Pulse Burst Research Articles

Abstract 13848: Establishing an Optogenetic in vitro Model to Study Atrial Fibrillation Based on Human Engineered Heart Tissue

Introduction: Atrial fibrillation (AF) is a type of sustained cardiac tachyarrhythmia in which the atrial rate can reach up to 300-600 beats per minute (BPM). The elucidation of the mechanisms of AF and the search for new therapeutic methods is hampered by a lack of suitable in vitro models. In previous studies, chronic optical tachypacing induced arrhythmia vulnerability in ventricular-like human engineered heart tissue (EHT). Here, we aimed to optimize an AF in vitro model based on optogenetics, EHT and a novel optogenetic pacing platform. Methods: Human induced pluripotent stem cells (hiPSC) stably expressing Volvox-Channelrhodopsin-1 (VChR1) were differentiated into atrial-like cardiomyocytes (CM) by treatment with 1 μM retinoic acid (RA) at the cardiac induction stage. CMs were used to cast fibrin-based 24-well format EHTs. The expression of atrial/ventricular markers was validated by qPCR, and action potential (AP) recordings with atrial-specific channel inhibitors were performed on at least one-month-old EHTs. A novel custom-designed circuit board with a programmable microcontroller and 24 RGB addressable light-emitting diodes (LED) was used for optical tachypacing. 25-30 days old EHTs were stimulated by 90 s bursts of blue light pulses (5 Hz), separated by 10 s breaks for 35-40 days. Non-paced and paced EHTs were characterized regarding contractility and gene expression. Results: EHTs had a high expression of atrial mRNA markers and displayed atrial-like AP. A flexible and straightforward optogenetic tool was built to make chronic tachypacing feasible. Using this novel platform, EHTs were successfully paced at 5 Hz for up to 40 days. Compared with non-paced EHTs, paced EHTs showed a different spontaneous contractility profile, higher expression of stress genes, and released more troponin I into the medium. Conclusions: We have successfully generated an in vitro model to mimic the status of atria with a high frequency (300 BPM) during AF. Paced EHTs displayed stress signs, partially resembling the changes of atria in AF. In the future, we could utilize this model to study the underlying mechanisms and assess proarrhythmic and antiarrhythmic drug effects.

Sub-ns-pulsed laser cleaning of an archaeological bone from the Sierra de Atapuerca, Spain: a case study

Controlled laser irradiation parameters using recently developed sub-nanosecond pulsed laser technology with emission wavelength in the near Infrared regime (1064 nm) have been assessed on a Pleistocene bone from the archaeological site of Sierra de Atapuerca, Spain. Burst pulse mode was employed to explore contaminant removal efficiency, while at the same time, assessing the degree of damage produced to the underlying original substrate surface. The surface morphology and composition of the deteriorated bone have been characterized, along with the effects of laser irradiation at 1064 nm, using Optical Microscopy (OM), Scanning Electron Microscopy–with Energy Dispersive X-ray Spectrometry (SEM–EDS), and X-ray Photoelectron Spectroscopy (XPS). The most effective laser cleaning parameters in burst mode have been identified in order to optimize the emission parameters of the laser, thus localizing its interaction within the outermost layers of contaminants and degradation products, avoiding damage to the underlying original bone surface. Hence, threshold cleaning and substrate damage values have been determined for this new sub-ns laser, paving the way to safer laser cleaning procedures that may be useful for the effective conservation of bone archaeological artifacts.

Ultrashort-pulsed laser processing with spatial and temporal beam shaping using a spatial light modulator and burst modes

We report on the effect of simultaneous spatial and temporal beam shaping on the ablation rate, ablation efficiency and the resulting surface characteristics of micromachined stainless steel using ultrashort-pulsed lasers. Beam shaping and the use of pulse bursts are promising methods to allocate the over the last decades increasing laser power of ultrashort-pulsed lasers in ablation processes. While the individual effects of beam shaping and pulse bursts on the ablation characteristics have recently been examined, the combination of both has not yet been adequately investigated. Using a spatial light modulator to generate different spot distributions with up to six spots and different separations it is possible to spatially distribute the available laser power. In combination with temporal beam shaping using a 200 kHz repetition rate and pulse bursts with a 40 MHz intra-burst rate, we investigate the influences in a scanning-based process and find an increasing ablation rate and efficiency for higher fluences. Subsequently using bursts in combination with a multi-spot beam profile, we found a distinctive emergence of cone like protrusions and a smoothing effect for fluences between 1.5 J/cm² and 3 J/cm² with six spot beam profile.

