Ultrashort Pulse Irradiation Research Articles

Self-enhanced coherent harmonic amplification in seeded free-electron lasers

High-intensity, ultrashort, fully coherent X-ray pulses hold great potential for advancing spectroscopic techniques to unprecedented levels. Here, we propose a novel scheme for generating high-brightness and femtosecond-scale soft X-ray radiation within a seeded free-electron laser (FEL) operating at MHz repetition rates. This scheme relies on the principles of self-modulation and superradiance. A relatively weak energy modulation of the pre-bunched electron beam is significantly amplified by the coherent radiation emitted in the self-modulator. Consequently, a coherent signal at ultra-high harmonics of the seed is achieved, and this signal is further amplified in the subsequent radiator through the fresh bunch and superradiant processes. Based on the parameters of the Shanghai soft X-ray FEL facility, three-dimensional simulations have been performed. The simulation results demonstrate that an electron beam with a laser-induced energy modulation as small as 2.3 times the slice energy spread can generate ultrashort coherent radiation pulses of around 2 GW within the water window spectral range. Moreover, the experimental results demonstrate that self-enhanced coherent energy modulation enables the production of coherent signals up to the 15th harmonic of a 266-nm seed laser. These findings indicate that the proposed scheme promises to generate ultrashort and coherent soft X-ray pulses at MHz repetition rates.

Discrete Wavelet Transformation for the Sensitive Detection of Ultrashort Radiation Pulse with Radiation-induced Acoustics.

Radiation-induced acoustics (RIA) shows promise in advancing radiological imaging and radiotherapy dosimetry methods. However, RIA signals often require extensive averaging to achieve reasonable signal-to-noise ratios, which increases patient radiation exposure and limits real-time applications. Therefore, this paper proposes a discrete wavelet transform (DWT) based filtering approach to denoise the RIA signals and avoid extensive averaging. The algorithm was benchmarked against low-pass filters and tested on various types of RIA sources, including low-energy X-rays, high-energy X-rays, and protons. The proposed method significantly reduced the required averages (1000 times less averaging for low-energy X-ray RIA, 32 times less averaging for high-energy X-ray RIA, and 4 times less averaging for proton RIA) and demonstrated robustness in filtering signals from different sources of radiation. The coif5 wavelet in conjunction with the sqtwolog threshold selection algorithm yielded the best results. The proposed DWT filtering method enables high-quality, automated, and robust filtering of RIA signals, with a performance similar to low-pass filtering, aiding in the clinical translation of radiation-based acoustic imaging for radiology and radiation oncology.

In vitro comparative study between adhesion forces obtained on zirconia ceramic micromechanically treated with femtosecond laser (1027nm), carbon dioxide laser (10,600nm), and aluminum-oxide particles.

Conventional surface roughening treatments used for silica-based ceramics in order to improve subsequent adhesion become unreliable for zirconia ceramics. Laser conditioning can be a good alternative. The purpose of this in vitro study was to compare conventional (macro) shear bond strength (SBS) values obtained between resin composite and zirconium oxide ceramic samples grouped according to different micromechanical treatments received, and examine differences in surface roughness. One-hundred and fifty disks of sintered zirconia were randomly divided into 5 groups and roughened as follows: (1) Group NOT, no surface treatment; (2) Group APA, abraded with 50-μm aluminum-oxide (Al2O3) particles; (3) Group TBS, abraded with 30-μm aluminum-oxide particles covered with silica; (4) Group CO2, irradiated with a CO2 laser which emitted in continuous wave mode at 3 W of power; and (5) Group FEM, irradiated with a pulsed femtosecond laser, with an incident energy of 10μJ, a frequency of 1000Hz, and a fluence of 1.3kJ/cm2. All surfaces were treated with a MDP-containing adhesive/silane coupling agent mixture upon which were prepared and light polymerized composite resin cylinders. Shear bond strength was measured and samples were observed by scanning electron microscopy (SEM). Statistically significant differences (p < 0.05) were found among all groups, except between CO2 and FEM, which showed the highest adhesion values (15.12 ± 2.35MPa and 16.03 ± 2.73MPa). SEM revealed differences in surface patterns. CO2 laser irradiation can be an alternative to sandblasting, although it could also weaken the ceramic. Suitable surface patterns on zirconia ceramics can be obtained with ultrashort pulsed radiation emitted by a pulsed femtosecond laser.

Time-Domain Rotational Raman Spectroscopy of the Ethylene Dimer and Trimer by Coulomb Explosion Imaging of Rotational Wave Packets.

