Strong Intensity Dependence Research Articles

Polarimetric second-harmonic generation microscopy of partially oriented fibers I: Digital modeling

Second-harmonic generation (SHG) in biological tissues originates predominantly from noncentrosymmetric fibrillar structures partially oriented within a focal volume (voxel) of a multiphoton excitation microscope. This study is aimed to elucidate fibrillar organization factors influencing SHG intensity, as well as achiral, R, and chiral, C, nonlinear susceptibility tensor component ratios. SHG response is calculated for various configurations of fibrils in a voxel using the digital nonlinear microscope. The R and C ratios are calculated using linear incident and outgoing polarization states that simulate polarization-in polarization-out polarimetric measurements. The investigation shows strong SHG intensity dependence on parallel/antiparallel fiber organization. The R and C ratios are strongly influenced by the fiber chirality, tilting of the fibers out of the image plane, and crossing of the fibers. The computational modeling provides the basis for the interpretation of polarimetric SHG microscopy images in terms of the ultrastructural organization of fibers in each voxel of the samples. The modeling results are employed in the accompanying paper to investigate the ultrastructures with parallel/antiparallel fibers and two-dimensional and tree-dimensional crossing fibers in biological and biomimetic structures.

Photo-scanning Electrochemical Microscopy Observation of Overall Water Splitting at a Single Aluminum-Doped Strontium Titanium Oxide Microcrystal

Particulate photocatalysts for the overall water splitting (OWS) reaction offer promise as devices for hydrogen fuel generation. Even though such photocatalysts have been studied for nearly 5 decades, much of the understanding of their function is derived from observations of catalyst ensembles and macroscopic photoelectrodes. This is because the sub-micrometer size of most OWS photocatalysts makes spatially resolved measurements of their local reactivity very difficult. Here, we employ photo-scanning electrochemical microscopy (photo-SECM) to quantitatively measure hydrogen and oxygen evolution at individual OWS photocatalyst particles for the first time. Micrometer-sized Al-doped SrTiO3/Rh2-yCryO3 photocatalyst particles were immobilized on a glass substrate and interrogated with a chemically modified SECM nanotip. The tip simultaneously served as a light guide to illuminate the photocatalyst and as an electrochemical nanoprobe to observe oxygen and hydrogen fluxes from the OWS. Local O2 and H2 fluxes obtained from chopped light experiments and photo-SECM approach curves using a COMSOL Multiphysics finite-element model confirmed stoichiometric H2/O2 evolution of 9.3/4.6 μmol cm-2 h-1 with no observable lag during chopped illumination cycles. Additionally, photoelectrochemical experiments on a single microcrystal attached to a nanoelectrode tip revealed a strong light intensity dependence of the OWS reaction. These results provide the first confirmation of OWS at single micrometer-sized photocatalyst particles. The developed experimental approach is an important step toward assessing the activity of photocatalyst particles at the nanometer scale.

Plasmon-Enhanced Electrochemiluminescence at the Single-Nanoparticle Level.

Plasmon-enhanced electrochemiluminescence (ECL) at the single-nanoparticle (NP) level was investigated by ECL microscopy. The Au NPs were assembled into an ordered array, providing a high-throughput platform that can easily locate each NP in sequential characterizations. A strong dependence of ECL intensity on Au NP configurations was observed. We demonstrate for the first time that at the single-particle level, the ECL of Ru(bpy)3 2+ -TPrA was majorly quenched by small Au NPs (<40 nm), while enhanced by large Au ones (>80 nm) due to the localized surface plasmon resonance (LSPR). Notably, the ECL intensity was further increased by the coupling effect of neighboring Au NPs. Finite Difference Time Domain (FDTD) simulations conformed well with the experimental results. This plasmon enhanced ECL microscopy for arrayed single NPs provides a reliable tool for screening electrocatalytic activity at a single particle.

