Function Of Incident Laser Fluence Research Articles

Intrusive effects of repetitive laser pulsing in high-speed tracer-LIF measurements

The effects of repetitive laser pulsing on laser-induced fluorescence (LIF) signals from three popular organic flow tracers, acetone, 3-pentanone, and biacetyl are examined experimentally in the context of high-speed PLIF imaging. The effects of varying the incident laser fluence, laser repetition rates, tracer mole fractions, and carrier gas (air or N2) are investigated. Repetitive laser pulsing leads to changes in the measured LIF signal as a function of laser pulse number for all three tracers. For biacetyl/air mixtures, the LIF signal increases as a function of pulse number and the LIF signal increase per pulse is observed to be a function of the incident laser fluence. For biacetyl/air mixtures at room temperature, the increase in LIF signal during repetitive laser pulsing is attributed solely to absorptive heating of the probe volume, which is confirmed by Rayleigh scattering thermometry measurements. For acetone and 3-pentanone mixtures in the air, the LIF signal decreases with increasing pulse number and the LIF signal depletion per pulse is a linear function of incident laser fluence. This allows the signal depletion per pulse from acetone and 3-pentanone to be normalized by laser fluence and generalized to a single parameter of 0.002%/pulse/(mJ/cm2). There is no discernable effect of varying the tracer mole fraction or the laser repetition rate over the range of values investigated. The substitution of N2 for the air as a carrier gas leads to a significant decrease in the signal depletion per pulse. The potential mechanisms for the enhanced signal depletion in the presence of oxygen are discussed. A likely source is “photo-oxidation”, where the products of laser photolysis react with the surrounding O2 to form the highly reactive hydroxyl (OH) radical, which then oxidizes the tracer. Overall, the current results indicate that under repetitive laser pulsing conditions (i.e., high-speed imaging), the tracer-LIF measurement techniques can be considered intrusive unless the laser fluences are kept sufficiently low. The implications for turbulent flow measurements are discussed including recommendations for minimizing the intrusive repetitive pulsing effects.

Femtosecond pulsed laser ablation dynamics and ablation morphology of nickel based superalloy CMSX-4

Pump-probe shadowgraphic imaging of single pulse femtosecond laser ablation was performed to investigate the dynamics of material removal during femtosecond laser machining of the intermetallic superalloy CMSX-4. Time-resolved shadowgraphic images were collected, which showed the presence of an expanding shock wave in the air in front of the target, following the onset of laser ablation. The dimensions of the shock wave were measured as a function of time (0–10.3ns), following the onset of ablation. The energy release associated with the observed shock wave and the pressure at the shock wave front versus time as a function of incident laser fluence (1.27–62.8J∕cm2) were inferred from the shock dynamics. The measured shock wave dynamics and inferred shock energy release are discussed in light of the evolving ablation morphology and ablated crater depth as a function of incident laser fluence.

Near-field thermal radiative transfer and thermoacoustic effects from vapor plumes produced by pulsed CO2 laser ablation of bulk water

Submillimeter deep heating of bulk water by thermal radiation from ablative water plumes produced by a 10.6μm transversely excited atmospheric CO2 laser and the related acoustic generation has been studied using a contact time-resolved photoacoustic technique. Effective penetration depths of thermal radiation in water were measured as a function of incident laser fluence and the corresponding plume temperatures were estimated. The near-field thermal and thermoacoustic effects of thermal radiation in laser-ablated bulk water and their potential near-field implications are discussed.

Dynamics of Ultrafast Laser Induced Damage in Single Crystal Ni-based Superalloy During Machining

AbstractPump-probe imaging of femtosecond pulsed laser ablation was performed to investigate the mechanical shock induced on an intermetallic superalloy CMSX-4 during femtosecond laser machining. Time resolved shadowgraphic images were collected of the shock wave produced in the air above the target following laser exposure (0-10.3 nanoseconds). The dimensions of the shock wave were measured as a function of delay time and laser fluence (1.27 J/cm2 - 62.8 J/cm2). Time-resolved shadowgraphic images of the ablation event will be presented, and the corresponding damage morphology as a function of incident laser fluence will be discussed.

