Two-color Emission Research Articles

Frequency-resolved measurement of two-color air plasma terahertz emission

We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2-2.2 THz) and GaP (0.4-6.8 THz) crystals, and ultra-broadband ABCD technique (1-17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.

Supramolecular Assembly of Hydrogen-Bonded Organic Frameworks with Carbon Dots: Realizing Ultralong Aqueous Room-Temperature Phosphorescence for Anticounterfeiting.

Room-temperature phosphorescent carbon dots (RTP-CDs) have received increasing attention due to their excellent optical properties and potential applications. Nevertheless, the realization of RTP-CDs in aqueous solutions remains a considerable challenge due to the water-molecule- and oxygen-induced deactivation of the triplet excitons, which leads to phosphorescence quenching. In this study, ultralong phosphorescence in water was achieved by in situ self-assembly of CDs encapsulated in a rigid hydrogen-bonded organic framework (HOF). The phosphorescence lifetime reaches an impressive 956.96 ms and exhibits long-lasting optical and structural stability in water for more than 90 days. The composite material not only has ultralong luminescence life and excellent luminescence stability but also has two-color phosphorescence emission, as well as excellent antiphotobleaching and phosphorescence stability in aqueous solution, which can solve the current problem that RTP is easily burst out by water and moisture. In addition, this study introduced a fluorescent dye based on the triplet-singlet Förster resonance energy transfer system (TS-FRET) to fine-tune the afterglow properties. This work will inspire the design of RTP systems with dual phosphor light sources and provide new strategies for the development of smart RTP materials in water.

White-Light Emission from a Host-Guest Composite between Carboxylatopillar[5]arene-Modified N-Doped Carbon Dots and Rhodamine 6G for Rutin Detection.

The construction of tunable white-light-emitting materials has garnered increasing attention in the scientific community. In this study, N-doped carbon dots (N-CDs) were surface-modified with carboxylatopillar[5]arene (CP[5]) using an EDC-NHS coupling reaction to create CCDs. CCDs were then conjugated with rhodamine 6G (R6G) through host-guest interactions to fabricate the CCDs-R6G composites. These composites produced two-color fluorescence emission peaks at 447 and 557 nm when excited by a wavelength of 340 nm. Excitingly, white-light emission (0.28, 0.30) can be readily achieved by varying the R6G concentration. To further explore potential applications, a new detection method for rutin (RT) based on the inner filter effect (IFE) was developed. Experimental results verify the practicality and reliability of the fluorescence sensor based on CCDs-R6G composites for RT detection in real samples.

Electrically Tunable Two-Color Cholesteric Laser.

Two-color lasing emission from an asymmetric structure, consisting of two dye-doped cholesteric liquid crystal (DD-CLC) layers separated by a transparent interlayer, is demonstrated. The DD-CLC mixtures have different reflection bands with long-wavelength band edges located at the green and red wavelengths of the visible spectrum, respectively. For the laser action, the CLC hosts provide the feedback, and the fluorescent laser dyes represent the active medium. When the stacked structure is optically pumped above the threshold, two simultaneous laser lines separated by 123 nm are observed at the long-wavelength band edges of the DD-CLC mixtures. The influence of an electric field on lasing behavior is also analyzed and discussed in terms of the reflection spectrum and laser action. The results show a reversible tuning of the reflection band, accompanied by a modification of the lasing characteristics under the application of an external field. Above a specific threshold voltage, one of the emission lines is suppressed and the other is conserved. With a further increase in the voltage, both laser emissions are entirely inhibited. The investigated structure demonstrates a simple technique to obtain an electrically tunable multi-wavelength laser, which might pave the way for a new generation of organic laser sources.

Analyzing 3D fields of refractive index, emission and temperature in spray-flame nanoparticle synthesis via tomographic imaging using multi-simultaneous measurements (TIMes)