Plasmon‐Enhanced Electrochemiluminescence at the Single‐Nanoparticle Level

AbstractPlasmon‐enhanced electrochemiluminescence (ECL) at the single‐nanoparticle (NP) level was investigated by ECL microscopy. The Au NPs were assembled into an ordered array, providing a high‐throughput platform that can easily locate each NP in sequential characterizations. A strong dependence of ECL intensity on Au NP configurations was observed. We demonstrate for the first time that at the single‐particle level, the ECL of Ru(bpy)32+‐TPrA was majorly quenched by small Au NPs (&lt;40 nm), while enhanced by large Au ones (&gt;80 nm) due to the localized surface plasmon resonance (LSPR). Notably, the ECL intensity was further increased by the coupling effect of neighboring Au NPs. Finite Difference Time Domain (FDTD) simulations conformed well with the experimental results. This plasmon enhanced ECL microscopy for arrayed single NPs provides a reliable tool for screening electrocatalytic activity at a single particle.

Tip-enhanced Raman spectroscopy with amplitude-controlled tapping-mode AFM

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for analyzing chemical compositions at the nanoscale owing to near-field light localized at a metallic tip. In TERS, atomic force microscopy (AFM) is commonly used for tip position control. AFM is often controlled under the contact mode for TERS, whereas the tapping mode, which is another major operation mode, has not often been employed despite several advantages, such as low sample damage. One of the reasons is the low TERS signal intensity because the tip is mostly away from the sample during the tapping motion. In this study, we quantitatively investigated the effect of the tapping amplitude on the TERS signal. We numerically evaluated the dependence of the TERS signal on tapping amplitude. We found that the tapping amplitude had a significant effect on the TERS signal, and an acceptable level of TERS signal was obtained by reducing the amplitude to a few nanometers. We further demonstrated amplitude-controlled tapping-mode TERS measurement. We observed a strong dependence of the TERS intensity on the tapping amplitude, which is in agreement with our numerical calculations. This practical but essential study encourages the use of the tapping mode for further advancing TERS and related optical techniques.

Unexpected Phonon Behaviour in BiFexCr1-xO3, a Material System Different from Its BiFeO3 and BiCrO3 Parents.

The dielectric function and the bandgap of BiFe0.5Cr0.5O3 thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO3 and BiCrO3. The bandgap value of BiFe0.5Cr0.5O3 is lower than that of BiFeO3 and BiCrO3, due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm−1 with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe0.5Cr0.5O3 thin films and BiFexCr1−xO3 polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe0.3Cr0.7O3.

Zinc(II) picolinate-based coordination polymers as luminescent sensors of Fe3+ ions and nitroaromatic compounds

The synthesis, structure and photoluminescence properties of the zinc(II) picolinate-based coordination polymers [Zn(H2O)2(L1)] (1) and [Zn(L2)2]·0·.5H2O (2) (L12– = 5-((4-carboxyphenyl)ethynyl)picolinate; L2– = 5-(pyridin-3-ylethynyl)-picolinate) are reported. Both compounds exhibit a ligand-based blue emission under UV irradiation. 1 shows a strong enhancement of the emission intensity with respect to the free ligand with a high quantum yield value of 30 %. This luminescence is quenched by the presence of nitrobenzene or Fe3+ cations without loss of the integrity of the sample. The Stern-Volmer plot yields a value of KSV = 121 M−1, indicating a moderate ability for nitrobenzene sensing (limit of detection, LOD = 5.86·10–4 M). For Fe3+ detection, KSV and LOD values of 812 M−1 and 3.78·10–4 M are respectively obtained. 1 exhibits a strong dependence of the luminescent intensity on pH conditions, preserving its crystal integrity in the 2–12 pH range.

