Excitation Laser Intensity Research Articles

The electrochemical modulation of single molecule fluorescence.

Recently it has been shown that electrochemistry, instead of using high intensity lasers, can be used to modulate the intensity of emission of fluorophores and even switch fluorophores between their ON and OFF states as required for single molecule localisation microscopy. This modulation of fluorescence does not necessarily correlate with direct oxidation and reduction of the dyes. Questions arise from this unexpected observation related to what is the electrochemistry that occurs, what are the important variables in switching fluorophores electrochemically and what range of dyes can be modulated with electrochemistry. Herein we seek to answer some of these questions. We demonstrate how to effectively modulate the fluorescence intensity of organic dye-labelled cell samples on an indium tin oxide surface using electrochemistry with redox-active mediators present in an oxygen scavenger buffer. We showed the electrochemical fluorescence modulation is sensitive to the applied potential and the excitation laser intensity, indicating the possibility of coupled photochemical and electrochemical reactions occurring. We also compared the electrochemical fluorescence modulation of representative oxazine, rhodamine, and cyanine dyes using ATTO 655, Alexa Fluor 488, and Alexa Fluor 647. Different dyes with distinctly different structural cores show fluorescence modulation to different extents. The electrochemical fluorescence modulation will be applicable in fluorescence imaging techniques as well as biosensing.

Impact of Excitation Intensity-Dependent Fluorescence Intensity Ratio of Upconversion Nanoparticles on Wide-Field Thermal Imaging

Erbium ion (Er3+)-doped upconversion nanoparticles (UCNPs) are frequently used for nanothermometry because their fluorescence intensity ratio (FIR) between two green emission bands at ∼525 and ∼545 nm is sensitive to temperature variation. One of the prerequisites for nanothermometry is that the FIR be independent of excitation intensity at constant temperature. In this work, the effect of excitation intensity on the FIR of core–double-shell NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+@NaYF4 UCNPs was investigated in two environments. The first environment is in aqueous solution, and the second is a monolayer of UCNPs on top of a silica–silicon substrate in air. The experimental results showed that the FIR decreases with the excitation intensity at constant temperature in each case. We further found that the excitation intensity-dependent FIR indeed deteriorated the thermal images acquired by wide-field upconversion fluorescence microscopy, in which a Gaussian laser beam was used to excite UCNPs uniformly coated on a silica–silicon substrate. The nonuniform excitation intensity of the incident laser beam resulted in thermal images that showed nonuniform temperature distributions in a 100 μm range field of view, even though the whole sample was maintained at constant temperature in air. To tackle this problem, we first measured the excitation intensity and temperature dependence of the FIR and the excitation laser intensity distribution on the sample. We then developed a correction scheme to correct the thermal images. With our correction process, the temperature distribution on the sample can be accurately mapped even with nonuniform illumination.

Parametric Study of CPT Resonance in Rubidium Vapor Cell for Application in Atomic Clock

The performance of Coherent Population Trapping (CPT) based atomic clocks primarily depends on the characteristics of CPT resonance. We have performed experiments to study and optimize the characteristics of CPT resonance in 87Rb atoms by measuring its contrast and full-width-at-half maximum (FWHM) as function of laser excitation and temperature of atomic vapor cells with different dimensions. A four-level atomic model is used to simulate CPT resonance characteristics along the length of atomic vapor cell. The model incorporates scaling law to understand collision dynamics in cells with different radius for a range of laser excitation intensities and the results are compared with experimental data. The quality figure, calculated from the measured values of FWHM and contrast, decreases with increase in laser intensity and improves in cells with higher dimension (radius). The optimum temperature corresponding to maximum quality figure varies with laser excitation intensity as well as cell dimension. The underlying collision dynamics and density effects that are responsible for the observed resonance characteristics are discussed.

