Frequency Research Articles

Real-time 3D MR guided radiation therapy through orthogonal MR imaging and manifold learning.

In magnetic resonance image (MRI)-guided radiotherapy (MRgRT), 2D rapid imaging is commonly used to track moving targets with high temporal frequency to minimize gating latency. However, anatomical motion is not constrained to 2D, and a portion of the target may be missed during treatment if 3D motion is not evaluated. While some MRgRT systems attempt to capture 3D motion by sequentially tracking motion in 2D orthogonal imaging planes, this approach assesses 3D motion via independent 2D measurements at alternating instances, lacking a simultaneous 3D motion assessment in both imaging planes. We hypothesized that a motion model could be derived from prior 2D orthogonal imaging to estimate 3D motion in both planes simultaneously. We present a manifold learning technique to estimate 3D motion from 2D orthogonal imaging. Five healthy volunteers were scanned under an IRB-approved protocol using a 3.0 T Siemens Skyra simulator. Images of the liver dome were acquired during free breathing (FB) with a 2.6mm × 2.6mm in-plane resolution for approximately 10min in alternating sagittal and coronal planes at ∼5 frames per second. The motion model was derived using a combined manifold learning and alignment approach based on locally linear embedding (LLE). The model utilized the spatially overlapping MRI signal shared by both imaging planes to group together images that had similar signals, enabling motion estimation in both planes simultaneously. The model's motion estimates were compared to the ground truth motion derived in each newly acquired image using deformable registration. A simulated target was defined on the dome of the liver and used to evaluate model performance. The Dice similarity coefficient and distance between the model-tracked and image-tracked contour centroids were evaluated. Motion modeling error was estimated in the orthogonal plane by back-propagating the motion to the currently imaged plane and by interpolating the motion between image acquisitions where ground truth motion was available. The motion observed in the healthy volunteer studies ranged from 12.6 to 38.7mm. On average, the model demonstrated sub-millimeter precision and>0.95 Dice coefficient compared to the ground truth motion observed in the currently imaged plane. The average Dice coefficient and centroid distance between the model-tracked and ground truth target contours were 0.96±0.03 and 0.26mm±0.27mm respectively across all volunteer studies. The out-of-plane centroid motion error was estimated to be 0.85mm±1.07mm and 1.26mm±1.38mm using the back-propagation (BP) and interpolation error estimation methods. The healthy volunteer studies indicate promising results using the proposed motion modeling technique. Out-of-plane modeling error was estimated to be higher but still demonstrated sub-voxel motion accuracy.

Dynamic Graphical Models with Variable Selection for Effective Connectivity

This paper proposes a novel approach that combines dynamic linear models applied to graph data and variable selection through spike-and-slab priors. The new class of models, called Dynamic Graphical Variable Selection, is used to infer effective connectivity in certain brain regions allowing both connectivity weights and graphical structure to vary over time. One advantage of our method is that as the graphical structure is estimated inferentially, the computational cost is reduced. That way our methodology can accommodate high-dimensional data, such as large networks observed through long periods of time. We illustrate our methodology via numerical experiments with simulated and synthetic data, and then applied to fNIRS real data. The obtained results showed that the static version of our model is competitive against previous methodologies and demands a lower computational cost. Our model is more flexible than the previous methodologies by allowing the graphical structure to vary over time.

Aqueous bromide oxidized with pulsed corona discharge

Bromide in treated natural waters is oxidized to toxic bromate with ozone. For the novelty of pulsed corona discharge (PCD) in water treatment, it’s potential in bromide transformation needs clarification. Experimental research into bromide-bromate conversion dependent on control parameters, including pulse repetition frequency, pH, Br- concentration, and presence of organics, was undertaken. Bromide transformed to bromate in acidic and circum-neutral media; transformation of bromate to bromide was also observed. Bromide was poorly oxidized in alkaline media. Oxidation of bromide was obstructed with oxalate. Formation of nitrate was noticeably decelerated by bromate.