A reaction–diffusion model for grayscale digital light processing 3D printing

Digital light processing (DLP) 3D printing is an additive manufacturing process that utilizes light patterns to photopolymerize a liquid resin into a solid. Due to the accuracy of modern digital micromirror devices (DMD) and recent advances in resin chemistry, it is now possible to create functionally graded structures using different light intensity values, also known as grayscale DLP (g-DLP). Different intensities of light lead to differences in the polymer crosslinking density after curing, which ultimately produces a part with gradients of material properties. However, g-DLP is a complicated process. First, the DLP printing is a highly coupled chemical and physical process that involves light propagation, chemical reactions, species diffusion, heat transfer, volume shrinkage, and changes in mechanical behaviors of the curing resin. Second, in g-DLP, light gradients create strong in plane gradients of chemical species concentrations in the curing liquid resin due to the strong dependence of light intensity on the rate of monomer crosslinking. Furthermore, light gradients through the depth create concentration gradients due to the degree of cure dependent light absorption and the use of photoabsorbers. These complex physical features of the printing process must be understood in order to properly control printing parameters such as light exposure time, printing speed, and grayscale variations to achieve accurate mechanical properties. In this paper, a photopolymerization reaction–diffusion model is developed and used in conjunction with experiments to investigate the coupled effects of light propagation, chemical reaction rates, and species diffusion during g-DLP 3D printing. The model is implemented numerically utilizing the finite difference method and simulation results are compared to experimental findings of simple printed structures. The agreement between experimental and model predictions of simple quantities of interest, such as geometric feature sizes, shows that the model can capture the overcure due to free-radical and other species diffusion during printing when grayscale patterns are employed. This model lays the groundwork for future extensions that can incorporate more complex coupled physics such as heat transfer, volume shrinkage, and material property evolution, which are critically important in utilizing g-DLP 3D printing for the fabrication of high-performance parts which excellent geometric and material property tolerances.

2-Acyl-1-aryl-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazole derivatives: Versatile fluorescent probes with remarkably large Stokes shift

A series of thirteen 2-acyl-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazole derivatives were synthesized by using a copper-catalyzed azomethine-imine-alkyne cycloaddition as a key step. These bimane-related fluorescent compounds with λex = 338–400 nm and λem = 522–644 nm exhibit interesting and promising optical properties, including a remarkably large Stokes shift (150–268 nm), positive solvatochromism, and strong dependence of emission intensity on pH in a range of pH 2–10. The benzotriazolide- and N-succinimidyl ester-functionalized compounds were successfully used as fluorescent probes for protein labelling.

High-Resolution Phosphorescence Lifetime Imaging (PLIM) of Bones

For the first time, the time-resolved two-photon excited autophosphorescence of non-labeled biological specimens was investigated by phosphoresce lifetime imaging with microscopic spatial resolution. A modified multiphoton tomograph was employed to record both photoluminescence contributions, autofluorescence and autophosphorescence, simultaneously, induced by two-photon excitation using an 80 MHz near infrared femtosecond-pulse-laser scanning beam, an acousto-optic modulator, and a time-correlated single-photon counting module for lifetime measurements from the picosecond to the microsecond range. In particular, the two-photon-excited luminescence of thermally altered bones was imaged. A strong dependence of the phosphorescence intensity on exposure temperature, with a maximum emission for an exposure temperature of approximately 600 °C was observed. Furthermore, the phosphorescence lifetime data indicated a bi-exponential signal decay with both a faster few µs decay time in the range of 3–10 µs and a slower one in the range of 30–60 µs. The recording of fluorescence and phosphorescence allowed deriving the relative signal proportion as an unbiased measure of the temperature dependence. The measurements on thermally altered bones are of particular interest for application to forensic and archeological investigations.

Wide gamut white LED device using green CsPbBr3 quantum dots glass and red K2SiF6: Mn4+ phosphor

Wide gamut white LED device is fabricated by combining green CsPbBr3 quantum dots glass (QDG) and red K2SiF6: Mn4+ phosphor with blue LED. The green CsPbBr3 QDG shows steep absorption spectra, narrow and symmetrical photoluminescence (PL) spectra, broad band excitation spectra and bi-exponential fluorescence decay. PL peak, full width at half maximum and average lifetime are 515 nm, 18 nm and 94.9 ns respectively. The red K2SiF6: Mn4+ phosphor shows narrow red emission peaks of Mn4+, broad excitation peaks, and single exponential fluorescence decay with lifetime of 8.5 ms. The green CsPbBr3 QDG has strong temperature dependence of PL intensity. The red K2SiF6: Mn4+ phosphor has negligible temperature dependence. The white LED device has wide color gamut of 134% NTSC and high color purity of 98%, 89% and 99% for the blue, green and red lights. The white light of white LED device has been realized by controlling thickness of the green CsPbBr3 QDG and content of the red K2SiF6: Mn4+ phosphor. We have evaluated the chromatic coordinate stability at different operating powers and times, and the maximum chromatic changes are as small as (0.004, 0.012) and (0.009, 0.026) for (x, y) respectively in CIE 1931 (Commission International de l′Eclairage 1931) chromatic coordinate.

