Absorption Of Laser Pulses Research Articles

Collisionless absorption, hot electron generation, and energy scaling in intense laser-target interaction

Among the various attempts to understand collisionless absorption of intense and superintense ultrashort laser pulses, a whole variety of models and hypotheses has been invented to describe the laser beam target interaction. In terms of basic physics, collisionless absorption is understood now as the interplay of the oscillating laser field with the space charge field produced by it in the plasma. A first approach to this idea is realized in Brunel's model the essence of which consists in the formation of an oscillating charge cloud in the vacuum in front of the target, therefore frequently addressed by the vague term “vacuum heating.” The investigation of statistical ensembles of orbits shows that the absorption process is localized at the ion-vacuum interface and in the skin layer: Single electrons enter into resonance with the laser field thereby undergoing a phase shift which causes orbit crossing and braking of Brunel's laminar flow. This anharmonic resonance acts like an attractor for the electrons and leads to the formation of a Maxwellian tail in the electron energy spectrum. Most remarkable results of our investigations are the Brunel like spectral hot electron distribution at the relativistic threshold, the minimum of absorption at Iλ2≅(0.3−1.2)×1021 Wcm−2μm2 in the plasma target with the electron density of neλ2∼1023cm−3μm2, the drastic reduction of the number of hot electrons in this domain and their reappearance in the highly relativistic domain, and strong coupling, beyond expectation, of the fast electron jets with the return current through Cherenkov emission of plasmons. The hot electron energy scaling shows a strong dependence on intensity in the moderately relativistic domain Iλ2≅(1018−1020) Wcm−2μm2, a scaling in vague accordance with current published estimates in the range Iλ2≅(0.14−3.5)×1021 Wcm−2μm2, and again a distinct power increase beyond I=3.5×1021 Wcm−2μm2. The low energy electrons penetrate normally to the target surface, the energetic electrons propagate in laser beam direction.

Simulation of Excitation and Propagation of Pico-Second Ultrasound

This paper presents an analytic and numerical simulation of the generation and propagation of pico-second ultrasound with nano-scale wavelength, enabling the production of bulk waves in thin films. An analytic model of laser-matter interaction and elasto-dynamic wave propagation is introduced to calculate the elastic strain pulse in microstructures. The model includes the laser-pulse absorption on the material surface, heat transfer from a photon to the elastic energy of a phonon, and acoustic wave propagation to formulate the governing equations of ultra-short ultrasound. The excitation and propagation of acoustic pulses produced by ultra-short laser pulses are numerically simulated for an aluminum substrate using the finite-difference method and compared with the analytical solution. Furthermore, Fourier analysis was performed to investigate the frequency spectrum of the simulated elastic wave pulse. It is concluded that a pico-second bulk wave with a very high frequency of up to hundreds of gigahertz is successfully generated in metals using a 100-fs laser pulse and that it can be propagated in the direction of thickness for thickness less than 100 ㎚.

Optical and spectroscopic diagnostics of laser-induced air breakdown and kerosene spray ignition

This study focuses on the transition from a laser-induced breakdown plasma to a flame kernel in two-phase flows. The test rig was a vertical flow channel with a full-cone spray nozzle installed inside. The fuel was Jet A-1 aviation kerosene. Breakdowns were generated by the focused laser pulses from a frequency-doubled and Q-switched Nd:YAG laser. The investigation of laser-induced breakdowns in ambient air provided valuable supplementary data to understand the interaction of the breakdown plasma and the fuel spray. To determine the breakdown energy, the amount of absorbed laser pulse energy was measured, and the blast wave energy consumption was estimated. Blast waves were visualized with high-speed schlieren imaging. Their energies were estimated by the application of Jones’ blast wave expansion model. High-speed imaging of air and spray breakdowns visualized their transient morphologies. Expansion velocities of air breakdowns were determined and revealed a supersonic expansion during the first few microseconds. Air breakdowns decayed and disappeared within 30μs. Spray breakdowns were observed over a period of 90μs, which covered their transition into flame kernels. Optical emission spectroscopy was applied to ambient air breakdowns, spray ignitions and spray breakdowns in nitrogen. The temporal decrease of nitrogen ion and atom lines was investigated, and mean lifetimes were determined. CN∗, C2∗ and CH∗ radicals were observed in spray ignitions, but no CH∗ was confirmed in spray breakdowns in nitrogen, while CN∗ and C2∗ occurred with a similar intensity as in spray ignitions. Simulated spectra were fitted to the CN∗ B2Σ+–X2Σ+ band between 384.2 and 388.4nm to determine temperatures at the breakdown region during the transition from breakdown plasma into spray flame kernels.

