Low Fluence Regime Research Articles

Combining performances of E(m)-corrected LII and absorption for in situ measurements of the volume fraction of 2–4 nm soot particles.

Determining the soot volume fraction (fv) in combustion environments requires detailed knowledge of the optical properties of the soot particles, and in particular of their absorption function E(m). This study addresses a fundamental lack of information on the optical properties of 2–4 nm soot particles. Recent works based on the modeling of the photoelectron emission yields and UV-vis-NIR-absorption measurements found a sharp decrease of E(m) with the particle size in the vis-NIR spectral region, which is inconsistent with the in situ detection of 2–4 nm particles in the near-infrared region by laser-induced incandescence (LII) or sensitive absorption methods like cavity ring-down extinction (CRDE). The objective of this study is twofold: first, an original method for the determination of E(m) of soot particles, including 2–4 nm particles is proposed. Then, the dynamic of two widespread in situ diagnostics, LII and CRDE, are compared over three orders of magnitude of fv in atmospheric premixed ethylene/air flames with different flow rates and C/O. The determination of the absolute value of E(m) and of its variation in the flames is derived from an original analysis, which does not require complex LII modeling. This analysis is based on the comparison between the experimental and calculated LII/LIImax signals in the low fluence regime, LIImax being the plateau value of the fluence curve, which is reached at fluence larger than 1 J/cm2 for the smallest C/O. E(m) is found to vary between 0.15 at low C/O up to 0.36 for the richest flames. Concerning the comparison of the dynamics of LII and CRDE, an excellent agreement is found above a threshold (C/O)limit, while LII exhibits a stronger decrease with C/O below (C/O)limit. This discrepancy is attributed to the spectral dependence of E(m) which is negligible above (C/O)limit, but increases when C/O decreases below (C/O)limit. The particle size distribution function (PSD), measured by scanning mobility particle sizing, reveals monomodal or bimodal PSDs with soot having mobility diameter in the range 2.3–7.5 nm depending on the flame conditions. It is suggested that the particles contained in the first PSD mode, which is dominant in the low C/O range, could be affected by a significant spectral dependence of E(m) in comparison with the second PSD mode.

Two-temperature model for ultrafast melting of Au-based bimetallic films interacting with single-pulse femtosecond laser: Theoretical study of damage threshold

Ultrashort-pulse laser excitation and associated thermal damage of single-layer metallic films have been extensively studied both theoretically as well as experimentally. However, study of ultrafast heat transfer mechanism in bimetallic films, a topic of immense current interest for designing next-generation optical components with better thermal resilience, is limited to a low fluence regime. Moreover, available results for damage threshold (DT) of bimetallic films have several ambiguities. The current work is an attempt towards development of comprehensive theory for the DT of Au-based bimetallic films, analyzing its dependence on associated physical parameters and bridging the gap in existing literature. This has been achieved by developing fully implicit two-temperature model-based Python code. Excellent agreement of the predicted temperature profiles with recently reported [E. L. Gurevich, Y. Levy, S. V. Gurevich, and N. M. Bulgakova, Phys. Rev. B 95, 054305 (2017)] femtosecond laser irradiated Au film validates the code. Subsequently, the code is utilized to investigate the influence of different metallic substrates, $M$ ($M$ = Ni, Cr, Cu) on ultrafast melting damage of $\mathrm{Au}\text{/}M$ films excited by single-pulse femtosecond laser. Simulations carried out for increasing Au layer thickness revealed a nonmonotonic variation of DT for all substrates. This study also brought out an important observation that for every substrate metal, there exists an optimum Au thickness for maximizing the incipient and the complete melting threshold. Maximum achievable enhancement for $\mathrm{Au}\text{/}M$ films over pure Au are found to be 30%, 22%, and 14% with Cu, Ni, and Cr substrates, respectively. The observed distinction in maximum DT among three different $\mathrm{Au}\text{/}M$ targets is explained by developing appropriate thermophysical models. Finally, an empirical function is proposed here for expressing nonmonotonic variation of two-layer DTs. Very good agreement between the substrate-specific peak positions predicted by the proposed function and effective electron diffusion length arising from thermophysical analysis makes the modeling robust and applicable for a wide range of laser wavelengths as well as arbitrary combinations of bimetallic films.