Turbulent spray flames issuing from the SpraySyn 2.0 burner were investigated in the present work by means of tomographic imaging using multi-simultaneous measurements (TIMes). This small-scale standard burner is designed to study flame-spray synthesis of tailored nanoparticle systems. The applied measurement technique combined background-oriented schlieren tomography, computed tomography of light emission and two-color tomographic salt emission thermometry to reveal single-shot and time-averaged 3D fields of refractive index (linked to temperature, pressure and species concentration), CH* chemiluminescence (flame front indicator), SrOH emission from synthesized trace amount of Sr(NO3)2 dissolved in the liquid precursor solution (marker of the hot spray-flame products), and temperature of the hot spray-flame products, respectively. The salt emission thermometry technique was extended to tomographic measurements for the first time. Its implementation was based on the voxel-wise intensity ratio of reconstructed thermally-excited SrOH signals at two different regions in the visible spectrum that was converted to temperature using a single calibration curve based on multi-line OH planar laser-induced fluorescence measurements. The cost-effective setup for TIMes was composed of 43 machine-vision cameras equipped with bandpass spectrum filters, an innovative continuous semi-circular background pattern and LED light sources. The combined reconstructed fields provided information on the 3D flame structure in detail, showing the interaction between the flame front and spray stream, and the hot temperature regions. Spray-flame features were quantitatively analyzed, namely temperature, height, width, 3D tilt angle, and spreading half-angle of the flame plume. The temperature measurements proved to be robust against variations in the fuel/oxidizer equivalence ratio for the four investigated operating conditions. The spray flame issuing from the SpraySyn 2.0 burner at the standard operating condition was more stable than that in the previous burner generation, although more asymmetric near the nozzle exit and tilted, which might still cause heterogeneity of the synthesized nanoparticle system.

Polymorphous meso-Dicyanovinyl-Substituted Tetraphenylethylene Derivative Showing Distinguishing Two-Color Emission and Ultrahigh-Contrast Mechano-Responsive Luminescence Behavior

Polymorphous <i>meso</i>-Dicyanovinyl-Substituted Tetraphenylethylene Derivative Showing Distinguishing Two-Color Emission and Ultrahigh-Contrast Mechano-Responsive Luminescence Behavior

Intercalation of Mordant orange 1 into ZnAl-layered double hydroxide and its dual-color photoluminescence emission

This paper reports the preparation of dual-color emitting nanohybrid based on ZnAl-layered double hydroxide (LDH). Mordant orange 1(MO1) was intercalated into the interlayer space of LDH, using the microwave-assisted ion-exchange method. Dye/ZnAl-LDH-MA nanohybrid was characterized by X-ray diffraction pattern (XRD), Fourier transform-infrared spectroscopy (FT-IR), CHN elemental analysis, inductively coupled plasma spectrometry (ICP), thermogravimetric (TGA-DTG) analysis, and field-emission scanning electron microscopy (FE-SEM) method. Based on XRD results, the basal spacing of dye/ZnAl-LDH nanohybrid confirms the bilayer intercalation of MO1 into the interlayer gallery of ZnAl-LDH with good c-orientation. The photonic properties of dye/ZnAl-LDH-MA nanohybrid were investigated by photoluminescence (PL) curves. The PL emission of dye/ZnAl-LDH-MA displays a dual-color photoluminescence emission due to fluorescence resonance energy transfer (FRET). Also, the intensity of PL emission of dye/ZnAl-LDH is higher than that of ZnAl-LDH and MO1 due to a decrease in the dye aggregation after the intercalation process. Therefore, using the microwave-assisted ion-exchange method, an inorganic-organic matrix with two-color emission, and high stability can be designed which can be used in optoelectronic and dual-color optical devices.

Ratiometric Oxygen Sensing with H-NOX Protein Conjugates.

Ratiometric sensors are self-referencing constructs that are functional in cells and tissues, and the read-out is independent of sensor concentration. One strategy for ratiometric sensing is to utilize two-color emission, where one component possesses analyte-dependent emission and the other is independent of analyte concentration, serving as an internal standard. In this way, the intensity ratio of the two components is a quantitative measure of the analyte. In this study, protein-based ratiometric oxygen sensors are prepared using the heme nitric oxide/oxygen-binding protein (H-NOX) from the thermophilic bacterium Caldanaerobacter subterraneus. The native heme cofactor is replaced with a Pd(II) or Pt(II) porphyrin as the oxygen-responsive phosphor. Mutagenesis is performed to incorporate a cysteine residue on the protein surface for thiol/maleimide coupling of the oxygen-insensitive dye, which serves as a Förster resonance energy transfer (FRET) donor for the porphyrin. While both Pd(II)- and Pt(II)-based sensors are responsive over biologically relevant ranges, the Pd sensor exhibits greater sensitivity at lower oxygen concentrations. Together, these sensors represent a new class of protein-based ratiometric oxygen sensors, and the modular platform allows the oxygen sensitivity to be tailored for a specific application. This proof-of-principle study has identified the key considerations and optimal methodologies to develop and subsequently refine protein-based ratiometric oxygen sensors.