X-ray generation under oblique incidence of femtosecond laser pulses with kHz pulse repetition rate on a metal foil with irregular relief

The possibility of improving the efficiency of X-ray generation under the interaction of femtosecond laser pulses (exhibiting high repetition rate and subrelativistic intensity) with a thin copper foil by changing the angle of incidence of the laser radiation is studied both experimentally and theoretically. The brightness of such an X-ray source in the conducted experiments considerably exceeded that of a conventional X-ray tube. The structure of the target surface before and after interaction is described in detail, and specific features of the influence of this structure on absorption of laser pulses are analyzed.

A spatiotemporal study of the relativistic nonlinear effects on laser absorption by a finite-size magneto plasma

In this paper, the relativistic electromagnetic 1.5 dimensional particle-in-cell code has been employed to investigate the temporal behavior of nonlinear absorption of a short intense laser pulse in finite-size magnetized sub-critical (i.e. under the critical density) plasma. This work has been done by relying on a detailed temporal study of the phase-mixing and scattering processes occurring in the finite-size system of laser-plasma interaction. Part of the results obtained in this manuscript confirm recent important theoretical findings, including the wave-breaking or phase-mixing phenomena of relativistic upper-hybrid (UH) oscillations in magnetized plasma [C. Maity, A. Sarka, P.K. Shukla, N. Chakrabarti, Phys. Rev. Lett. 110, 215002 (2013)]. Moreover, we found the explicit reasons for the anomalous behavior of the absorption rate in magnetized plasma by scrutinising the phase-mixing time and scattering process in finite-size magnetized plasma for different values of laser and plasma parameters.

Enhanced light absorption in nanotextured amorphous thin-film silicon caused by femtosecond-laser materials processing

Efficient thin-film solar cells balance the reduced absorption occurring in thin absorber layers by means of various photon management strategies that often involve randomly nanotextured interfaces. We report on broadband absorption enhancement in nanotextured amorphous silicon processed by femtosecond laser materials processing. As identified by micro-Raman spectroscopy and surface profilometry, the absorption of a single femtosecond amplifier laser pulse (30fs, 795nm, 75mJcm−2) creates a thin nanotextured micro-crystalline surface layer. Optical microscopy in transmission and reflection geometry reveals a broadband absorption enhancement in the visual spectrum range for the nanotextured area. Scattered light spectroscopy in combination with spectral interferometry indicates that light trapping for about 100fs is achieved in the femtosecond-laser processed amorphous silicon area and thus is responsible for the observed enhanced absorption and locally enhanced Raman yields. Thus fs-laser materials processing offers an interesting pathway towards advanced photon management in amorphous silicon based thin-film solar cells.

Indication of Te segregation in laser-irradiated ZnTe observed by in situ coherent-phonon spectroscopy

We irradiate a ZnTe single crystal with 10-fs laser pulses at a repetition rate of 80 MHz and investigate its resulting gradual modification by means of coherent-phonon spectroscopy. We observe the emergence of a phonon mode at about 3.6 THz whose amplitude and lifetime grow monotonously with irradiation time. The speed of this process depends sensitively on the pump-pulse duration. Our observations strongly indicate that the emerging phonon mode arises from a Te phase induced by multiphoton absorption of incident laser pulses. A potential application of our findings is laser-machining of microstructures in the bulk of a ZnTe crystal, a highly relevant electrooptic material.

