Excitation Pulse Duration Research Articles

Numerical and experimental study of thermography parameters for inspection of composites repaired by CFRP patches

ABSTRACT Considering the ever-increasing importance of composite patches applied to repair or reinforce composite structures, radical health monitoring of these patches seems inescapable. Thermography method is one of the most influential non-destructive methods applied to inspect the repaired structures. Since there are many parameters influencing defects detection in repaired composite structures, extracting the most influential method to determine these parameters is significantly important. In the present study, a repaired carbon fiber reinforced plastic (CFRP) composite structure having defects of different dimensions, types and features located in different depths, is investigated by experiment and finite element simulations to assess the influence of the experimental parameters, like the thermal excitation pulse duration and the best image frame. In thermal simulation, two criteria, maximum thermal variations versus time and path length, are developed to derive the most optimum heating duration and the best thermal frames. The obtained results are validated through defining direct and diagonal paths passing through defects demonstrated on thermography images and thermal map images. The average error of thermal variations on different paths displayed on thermography and simulation images is less than 4.5 percent which confirms the correspondence of obtained results of both mentioned methods.

Unified and vector theory of Raman scattering in gas-filled hollow-core fiber across temporal regimes.

Raman scattering has found renewed interest owing to the development of gas-filled hollow-core fibers, which constitute a unique platform for exploration of novel ultrafast nonlinear phenomena beyond conventional solid-core-fiber and free-space systems. Much progress has been made through models for particular interaction regimes, which are delineated by the relation of the excitation pulse duration to the time scales of the Raman response. However, current experimental settings are not limited to one regime, prompting the need for tools spanning multiple regimes. Here, we present a theoretical framework that accomplishes this goal. The theory allows us to review recent progress with a fresh perspective, makes new connections between distinct temporal regimes of Raman scattering, and reveals new degrees of freedom for controlling Raman physics. Specific topics that are addressed include transient Raman gain, the interplay of electronic and Raman nonlinearities in short-pulse propagation, and interactions of short pulses mediated by phonon waves. The theoretical model also accommodates vector effects, which have been largely neglected in prior works on Raman scattering in gases. The polarization dependence of transient Raman gain and vector effects on pulse interactions via phonon waves is investigated with the model. Throughout this Perspective, theoretical results are compared to the results of realistic numerical simulations. The numerical code that implements the new theory is freely available. We hope that the unified theoretical framework and numerical tool described here will accelerate the exploration of new Raman-scattering phenomena and enable new applications.

Deciphering the emission dynamics of colloidal PbS quantum dots: a spectroscopic exploration under diverse environmental and experimental conditions.

This study employs a combination of spectroscopic techniques, including absorption, photoluminescence (PL), time-resolved PL kinetics, and transient absorption measurements, to elucidate the dynamics of excited states of oleic acid (OA)-capped colloidal PbS quantum dots (QDs) in toluene with a mean size of approximately 3 nm. PL decay profiles were properly treated employing either fitting or fitting-free models to extract average decay lifetimes and lifetime distributions. Our findings highlight the profound impact of factors such as particle concentration, size, surface treatment, and the surrounding environment on the optical properties of PbS QDs. We observed a notable blue shift in both absorption and emission spectra, along with shorter decay lifetimes, when the concentration of the particles was reduced. This effect was attributed to the dynamics of concentration-selective ligand desorption on the surface of PbS QDs. An observed distinct peak in the spectral dispersion of average lifetimes was linked to an external origin, which was proved by the addition of varying OA volumes, evidently affecting these features. This study also explores how the PL emission of PbS QD suspensions varies with experimental parameters, such as excitation pulse duration and laser power. A gradual reduction in average lifetimes was demonstrated under the modulation of pulse duration from 15 μs to 150 ns, illustrating a transition from continuous-wave (cw) excitation to quasi-pulsed excitation. This transition was shown to be accompanied by a noticeable shift in the lifetime distributions towards shorter durations, as a consequence of various onset components in the QD ensemble. Comparing with truly pulsed excitation, we experimentally confirmed that the amplitude-average lifetime under cw excitation matches the intensity-average lifetime under pulsed excitation. Lastly, we noted linear and nonlinear behaviors in the PL amplitude as a function of excitation power, with the average lifetime remaining nearly constant under pulsed excitation conditions.