Features of the Dynamics of the Photoluminescence Spectrum of the Cu3In5S9 Crystal with a Change in the Intensity of Laser Excitation

Features of the Dynamics of the Photoluminescence Spectrum of the Cu3In5S9 Crystal with a Change in the Intensity of Laser Excitation

Unified potential fluctuations model for photoluminescence spectra at room temperature—Cu(In,Ga)Se2 thin films

Room temperature photoluminescence (PL) is a powerful technique to study the properties of semiconductors. However, the interpretation of the data can be cumbersome when non-ideal band edge absorption takes place, as is the case in the presence of potential fluctuations. In this study, PL measurements are modeled to quantify potential fluctuations in Cu(In,Ga)Se2 (CIGS) absorber layers for photovoltaic applications. Previous models have attributed these variations to either bandgap fluctuations (BGFs) or electrostatic fluctuations (EFs). In reality, these two phenomena happen simultaneously and, therefore, affect the PL together. For this, the unified potential fluctuation (UPF) model is introduced. This model incorporates the effect of both types of fluctuations on the absorptance of the material and subsequently the PL spectra. The UPF model is successfully used to fit both single- and three-stage co-evaporated ultrathin (around 500 nm) CIGS samples, showing a clear improvement with respect to the previous BGF and EF models. Some PL measurements show possible interference distortions for which an interference function is used to simultaneously correct the PL spectra of a sample measured with several laser excitation intensities. All the models used in this work are bundled into a user-friendly, open-source Python program.

Mesoscopic clustering behavior of thermophoretic-type active particle suspension under quasi-one-dimensional microconfinement.

We experimentally investigate the mesoscopic clustering behavior of thermophoretic-type active particle suspension under quasi-one-dimensional spatial confinement (high aspect ratio microchannel). The microchannel enhances the viscous dissipation to operate the system in subpropulsion regime. We find that, in the subpropulsion regime, the steady-state configuration of active particle suspension exhibits a transition from homogeneous state to sausagelike clustering bundle located at the channel center, quasiperiodic isolated clusters at the channel center, aperiodic isolated cluster deviated from channel center, and finally to the typical propulsion-induced accumulation around the channel boundary as increasing the excitation laser intensity. The formation of those patterns is under the interplay of outward-pointing mesoscopic scaled thermophoretic force and the applied spatial confinement. The finding of those special patterns can provide some further possibilities of particle manipulation at mesoscopic scale.

Laser-induced assembly of biological cells and colloids onto a candle soot coated substrate

Recently, laser-induced heating assisted assembly of colloidal particles is emerging as a popular technique against direct light-assisted particle assembling methods. Herein, we demonstrate the trapping and assembly of yeast cells and upconversion particles at laser excitation intensities as low as 11 μW/μm2 via localized heating of non-plasmonic candle soot coated biocompatible polymer, polydimethylsiloxane. The thermal convective flow around the irradiation zone plays a vital role in trapping and assembling of the objects. The flow velocity profile is symmetrical around the irradiation zone, and the velocity increases drastically near the irradiation zone and with laser intensity. An increase in laser intensity leads to formation of an air bubble at the liquid-substrate interface and subsequent assembly of the objects at the interface. The tunneling of objects towards the irradiation zone across the barrier is demonstrated. Finally, with the localized laser-induced heating, the surface charge independent swarm of upconversion particle is shown, and the size and range of which found to be laser intensity dependent. With the wavelength-independent low power operation in the near-infrared range through simple optics, the results presented here may open avenues for a wide range of applications in diverse areas, including life sciences, colloidal science, nanoscience, photonics, and material science.

Синтез и характеризация наноразмерных апконвертирующих фосфоров NaYF4:Yb3+:Er3+/NaYF4

We carried out the solvothermal synthesis of UCNPs with the core/shell structure NaYF4:Yb3+:Er3+/NaYF4 and analyzed their photoluminescent properties depending on the nanoparticle size and preparative synthesis conditions. It has been shown that intensity and lifetime of the photoluminescence of the upconversion nanophosphors depend on the intensity of laser excitation, size of the nanoparticles and presence of an inert shell. Upon the formation of an inert shell on the surface of NaYF4:Yb3+:Er3+ nanoparticles the conversion efficiency of visible and NIR radiation in different spectral regions can increase by more than an order of magnitude due to shielding of luminescent centers from nonradiative losses at surface defects.

Light-Tunable Ferromagnetism in Atomically Thin Fe_{3}GeTe_{2} Driven by Femtosecond Laser Pulse.

