Single-photon Excitation Research Articles

Application of confocal laser scanning microscopy to investigation of micro crystals in transparent amorphous media: Photoluminescence tomography and spectroscopy of CdZnSSe crystallites in historical silicate glass

Recently, photoluminescence tomography based on the confocal laser scanning microscopy with two-photon excitation has been developed to study the distribution of point and extended defects in the bulk of ZnSe laser crystals. This article presents the use of the tomography to investigate luminescent micro inclusions in transparent amorphous media such as silicate glass. Studies of CdZnSSe crystals synthesized in a silicate glass melt have been carried out using the tomography with both two-photon and single-photon excitation of luminescence. Zn-rich glass manufactured in the 19th century has been found to contain micron-sized crystals of CdξZn1−ξSζSe1−ζ (ξ≈0.5, ζ≈0.5) of hexagonal crystal system exhibiting intense photoluminescence. The photoluminescence band of these crystals has been found to peak at about 2.1 eV (∼590nm) at 300 K. Minor shifts in the maximum of bands and changes in their shape in individual crystals or at tomogram points within the crystal are associated with variations in their composition. Changes in the photoluminescence band shape and maximum due to the excitation of luminescence in CdZnSSe crystallites by laser radiation of different energies have also been observed in this study.

Two-photon photodynamic therapy with curcumin nanocomposite

Two-photon photodynamic therapy (TP-PDT) offers an innovative approach to cancer treatment that utilizes near-infrared light to activate photosensitizers and generate reactive oxygen species (ROS) for targeted cancer cell elimination. TiO2-CUR-Sofast (TCS), which uses TiO2 nanoparticles and Sofast cationic polymer to modify curcumin (CUR), has demonstrated potential as a photosensitizer under visible light irradiation, addressing the limitations of CUR's narrow spectral range and low bioavailability. This study explores the utility of the two-photon technique to activate TCS within the infrared spectrum, aiming to enhance ROS production and penetration depth compared to traditional CUR. TCS exhibits a significantly higher ROS production at 900 nm excitation wavelength, approximately 6–7 times that of CUR, signifying a substantial increase in efficiency. In TP-PDT, TCS showed significant phototoxicity against HeLa and T24 cell lines compared to CUR. Furthermore, TCS's photodynamic efficacy is further confirmed by cell apoptosis and necrosis studies, where approximately 89 % of cells treated with TCS under 900 nm light irradiation were observed in an apoptosis/necrosis state. And the TP-PDT effect in deep tissue was simulated using pig skin. It shows that the two-photon excitation has a significant penetration depth advantage over the single-photon excitation. These results indicate that the two-photon PDT scheme of TCS has greater potential than the single-photon PDT scheme in the treatment of cancer, and provides an experimental foundation for the effective treatment of deep lesions.

Analogical atomic effects of graphene nanostructure induced by nonlinearity.

A scheme to construct analogical atoms using graphene nanostructures via quantum nonlinearity is proposed. Due to the strong field localization capability of graphene plasmons and the significant intrinsic nonlinearity of graphene, a strong nonlinear optical response can be realized even with single-photon excitation. In this process, the quantum vacuum localized plasmonic mode plays a crucial role in achieving the third-order multi-photon nonlinear effect. The eigenfrequency of the nanostructure can shift by an amount tens of times larger than the decay rate, causing the nanostructure cavity to exhibit atomic-like behaviors. Furthermore, multilevel atoms can be constructed through the interaction of composite graphene nanostructures. The parameters of these atoms can be manipulated by adjusting the nanostructure size, Fermi energy, and other parameters. This research holds significant potential for applications in highly integrated, controllable nanophotonics.

Impact of Classical and Quantum Light on Donor-Acceptor-Donor Molecules.

Investigations of entangled and classical two-photon absorption have been carried out for six donor (D)-acceptor (A)-donor (D) compounds containing the dithieno pyrrole (DTP) unit as donor and acceptors with systematically varied electronic properties. Comparing ETPA (quantum) and TPA (classical) results reveals that the ETPA cross section decreases with increasing TPA cross section for molecules with highly off-resonant excited states for single-photon excitation. Theory (TDDFT) results are in semiquantitative agreement with this anticorrelated behavior due to the dependence of the ETPA cross section but not TPA on the two-photon excited state lifetime. The largest cross section is found for a DTP derivative that has a single photon excitation energy closest to resonance with half the two-photon excitation energy. These results are important for the possible use of quantum light for low-intensity energy-conversion applications.

