Multipulse Excitation Research Articles

Enhanced Ultrafast Broadband Reverse Saturable Absorption in Twistacenes with Enlarged π-Conjugated Central Bridge.

Optical nonlinearities of two all-carbon twistacenes, DPyA and DPyN, with the different π-conjugated central bridges were investigated. The nonlinear absorption properties of these compounds were measured using the femtosecond Z-scan with wavelengths between 650 and 900 nm. It has been found that the nonlinear absorption originated from two-photon absorption (TPA) and TPA-induced excited state absorption (ESA), wherein DPyA demonstrates higher performance than DPyN. The TPA cross section of DPyA (4300 GM) is nearly 4.3 times larger than that of DPyN at 650 nm. Moreover, the different central structures modulate the intensity of ESA at 532 nm, and DPyA exhibits an excellent ESA at 532 nm with multi-pulse excitation. Meanwhile, the result of data fitting and quantum chemistry calculation shows that the enhancement of nonlinear absorption in DPyA is due to the extended π- conjugated bridge and improved delocalization of π-electrons. These all-carbon twistacenes could yield potential applications in optical power limiting (OPL) technology.

Imaging and detection of long-lived fluorescence probes in presence of highly emissive and scattering background.

Optical biomedical imaging and diagnostics is a rapidly growing field that provides both structural and functional information with uses ranging from fundamental to practical clinical applications. Nevertheless, imaging/visualizing fluorescence objects with high spatial resolution in a highly scattering and emissive biological medium continues to be a significant challenge. A fundamental limiting factor for imaging technologies is the signal-to-background ratio (SBR). For a long time to improve the SBR, we tried to improve the brightness of fluorescence probes. Many novel fluorophores with improved brightness (almost reaching the theoretical limit), redshifted emission, highly improved photostability, and biocompatibility greatly helped advance fluorescence detection and imaging. However, autofluorescence, scattering of excitation light, and Raman scattering remain fundamental limiting problems that drastically limit detection sensitivity. Similarly, significant efforts were focused on reducing the background. High-quality sample purification eliminates the majority of autofluorescence background and in a limited confocal volume allows detection to reach the ultimate sensitivity to a single molecule. However, detection and imaging in physiological conditions does not allow for any sample (cells or tissue) purification, forcing us to face a fundamental limitation. A significant improvement in limiting background can be achieved when fluorophores with a long fluorescence lifetime are used, and time-gated detection is applied. However, all long-lived fluorophores present low brightness, limiting the potential improvement. We recently proposed to utilize multipulse excitation (burst of pulses) to enhance the relative signal of long-lived fluorophores and significantly improve the SBR. Herein, we present results obtained with multipulse excitation and compare them with standard single-pulse excitation. Subtraction of images obtained with a single pulse from those obtained with pulse burst (differential image) highly limits background and instrumental noise resulting in more specific/sensitive detection and allows to achieve greater imaging depth in highly scattering media, including skin and tissue.

Methods and apparatus for comprehensive characterization of performance attributes and damage thresholds of ultrafast laser optics

It is presently well understood that the operational performance limits of optics are determined by three fundamental attributes: the initiation of laser-induced damage, the growth of damage sites, and the transient (nondamaging) modification of optical parameters. The comprehensive characterization of the performance limitations of ultrafast optics requires consideration of all three fundamental attributes. The vast majority of the literature to date, however, has focused primarily on damage-initiation and testing systems that are largely focused on determining the damage-initiation threshold under single- and multipulse excitation. We discuss a testing apparatus that was designed to offer the capability to adequately characterize all three of these performances attributes under femtosecond, near-infrared laser irradiation. Key aspects of the methodology are discussed; these include high-dynamic-range energy control, variable beam size, wavelength tunability, B-integral management, and functional performance characterization to explore the true operational limits of the components. Example results for a metal-dielectric mirror demonstrate the test station’s operation.

