Long-wavelength Illumination Research Articles

Improvement in Performance and Stability of PbS QD/IGZO Phototransistors Through the Introduction of Ga2O3 Film for Broadband Sensor Applications.

The development of broadband photosensors has become crucial in various fields. Indium-gallium-zinc oxide (IGZO, In:Ga:Zn = 1:1:1) phototransistors with PbS quantum dots (QDs) have shown promising features for such sensors, such as reasonable mobility, low leakage current, good photosensitivity, and low-cost fabrication. However, the instability of PbS QD/IGZO phototransistors under an air atmosphere and prolonged storage remain serious concerns. In this article, two concepts to improve the reliability of PbS QD/IGZO phototransistors were implemented. P-type doping in the PbS QD layer through oxidation allows increasing the built-in potential between IGZO and PbS QDs, leading to enhancement in photoinduced electron-hole pair creation. Second, agglomeration and fusion of a PbS QDs layer were controlled via thermal annealing, which facilitated the transport of photocreated carriers. The p-type doping and interconnection of a PbS QD layer can be achieved by deposition and subsequent thermal annealing of gallium oxide (Ga2O3) on PbS QD/IGZO stacks. The resulting Ga2O3/PbS QD/IGZO phototransistors exhibited high-performance switching characteristics under dark conditions. Notably, they showed a remarkable photoresponsivity of 196.69 ± 4.05 A/W and a detectivity of (5.47 ± 1.4) × 1012 Jones even at a long-wavelength illumination of 1550 nm. While the unpassivated PbS/IGZO phototransistor suffered serious degradation in optical performance after 2 weeks of storage, the Ga2O3/PbS QD/IGZO phototransistor demonstrated enhanced stability, maintaining high performance for over 5 weeks. These findings suggest that Ga2O3/PbS QD/IGZO phototransistors offer a feasible approach for the fabrication of large-scale active matrix broadband photosensor arrays, potentially revolutionizing optical sensing in various cutting-edge applications.

324-OR: VEGF Is a Contributing Factor for the Reduction of Retinal Function in Diabetic Retinopathy and Hypoxic Retinal Disorders

Vascular endothelial growth factor (VEGF or VEGF-A), a major therapeutic target for blood-retina barrier breakdown (BRB) in diabetic retinopathy (DR), neovascular age-related macular degeneration (AMD), and other and hypoxic retinal diseases, has been shown to act as a direct functional regulator for peripheral and central nervous system (CNS) neurons. To investigate the direct effect of VEGF on photoreceptor function in DR and hypoxic retinal diseases, we examined and compared the effect of recombinant (rVEGF) on retinal function with electroretinography (ERG) in Akita spontaneous diabetic mice and wild-type (WT) mice kept under normal environment and hypoxia (8 percent oxygen for 5 days). Retinal function was measured with ERG in dark-adapted mice immediately after rVEGF intravitreally delivery under long-wavelength illumination. While the rVEGF caused a significant reduction of scotopic ERG a-wave and b-wave amplitudes and photopic ERG b-wave amplitudes in a dose-dependent manner in WT mice kept under normal conditions, the effect of rVEGF on the alteration of ERG amplitudes was diminished in age-matched Akita diabetic and hypoxic mice in which retinal VEGF levels are upregulated. Our results suggest that VEGF is a most likely direct functional regulator of photoreceptors. Our data strongly support that VEGF upregulation is a likely cause of the reduced photoreceptor function in DR and hypoxic retinal disorders. As VEGF is a major therapeutic target for BRB breakdown in DR, AMD, and other hypoxic retinal BRB diseases, our work provides the mechanistic insights for the therapeutic effects on visual acuity and for the care of anti-VEGF drug treated patients. Disclosure Y.Le: None. M.Zhu: None. Funding National Institutes of Health (R01EY26970, P30EY021725, P30GM122744); Oklahoma Center for the Advancement of Science and Technology (HR20-065)

Triplet Shelving in Fluorescein and Its Derivatives Provides Delayed, Background-Free Fluorescence Detection

Fluorescence from the xanthene dyes rose bengal, erythrosine B, eosin Y, and fluorescein is modulated by reversibly optically populating and depopulating their long-lived triplet states through coillumination with a second, long-wavelength laser. Here, we show that repumping the S1 state from the triplet generates strong, nanosecond-lived optically activated delayed fluorescence (OADF), microseconds to milliseconds after primary pulsed excitation. This time-delayed emission upon long-wavelength illumination generates fluorescence after triplet shelving and is a major contribution to fluorescence enhancement/modulation. The time-delayed and background-free OADF component is further increased using a >1 μs burst continuous wave excitation scheme to increase the steady-state triplet populations, yielding strong OADF even from strongly emissive fluorescein. Because emission is delayed long after the high-energy primary excitation, yellow-orange fluorescence is readily observed on zero background. As OADF generation depends on the triplet quantum yields and the reverse intersystem crossing rates, we directly probe the usually difficult-to-measure photophysics, create new zero-background detection schemes, and increase OADF through tailored excitation schemes, all improving sensitivity. The excellent match between experiments and simulations demonstrates the promise of these studies for OADF characterization, while enabling us to determine that OADF (in contrast to ground-state recovery and re-excitation) is the major component of fluorescence enhancement for xanthenes studied with triplet quantum yields exceeding 0.1.