Coherent excitation of three-atom entangled states near a two-body Förster resonance

We experimentally study three-body energy exchange during Rydberg excitation near a two-body F\"orster resonance. By varying the excitation pulse duration or Rabi frequency, we coherently control the excitation of three-atom entangled states. We prove coherence using an optical rotary echo technique and compare with a model for excitation in a three-atom basis. Our results suggest a robust way to implement a three-body entangling operation.

The effect of source backing materials and excitation pulse durations on laser-generated ultrasound waveforms.

In this article, it is shown experimentally that a planar laser-generated ultrasound source with a hard reflective backing will generate higher acoustic pressures than a comparable source with an acoustically matched backing when the stress confinement condition is not met. Furthermore, while the source with an acoustically matched backing will have a broader bandwidth when the laser pulse is short enough to ensure stress confinement, the bandwidths of both source types will converge as the laser pulse duration increases beyond stress confinement. The explanation of the results is supported by numerical simulations.

Excitation Pulse Duration Response of Upconversion Nanoparticles and Its Applications.

Lanthanide-doped upconversion nanoparticles (UCNPs) have rich photophysics exhibiting complex luminescence kinetics. In this work, we thoroughly investigated the luminescence response of UCNPs to excitation pulse durations. Analyzing this response opens new opportunities in optical encoding/decoding and the assignment of transitions to emission peaks and provides advantages in applications of UCNPs, e.g., for better optical sectioning and improved luminescence nanothermometry. Our work shows that monitoring the UCNP luminescence response to excitation pulse durations (while keeping the duty cycle constant) by recording the average luminescence intensity using a low-time resolution detector such as a spectrometer offers a powerful approach for significantly extending the utility of UCNPs.

AP/HTPB combustion response simulation with a new equivalent linear system method

In the study of solid rocket motor (SRM) combustion instability, modeling and analyzing the combustion response of the solid propellant is a laborious task. We propose a new idea, an equivalent linear system method, for modeling solid propellant combustion response with three steps: first, simulating the transient combustion process under pulse pressure excitation for only one CFD case; then, treating the combustion process as a black-box and obtaining the transfer function of the combustion process from the CFD result; finally, predicting and analyzing the combustion response based on the transfer functions. For the AP/HTPB sandwich propellant withp¯=3MPa, the amplitude, duration, and direction of excitation pulses are studied. The new method is also verified, with a maximum relative error of 4.3% for burning rate response under p˜≤1MPa (as high as one-third of the pressure DC). Applicability of this new method is demonstrated for another two AP/HTPB sandwich propellants with different configuration under p¯=8MPa and p¯=10MPa, showing that the system-simulated combustion responses of burning surface temperature and burning rate are consisted well with the CFD results and the maximum relative error of burning rate response is 2.1%. Asymmetric combustion response for positive and negative pressure perturbations is a new finding based on the CFD and simulation results. Besides, a new approach using dual transfer functions is proposed to ensure high accuracy for the simulation of solid propellant combustion. The pressure-coupled response function is predicted by this method for the sandwich propellant under p¯=5MPa and p˜=20kPa within a wide continuous frequency range (0–3000 Hz). The comparison with CFD results indicates that the relative bias of Rp is less than 2%. With the proposed method, the combustion response to arbitrary pressure excitations can be simulated with high accuracy, high time-resolution, and high speed. The method shows great advantage and potential in simulating and predicting combustion instability.