Fluorescence intensity ratio optical thermometer YNbO4: Pr3+, Tb3+ based on intervalence charge transfer

Currently, due to the advantages of fast response, remote operations and good stability, non-contact optical temperature measurement technology based on fluorescence intensity ratio (FIR) has attracted intensive attention. Here, we successfully developed a novel Pr3+, Tb3+ co-doped YNbO4 phosphor which can be used as a non-contact optical thermometer. Under excitation at 295 nm, Pr3+, Tb3+ co-doped YNbO4 emits red light from Pr3+ (1D2 → 3H4) and green light from Tb3+ (5D4 → 7F5), simultaneously. Based on strong dependence of the emission intensity on temperature, the FIR of red and green emissions is designed as detection signal to accurately monitor temperature. The maximum absolute and relative sensitivities reached as high as 0.0125 K−1 at 508 K and 1.01% K−1 at 478 K, respectively. The luminous color of this phosphor is sensitive to the temperature, which provides the possibility for visualization of temperature measurement. There was little change in the luminescence intensity of Pr3+, Tb3+ co-doped phosphors after they went through five heating-cooling cycles. It is proved that the multi-phonon relaxation from 3P0 to 1D2 and intermediate charge transfer are responsible to the temperature-dependent luminescence of the phosphors. Optical thermometer based on the current phosphors show the advantages of high sensitivity, high precision and good repeatability. This indicates Pr3+, Tb3+ co-doped phosphors have potential application value in optical temperature sensing field.

Remanent Polarization and Strong Photoluminescence Modulation by an External Electric Field in Epitaxial CsPbBr3 Nanowires.

Metal halide perovskites (MHPs) have unique characteristics and hold great potential for next-generation optoelectronic technologies. Recently, the importance of lattice strain in MHPs has been gaining recognition as a significant optimization parameter for device performance. While the effect of strain on the fundamental properties of MHPs has been at the center of interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent photoluminescence study on heteroepitaxially strained surface-guided CsPbBr3 nanowires. We reveal an unexpected strong linear dependence of the photoluminescence intensity on the alternating field amplitude, stemming from an induced internal dipole. Using low-frequency polarized-Raman spectroscopy, we reveal structural modifications in the nanowires under an external field, associated with the observed polarity. These results reflect the important interplay between strain and an external field in MHPs and offer opportunities for the design of MHP-based optoelectronic nanodevices.

Generation of Intense Sub-10 fs Pulses at 385nm* *Supported by the National Key Research and Development Program of China (Grant No. 2019YFA0307703), the Major Research Plan of NSF of China (Grant No. 91850201), and the National Natural Science Foundation of China (Grant Nos. 11974426, 11974425, U1830206 and 11604386).

We demonstrated the generation and characterization of 9.7 fs, 180 μJ pulses centered at 385 nm via the frequency doubling of few-cycle near-infrared pulses. Both moderate conversion efficiency (9.5%) and broad phase matching bandwidth (20 nm) were achieved by shaping the spectra of the fundamental pulses. The strong intensity dependence of second-order harmonic generation and well controlled material dispersion ensured the inexistence of satellite pulses, which was confirmed by the self-diffraction frequency resolved optical gating measurement.

Characterization of thymine microcrystals by CARS and SHG microscopy

Identification of chemically homologous microcrystals in a polycrystal sample is a big challenge and requires developing specific highly sensitive tools. Second harmonic (SHG) and coherent anti-Stokes Raman scattering (CARS) spectroscopy can be used to reveal arrangement of thymine molecules, one of the DNA bases, in microcrystalline sample. Strong dependence of CARS and SHG intensity on the orientation of the linear polarization of the excitation light allows to obtain high resolution images of thymine microcrystals by additionally utilizing the scanning microscopy technique. Experimental findings and theoretical interpretation of the results are compared. Presented experimental data together with quantum chemistry-based theoretical interpretation allowed us to determine the most probable organization of the thymine molecules.

