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Chernysheva, Maria, Aleksey Rozhin, Yuri Fedotov, Chengbo Mou, Raz Arif, Sergey M. Kobtsev, Evgeny M. Dianov y Sergei K. Turitsyn. "Carbon nanotubes for ultrafast fibre lasers". Nanophotonics 6, n.º 1 (6 de enero de 2017): 1–30. http://dx.doi.org/10.1515/nanoph-2015-0156.
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AbstractCarbon nanotubes (CNTs) possess both remarkable optical properties and high potential for integration in various photonic devices. We overview, here, recent progress in CNT applications in fibre optics putting particular emphasis on fibre lasers. We discuss fabrication and characterisation of different CNTs, development of CNT-based saturable absorbers (CNT-SA), their integration and operation in fibre laser cavities putting emphasis on state-of-the-art fibre lasers, mode locked using CNT-SA. We discuss new design concepts of high-performance ultrafast operation fibre lasers covering ytterbium (Yb), bismuth (Bi), erbium (Er), thulium (Tm) and holmium (Ho)-doped fibre lasers.
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Nagatsuma, T., K. Iwatsuki, M. Shinagawa, A. Kozen, M. Yaita, K. Suzuki y K. Kato. "Electro-optic characterisation of ultrafast photodetectors using adiabatically compressed soliton pulses". Electronics Letters 30, n.º 10 (12 de mayo de 1994): 814–16. http://dx.doi.org/10.1049/el:19940433.
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Hu, Xinyu, Rui Pan, Mingyong Cai, Weijian Liu, Xiao Luo, Changhao Chen, Guochen Jiang y Minlin Zhong. "Ultrafast laser micro-nano structured superhydrophobic teflon surfaces for enhanced SERS detection via evaporation concentration". Advanced Optical Technologies 9, n.º 1-2 (25 de febrero de 2020): 89–100. http://dx.doi.org/10.1515/aot-2019-0072.
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AbstractEvaporation concentration of target analytes dissolved in a water droplet based on superhydrophobic surfaces could be able to break the limits for sensitive trace substance detection techniques (e.g. SERS) and it is promising in the fields such as food safety, eco-pollution, and bioscience. In the present study, polytetrafluoroethylene (PTFE) surfaces were processed by femtosecond laser and the corresponding processing parameter combinations were optimised to obtain surfaces with excellent superhydrophobicity. The optimal parameter combination is: laser power: 6.4 W; scanning spacing: 40 μm; scanning number: 1; and scanning path: 90 degree. For trapping and localising droplets, a tiny square area in the middle of the surface remained unprocessed for each sample. The evaporation and concentration processes of droplets on the optimised surfaces were performed and analyzed, respectively. It is shown that the droplets with targeted solute can successfully collect all solute into the designed trapping areas during evaporation process on our laser fabricated superhydrophobic surface, resulting in detection domains with high solute concentration for SERS characterisation. It is shown that the detected peak intensity of rhodamine 6G with a concentration of 10−6m in SERS characterisation can be obviously enhanced by one or two orders of magnitude on the laser fabricated surfaces compared with that of the unprocessed blank samples.
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Tan, T. Y. T. y G. S. H. Yeo. "Advances in Imaging in Prenatal Diagnosis and Fetal Therapy". Annals of the Academy of Medicine, Singapore 32, n.º 3 (15 de mayo de 2003): 289–93. http://dx.doi.org/10.47102/annals-acadmedsg.v32n3p289.
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Technological advances in ultrasound have contributed to improvements in prenatal diagnosis. Transvaginal scanning and harmonic imaging have allowed better resolution and improved structural characterisation. Doppler techniques have further been improved, which allow accurate flow studies of vessels in the placenta and fetus. These have contributed much to the management of intrauterine growth restriction, fetal anaemia and twin-twin transfusion syndrome (TTTS). Three-dimensional sonography and magnetic resonance imaging ultrafast sequences are useful adjuncts to conventional 2-dimensional sonography, increasing the confidence and diagnostic accuracy of prenatal diagnosis. Fetal therapy has seen major advances in recent years as well, secondary to improvements in endoscopic instruments and surgical techniques. Selective Nd:YAG laser photocoagulation of communicating vessels and cord occlusion have been used to treat complications of monochorionic twins like TTTS and twin-reversed arterial perfusion sequence.