The influence of Ce3+ codoping on upconversion in nanocrystalline NaYF4:Yb3+,Tm3+

Usually, to obtain multicolor upconversion in lanthanides, materials with different codopants, different size, chemical composition or compositional architecture are required. However, designing a material whose emission color can be tuned in-situ by an external stimulus, such as temperature, excitation pulse duration or photoexcitation density, is of great interest for many applications, but still remains a challenge. The possible solution for this issue can be achieved by intentional codoping the luminescent materials with optically active ions which can disturb or compete with other energy transfer processes, and in consequence lead to changes in relative emission intensity, thus the dominant emission color. Here we synthesized and investigated colloidal luminescent Yb3+ and Tm3+ doped NaYF4 nanoparticles which were codoped with optically active Ce3+ ions, whose relative emission intensity ratios between 1D2→3F4 (at 450 nm), 1G4→3H6 (at 475 nm), 1G4→3F4 (at 650 nm), and 3H4→3H6 (at 800 nm) were significantly dependent from external stimulus – namely temperature and/or excitation pulse length. Our results shows two fold increase in 450/800, 470/800 and 650/800 luminescence intensity bands ratios (LIR) only after incorporation of Ce3+ ions to the material, and even three-fold increase in those LIR after changing the excitation conditions (i.e. pulse width). Moreover, temperature dependent luminescence properties revealed relative sensitivity of 4%/K for the 450/800 and 470/800 bands ratio and almost 5.5%/K for the 650/800 band ratio close to 0 °C. By switching to physiological range of temperatures, the thermal sensitivity decreased to around 2%/K for the 450/800 and 470/800 bands ratio and to over 2.5%/K for 650/800 bands ratio, which should be considered as significant values and suitable for temperature sensing in practical applications. It is worth noting that the thermal sensitivity values can be further enhanced by applying different excitation pulse width. The proposed materials allows to tune its emissions as well as thermal properties by changing the dopant concentration or more importantly also in situ, through excitation scheme modifications.

Dataset to manuscript "Excitation pulse duration response of upconversion nanoparticles and its applications"

Dataset to manuscript "Excitation pulse duration response of upconversion nanoparticles and its applications"

Electric Field Tuned Dipolar Interaction Between Rydberg Atoms

We demonstrated a tuned dipole interaction between Rydberg atoms by employing a controllable electric field in a cold cesium ensemble. The |nP3/2⟩ (n = 38–40) Rydberg pairs are prepared with a three-photon scheme and detected via the state-selective field ionization technique. A weak DC electric field is used to tune the Rydberg pair interaction from the van der Waals interaction regime to the dipole–dipole interaction regime. The Förster resonant interaction and an adiabatic resonance energy transfer between the nP and nS Rydberg states are attained by precisely tuning the electric field. Rydberg excitation blockade with and without the electric field is investigated by changing the excitation pulse duration, which demonstrates that the dipole interaction–induced blockade effect is stronger than the van der Waals interaction–induced blockade effect. The precise control of the Rydberg interaction is of great significance to the coherent interaction in many-body systems and non-radiative collision processes.

Nonlinear optical absorption characteristics and ultrafast carrier dynamics of one-dimensional strongly correlated cuprate nanosheets

Developing novel optical materials with large and ultrafast nonlinear optical (NLO) response is significant for photonics and optoelectronics. Cuprates as the representative one-dimensional (1D) strongly correlated material show promising NLO properties, but relevant research is quite rare. Here, using Ca2CuO3 nanosheets as the prototype, we systematically explored the NLO performance and ultrafast carrier dynamics of these cuprate nanosheets prepared by a ball-milling process. The obtained Ca2CuO3 nanosheets exhibited convertible NLO behavior from saturable absorption (SA) to reverse saturable absorption (RSA) with strong absorption coefficient, induced by increasing wavelengths and duration of excitation pulse. In addition, we revealed their ultrashort relaxation time in (sub-)picosecond scale for photogenerated carriers, mainly owing to the midgap state-assisted recombination by emitting multiple phonons and spinons. Furthermore, the similar NLO feature also investigated in isostructural Sr2CuO3 and Ba2CuO3 nanosheets further indicated the excellent NLO performance of 1D strongly correlated cuprates. This work opens up a new avenue for future photonics with 1D strongly correlated materials as the novel platforms.