Enhanced relaxor behavior and excellent anti-fatigue performance in lanthanum-modified Sr0.7Bi0.2TiO3 ceramics

Ferroelectric ceramics have attracted increasing attention for their wide applications in pulsed-power systems. Wherein relaxor ferroelectric ceramics with fast discharge speed and great stability are considered promising candidate materials. In this study, we introduced the rare-earth element La into Sr0.7Bi0.2TiO3 relaxor ferroelectric ceramics. Due to the lattice distortion triggered by La ions, the microdomains broke and transformed into high dynamic polar nano-regions (PNRs), thus improving the relaxor characteristic. We have identified this phenomenon through various characterizations, including the continuous waveform P-E (CWPE) test and the first-order reversal curve (FORC) test. With the high dynamic PNRs replacing the original domains, a better energy storage performance was achieved (Wrec = 3.22J/cm3 and η = 88.83% under 330kV/cm), and the pulse charge-discharge speed has been further improved (t0.9 = 70-81ns under 40-200kV/cm). Simultaneously, Sr0.658Bi0.188La0.04TiO3 ceramics owed remarkable anti-fatigue characteristics (0.40% attenuation after 107 cycles in the P-E test, 5.82% attenuation after 106 times in the pulse repetition frequency discharge test) due to the weak links of PNRs, revealing the great application potential of the Sr0.658Bi0.188La0.04TiO3 ceramic.

Spatially Explicit Assessments of Heat-Related Health Risks: A Literature Review

Extreme heat events, or heatwaves, exert significant impacts on human society, ecosystems, and the economy. The continuous development of remote sensing technology has facilitated the acquisition of high-quality data for assessing health risks associated with these extreme heat events. This study systematically reviews the evaluation factors and assessment framework for a spatially explicit assessment of heat-related health risks. The contribution of geospatial big data, with a particular focus on satellite observations, to these assessments was investigated. The Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat surface temperature (LST) are identified as the two most widely utilized data sources for mapping heat hazards. The incorporation of multi-sensor observations, along with the implementation of spatiotemporal fusion and downscaling techniques, enhances both the spatial resolution and temporal frequency of heat hazard characterization. It is essential to consider issues of justice and equality in heat-resilient planning and mitigation practices. Integrating heatwave risk assessment results with analyses of urban morphology, land use functions and infrastructure can provide critical information for government agencies to strategically plan urban layout, functions, and public service facilities while optimizing and enhancing urban green infrastructures.

Hollow MoS2-Co3S4 heterostructures derived from ZIF-67 as saturable absorber for generating gigahertz repetition frequency mode-locked fiber lasers

In practical applications, high repetition frequency (HRF) lasers are widely used in the fields of material processing, laser communication and medical therapy. In this work, the passive harmonic mode-locked (HML) technique is used to realize a HRF fiber laser in GHz level, effectively overcoming the limitations of traditional fiber laser structures. In order to maximize the advantages of nanomaterials, we synthesized high-purity hollow MoS2-Co3S4 heterojunction materials by hydrothermal method. Then, a “sandwich” structure saturable absorber (SA) was prepared by photodeposition, and its nonlinear optical properties were investigated. The results demonstrate that the modulation depth and saturation intensity of MoS2-Co3S4 SA are 24 % and 4.53 MW/cm2, respectively. The fiber laser based on MoS2-Co3S4 SA could realize the fundamental frequency mode-locked at 23.11 MHz with a pulse width of 0.99 ps. In addition, various high-order HML are realized with this laser system capable of stable operation for extended periods up to the highest repetition frequency of 1.109 GHz achieved thus far in this field. This research provides strong support to promote the development and commercialization of HRF fiber lasers.

Precise passive synchronization of dissipative soliton resonance lasers via cascaded cross-phase- and cross-absorption-modulation for mid-infrared mode-locked pulse generation

We present the first demonstration of utilizing cascaded cross-phase and cross-absorption modulation effects for synchronizing the pulse parameters of two mode-locked fiber lasers operating in the dissipative soliton resonance regime. By utilizing the cross-absorption modulation effect we achieve a significant improvement in pulse repetition frequency synchronization dynamic range (13.5 cm cavity optical path mismatch, or 1.87 kHz pulse repetition frequency mismatch) while simultaneously achieving passive pulse duration synchronization through injection of synchronization pulses into the gain fiber of one of the Slave lasers. This approach leads to a record pulse repetition frequency synchronization dynamic range. Subsequently, the synchronized DSR lasers at 1.06 µm and 1.56 µm are employed in a nonlinear difference frequency generation stage to generate mode-locked mid-infrared pulses with a pulse repetition rate of ∼ 2 MHz and an exceptional fractional instability of 3.60 × 10-13, measured for a 1000 s integration time.

