X-ray emission during ultrafast laser grooving

Xe(L) X-ray emission from laser–cluster interaction

Ultrashort pulse laser processing of materials allows for precise machining with high accuracy. By increasing the repetition rate to several 100 kHz, laser machining becomes quick and cost-effective. Ultrafast laser processing at high repetition rates and peak intensities above 1013 W/cm2 can cause a potential hazard by generation of unwanted x-ray radiation. Therefore, radiation protection must be considered. For 925 fs pulse duration at a center wavelength of 1030 nm, the x-ray emission in air at a repetition rate of 400 kHz was investigated up to a peak intensity of 2.6 × 1014 W/cm2. Based on the presented measurements, the properties of potential shielding materials will be discussed. By extending our previous works, a scaling of the x-ray radiation emission to higher peak intensities up to 1015 W/cm2 is described, and emitted x-ray doses are predicted.

A 0.5 PW 15 J/pulse, 30 fs, 1 shot/min, Ti:Sapphire based high pulse contrast laser system is being constructed at OSU. The laser peak intensity will be measured with a proton acceleration based technic.

The ongoing trend in the development of powerful ultrashort pulse lasers has attracted increasing attention for this technology to be applied in large-scale surface engineering and modern microfabrication. However, the emission of undesired X-ray photon radiation was recently reported even for industrially relevant laser irradiation regimes, causing serious health risks for laser operators. In the meantime, more than twenty influencing factors have been identified with substantial effects on X-ray photon emission released by ultrashort pulse laser processes. The presented study on enhanced X-ray emission arising from high pulse repetition frequency ultrashort pulse laser processing provides new insights into the interrelation of the highest-contributing parameters. It is verified by the example of AISI 304 substrates that X-ray photon emission can considerably exceed the legal dose rate limit when ultrashort laser pulses with peak intensities below 1 × 1013 W/cm² irradiate at a 0.5 MHz pulse repetition frequency. The peak intensity threshold value for X-ray emissions decreases with larger laser spot sizes and longer pulse durations. Another key finding of this study is that the suction flow conditions in the laser processing area can affect the released X-ray emission dose rate. The presented results support the development of effective X-ray protection strategies for safe and risk-free ultrashort pulse laser operation in industrial and academic research applications.

Objective: To explore the difference of perfusion parameters between gastric cancer (GC) and gastric stromal tumors (GSTs) by using oral contrast plus contrast-enhanced ultrasonography (OC+CEUS).Methods: We retrospectively reviewed 149 patients with histologically confirmed gastric lesions (80 patients with GC and 69 patients with GST). OC+CEUS was performed in all patients in the GC group and the GST group before surgery. The cine loops of OC+CEUS of all cases were analyzed. The perfusion parameters including arrival time (AT), time to peak (TTP), basal intensity (BI), and peak intensity (PI) were obtained via a program designed for autotracking contrast quantification (ACQ). The between-group differences in these parameters were compared.Results: According to time-intensity curve (TIC) analysis, high-risk GST had higher PI than low-risk GST (P < 0.05). GC had faster AT and higher PI than normal gastric wall (P < 0.05); GST had higher PI than normal gastric wall (P < 0.05). Furthermore, the GC group had faster AT and higher PI than the GST group (P < 0.05). In contrast, the difference in BI and peak time (TTP) between the groups was not significant (P > 0.05).Conclusion: AT and PI differ significantly between the GC group and the GST group. As a new method, OC+CEUS has value for the differential diagnosis of GC and GST.

Investigations on laser micro ablation of steel have been made using multipulses instead of single pulses. The aim is to improve the efficiency of laser micro ablation by increasing the ablation rate and quality at the same time. For the improvement of the multipulse laser ablation a flash-lamp pumped Nd:YAG laser with 20 to 100 ns pulse duration is used which can supply multipulses with a specially triggered pockels cell at 10 Hz. A mayor increase of ablation by distribution of energy on several pulses is observed. Further investigations have been made with a commercial ps laser system with a pulse duration of 12 ps at a repetition rate of 100 kHz. The used bursts consisted of up to four pulses with interpulse separations Δt between 20 ns and 100 μs. A cut-off frequency νcut-off up to which the double pulse ablation equals single pulse ablation is defined by plasma enhanced absorption of the second laser pulse. For the comparison of multipulse and single pulse ablation the geometry and the roughness of the irradiated area have been detected by white light interference microscopy.

We studied the highest harmonic photon energies of high-order harmonic generation (HHG) for He, Ne, Ar, Kr, Xe, and N2. This research employed 65-fs and 150-fs Ti:sapphire laser pulses, of which the peak intensities in a vacuum are higher than the barrier-suppression-ionization (BSI) intensities. We fitted two analytical formulae to the experimental results of HHG. One formula, which was obtained by fitting an analytical formula to the results by a quantum mechanical simulation of HHG, expresses the highest interaction intensity by a function of the BSI intensity and the Keldysh γ-parameter. The other formula is derived by introducing the saturation intensity of HHG, which was proposed by Chang et al. [Phys. Rev. Lett. 79, 2967 (1997)]. We discuss the highest interaction intensity for the condition when the peak laser intensities are higher than the BSI intensities.

Laser processing of materials always was the important field for laser applications. Copper vapor laser (CVL) system are widely used in micromechanical engineering where optical system may provide high image quality. That allows us to concentrate the energy on a small surface and to produce very tiny holes and very thin cutting edges. The possibility to use "generator-amplifier" laser system (copper vapor elements LT-5Cu and LT-30Cu) for processing material without mechanical movements was investigated. As the pumping generator was used the scheme with the current pulse duration about 80 - 100 ns and the laser pulse duration may vary up to 25 ns. In the unstable resonator scheme the special plane mirror with reflecting coating was used. With the help of this system a number of materials were processed, namely: copper, stainless steel, gold, aluminum and nonmetals: sapphire, ceramics, various rocks, plastics etc.