Two-color Emission in Dy3+-activated CaZnP2O7 Pyrophosphate for White LED

Abstract: CaZnP2O7 pyrophosphate phosphor doped with Dy3+ ions was synthesized by modified solid-state reaction. The crystalline phases were recognized using X-ray diffraction. Surface morphology was studied by scanning electron microscopy. Furthermore, the chromaticity coordinates' values were estimated from the emission spectra of CaZnP2O7:Dy3+. The phosphor photoluminescence emission spectra were found to have an excitation at around 353 nm showing two distinguishing bands centered at about 482 nm (Blue) and 575 nm (Yellow) corresponding to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+, respectively. These phosphors have strong absorption in the near-UV region. The intact study reveals that the present phosphors are suitable for color converter using UV light as the primary light source, which can be used as a blue/yellow phosphor excited by the n-UV LED chip and mixed with other color emission phosphors to obtain white light. Keywords: Chemical synthesis, Luminescence, X-ray diffraction, Optical properties.

Three-wavelength broadband soot pyrometry technique for axisymmetric flames.

Soot temperature measurements in laminar flames are often performed through two-color broadband emission pyrometry (BEMI) or modulated absorption/emission (BMAE) techniques, using models to relate the ratio between flame intensities at two different wavelengths with soot temperature. To benefit from wider spectral range and increase the accuracy of experimental estimation of soot temperature, this work proposes a new approach that uses three-color broadband images captured with a basic color camera. The methodology is first validated through simulations using numerically generated flames from the CoFlame code and then used to retrieve soot temperature in an experimental campaign. The experimental results show that using three-color and BEMI provides smoother reconstruction of soot temperature than two-color and BMAE when small disturbances exist in the measured signals due to a reduced experimental noise effect. A sensitivity analysis shows that the retrieved temperature from three-color BEMI is more resilient to variations on the ratio of measured signals than BMAE, which is confirmed by an error propagation analysis based on a Monte Carlo approach.

Photochromism and photoswitchable luminescence in a Zn7 cluster-based metal-organic framework with an organic guest.

Photochromic materials coupled with photoswitchable luminescence functionalities are of particular interest due to their potential applications in switches and optical memory devices. However, the construction of such materials, especially those with two-color emission states, is still challenging. In this context, a rare Zn7 cluster-based host-guest MOF material, (bbmp)[Zn7(IPA)6(OH)4(H2O)2] (1) (H2IPA = isophthalic acid, bbmp·2I = 4,4'-([1,1'-biphenyl]-4,4'-diyl)bis(1-methylpyridin-1-ium) diiodide), was prepared by encapsulating an organic cation into an anionic MOF produced from zinc cations and isophthalic acid ligands, which exhibits reversible naked detectable photochromic properties varying from yellow to green upon UV-Vis light irradiation. The photoactive guest bbmp2+ and the short O⋯N+ distances between the oxygen atoms of the carboxylate groups and the pyridine ring play a crucial role in the photochromism of this compound. More interestingly, the luminescence color of this cluster-based host-guest material can be reversibly switched from green to blue upon irradiation, exhibiting photoswitchable luminescence properties.

All-Optical Two-Color Terahertz Emission from Quasi-2D Nonlinear Surfaces.

Two-color terahertz (THz) generation is a field-matter process combining an optical pulse and its second harmonic. Its application in condensed matter is challenged by the lack of phase matching among multiple interacting fields. Here, we demonstrate phase-matching-free two-color THz conversion in condensed matter by introducing a highly resonant absorptive system. The generation is driven by a third-order nonlinear interaction localized at the surface of a narrow-band-gap semiconductor, and depends directly on the relative phase between the two colors. We show how to isolate the third-order effect among other competitive THz-emitting surface mechanisms, exposing the general features of the two-color process.