Probe and control of photo-excited magnetization precession in Co/Pd multilayer films at low laser fluence regime

We present femtosecond-pulse-induced precession of magnetization at low laser fluence (<5μJ/cm2) regime as a function of magnetic field and laser fluence in three Co/Pd multilayer (ML) systems. These systems belong to three different regimes of magnetic anisotropy that vary with Co thickness (tCo): in-plane (sample 1, tCo = 0.74 nm), weakly out-of-plane (sample 2, tCo = 0.6 nm), and out-of-plane (sample 3, tCo = 0.40 nm). Interestingly, we observed that the precession amplitudes increase significantly with decreasing the Co layer thickness. In this study, the influence of various spin dynamics and static magneto-optical parameters on precession amplitude is examined critically and compared with a previously proposed analytical expression that connects those quantities. It is found that the enhancement of structural-dependent energy transfer efficiency between charge and spin subsystems is indeed responsible for the observed variations in precession amplitudes. On the basis of this fact, we discuss that the spin–orbit interaction that yields perpendicularly spin-polarized electrons in the MLs through the Co/Pd interface is responsible for the observed increase in precession amplitudes of locally excited magnetization. Our approach of employing low-fluence laser excitation of magnetization precession could be practical for developing a non-thermal, all-optical magnetic switching toward photonic memory applications.

Energetic electron irradiations of amorphous and crystalline sulphur-bearing astrochemical ices.

Laboratory experiments have confirmed that the radiolytic decay rate of astrochemical ice analogues is dependent upon the solid phase of the target ice, with some crystalline molecular ices being more radio-resistant than their amorphous counterparts. The degree of radio-resistance exhibited by crystalline ice phases is dependent upon the nature, strength, and extent of the intermolecular interactions that characterise their solid structure. For example, it has been shown that crystalline CH3OH decays at a significantly slower rate when irradiated by 2 keV electrons at 20 K than does the amorphous phase due to the stabilising effect imparted by the presence of an extensive array of strong hydrogen bonds. These results have important consequences for the astrochemistry of interstellar ices and outer Solar System bodies, as they imply that the chemical products arising from the irradiation of amorphous ices (which may include prebiotic molecules relevant to biology) should be more abundant than those arising from similar irradiations of crystalline phases. In this present study, we have extended our work on this subject by performing comparative energetic electron irradiations of the amorphous and crystalline phases of the sulphur-bearing molecules H2S and SO2 at 20 K. We have found evidence for phase-dependent chemistry in both these species, with the radiation-induced exponential decay of amorphous H2S being more rapid than that of the crystalline phase, similar to the effect that has been previously observed for CH3OH. For SO2, two fluence regimes are apparent: a low-fluence regime in which the crystalline ice exhibits a rapid exponential decay while the amorphous ice possibly resists decay, and a high-fluence regime in which both phases undergo slow exponential-like decays. We have discussed our results in the contexts of interstellar and Solar System ice astrochemistry and the formation of sulphur allotropes and residues in these settings.

Dual-pulse laser-induced incandescence to quantify carbon nanostructure and related soot particle properties in transient flows – Concept and exploratory study

In this work a dual-pulse laser-induced incandescence method is presented, which is potentially suitable for the in-situ determination of E(m,λUV)/E(m,λNIR) in transient flows. The dual-pulse LII is irradiating a particle ensemble by two subsequent laser pulses of different wavelengths with a time delay long enough to ensure cooling of the particles after the first pulse, but short enough to prevent the particles from moving out of the probe volume. In the experiment, the first pulse is kept at low and constant fluence, whereas the fluence of the second pulse is periodically modulated. If coincidence of the generated two LII signals is repeatedly achieved, E(m,λi)/E(m,λj) can be evaluated. The synchronous temperature determination of the laser-pulse heated particles even enables the quantification of the absolute values of E(m,λi) and E(m,λj). In an exploratory study using model soot particles generated in a spark discharge generator, the applicability of the method is investigated in detail. The nanostructural and optical properties of the model particles, the influence of the first pulse and other boundary conditions are studied comprehensively. Both, E(m,λ) and E(m,λi)/E(m,λj) are in excellent agreement with ex-situ analyses of the nanostructure and data reported in the literature. In the low-fluence regime of the first pulse, nanostructural modifications of the particles can be ruled out. It is finally shown that the dual-pulse LII not only offers a rapid, non-intrusive diagnostic method of averaged E(m,λi)/E(m,λj) and absolute E(m,λ) values as well as primary particle sizes, but also enables the determination of their statistical distributions, which in turn provides inferences about the nano structure and properties such as graphene-like layer lengths distribution and characteristics inferred thereof.