The recent discovery of intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has opened up a new arena for spintronics, raising an opportunity of achieving tunable intrinsic 2D vdW magnetism. Here, we show that the magnetization and the magnetic anisotropy energy (MAE) of few-layered Fe_{3}GeTe_{2} (FGT) is strongly modulated by a femtosecond laser pulse. Upon increasing the femtosecond laser excitation intensity, the saturation magnetization increases in an approximately linear way and the coercivity determined by the MAE decreases monotonically, showing unambiguously the effect of the laser pulse on magnetic ordering. This effect observed at room temperature reveals the emergence of light-driven room-temperature (300K) ferromagnetism in 2D vdW FGT, as its intrinsic Curie temperature T_{C} is ∼200 K. The light-tunable ferromagnetism is attributed to the changes in the electronic structure due to the optical doping effect. Our findings pave a novel way to optically tune 2D vdW magnetism and enhance the T_{C} up to room temperature, promoting spintronic applications at or above room temperature.

Notes on thermometric artefacts by Er3+ luminescence band interference

Optical thermometry based on the intensity ratio of the 2H11/2 → 4I15/2 (525 nm) and 4S3/2 → 4I15/2 (545 nm) emission of Er3+ provides a powerful tool for microscale temperature sensing. Crystalline β-NaYF4:Er,Yb is an excellent thermometry material for green upconversion emission upon NIR laser excitation. However, interfering 2H9/2 → 4I13/2 emission is observed around 555 nm for continuous-wave laser excitation intensities above 10 W/cm2. In this study, the green Er3+ emission bands are characterized and it is demonstrated how the true sample temperature is determined circumventing possible artefacts.

Cold white light emission in tellurite-zinc glasses doped with Er3+–Yb3+–Tm3+ under 980 nm

Tellurite-zinc glasses doped with Er3+, Yb3+ and Tm3+ were fabricated by conventional melt-quenching technique and characterised by absorption spectroscopy, refractive index, lifetime measurements, excitation, luminescence and up-conversion spectroscopy. Tunable blue to cold white light emission based on the colour mixing of red, green and blue light was obtained via up-conversion under 980 nm and by adjusting the laser excitation intensity. The balancing of the relative intensity of each colour is provided by the energy transfer process between the rare-earth ions. Moreover, luminescence spectroscopy was performed with a 405 nm laser to further understand the energy transfer mechanism that produce the white light. The Judd-Ofelt intensity parameters (Ω2, Ω4 and Ω6) were calculated for all samples and present the trend Ω2>Ω4>Ω6. The spectroscopic parameters were computed with the Judd-Ofelt theory as well as the energy transfer micro-parameters (critical radius of interaction and energy transfer coefficient). Such results shall be used to give a comprehensive explanation for the energy transfer process between these rare earth ions.

Self-Blinking Dyes Unlock High-Order and Multiplane Super-Resolution Optical Fluctuation Imaging.

Most diffraction-unlimited super-resolution imaging critically depends on the switching of fluorophores between at least two states, often induced using intense laser light and specialized buffers or UV radiation. Recently, so-called self-blinking dyes that switch spontaneously between an open, fluorescent "on" state and a closed, colorless "off" state were introduced. Here, we exploit the synergy between super-resolution optical fluctuation imaging (SOFI) and spontaneously switching fluorophores for 2D and 3D imaging. SOFI analyzes higher order statistics of fluctuations in the fluorophore emission instead of localizing individual molecules. It thereby tolerates a broad range of labeling densities, switching behavior, and probe brightness. Thus, even dyes that exhibit spontaneous blinking characteristics that are not suitable or suboptimal for single molecule localization microscopy can be used successfully for SOFI-based super-resolution imaging. We demonstrate 2D imaging of fixed cells with almost uniform resolution up to 50-60 nm in 6th order SOFI and characterize changing experimental conditions. Next, we investigate volumetric imaging using biplane and eight-plane data acquisition. We extend 3D cross-cumulant analysis to 4th order, achieving super-resolution in 3D with up to 29 depth planes. Finally, the low laser excitation intensities needed for single and biplane self-blinking SOFI are well suited for live-cell imaging. We show the perspective for time-resolved imaging by observing slow membrane movements in cells. Self-blinking SOFI thus provides a more robust alternative route for easy-to-use 2D and 3D high-resolution imaging.

