Single Ultrashort Pulse Research Articles

Transcranial pulse stimulation in the treatment of mild neurocognitive disorders.

There are limited effectiveness and potential side effects of existing pharmacological approach in treating mild neurocognitive disorder (NCD). Transcranial pulse stimulation (TPS) applies repetitive single high-pressure ultrashort shockwave pulses to stimulate the brain, which has been shown to effectively improve cognition in major NCD. However, the effectiveness of TPS in mild NCD patients remained unknown. This study aims to assess the effectiveness and tolerability of TPS with neuro-navigation in old-age adults with mild NCD by both clinical and biochemical assessments. An open-label study recruited older adults with mild NCD to receive neuro-navigated TPS intervention for two weeks with three sessions per week. Assessments included detailed cognitive assessments, APOE genotype, and brain-derived neurotrophic factor (BDNF). Nineteen participants (12 females and 7 males) completed the whole TPS interventions with no serious adverse effects reported. Repeated measures ANOVA showed statistically significant effects of time on HK-MoCA (F (3, 54) = 4.99, P = 0.004), 30-sec interval of Verbal Fluency Test (F (3, 54) = 2.94, P = 0.041), Stroop interference (F (3, 54) = 3.46, P = 0.023), and Chinese IADL (F (3, 54) = 2.78, P = 0.050) after receiving the intervention. Bonferroni post hoc comparisons on HK-MoCA showed a significant improvement after intervention. There was no significant change in serum BDNF level. TPS has brought significant improvement in cognition of elderly with mild NCD. It has a great potential to delay the deterioration of cognition in older adults. The long-term effect of TPS in cognition would benefit from further large-scale, randomized, sham-controlled trials.

Investigation of Ablation Efficiency of Stainless Steel Using Pulsed Lasers in Burst Mode

Due to its high corrosion resistance and mechanical properties, stainless steel is commonly used in various industrial applications. Although different types of stainless steel are similar in their chemical composition, they can differ significantly in their thermal diffusivity. This property is relevant in the ability of a material to conduct heat and thus, in laser processing. In this frame, this study compares the ablation efficiency and characteristics of polished stainless steel samples of the alloys AISI 304, AISI 420, and AISI 316Ti. They are irradiated with single ultrashort pulses having pulse durations between 250 fs and 10 ps as well as using GHz burst modii. The goal is to investigate the differences in both the ablation threshold and the ablation rate to improve the ablation efficiency. The results show that shorter pulse durations lead to a more efficient ablation process. On the other hand, GHz bursts are found to be, in general, less efficient. In addition, there is a significant difference in the surface morphology depending on the process parameters. The differences in the thermal diffusivity do not significantly influence the ablation threshold fluence but surface morphology and the ablation rate.

Self-Separation of a Single Ultrashort Light Pulse in the Parametric Raman Anti-Stokes Laser Based on a CaMoO4 Crystal under Intracavity Synchronous Pumping

The effect of the self-separation of a single ultrashort light pulse of a parametric Raman CaMoO4 laser with birefringence phase matching under intracavity synchronous pumping by a mode-locked 1064 nm Nd:YAG laser is experimentally investigated and theoretically simulated. The conditions for the self-separation effect for the single ultrashort pulse at an anti-Stokes wavelength of 973 nm with the pulse duration of 9 ps and the pulse energy of up to 9 μJ are defined.