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Donis-González, Irwin R., Daniel E. Guyer, Anthony Pease y Frank Barthel. "Internal characterisation of fresh agricultural products using traditional and ultrafast electron beam X-ray computed tomography imaging". Biosystems Engineering 117 (enero de 2014): 104–13. http://dx.doi.org/10.1016/j.biosystemseng.2013.07.002.
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Bhudolia, Somen K., Goram Gohel, Jayaram Kantipudi, Kah Fai Leong y Robert J. Barsotti. "Ultrasonic Welding of Novel Carbon/Elium® Thermoplastic Composites with Flat and Integrated Energy Directors: Lap Shear Characterisation and Fractographic Investigation". Materials 13, n.º 7 (1 de abril de 2020): 1634. http://dx.doi.org/10.3390/ma13071634.
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The current research work presents a first attempt to investigate the welding attributes of Elium® thermoplastic resin and the fusion bonding using ultrafast ultrasonic welding technique. The integrated energy director (ED) polymer-matrix composites (PMCs) panel manufacturing was carried out using the Resin Transfer Moulding (RTM) technique and the scheme is deduced to manufacture a bubble-free panel. Integrated ED configurations and flat specimens with Elium® film of different thickness at the interface were investigated for ultrasonic welding optimization. Optimised weld time for integrated ED and flat Elium® panels with film (0.5 mm thick) configuration was found to be 1 s and 5.5 s, respectively. The ED integrated configuration showed the best welding results with a lap shear strength of 18.68 MPa. The morphological assessment has shown significant plastic deformation of Elium® resin and the shear cusps formation, which enhances the welding strength. This research has the potential to open up an excellent and automated way of joining Elium® composite parts in automotive, wind turbines, sports, and many other industrial applications.
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Lau, Y. M., F. Möller, U. Hampel y M. Schubert. "Ultrafast X-ray tomographic imaging of multiphase flow in bubble columns – Part 2: Characterisation of bubbles in the dense regime". International Journal of Multiphase Flow 104 (julio de 2018): 272–85. http://dx.doi.org/10.1016/j.ijmultiphaseflow.2018.02.009.
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Juodkazis, Saulius, Arturas Vailionis, Eugene G. Gamaly, Ludovic Rapp, Vygantas Mizeikis y Andrei V. Rode. "Femtosecond laser-induced confined microexplosion: tool for creation high-pressure phases". MRS Advances 1, n.º 17 (30 de diciembre de 2015): 1149–55. http://dx.doi.org/10.1557/adv.2015.41.
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ABSTRACTNew material phases formed under non-equilibrium conditions at pressures above 100 GPa and temperatures exceeding 104K, the conditions of the warm dense matter (WDM), have become accessible using micro-explosions triggered by ultra-short sub-1 ps pulses tightly focused into micro-volume with cross sections comparable with the wavelength of light. Laser-induced micro-explosions convert a material in a focal volume into a non-equilibrium disordered plasma state confined inside the bulk of pristine crystal. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable high pressure phases, which are preserved in the surrounding pristine crystal for following recovery and exploitation. Direct laser writing was used to pattern large areas by closely packed arrays of the microexplosion modified sites for structural characterisation of the minute volumes of nano-materials with transmission electron microscopy, diffraction and synchrotron X-ray diffraction. The method of ultrafast-laser induced confined microexplosion is demonstrated for modification and creation of new phases in case of bcc-Al inside sapphire, valence change of Fe-ions in olivine, formation of new tetragonal bt8 and st12 phases of silicon, Ge and O separation in GeO2 glass and molecular oxygen formation inside voids at the site of microexplosion inside glasses.
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Costa, P. M. F. J. "Imaging the stimuli response of nanostructured materials inside a transmission electron microscope: from today’s sub-second recording to ultrafast phenomena". Microscopy and Microanalysis 19, S4 (agosto de 2013): 93–94. http://dx.doi.org/10.1017/s1431927613001086.