Insights into the Mechanisms of Single-Photon and Two-Photon Excited Surface Enhanced Fluorescence by Submicrometer Silver Particles.

Surface enhanced fluorescence (SEF) based on noble metal nanoparticles is an effective means to achieve high sensitivity in fluorescence detection. Currently, the physical mechanism behind enhanced fluorescence is not fully understood. This paper measures the fluorescence signals of Dihydroporphyrin f methyl ether (CPD4) under both single-photon and two-photon excitation based on submicrometer silver particles with rough morphologies, achieving enhancement factors of 34 and 45 times, respectively. On this basis, by combining the radiative field characteristics produced by the silver particles, a stimulated radiation model of molecules is established to elucidate the changes in the molecular photophysical process when influenced by silver particles. Moreover, the fluorescence lifetime of the molecules was measured, showing that the presence of silver particles induces an increase in the molecular radiative decay rate, causing the fluorescence lifetime to decay from 3.8 ns to 3 ns. The results indicate that the fluorescence enhancement primarily originates from the submicrometer silver particles' enhancement effect on the excitation light. Additionally, the fluorescence signal emitted by the molecules couples with the silver particles, causing the local surface plasmon resonances generated by the silver particles to also emit light signals of the same frequency. Under the combined effect, the fluorescence of the molecules is significantly enhanced. The findings provide a theoretical foundation for understanding the fluorescence enhancement mechanism of silver particles, adjusting the enhancement effect, and developing enhanced fluorescence detection devices based on submicrometer silver particles, holding significant practical importance.

Modifying Parallel Excitations into One Framework for C(sp3)─H Bond Activation with Energy Combined More Than Two Photons.

Natural photosynthesis enzymesutilize energies of several photons for challenging oxidation of water, whereas artificial photo-catalysis typically involves only single-photon excitation. Herein, a multiphoton excitation strategy is reported that combines parallel photo-excitations with a photoinduced electron transfer process for the activation of C(sp3)─H bonds, including methane. The metal-organic framework Fe3-MOF is designed to consolidate 4,4',4″-nitrilotrisbenzoic units for the photoactivation of dioxygen and trinuclear iron clusters as the HAT precursor for photoactivating alkanes. Under visible light irradiation, the dyes and iron clusters absorbed parallel photons simultaneously to reach their excited states, respectively, generating 1O2 via energy transfer and chlorine radical via ligand-to-metal charge transfer. The further excitation of organic dyes leads to the reduction of 1O2 into O2 •- through a photoinduced electron transfer, guaranteeing an extra multiphoton oxygen activation manner. The chlorine radical abstracts a hydrogen atom from alkanes, generating the carbon radical for further oxidation transformation. Accordingly, the total oxidation conversion of alkane utilizing three photoexcitation processes combines the energies of more than two photons. This new platform synergistically combines a consecutive excited photoredox organic dye and a HAT catalyst to combine the energies of more than two photons, providing a promising multiphoton catalysis strategy under energy saving, and high efficiency.

Autofluorescence lifetime flow cytometry with time-correlated single photon counting.