Investigations on a Passive Real-Time Magnetic Flux Density Amplifier Made From Eight-Shaped HTS Loops

The superconducting devices made by high temperature superconducting (HTS) tapes attract great significance in high magnetic fields and at low temperature. A configuration of the magnetic flux density amplifier (MFDA) composed of rare earth-Ba-Cu-O (REBCO) HTS tapes with one central slit and twisted to form an “eight-shaped” loop with two different sizes of “closed-loops” is reported by our group in recent years. However, it can only achieve the fixed amplified function of magnetic flux density. This article proposes a passive dynamic MFDA made by the same HTS tape as an “eight-shaped” loop, which abides by the principle of flux conservation. An experimental setup is established to investigate the real-time amplification of magnetic flux density, and numerical method is used to simulate the process of dynamic magnetic flux density. Five kinds of multipulse excitation amplitudes are utilized in the experiment and a real-time process of magnetic flux density is displayed within a period. The feasibility of amplified magnetic flux density in the HTS loop is separately verified by experiment and simulation in 77 K. In order to improve the accuracy of ac loss calculation, analytical method and finite element method are compared with each other. Temperature distribution of the real-time MFDA is obtained and thermal stability is also analyzed by comparing the effects of different transmission currents and frequencies to the HTS loop. The results show that REBCO loops achieve the real-time amplified function of magnetic flux density and it has the ability of passive long-term stable operation, which is potential for sensor of flux density.

Ultrafast spectroscopy of C-H vibrations in pathogenic bacteria in 3-μm spectral range

Dynamic optical density spectra were obtained under multipulse excitation of bacterial cultures of S. aureus and P. aeruginosa by 3 μm mid-infrared ultrashort laser pulses, corresponding to the vibrational excitation of the C–H bonds of the bacterial cell. These spectra demonstrated pronounced laser intensity-dependent blue spectral shift, presumably associated with the breaking of hydrogen bonds, which are responsible for the formation of secondary and tertiary protein structures.

Rydberg blockade in an ultracold strontium gas revealed by two-photon excitation dynamics

We demonstrate the interaction-induced blockade effect in an ultracold $^{88}$Sr gas via studying the time dynamics of a two-photon excitation to the triplet Rydberg series $5\mathrm{s}n\mathrm{s}\, ^3\textrm{S}_1$ for five different principle quantum numbers $n$ ranging from 19 to 37. By using a multi-pulse excitation sequence to increase the detection sensitivity we could identify Rydberg-excitation-induced atom losses as low as $<1\%$. Based on an optical Bloch equation formalism, treating the Rydberg-Rydberg interaction on a mean-field level, the van der Waals coefficients are extracted from the observed dynamics, which agree fairly well with \emph{ab initio} calculations.

Coherent 2D electronic spectroscopy with complete characterization of excitation pulses during all scanning steps.

Coherent two-dimensional (2D) electronic spectroscopy has become a standard tool in ultrafast science. Thus it is relevant to consider the accuracy of data considering both experimental imperfections and theoretical assumptions about idealized conditions. It is already known that chirped excitation pulses can affect 2D line shapes. In the present work, we demonstrate performance-efficient, automated characterization of the full electric field of each individual multipulse sequence employed during a 2D scanning procedure. Using Fourier-transform spectral interferometry, we analyze how the temporal intensity and phase profile varies from scanning step to scanning step and extract relevant pulse-sequence parameters. This takes into account both random and systematic variations during the scan that may be caused, for example, by femtosecond pulse-shaping artifacts. Using the characterized fields, we simulate and compare 2D spectra obtained with idealized and real shapes obtained from an LCD-based pulse shaper. Exemplarily, we consider fluorescence of a molecular dimer and multiphoton photoemission of a plasmonic nanoslit. The deviations from pulse-shaper artifacts in our specific case do not distort strongly the population-based multidimensional data. The characterization procedure is applicable to other pulses-shaping technologies or excitation geometries, including also pump-probe geometry with multipulse excitation and coherent detection, and allows for accurate consideration of realistic optical excitation fields at all inter-pulse time-delays.

