Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The ...simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface "swelling" occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10 super(11) K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.
In this paper we present femtosecond laser ablation studies of the metals copper, silver and tungsten. Measurements of the threshold fluence determined from the hole diameters versus fluence provide ...incubation coefficients of the three materials, which are found to be equal within one standard deviation. Furthermore, we have determined the single-shot threshold fluences to be 1.7±0.3 J/cm
2
, 1.5±0.4 J/cm
2
and 0.44±0.02 J/cm
2
for copper, silver and tungsten, respectively. These are in good agreement with theoretical values calculated neglecting heat diffusion.
The ablation rates in the regimes of high and low fluence have been examined. In the low-fluence regime, a logarithmic dependence has been found, while the high-fluence data are consistent with a linear dependence. A comparison of the high-fluence data with an approximate analytical solution provides the effective absorptance of the samples, i.e. the fraction of the laser energy that is contributing to the ablation process.
Experimental results of femtosecond laser ablation of the metals copper, silver and tungsten are compared to simulations based on the two-temperature model. The comparison provides new information ...about the laser-heating process: For the noble metals (Cu, Ag), the energy transport via ballistic electrons must be included, while this effect is negligible for a transition metal (W). The comparison provides values for the range of ballistic electrons in the noble metals. The model calculation is also employed to investigate the dependence of the threshold fluence and melting depth on pulse duration. It is observed that for pulses shorter than approximately 1 ps the threshold fluence and melting depth are independent of the pulse duration, while they increase as
τ
0.47
and
τ
0.51
, respectively, for pulses longer than ∼40 ps, in good agreement with approximate analytical expressions predicting a
dependence.
The ablation from a single-crystal Al(111) surface with single ultrashort near-infrared laser pulses has been investigated under ultrahigh vacuum conditions. Scanning-electron and atomic-force ...microscopy of the irradiated surface reveal a surprising development of the material response at increasing fluence: at low fluence, swelling exceeding one hundred nanometers dominates. At higher fluences, a hole is gradually formed in the swollen material, which eventually reaches below the original surface level. The observations indicate the significance of mechanical effects during ablation.
In this contribution, we study the optically stimulated luminescence (OSL) exhibited by commercial Formula: see text:Ce crystals. This photon emission mechanism, complementary to scintillation, can ...trap a fraction of radiation energy deposited in the material and provides sufficient signal to develop a novel post-irradiation 3D dose readout. We characterize the OSL emission through spectrally and temporally resolved measurements and monitor the dose linearity response over a broad range. The measurements show that the Formula: see text centers responsible for scintillation also function as recombination centers for the OSL mechanism. The capture to OSL-active traps competes with scintillation originating from the direct non-radiative energy transfer to the luminescent centers. An OSL response on the order of 100 ph/MeV is estimated. We demonstrate the imaging capabilities provided by such an OSL photon yield using a proof-of-concept optical readout method. A 0.1 Formula: see text spatial resolution for doses as low as 0.5 Gy is projected using a cubic crystal to image volumetric dose profiles. While OSL degrades the intrinsic scintillating performance by reducing the number of scintillation photons emitted following the passage of ionizing radiation, it can encode highly resolved spatial information of the interaction point of the particle. This feature combines ionizing radiation spectroscopy and 3D reusable dose imaging in a single material.
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•Laser-induced optical-change threshold of GST depends strongly on film thickness.•Amorphization requires slightly higher laser fluences.•Two distinct low- and high-fluence ablation ...thresholds are present for thick films.
Modifications induced by ultrashort laser pulses have been investigated in crystalline Ge2Sb2Te5 thin films. The observations comprise optical and structural changes, amorphization, and ablation. By combining optical microscopy and cross-sectional scanning- and transmission electron microscopy, it is found that the threshold for permanent change of the optical properties of Ge2Sb2Te5 – responsible for the reduced optical reflectivity – is slightly lower than that for amorphization; no further change in reflectivity is seen upon amorphization. The laser-fluence thresholds for amorphization and change of the optical properties both show a strong dependence on film thickness that can be explained thermally by two-temperature simulations. In the case of sufficiently thick films, two distinct low- and high-fluence ablation thresholds are found. The mechanisms of the ablation thresholds are discussed, and it is concluded that ablation in the low-fluence regime proceeds by photomechanical spallation.
The ultrafast dynamics of short-pulse-laser-excited fused silica is investigated by measuring polarization-resolved reflectances of a wavelength-tunable probe pulse following a near-infrared pump ...pulse. These data, which should, in principle, allow the determination of both real and imaginary parts of the time-dependent electric permittivity, turn out to become incompatible with an isotropic optical response after ∼200 fs. This is reconciled with a uniaxial permittivity tensor, which predicts greatly enhanced absorption of p-polarized light, suggesting that an absorption channel is missing in the standard optical models for strongly excited dielectrics. The timescale for the excitation-induced variations in the optical properties exhibits a wavelength dependence that can be explained qualitatively by Pauli blocking in the electronic band structure.
Most solid-state detectors, including 3D dosimeters, show lower signal in the Bragg peak than expected, a process termed quenching. The purpose of this study was to investigate how variation in ...chemical composition of a recently developed radiochromic, silicone-based 3D dosimeter influences the observed quenching in proton beams. The dependency of dose response on linear energy transfer, as calculated through Monte Carlo simulations of the dosimeter, was investigated in 60 MeV proton beams. We found that the amount of quenching varied with the chemical composition: peak-to-plateau ratios (1 cm into the plateau) ranged from 2.2 to 3.4, compared to 4.3 using an ionization chamber. The dose response, and thereby the quenching, was predominantly influenced by the curing agent concentration, which determined the dosimeter's deformation properties. The dose response was found to be linear at all depths. All chemical compositions of the dosimeter showed dose-rate dependency; however this was not dependent on the linear energy transfer. Track-structure theory was used to explain the observed quenching effects. In conclusion, this study shows that the silicone-based dosimeter has potential for use in measuring 3D-dose-distributions from proton beams.
Three-dimensional dosimetry of proton therapy (PT) with chemical dosimeters is challenged by signal quenching, which is a lower dose-response in regions with high ionization density due to high ...linear-energy-transfer (LET) and dose-rate. This study aimed to assess the viability of an empirical correction model for 3D radiochromic silicone-based dosimeters irradiated with spot-scanning PT, by parametrizing its LET and dose-rate dependency.
Ten cylindrical radiochromic dosimeters (Ø50 and Ø75 mm) were produced in-house, andirradiated with different spot-scanning proton beam configurations and machine-set dose rates ranging from 56 to 145 Gy/min. Beams with incident energies of 75, 95 and 120 MeV, a spread-out Bragg peak and a plan optimized to an irregular target volume were included. Five of the dosimeters, irradiated with 120 MeV beams, were used to estimate the quenching correction factors. Monte Carlo simulations were used to obtain dose and dose-averaged-LET (LETd) maps. Additionally, a local dose-rate map was estimated,using the simulated dose maps and the machine-set dose-rate informationretrieved from the irradiation log-files. Finally, the correction factor was estimated as a function of LETdand local dose-rate and tested on the different fields.
Gamma-pass-rates of the corrected measurementswere >94% using a 3%-3 mm gamma analysis and >88% using 2%-2 mm, with a dose deviation of <5.6 ± 1.8%. Larger dosimetersshowed a 20% systematic increase in dose-response, but the same quenching in signalwhen compared to the smaller dosimeters.
The quenching correction model was valid for different dosimeter sizes to obtain relative dosimetric maps of complex dose distributions in PT.
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