Blue-shifted Spectrum Research Articles

Stable, precisely controlled, and switchable thulium-doped fiber laser based on cascaded mode interference filters.

This research experimentally demonstrates a switchable, single-wavelength, thulium-doped fiber laser based on the cascading of a multimode-single-mode-multimode (MSM) fiber filter and a two-mode fiber (TMF) filter. When the MSM fiber filter suffers from bending, the blue-shift of the output spectrum can be obtained. A switchable lasing wavelength output is realized by bending the MSM fiber filter to cover different channels of the TMF filter. The output wavelength can be switched from 1982.54 to 1938.81 nm with an optical signal-to-noise ratio of higher than 40 dB. The wavelength interval of the switchable output is an integral multiple of the wavelength interval of the TMF filter. The stability of the output wavelength was tested within 60 min, and the wavelength shift and output power fluctuation were found to be less than 0.01 nm and 0.31 dB, respectively, which demonstrates a stable output performance.

State filling effects on photoluminescence and photovoltaic characteristic of aluminium-doped CdTe colloidal quantum dots stabilized in aqueous medium

CdTe and aluminium (Al)-doped CdTe (Al:CdTe QDs) colloidal quantum dots (QDs) were synthesized through an aqueous route. CdTe and Al:CdTe QDs colloidal quantum dots of different size were obtained by collecting the samples at different refluxing time. The size of the synthesized QDs collected after 6 h of refluxing time was measured to 3.3 and 4.8 nm for Al:CdTe QDs and CdTe QDs, respectively. The size shrinkages of Al:CdTe QDs were due to chlorine ions from AlCl3 precursor, which controls the growth kinetics of QDs and stabilizes the QDs in the aqueous phase. The size shrinkages of Al:CdTe QDs result in an increase of bandgap from 2.13 to 2.3 eV and congruently blue-shifted absorbance and emission spectra were discerned. The state filling effect, gradual filling of energy state, observed clearly in photoluminescence spectra of Al:CdTe QDs for samples collected after 30 min of refluxing time. We also fabricated Gratzel-type QDs-sensitized solar cells by loading CdTe and Al:CdTe QDs on mesoporous TiO2 photoanode. Photovoltaic efficiency is marginally increased (13%) from 0.45 to 0.51% for Al-doped CdTe QDs as compared to CdTe QDs.

Leakage of holes induced by Si doping in the AlGaN first barrier layer in GaN/AlGaN multiple-quantum-well ultraviolet light-emitting diodes

The electroluminescence characteristics of GaN/AlGaN multiple-quantum-well ultraviolet light-emitting diodes (LEDs) with different concentrations of silicon impurities in the first AlGaN barrier layer near the N-type GaN region is investigated numerically. It is found that the LED's electroluminescence spectrum blueshifts and its peak intensity increases first and then decreases, as the Si-doping concentration increases. This is because that the effective potential height and width of the first barrier layer is reduced due to the screening effect by the ionization of silicon impurities. As a result, more electrons can be injected into the active region, enhancing the luminescence efficiency. However, when the impurity concentration is too high, the leakage of holes may become severe, leading to a reduction of the luminescence intensity for the most heavily doped sample at the injection current of 20 mA.

V OC variation with different molecular weight fractions in highly efficient organic photovoltaic bulk heterojunctions

Tuning molecular weight fractions is one of the most important strategies for optimizing the morphology of bulk heterojunctions (BHJs) to obtain high-performance organic solar cells. Here, we investigate the effects of molecular weight fractions on the performance of state-of-the-art PM6:Y6 BHJ solar cells. A remarkable blueshift in the photoluminescence spectra can be observed with the introduction of the PM6 component. The electroluminescent and morphological analyses indicate that this observation can be attributed to increased charge-transfer state energy and an intermolecular interaction between donor and acceptor, leading to open circuit voltage (V OC) variations in the photovoltaic device. The transient absorption spectra confirm that the optimized donor:acceptor (D:A) ratio can suppress the generation of polarons and achieve high efficiency exciton dissociation. The results of atomic force microscopy and neutron reflectometry reveal that higher acceptor contents promote the formation of a desirable interpenetrating network and enable high fill factors and short-circuit currents to be obtained.

