on Nb2CTxMXene Q-switched Tm:YAP laser at 2µm for the nonlinear optical response.

In this paper, the Nb2CTxMXene nanosheets were fabricated and the corresponding microstructures were investigated. The nonlinear optical response was illustrated by open aperture Z-scan and I-scan methods. The ground and the excited state absorption cross-sections of 2D Nb2CTxMXene were also investigated. As the saturable absorber (SA), the Nb2CTxMXene was applied in the passively Q-switched Tm:YAP laser. 1.96µs Q-switched pulses with 3.97 W peak power were achieved at the repetition frequency of 80 kHz. Further theoretical model was built by using the coupled rate equations in simulating the dynamic process of the passively Q-switched Tm:YAP laser. The numerical simulation results are fundamentally in agreement with the experimental results, which proves the Nb2CTxMXene can be a good potential nanomaterial for further optoelectronic applications.

Detection of Heavy Metals in Water Using Graphene Oxide Quantum Dots: An Experimental and Theoretical Study.

In this work, we investigate by ab initio calculations and optical experiments the sensitivity of graphene quantum dots in their use as devices to measure the presence, and concentration, of heavy metals in water. We demonstrate that the quenching or enhancement in the optical response (absorption, emission) depends on the metallic ion considered. In particular, two cases of opposite behaviour are considered in detail: Cd2+, where we observe an increase in the emission optical response for increasing concentration, and Pb2+ whose emission spectra, vice versa, are quenched along the concentration rise. The experimental trends reported comply nicely with the different hydration patterns suggested by the models that are also capable of reproducing the minor quenching/enhancing effects observed in other ions. We envisage that quantum dots of graphene may be routinely used as cheap detectors to measure the degree of poisoning ions in water.

Spin Crossover in Nickel(II) Tetraphenylporphyrinate via Forced Axial Coordination at the Air/Water Interface.

Coordination-induced spin crossover (CISCO) in nickel(II) porphyrinates is an intriguing phenomenon that is interesting from both fundamental and practical standpoints. However, in most cases, realization of this effect requires extensive synthetic protocols or extreme concentrations of extra-ligands. Herein we show that CISCO effect can be prompted for the commonly available nickel(II) tetraphenylporphyrinate, NiTPP, upon deposition of this complex at the air/water interface together with a ruthenium(II) phthalocyaninate, CRPcRu(pyz)2, bearing two axial pyrazine ligands. The latter was used as a molecular guiderail to align Ni···Ru···Ni metal centers for pyrazine coordination upon lateral compression of the system, which helps bring the two macrocycles closer together and forces the formation of Ni-pyz bonds. The fact of Ni(II) porphyrinate switching from low- to high-spin state upon acquiring additional ligands can be conveniently observed in situ via reflection-absorption UV-vis spectroscopy. The reversible nature of this interaction allows for dissociation of Ni-pyz bonds, and thus, change of nickel cation spin state, upon expansion of the monolayer.

Linear and Star-Shaped Extended Di- and Tristyrylbenzenes: Synthesis, Characterization and Optical Response to Acid and Metal Ions.

Two linear 1,4-distyrylbenzenes and five star-shaped 1,3,5-tristyrylbenzene derivatives (L2a and L2b , Y0 -Y3 and YNBu ) were synthesized and spectroscopically characterized. The photophysical properties, optical response to acid and metal ions were investigated. Upon backbone extension of linear distyrylbenzenes or the introduction of dibutylanilines, the electronic spectra are redshifted. Incorporation of electron-deficient pyridyl units does not significantly affect the optical properties. Variation of the number of pyridine rings and substitution pattern tune the fluorescence response to acids and metal ions. The novel arenes discriminate Al3+ , Mn2+ , Fe3+ , Fe2+ , Cd2+ , Ag+ and Hg2+ .

Single-Plasmon Thermo-Optical Switching in Graphene.

While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.

Optical Detection of Fe3+ Ions in Aqueous Solution with High Selectivity and Sensitivity by Using Sulfasalazine Functionalized Microgels.

A highly selective and sensitive optical sensor was developed to colorimetric detect trace Fe3+ ions in aqueous solution. The sensor was the sulfasalazine (SSZ) functionalized microgels (SSZ-MGs), which were fabricated via in-situ quaternization reaction. The obtained SSZ-MGs had hydrodynamic radius of about 259 ± 24 nm with uniform size distribution at 25 °C. The SSZ-MG aqueous suspensions can selectively and sensitively response to Fe3+ ions in aqueous solution at 25 °C and pH of 5.6, which can be quantified by UV-visible spectroscopy and also easily distinguished by the naked eye. Job's plot indicated that the molar binding ratio of SSZ moiety in SSZ-MGs to Fe3+ was close to 1:1 with an apparent association constant of 1.72 × 104 M-1. A linear range of 0-12 µM with the detection limit of 0.110 µM (0.006 mg/L) was found. The obtained detection limit was much lower than the maximum allowance level of Fe3+ ions in drinking water (0.3 mg/L) regulated by the Environmental Protection Agency (EPA) of the United States. The existence of 19 other species of metal ions, namely, Ag+, Li+, Na+, K+, Ca2+, Ba2+, Cu2+, Ni2+, Mn2+, Pb2+, Zn2+, Cd2+, Co2+, Cr3+, Yb3+, La3+, Gd3+, Ce3+, and Bi3+, did not interfere with the detection of Fe3+ ions.

Controlled Gold Nanorod Reorientation and Hexagonal Order in Micromolded Gold Nanorod@pNIPAM Microgel Chain Arrays.

A one-step soft lithography based pathway to manufacture aligned gold nanorod@poly-(N-isopropylacrylamide) (GNR@pNIPAM) hybrid chains with hexagonal arrangement of the nanorods and with an anisotropic optical response is presented. After demonstration of an efficient synthesis protocol, yielding uniform composite microgels in high concentration, a micromolding procedure using wrinkled polydimethylsiloxane (PDMS) templates to fabricate aligned hybrid chains is introduced. It is found that the self-assembled GNR@pNIPAM microgels inside the PDMS wrinkle grooves can be transferred onto solid substrates, on which they exhibit a hexagonal order, as confirmed by small-angle X-ray scattering. Further, it is shown that the application of minimized PDMS wrinkle dimensions aligns GNRs inside the pNIPAM microgels, and that the optical response of such molded assemblies is anisotropic.

Coupling localized plasmonic and photonic modes tailors and boosts ultrafast light modulation by gold nanoparticles.

Plasmonic nanoparticles offer a broad range of functionalities, owing to their ability to amplify light in the near-field or convert it into heat. However, their ultrafast nonlinear optical response remains too low to envisage all-optical high-rate photonic processing applications. Here, we tackle this challenge by coupling the localized plasmon mode in gold nanoparticles with a localized photonic mode in a 1D resonant cavity. Despite the nonradiative losses, we demonstrate that a strong, reversible, and ultrafast optical modulation can be achieved. By using a light pumping fluence of less than 1 mJ cm(-2), a change of signal transmittance of more than 100% is generated within a few picosecond time scale. The nanoparticle transient optical response is enhanced by a factor of 30 to 40 while its spectral profile is strongly sharpened. The large nonlinear response of such plasmonic cavities could open new opportunities for ultrafast light processing at the nanoscale.