Submicrometer surface shaping and surface repair of metals via laser ablation

Metal foils with precise thicknesses are required for use as laser targets in high energy density physics experiments. To achieve the micrometer scale tolerances accompanying these applications, the authors have developed a laser ablation technique utilizing burst pulses from a femtosecond laser operated in concert with a high-speed galvo scanning system. For authors’ application, an optimum laser processing format consisted of 250 fs laser pulses arranged in pairs, with the pulse separation in each pair equal to 400 ps and high galvo scanning speeds (∼2000 mm/s). These parameters enable the authors to thin and shape metals (depleted uranium, beryllium, tantalum, and iron) and remove unwanted variations in foil thickness over meaningful foil sizes. Double femtosecond pulses enable ablation that mitigates some undesirable pre-existing surface structures such as residual scalloping and scratches and lead to a surface roughness near 300 nm. Precise ablation rates were correlated to laser processing area and foil thickness. Finally, ablation rates were correlated to bulk surface temperature by tracking workpiece heating during ablation. These techniques yield the ability to apply highly controlled thickness maps with micrometer accuracy and tens of nanometer precisions. This control has demonstrated the ability to level foils with micrometer scale thickness deformities and create various programmed patterns that are difficult or out of reach of traditional machining methods.

Compensation of target signal migration in the multichannel matched filter

The study proposes a method of target range signal compensation within different pulse burst periods during in-period matched filtering. The proposed method allows to reduce losses in case of long-term signal integration. This is especially important for signals highly sensitive to Doppler frequency shift. In terms of this parameter, a multichannel configuration of the compression filter is a probable solution for such signals. Based on the example of a signal with non-linear frequency modulation and the results of simulation modelling, we analyse the effectiveness of migration compensation and give recommendations on channel multiplexing for the compression filter with an integrated migration compensator.

Effectiveness of target signal migration compensation at compression filter output

The paper describes a case study with the target moving at unknown radial velocity. Exemplified by the frequency modulation signal, the study presents the analysis of specific features of channel multiplexing for the target range migration compensator at the compression filter output within pulse burst periods. Channel multiplexing allows to reduces losses in case of long-term signal integration. The solution effectiveness is estimated and a method is proposed for compensator channel multiplexing ensuring protection against unwanted signal expansion.

Imaging with diffractive axicons rapidly milled on sapphire by femtosecond laser ablation

Fabrication of large area (sub-1 cm cross-section) micro-optical components in a short period of time (~ 10 min) and with lesser number of processing steps is highly desirable and cost-effective. In the recent years, femtosecond laser fabrication technology has revolutionized the field of manufacturing by offering the above capabilities. In this study, a fundamental diffractive optical element, binary axicon–axicon with two phase or amplitude levels, has been designed in three configurations namely conventional axicon, photon sieve axicon (PSA) and sparse PSA and directly milled onto a sapphire substrate. The fabrication results revealed that a single pulse burst fabrication can produce a flatter and smoother profile than pulse overlapped fabrication which gives rise to surface damage and increased roughness. The fabricated elements were processed in IsoPropyl alcohol and potassium hydroxide to remove debris and redeposited amorphous sapphire. An incoherent illumination was used for optical testing of the components and a non-linear optical filter was used for cleaning the noisy images generated by the diffractive optical elements.

Disrupting Short-Term Memory Maintenance in Premotor Cortex Affects Serial Dependence in Visuomotor Integration.

Human behavior is biased by past experience. For example, when intercepting a moving target, the speed of previous targets will bias responses in future trials. Neural mechanisms underlying this so-called serial dependence are still under debate. Here, we tested the hypothesis that the previous trial leaves a neural trace in brain regions associated with encoding task-relevant information in visual and/or motor regions. We reasoned that injecting noise by means of transcranial magnetic stimulation (TMS) over premotor and visual areas would degrade such memory traces and hence reduce serial dependence. To test this hypothesis, we applied bursts of TMS pulses to right visual motion processing region hV5/MT+ and to left dorsal premotor cortex (PMd) during intertrial intervals of a coincident timing task performed by twenty healthy human participants (15 female). Without TMS, participants presented a bias toward the speed of the previous trial when intercepting moving targets. TMS over PMd decreased serial dependence in comparison to the control Vertex stimulation, whereas TMS applied over hV5/MT+ did not. In addition, TMS seems to have specifically affected the memory trace that leads to serial dependence, as we found no evidence that participants' behavior worsened after applying TMS. These results provide causal evidence that an implicit short-term memory mechanism in premotor cortex keeps information from one trial to the next, and that this information is blended with current trial information so that it biases behavior in a visuomotor integration task with moving objects.SIGNIFICANCE STATEMENT Human perception and action are biased by the recent past. The origin of such serial bias is still not fully understood, but a few components seem to be fundamental for its emergence: the brain needs to keep previous trial information in short-term memory and blend it with incoming information. Here, we present evidence that a premotor area has a potential role in storing previous trial information in short-term memory in a visuomotor task and that this information is responsible for biasing ongoing behavior. These results corroborate the perspective that areas associated with processing information of a stimulus or task also participate in maintaining that information in short-term memory even when this information is no longer relevant for current behavior.