Dual phase two-color emission observed in van der Waals GaTe planes

After the eagerly study on two-dimensional (2D) electronic devices made by layered semiconductors, light-emitting devices based on 2D semiconductors have also flourished in recent years due to their special characters of flexibility, flatness surface, and thickness-tunable band gap modulation, etc. Among the 2D materials, III-VI layer compounds with hexagonal structure like GaSe and InSe are essentially direct semiconductors that own high band-edge luminescence efficiency and wide thickness tunable range from bulk to thin few-layer scale. In this work, dual-phase two-color emissions of multilayered gallium monotelluride positioned at 1.588 and 1.652 eV are simultaneously detected by micro-photoluminescence (μPL) measurement at 300 K. The lower-energy peak may originate from the hexagonal GaTe (H-GaTe) phase while the higher-energy luminescence might come from monoclinic GaTe (M-GaTe) phase and which are verified by micro-thermoreflectance (μTR) and high-resolution transmission electron microscopy (HRTEM). Micro-time-resolved photoluminescence (TRPL) and area fluorescence lifetime mapping (AFLM) of the multilayer GaTe indicated that the luminescence decay time constant of the H-GaTe is larger than that of the M-GaTe for verification of the dominant phase in multilayered GaTe is the monoclinic phase. In-plane structural and optical anisotropy of the multilayered GaTe were characterized by using polarized micro-Raman and polarized μPL measurements. The results clearly indicate that the monoclinic-phase GaTe possesses higher polarized extinction ratio with respect to that of the hexagonal GaTe for fully extinction of the Raman vibration mode and band-edge emission in the dual-phase gallium telluride. The existence of dual crystalline phases in multilayer GaTe may render it possible for fabrication of two-color emission device available for near-infrared (NIR) optical communication use.

Two-color emission of BaWO4 crystals pumped by 472 nm 0.11 ps laser pulses for frequency conversion into the longwave IR domain

The frequency conversion of Ti:Sapphire laser second harmonic pulses into two-color emission using a tandem of BaWO4 Raman crystals was experimentally demonstrated with an internal efficiency of ∼17%. The output emission contained symmetrical Stokes and anti-Stokes spectral components separated by 902 cm−1 with a full-width half-maximum bandwidth of ≥110 cm−1 each. The applicability of this two-color emission was numerically verified for the production of longwave infrared ultrashort pulses suitable for seeding a high-pressure CO2 laser amplifier via difference frequency generation in LiGaS2 crystal.

Simultaneous two-color snapshot view on ultrafast charge and spin dynamics in a Fe-Cu-Ni tri-layer

Ultrafast phenomena on a femtosecond timescale are commonly examined by pump-probe experiments. This implies multiple measurements, where the sample under investigation is pumped with a short light pulse and then probed with a second pulse at various time delays to follow its dynamics. Recently, the principle of streaking extreme ultraviolet (XUV) pulses in the temporal domain has enabled recording the dynamics of a system within a single pulse. However, separate pump-probe experiments at different absorption edges still lack a unified timing, when comparing the dynamics in complex systems. Here, we report on an experiment using a dedicated optical element and the two-color emission of the FERMI XUV free-electron laser to follow the charge and spin dynamics in composite materials at two distinct absorption edges, simultaneously. The sample, consisting of ferromagnetic Fe and Ni layers, separated by a Cu layer, is pumped by an infrared laser and probed by a two-color XUV pulse with photon energies tuned to the M-shell resonances of these two transition metals. The experimental geometry intrinsically avoids any timing uncertainty between the two elements and unambiguously reveals an approximately 100 fs delay of the magnetic response with respect to the electronic excitation for both Fe and Ni. This delay shows that the electronic and spin degrees of freedom are decoupled during the demagnetization process. We furthermore observe that the electronic dynamics of Ni and Fe show pronounced differences when probed at their resonance, while the demagnetization dynamics are similar. These observations underline the importance of simultaneous investigation of the temporal response of both charge and spin in multi-component materials. In a more general scenario, the experimental approach can be extended to continuous energy ranges, promising the development of jitter-free transient absorption spectroscopy in the XUV and soft X-ray regimes.

Trion Emission Dominates the Low-Temperature Photoluminescence of CdSe Nanoplatelets.

Colloidal nanoplatelets (NPLs) are atomically flat, quasi-two-dimensional particles of a semiconductor. Despite intense interest in their optical properties, several observations concerning the emission of CdSe NPLs remain puzzling. While their ensemble photoluminescence spectrum consists of a single narrow peak at room temperature, two distinct emission features appear at temperatures below ∼160 K. Several competing explanations for the origin of this two-color emission have been proposed. Here, we present temperature- and time-dependent experiments demonstrating that the two emission colors are due to two subpopulations of uncharged and charged NPLs. We study dilute films of isolated NPLs, thus excluding any explanation relying on collective effects due to NPL stacking. Temperature-dependent measurements explain that trion emission from charged NPLs is bright at cryogenic temperatures, while temperature activation of nonradiative Auger recombination quenches the trion emission above 160 K. Our findings clarify many of the questions surrounding the photoluminescence of CdSe NPLs.