Femtosecond study of surface structure and composition and time-resolved spectroscopy in metals

Surface structuring and compositioning in aluminum alloy 2024-T3 were demonstrated using a femtosecond pulse laser. Surface nanostructuring was developed as a function of laser parameters and the surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. Surface compositioning was performed by selectively removing the elements on the surface of the sample. Femtosecond studies of highly excited electrons were performed by a pump–probe technique, and the thermalization time was found to be in a range of 1.5–3 ps, increasing with incident fluence. The time-resolved measurement is well matched to the numerical calculation.

Singlet-singlet annihilation in ultraviolet matrix-assisted laser desorption/ionization studied by fluorescence spectroscopy.

Laser-induced fluorescence spectroscopy was carried out on microcrystalline samples of three typical matrices under conditions of matrix-assisted laser desorption/ionization (MALDI). The emitted fluorescence intensity was determined as a function of incident laser fluence and a sublinear increase of the fluorescence intensity with laser fluence was found. A very good fit was obtained when the experimental fluorescence vs. fluence data were compared with a numerical model assuming that under typical MALDI fluence conditions a large fraction of molecules in the excited singlet state undergoes singlet-singlet annihilation. Throughout the fluence range relevant for MALDI, however, the experimental data could not be fit well to a model assuming resonant two-photon absorption as the process depopulating the singlet state. In a separate set of experiments, the singlet lifetimes of several typical crystalline MALDI matrices were determined and found to be considerably shorter than previously reported. While both singlet-singlet annihilation and resonant two-photon absorption have been discussed in the literature as candidates for pathways to primary matrix ion generation in MALDI, the data presented here suggest that singlet-singlet annihilation is the dominant mechanism for depopulating the singlet state in a matrix crystal excited at typical MALDI fluences.

Femtosecond studies of highly excited electrons in metals

Time-resolved measurement of electron temperature was performed through femtosecond studies of highly excited electrons in aluminum alloy, and the process of electron–lattice energy transfer was measured to be 1–3 ps, increasing with the laser fluence. Two ablation regimes were found in the logarithmic dependence of the ablated depth on laser fluence, and two components were attributed to the optical and energy penetration. The surface micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. The surface features ranging from submicron to microns were developed through the variation in laser fluence and count of laser pulse.

Surface microstructuring of aluminum alloy 2024 using femtosecond excimer laser irradiation

Surface microstructuring and modification of aluminum alloy 2024-T3 were investigated using a femtosecond pulse laser irradiation. The surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. The surface features ranging from submicrometer to micrometers were developed through the variation in laser fluence and laser pulse count. Results indicated that surface patterns were uniquely constructed in a controlled use of pulsed laser parameters for the ablation. Two ablation regimes were found in the logarithmic dependence of the ablated depth on laser fluence and two components were attributed to the optical and energy penetration. A thermal dynamic model was adopted to analyze the ablation processes, and the theoretical analysis agreed well with the experimental results.

Surface texturing of aluminum alloy 2024-T3 via femto- and nanosecond pulse excimer laser irradiation

Surface texturing effects of aluminum alloy 2024-T3 using femtosecond and nanosecond pulse laser irradiation were studied. The micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. Results indicated that the surface features, ranging from nano to microdimension, can be developed through variation in laser fluence intensities. Two ablation regimes in the logarithmic fluence dependence of the ablated depth for the 500-fs-pulse irradiation were observed. The theoretical analysis for ablation processes is in good agreement with the experimental results.

Excimer-laser-induced densification of fused silica:laser-fluence and material-grade effects on the scaling law

We present results of the study of 193 nm laser-induced densification of fused silica as a function of incident laser fluence and pulse count. We describe an internally self-consistent experiment involving the simultaneous irradiation of 12 samples of two different grades, at fluences varying by two orders of magnitude and pulse counts exceeding 1.5 × 10 9. We are able to establish the existence of a dose-rate dependence of the densification phenomenon and to characterize the empirical rate constants as a function of fluence for two different grades of fused silica.