Effects of low-frequency repetitive transcranial magnetic stimulation combined with cerebellar continuous theta burst stimulation on spasticity and limb dyskinesia in patients with stroke

BackgroundRepetitive transcranial magnetic stimulation (rTMS) has been reported to treat muscle spasticity in post-stroke patients. The purpose of this study was to explore whether combined low-frequency rTMS (LF-rTMS) and cerebellar continuous theta burst stimulation (cTBS) could provide better relief than different modalities alone for muscle spasticity and limb dyskinesia in stroke patients.MethodsThis study recruited ninety stroke patients with hemiplegia, who were divided into LF-rTMS+cTBS group (n=30), LF-rTMS group (n=30) and cTBS group (three pulse bursts at 50 Hz, n=30). The LF-rTMS group received 1 Hz rTMS stimulation of the motor cortical (M1) region on the unaffected side of the brain, the cTBS group received cTBS stimulation to the cerebellar region, and the LF-rTMS+cTBS group received 2 stimuli as described above. Each group received 4 weeks of stimulation followed by rehabilitation. Muscle spasticity, motor function of limb and activity of daily living (ADL) were evaluated by modified Ashworth Scale (MAS), Fugl-Meyer Assessment (FMA) and Modified Barthel Index (MBI) scores, respectively.ResultsThe MAS score was markedly decreased, FMA and MBI scores were markedly increased in the three groups after therapy than before therapy. In addition, after therapy, LF-rTMS+cTBS group showed lower MAS score, higher FMA and MBI scores than the LF-rTMS group and cTBS group.ConclusionMuscle spasticity and limb dyskinesia of the three groups are all significantly improved after therapy. Combined LF-rTMS and cTBS treatment is more effective in improving muscle spasticity and limb dyskinesia of patients after stroke than LF-rTMS and cTBS treatment alone.

Chemical etching of fused silica after modification with two-pulse bursts of femtosecond laser.

Bursts of femtosecond laser pulses were used to record internal modifications inside fused silica for selective chemical etching. Two-pulse bursts with a variable energy ratio between those pulses at a fixed inter-pulse duration of 14.5 ns were applied for the first time. The selective chemical etching rate of the laser-modified material with the burst of two pulses was compared to the single-pulse regime when etching in HF and KOH etchants. The advantage of the burst-mode processing was demonstrated when etching was performed in the KOH solution. More regular nanogratings were formed, and the etching initiation was more stable when burst pulses were applied for fused silica modification. The vertical planar structures were obtained using the two-pulse bursts with an energy ratio of 1:2, increasing the etching rate by more than 35% compared to the single-pulse processing. The highest ever reported selectivity of 1:2000 was demonstrated by introducing the two-pulse burst mode.

Investigating Spinner Dolphin (Stenella longirostris) Occurrence and Acoustic Activity in the Maui Nui Region

Hawaiian spinner dolphins (Stenella longirostris) rest during the day in the islands’ coastal waters where they are susceptible to human disturbance. Due to concerns over the negative impacts of human activity, the Pacific Islands Regional Office of the National Oceanic and Atmospheric Administration (NOAA) has proposed a 50-yard approach rule for spinner dolphins and is also considering time-area closures of certain important spinner dolphin resting areas. However, gaps still persist in the understanding of how spinner dolphin populations on different Hawaiian Islands use coastal waters, raising questions about the efficacy of the proposed rules. To better understand how spinner dolphins use the waters in the Maui Nui region (Maui, Moloka‘i, Lāna‘i, and the ‘Au‘au channel), a study was conducted using a combination of passive acoustic monitoring and vessel surveys to document spinner dolphin occurrence and movements. Bottom-moored acoustic recorders were deployed at eight locations in Maui Nui, and at one previously established resting bay off west O‘ahu for comparison. The amount of whistles, clicks, and burst pulses at each location was quantified and averaged by the hour of the day. Acoustic activity was greater at the O‘ahu site than at any of the Maui Nui sites, and was greatest between sunrise and noon. Acoustic activity and vessel surveys both reveal that spinner dolphins occur and exhibit resting behaviors in the ‘Au‘au channel between Maui and Lāna‘i, and also along west Maui and southeast Lāna‘i. Spinner dolphins resting in a channel between islands appears to be unique to Maui Nui and differs from resting patterns described along Hawai‘i Island and O‘ahu. Because spinner dolphins appear to use both the coastlines and the channel to rest in Maui Nui, the 50-yard approach rule appears to be a more suitable management option for the region than time-area closures.