Double electron emission from surfaces via low-energy positrons

We have studied the electron pair emission process from surfaces due to the impact of a primary low-energy positron. The existence of this process implies a two-step scattering event. We find that the electron pair intensity is on the same scale as the positron-electron pair intensity. This suggests that a significant contribution of the pair intensity is actually due to the emission of three particles, of which two are detected. We also observe a strong material dependence of the coincidence intensity. Similarly to our previous studies with primary electron excitation or photon absorption, NiO is the material that displays the highest coincidence rates. We also note that the electron pair intensity scales with the single electron rate. This was also observed in electron pair emission with primary photons or electrons. This suggests that the coincidence intensity may provide a measure of the electron correlation strength.

Effect of heat treatment in vacuum on photoluminescence of anodic alumina.

The results are presented of a study of the photoluminescent (PL) properties of an undoped porous anodic alumina (PAA) and PAA doped with manganese ions. The PAA samples were prepared by anodization of aluminum. The effect of annealing conditions in vacuum on the PL spectra was studied for the first time and a comparative analysis was made with the spectra of the PAA annealed in air. Vacuum annealing was used to obtain oxygen-deficient alumina. A strong dependence of the PAA PL intensity on the annealing temperature in vacuum has been found: for the samples annealed at 600°С, the PL intensity is 15 times higher than that measured on the initial samples, whereas for the samples annealed in air it increases only 4.5-fold with excitation at the wavelengths of 275nm. This is the result of the formation of a high concentration of oxygen vacancies during annealing in vacuum under conditions of oxygen deficiency as compared with the samples annealed in air, where diffusion of oxygen from air leads to a decrease in vacancies. A significant increase in the PL intensity permits consideration of the vacuum-annealed PAA as a promising material for dosimetry.

Luminescence of Pr3+ and Nd3+ Ions in Double Molybdates

The optical spectra of samples of double zirconium molybdates activated by rare-earth Pr3+ and Nd3+ ions are studied. The presence of bands corresponding to the 4f–4f transitions of Pr3+ and Nd3+ trivalent ions in the absorption, excitation, and luminescence spectra is shown. A strong dependence of the luminescence intensity of Pr3+ ions on temperature is revealed.

(Invited) Using Surface Photovoltage Spectroscopy to Observe Photovoltage Generation at the Interfaces of Cu2o, BiVO4, and Rh:SrTiO3 Particles

Surface photovoltage spectroscopy (SPS) allows contact-less measurement of the Galvani photopotential at the interfaces of particles under illumination. Here we use SPS to observe the light intensity and wavelength dependent photopotential in a Ru–SrTiO3:Rh/BiVO4 particle tandem overall water splitting photocatalyst and in particle layers made of Cu2O and BiVO4. For the tandem, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of −2.45 V at 435 nm (2.44 mW cm–2), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO4 microcrystals. Isolated Cu2O particle layers on FTO show p-type behavior and BiVO4 particles show n-type behavior. In a FTO/Cu2O/BiVO4 particle stack, both particles act in tandem, producing up to -2.1 V photovoltage under 2.5 eV excitation, which corresponds to 90% of the ideal limit. The photovoltage spectra also reveal the effects of shading at high excitation energy and the effects of low absorber strength at low excitation energy. These results are relevant to the understanding of photochemical charge transfer at irregular particle surfaces and to the construction of devices for artificial photosynthesis and water splitting. Figure 1

Люминесценция ионов Pr-=SUP=-3+-=/SUP=- и Nd-=SUP=-3+-=/SUP=- в двойных молибдатах

AbstractThe optical spectra of samples of double zirconium molybdates activated by rare-earth Pr^3+ and Nd^3+ ions are studied. The presence of bands corresponding to the 4 f –4 f transitions of Pr^3+ and Nd^3+ trivalent ions in the absorption, excitation, and luminescence spectra is shown. A strong dependence of the luminescence intensity of Pr^3+ ions on temperature is revealed.