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Over the last decade, with the advent of aberration correctors and energy-filters, transmission electron microscopes (TEMs) have seen phenomenal developments both in regards to spatial and spectral resolutions. Today, the TEM is a characterisation tool pervasive to nanoscaled sciences and technologies. Imaging and identifying single atoms, including low Z elements such as B or C, is within reach of a skilful operator with access to this new generation of microscopes.Parallel to these advances, sample holders have also been evolving. Instead of just fixing and orienting the sample for observation, these ancillary components of TEMs have gradually been morphing into full-fledged instruments. Increasingly sophisticated devices enable a multitude of experiments performed inside the column of the TEM which includes the manipulation and exposure of discrete nanostructures to a variety of stimuli (heat, pressure, electrical current…). An example is the real-time observation of solid-vapour phase transitions in filled carbon nanotubes acting as interconnects and experiencing the dynamical effects of Joule heating. Together with resolution improvements, these added capabilities represent a paradigmatic shift in how we perceive and work with TEMs. No longer is the electron microscopist limited to simply analyse specimens in “post-mortem” state but (s)he can now take hold of a full set of physical and chemical experiments that encompass a much broader range of scientific interests.Despite all progress, there is a crucial part of the TEM capabilities that is still awaiting a similar leap in resolution: the time-domain. In result of the past dominance of “post-mortem” analysis, a temporal resolution enhancement has been overlooked by most instrument manufacturers and researchers. However, with the growing interest on in situ methodologies, the ability to perform time-resolved experiments has suddenly become a fundamental necessity. Classically, registering tools such as photographic films, charge-coupled devices or video-rated cameras have been adequate to acquire data on events that take place on timescales ranging from seconds to hours. Such temporal resolution is not sufficient for an immensity of life- and materials science-related processes. To illustrate this, Figure 1 shows the before and after states of a ZnS-filled carbon nanotube that was subjected to a pulse of electrical current. Although it is possible to observe clearly the end result, one can only speculate on how the confined semiconductor was released, transported and delivered to the receiving substrate. Considering that the pulse lasted for 250 ms, a temporal resolution of at least 50 ms (or 20 frames per second) for the registering device would be necessary to build a reasonable impression of the mechanism behind this delivery-procedure.PMFJC acknowledges the support from the Fundação para a Ciência e Tecnologia (Ciencia2007 Fellowship and project grant PTDC/EME-PME/112073/2009) and Alexander von Humboldt Foundation (Advanced Researcher Fellowship).
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Peralta, L., E. Mourier, C. Richard, P. Chavette-Palmer, M. Muller, M. Tanter y G. Rus. "117 IN VIVO EVALUATION OF THE CERVICAL STIFFNESS EVOLUTION DURING INDUCED LABOR IN EWES USING ELASTOGRAPHY". Reproduction, Fertility and Development 27, n.º 1 (2015): 150. http://dx.doi.org/10.1071/rdv27n1ab117.
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Despite numerous advances and intensive research in perinatal medicine, spontaneous preterm birth (PTB) is the leading global cause of neonatal mortality and morbidity. On the other hand, labour has to be induced in ~23% of pregnancies worldwide. Both issues may be related to the distensibility of the cervical tissue. Quantitative and objective monitoring of the cervix ripening may provide a complementary method to identify cases at risk of PTB and assess the likelihood of successful induction of labour. Currently, however, no reliable clinical tools for such a quantitative and objective evaluation exist. Elastography aims at imaging tissue stiffness. All elastography techniques rely on the same basics: an external force is applied to the tissue and the resulting movements are then followed. Supersonic shear imaging (SSI) is a dynamic method that uses the propagation of mechanical waves to excite the tissue. Its speed is tracked then by ultrafast imaging, allowing characterisation of stiffness [Bercoff et al. 2004 IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 51, 396–409]. Understanding the mechanisms that take place in normal pregnancy will allow a better comprehension of the cervical remodelling and lead to better methods of diagnosis of PTB and successful induction of labour. In this work, we propose a preliminary assessment of the evolution of stiffness during the cervical maturation process in the sheep. The main goal was to study the feasibility of elastography using SSI to quantify cervical stiffness during the maturation process and to assess the potential of this technique for diagnosis of preterm labour and for labour induction success. Cervical stiffness was quantified, by 2 different operators, in 9 pregnant ewes in vivo by using SSI. The cervical ripening was induced by a dexamethasone injection in 5 animals, and 4 animals constituted the control group. The stiffness of the second ring of the cervix was quantified over a circular region of interest of 5 mm of diameter during vaginal ultrasound examination. Images were acquired every 4 h during 24 h to monitor the cervical maturation induced by the dexamethasone injection. Cervical stiffness was found to decrease significantly throughout the cervical ripening (from 9.5 ± 0.9 kPa to 5.0 ± 0.8 kPa; P = 2.7e–5). The intraobserver and interobserver repeatability of measurements were assessed using Bland-Altman analysis with 95% CI. The principal findings of the study were that elastography measurements using SSI technique were highly reproducible in all cases. Second, stiffness of the uterine cervix decreases throughout the maturation process induced by the dexamethasone injection. Finally, it was possible to quantify the decrease of stiffness through the cervical maturation process. Elastography may be a valuable method to quantify objectively and noninvasively the cervical stiffness in vivo, and ultimately could be a useful tool for the diagnosis of PTB and the assessment of labour induction success.