Thermal and non-thermal ablation mechanisms in crystalline silicon by femtosecond laser pulses: classical approach of the carrier density two temperature model

Laser micromachining has attracted considerable interest because of its wide range of applications across nearly all manufacturing sectors and mostly in semiconductors such as silicon. However, modern micro-manufacturing demands progressive product miniaturization, high accuracy, and high-precision material removal. For this purpose, a fundamental study of the interaction between ultrashort laser pulses and silicon will be valuable for studying ablation characteristics and ablation performance. The femtosecond laser pulse interaction with the silicon is divided into five parts: (a) the interaction of laser light with the carriers, (b) variation of the carrier density and carrier temperature, (c) energy exchange between carriers and lattice, (d) thermomechanical response of the material, and (e) ablation. The evolution of the carrier density and carrier-lattice energy coupling equation is solved simultaneously to determine the optimum value of the ablation width and ablation depth of femtosecond laser pulses on the silicon. The first time, 2D axial symmetry thermal and non-thermal ablation profiles were compared with the experimental result at fluence ranging from 0.75 to 9 J cm−2 at the wavelength of 515 nm and 180 fs laser on the silicon sample. A comparative study of damage thresholds from experiments and simulations is presented. The concordance between model calculations and experimental data demonstrates that fs laser ablation is thermal in nature in low fluence regime, whereas it is non- thermal in a high-fluence regime. Fundamental information such as the time evolution of the carrier density, carrier temperature evolution, and lattice temperature evolution can be obtained from the simulation results.

Dynamic strain evolution in an optically excited Pt thin film.

The structural evolution of a Pt thin film following photo-thermal excitation by 1 ps optical laser pulses was studied with a time resolution of 100 ps over a total time period of 1 ms. Laser pulse fluences below 50 mJ/cm2 were insufficient to relax the residual stress state of the as-prepared film even after 10 000 pulses. In this fluence regime, a rapid initial lattice expansion and a decrease in the lattice coherence length due to ultrafast photo-thermal heating were observed. The lattice expansion reached a maximum, and the coherence length reached a minimum, 100–200 ps after excitation before monotonically decaying back to their initial values in about 1 µs. Laser pulse fluences greater than 50 mJ/cm2 produced irreversible stress relaxation within the first 10 optical pulses. In this regime, the lattice expansion was qualitatively similar to that in the low fluence regime, except that the initial structural state was not recovered. The evolution in the coherence length, however, was more complex. Following an initial decrease similar to that observed at low fluence, the coherence length then increased to a broad maximum greater than the initial value, before recovery.

Laser-induced incandescence (2\u03bb and 2C) for estimating absorption efficiency of differently matured soot

Two-wavelength and two-color laser-induced incandescence (2λ–2C-LII) was used to study the absorption properties of three types of cold soot of different maturity from a mini-CAST soot generator. LII fluence curve analysis allowed for estimating absorption wavelength dependence in terms of dispersion coefficients ξ by the use of two excitation wavelengths (532 and 1064 nm). The estimated ξ (based on E(m, λ) ∝ λ1−ξ) spanned from ~ 1.2 for the mature soot, up to 2.3 for the young soot. The results for the mature soot showed good agreement with previous measurement using multi-wavelength extinction. For the young soot, however, some discrepancy was observed suggesting a weaker wavelength dependence (lower ξ) from the LII fluence analysis. Furthermore, an estimation of the E(m, λ) for the different types of soot was done from the experimental fluence curves with temperature analysis in the low-fluence regime and simulations using an LII model. Additionally, uncertainties and limitations were discussed. Finally, it should be pointed out that caution has to be taken when interpreting 2λ-LII results to obtain quantitative absorption properties of less mature soot, which may be influenced by thermal annealing during the laser pulse and by absorption from non-refractory species externally/internally mixed with the soot.

On thermal regime of nanoparticles in synthesis flame

Flame Synthesis conditions significantly affect nanoparticles properties. Understanding thermal regime in flames can be very useful for tuning nanoparticles with specific properties. In this work, information on thermal status is obtained through the analysis of light emission from nanoparticles during synthesis and under an external laser irradiation. We focus on the dependence of the particle temperature on laser fluence as reported in our previous work. We discuss temperature plateau observed in the low laser fluence regime as attributed to non-isothermality, that means the local coexistence of partially solidified particles at melting point and much hotter particles in a flame spray.

Simultaneous determination of primary particle size distribution and thermal accommodation coefficient of soot aggregates using low-fluence LII.

For the ill-posed inverse problem of LII-based nanoparticle size measurement, recovered primary particle size distribution (PPSD) is sensitive to the uncertainty of LII model parameters. In the absence of reliable prior knowledge, the thermal accommodation coefficient (TAC) and fractal-dependent shielding factor are often required to be inferred simultaneously with the PPSD. In the simplified LII model for low fluence regime, TAC and fractal-dependent shielding factor are combined to define a new fractal-dependent TAC. The present study theoretically verified the feasibility of inferring PPSD and fractal-dependent TAC from the normalized LII signals. Moreover, the inversion is independent of prior knowledge of most full LII model parameters, which is attributed to low laser fluence, normalized signal, and fractal-dependent TAC.