High-Efficiency Nd3+-Doped Phosphate Fiber Laser at 880 nm

Spectroscopic properties of Nd3+-doped phosphate glass for three-level laser were investigated for the first time. A 60 nm blue-shift of the three-level transition fluorescence spectrum was achieved by changing the host material from traditional silica glass to phosphate glass. An 880-nm all-fiber laser with a world-record slope efficiency of 42.8% was demonstrated with 25-cm-long 0.25-wt.% Nd3+-doped phosphate fiber. Our research shows that Nd3+-doped phosphate glass is a high-efficiency material for laser operation below 900 nm.

Ultrafast Excited-State Dynamics of Tricarbocyanine Dyes Probed by Two-Dimensional Electronic Spectroscopy: Polar Solvation vs Photoisomerization.

Photophysical properties of tricarbocyanine dyes in various solvents have been widely investigated using a variety of spectroscopic tools. However, the presence of several ground-state isomers and interconversion between these isomers on an ultrafast timescale upon photoexcitation render unambiguous assignment of spectral features quite difficult. In this work, ultrafast excited-state dynamics of two tricarbocyanine dyes in two solvents, DNTTCI and IR140, in ethanol and ethylene glycol, are studied by two-dimensional electronic spectroscopy (2DES). We present a detailed discussion on design and calibration of the 2DES apparatus and on the method for data processing by phase-cycling. For DNTTCI we report a method to obtain solvation correlation function, the nature of which is found to be strongly dependent on the excitation frequencies; a blue-shifted spectrum at early time is observed and explained based on preferential emission from a subset among various isomers having overlapping spectral features. For IR140 in ethanol, four isomers with distinct spectral features are identified, and most importantly, three of these isomers were found to interconvert upon photoexcitation which completes within 100 fs and is explained based on a kinetic model of consecutive chemical reaction. Density functional theory calculations show the presence of several ground-state isomers for both these dyes. Through this work we demonstrate how 2DES can help us to decipher distinct excited-state photophysics in two carbocyanine dyes, polar solvation and photoisomerization, by resolving spectral congestion without sacrificing time resolution.

Strategy to modify intrinsic luminescence via post-annealing treatment and impurity doping in SbNb3(PO4)6

Self-activated luminescence of a phosphor has unique advantages such as natural homogeneity, wide wavelength, without rare-earth dopant, easy preparation, etc. However, its luminescence efficiency usually is not satisfied. This work reports several strategies to modify the intrinsic luminescence intensity of SbNb3(PO4)6 synthesized via the sol–gel method. XRD measurements were conducted to verify phase formations and structural characteristics. The transition natures and band energy were determined on the basis of the optical absorption. SbNb3(PO4)6 shows a broad self-activated luminescence (400–750 nm) peaked at 485 nm. Impurity ion V5+ doping improves the intrinsic luminescence of SbNb3(PO4)6. The optimal efficiency of 31.63% was observed in 7.0 mol% V5+-doped SbNb3(PO4)6. Post-annealing of as-prepared SbNb3(PO4)6 in the air can enhance the luminescence, while, annealing in CO atmosphere deteriorates the intensity. The luminescence thermal stabilities and activation energies were evaluated via the temperature-dependent emission intensities and decay lifetimes. The intrinsic defects such as oxygen vacancies (VO••) were confirmed via XPS measurements, which exerted great influences on the emission and thermal stabilities of the emission. The abnormal blue-shift of the emission spectra was presented from 10 to 350 K. The luminescence mechanism was proposed for the self-activated emission based on the dynamic emission spectra, decay time, and multiple color centers.