Research on Fault Diagnosis of Analog Circuit Based on Volterra Theory and Higher-Order Spectrum Analysis

To diagnose fault of nonlinear components in analog circuits, a method of fault diagnosis based on Volterra series theory, high-order spectrum and kernel assurance criterion (KAC) analysis was proposed. Firstly, input and output signals of the analog circuit were used to identify a Volterrra model. Secondly, the kernel function was solved by the improved multi-pulse excitation method. The high-order spectrum and its slice method were used to qualitatively characterize the kernel function. Finally, the concept of KAC indicator was introduced to quantitatively compare the characteristics of soft faults in the analog circuit, and the fault degree information was extracted and diagnosed. Taking the Sallen-key filter circuit as the research object, the simulation of the method shows that the high-order spectrum and KAC index can be used to quantify the Volterra system with different fault levels, which can effectively distinguish analog circuit with different soft faults, and have certain practical significance.

Transducer-matched multipulse excitation for signal-to-noise ratio improvement in diode laser-based photoacoustic systems.

.We analyze transducer-matched multipulse excitation as a method for improving of the signal-to-noise ratio (SNR) for diode laser-based photoacoustic systems. We discuss the principle of the technique, its advantages, and potential drawbacks and perform measurements to analyze the obtainable SNR increase. We show in experiment and computationally that a lower boundary estimate of 1.2 to 1.8 fold SNR improvement can be provided using transducer-matched pulse bursts, depending on the transducer and particular arrangement. Finally, we analyze implications that the transducer resonance effects may have on the recently introduced advanced photoacoustic techniques. The findings are of immediate interest to modalities utilizing dense pulse sequences and systems possessing limited pulse energy. In particular, transducer-matched multipulse excitation may be beneficial for diode-based photoacoustic systems operated with transducers in the range of 1 to 5 MHz since the required hardware is readily available.

Mechanism of hot spots formation in pentaerythritol tetranitrate under pulsed laser irradiation

The spectral, kinetic, spatial, and amplitude characteristics of emissions of pressed samples of PETN (pentaerythritole tetranitrate) by the first harmonic of a neodymium laser (1064 nm, 12 ns) are studied. It is found that at the moment of irradiation of PETN by laser pulses, hot spots are formed on the surface of the samples. The energy density of laser radiation varies in the range of 0.5–3 J/cm2. The characteristics of hot spots are determined by absorbing inhomogeneities and parameters of laser radiation. Multi-pulse excitation leads to the annealing of absorbing irregularities and increased threshold of optical breakdown. The received results are interpreted on the basis of ideas of the low-threshold optical breakdown as evolving within local (defective) regions of the PETN in the electric field of the light wave.

Lock-in inductive thermography for surface crack detection in different metals

ABSTRACTInductive thermography is a non-destructive technique for detecting surface cracks in electrically conductive materials. The induced eddy currents and the heat diffusion are disturbed by surface cracks, which makes the defects visible in infrared images. In non-magnetic materials, with high electrical and thermal conductivity, a short heating pulse is necessary, otherwise the thermal signal diminishes too quickly. However, with a short heating pulse only a little amount of heat is induced into the material, so the noise may be too high compared to the signal, causing a too low signal-to-noise (SNR) ratio for reliable defect detection. Applying a sequence of short pulses offers a solution. The evaluation of this multi-pulse excitation is in principle identical to the well-known lock-in technique, which is usually applied for sinusoid modulated heating. The main goal of the paper is to investigate how the temperature and phase noise can be reduced by several factors. The influence of the number of pulses, the heating power, the pulse duration, and the crack depth on the SNR is investigated. Theoretical considerations and experimental results are presented and compared to the derived equations. Aluminum and non-magnetic steel samples with artificial cracks were inspected and compared to the theoretical results.

Resonator neuron and triggering multipulse excitability in laser with injected signal

Semiconductor lasers with coherent forcing are expected to behave similarly to simple neuron models in response to external perturbations, as long as the physics describing them can be approximated by that of an overdamped pendulum with fluid torque. Beyond the validity range of this approximation, more complex features can be expected. We perform experiments and numerical simulations which show that the system can display resonator and integrator features depending on parameters and that multiple pulses can be emitted in response to larger perturbations.