Autofluorescence lifetime imaging microscopy (FLIM) is sensitive to metabolic changes in single cells based on changes in the protein-binding activities of the metabolic co-enzymes NAD(P)H. However, FLIM typically relies on time-correlated single-photon counting (TCSPC) detection electronics on laser-scanning microscopes, which are expensive, low-throughput, and require substantial post-processing time for cell segmentation and analysis. Here, we present a fluorescence lifetime-sensitive flow cytometer that offers the same TCSPC temporal resolution in a flow geometry, with low-cost single-photon excitation sources, a throughput of tens of cells per second, and real-time single-cell analysis. The system uses a 375 nm picosecond-pulsed diode laser operating at 50 MHz, alkali photomultiplier tubes, an FPGA-based time tagger, and can provide real-time phasor-based classification (i.e., gating) of flowing cells. A CMOS camera produces simultaneous brightfield images using far-red illumination. A second PMT provides two-color analysis. Cells are injected into the microfluidic channel using a syringe pump at 2-5 mm/s with nearly 5 ms integration time per cell, resulting in a light dose of 2.65 J/cm2 that is well below damage thresholds (25 J/cm2 at 375 nm). Our results show that cells remain viable after measurement, and the system is sensitive to autofluorescence lifetime changes in Jurkat T cells with metabolic perturbation (sodium cyanide), quiescent versus activated (CD3/CD28/CD2) primary human T cells, and quiescent versus activated primary adult mouse neural stem cells, consistent with prior studies using multiphoton FLIM. This TCSPC-based autofluorescence lifetime flow cytometer provides a valuable label-free method for real-time analysis of single-cell function and metabolism with higher throughput than laser-scanning microscopy systems.

High-Resolution Photoelectron Spectroscopy of the Ground and First Excited Electronic States of MgXe.

We report on the characterization of the X+ 2Σ+ ground and the A+ 2ΠΩ (Ω = 1/2, 3/2) and B+ 2Σ+ electronically excited states of MgXe+. Rotationally cold MgXe in the a 3Π0(v″ = 0) metastable electronic state was generated in a laser-ablation supersonic-beam source. Following single-photon excitation from the metastable state, the vibrational structure of the X+ state of MgXe+ was measured by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy, and the adiabatic ionization energy of the X+ ← a ionizing transition was determined to be EI/(hc) = 37,468.3(6) cm-1. Spectra of the A+ ← X+ and B+ ← X+ transitions were recorded by using the method of isolated-core Rydberg-dissociation spectroscopy. The observation of the Mg+(3p) 2P1/2 + Xe 1S0 dissociation limit enabled the determination of the dissociation energies of the X+ [D0(X+) = 2970(7) cm-1] and A+ states [D0(A1/2+) = 9781(7) cm-1 and D0(A3/2+) = 9603(7) cm-1]. We compare these results with those of earlier experimental studies and ab initio quantum-chemical calculations.

Multiline molecular tagging velocimetry of nitric oxide at 100 kHz using an injection-seeded burst-mode OPO.

An injection-seeded, burst-mode optical parametric oscillator (OPO) operating at a repetition rate of 100 kHz is used to demonstrate the multiline molecular tagging velocimetry of an underexpanded jet using nitric oxide fluorescence. The very narrow linewidth of the OPO system, along with the relatively high pulse energies of the burst-mode system, enables efficient single-photon excitation of nitric oxide along multiple laser beam lines at a high repetition rate. Simultaneous one-dimensional velocity profile measurements were obtained of an underexpanded jet system at six different locations using a reference initial image and single-shot delayed images. A methodology for calculating the uncertainty of single-shot velocity is also described. Mean and root-mean-square velocity profiles are obtained at multiple locations simultaneously over a sampling time of 1 ms. The high-repetition-rate velocity measurements also appear to capture the onset of velocity oscillations and has the potential to reveal velocity frequency content occurring in the tens of kHz. The demonstrated velocimetry technique could be paired with other emerging burst-mode laser capabilities for a quantitative multiparameter gas property or multicomponent gas velocity measurements for supersonic and hypersonic flows, especially within ground test facilities that are limited to very short run durations.

Simultaneous Two- and Three-Photon Deep Imaging of Autofluorescence in Bacterial Communities.

The intrinsic fluorescence of bacterial samples has a proven potential for label-free bacterial characterization, monitoring bacterial metabolic functions, and as a mechanism for tracking the transport of relevant components through vesicles. The reduced scattering and axial confinement of the excitation offered by multiphoton imaging can be used to overcome some of the limitations of single-photon excitation (e.g., scattering and out-of-plane photobleaching) to the imaging of bacterial communities. In this work, we demonstrate in vivo multi-photon microscopy imaging of Streptomyces bacterial communities, based on the excitation of blue endogenous fluorophores, using an ultrafast Yb-fiber laser amplifier. Its parameters, such as the pulse energy, duration, wavelength, and repetition rate, enable in vivo multicolor imaging with a single source through the simultaneous two- and three-photon excitation of different fluorophores. Three-photon excitation at 1040 nm allows fluorophores with blue and green emission spectra to be addressed (and their corresponding ultraviolet and blue single-photon excitation wavelengths, respectively), and two-photon excitation at the same wavelength allows fluorophores with yellow, orange, or red emission spectra to be addressed (and their corresponding green, yellow, and orange single-photon excitation wavelengths). We demonstrate that three-photon excitation allows imaging over a depth range of more than 6 effective attenuation lengths to take place, corresponding to an 800 micrometer depth of imaging, in samples with a high density of fluorescent structures.