We report rotational Raman spectroscopy of the ethylene dimer and trimer, based on time-resolved Coulomb explosion imaging of rotational wave packets. Rotational wave packets were created in the gas-phase ethylene clusters upon nonresonant ultrashort pulse irradiation. The subsequent rotational dynamics were traced as spatial distribution of monomer ions ejected from the clusters via the Coulomb explosion process induced by a strong probe pulse. The observed images of monomer ions show multiple kinetic energy components. The time-dependence of the angular distribution for each component was analyzed, and the Fourier transformation spectra, which correspond to rotational spectra, were obtained. A lower kinetic energy component was mainly attributed to a signal from the dimer and a higher energy component mainly from the trimer. We have successfully observed rotational wave packets up to a delay time of ∼20 ns and achieved a spectral resolution of 70 MHz after Fourier transformation. Owing to this higher resolution than the previous studies, improved rotational and centrifugal distortion constants were obtained from the spectra. In addition to improving the spectroscopic constants, this study opens the way for rotational spectroscopy of larger molecular clusters than dimers through Coulomb explosion imaging of rotational wave packets. Details of spectral acquisition and analyses of each kinetic energy component are also reported.

Ultrashort pulses of ultraviolet and vacuum-ultraviolet radiation of diffuse high-pressure gas discharges of picosecond duration

The emissivity of a discharge with an extreme high energy input rate of more than 5 × 1013 A/s has been studied. The diffuse discharge in xenon and nitrogen media at a pressure in the range of 0.1–3 atm had a pulse duration (FWHM) of 200 ps, a voltage and current amplitude of 280 kV and about 5 kA, respectively. It was shown that the duration of the radiation pulse in both gases was close to the duration of the discharge pulse, with the radiation power being 60 MW in xenon and 1.2 MW in nitrogen. The emission spectrum of the discharge in xenon was mainly in the form of a continuum in the wavelength range of 110–440 nm, therewith the maximum of the spectrum was located beyond the short-wavelength boundary of this range. The emission spectrum of the discharge in nitrogen, in addition to a similar continual component, also contained an intense line at a wavelength of λ = 337 nm. Possible mechanism that determines the shape of the recorded spectra was discussed.

Ex Vivo Exposure to Soft Biological Tissues by the 2-μm All-Fiber Ultrafast Holmium Laser System

We present the results of ex vivo exposure by an ultrafast all-fiber Holmium laser system to porcine longissimus muscle tissues. A simple Ho-doped laser system generated ultrashort pulsed radiation with less than 1 ps pulse width and a repetition rate of 20 MHz at a central wavelength of 2.06 μm. Single-spot ex vivo experiments were performed at an average power of 0.3 W and different exposure times of 5, 30 and 60 s, varying the total applied energy in the range of 1.5–18 J. Evaluation of laser radiation exposure was performed according to the depth and diameter of coagulation zones, ablation craters and thermal damage zones during the morphological study. Exposure by ultrashort pulsed radiation with an average power of 0.3 W showed destructive changes in the muscle tissue after 5 s and nucleation of an ablative crater. The maximum ablation efficiency was about 28% at the ablation depth and diameter of 180 μm and 500 μm, respectively. The continuous-wave radiation impact at the same parameters resulted only in heating of the near-muscular tissue, without ablation and coagulation traces. Exposure to tissue with an average power at 0.3 W of ultrashort pulsed radiation led, within 30 and 60 s, to similar results as caused by 0.5 W of continuous-wave radiation, although with less carbonization formation.

‘Cookies on a tray’: Superselective hierarchical microstructured poly(l-lactide) surface as a decoy for cells

In this research we developed a micro-sized hierarchical structures on a poly(l-lactide) (PLLA) surface. The obtained structures consist of round-shaped protrusions with a diameter of ~20 μm, a height of ~3 μm, and the distance between them ~30 μm. We explored the effect of structuring PLLA to design a non-cytotoxic material with increased roughness to encourage cells to settle on the surface. The PLLA films were prepared using the casting melt extrusion technique and were modified using ultra-short pulse irradiation – a femtosecond laser operating at λ = 1030 nm. A hierarchical microstructure was obtained resembling ‘cookies on a tray’. The cellular response of fibro- and osteoblasts cell lines was investigated. The conducted research has shown that the laser-modified surface is more conducive to cell adhesion and growth (compared to unmodified surface) to such an extent that allows the formation of highly-selectively patterns consisting of living cells. In contrast to eukaryotic cells, the pathogenic bacteria Staphylococcus aureus covered modified and unmodified structures in an even, non-preferential manner. In turn, adhesion pattern of eukaryotic fungus Saccharomyces boulardii resembled that of fibro- and osteoblast cells rather than that of Staphylococcus. The discovered effect can be used for fabrication of personalized and smart implants in regenerative medicine.