Cellular irradiations with laser-driven carbon ions at ultra-high dose rates

Objective. Carbon is an ion species of significant radiobiological interest, particularly in view of its use in cancer radiotherapy, where its large Relative Biological Efficiency is often exploited to overcome radio resistance. A growing interest in highly pulsed carbon delivery has arisen in the context of the development of the FLASH radiotherapy approach, with recent studies carried out at dose rates of 40 Gy s−1. Laser acceleration methods, producing ultrashort ion bursts, can now enable the delivery of Gy-level doses of carbon ions at ultra-high dose rates (UHDRs), exceeding 109 Gy s−1. While studies at such extreme dose rate have been carried out so far using low LET particles such as electrons and protons, the radiobiology of high-LET, UHDR ions has not yet been explored. Here, we report the first application of laser-accelerated carbon ions generated by focussing 1020 W cm−2 intense lasers on 10–25 nm carbon targets, to irradiate radioresistant patient-derived Glioblastoma stem like cells (GSCs). Approach. We exposed GSCs to 1 Gy of 9.5 ± 0.5 MeV/n carbon ions delivered in a single ultra-short (∼400-picosecond) pulse, at a dose rate of 2 × 109 Gy s−1, generated using the ASTRA GEMINI laser of the Central Laser Facility at the Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK. We quantified carbon ion-induced DNA double strand break (DSB) damage using the 53BP1 foci formation assay and used 225 kVp x-rays as a reference radiation. Main Results. Laser-accelerated carbon ions induced complex DNA DSB damage, as seen through persistent 53BP1 foci (11.5 ± 0.4 foci/cell/Gy) at 24 h and significantly larger foci (1.69 ± 0.07 μm2) than x-rays induced ones (0.63 ± 0.02 μm2). The relative foci induction value for laser-driven carbon ions relative to conventional x-rays was 3.2 ± 0.3 at 24 h post-irradiation also confirming the complex nature of the induced damage. Significance. Our study demonstrates the feasibility of radiobiology investigations at unprecedented dose rates using laser-accelerated high-LET carbon ions in clinically relevant models.

Bragg order effect on ultrashort pulse generation by distributed feedback dye lasers under subnanosecond excitation: Numerical study

A comparative numerical study of the 1st- and 2nd-order distributed feedback (DFB) dye lasers pumped with 0.5 ns, 532 nm laser pulses is performed. A dynamic model of the gain-coupled DFB dye laser based on a coupled-wave theory and taking into account Bragg diffraction order is used. Due to the periodical distribution of the pump field and stimulated emission intensities along the excited dye volume, the rate equations of the population densities are turned into the equations of the Fourier coefficients. To simulate the DFB lasing, coupled-wave equations of optical fields are used. Effect of the pumping intensity on the temporal and energy characteristics of DFB dye lasers is studied depending on the dye concentration and the length of the excited dye volume. Influence of the lasing seed type (constant or noise-like) on the simulation results is also analyzed. A special emphasis is on the characteristics of single ultrashort (US) picosecond pulses generated by the relaxation oscillations mechanism. The results obtained indicate that for both types of DFB dye lasers the temporal course of the output intensity is governed by the pumping strength. Depending on the excess above the threshold (high or low), either US pulse trains or single US pulses are generated. The influence of the active medium parameters on their characteristics has been studied in detail. In addition, it was established that unlike the 1st-order DFB lasing, where the simulation results are independent of the seed type and smooth US pulses are always obtained, for the 2nd-order regime, to get smooth rather than structural single US pulses, a noise-like seed should be used. However, the most important finding is that the changeover from the 1st to the 2nd order leads to ∼1.1–3.2 (∼1.13–1.46) times reduction in duration, ∼1.4–3.4 (∼1.4–3.0)-fold increase in energy and ∼2.8–9.7 (∼1.5–4.8)-fold growth in peak power of single US pulses for the case of constant (noise-like) lasing seed. Hence, the use of the 2nd-order regime represents a simple and convenient way to improve the energy characteristics of single US pulses of DFB dye lasers generated by the relaxation oscillations mechanism.

The role of fluence in determining the response of thin molybdenum films to ultrashort laser irradiation; from laser-induced crystallization to ablation via photomechanical ablation and nanostructure formation

The selective processing of Mo at low temperatures is challenging, especially in advanced manufacturing on flexible and heat-sensitive substrate due to its higher melting temperature. The key role of fluence in determining the response of thin Mo films to ultrashort laser irradiation is considered in this study. At low fluences, the electrical properties of Mo are enhanced by a localized laser-induced crystallization mechanism; the electrical mobility of Mo is increased and the contact resistance between Mo-Si interface is reduced. At higher fluences, selective patterning of Mo proceeds without impacting the Si layer and the threshold fluence for ablation increases with the film thickness of Mo. Two fluence dependent ablation mechanisms are observed depending on the Mo film thickness. For thin films of thicknesses 20 nm and 40 nm, selective ablation proceeds only by a photothermal interaction. For 60 nm and 80 nm thick films, selective ablation proceeds by both photomechanical and photothermal interactions at two-separate higher fluence regimes, respectively. Interestingly, between these two ablation regimes, a non-ablative nanostructuring regime occurs. The study provides a concise overview of the process window for implementing the laser-induced modifications to Mo layers with minimal impact to the substrate using single wavelength ultrashort pulse laser.