High-resolution photoelectron spectroscopy of the very weakly bound MgNe molecule

ABSTRACT We report on the characterisation of the X + 2 Σ + ground electronic state of MgNe + by photoionisation, mass-analysed threshold-ionisation (MATI) and pulsed-field-ionisation zero-kinetic-energy photoelectron (PFI-ZEKE-PE) spectroscopy and the observation of the metastable a 3 Π 0 state of MgNe. Rotationally cold MgNe was generated in a laser-ablation supersonic-beam source in both the X 1 Σ + ( v ″ = 0 ) ground state and a 3 Π 0 ( v ″ = 0 ) metastable electronic state. PFI-ZEKE-PE spectra with full resolution of the vibrational structure and partial resolution of the rotational structure were recorded from the X 1 Σ + ground state following resonant ( 1 + 1 ′ ) two-photon excitation via selected rovibrational levels of the C 1 Π intermediate state and from the a 3 Π 0 metastable state following single-photon excitation. The lowest six vibrational levels of the X + 2 Σ + state were observed, covering 80% of the X + 2 Σ + potential-well depth. The experimental data were used to determine potential-energy functions, dissociation energies, and molecular constants for the X 1 Σ + , a 3 Π 0 and C 1 Π states of MgNe and the X + 2 Σ + state of MgNe + . With dissociation energies D 0 of 12(3) cm−1 and 10(3) cm−1, respectively, the X 1 Σ + and a 3 Π 0 states of 24 Mg 20 Ne are extremely weakly bound. In contrast, the X + 2 Σ + state of 24 Mg 20 Ne + has a dissociation energy D 0 of 185(3) cm − 1 and 12 bound vibrational levels. From the difference between the adiabatic ionisation energies of the X 1 Σ + (61 498.5(4) cm − 1 ) and the a 3 Π 0 (39 645.1(3) cm − 1 ) states, the term value of the a 3 Π 0 ground vibrational level is determined to be 21 853.4(5) cm − 1 . These results are compared with those of earlier experimental studies and ab initio quantum-chemical calculations.

Investigation of photoluminescence properties of a DPA functionalized tris-cyclometalated iridium complex and its potential for metal sensing

Herein we report the synthesis and investigation of properties of a luminescent tris-cyclometalated complex [Ir(ppy)2ppy-dpa] (1), derived from fac-[Ir(ppy)3]. The complex carries a dipicolylamine (dpa) chelator on one of the 2-phenylpyridine (ppy) ligands. The dpa chelator exhibits minimal effect on the photophysical properties of the core iridium luminophore. The complex exhibits bright photoluminescence, which can be elicited with single photon and two-photon excitation. The two-photon excitation is achieved with light in the NIR region. Aggregation in aqueous solution has an effect on the photoluminescence properties of complex 1. The emission of complex 1 is quenched nearly completely in the presence of Cu2+ ions with a detection limit of 3.5 × 10−7 M. The luminescence of iridium complex 1 is unaffected by the presence of Co3+ and Zn2+. However, the emission of complex 1 is moderately quenched in the presence of Mn2+ and Fe2+ ions, and greatly quenched in the presence of Fe3+, Co2+, and Ni2+. However, Cu2+ causes its effect even when these competing cations are present. Complex 1 exhibits low cytotoxicity towards HeLa cells and can be imaged with single photon excitation at 405 nm or two-photon excitation at 800 nm.