Influence of laser pulse regime on the structure and optical properties of TiO2 nanolayers

TiO2 films were deposited onto glass substrates by pulsed laser deposition (PLD) in high vacuum under monopulse and multipulse excitation. After deposition, a thermal treatment on air atmosphere was done to promote crystallization. Films were studied by x-ray diffraction, energy dispersive spectroscopy, scanning electron, atomic force microscopy, UV–vis spectroscopy, and ellipsometry. After air annealing, films gain a different amount of oxygen: TiO1.9 in multipulse regime versus TiO1.7 in the monopulse one. Splashing is observed in both regimes although in the multipulse mode greater particles are found, that derived in a less compact film after annealing which could be the cause of the better oxygen diffusion. The optical band gap of the film prepared with monopulse excitation is 3.09 eV. This value increased to 3.34 eV with annealing, corresponding to that of anatase. The film made with multipulses has an Eg = 3.12 eV which was invariant upon annealing. The difference in the properties of the films grown in the different regimes was attributed to the re-excitation of the plasma during the ablation process in the multipulse ablation that leads to an increased splashing density and thereafter a less compact film and the presence of off-stoichiometry inclusions within the film bulk.

Nonlinear and Quantum Optics on the Nature of Sugar Emission Excited by the First Harmonic of a Nd Laser

The physical nature of emissions excited in pressed-sugar samples by the first harmonic of a neodymium laser (1064 nm, 12 ns) is studied. The spectral, kinetic, spatial, and amplitude characteristics of emissions upon single- and multipulse irradiation with energy densities of 0.012–3.2 J/cm2 are measured. It is found that excitation of a sample by the first pulse causes two emission types, namely, emission of the second harmonic of a Nd laser (532 nm) and glowing of dense low-temperature plasma formed as a result of a low-temperature optical breakdown on absorbing inhomogeneities. Multipulse excitation leads to annealing of absorbing inhomogeneities and to an increase in the optical breakdown threshold.

Research and application of Volterra series theory in rolling bearing fault state feature extraction

Due to the generally strong non-linear characteristics of bearing failure, leading to overall mechanical system failure, fault state feature extraction is difficult. In this paper, a fault feature extraction method based on the Volterra series kernel under multi-pulse excitation is proposed. To avoid reliance on simplified models based on traditional mechanics, a nonlinear Volterra series model was constructed by introducing the input and output signals of the system, and using a low-order Volterra series kernel from the time domain and frequency domain, which was then solved using a multi-pulse excitation method. Furthermore, the state of the rolling bearing was determined using different characteristics of the corresponding generalized frequency response, and the current fault stage was inferred. The rolling bearing failure was validated experimentally, and it was shown that the Volterra series model can be more easily used to extract fault characteristics and trends of a rolling bearing in comparison to the traditional wavelet algorithm, therefore serving as a better method for fault prediction.

Simultaneous Phosphorescence and Fluorescence Lifetime Imaging by Multi-Dimensional TCSPC and Multi-Pulse Excitation.

TCSPC FLIM/PLIM is based on a multi-dimensional time-correlated single-photon counting process. The sample is scanned by a high-frequency-pulsed laser beam which is additionally modulated on/off synchronously with the pixels of the scan. FLIM is obtained by building up the distribution of the photons over the scanning coordinates and the times of the photons in the excitation pulse sequence, PLIM is obtained by building up the photon distribution over the scanning coordinates and the photon times in the modulation period. FLIM and PLIM data are thus obtained simultaneously within the same imaging process. Since the technique uses not only one but many excitation pulses for every phosphorescence signal period the sensitivity is much higher than for techniques that excite with a single pulse only. TCSPC FLIM/PLIM works both with one-photon and two-photon excitation, does not require a reduction of the laser pulse repetition rate by a pulse picker, and eliminates the need of high pulse energy for phosphorescence excitation.