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Kranz, Christine y Maria Wächtler. "Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes". Chemical Society Reviews, 2021. http://dx.doi.org/10.1039/d0cs00526f.
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This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.
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Bossini, Davide, Dominik Juraschek, Matthias Geilhufe, Naoto Nagaosa, Alexander V. Balatsky, Marija Milanovic, Vladimir Srdic et al. "Magnetoelectrics and Multiferroics: Theory, Synthesis, Characterisation, Preliminary Results and Perspectives for All-Optical Manipulations." Journal of Physics D: Applied Physics, 30 de marzo de 2023. http://dx.doi.org/10.1088/1361-6463/acc8e1.
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Abstract Solid state compounds exhibiting multiple and coupled macroscopic orders, named multiferroics, represent a challenge for both theoretical and experimental modern condensed-matter physics. Spins and the electric polarisation in conventional magnetic and ferroelectric materials can be manipulated on their fundamental timescales, by means of femtosecond laser pulses. In view of the resounding success and popularity of the all-optical approach, it is only natural to wonder about the application of this scheme to study the intrinsic coupling between spins and charges in multiferroics. Deeply fundamental questions arise: can ultrashort laser pulses deterministically activate, enhance or suppress the magnetoelectric coupling on the femtosecond timescale? Can these processes be triggered in a fully coherent fashion, thus being unrestrained by any thermal load? Which mechanism of spin-charge coupling is most favourable to overcome these overarching and daunting challenges? This problem is interdisciplinary in nature, requiring contributions from materials science and condensed matter physics from both theoretical and experimental perspectives. High-quality materials suitable for optical investigations have to be identified, synthetized and characterised. General and valid models offer then a guide to the plethora of possible light-induced processes, resulting in the desired ultrafast multiferroic manipulations. Finally, healthy experimental schemes, able to unambiguously track the ultrafast dynamics of either the ferroelectric or the magnetic order parameter have to be developed and implemented. 

Our motivation to write this review is to lay a broad and multidisciplinary foundation, which may be employed as a starting point for non-equilibrium approaches to the manipulation of the multiferroicity on the femtosecond timescale. This was also one of the main goals of the COST Action MAGNETOFON, whose network constitutes the core of the authors of this review. The present work thus represents a part of the scientific legacy of MAGNETOFON itself.
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Toppi, Marco, Giuseppe Battistoni, Alessandro Bochetti, Patrizia De Maria, Micol De Simoni, Yunsheng Dong, Marta Fischetti et al. "The MONDO Tracker: Characterisation and Study of Secondary Ultrafast Neutrons Production in Carbon Ion Radiotherapy". Frontiers in Physics 8 (19 de noviembre de 2020). http://dx.doi.org/10.3389/fphy.2020.567990.
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Vermeersch, Bjorn, Je-Hyeong Bahk, James Christofferson y Ali Shakouri. "Design and thermoreflectance imaging of high-speed SiGe superlattice microrefrigerators". MRS Proceedings 1329 (2011). http://dx.doi.org/10.1557/opl.2011.1232.
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ABSTRACTOver the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterisation techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium. In this paper, we present the design and thermal characterisation of integrated highspeed microcoolers based on SiGe superlattices. The electrode metalisation is laid out as a coplanar waveguide, enabling to supply electrical pulses with short rise times to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ∼100ns temporal and submicron spatial resolution without the need to scan the sample. Net cooling in the 2 degree range is observed, with response times well below 1μs. This is almost two orders of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. This provides potential to study ultrafast electron-phonon interactions during Peltier effects.