Rapid modeling of borehole measurements of nuclear magnetic resonance via spatial sensitivity functions

Borehole measurements of nuclear magnetic resonance (NMR) are routinely used to estimate in situ rock and fluid properties. Conventional NMR interpretation methods often neglect bed-boundary and layer-thickness effects in the calculation of fluid volumetric concentrations and NMR relaxation-diffusion correlations. Such effects introduce notable spatial averaging of intrinsic rock and fluid properties across thinly bedded formations or in the vicinity of boundaries between layers exhibiting large property contrasts. Forward modeling and inversion methods can mitigate the aforementioned effects and improve the accuracy of true layer properties in the presence of mud-filtrate invasion and borehole environmental effects across spatially complex formations. We have developed a fast and accurate algorithm to simulate borehole NMR measurements using the concept of spatial sensitivity functions (SSFs) that honor NMR physics and incorporate tool, borehole, and formation geometry. Tool sensitivity maps are derived from a 3D multiphysics forward model that couples NMR tool properties, magnetization evolution, and electromagnetic propagation. In addition, a multifluid relaxation model based on Brownstein-Tarr’s equation is introduced to estimate layer NMR porosity decays and relaxation-diffusion correlations from pore-size-dependent rock and fluid properties. The latter model is convolved with the SSFs to reproduce borehole NMR measurements. The results indicate that NMR spatial sensitivity is controlled by porosity, electrical conductivity, excitation pulse duration, and tool geometry. We benchmark and verify the SSF-derived forward approximation against 3D multiphysics simulations for a series of synthetic cases with variable bed thickness and petrophysical properties, and in the presence of mud-filtrate invasion in a vertical well. Results indicate that the approximation can be executed in a few seconds in a central processing unit, by a factor of 1000 times faster than rigorous multiphysics calculations, with maximum root-mean-square errors of 1%.

Minimisation of slab-selective radiofrequency excitation pulse durations constrained by an acceptable aliasing coefficient

A technical note is presented on the slab-direction aliasing of 3D imaging, introducing a simple methodology for determining the minimised duration of low flip-angle sinc radiofrequency (RF) excitation pulses, with respect to a required slab profile accuracy. The various interdependent factors affected in modifying an RF pulse duration are considered and analysed in the context of a new metric for quantifying the levels of permitted slab-aliasing. A general framework is presented for the selection of standard sinc RF excitation pulses with system-minimised durations, as well as their analysis and validation, and a demonstration of this methodology is performed for an example requirement and scanner. This methodology enables implementation of standard (vendor-generated) RF pulses with minimised duration for a required application, with high confidence in their operational reliability. Parts of such a methodology may also in theory be extended to more advanced RF pulse designs.

On the role of excitation pulse duration on luminescence measurements

Described here is an investigation into the effects of excitation pulse duration on the temporal character of luminescence of three characteristic thermographic phosphors: Mg4FGeO6:Mn, La2O2S:Eu, and YAG:Dy. Attention is first given to the build-up of the emission while the excitation pulse is on and its relation to temperature. Next, effects after pulse termination are addressed. For emission comprised of two or more characteristic decays, increasing the pulse duration will enhance the longer components relative to the shorter. For La2O2S:Eu, increasing the duration increases emission of successively longer-lived emitting states; and they display pronounced temperature dependence. For YAG:Dy, as excitation duration is increased, the main decay component used for thermometry is enhanced relative to two shorter components of less interest.

Feasibility of Harmonic Motion Imaging Using a Single Transducer: In Vivo Imaging of Breast Cancer in a Mouse Model and Human Subjects.