Theoretical study of high-order harmonics and single ultrashort attosecond pulse generated by optimized combination of laser field

High-order harmonic generation, which is a hot topic of strong ultrafast fields, is one of the most important ways for obtaining the ultraviolet attosecond sources, and has a very wide application prospect. This work focuses on the challenges of the generation of either short or high attosecond pulses. We present the research progress of the high-order harmonics and attosecond pulse generation, and propose an effective and feasible method, and show some results. Specifically, combining the time-dependent Schrödinger equation and new unconstrained optimization algorithm, the objective function with the aim of the widest supercontinuum plateau of He atom is designed and the optimized two-color and three-color laser fields are obtained. The supercontinuum spectra extend up to 100 harmonic orders for the case of the optimized two-color laser field. As a result, a single ultrashort attosecond pulse of 25 as is produced. For the three-color case, the supercontinuum spectra reach up to 170 harmonic orders, and the width of single shortest attosecond pulse obtained by superposing pulses from low order (110 order) to high order (280 order) is obtained to be 17 as . Taking the optimized two-color laser field for example, the macroscopic medium propagation is discussed by solving the Maxwell equation. The results show that the selectivity of quantum trajectories from far-field space distribution can obtain the single ultra-short attosecond pulse.

Current state of ultrasound neuromodulation therapy

Abstract Recently the very first patient studies on clinical effects of ultrasound based neuromodulation therapies, targeting deep and superficial brain areas, have been published. Various technologies meanwhile exist, including focused ultrasound techniques (FUS) applying repeated sonication periods of several 100 milliseconds as well as transcranial pulse stimulation (TPS) applying single ultrashort pressure pulses in the microsecond range, which are repeated with a frequency of 4 to 5 Hz. Ultrasound based neuromodulation techniques include 2 important advantages for clinical application: (1) Pathological conductivities in patient brains do not have a relevant impact on ultrasound targeting, (2) Ultrasound techniques are the first which allow focal non-invasive deep brain stimulation. This talk will summarize the recent clinical ultrasound developments and current state of ultrasound neuromodulation therapy. Keywords: Transcranial Pulse Stimulation, Focused Ultrasound, Therapy

Generation of quantum vortices in photodetachment: The role of the ground-state wave function

Formation of quantum vortices in laser-induced photodetachment from negative ions is analyzed. The driving laser field consists of a single ultrashort pulse of circular polarization and the unperturbed ground-state wave function of the anion is found in either the $s$ or $p$ state. In particular, numerical illustrations for the photodetachment from ${\mathrm{H}}^{\ensuremath{-}},$ ${\mathrm{O}}^{\ensuremath{-}},$ ${\mathrm{K}}^{\ensuremath{-}},$ and a model ${\mathrm{A}}^{\ensuremath{-}}$ anion are presented. Special attention is paid to the symmetry of the ground-state wave function and ionization potential over the final vortex pattern. It is shown that the two-dimensional spectra of photoelectrons in momentum space comprise three well-defined regions: The low-energy (central) region, multiphotonlike zone, and supercontinuum. While the supercontinuum does not contribute to vorticity and the multiphoton zone depends only on the laser field characteristics, vortices in the low-energy region strongly depend on the bound-state wave function and its ionization potential.

Numerical simulation of a laser with a Q-switched cavity for production of ultrashort pulses

A theoretical model was developed to calculate lasing modes for giant pulses in solid-state lasers with Q-switched cavities. The lasing process begins with steady-state mode locking. Then, when a control pulse is fed to the cavity electro-optic modulator, the light propagating in the cavity is amplified. When the laser pulse reaches maximum energy, the cavity opens, producing a single ultrashort pulse. Simulations performed using a commercial 50 W Nd:YAG solid-state CW laser diode pump module indicated that, when the driver parameters for the electro-optical modulator are optimized, the cavity switching mode can be used for high-efficiency extraction of almost all the energy (up to 20 mJ) stored in the laser medium in steady-state mode in the form of a single pulse with a duration in the hundreds of picoseconds.