How Temporal Symmetry Defines Morphology in BATSE Gamma-Ray Burst Pulse Light Curves

Abstract We present compelling evidence that most gamma-ray burst (GRB) pulse light curves can be characterized by a smooth single-peaked component coupled with a more complex emission structure that is temporally symmetric around the time of the pulse peak. The model successfully fits 86% of Burst and Transient Source Experiment GRB pulses bright enough for structural properties to be measured. Surprisingly, a GRB pulse’s light-curve morphology can be accurately predicted by the pulse asymmetry and the stretching/compression needed to align the structural components preceding the temporal mirror with the time-reversed components following it. Such a prediction is only possible because GRB pulses exhibit temporal symmetry. Time-asymmetric pulses include fast rise exponential decays, rollercoaster pulses, and asymmetric u-pulses, while time-symmetric pulses include u-pulses and crowns. Each morphological type is characterized by specific asymmetries, stretching parameters, durations, and alignments between the smooth and structured components, and a delineation in the asymmetry/stretching distribution suggests that symmetric pulses and asymmetric pulses may belong to separate populations. Furthermore, pulses belonging to the short GRB class exhibit similar morphologies to the long GRB class, but appear to simply occur on shorter timescales.

Study on X-ray Emission Using Ultrashort Pulsed Lasers in Materials Processing.

The interaction of ultrashort pulsed laser radiation with intensities of 1013 W cm−2 and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 1012 W∙cm−2 < I0 < 5.2 × 1016 W∙cm−2, up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as > 45 mSv h−1 can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.

X-ray generation by laser ablation using MHz to GHz pulse bursts

Ultrashort pulsed laser sources generating pulse trains (bursts) with intra-burst repetition rates in the MHz and the GHz regime enable an efficient production of microstructures with a high surface quality. However, x-ray radiation can be generated during the laser micromachining using large intensities of the laser radiation and its interaction with the ablation cloud or high-density plasma. Therefore, the authors report on the interaction of bursts with a wavelength of 1030 nm and pulse durations of 0.24 and 10 ps with intra-burst repetition rates of 65 MHz (MHz-burst mode) and 2.5 GHz (GHz-burst mode) as well as a combination of both burst modes, called BiBurst mode, with stainless steel, and the x-rays are generated. The x-ray dose rates determined in the respective burst modes are compared and discussed with those of conventional ultrafast laser radiation (single-pulse mode). Furthermore, a theoretical model is used to calculate the expected x-ray dose rates. In the investigated parameter range, the highest dose rates of more than 105μSv/h are determined at a specific burst setting. Compared to the single-pulse mode, significantly higher dose rates are determined using the burst mode with the same total intensity. Based on the results of this study, it can be stated that the interaction of ultrafast laser radiation in the burst mode with a generated ablation cloud or high-density plasma plays a major role in x-ray generation and the resulting x-ray dose rates.

Multi beam microprocessing for printing and embossing applications with high power ultrashort pulsed lasers

Abstract Micro structuring of surfaces is of great interest for various applications, e.g. for the tooling industry, the printing industry and for consumer goods. In suitable mass production applications, such as injection molding or roll-to-roll processing for various markets, the final product could be equipped with new properties, such as hydrophilic behavior, adjustable gloss level, soft-touch behavior, light management properties etc. To generate functionalities at reasonable cost, embossing dies can be augmented with additional micro/nano-scale structure using laser ablation technologies. Despite the availability of ultrashort pulsed (USP) high power lasers (up to several hundred watts), it is still a challenge to structure large areas, as required on embossing rolls, in an acceptable processing time for industrial production. In terms of industrial implementation, direct digital transfer is a limiting factor for ultrahigh resolution. Shorter machining times by further increasing spot or workpiece motion are limited. Enlarging the ablation diameter, and thus the tool diameter, delivers a higher ablation rate with the comparable ablation quality, but entails a reduction in resolution. While maintaining the achieved state-of-the-art performance, upscaling of single modulated lasers provides a less demanding way to increase productivity. In the processing of steel surfaces, an increase in material removal can also be achieved by using pulse burst. In this work, the parallel process of single modulated multi laser sources is compared with a laser source split by diffractive optical elements (DOE) for applications in a cylinder micro patterning system. A newly developed highly compact ps laser with repetition rates up to 8 MHz and an average power of 300 or 500 W was divided into 8 or 16 parallel beamlets by a DOE. The ablation rate of each approach was investigated by typical microstructures on copper surfaces. At surface speeds of 10 m/s and a resolution of 5080 dpi, an ablation rate of up to 27 mm³/min was achieved. Different functional surface geometries were realized on an embossing roll as master, which is used for replication of the structures in roll-to-roll processes. Functional structures, such as friction reduction, improved soft touch or light guiding elements on large surfaces are demonstrated.