Harmonic motion imaging (HMI) interrogates the mechanical properties of tissues by simultaneously generating and tracking harmonic oscillation using focused ultrasound and imaging transducers, respectively. Instead of using two transducers, the objective of this work is to develop a single transducer HMI (ST-HMI) to both generate and track harmonic motion at "on-axis" to the force for facilitating data acquisition. In ST-HMI, the amplitude-modulated force was generated by modulating excitation pulse duration and tracking of motion was performed by transmitting tracking pulses interleaved between excitation pulses. The feasibility of ST-HMI was performed by imaging two elastic phantoms with three inclusions (N = 6) and comparing it with acoustic radiation force impulse (ARFI) imaging, in vivo longitudinal monitoring of 4T1, orthotropic breast cancer mice (N = 4), and patients (N = 3) with breast masses in vivo. Six inclusions with Young's moduli of 8, 10, 15, 20, 40, and 60 kPa were embedded in a 5 kPa background. The ST-HMI-derived peak-to-peak displacement (P2PD) successfully detected all inclusions with [Formula: see text] of the linear regression between the P2PD ratio of background to inclusion versus Young's moduli ratio of inclusion to background. The contrasts of 10 and 15 kPa inclusions were higher in ST-HMI than ARFI-derived images. In the mouse study, the median P2PD ratio of tumor to non-cancerous tissues was 3.0, 5.1, 6.1, and 7.7 at 1, 2, 3, and 4 weeks post-injection of the tumor cells, respectively. In the clinical study, ST-HMI detected breast masses including fibroadenoma, pseudo angiomatous stromal hyperplasia, and invasive ductal carcinoma with a P2PD ratio of 1.37, 1.61, and 1.78, respectively. These results indicate that ST-HMI can assess the mechanical properties of tissues via generation and tracking of harmonic motion "on-axis" to the ARF. This study is the first step towards translating ST-HMI in clinics.

Stable and transient bubble formation in acoustically-responsive scaffolds by acoustic droplet vaporization: theory and application in sequential release

Acoustically-responsive scaffolds (ARSs), which are fibrin hydrogels containing monodispersed perfluorocarbon (PFC) emulsions, respond to ultrasound in an on-demand, spatiotemporally-controlled manner via a mechanism termed acoustic droplet vaporization (ADV). Previously, ADV has been used to control the release of bioactive payloads from ARSs to stimulate regenerative processes. In this study, we used classical nucleation theory (CNT) to predict the nucleation pressure in emulsions of different PFC cores as well as the corresponding condensation pressure of the ADV-generated bubbles. According to CNT, the threshold bubble radii above which ADV-generated bubbles remain stable against condensation were 0.4 µm and 5.2 µm for perfluoropentane (PFP) and perfluorohexane (PFH) bubbles, respectively, while ADV-generated bubbles of any size in perfluorooctane (PFO) condense back to liquid at ambient condition. Additionally, consistent with the CNT findings, stable bubble formation from PFH emulsion was experimentally observed using confocal imaging while PFO emulsion likely underwent repeated vaporization and recondensation during ultrasound pulses. In further experimental studies, we utilized this unique feature of ADV in generating stable or transient bubbles, through tailoring the PFC core and ultrasound parameters (excitation frequency and pulse duration), for sequential delivery of two payloads from PFC emulsions in ARSs. ADV-generated stable bubbles from PFH correlated with complete release of the payload while transient ADV resulted in partial release, where the amount of payload release increased with the number of ultrasound exposure. Overall, these results can be used in developing drug delivery strategies using ARSs.

Asymmetric broadening and blue shift of the stimulated Raman scattering spectrum in water under chirped picosecond laser pulse train excitation

For the first time, we have revealed an asymmetric broadening and a 35 cm−1 blue shift (up to 3400 cm−1) of the stimulated Raman scattering OH band in water under chirped picosecond pulse train excitation (pulse duration ∼60 ps with 8 ns interval between pulses) while increasing the train length to 25 pulses. From the observed broadening and shift of the OH band centroid, a lower estimate (≥8 ns) of molecular complexes spectral states relaxation time at room temperature was obtained.