1.45 ps All-solid-state Q-switched mode-locked laser based on new material Bi2Te3/Sb2Te3

In this work, the improvement of material properties and the optimization of mode-locked devices are discussed. The lateral heterojunction Bi2Te3/Sb2Te3 is prepared by a simple solvothermal method to overcome the limitation of traditional single material and improve the nonlinear optics properties. Based on a Bi2Te3/Sb2Te3 saturable absorber, the stable passive Q-switched mode-locked operation with wavelength of 1064 nm is achieved in Nd: YVO4 crystal. When the pump power reaches 5.5 W, a Q-switched mode-locked output with a maximum power of 910 mW is obtained, and the corresponding optical–optical conversion efficiency is 16.5 %. The repetition frequency of Q-switched pulse envelopes is 83.81 kHz, the pulse width is 1.8 μs, and the pulse energy is 10.8 μJ. The repetition frequency of the mode-locked pulse in the Q-switched envelope is 312.8 MHz and the pulse width is 1.45 ps. This flexible tunable laser provides high peak power, narrow pulse width, and highly stable laser output, broadening its application prospects in high-performance pulsed lasers.

An experimental study on laser cleaning of the soot deposition layer on a white marble surface

A coordinated application of the image binarization, area extrapolation, and laser-induced breakdown spectroscopy (LIBS) detection for monitoring and controlling the laser cleaning soot deposition layer process on a white marble surface was reported. The laser cleaning process used a fiber pulsed laser beam with a fixed wavelength of 1064 nm, a pulse repetition frequency of 100 kHz, and a focal spot diameter of 95 μm. Image binarization was conducted to determine 150 ns as the appropriate fixed pulse width for laser cleaning of the soot deposition layers from four discrete values of 50 ns, 100 ns, 150 ns, and 200 ns. The optimal laser energy density of 2.82 J·cm-2 was obtained through area extrapolation, while the optimal spot overlap rate of 20 % was determined to maximize the effective laser cleaning speed. The optimal number of cleanings for a soot deposition layer with a thickness of 77μm was determined to be 5 using LIBS detection and to monitor the laser cleaning process in real time for each small area of the soot deposition layer. Image binarization was used to evaluate the overall cleaning quality of large marble surfaces. By dynamically adjusting the optimal number of cleanings, the cleaning efficiency of the soot deposition layers on half of the surface area of a 20 cm diameter white marble artifact exceeded 95 %. The results indicate that the synergistic use of various online monitoring methods can optimize multiple laser parameters, enabling precise removal of the soot deposition layer on the white marble surfaces caused by anthropic factors, without damaging its surface.

The Dynamic Atom-Probe: Past, Present, and Perspectives.

The present communication aims at demonstrating the wealth of information accessible by 1D-atom probe experiments using pulsed field desorption mass spectrometry (PFDMS), ultimately combined with video-field ion microscopy, while subjecting metallic samples to elevated gas pressures and studying surface reaction kinetics. Two case studies are being presented here: (a) the microkinetics of nickel tetracarbonyl (Ni(CO)4) formation through reaction of carbon monoxide with nickel and (b) the nitric oxide decomposition and reaction with hydrogen on platinum at variable steady electric fields mimicking electrocatalytic conditions. In both cases, surface areas with 140-150 atomic sites of the stepped Ni (001) and Pt (111) sample surfaces were probed. Under (a), we demonstrate variable repetition frequencies of field pulses to inform kinetic and mechanistic details of the surface reaction while under (b), we reveal the occurrence of field-induced processes impacting the surface reaction mechanism of nitric oxide with hydrogen and therefore opening new pathways not available under purely thermal conditions (in the absence of electric fields). Some aspects of PFDMS technical achievements will be discussed as they may provide clues for designing dynamic atom probe tomography instrumentation.

Enhancing Weather Target Detection with Non-Uniform Pulse Repetition Time (NPRT) Waveforms

The velocity/distance trade-off poses a fundamental challenge in pulsed Doppler weather radar systems and is known as the velocity/distance dilemma. Techniques such as multiple-pulse repetition frequency, staggered pulse repetition time (PRT), and pulse phase coding are commonly used to mitigate this issue. The current study evaluates the adaptability/capability of a specific type of low-capture signal called the non-uniform PRT (NPRT) through analyzing the weather target characteristics of typical velocity distributions. The spectral moments estimation (SME) signal-processing algorithm of the NPRT weather echo is designed to calculate the average power, velocity, and spectrum width of the target. A comprehensive error analysis is conducted to ascertain the efficacy of the NPRT processing algorithm under influencing factors. The results demonstrate that the spectral parameters of weather target echo with a velocity of [−50,50] m/s through random-jitter NPRT signals align with radar functionality requirements (RFRs). Notably, the NPRT waveform resolves the inherent conflicts between the maximum unambiguous distance and velocity and elevates the upper limit of the maximal observation velocity. The evaluation results confirm that nonlinear radar signal processing technology can improve a radar’s detection performance and provide a new method for realizing the multifunctional observation of radar in different applications.