Generating bursts of femtosecond laser pulses with a tunable delay and envelope in a folded Michelson interferometer.

Bursts of 16 femtosecond laser pulses are generated in a fourfold Michelson interferometer with a tunable delay and envelope. Solutions are given to solve the "forward problem" (bursts from a given parameter set) and "inverse problem" (obtain parameter set from a given burst). Three types of bursts are generated experimentally with envelopes suitable for applications in laser materials processing and the generation of terahertz radiation.

Laser-based material interactions and ablation processes by bursts of 70 ps pulses.

The intermediate pulse duration regime between typical ultra-short and nanosecond pulses has been investigated using MHz-range bursts of 70 ps pulses emitted from a custom-made fiber laser source. The goal of this study was to observe and understand the processes involved during laser ablation on the timescales from picoseconds to nanoseconds, relevant due to pulses in bursts. We developed material processing approaches that enable similar behaviour as single 70 ps pulse ablation to ultra-short pulses in terms of quality and burst-mode behaviour like nanosecond pulses in terms of efficiency. The variability of the fiber laser operation modes was studied and compared to both ultra-short and nanosecond pulses from standard laser sources.

Acoustic classification of false killer whales in the Hawaiian islands based on comprehensive vocal repertoire.

Use of underwater passive acoustic datasets for species-specific inference requires robust classification systems to identify encounters to species from characteristics of detected sounds. A suite of routines designed to efficiently detect cetacean sounds, extract features, and classify the detection to species is described using ship-based, visually verified detections of false killer whales (Pseudorca crassidens). The best-performing model included features from clicks, whistles, and burst pulses, which correctly classified 99.6% of events. This case study illustrates use of these tools to build classifiers for any group of cetacean species and assess classification confidence when visual confirmation is not available.

Laser ablation of silicon with THz bursts of femtosecond pulses

In this work, we performed an experimental investigation supported by a theoretical analysis, to improve knowledge on the laser ablation of silicon with THz bursts of femtosecond laser pulses. Laser ablated craters have been created using 200 fs pulses at a wavelength of 1030 nm on silicon samples systematically varying the burst features and comparing to the normal pulse mode (NPM). Using bursts in general allowed reducing the thermal load to the material, however, at the expense of the ablation rate. The higher the number of pulses in the bursts and the lower the intra-burst frequency, the lower is the specific ablation rate. However, bursts at 2 THz led to a higher specific ablation rate compared to NPM, in a narrow window of parameters. Theoretical investigations based on the numerical solution of the density-dependent two temperature model revealed that lower lattice temperatures are reached with more pulses and lower intra-burst frequencies, thus supporting the experimental evidence of the lower thermal load in burst mode (BM). This is ascribed to the weaker transient drop of reflectivity, which suggests that with bursts less energy is transferred from the laser to the material. This also explains the trends of the specific ablation rates. Moreover, we found that two-photon absorption plays a fundamental role during BM processing in the THz frequency range.

High speed pulsed laser cutting of anode material for a Li-ion battery in burst mode

The bursts of picosecond laser pulses have nanosecond-level short interval delay. These bursts contain a variable number of sub-pulses, which are used for laser cutting of copper current collector and graphite anode material for Li-ion battery anode. The influences of 2–10 sub-pulses on kerf edges were studied and were compared with that of a single pulse. The shapes of anode edge cut under different conditions, obtained using scanning electron microscopy (SEM), revealed that using burst mode would yield a smaller heat-affected zone (HAZ) of the copper current collector and smaller delamination width of graphite anode material. The capability of laser cutting of anode was characterized with maximum single-time cutting speed. Results showed that the cutting efficiency was raised evidently with the increase in the number of pulses in a burst, and the maximum cutting speeds for the copper current collector and graphite anode material could reach 3,800 mm/s and 500 mm/s respectively.