Feasibility of a single-transducer harmonic motion imaging using frequency-based simultaneous multiple harmonic oscillation excitation pulses

Single-transducer harmonic motion imaging (ST-HMI) uses a single transducer to generate the harmonic motion at a particular frequency by modulating excitation pulse duration and then, estimate the motion by collecting tracking pulses in-between the excitation pulses, unlike the conventional HMI which uses focused ultrasound and an imaging transducer. Instead of oscillating at a single frequency, the objective is to use ST-HMI for simultaneously exciting the tissue at multiple frequencies. Six excitation pulses per period were selected by sampling a continuous signal which was generated by summing sinusoids with fundamental and harmonics of 100 Hz and 200–1000 Hz, respectively. A mean peak to peak displacement (MP2PD) image was generated by a weighted average of P2PD at each frequency with a weighting factor derived from the Fourier transform. The new method is evaluated by imaging four inclusions (INC, Young's modulus: 6, 9, 32, and 75 kPa) embedded in 18 kPa background (BKD) and HIFU ablated excised canine liver. The MP2PD image successfully differentiated all four inclusions with R2 = 0.99 of the linear regression between the MP2PD ratio of BKD to INC versus Young's modulus ratio of INC to BKD. The MP2PD image detected the ablated region with the MP2PD ratio of non-ablated versus ablated regions of 1.64. These results demonstrate the feasibility of simultaneously exciting the tissue at multiple frequencies. Ongoing studies entail the translation of this new elasticity imaging method of detecting breast masses in humans.

Excitation of molecular rotational levels by unipolar subcycle pulses

We study theoretically the action of subcycle unipolar pulses on the rotational dynamics of polar molecules. We show that efficient excitation of molecular rotational resonances by subcycle pump pulses with respect to bipolar single-cycle one is achieved, provided that duration of excitation pulses is much smaller than the rotational period of the molecule. The excitation probability for different rotational levels is analysed in dependence on the parameters of molecules and pump pulses. Remarkably, our analysis revealed the possibility of the almost single level excitation and the formation of the population inversion in the rotational levels excited by unipolar pulses, in spite of the nonresonant interaction and ultrabroad spectrum of subcycle pulses. This approach is entirely different from the traditional resonant excitation techniques with long quasi-monochromatic pulses tuned to the molecular transition. And from the other methods, when multi-color laser pulses or single-cycle THz pulses are used for control of the molecular properties. This opens new avenues in applications related to control of molecular systems and lasing creation, since it allows efficient ultrafast control over different rotational levels.

The influence of Ce3+ codoping and excitation scheme on spectroscopic properties of NaYF4:Yb3+,Ho3+

The possibility to design and intentionally tune the luminescent properties of phosphors, e.g. their emission colour, quantum yield and kinetic behavior, is of fundamental significance and a challenge for numerous applications. Usually, to obtain multicolour emission, multiple nanomaterials with different co-dopants combinations, sizes or chemical architectures are synthesized and characterized. Of special interest would be those materials, where on-demand and in-situ modifications of multicolour emission can be obtained by changing the external stimulus, such as magnetic field, temperature, excitation intensity or pulse width. Here we synthesize and evaluate luminescent NaYF4 nanoparticles doped Yb3+/Ho3+, whose red and green emission colour is modulated by variable concentration of Ce3+ admixture, excitation pulse duration and temperature. In the first scenario (Yb3+/Ho3+ only doping), such materials exhibit strong and stable green upconversion emission. Upon Ce3+ co-doping, the red to green ratio (RGR) changed from 0,43 (for pristine dopants) to 4,23 (for 15%Ce3+ co-doped samples). Moreover, the measurement performed with excitation pulse width modulation (PWM) enabled to increase contribution of red emission in-situ. For short excitation pulses, the green emission was observed, while for longer pulses the red emission started to dominate. Finally, the energy transfer in this tri-doped system demonstrated to be susceptible to external temperature as well, with relative thermal sensitivity of 0,8%/K in biological range for 5%Ce3+ doped sample. Detailed spectroscopic analysis including temperature dependent emission spectra, rise times and emission kinetics allow us to understand and propose the possible energy transfer mechanisms in this system.