Adjustable repetition rate mode-locked fiber laser using a ZnO nanolaminate

This paper experimentally demonstrates the use of a zinc oxide (ZnO) nanolaminate as a polarization-dependent loss element to control the frequency repetition rate in a passive mode-locked fiber laser. The ZnO nanolaminate was deposited via the atomic layer deposition technique. The system achieves repetition frequencies ranging from 2.2 to 6.6 MHz, with emission peaks occurring at wavelengths between 1569.1 nm and 1570.8 nm, exhibiting three to four peaks. The fiber exhibits a minimal pulse duration of around 9.78 ns; the maximal pulse-to-pulse separation was 452 ns. The fiber laser demonstrates excellent stability in both power and wavelength emission. The compact experimental setup leverages the properties of the thin film of zinc oxide, offering a versatile laser system capable of quickly altering pulse repetition rates by simply adjusting the polarization state.

Simultaneous orthogonally polarized dual-wavelength actively Q-switched Ho:YLF laser at 2063 nm and 2065 nm

The simultaneous and output-balanced orthogonally polarized dual-wavelength actively Q-switched Ho:YLF laser at 2063 nm and 2065 nm was realized for the first time. In the continuous wave orthogonally polarized dual-wavelength operation, the total maximum output power of 4.6 W was achieved. In the actively Q-switching orthogonally polarized dual-wavelength operation, at the pulse repetition frequency of 5 kHz, the total maximum average output power, pulse energy, and peak power were 2.37 W, 0.47mJ, and 4.32 kW, respectively. As far as we know, this is the first simultaneous orthogonally polarized dual-wavelength actively Q-switched 2 μm Ho-doped solid-state laser.

Influence of Tropical Cyclones and Cold Waves on the Eastern Guangdong Coastal Hydrodynamics: Processes and Mechanisms

Influence of Tropical Cyclones and Cold Waves on the Eastern Guangdong Coastal Hydrodynamics: Processes and Mechanisms

Puzzlingly low utilization of solar irrigation pumps by smallholders in Nepal undermines cost-effectiveness

Abstract Solar Powered Irrigation Pumps (SIPs) hold substantial potential for low carbon irrigation expansion, particularly where affordable electricity is limited. In contrast to diesel-based irrigation, which carries steep fuel costs, irrigation by SIPs requires zero marginal costs, but high initial investments. This makes their competitiveness with diesel pumps highly dependent on the temporal frequency of their usage. Using unique and detailed data on SIP usage by smallholders in Nepal, we show SIP usage frequency is low, making it financially competitive with Diesel for only a small fraction of farmers. We analyze characteristics of farmers who make low/high usage of the SIP, and explore potential explanations for the puzzling low level of SIP use.

ERG responses and the Ferry-Porter law

The Ferry-Porter law states that the psychophysically measured critical flicker fusion frequency (CFF) is linearly correlated with the logarithm of the mean luminance. We studied the relationship between the CFFs of L- and M-cone driven ERGs (five normal trichromats; sinusoidal modulation; 18% cone contrast) and mean luminance. The measurements were performed at seven mean luminances between 2.84 and 284cd/m2. Close to the CFF, the ERG amplitudes versus temporal frequency could be described by a simple exponential function. The CFFs were higher for L- than for M-cone driven ERGs and depended linearly on the logarithm of the mean luminance (i.e., adhered to the Ferry-Porter law) with steeper slopes compared to psychophysical measurements.

Enhancing lung cancer growth inhibition with calcium ions: Role of mid- and high-frequency electric field pulses