Prospect for precision quantum logic spectroscopy of vibrational overtone transitions in molecular oxygen ions

Precision spectroscopy has been the driving force for progress of our physical understanding and still is a promising tool for the investigation of new physics. Molecules offer transitions which allow tests that are not possible in atomic systems. However, usually precision spectroscopy of molecules is challenging due to the lack of cycling transitions for state preparation and state detection. For molecular ions, this obstacle can be overcome by quantum logic spectroscopy, where dissipation for state preparation and detection is provided by a co-trapped atomic ion exploiting the shared eigenstates of motion. Here, we propose a full quantum logic spectroscopy scheme for molecular oxygen ions and theoretically investigate the feasibility of quantum logic-assisted state detection and preparation. Furthermore, we provide coupling rates for a direct single-photon quadrupole excitation of a vibrational overtone transition that can serve as a sensitive reference for tests of a possible variation of the proton-to-electron mass ratio.

Two-photon chemistry of tetrahydrofuran in clathrate hydrates.

High-lying electronic states hold the potential for new and unusual photochemical reactions. However, for conventional single-photon excitation in the condensed phase, reaching these states is often not possible because the vacuum-ultraviolet (VUV) light required is competitively absorbed by the surrounding matrix rather than the molecule of interest. Here, this hurdle is overcome by leveraging nonresonant two-photon absorption (2PA) at 265 nm to achieve preferential photolysis of tetrahydrofuran (THF) trapped within a clathrate hydrate network at 77 K. Electron spin resonance (ESR) spectroscopy enables direct observation and identification of otherwise short-lived organic radicals stabilized by the clathrate cages, providing clues into the rapid dynamics that immediately follow photoexcitation. 2PA induces extensive fragmentation of enclathrated THF yielding 1-alkyl, acyl, allyl and methyl radicals-a stark departure from the reactive motifs commonly reported in γ-irradiated hydrates. We speculate on the undetected transient dynamics and explore the potential role of trapped electrons generated from water and THF. This demonstration of nonresonant two-photon chemistry presents an alternative approach to targeted condensed phase photochemistry in the VUV energy range.

Theoretical study of the properties of multiphoton MINFLUX and RASTMIN super-resolution imaging

Fluorescence microscopy is an important tool for studying cellular physiological functions, and the precise localization of the individual fluorescent molecules is crucial for resolution. In recent years, researchers have proposed various single-molecule localization methods with molecular-level precision. Here, we conducted simulation calculations and feasibility studies on two imaging techniques of minimal emission fluxes (MINFLUX) and RASTer scanning a minimum of light (RASTMIN) for single-photon and multiphoton imaging. Our results indicated that the single-photon excitation wavelength had little influence on the localization precision of MINFLUX and RASTMIN, but multiphoton MINFLUX and RASTMIN significantly improved the center localization precision. However, in multiphoton MINFLUX, the average localization precision was slightly compromised. In contrast, benefiting from the advantages of raster scanning, the average localization precision of multiphoton RASTMIN was superior to that of single-photon RASTMIN. Furthermore, to address the issues of reduced average localization accuracy with multiphoton MINFLUX, we propose an increase in the number of scanning points to enhance the performance of multiphoton MINFLUX imaging.

Room temperature amplified spontaneous emissions in a sub-centimeter sized CsPbBr3 bulk single crystal.

All inorganic perovskite CsPbBr3 shows great potential in laser device because of its excellent luminescence characteristics, while the room temperature amplified spontaneous emission (ASE) in a large size CsPbBr3 bulk single crystal is still quite difficult. Herein, we have obtained the room temperature ASE in a sub-centimeter size CsPbBr3 bulk single crystal pumped with the single-photon excitation. Based on the reproducible light path within the CsPbBr3 bulk single crystal, the photonic feedback between the bottom and top facets naturally enhances the population inversion, which exhibits an amplified spontaneous emission threshold of ∼320 µJ/cm2. The blue shift of the ASE peak along with the increased pumping intensity is also observed and ascribed to the reduction of the refractive index and the energy band filling effect. These findings demonstrate the sub-centimeter size CsPbBr3 bulk single crystal to be an excellent candidate as an optical gain media for crystal lasers.