Thermionic electron emission from narrow band-gap semiconductors under picosecond laser excitation

A model is presented to relate picosecond laser induced thermionic electron emission to the carrier and phonon dynamics of semiconductors. Silicon is chosen as a model material since its optical and thermal properties are well characterized. The temporal profiles of thermionic emission current and total electron yield are obtained as a function of incident laser fluence for different surface conditions, below the silicon surface melting threshold. Two distinct regimes have been found for the dependence of the electron yield on the laser fluence due to thermionic emission and this behavior is related to Auger recombination.

Mechanism of laser ablation for aqueous media irradiated under confined-stress conditions

Pulsed laser ablation of aqueous medium irradiated under conditions of temporal confinement of thermal stress is described. Time-resolved measurements of laser-induced transient stress waves with simultaneous imaging of ablation process by laser-flash photography were performed. Stress transients induced in aqueous solution of K2CrO4 by ablative nanosecond laser pulses at 355 nm were studied by a broad-band lithium niobate acoustic transducer. Recoil momentum upon material ejection was measured from the temporal profiles of the acoustic transducer signal as a function of incident laser fluence. Cavitation bubbles produced in the irradiated volume during the tensile phase of thermoelastic stress were shown to drive material ejection at temperatures substantially below 100 °C. Experimental data are evident that nanosecond-pulse laser ablation of aqueous media (when temporal stress-confinement conditions are satisfied) include the following two main stages of material ejection: (1) ejection of water microdroplets due to expansion and rupture of subsurface cavitation bubbles; (2) ejection of liquid streams with substantial volume upon collapse of initial crater and large cavitation bubbles in the depth of irradiated volume (after coalescence of smaller bubbles).

Enhanced ArF laser absorption in a collagen target under ablative conditions.

The time-resolved transmission of collagen targets during ArF excimer laser ablation has been measured. The transmitted excimer pulse measurements demonstrate enhanced media attenuation that is a nonlinear function of incident laser fluence. Forward scattering of the transmitted pulse has been assessed to be a negligible contributor to the observed phenomena. Results of pump/probe interrogation of the ablation site indicate that the onset of enhanced attenuation occurs on the time course of the ablating laser pulse and persists for times on the order of hundreds of microseconds.

Ablation of gallstones using excimer laser pulses

Ablation of gallstones using a pulsed KrF excimer laser was studied in both air and water. Acoustic and optical signals were monitored as a function of incident laser fluence. The etch depth per pulse was found to be proportional to the logarithm of the laser fluence. A threshold for plasma generation was observed at a fluence of 0.5 J cm−2, beyond which the etch rate saturates and eventually decreases. Considerable etching was seen below this fluence. This process may have applications in a clinical setting.

Acoustic signals from laser-annealed amorphous silicon

Acoustic signals generated by Q-switch laser irradiation of amorphous Si have been investigated as a function of incident laser fluence. Marked discontinuities in the signal at the recrystallization threshold and at the damage threshold are found. The results indicate that the first effect is due to melting and the second is due to evaporation.

Infrared Laser Photolysis Of Polyatomic Molecules: A Powerful Technique For Studying Elementary Processes In The Gas Phase

The infrared photolysis of polyatomic molecules is discussed. Several examples are presented, with products being detected either by chemiluminescence or by laser-induced fluorescence. The technique allows a study both of the dynamics of unimolecular decomposition and of the reaction kinetics and dynamics of the resulting free radicals. Information on the former is deduced from such features as product energy partitioning, product yield as a function of incident laser fluence, and the effect of collisions on the degree and rate of the dissociation. The latter aspect of the technique is powerfully illustrated by the reactions of C2 (X,a) radicals (produced by photolysis of several organic molecules) with such gases as 02 and NO, from which chemiluminescent reaction products have been detected.