On-resonance islands of Ag-nanowires sense the level of glycated hemoglobin for diabetes diagnosis

Glycated hemoglobin (HbA1c) is one of the most important indicators for the diabetes diagnosis. In this study, we designed and characterized a disordered silver nanowire-based chip for the detection of HbA1c level in human blood samples upon localized surface plasmon resonance (LSPR). The architecture of the chip intended to enhance the LSPR shift by the combination of two mechanisms: (1) an overall variation in refractive index change upon the protein binding due to the presence of the nanowires, and (2) a plasmonic coupling between the Ag nanowires. The hydrophobic surface made by the disordered nanowires retained the sample during the testing. Samples with different HbA1c levels, including the ones recommended by the American Diabetes Association for diabetes diagnosis, displayed a blueshift and bandwidth narrowing of the absorbance spectra, signifying the sensitivity of the chip. Besides, the sensorgrams demonstrated peaks with intensities varying according to the characteristics of the samples. The analysis of the HbA1c-specific-peak displayed that the values similar to HbA1c levels in calibrated blood samples with an error of less than 1%.This technique helps to open up a new avenue toward highly sensitive trace protein detection via the plasmonic method.

Synthesis, Characterization and Dye-Sensitized Solar Cell Application of a Novel Symmetric Diphenylpyran Dye with Dual Rhodamine-3-acetic Acid Anchors

With simple three-step method, a new symmetrical metal free organic diphenylpyran dye containing dual rhodamine-3-acetic acid anchors was successfully synthesized, characterized and applied to dye-sensitized solar cells (DSSC). The dye has saturated methylene in its acceptor part, which makes the electron withdrawing group and the anchor group cannot be conjugated well. In addition, the exhibits lower molar extinction coefficients of the dye and its blue-shifted absorption spectrum also influenced the light capture ability. The resulting weaker intramolecular charge transfer and the electron coupling effect of the dye hindered the dye charge injection and shorten the electron lifetime, leading to an average power conversion efficiency of 0.65 % (Jsc = 2.32 mA cm−2, Voc = 448 mV and FF = 0.63). Further design of an efficient dye molecule needs efficient conjugation between the electron-withdrawing group and anchoring group, as well as an effective light harvest capability.

Lasing from a conjugated polymer at selectable areas and at tunable wavelengths via electron irradiation

A simple approach to producing tunable lasing is explored. In this work, a basic planar lasing system with a layer of light emitting conjugated polymer, MEH-PPV, sandwiched between a distributed Bragg reflector (DBR) and silver is utilized. By irradiating MEH-PPV with electrons, the photoluminescence (PL) spectrum of MEH-PPV undergoes blueshift and peak broadening, which in turn alters the lasing wavelength. Furthermore, patterning the MEH-PPV layer by electron irradiation and employing filters allow lasing to be controlled to occur only at certain areas. Thus, our approach allows the fabricated system to possess significant tunability, a highly sought out trait in the field of lasing. The detailed methodology by which tunable lasing is achieved is discussed along with the mechanism behind the irradiation-induced blueshift of the PL spectrum.

Star-shaped benzotriindole-based donor-acceptor molecules: Synthesis, properties and application in bulk heterojunction and single-material organic solar cells

The search of the novel building blocks for π-conjugated donor-acceptor (D-π-A) molecules remains an urgent task to design promising materials for organic solar cells (OSCs) and other electronic devices. Here we report on the design and synthesis of two star-shaped D-π-A small molecules based on benzotriidole (BTI) electron-donating core, BTI(2T-DCV-Hex)3 and BTI(2T-CNA-EHex)3, end-capped with either hexyldicyanovinyl or 2-ethylhexylcyanoacetate acceptor groups. Comprehensive investigation and comparison of the optical, thermal and physicochemical properties of these molecules and their analogue with the triphenylamine (TPA) core, N(Ph-2T-DCV-Hex)3, revealed the effect of the electron-withdrawing groups and type of the donor core on their properties. The BTI-based material BTI(2T-DCV-Hex)3 differs from the amorphous TPA-based analogue by high crystallinity and blue-shifted absorption and luminescence spectra. The change of electron-withdrawing group from hexyldicyanovinyl to 2-ethylhexylcyanoacetate leads to a higher energy of the lowest unoccupied molecular orbital, lower solubility and several times higher photoluminescence quantum yield in solutions achieving 67%. Evaluation of the photovoltaic performance of these materials in single-material OSCs and as a donor material in bulk heterojunction OSCs with PC71BM as an acceptor revealed that the devices based on BTI(2T-DCV-Hex)3 are more efficient as compared to those based on BTI(2T-CNA-EHex)3. In comparison to N(Ph-2T-DCV-Hex)3, the photovoltaic devices based on BTI(2T-DCV-Hex)3 showed the comparable performance in bulk heterojunction OSCs and two times higher performance (about 1%) in single-material OSCs. As a result, we conclude that the BTI core is a promising block for the design of semiconducting materials for organic photovoltaics and other related applications.