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Hajivassiliou, George, Marios Georgios Kassapis y John Tisch. "Rapid retrieval of femtosecond and attosecond pulses from streaking traces using convolutional neural networks". New Journal of Physics, 29 de agosto de 2023. http://dx.doi.org/10.1088/1367-2630/acf4b8.
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Abstract Attosecond streaking is a powerful and versatile technique that allows the full-field characterisation of femtosecond to attosecond optical pulses. It has been instrumental in the verification of attosecond pulse generation and probing of ultrafast dynamics in matter. Recently, machine learning (ML) has been applied to retrieve the fields from streaking data. This offers a number of advantages compared with traditional iterative algorithms, including faster processing and better resilience to noise. Here, we implement a ML approach based on convolutional neural networks (CNNs) and limit the search to physically realistic pulses that can be specified with a small number of parameters. This leads to substantial reductions in both training and retrieval times, enabling near kHz retrieval rates. We examine how the retrieval performance is affected by noise, and for the first time in this context, study the effect of missing data. We show that satisfactory retrievals are still possible with signal to noise ratios as low as 10, and with up to 40% of data missing.
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Reavette, Ryan M., Anenta Ramakrishnan, Ethan M. Rowland, Meng-Xing Tang, Jamil Mayet y Peter D. Weinberg. "Detecting heart failure from B-mode ultrasound characterisation of arterial pulse waves". American Journal of Physiology-Heart and Circulatory Physiology, 24 de mayo de 2024. http://dx.doi.org/10.1152/ajpheart.00219.2024.
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This study investigated the sensitivity and specificity of identifying heart failure with reduced ejection fraction (HFrEF) from measurements of the intensity and timing of arterial pulse waves. Previously validated methods combining ultrafast B-mode ultrasound, plane-wave transmission, singular value decomposition and speckle tracking were used to characterise the compression and decompression ("S" and "D") waves occurring in early and late systole, respectively, in carotid arteries of outpatients with left ventricular ejection fraction (LVEF) <40%, determined by echocardiography, and signs and symptoms of heart failure, or with LVEF ≥50% and no signs or symptoms of heart failure. On average, the HFrEF group had significantly reduced S-wave intensity and energy, a greater interval between the R wave of the ECG and the S wave, a reduced interval between the S and D waves, and an increase in the S-wave shift, a novel metric that characterises the shift in timing of the S-wave away from the R wave of the ECG and towards the D wave (all p<0.01). Receiver operating characteristics (ROCs) were used to quantify for the first time how well wave metrics classified individual participants. S-wave intensity and energy gave areas under the ROC of 0.76-0.83, the ECG- S wave interval gave 0.85-0.88, and the S-wave shift gave 0.88-0.92. Hence the methods, which are simple to use and do not require complex interpretation, provide sensitive and specific identification of HFrEF. If similar results were obtained in primary care, they could form the basis of techniques for heart failure screening.
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Jagt, Robert A., Ivona Bravić, Lissa Eyre, Krzysztof Gałkowski, Joanna Borowiec, Kavya Reddy Dudipala, Michał Baranowski et al. "Layered BiOI single crystals capable of detecting low dose rates of X-rays". Nature Communications 14, n.º 1 (28 de abril de 2023). http://dx.doi.org/10.1038/s41467-023-38008-4.
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AbstractDetecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1 × 103 μC Gyair−1 cm−2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s−1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications.
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Wei qian-yi, Ni jie-lei, Li Ling, Zhang yu-quan, Yuan xiao-cong y Min chang-jun. "Research progress of ultra-high spatiotemporal resolved microscopy". Acta Physica Sinica, 2023, 0. http://dx.doi.org/10.7498/aps.72.20230733.