Laser Pulse Shortening via Zero-Dispersion Phase Matching of Parametric Raman Interactions in Crystals

We propose and study the conditions of zero-dispersion phase matching for parametric Raman interactions in birefringent crystals differing by anisotropy of zero-dispersion wavelength and allowing for the spectral tuning of the zero-dispersion phase-matching condition. We choose a highly birefringent crystal of calcite having a wide zero-dispersion anisotropy range for the demonstration of new effects of laser pulse shortening in parametric Raman lasers with spectrally tunable zero-dispersion phase matching. We demonstrate the anti-Stokes (1168 nm) and multi-Stokes (1629 nm) picosecond pulse shortening and self-separation of single 80-ps ultra-short pulse from the zero-dispersion phase-matched parametric Raman lasers that are based on the calcite crystal without using any electro-optical device.

Engineering Single-Shot All-Optical Switching of Ferromagnetic Materials.

Since it was recently demonstrated in a spin-valve structure, magnetization reversal of a ferromagnetic layer using a single ultrashort optical pulse has attracted attention for future ultrafast and energy-efficient magnetic storage or memory devices. However, the mechanism and the role of the magnetic properties of the ferromagnet as well as the time scale of the magnetization switching are not understood. Here, we investigate single-shot all-optical magnetization switching in a GdFeCo/Cu/[CoxNi1-x/Pt] spin-valve structure. We demonstrate that the threshold fluence for switching both the GdFeCo and the ferromagnetic layer depends on the laser pulse duration and the thickness and the Curie temperature of the ferromagnetic layer. We are able to explain most of the experimental results using a phenomenological model. This work provides a way to engineer ferromagnetic materials for energy efficient single-shot all-optical magnetization switching.

Combination and Compression of Multiple Pulses With Same or Different Wavelengths

We propose a practical scheme to demonstrate the combination and subsequent self-similar compression of two pulses with the same or different central wavelengths while propagating through a nonlinear fiber with exponentially decreasing dispersion. To initiate these processes, two raised cosine pulses with the same or different wavelengths is modulated using a phase modulator to acquire the same chirp at the input of the fiber. While propagating through the nonlinear fiber, these chirped pulses first coalesce into a single pulse and during further propagation get compressed into a single ultrashort high-power pulse. The output pulse possesses a large compression factor, high proportion of energy and peak power compared to a single input pulse. We also report the combination and compression of five raised cosine pulses with different wavelengths to achieve an appreciable compression effect, indicating that this system works well even with a small number of input pulses. The proposed scheme provides a simple way to generate high power ultrashort pulse with high energy and good quality in a short length of fiber.

Transcranial Pulse Stimulation with Ultrasound in Alzheimer's Disease-A New Navigated Focal Brain Therapy.

Ultrasound‐based brain stimulation techniques may become a powerful new technique to modulate the human brain in a focal and targeted manner. However, for clinical brain stimulation no certified systems exist and the current techniques have to be further developed. Here, a clinical sonication technique is introduced, based on single ultrashort ultrasound pulses (transcranial pulse stimulation, TPS) which markedly differs from existing focused ultrasound techniques. In addition, a first clinical study using ultrasound brain stimulation and first observations of long term effects are presented. Comprehensive feasibility, safety, and efficacy data are provided. They consist of simulation data, laboratory measurements with rat and human skulls and brains, in vivo modulations of somatosensory evoked potentials (SEP) in healthy subjects (sham controlled) and clinical pilot data in 35 patients with Alzheimer's disease acquired in a multicenter setting (including neuropsychological scores and functional magnetic resonance imaging (fMRI)). Preclinical results show large safety margins and dose dependent neuromodulation. Patient investigations reveal high treatment tolerability and no major side effects. Neuropsychological scores improve significantly after TPS treatment and improvement lasts up to three months and correlates with an upregulation of the memory network (fMRI data). The results encourage broad neuroscientific application and translation of the method to clinical therapy and randomized sham‐controlled clinical studies.

Noise correlations in time- and angle-resolved photoemission spectroscopy

In time-resolved photoemission experiments, more than one electron can be emitted from the solid by a single ultra-short pulse. We theoretically demonstrate how correlations between the momenta of outgoing electrons relate to time-dependent two-particle correlations in the solid. This can extend the scope of time- and angular-resolved photoemission spectroscopy to probe superconducting and charge density fluctuations in systems without long-range order, and to reveal their dynamics independent of the electronic gap and thus unrestricted by the energy-time uncertainty. The proposal is illustrated for superconductivity in a BCS model. An impulsive perturbation can quench the gap on ultrafast timescales, while non-equilibrium pairing correlations persist much longer, even when electron-electron scattering beyond mean-field theory is taken into account. There is thus a clear distinction between a dephasing of the Cooper pairs and the thermalization into the normal state. While a measurement of the gap would be blind to such pairing correlations, they can be revealed by the angular correlations in photoemission.

