Autofluorescence spectroscopy and imaging of Platymonas subcordiformis irradiated by diode laser based on LSCM

Autofluorescence spectra and optical imaging of Platymonas subcordiformis after irradiation of diode laser were observed via laser scanning confocal microscopy (LSCM). With 488 nm Ar(+) laser excitation, the horizontal and vertical dimensions of a cup-shaped chloroplast of the irradiation group increased about 10% compared with the control group. The fluorescence spectra were similar between irradiation group and control group with a maximum fluorescence band around 682 nm, whereas the former has a higher intensity. Image of a small circular substance with stronger two-photon autofluorescence (TPA) was obtained when using two-photon excitation wavelength of 800 nm in single-channel mode. Further analysis by the 800 nm excitation based on two independent-channels mode showed an emission band of the small circular substance around 376-505 nm, which corresponded to the eyespot of P. subcordiformis. In lambda scanning mode, with two-photon wavelength of 800 nm excitation, six fluorescence peaks that are located at 465, 520, 560, 617, 660 and 680 nm were observed; the fluorescence intensity of the irradiation group was higher than that of the control group, especially at 520, 560 and 617 nm. As a conclusion, diode laser irradiation can promote chloroplast growth of P. subcordiformis cells in the form of expanding area and the increasing content of protein, phospholipids and chlorophyll. LSCM, especially TPA imaging based on femtosecond laser excitation, provides a nondestructive, real-time and accurate method to study changes of living algal cells under laser irradiation and other environmental factors.     

Mapping piezoelectric-field distribution in gallium nitride with scanning second-harmonic generation microscopy

Taking advantage of the electric field-enhanced second-harmonic generation effect in bulk gallium nitride (GaN) and indium gallium nitride (InGaN) quantum wells, we demonstrated the piezoelectric field distribution mapping in bulk GaN and InGaN multiple-quantum-well (MQW) samples using scanning second-harmonic generation (SHG) microscopy. Scanning SHG microscopy and the accompanying third-harmonic generation (THG) microscopy of the bulk GaN sample were demonstrated using a femtosecond Cr:forsterite laser at a wavelength of 1230 nm. Taking advantage of the off-resonant electric field-enhanced SHG effect and the bandtail state-resonance THG effect, the second- and third-harmonic generation microscopic images obtained revealed the piezoelectric field and bandtail state distributions in a GaN sample. Combined with 720 nm wavelength excited two-photon fluorescence microscopy in the same sample, the increased defect density around the defect area was found to suppress bandedge photoluminescence, to increase yellow luminescence, to increase bandtail state density, and to decrease residue piezoelectric field intensity. Scanning SHG microscopy of the InGaN MQW sample was resonant excited with 800 nm femtosecond pulses from a Ti:sapphire laser in order to suppress SHG contribution from the bulk GaN substrate. Taking advantage of the strong piezoelectric field inside the InGaN quantum well, the wavelength resonant effect, and the electric field-enhanced SHG effect of InGaN quantum wells, resonant scanning SHG microscopy revealed the piezoelectric field distribution inside the wells. Combined with accompanying three-photon fluorescence microscopy from the bulk GaN substrate underneath the quantum wells, the direct correspondence between the piezoelectric field strength inside the quantum well and the substrate quality can be obtained. According to our study, the GaN substrate area with bright bandedge luminescence corresponds to the area with strong SHG signals indicating a higher stained-induced piezoelectric field. These scanning harmonic generation microscopies exhibit superior images of the piezoelectric field and defect state distributions in GaN and InGaN MQWs not available before. Combining with scanning multiphoton fluorescence microscopy, these techniques open new ways for the physical property study of this important material system and can provide interesting details that are not readily available by other microscopic techniques.     

Inverse problems

Positron emission tomography involves constructing an image of brain tissue from gamma rays counted at detectors surrounding the head. This is an inverse problem: how to measure a phenomenon from data taken from a derived distribution. We have noise and a loss of high frequency signal, both of which contribute to ill-conditioning. A brief and informal review of the mathematical and statistical methods available for handling inverse problems which are linear or linearizable is given.     

Multiphoton fluorescence lifetime imaging of human hair

In vivo and in vitro multiphoton imaging was used to perform high resolution optical sectioning of human hair by nonlinear excitation of endogenous as well as exogenous fluorophores. Multiphoton fluorescence lifetime imaging (FLIM) based on time-resolved single photon counting and near-infrared femtosecond laser pulse excitation was employed to analyze the various fluorescent hair components. Time-resolved multiphoton imaging of intratissue pigments has the potential (i) to identify endogenous keratin and melanin, (ii) to obtain information on intrahair dye accumulation, (iii) to study bleaching effects, and (iv) to monitor the intratissue diffusion of pharmaceutical and cosmetical components along hair shafts.     

Ultrafast optical pulse delivery with fibers for nonlinear microscopy

Nonlinear microscopies including multiphoton excitation fluorescence microscopy and multiple-harmonic generation microscopy have recently gained popularity for cellular and tissue imaging. The optimization of these imaging methods for minimally invasive use requires optical fibers to conduct light into tight space, where free-space delivery is difficult. The delivery of high-peak power laser pulses with optical fibers is limited by dispersion resulting from nonlinear refractive index responses. In this article, we characterize a variety of commonly used optical fibers in terms of how they affect pulse profile and imaging performance of nonlinear microscopy; the following parameters are quantified: spectral bandwidth and temporal pulse width, two-photon excitation efficiency, and optical resolution. A theoretical explanation for the measured performance of these fibers is also provided.     