In vivo label-free confocal imaging of the deep mouse brain with long-wavelength illumination.

Optical microscopy is a valuable tool for in vivo monitoring of biological structures and functions because of its non-invasiveness. However, imaging deep into biological tissues is challenging due to the scattering and absorption of light. Previous research has shown that 1300 nm and 1700 nm are the two best wavelength windows for deep brain imaging. Here, we combined long-wavelength illumination of ~1700 nm with reflectance confocal microscopy and achieved an imaging depth of ~1.3 mm with ~1-micrometer spatial resolution in adult mouse brains, which is 3-4 times deeper than that of conventional confocal microscopy using visible wavelength. We showed that the method can be added to any laser-scanning microscopy with simple and low-cost sources and detectors, such as continuous-wave diode lasers and InGaAs photodiodes. The long-wavelength, reflectance confocal imaging we demonstrated is label-free, and requires low illumination power. Furthermore, the imaging system is simple and low-cost, potentially creating new opportunities for biomedical research and clinical applications.

Spectrum-Dependent Spiro-OMeTAD Oxidization Mechanism in Perovskite Solar Cells

We propose a spectrum-dependent mechanism for the oxidation of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), which is commonly used in perovskite solar cells as the hole transport layer. The perovskite layer plays different roles in the Spiro-OMeTAD oxidization for various spectral ranges. The effect of oxidized Spiro-OMeTAD on the solar cell performance was observed and characterized. With the initial long-wavelength illumination (>450 nm), the charge recombination at the TiO2/Spiro-OMeTAD interface was increased due to the higher amount of the oxidized Spiro-OMeTAD. On the other hand, the increased conductivity of the Spiro-OMeTAD layer and enhanced charge transfer at the Au/Spiro-OMeTAD interface facilitated the solar cell performance.

Is it effective to harvest visible light by decreasing the band gap of photocatalytic materials?

In situ variations in the electron work function and photo-current of TiO2 nanotubes demonstrate that long-wavelength illumination only has a minor effect on the excitation of electrons in the nanotubes after being exposed to short-wavelength light or when the short-wavelength light coexisted, indicating that the solar spectrum may not be utilized as efficiently as expected by extending the absorption spectrum of the photocatalytic material to visible light range with decreased band gaps.

Thermally and photostimulated recombination processes in PbWO4 crystals at low temperatures

We have studied thermally and photostimulated recombination processes in PbWO4 crystals in the temperature interval 90–295 K. We have shown that after x-ray excitation of the crystal at 90 K, the thermally stimulated luminescence curve shows a relatively intense peak at 110 K and weak peaks at about 173, 191, and 232 K. Exposure of the PbWO4 crystal to IR radiation after excitation by x-ray photons leads to the appearance of photostimulated flash luminescence, a significant decrease in the intensity of the 110 K peak, and a shift of that peak to the 116 K region. But long-wavelength illumination has less of an effect on the intensity and position of the higher temperature peaks. It is assumed that F− centers are formed in the PbWO4 crystal at 90 K during x-ray exposure. The thermal light sum released by IR illumination in the temperature region 90–150 K has a maximum at ≈108 K. The nature of this maximum is connected with the complex centers [Pb3+ + (WO3-F−)]. Optical and thermal degradation of these centers leads to the appearance of emission in the green region of the localized exciton spectrum.

Photo-hydrogen production rate of a PVA-boric acid gel granule containing immobilized photosynthetic bacteria cells

Abstract The polyvinyl alcohol (PVA)-boric acid gel granule facilitates the light penetration and mass transport as it has the features of the transparency and adequate porous structure. In this work, a hydrogen production bioreactor with the indigenous photosynthetic bacteria (PSB) Rhodopseudomonas palustris CQK 01 immobilized in a PVA-boric acid gel granule is developed to enhance the rate of photo-hydrogen production. Particular attention is paid to exploring the effects of illumination wavelength and intensity, as well as the effects of concentration, flow rate, pH, and temperature of influent substrate solution on the hydrogen production rate. The immobilized PSB gel granule exhibited the maximum hydrogen production rate of 3.6 mmol/g cell dry weight/h in all tests. The experimental results show that the hydrogen production rate of an immobilized PSB granule illuminated at 590 nm is distinctly higher than that at 470 and 630 nm. Photo-inhibition of the gel granule occurs as the long-wavelength illumination intensity exceeds 7000 lux. In addition, there exists an optimal pH of 7.0 and temperature of 30 °C for PSB immobilized in the granule to produce hydrogen. More importantly, the feasibility of PSB immobilized in the PVA-boric acid gel granule for the enhancement of the photo-hydrogen production is demonstrated.