Single AIEgen for multiple tasks: Imaging of dual organelles and evaluation of cell viability

Fully understanding the complicated interplays among various chemical species and organelles is greatly important to unravel the mystery of life. However, fluorescent probes capable of visualizing multiple targets discriminatively are severely deficient, which extremely limit the investigation on intracellular interplays among various species. Towards this end and in consideration of the unique advantages of aggregation-induced emission luminogens (AIEgens), here we rationally designed and presented a single AIEgen, named TVQE, bearing lipophilic, cationic and hydrolyzable moieties, and this AIEgen was capable of illuminating mitochondria and lipid droplets with red and blue emission, respectively. In addition, TVQE was successfully used for evaluating cell viability due to its distinct two-color emission changes tuned by esterase-mediated hydrolysis. Of particular importance is that TVQE can selectively differentiate live, early apoptotic, late apoptotic, and dead cells by confocal microscopy and quantify cell viability statistically by flow cytometry.

Switchable mechanoresponsive luminescence from traditional triphenylamine-thiophene carbaldehyde luminogens

Two donor-π-acceptor molecules (TPA-TC and 2Br-TPA-TC) with the common backbone of triphenylamine-thiophene carbaldehyde are used to explore the mechanoresponsive luminescence (MRL) properties. Both of them show remarkable solvatochromic effects, and their emission colors in solution could be changed from blue to yellow depending on the solvent polarity. Density function theory calculations indicate that the energy band gap of 2Br-TPA-TC is slightly larger than that of TPA-TC, which could explain their UV/Vis and fluorescence behaviors. It is noted that the green emission of TPA-TC and 2Br-TPA-TC is turned into deeper yellow-green after grinding and recovered via heating, and the two-color emission cycles are reversible. The MRL behaviors associated with the morphology transformation between crystalline and amorphous states could be monitored by powder X-ray diffraction spetra. Single-crystal structural analyses for TPA-TC and 2Br-TPA-TC help to understand the MRL mechanisms, in which the twisted molecules are loosely packed with the affixation of multiple H-bonding interactions and they are susceptible to grinding-induced photoluminescence changes.

Facile synthesis of multifunctional nanoparticles encoded with quantum dots and magnetic nanoparticles: cell tagging and MRI

In this study, fluorescence-encoded magnetic biocompatible nanoparticles (NPs) were constructed from CdSe@ZnS quantum dots (QDs) and Fe3O4 nanoparticles with a one-step reprecipitation-encapsulation method. The resultant hybrid NPs exhibit small size (∼130 nm in diameter), highly bright QDs, two-color emissions (green and red) under single-wavelength excitation, easy separation with a magnet and efficient cellular internalization. Energy transfer between the incorporated QDs was studied to better tailor the encoded fluorescence, and 11 barcodes were obtained by adjusting the ratio of green and red QDs. We used four sets of the barcodes to tag specific cancer cells (HepG2) as a proof-of-concept, and distinguished each set according to respective overlayed fluorescence images using laser confocal microscopy. Moreover, the incorporated Fe3O4 NPs endowed as-constructed optical barcode superparamagnetic property by T2-enhanced magnetic resonance effect with an r2 value of 145.25 s−1 mM−1 at 3 T. These results suggest that the multifunctional NPs are very promising for discriminating different cells and dual-modality imaging.

Analysis of two-color laser-induced electron emission from a biased metal surface using an exact quantum mechanical solution

Photoelectron emission from nanotips via strong-field lasers is a powerful technique for coherent control of ultrafast electron dynamics, and so is important in many areas, including time-resolved electron microscopy and free-electron lasers. The effects of dc bias are typically ignored, though. Exact solution of the time-dependent Schr\odinger equation reveals that, under large dc bias, strong modulation persists in the emission current, the magnitude of which increases significantly. In the authors' model, the dynamics of the multiphoton excited states depend strongly on the applied dc field. This work suggests a practical approach to strong control of high-current photoemission.