Enhancement of coloring under ultraviolet irradiation in photochromic diarylbenzenes

1,2-Diarylbenzenes have been developed as a new family of T-type photochromic switches that have potential applications such as ophthalmic lenses, real-time holography, and super-resolution microscopy. In this work, we have designed and synthesized novel diarylbenzenes bearing thiazole and oxazole rings as the aryl groups to improve the coloring under ultraviolet (UV) irradiation. The substitution of the aryl groups with thiazole and oxazole rings having lower aromaticity than thiophene brought about the deceleration of the thermal bleaching reaction, the blue-shift of the absorption spectra of the closed-ring isomers, and the increase of the photocyclization reactivity, which results in the enhancement of the coloring when exposed to UV light. The results would provide useful information for the development of practical applications using diarylbenzene chromophores.

A Solid-State Luminescent Cd(II) Supramolecular Coordination Framework Based on Mixed Luminophores as a Sensor for Discriminatively Selective Detection of Amine Vapors.

A novel Cd(II) supramolecular coordination framework containing mixed functionalized luminophore ligands, namely, [Cd(AS)2(phen)2]EtOH or 1 (where AS = 4-aminosalicylate, phen = 1,10-phenanthroline, EtOH = ethanol), was successfully synthesized as a solid-state luminescent sensor for the detection of amine vapors. The single-crystal X-ray diffraction analysis revealed that 1 possesses a three-dimensional (3D) supramolecular framework enclosing ethanol molecule in the lattice. The supramolecular structure is well-stabilized by various noncovalent intermolecular interactions through functional groups of ligands. Compound 1 shows an intense yellow solid-state emission and displays a reversibly discriminative luminescent response to NH3 and ethylenediamine (EDA) vapors through very large blue-shifted luminescent spectra with distinguishable emission colors under UV light. This work reports the first time for selective luminescent sensing of NH3 and EDA vapors with considerably different emission color change. A sensing mechanism has been confirmed by density functional theory and time-dependent density functional theory calculations that agrees well with the experimental results. Also, 1 exhibits a good recyclability over five cycles for sensing of NH3 and EDA vapors.

Magneto-luminescent zinc/iron oxide core-shell nanoparticles with tunable magnetic properties

Abstract Core-shell nanospheres with tailorable magnetic properties and strong luminescence promise versatile and efficient biomedical and electronic applications. We report a low-cost sonochemical synthesis of ZnO/iron oxide core-shell nanostructures with tunable magnetic properties. The high-resolution transmission electron microscopy (HRTEM) illustrates the distinguished dispersity of the oxide nanoparticles, whereas elemental profiling using line scan data confirm the formation of the distinguished core and shell phases. Annealing in N2–H2 and Ar–H2 ambient alters the magnetic properties such as coercive field, saturation magnetization, and remnant magnetization within the inverse spinel iron oxide with space group Fd3m. Subtle structural changes between maghemite (γ-Fe2O3) and its reduced form, magnetite (Fe3O4), have been identified in the thin (0.5–3 nm) shell region of atomic dimensions. Structural composition including phase transitions and defect states are investigated using XRD, XPS, and Raman measurements. Coexistence of ferromagnetism (from the shell and characterized by magnetic measurements) and strong luminescence (from ZnO core and obtained from photoluminescence spectroscopy) at room temperature indicates exotic magneto-optical coupling, which is supported by the blue-shifted luminescence spectra. Our approach combining nanotechnology and solid-state chemistry opens a new frontier to the building of innovative nanomaterials for integration, especially multifunctional core-shell oxide nanomaterials.