Flash method experiment design for the thermal characterization of orthotropic materials

In this paper an experiment design of the flash method dedicated to orthotropic material thermal characterization is considered. The present study relies on a comparative evaluation of the laser excitation duration time and intensity level, with respect to the measurement face, and their effects on the estimation accuracy of orthotropic material thermal diffusivities. The present estimation method, based on the well-known flash method concept, allows the simultaneous estimation of the three main thermal diffusivities. The identification method relies on the resolution of an inverse problem by means of a stochastic optimization algorithm retrieving the thermal properties by minimization of the least-squares criterion between the outputs of an analytical model and numerical predictions or experimental measurements. Both the direct analytical model and the estimation strategy are validated using an experimental test bench conducted on a carbon fiber reinforced polymer composite sample. The evaluation of various experiment designs, corresponding to different combinations of laser spot intensity and duration time, is conducted according to the face of the observation. A sensitivity analysis is conducted to complete the search of the optimal configuration. The present numerical study has enabled us to identify the rear face to be the most effective in order to successfully estimate the thermal diffusivities of such materials, especially the in-depth thermal diffusivity. For this configuration, excitation with a relatively long duration time (about 10 s in the present relatively low-diffusivity-material case), has been proved to be more convenient for such identification problems, compared to the impulse or very short pulse types.

Simulation of the transient current of radiation detector materials using the constrained profile interpolation method

A semi-Lagrangian-type advection scheme, the constrained profile interpolation (CIP) method, is adopted to obtain a numerical solution of a drift–diffusion equation of photo-generated charge carriers in negatively biased, high-resistivity semiconductor detector materials. Transient current waveforms of high-resistivity CdZnTe measured by a time-of-flight (TOF) technique under different bias voltages and various laser excitation intensities, are theoretically reproduced from the temporal evolution of the charge distribution and corresponding internal electric-field evolution, which are derived from the solution of the drift–diffusion equation, in combination with Poisson’s equation. The temporal evolution of the charge distribution and internal electric-field distribution obtained by the present theoretical technique agree very well with those obtained by a Monte Carlo (MC) simulation, an independent numerical technique. We demonstrate the validity of the present numerical technique by reproducing the excitation-intensity dependence of experimental TOF current waveforms measured in a CdZnTe crystal.

Tunable excitonic properties in two-dimensional heterostructures based on solution-processed PbI2 flakes

We investigate the manifestations of band structure engineering in few-layer PbI2-based heterostructures by probing their tunable optical properties. First, we have successfully prepared atomically thin flakes from PbI2 solution by two distinct approaches. A drop-casting of PbI2 solution onto various substrates followed by a simple heating process yields abundant flakes with different thickness and regular shape. Mechanical exfoliation of PbI2 bulk crystals, obtained from a low-temperature recrystallization process of PbI2 solution, also gives ultrathin PbI2 flakes of high quality. Moreover, these PbI2 flakes are employed to construct various van de Waals heterostructures. A significant enhancement of photoluminescence in MoSe2 interfaced with PbI2 was observed at different laser excitation intensity, due to the forming of type-I band alignment. Type-I band alignment can also be investigated in MoS2/PbI2 heterostructure, while type-II band alignment is built-in WSe2/PbI2 heterostructure. These results demonstrate that the strong interfacial coupling between PbI2 and other two-dimensional semiconductors can modulate their band alignment, and as a result, the exciton properties noticeably, which provides new insights of building a designer heterostructure device at the atomic level.

Crystallographic phase changes and damage thresholds of CsPbI3 microwire waveguides through continuous wave photoablation

We investigate waveguide efficiency of CsPbI3 microwire waveguides and their photodegradation over a range of continuous wave laser excitation energies and intensities.

Synthesis, structure and femtosecond nonlinear absorption properties of Ce-ZnO films

In this work, Ce-ZnO (CZO) films with excellent nonlinear absorption (NA) properties were successfully prepared via the magnetron sputtering process, where the conditions depended morphology were characterized. The cluster surface appeared with the increased sputtering time at low power, and disappeared at high power. Energy dispersive spectrometer and X-ray photoelectron spectroscopy analysis confirmed the presence of Ce ions into ZnO lattice. The functional groups, optical bandgap value and NA properties were revealed by Raman spectroscopy studies, UV–Visible absorption spectra and Z-scan technique, respectively. By adjusting the direct current sputtering power and excitation intensities, the NA mechanism of samples was converted from saturable absorption (SA) to reverse saturable absorption (RSA) behavior. Under different laser excitation intensities, the NA mechanism of samples was that TPA induced RSA. Furthermore, the results show that the order of magnitude of the NA coefficient for samples is ~10−8 m/W. In addition, the ultrafast carrier dynamics was studied using two-color pump-probe technique. When excited with 380 and 400 nm pumps and probed over a broad range in the visible region, the biexponential decay of positive signal is same to the mechanism of pump-induced deep-level or excited state absorption. CZO films will be promising candidate for developing optoelectronic devices.