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High-resolution microscopy has opened the door to the exploration of the micro-world, while femtosecond laser has provided a measurement method for the detection of ultrafast physical/chemical phenomena. Combination of these two techniques can produce new microscopic techniques with both ultra-high spatial resolution and ultra-fast temporal resolution, and thus has great importance for exploring new scientific phenomena and mechanisms at extremely small spatial and temporal scales. This paper reviews the basic principles and properties of main international microscopic techniques with ultra-high time- and space-resolution, and introduces the latest research progress of their applications in varies fields such as characterization of optoelectronic materials and devices, monitoring of femtosecond laser micromachining, and detection of surface plasmon excitation dynamics. In order to present these research works systematically, we classify these techniques based on time and space dimensions, including the near-field multi-pulse imaging techniques, the far-field multi-pulse imaging techniques, and the far-field single-pulse imaging techniques. In chapter 2, we introduce the principles and characteristics of the ultra-high spatiotemporal resolved microscopic techniques. The near-field multi-pulse spatiotemporal microscopic techniques based on nano-probe are described in Section 2.1, which show the combination of common near-field imaging techniques such as AFM(Atomic Force Microscopy, AFM), NSOM(near-field scanning optical microscopy, NSOM), STM(Scanning Tunneling Microscope, STM) and the ultra-fast temporal detection of pump-probe technique. In section 2.2 we introduce the far-field multi-pulse spatiotemporal microscopic techniques. In contrast to near-field cases, the far-field spatiotemporal microscopic techniques have lower spatial resolution but bring more advantages of being non-invasive, non-contact, wider field of view, and faster imaging speed. In section 2.3 we introduce the far-field single-pulse spatiotemporal microscopic techniques, which use a single ultrafast light pulse to capture dynamic process at different moments in time, enabling real-time imaging of ultrafast phenomena. In chapter 3, the advances in the application of the ultra-high spatiotemporal resolved microscopic techniques have been introduced in many frontier areas, including the monitoring of femtosecond laser micromachining in section 3.1, the detection of optoelectronic materials/devices in section 3.2, the characterization of surface plasmon dynamics in section 3.3. Finally, in chapter 4, we summarize the features of all above introduced spatiotemporal microscopic techniques in a table, including the spatial/temporal resolution, advantages and disadvantages of each technique, and provides an outlook on future development trends in this research field. Looking ahead, ultra-high spatiotemporal resolved microscopy is rapidly evolving towards the trend of "smaller, faster, smarter and more extensive". Its development not only promotes the research progress of the microscopy technology, but also provides a powerful tool for various applications such as precision machining, two-dimensional material dynamics, optoelectronic device design and characterisation.
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Lubel, Emma, Bruno Grandi-Sgambato, Deren Y. Barsakcioglu, Jaime Ibanez, Meng-Xing Tang y Dario Farina. "Kinematics of individual muscle units in natural contractions measured in vivo using ultrafast ultrasound". Journal of Neural Engineering, 24 de agosto de 2022. http://dx.doi.org/10.1088/1741-2552/ac8c6c.
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Abstract Objective: The study of human neuromechanical control at the motor unit (MU) level has predominantly focussed on electrical activity and force generation, whilst the link between these, i.e., the muscle deformation, has not been widely studied. To address this gap, we analysed the kinematics of muscle units in natural contractions. Approach: We combined high-density surface electromyography (HDsEMG) and ultrafast ultrasound (US) recordings, at 1000 frames per second, from the tibialis anterior muscle to measure the motion of the muscular tissue caused by individual MU contractions. The MU discharge times were identified online by decomposition of the HDsEMG and provided as biofeedback to 12 subjects who were instructed to keep the MU active at the minimum discharge rate (9.8 ± 4.7 pulses per second; force less than 10% of the maximum). The series of discharge times were used to identify the velocity maps associated with 51 single muscle unit movements with high spatio-temporal precision, by a novel processing method on the concurrently recorded US images. From the individual MU velocity maps, we estimated the region of movement, the duration of the motion, the contraction time, and the excitation-contraction (E-C) coupling delay. Main results: Individual muscle unit motions could be reliably identified from the velocity maps in 10 out of 12 subjects. The duration of the motion, total contraction time, and E-C coupling were 17.9 ± 5.3 ms, 56.6 ± 8.4 ms, and 3.8 ± 3.0 ms (n = 390 across 10 participants). The experimental measures also provided the first evidence of muscle unit twisting during voluntary contractions and MU territories with distinct split regions. Significance: The proposed method allows for the study of kinematics of individual MU twitches during natural contractions. The described measurements and characterisations open new avenues for the study of neuromechanics in healthy and pathological conditions.