An Ejecta Kinematics Study of Kepler’s Supernova Remnant with High-resolution Chandra HETG Spectroscopy

Abstract We report our measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Kepler’s supernova remnant (SNR). We measure the Doppler shift of the He-like Si Kα line-center energy in the spectra of these knots based on our Chandra High-Energy Transmission Grating Spectrometer observation to estimate their radial velocities. We estimate high radial velocities of up to ∼8000 km s−1 for some of these ejecta knots. We also measure proper motions for our sample based on the archival Chandra Advanced CCD Imaging Spectrometer data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are almost freely expanding after ∼400 yr since the explosion. The fastest moving knots show proper motions of up to ∼0.″2 per year. Assuming that these high-velocity ejecta knots are traveling ahead of the forward shock of the SNR, we estimate the distance to Kepler’s SNR d ∼ 4.4–7.5 kpc. We find that the ejecta knots in our sample have an average space velocity of v s ∼ 4600 km s−1 (at a distance of 6 kpc). We note that 8 of the 15 ejecta knots from our sample show a statistically significant (at the 90% confidence level) redshifted spectrum, compared to only two with a blueshifted spectrum. This may suggest an asymmetry in the ejecta distribution in Kepler’s SNR along the line of sight; however, a larger sample size is required to confirm this result.

Exciplex Formation and Electromer Blocking for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs with All-Solution-Processed Organic Layers.

Highly efficient solution-processable emitters are greatly desired to develop low-cost organic light-emitting diodes (OLEDs). The recently developed thermally activated delayed fluorescence (TADF) materials are promising candidates, but blue TADF materials compatible with the all-solution-process have still not been achieved. Here, a series of TADF materials, named X-4CzCN, are developed by introducing the bulky units through an unconjugated linker, which realizes high molecular weight to enhance the solvent resistance ability without disturbing the blue TADF feature. Meanwhile, the peripheral wrapping groups efficiently inhibit the triplet-triplet and triplet-polaron quenching by isolating the energy-transfer and charge-transporting channels. The photophysical measurements indicate that a small variation in peripheral unit will have a noticeable effect on the luminescence efficiency. The enlarged volume of peripheral units will make the electroluminescent spectra blueshift, while enhancing the energy transfer of exciplex and blocking the energy leakage of electromer can facilitate the exciton utilization. As a result, the fully solution-processed blue OLED achieves a CIE of (0.16, 0.27), a low turn on voltage of 2.9 eV, and a high external quantum efficiency of 20.6 %. As far as we known, this is the first report of all-solution-processed TADF OLEDs with blue emission, which exhibits a high efficiency even comparable to the vacuum-deposited devices.

A new colorimetric and ratiometric probe for highly selective recognition and bioimaging of ClO- and Al3.

In this study, a new fluorescence probe HMAQ based on quinazoline and diaminomaleonitrile was constructed for sensing ClO- and Al3+. A fluorescence blue-shift with 102nm together with a color change from golden-yellow to colorless was found by hypochlorite-induced hydrolysis of -CH=N- group to release the initial fluorophore. Besides, Al3+ could cause a 72-nm blue-shifted emission spectra and a color change from golden-yellow to brown. As expected, HMAQ exhibited a satisfactory selectivity and sensitivity to ClO-/Al3+ with a quick response. Most notably, the reversibility of the [HMAQ+Al3+] complex could be used to detect ClO- and Al3+ simultaneously without mutual interferences. The detection limits of HMAQ for ClO- and Al3+ were turned out to be 10.2nM and 1.56nM, respectively. The high-performance results of real-time detections demonstrated the enormous potential of HMAQ in real-water samples and living cells.