Fine-structured polymerized cholesteric liquid crystal (PCLC) wedge laser devices have been realized, with high fine spatial tunability of the lasing wavelength. With resolution less than 0.3 nm in a broad spectral range, more than one hundred laser lines could be obtained in a PCLC cell without extra devices. For practical device application, we studied the stability of the device in detail over time, and in response to strong external light sources, and thermal perturbation. The PCLC wedge cells had good temporal stability for 1 year and showed good stability for strong perturbations, with the lasing wavelength shifting less than 1 nm, while the laser peak intensities decreased by up to 34%, and the high energy band edge of the photonic band gap (PBG) was red shifted 3 nm by temperature perturbation. However, when we consider the entire lasing spectrum for the PCLC cell, the 1-nm wavelength shift may not matter. Although the laser peak intensities were decreased by up to 34% in total for all of the perturbation cases, the remaining 34% laser peak intensity is considerable extent to make use. This good stability of the PCLC laser device is due to the polymerization of the CLC by UV curing. This study will be helpful for practical CLC laser device development.

An extension of the Keldysh formulation for electron tunnel ionization has resulted in a description of laser-induced dielectric breakdown with the following features: threshold optical field is predictive based on material properties such as ionization potential, number density, and refractive index. Time to damage is predicted to occur at or slightly beyond the peak laser intensity. Threshold breakdown fields depend on gas pressure as p<SUP>-1/3</SUP>. Threshold field varies with pulse duration as t<SUP>-1/4</SUP>. The perceived anomalously high strong-field ionization rate of O<SUB>2</SUB> as compared to Xe is explained. Acceptable agreement with measured threshold is seen for a range of materials with 0.000036 &lt; n - 1 &lt; 1.4, and threshold field varies approximately as (Lambda) <SUP>0.77</SUP> for 0.248 micrometers &lt; 1 &lt; 2.94 micrometers . Specific pulse shapes can be simulated and their effects predicted.

The effect of dose rate on the response of austenitic stainless steels to neutron radiation

\nField ionization of hydrocarbon molecules to high charge states is studied as a function of laser pulse duration, peak intensity and molecular alignment. Results are in agreement with the recently proposed mechanism of multi-bond enhanced ionization.\n

Dose rate effects on shape memory epoxy resin during 1 MeV electron irradiation in air

Experimental measurements of K-shell x-ray emission from the front of Cu target irradiated by intense ultra-short duration Ti:Sapphire laser pulse have been performed using x-ray crystal (HAPG/HOPG) spectrograph, for different thicknesses of target, laser angle of incidence and polarization. A wide range of laser intensity (non-relativistic to relativistic) was covered through variation of laser pulse energy, pulse duration and target position with respect to best focus. Maximum x-ray emission flux was observed for 7 µm thick Cu foil for p-polarized 25fs duration laser pulse, for angle of incidence of 550 and maximum laser intensity of ~4x1019 W/cm2 used in the experiment. For both thinner and thicker targets x-ray flux reduced. Interestingly, x-ray flux variation with laser intensity in the relativistic regime (~4x1018- 4x1019 W/cm2), for shortest 25fs duration laser pulse, showed two slightly different scalings i.e. slower/faster variation towards lower/higher end of the intensity regime, and measurement with thicker target of 50µm also showed two scalings but with opposite trend i.e. faster/ slower variation towards lower/higher end. X-ray emission flux decreased with decrease in laser intensity (up to ~1x1017 W/cm2) for longer laser pulse duration (up to 9.8ps), but increased slightly for larger laser spot size ( intensity ~5x1017 W/cm2) on the target and reduced subsequently. Maximum Kα x-ray flux of ~1.4×1010 photons/sr. was estimated for 7 μm target and laser to Kα conversion efficiency is found to be ~1.1x10-5. Geant4 simulations performed to study Kα x-ray flux variation with target thickness suggest that refluxing of electrons lead to ~2.9x enhancement in x-ray flux for the 7 µm foil compared to the 50 µm foil. Further, laser-to-electron conversion efficiency was also estimated to be ~6.5%, which was found to be similar with simulations performed with PrismSPECT spectroscopic code. X-ray flux variations with laser and target parameters are discussed in terms of applicable fast electron generation mechanisms and associated processes of refluxing and reacceleration, along with x-ray reabsorption.

Radiation reaction (RR) is the oldest still-unsolved problem in electrodynamics. In addition to conceptual difficulties in its theoretical formulation, the requirement of exceedingly large charge accelerations has thus far prevented its unambiguous experimental identification. Here, we show how measurable RR effects in a laser-electron interaction can be achieved through the use of flying focus pulses (FFPs). By allowing the focus to counterpropagate with respect to the pulse phase velocity, a FFP overcomes the intrinsic limitation of a conventional laser Gaussian pulse (GP) that limits its focus to a Rayleigh range. For an electron initially also counterpropagating with respect to the pulse phase velocity, an extended interaction length with the laser peak intensity is achieved in a FFP. As a result, the same RR deceleration factors are obtained, but at FFP laser powers orders of magnitude lower than for ultrashort GPs with the same energy. This renders the proposed setup much more stable than those using GPs and allows for more accurate in situ diagnostics. Using the Landau-Lifshitz equation of motion, we show numerically and analytically that the capability of emerging laser systems to deliver focused FFPs will allow for a clear experimental identification of RR.