Pulsed laser deposition of PbTe under monopulse and multipulse regime

The aim of this paper is to compare PLD technique with monopulse and multipulse nanosecond laser excitation. We show the feasibility of depositing nanometric layers of PbTe employing the regimes already mentioned. Each of the grown layers were characterized by XRD, EDXS, SEM, Spectroscopic Elipsometry, AFM and the thickness was measured by mechanic profilometry. We have conducted comparative experiments to show the advantages and drawbacks of making PLD with mono and multipulse nanosecond laser.

Extensive theoretical study on electronically excited states of calcium monochloride: Molecular laser cooling and production of ultracold chlorine atoms.

Nine doublet Λ-S states of calcium monochloride (CaCl) are calculated using the internally contracted multireference configuration interaction method with the Davidson correction. Both the core subvalence and spin-orbit coupling effects are taken into account. Laser cooling of CaCl and production of ultracold chlorine atoms are investigated and assessed. Our computed spectroscopic constants and radiative lifetimes match the available experimental data very well. The determined Franck-Condon factors and vibrational branching ratios of the A(2)Π1/2(ν('))←X(2)Σ1/2 (+)(ν) transition are highly diagonally distributed and the evaluated radiative lifetime for the A(2)Π1/2(ν' = 0) state is 28.2 ns, which is short enough for rapid laser cooling. Subsequently, detection of cold molecules via resonance enhanced multiphoton ionization to determine the final quantum state populations is discussed and the ionization energy calculated. A multi-pulse excitation scheme is proposed for producing ultracold chlorine atoms from zero-energy photodissociation of the cooled CaCl. Our results demonstrate the possibility of producing ultracold CaCl molecules and Cl atoms.

Multi-Pulse Excitation for Underwater Analysis of Copper-Based Alloys Using a Novel Remote Laser-Induced Breakdown Spectroscopy (LIBS) System.

In this work, the use of multi-pulse excitation has been evaluated as an effective solution to mitigate the preferential ablation of the most volatile elements, namely Sn, Pb, and Zn, observed during laser-induced breakdown spectroscopy (LIBS) analysis of copper-based alloys. The novel remote LIBS prototype used in this experiments featured both single-pulse (SP-LIBS) and multi-pulse excitation (MP-LIBS). The remote instrument is capable of performing chemical analysis of submersed materials up to a depth of 50 m. Laser-induced breakdown spectroscopy analysis was performed at air pressure settings simulating the conditions during a real subsea analysis. A set of five certified bronze standards with variable concentration of Cu, As, Sn, Pb, and Zn were used. In SP-LIBS, signal emission is strongly sensitive to ambient pressure. In this case, fractionation effect was observed. Multi-pulse excitation circumvents the effect of pressure over the quantitative analysis, thus avoiding the fractionation phenomena observed in single pulse LIBS. The use of copper as internal standard minimizes matrix effects and discrepancies due to variation in ablated mass.

Elemental analysis of materials in an underwater archeological shipwreck using a novel remote laser-induced breakdown spectroscopy system

LIBS analysis of submerged materials in an underwater archeological site has been performed for the first time. A fiber-optics-based remote instrument was designed for the recognition and identification of archeological assets in the wreck of the Bucentaure (Bay of Cadiz, South of Spain). The LIBS prototype featured both single-pulse (SP-LIBS) and multi-pulse excitation (MP-LIBS). The use of multi-pulse excitation allowed an increased laser beam energy (up to 95mJ) transmitted through the optical fiber. This excitation mode results in an improved performance of the equipment in terms of extended range of analysis (to a depth of 50m) and a broader variety of samples to be analyzed (i.e., rocks, marble, ceramics and concrete). Compared to single-pulse, an intensity enhancement factor of 15× was observed at the same irradiance value, 1.89GW/cm2. Thus, a longer pulse duration promotes the heating and melting of the sample, resulting in a greater mass ablated. As a consequence of the optimization of experimental conditions performed in laboratory, underwater characterization of ancient pottery was achieved.