Imaging Deep Optical Sections with Two-Photon Excitation Fluorescence Microscopy

The technique of optical sectioning allows the visualization of a succession of images of parallel planes within a thick specimen with little or no out-of-focus interference. Ultimately, a limit is reached on the depth to which optical sections can be obtained from a given sample. This limit, up to the working distance of the objective, is largely determined by the degree of light scattering encountered by the incident excitation beam as well as the returning emission signal.Confocal imaging was one of the first optical sectioning techniques applied to fluorescence imaging. Two-photon excitation imaging is a recently developed alternative optical sectioning technique for fluorescence imaging where an excitation wavelength of around twice the excitation peak of the fluorophore is used in a laser-scanning microscope. This excitation wavelength produces very little fluorophore excitation in the bulk of the sample, but when the incident photons are confined in space and time sufficient two-photon absorption events can take place to obtain rapid imaging of fluorophores. With high peak powers—obtained with a sub-picosecond pulsed laser source focused by a lens—sufficient photon density can be obtained for easily detectable two-photon events. Thus fluorophore excitation occurs as two photons are absorbed essentially simultaneously, which act effectively as a single photon of twice the energy (half the wavelength). Two-photon events have a quadratic dependence on intensity, and, therefore, decrease rapidly away from the focal volume of the lens. In a raster scanning system, fluorophore excitation is confined to the optical section being viewed as fluorophore away from the lens focal volume is not excited by the long-wavelength illumination.

Effect of Fluorescence Quenching by Stimulated Emission on the Spectral Properties of a Solvent-Sensitive Fluorophore

We examined the emission spectra and wavelength-dependent anisotropies of the solvent-sensitive fluorophore 4-(dimethylamino)-4‘-cyanostilbene (DCS) under condition of fluorescence quenching by stimulated emission. The sample was illuminated with a train of 10 ps pulses at 285 nm, and a train of stimulating pulses at 570 nm which were delayed by a time td relative to the excitation. Stimulated emission of DCS was demonstrated to occur by observation of gain in the long-wavelength beam. Illumination on the long-wavelength side of the emission spectrum with the long-wavelength time-delayed pulses resulted in a blue shift of the emission spectrum, and a progressive decrease of the emission anisotropy as the observation wavelength increased toward the stimulating wavelength. The spectral shifts and wavelength-dependent anisotropies of DCS were more pronounced in more viscous solvents where spectral relaxation is incomplete during the excited state lifetime. Light quenching of DCS in a low-viscosity solvent revealed no spectral shifts or wavelength-dependent anisotropies. Control measurements using acridine orange, which is relatively insensitive to solvent polarity, did not show any spectral shift or wavelength-dependent anisotropy with light quenching. The data for DCS can be explained by the presence of a time-dependent spectral shift and wavelength-selective quenching of the longer wavelength emission. In this model the relaxed state is formed following excitation of the unrelaxed state, and the relaxed state is preferentially quenched by long-wavelength illumination. Comparison of the data with model calculations indicates the presence of at least two spectral relaxation times. These results demonstrate that light quenching by stimulated emission acts selectively based on overlap of the stimulating wavelength with the emission spectrum. Observation of the emission spectrum in the presence of time-delayed and power-controlled long-wavelength pulses can be used to study time-dependent excited state processes.

Electronic transport and cyclotron resonance in 2H-PbI2

Photoexcited electrons and holes in 2H-PbI2 are studied by measurements of the Hall effect magnetoconductivity and cyclotron resonance. Transport by holes is dominant below 20K under long-wavelength illumination. Electron transport is dominant under white light illumination below 20K and under all illumination conditions above 20K. Hole mobility is anisotropic with 2< mu perpendicular to c/ mu /sub //c/<4. The mobility of electrons above 30K, when limited by optical phonon scattering may be fitted by an effective band mass of 0.55 m0 where m0 is the mass of the free electron. Cyclotron resonance at 137 GHz gives a polaron mass m/sub ///mperpendicular to )12/=(0.68+or-0.16)m0, or a band mass (mb//mb perpendicular to )12/=(0.46+or-0.15)m0.