High-order harmonic generation of Li+ with combined infrared and extreme ultraviolet fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11465016, 11664035, 11764038, and 11765018) and the Foundation of Northwest Normal University, China (Grant No. NWNU-LKQN-17-1).

We investigate high-order harmonic generation (HHG) of Li+ ion driven by an intense infrared (IR) laser field in combination with a weak XUV pulse. To achieve this, we first construct an accurate single-active electron angular-momentum-dependent model potential of Li+ ion, by which the accurate singlet energy levels of Li+ for the ground state and excited states with higher quantum numbers can be obtained. Then, we solve numerically the three dimensional time-dependent Schrödinger equation of Li+ ion by means of the generalized pseudospectral method to obtain HHG. Our results show that the strength of assisted XUV is not amplified during the harmonic generation process, but the yield of HHG power spectrum in the whole plateau has a significant enhancement. Furthermore, the optimal phase delay between the IR and XUV pulses allows the production of ultrabroadband supercontinuum spectra. By superposing some harmonics, a strong new single 27-attosecond ultrashort pulse can be obtained.

Correlation dynamics in double photoionization of excited helium atom by a single ultrashort XUV pulse

SynopsisWe study single- and two-photon double ionization of helium by short XUV pulses by numerically solving the time-dependent Schrödinger equation in full dimensionality within a finite element discrete variable representation scheme. We discuss the joint energy and angular distributions, identify sequential and non-sequential contributions in the double ionization by ultrashort pulse, and track the dynamics of the ionization process in distinct double ionization regimes.

Spatial and temporal control of populations, branching ratios, and electronic coherences in LiH by a single one-cycle infrared pulse

Dynamical computations demonstrate considerable selectivity over the fragmentation channels of the LiH molecule via the polarization and the carrier envelope phase (CEP) of a single ultrashort one-cycle strong IR pulse. For aligned molecules, control of the CEP allows building nonstationary coherent electronic wave packets of contrasting ionic character, either $\mathrm{L}{\mathrm{i}}^{\ensuremath{\delta}+}{\mathrm{H}}^{\ensuremath{\delta}\ensuremath{-}}$ or $\mathrm{L}{\mathrm{i}}^{\ensuremath{\delta}\ensuremath{-}}{\mathrm{H}}^{\ensuremath{\delta}+}$. The complementary coherences are maintained all the way to dissociation. The direction of the electric field at its maximum points either towards the Li or towards the H atom, which selectively steers the nuclear dynamics to specific dissociation products.

Imaging ultrafast molecular wave packets with a single chirped UV pulse

We show how to emulate a conventional pump-probe scheme using a single frequency-chirped ultrashort UV pulse to obtain a time-resolved image of molecular ultrafast dynamics. The chirp introduces a spectral phase in time that encodes the delay between the pump and the probe frequencies contained in the pulse. By comparing the results of full dimensional ab initio calculations for the H$^+_2$ molecule with those of a simple sequential model, we demonstrate that, by tuning the chirp parameter, two-photon energy-differential ionization probabilities directly map the wave packet dynamics generated in the molecule. As a result, one can also achieve a significant amount of control of the total ionization yields, with a possible enhancement by more than an order of magnitude.

Ultra-short and ultra-intense X-ray free-electron laser single pulse in one-dimensional photonic crystals.

The propagation within a one-dimensional photonic crystal of a single ultra-short and ultra-intense pulse delivered by an X-ray free-electron laser is analysed with the framework of the time-dependent coupled-wave theory in non-linear media. It is shown that the reflection and the transmission of an ultra-short pulse present a transient period conditioned by the extinction length and also the thickness of the structure for transmission. For ultra-intense pulses, non-linear effects are expected: they could give rise to numerous phenomena, bi-stability, self-induced transparency, gap solitons, switching, etc., which have been previously shown in the optical domain.