Comparison of the Energy Transfer Dynamics in Structural and Spectral Variants of the Light-Harvesting Complex 2 of Purple Bacteria

Light-harvesting complex 2 (LH2) of purple bacteria is a well-known model to study the initial light-harvesting steps of photosynthesis. It is organized into two rings: eight or nine bacteriochlorophyll collectively absorb at 800 nm (the B800 ring), while sixteen or eighteen bacteriochlorophyll absorb at 850 nm (the B850 ring). LH2 is produced in several variants both within and across species. While the dynamics of some of these variants have been characterized, the energy transfer rates within many of these variants have not been measured. In this work, we utilize ultrafast transient absorption measurements to compare the energy transfer dynamics of three variants of LH2: LH2 R. sphaeroides, a nonamer, LH2 Ph. molischianum, an octamer, and LH3 Ph. molischianum, which has a blue-shifted absorption spectrum to 820 nm compared to the B850 ring of LH2 Ph. molischianum. These results provide insight into how variations between and within species impact energy flow in photosynthetic systems.

Control of the terahertz spectra generated from laser induced plasma

Broadband terahertz (THz) emission generated from laser induced gas plasma provides an effective tool for studying nonlinear spectrum, imaging and remote sensing. Recently, the contribution of plasma oscillation to the THz emission was revealed from the nitrogen molecules pumped by intense two-color laser pulses. Plasma oscillation contributes only to the THz emission at relatively low plasma density due to negligible plasma absorption. More generally, with the THz emission generated from the ionizing gaseous medium, the surrounding plasma is expected to play an important role in the generation process. For the THz radiation from laser filament, the plasma region is extended in the laser propagation direction, and the effect of surrounding plasma on the emitted THz spectrum needs studying. In this work, we investigate the relation between pump power and filament length from THz spectrum emitted by air filament driven by two-color laser pulse. The time domain spectrum of THz field is recorded by an electro-optic (EO) sampling technique. In our experiments, significant frequency shifts are observed as the pump power and the filament length increase, and we find that the center frequency of the THz radiation is shifted towards longer wavelength, which is the so called red-shift of the THz spectrum. This red-shift is independent of THz radiation angle. The observations are explained by the plasma absorption inside the air filament. Our theoretical model is based on three mechanisms: the ionization-induced photocurrent, the plasma current oscillation and the plasma absorption. We coherently add up all the local THz fields inside the air filament, and simultaneously consider the plasma absorption induced correction of the THz spectrum. The simulation well reproduces the experimental observation. The skin depth decreases as the plasma density increases, thus the plasma absorption dominates the red-shift process. If the skin depth is larger than the filament length, the plasma oscillation contributes to the THz spectrum dominantly, and thus leading to the blue-shift of THz spectrum. Our results indicate that for the extended filament length or higher plasma density, the combining effect of photocurrent, plasma oscillation and absorption, results in the observed low-frequency broadband THz spectrum. Our study offers a method of coherently controlling the broadband THz spectrum.

Effect of Li doping and point vacancy on photocatalytic properties and mechanism of ZnO

Controlling the point defect in Li-doped ZnO is difficult. In addition, the absorption spectrum distribution in Li-doped ZnO with O vacancy (VO) has two reports of red- and blue-shift experimental predictions. The photocatalytic properties of Li doping and point defect on ZnO was studied using first-principles method based on generalized gradient approximation (GGA) plane-wave super-soft pseudopotential + U (GGA + U) to solve this problem. Compared with undoped ZnO, ZnO supercell with Li replacing Zn (LiZn) and VO, LiZn and Zn vacancy (VZn), interstitial Li (Lii) and VZn, and ZnO supercell with LiZn, Lii, and VZn have a narrower band gap and a red-shifted absorption spectrum distribution. By contrast, ZnO supercell with Lii and VZn have a wider band gap and a blue-shifted absorption spectrum distribution. The effective mass calculations show that the ZnO supercell with LiZn and VO has the smallest effective mass of carriers and can thus accelerate the separation of electron–hole pairs, thereby improving photocatalytic activity. The analysis of the electric dipole moment showed that ZnO supercell with LiZn and VO has the strongest activity of carries. This condition is advantageous for designing and preparing novel photocatalysts.