Evolution of the magnetic and polaronic order of Pr1/2Ca1/2MnO3 following an ultrashort light pulse

The dynamics of electrons, spins and phonons induced by optical femtosecond pulses has been simulated for the polaronic crystal $\rm{Pr_{1/2}Ca_{1/2}MnO_3}$. The model used for the simulation has been derived from first-principles calculations. The simulations reproduce the experimentally observed melting of charge/orbital order with increasing fluence. The loss of charge order in the high-fluence regime induces a transition to a ferromagnetic metal. At low fluence, the dynamics is deterministic and coherent phonons are created by the repopulation of electronic orbitals, which are strongly coupled to the phonon degrees of freedom. In contrast to the low-fluence regime, the magnetic transitions occurring at higher fluence can be attributed to a quasi-thermal transition of a cold-plasma-like state with hot electrons and cold phonons and spins. The findings can be rationalized in a more complete picture of the electronic structure that goes beyond the simple ionic picture of charge order.

Progressive Self-Boosting Anapole-Enhanced Deep-Ultraviolet Third Harmonic During Few-Cycle Laser Radiation

Nanoantennas made of high-index semiconductors with a strong nonlinearity and supported optical Mie-type resonances offer a promising alternative platform for nonlinear nanophotonics. In this Letter, we employ an array of amorphous silicon nanodisks with varying diameters to produce a broadband deep-ultraviolet third harmonic of a few-cycle Ti:sapphire oscillator. Ultrashort light pulses efficiently deposit their energy at the center of the disks where the electric field is strongly amplified by the anapole states. This leads to a progressive material modification in an extreme multishot (>1010 pulses) and a rather low fluence (<10–3 J/cm2) regime, drastically differing from other known mechanisms, such as nonthermal plasma annealing or thermal melting-induced recrystallization. We suggest that the material modification is due to femtosecond laser-induced excitation of dangling bonds, which leads to a gradual boosting of the third harmonic conversion efficiency and broadening of its spectral bandwidth.

Femtosecond laser induced surface nanostructures on SrTiO3

SrTiO3 (STO) is one of the key perovskite oxides with fascinating properties in electronics and optoelectronics. The generation and evolution of surface nanostructures induced by femtosecond lasers on STO surfaces are studied, showing distinct features with a clear fluence and pulse number dependence. Unlike regular ripples, which are either parallel or perpendicular to the laser polarization, the ripples coexist in both directions after irradiated with four pulses, and they cover the whole structured area. When increasing the number of laser pulses, the ripples parallel to the laser polarization dominate at a lower fluence regime, while the ripples perpendicular to the laser polarization are the only surface structures formed at a higher fluence regime. The finite-difference time-domain method and the Drude model have been adopted to analyze energy deposition on the STO surface. The results demonstrate that the combination of energy deposition at different excitation levels and the feedback effect is the origin of the formation and evolution behavior of the surface nanostructures on STO.

Femtosecond-laser-induced modifications of Ge2Sb2Te5 thin films: Permanent optical change without amorphization

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.

Ultrashort pulsed laser ablation of stainless steels

Abstract This research investigated the ablation process of commercial stainless steels with ultrashort pulsed laser. Square hollows were ablated on stainless steel sheets using a picosecond laser with pulse duration of 0.25, 1 and 10 ps, respectively, and fluence ranging from 0.125 J/cm2 to 5 J/cm2. For each setting the surface quality and ablation efficiency were determined by optical microscopy and scanning electron microscopy (SEM). Processing windows producing high quality surfaces at high ablation efficiency were found at a fluence around 0.75 J/cm2 for the two shorter pulse lengths tested. Individual cones and periodic cone-like protrusions were found in the low fluence regime ( 0.875 J/cm2), respectively, both of which make the ablated surface rough. Emphasizing on the individual cones, it is found that the cones are caused by inclusions in the base materials, attributed to a higher fluence threshold required for ceramic-like inclusions. This novel theory explaining the creation mechanism is verified by multiple analysis methods like SEM and energy dispersive X-ray spectroscopy and stop-motion SEM imaging.