Nonlinear Absorption Applications of CH3NH3PbBr3 Perovskite Crystals

Researchers have recently revealed that hybrid lead halide perovskites exhibit ferroelectricity, which is often associated with other physical characteristics, such as a large nonlinear optical response. In this work, the nonlinear optical properties of single crystal inorganic–organic hybrid perovskite CH₃NH₃PbBr₃ are studied. By exciting the material with a 1044 nm laser, strong two‐photon absorption‐induced photoluminescence in the green spectral region is observed. Using the transmission open‐aperture Z‐scan technique, the values of the two‐photon absorption coefficient are observed to be 8.5 cm GW^?1, which is much higher than that of standard two‐photon absorbing materials that are industrially used in nonlinear optical applications, such as lithium niobate (LiNbO₃), LiTaO₃, KTiOPO₄, and KH₂PO₄. Such a strong two‐photon absorption effect in CH₃NH₃PbBr₃ can be used to modulate the spectral and spatial profiles of laser pulses, as well as to reduce noise, and can be used to strongly control the intensity of incident light. In this study, the superior optical limiting, pulse reshaping, and stabilization properties of CH₃NH₃PbBr₃ are demonstrated, opening new applications for perovskites in nonlinear optics.     

Time‐Resolved Studies of Energy Transfer in Thin Films of Green and Red Fluorescent Proteins

Biologically derived fluorescent proteins are attractive candidates for lasing and sensing due to their excellent optical properties, including their high quantum yield, spectral tunability, and robustness against concentration quenching. Here, a time‐resolved study of the fluorescence dynamics of protein thin films is reported for the enhanced green fluorescent protein (EGFP), the red‐emitting tandem‐dimer protein tdTomato, and blends of EGFP and tdTomato. The exciton dynamics are characterized by using spectrally and time‐resolved measurements of fluorescence and a threefold reduction in lifetime is observed when going from solution to thin film, down to 1 and 0.6 ns for EGFP and tdTomato, respectively. This finding is attributed to a dipole–dipole nonradiative Förster resonant energy transfer (FRET) in solid state. The temporal characteristics of FRET in blended thin films are also studied and increased nonradiative transfer rates are found. Finally, efficient sensitization of a semiconductor surface with a protein thin film is reported. Such a configuration may have important implications for energy harvesting in hybrid organic–inorganic solar cells and other hybrid optoelectronic devices.     

Porous Porphyrin‐Based Organosilica Nanoparticles for NIR Two‐Photon Photodynamic Therapy and Gene Delivery in Zebrafish

Periodic mesoporous organosilica nanoparticles emerge as promising vectors for nanomedicine applications. Their properties are very different from those of well‐known mesoporous silica nanoparticles as there is no silica source for their synthesis. So far, they have only been synthesized from small bis‐silylated organic precursors. However, no studies employing large stimuli‐responsive precursors have been reported on such hybrid systems yet. Here, the synthesis of porphyrin‐based organosilica nanoparticles from a large octasilylated metalated porphyrin precursor is described for applications in near‐infrared two‐photon‐triggered spatiotemporal theranostics. The nanoparticles display unique interconnected large cavities of 10–80 nm. The framework of the nanoparticles is constituted with J‐aggregates of porphyrins, which endows them with two‐photon sensitivity. The nanoparticle efficiency for intracellular tracking is first demonstrated by the in vitro near‐infrared imaging of breast cancer cells. After functionalization of the nanoparticles with aminopropyltriethoxysilane, two‐photon‐excited photodynamic therapy in zebrafish is successfully achieved. Two‐photon photochemical internalization in cancer cells of the nanoparticles loaded with siRNA is also performed for the first time. Furthermore, siRNA targeting green fluorescent protein complexed with the nanoparticles is delivered in vivo in zebrafish embryos, which demonstrates the versatility of the nanovectors for biomedical applications.     

Long‐Lived and Highly Efficient TADF‐PhOLED with “(A)n–D–(A)n” Structured Terpyridine Electron‐Transporting Material

The electron‐transporting material (ETM) is one of the key factors to determine the efficiency and stability of organic light‐emitting diodes (OLEDs). A novel ETM with a “(Acceptor)_n–Donor–(Acceptor)_n” (“(A)_n–D–(A)_n”) structure, 2,7‐di([2,2′:6′,2″‐terpyridin]‐4′‐yl)‐9,9′‐spirobifluorene (27‐TPSF), is synthesized by combining electron‐withdrawing terpyridine (TPY) moieties and rigid twisted spirobifluorene, in which the TPY moieties facilitate electron transport and injection while the spirobifluorene moiety ensures high triplet energy (T₁ = 2.5 eV) as well as enhances glass transition temperature (T_g = 195 °C) for better stability. By using tris[2‐(p‐tolyl)pyridine]iridium(III) (Ir(mppy)₃) as the emitter, the 27‐TPSF‐based device exhibits a maximum external quantum efficiency (η_{ext, max}) of 24.5%, and a half‐life (T₅₀) of 121, 6804, and 382 636 h at an initial luminance of 10 000, 1000, and 100 cd m^?2, respectively, which are much better than the commercialized ETM of 9,10‐bis(6‐phenylpyridin‐3‐yl)anthracene (DPPyA). Furthermore, a higher efficiency, a η_{ext, max} of 28.2% and a maximum power efficiency (η_PE, _max) of 129.3 lm W^?1, can be achieved by adopting bis(2‐phenylpyridine)iridium(III)(2,2,6,6‐tetramethylheptane‐3,5‐diketonate) (Ir(ppy)₂tmd) as the emitter and 27‐TPSF as the ETM. These results indicate that the derivative of TPY to form “(A)_n–D–(A)_n” structure is a promising way to design an ETM with good comprehensive properties for OLEDs.