Photophysical properties of nitrogen-doped carbon quantum dots synthesized by graphite

Here, nitrogen-doped carbon quantum dots (N-CQDs) were successfully synthesized by the solvothermal method using graphite as the carbon source and N,N-dimethylformamide as the nitrogen source. We characterized the structure and chemical constitution of N-CQDs using X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. We investigated the pump- and temperature-dependent photoluminescence (PL) properties and the dynamic exciton recombination processes of N-CQDs, using both steady-state and time-resolved PL techniques. The spectral results show that the PL emission peak located at 518 nm at room temperature, mainly originates from the n-π∗ transition on the surface of N-CQDs. The pump fluence and PL integral intensity were analyzed to demonstrate the existence of single-photon excitation under the 405 nm laser excitation. As the temperature increases, the non-radiative transition gradually increases, which decreases the PL intensity, the full width at half maxima first narrows and then widens and the PL lifetime gradually decreases. Furthermore, we combined the N-CQDs with chip to prepare light-emitting diode (LED). The resulting chromaticity coordinate was obtained to be (0.29, 0.40). This study offers a comprehensive understanding of the luminescence mechanism in N-doped CQDs and introduces a novel approach for the quickly fabrication of full-color display LEDs.

Precise determination of adiabatic ionisation energies of large polyatomic molecules: p-diaminobenzene

We present a procedure, based on established principles, to accurately determine the ionisation energies of medium-sized to large polyatomic molecules in supersonic beams. The method combines single-photon excitation from the electronic ground state using tunable vacuum-ultraviolet laser radiation, the delayed pulsed-field ionisation of high Rydberg states with sequences of pulsed electric fields, and the modelling of the lineshapes from calculations of the field-ionisation dynamics of high Rydberg states and of the rotational contours of the photoelectron bands. The method is illustrated by a measurement of the adiabatic ionisation energy of p-diaminobenzene (p-DB), for which we obtain the value of 54,617.47(15) cm , which represents a more than 20-fold accuracy improvement over previous determinations.

Investigation of the deposition of α-tantalum (110) films on a-plane sapphire substrate by molecular beam epitaxy for superconducting circuit

Polycrystalline α-tantalum (110) films deposited on the c-plane sapphire substrate by sputtering are used in superconducting qubits nowadays. However, these films always occasionally form other structures, such as α-tantalum (111) grains and β-tantalum grains. To improve the film quality, we investigate the growth of α-tantalum (110) films on the a-plane sapphire substrate under varying conditions by molecular beam epitaxy technology. The optimized α-tantalum (110) film is a single crystal, with a smooth surface and atomically flat metal–substrate interface. The film with thickness of 30 nm shows a Tc of 4.12 K and a high residual resistance ratio of 9.53. The quarter wavelength coplanar waveguide resonators fabricated with the 150 nm optimized α-tantalum (110) film exhibit intrinsic quality factor of over one million under single photon excitation at millikelvin temperature.

Structural and optical properties of europium-doped sodium-lead-antimony glasses

Antimony-sodium-lead glasses doped with varying amounts of europium were synthesized using the melt quenching technique. To investigate the mutual effects between the dopant and the glass matrix, various methods and techniques were employed to analyze and characterize the glass samples. These methods included X-ray diffraction (XRD) for justifying the amorphous nature of the samples, differential scanning calorimetry (DSC) for analyzing thermal properties, Fourier transform infrared (FTIR) spectroscopy for studying functional groups in the 1400–400 cm-1 range, and peak deconvolution techniques used to analyze the defect impact in the 1100–800 cm-1 band region. UV–VIS spectroscopy was utilized for absorption analysis, optical band gap estimation, and Urbach energy calculation. Photoluminescence spectroscopy was employed in the UV–VIS-NIR range to investigate excitation, emission, lifetime decay, and local structure of Eu3+ ions within the disordered matrix, characterized by high polarizability content. Judd-Ofelt analysis and radiative parameters were obtained from emission spectra using a 464 nm excitation wavelength. The estimation of the optical band gap revealed broadening and shrinking with increasing Eu3+ ion concentration. The excitation spectra showcased the potential for emission under two distinct excitation mechanisms: single-photon excitation (1PE) and two-photon excitation (2PE). The quality of emitted light color was characterized using CIE chromaticity coordinates.