Calcium electroporation (CaEP) involves the combination of calcium ions with electroporation, which is induced by pulsed electric fields (PEFs). This study explores the application of high-frequency unipolar nanosecond pulsed electric fields (nsPEFs: 8–14 kV/cm, 200 ns, 10 kHz, 100 kHz, 1 MHz repetition frequency pulse bursts, n = 100) and their potential in inhibiting lung cancer cell growth. As a reference, standard microsecond range parametric protocols were used (100 µs x 8 pulses). Methods included cell permeability quantification through Yo-Pro-1 uptake, cell viability assays, immunofluorescence studies for apoptosis and EMT markers, analysis of cell death types depending on repetition frequency pulse bursts. We determined the susceptibility of human lung cancer to electric pulses, characterized the efficacy of CaEP, and investigated cell death types depending on repetition frequency pulse bursts. We have shown that adding calcium ions to the applied nsPEF protocol increases cytotoxicity. Additionally, the use of these electroporation parameters can modulate key cellular processes, such as the epithelial-mesenchymal transition and apoptosis, as indicated by changes in the expression of markers such as E-cadherin, N-cadherin, BCL-2, and p53. Changes in cell morphology over time were observed using holotomographic microscopy. The study provides insights into the modulation of key cellular processes, indicating that nsPEF technology could improve the outcomes of conventional cancer treatments through enhanced efficacy and potentially mitigating drug resistance mechanisms. The promising results advocate for further research to optimize nsPEF protocols for clinical application, highlighting the potential of electrical fields in advancing cancer therapy.

Stabilized 30 µJ dissipative soliton resonance laser source at 1064 nm

We demonstrate the first successful stabilization of a dissipative soliton resonance (DSR) mode-locked (ML) laser source using straightforward techniques. Our setup employed a figure-8 (F8) resonator configuration and a nonlinear optical loop mirror (NOLM) to achieve stable mode-locking, generating 1064 nm rectangular pulses with a 3 ns duration at a repetition frequency of ~ 1 MHz. The pulses were boosted in an all-fiber amplifier chain and reached 30 µJ and 10 kW peak power per pulse at 30 W average output power. We addressed a critical gap in the literature by actively stabilizing key DSR pulse parameters: average output power (improved by a factor of 51), pulse repetition frequency (improved by 7583 using cross-phase modulation for synchronization), and pulse duration (improved by a factor of ~ 4). Additionally, we included a numerical analysis to explore the pulse formation mechanisms in DSR ML lasers working in a F8 configuration. Our findings show that non-complex all-in-fiber DSR ML lasers can reliably produce high-energy pulses with stable, repeatable parameters, making them suitable for future applications e.g. in nonlinear frequency conversion, laser micromachining, or LIDAR.

Processing characteristics of C/C composites with nanosecond pulse laser

This study investigates the effects of the nanosecond pulse laser parameters on the processing characteristics of carbon fiber-reinforced carbon matrix (C/C) composites through a series of laser cutting experiments. It compares the cutting width, depth, heat-affected zone (HAZ) width, processed surface micro-morphology, as well as the oxidation characteristics of the material under different processing parameters. The HAZ width is analyzed in relation to the variations in the oxygen content. The results indicate that both the cutting width and depth initially increase and then decrease with increasing repetition frequency. Moreover, increasing the laser power and pulse width gradually increases the cutting width, depth, and HAZ width, whereas a higher scanning speed tends to reduce the cutting width and depth. Since changes in the processing power, scanning speed, and pulse width affect the HAZ, the oxygen content exhibits a proportional relationship with the HAZ width. In addition, an increase in the repetition frequency can expand the HAZ while reducing the oxygen content. A careful increase in the repetition frequency aids in lowering the ablation threshold of the material, leading to more material removal. Nevertheless, a continuous increase in energy density can trigger a fiber pull-out phenomenon and cause degradation of the matrix.

Mo2TiAlC2 as a Saturable Absorber for a Passively Q-Switched Tm:YAlO3 Laser.

Owing to their remarkable characteristics, two-dimensional (2D) layered, MAX phase materials have garnered significant attention in the field of optoelectronics in recent years. Herein, a novel MAX phase ceramic material (Mo2TiAlC2) was prepared into a saturable absorber (SA) by the spin-coating method for passively Q-switching (PQS), and its nonlinear optical absorption properties were characterized with a Tm:YAlO3 (Tm:YAP) nanosecond laser. The structure characteristics and composition analysis revealed that the Mo2TiAlC2 material exhibits a well-defined and stable structure, with a uniform thin film successfully obtained through spin coating. In this study of a PQS laser by employing a Mo2TiAlC2-based SA, an average output power of 292 mW was achieved when the absorbed pump power was approximately 4.59 W, corresponding to a central output wavelength of 1931.2 nm. Meanwhile, a stable pulse with a duration down to 242.9 ns was observed at a repetition frequency of 47.07 kHz, which is the narrowest pulse width recorded among PQS solid-state lasers using MAX phase materials as SAs. Our findings indicate that the Mo2TiAlC2 MAX phase ceramic material is an excellent modulator and has promising potential for ultrafast nonlinear photonic applications.