Tuning electron bunch with a longitudinally shaped laser to generate half-cycle terahertz radiation pulse

We present a new scheme for the realization of ultrashort terahertz (THz) radiation pulses by electron beam with a frequency-chirped microbunching structure travelling in a tapered undulator. In this proposal, a longitudinally modulated laser pulse with a chirp rate in the instantaneous modulation frequency is obtained through the optical beating of dual-chirp pulses; then the chirped micro-structure is converted to a chirped density modulation on electron beams by the laser-electron interaction and dispersion effect; finally by sending this electron beam into a designed undulator, ultrashort pulse and broad-band THz radiation can be produced. Considering the fact that the density modulation of the electron beam is directly inherited from the shaped laser, the central frequency and chirp rate can be easily tuned by the optical elements. Numerical simulations confirm that radiation pulses with ultrashort duration can be generated from such a microbunched electron beam. This technique may open new research avenues for strong-field few-cycle THz photonics.

Antenna Array with TEM-Horn for Radiation of High-Power Ultra Short Electromagnetic Pulses

An antenna array with short shielded transverse electromagnetic horns (S-TEM-horns) for emitting high-power radiation of ultra-short electromagnetic pulses (USEMP) has been created and researched. The antenna unit consists of an ultra-wideband antenna array with four S-TEM horns, with each connected to a two-wire HF transmission line, and these four lines are connected to an antenna feeder. This feeder is connected to a semiconductor generator with the following parameters: a 50 Ohm connector, 10–100 kV high-voltage monopolar pulses, a rise time of about 0.1 ns, FWHM = 0.2–1 ns, and pulse repetition rates of 1–100 kHz. The antenna array was designed and optimized to achieve a high efficiency of about 100% for the antenna aperture by using a 2 × 2 array with S-TEM-horns, with shielding rectangular plates for the return current. The transient responses were studied by simulation using the electromagnetic 3D code “KARAT” at the time domain and experimentally with the use of our stripline sensor for measurement of the impulse electrical field with a 0.03 ns rise time and a 7 ns duration at the traveling wave. The radiators were emitting USEMP waves with a hyperband frequency spectrum of 0.1–6 GHz. The radiation with an amplitude of 5–30 kV/m of the E-field strength at a distance of up to 20 m was successfully applied to test the electronics for immunity to electromagnetic interference.

Low Repair Capacity of DNA Double-Strand Breaks Induced by Laser-Driven Ultrashort Electron Beams in Cancer Cells.

Laser-driven accelerators allow to generate ultrashort (from femto- to picoseconds) high peak dose-rate (up to tens of GGy/s) accelerated particle beams. However, the radiobiological effects of ultrashort pulsed irradiation are still poorly studied. The aim of this work was to compare the formation and elimination of γH2AX and 53BP1 foci (well known markers for DNA double-strand breaks (DSBs)) in Hela cells exposed to ultrashort pulsed electron beams generated by Advanced Research Electron Accelerator Laboratory (AREAL) accelerator (electron energy 3.6 MeV, pulse duration 450 fs, pulse repetition rates 2 or 20 Hz) and quasi-continuous radiation generated by Varian accelerator (electron energy 4 MeV) at doses of 250–1000 mGy. Additionally, a study on the dose–response relationships of changes in the number of residual γH2AX foci in HeLa and A549 cells 24 h after irradiation at doses of 500–10,000 mGy were performed. We found no statistically significant differences in γH2AX and 53BP1 foci yields at 1 h after exposure to 2 Hz ultrashort pulse vs. quasi-continuous radiations. In contrast, 20 Hz ultrashort pulse irradiation resulted in 1.27-fold higher foci yields as compared to the quasi-continuous one. After 24 h of pulse irradiation at doses of 500–10,000 mGy the number of residual γH2AX foci in Hela and A549 cells was 1.7–2.9 times higher compared to that of quasi-continuous irradiation. Overall, the obtained results suggest the slower repair rate for DSBs induced by ultrashort pulse irradiation in comparison to DSBs induced by quasi-continuous irradiation.