Low damage electrical modification of 4H-SiC via ultrafast laser irradiation

The electrical properties of 4H-SiC under ultrafast laser irradiation in the low fluence regime (&amp;lt;0.50 J/cm2) are presented. The appearance of high spatial frequency laser induced periodic surface structures is observed at a fluence near 0.25 J/cm2 and above, with variability in environments like in air, nitrogen, and a vacuum. In addition to the formation of periodic surface structures, ultrafast laser irradiation results in possible surface oxidation and amorphization of the material. Lateral conductance exhibits orders of magnitude increase, which is attributed to either surface conduction or modification of electrical contact properties, depending on the initial material conductivity. Schottky barrier formation on ultrafast laser irradiated 4H-SiC shows an increase in the barrier height, an increase in the ideality factor, and sub-bandgap photovoltaic responses, suggesting the formation of photo-active point defects. The results suggest that the ultrafast laser irradiation technique provides a means of engineering spatially localized structural and electronic modification of wide bandgap materials such as 4H-SiC with relatively low surface damage via low temperature processing.

Laser-ablation-based ion source characterization and manipulation for laser-driven ion acceleration

For laser-driven ion acceleration from thin foils (∼10 μm–100 nm) in the target normal sheath acceleration regime, the hydro-carbon contaminant layer at the target surface generally serves as the ion source and hence determines the accelerated ion species, i.e. mainly protons, carbon and oxygen ions. The specific characteristics of the source layer—thickness and relevant lateral extent—as well as its manipulation have both been investigated since the first experiments on laser-driven ion acceleration using a variety of techniques from direct source imaging to knife-edge or mesh imaging. In this publication, we present an experimental study in which laser ablation in two fluence regimes (low: F ∼ 0.6 J cm−2, high: F ∼ 4 J cm−2) was applied to characterize and manipulate the hydro-carbon source layer. The high-fluence ablation in combination with a timed laser pulse for particle acceleration allowed for an estimation of the relevant source layer thickness for proton acceleration. Moreover, from these data and independently from the low-fluence regime, the lateral extent of the ion source layer became accessible.

Effects of pulse durations and environments on femtosecond laser ablation of stainless steel

The influence of pulse durations (35fs and 260 fs) and environments (air and vacuum) on the laser-induced damage thresholds (LIDTs) and ablation rates of 304 stainless steel were studied. Two distinct ablation regimes were obtained from the ablation rate curves. At low fluence regime, the ablation rates were similar in spite of the differences of pulse durations and experiment environments. At high fluence regime, the ablation rates of 35 fs pulse duration in vacuum were obviously higher than others. The ablation craters showed smooth edges, moth-eye such as structures, and laser-induced periodic surface structures (LIPSSs). At a fixed fluence, the periods of LIPSSs decreased monotonously in their mean spatial period between ~ 700 nm (5 pulses) and 540 nm (200 pulses) with the increase of pulse numbers in air with 35 fs pulse duration. The formation mechanisms of moth-eye like structures and LIPSSs were also discussed.

Engineering of electronic properties of single layer graphene by swift heavy ion irradiation

In this work, swift heavy ion irradiation induced effects on the electrical properties of single layer graphene are reported. The modulation in minimum conductivity point in graphene with in-situ electrical measurement during ion irradiation was studied. It is found that the resistance of graphene layer decreases at lower fluences up to 3 × 1011 ions/cm2, which is accompanied by the five-fold increase in electron and hole mobilities. The ion irradiation induced increase in electron and hole mobilities at lower fluence up to 1 × 1011 ions/cm2 is verified by separate Hall measurements on another irradiated graphene sample at the selected fluence. In contrast to the adverse effects of irradiation on the electrical properties of materials, we have found improvement in electrical mobility after irradiation. The increment in mobility is explained by considering the defect annealing in graphene after irradiation at a lower fluence regime. The modification in carrier density after irradiation is also observed. Based on findings of the present work, we suggest ion beam irradiation as a useful tool for tuning of the electrical properties of graphene.

Modification of magnetization orientation in Pt/Co/Pt ultrathin films by femtosecond laser pulses

Ultrathin Pt/Co/Pt films were irradiated with femtosecond laser pulses of sufficiently high energy to create irreversible structural and in consequence magnetic modifications. The effect was studied as a function of an initial structure of the Co film, determined by different temperatures during preparation of the Pt buffer layer, and a Co film thickness, which determines the initial out-of-plane or in-plane magnetization orientation. A creation of many magnetic phases is shown. The out-of-plane magnetization state is found to be created for significantly broader fluence range for the Co film deposited on the Pt buffer layer grown at 750°C in comparison to that grown at room temperature. Reversible magnetization dynamics was also studied in the low fluence regime, using a pump-probe setup. The results were discussed using simulations of temperature dynamics.