Combined High-Resolution Optical Tweezers and Multicolor Single-Molecule Fluorescence with an Automated Single-Molecule Assembly Line.

We present an instrument that combines high-resolution optical tweezers and multicolor confocal fluorescence spectroscopy along with automated single-molecule assembly. The multicolor allows the simultaneous observation of multiple molecules or multiple degrees of freedom, which allows, for example, the observation of multiple proteins simultaneously within a complex. The instrument incorporates three fluorescence excitation lasers, with a reliable alignment scheme, which will allow three independent fluorescent probe or FRET measurements and also increases flexibility in the choice of fluorescent molecules. We demonstrate the ability to simultaneously measure angstrom-scale changes in tether extension and fluorescence signals. Simultaneous tweezers and fluorescence measurement are particularly challenging because of fluorophore photobleaching, even more so if multiple fluorophores are to be measured. Therefore, (1) fluorescence excitation and detection is interlaced with time-shared dual optical traps. (2) We investigated the photostability of common fluorophores. The mean number of photons emitted before bleaching was unaffected by the trap laser and decreased only slightly with increasing excitation laser intensity. Surprisingly, we found that Cy5 outperforms other commonly used fluorophores by more than fivefold. (3) We devised computer-controlled automation, which conserves fluorophore lifetime by quickly detecting fluorophore-labeled molecule binding, turning off lasers, and moving to add the next fluorophore-labeled component. The single-molecule assembly line enables the precise assembly of multimolecule complexes while preserving fluorophores.

Saturated laser-induced fluorescence in liquid kerosene seeded with a dye: influence of temperature and excitation intensity

Fluorescence-based techniques are commonly implemented to characterize droplet sprays in flows at temperature above 293 K. Nevertheless, in the case of gas turbine altitude relight, aeroengines are exposed to critical operating conditions where the liquid fuel temperature can reach values as low as − 40 °C. As a result, it is necessary to address the influence of this parameter on fluorescence intensity for temperatures below room values. In this paper, an innovative saturated laser-induced fluorescence (SLIF) excitation scheme is proposed to overcome the strong temperature dependence of fluorescence in liquid kerosene seeded with a dye. The findings of this work are based on gated sequential images recorded with an intensified camera during and after the laser pulse duration so as to sample the temporal evolution of the fluorescence emission. The results show that, for low laser excitation intensity, the fluorescence intensity remains proportional to the laser power. However, a non-linear response is observed in the case of high excitation intensity. This difference can be used to limit the fluorescence dependence on temperature by carefully selecting the temporal window where fluorescence is detected. Finally, although the method presented in this paper is applied to cold liquid kerosene, its range of application can be extended to other liquids and other temperature ranges.

Plasmon Enhanced Two‐Photon Probing with Gold and Silver Nanovoid Structures

AbstractNonlinear optical signals benefit greatly from the enhanced local optical fields in the vicinity of plasmonic nanostructures. Gold and silver nanovoid arrays of varying size and thickness, fabricated by electrochemical deposition are shown here to act as stable plasmonic nanostructures and to enhance the weak, incoherent two‐photon excited process of surface‐enhanced hyper Raman scattering (SEHRS) with high microscopic homogeneity and reproducibility that typical SEHRS experiments have not been addressing so far. Silver nanovoids yield stronger enhancement than gold voids, but gold nanovoid arrays show improved stability at high laser excitation intensities. Combined screening experiments using SEHRS and second‐harmonic generation (SHG) reveal a dependence of the enhancement of both signals on void structural parameters and similar optimum geometries for both two‐photon processes. The results confirm the suggested important role for the enhancement of the near‐infrared excitation field in SEHRS and suggest SHG as a fast screening tool to identify nanostructures that can support high SEHRS enhancement.