Amplification of radially polarized ultra-short pulsed radiation to average output powers exceeding 250 W in a compact single-stage Yb:YAG single-crystal fiber amplifier

A simple and compact single-stage Yb:YAG single-crystal fiber amplifier was setup to amplify 784 fs long seed pulses to an output energy of 6 upmuJ and an average output power of 290 W. The experimental results are verified by numerical models to estimate the limitations of the SCF technology with regards to beam quality and average output power.

Minimum dose rate estimation for pulsed FLASH radiotherapy: A dimensional analysis.

To provide an order of magnitude estimate of the minimum dose rate ( ) required by pulsed ultra-high dose rate radiotherapy (FLASH RT) using dimensional analysis. In this study, we postulate that radiation-induced transient hypoxia inside normal tissue cells during FLASH RT results in better normal tissue sparing over conventional dose rate radiotherapy. We divide the process of cell irradiation by an ultra-short radiation pulse into three sequential phases: (a) The radiation pulse interacts with the normal tissue cells and produces radiation-induced species. (b) The radiation-induced species react with oxygen molecules and reduce the cell environmental oxygen concentration ( ). (c) Oxygen molecules, from nearest capillaries, diffuse slowly back into the resulted low regions. By balancing the radiation-induced oxygen depletion in phase II and diffusion-resulted replenishment in phase III, we can estimate the maximum allowed pulse repetition interval to produce a pulse-to-pulse superimposed reduction against the baseline . If we impose a threshold in radiosensitivity reduction to achieve clinically observable radiotherapy oxygen effect and combine the processes mentioned above, we could estimate the required for pulsed FLASH RT through dimensional analysis. The estimated required for pulsed FLASH RT is proportional to the product of the oxygen diffusion coefficient and inside the cell, and inversely proportional to the product of the square of the oxygen diffusion distance and the drop of intracellular per unit radiation dose. Under typical conditions, our estimation matches the order of magnitude with the dose rates observed in the recent FLASH RT experiments. The introduced in this paper can be useful when designing a FLASH RT system. Additionally, our analysis of the chemical and physical processes may provide some insights into the FLASH RT mechanism.

Coherent electron dynamics in monolayer MoS2 under ultrashort optical pulse

A theoretical investigation on the coherent Dirac-like quasiparticle dynamics in monolayer $\text{MoS}_2$ under an ultrashort optical pulse irradiation is presented. Particularly, we remain specific features of ML-MDS associated with the mass asymmetry $\alpha$ and topological aspect $\beta$ parameters resulting in Schr$\stackrel{..}{\text{o}}$dinger type wavevector of charge carriers. The direct band gap, spin-resolved valence band spilitting and valley degeneracy breaking due to strong spin-orbit coupling affect ultrafast dynamics of Dirac fermions. Because the duration of the optical pulse is less than the electron scattering time, which is $\sim 10-100\ fs$, the electron dynamics in electric field of the optical pulse is coherent, and consequently, we can describe the coupling of electron with strong electromagnetic field by the time-dependent Schr$\stackrel{..}{\text{o}}$dinger equation. The conduction band and valence band coupling via the strong electric field of pulse ($0.2-2.5 \ V/\text{\AA}$) gives rise to appearance of a dipole moment during the pulse is applied. We find that the dipole is complex, originating from the existence of band gap. We show an asymmetric singularities in Dirac points for absolute of dipole moment. We solve the resulting coupled evolution equations for the expansion coefficient of valence and conduction bands to obtain probability of population transition between valence and conduction bands. The irreversible electron dynamics as a key feature of two-dimensional Dirac matters in interacting with an ultrashort pulse strongly depends on the electronic structure of $\text{MoS}_2$. Furthermore, we present conduction band population distribution with an asymmetric exhibition at the each pair of Dirac points ($K$ and $K'$), when the pulse ends. This leads to valley polarization effect.

Generation of coherent broadband high photon flux continua in the XUV with a sub-two-cycle fiber laser.

High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.

Generation of frequency-chirped density modulation electron beam for producing ultrashort THz radiation pulse

A new scheme is proposed to generate electron beams with a frequency-chirped density modulation for producing ultrashort THz radiation pulse in an optimally tapered undulator. In this scheme, an energy chirped electron beam is sequentially modulated in two modulators by interacting with two laser pulses with the same central frequency but different chirp rates, practically one laser pulse is linearly chirped and the other is chirpfree. Two dispersion sections are applied to transfer the energy modulation formed in the modulators to the desired density modulation. The central frequency of the density modulation of the electron beam can be continuously tuned by changing the compression coefficients of the dispersion sections, and the chirp rate of the final modulation frequency can also be tuned by either varying the compression coefficients or the linear chirp rates of the modulation laser pulses. The generation of ultrashort and broadband THz radiation pulses is also briefly discussed.

Coherent master equation for laser modelocking

Modelocked lasers constitute the fundamental source of optically-coherent ultrashort-pulsed radiation, with huge impact in science and technology. Their modeling largely rests on the master equation (ME) approach introduced in 1975 by Hermann A. Haus. However, that description fails when the medium dynamics is fast and, ultimately, when light-matter quantum coherence is relevant. Here we set a rigorous and general ME framework, the coherent ME (CME), that overcomes both limitations. The CME predicts strong deviations from Haus ME, which we substantiate through an amplitude-modulated semiconductor laser experiment. Accounting for coherent effects, like the Risken-Nummedal-Graham-Haken multimode instability, we envisage the usefulness of the CME for describing self-modelocking and spontaneous frequency comb formation in quantum-cascade and quantum-dot lasers. Furthermore, the CME paves the way for exploiting the rich phenomenology of coherent effects in laser design, which has been hampered so far by the lack of a coherent ME formalism.

The universal character of the breakdown of condensed media by powerful terahertz radiation and the Abbe criterion

This paper analyzes new experimental results on the surface breakdown of semiconductors and thin metallic films by ultrashort pulses of plane-polarized terahertz radiation. It shows that the character of the observed breakdown is satisfactorily explained by the universal polariton model, tested with ultrashort pulses of visible and near-IR radiation interacting with condensed media. By using a nonlinear mathematical model of the formation of the spatial periods of a nanograting that are multiples of the wavelength of the exciting radiation, it is shown that the periods of the gratings thus formed are substantially shorter than the linear diffraction limit in the optical and terahertz regions. It is demonstrated that the diffraction limit can be overcome in nonlinear processes by which interference nanogratings are formed with normal and anomalous orientations.

Energy and Time Distribution of Photoelectrons Released in Water under Ultra-Short X-Ray Pulse Irradiation

Energy and Time Distribution of Photoelectrons Released in Water under Ultra-Short X-Ray Pulse Irradiation

Picosecond Laser-Induced Photothermal Skin Damage Evaluation by Computational Clinical Trial.

Computational clinical trial (CCT) in the field of laser medicine promotes clinical application of novel laser devices, because this trial carried out based on numerical modeling of laser-tissue interactions and simulation of a series of treatment process. To confirm the feasibility of the computational clinical trial of skin treatment with a novel picosecond laser, this paper presents an evaluation method of the safety. In this method, the light propagation and thermal diffusion process after ultrashort light pulse irradiation to a numerical skin model is calculated and the safety based on the photothermal damage is evaluated by computational modeling and simulation. As an example, the safety of a novel picosecond laser device was examined by comparing with several laser devices approved for clinical use. The ratio of the maximum thermal damage induced by picosecond laser irradiation was 1.2 × 10-2 % at the epidermis, while that caused by approved laser irradiation was 99 % at the capillary vessels. The numerical simulation demonstrated that less thermal damage was observed compared with the approved devices. The results show the safety simulated by photothermal damage calculation was consistent with the reported clinical trials. This computational clinical trial shows the feasibility of applying computational clinical trials for the safety evaluation of novel medical laser devices. In contrast to preclinical and clinical tests, the proposed computational method offers regulatory science for appropriately and quickly predicting and evaluating the safety of a novel laser device.

Controlled drug delivery system by fs-laser micromachined biocompatible rubber latex membranes

The concept behind drug delivery systems is to provide the correct amount of pharmaceutical compound to a desired body part within a predefined timespan. It can be made of various materials such as liposomes, niosomes, nano/microcapsules, cyclodextrins, polymers and many more. Natural rubber latex is used as solid matrix for drug delivery because it stimulates the healing process of wounds and angiogenesis. In this context, we have produced natural rubber latex membranes with of 2000, 6000 and 10,000 pore/cm2 via fs-laser micromachining and studied its influence on the drug delivery experiments. Drug release kinetics of Ciprofloxacin-loaded microdrilled biopolymer samples showed that the final concentration of drug released has a linear dependence with the pore density. According to infrared spectroscopy analysis, latex membranes maintained their structural properties after ultrashort pulse irradiation and ciprofloxacin adsorption. The demonstration of a fast and reliable production of a controlled drug delivery system puts fs-laser micromachined porous natural rubber latex as a promising candidate for in vivo application in which a proper amount of drug needs to be released.