Two-photon-induced internal modification of silicon by erbium-doped fiber laser.

Three-dimensional bulk modification of dielectric materials by multiphoton absorption of laser pulses is a well-established technology. The use of multiphoton absorption to machine bulk silicon has been investigated by a number of authors using femtosecond laser sources. However, no modifications confined in bulk silicon, induced by multiphoton absorption, have been reported so far. Based on results from numerical simulations, we employed an erbium-doped fiber laser operating at a relatively long pulse duration of 3.5 nanoseconds and a wavelength of 1549 nm for this process. We found that these laser parameters are suitable to produce modifications at various depths inside crystalline silicon.

Optical discharge with absorption of repetitive laser pulses in supersonic air flow: wave structure and condition of a quasi-steady state

We report a study of the wave structure formed by an optical discharge plasma upon the absorption of repetitively pulsed laser radiation in a supersonic air flow. Experimental data are presented on the configuration of the head shock wave and the geometry and characteristic dimensions of breakdown regions behind a laser plasma pulsating in the flow at a frequency of up to . The data are compared to calculation in a point explosion model with allowance for counterpressure, which makes it possible to identify the relationship between laser radiation and supersonic flow parameters that ensures quasi-steady-state energy delivery and is necessary for extending the possibilities of controlling the structure of supersonic flows.

Heating of a metal nanofilm during femtosecond laser pulse absorption

We have studied the temperature evolution of electrons and the lattice of a metal nanofilm interacting with a femtosecond s- or p-polarised pulse. It is shown that even if the film thickness is greater than the skin-layer depth, the temperature distribution during the pulse action may be close to the uniform one because of the high electron thermal conductivity, which leads to a rapid redistribution of energy over the film thickness.

Nonlinear absorption and optical damage threshold of carbon-based nanostructured material embedded in a protein

Physical processes in laser–matter interaction used to be determined by generation of fast electrons resulting from efficient conversion of the absorbed laser radiation. Composite materials offer the possibility to control the absorption by choice of the host material and dopants. Reported here strong absorption of ultrashort laser pulse in a composite carbon-based nanomaterial including single-walled carbon nanotubes (SWCNTs) or multilayer graphene was measured in the intensity range between 1012 and 1016 W cm−2. A protein (lysozyme) was used as the host. The maximum absorption of femtosecond laser pulse has reached 92–96 %. The optical damage thresholds of the coatings were registered at an intensity of (1.1 ± 0.5) × 1013 W cm−2 for the embedded SWCNTs and at (3.4 ± 0.3) × 1013 W cm−2 for the embedded graphene. Encapsulated variant of the dispersed nanomaterial was investigated as well. It was found that supernatant protein in the coating material tends to dominate the absorption process, independently of the embedded nanomaterial. The opposite was observed for the encapsulated material.

Artifact removal in photoacoustic section imaging by combining an integrating cylindrical detector with model-based reconstruction

Photoacoustic section imaging reveals optically absorbing structures within a thin slice of an object. It requires measuring acoustic waves excited by absorption of short laser pulses with a cylindrical acoustic lens detector rotating around the object. Owing to the finite detector size and its limited depth of focus, various artifacts arise, seen as distortions within the imaging slice and cross-talk from neighboring areas of the object. The presented solution aims at avoiding these artifacts by a special design of the sensor and by use of a model-based reconstruction algorithm that improves section images by incorporating information from neighboring sections. The integrating property of the cylindrical detector, which exceeds in direction of the cylinder axis the size of the imaged object, avoids the lateral blurring that normally results from the finite width of a small detector. Applying a maximum likelihood reconstruction method for the inversion of the imaging system matrix to the temporal pressure signals yields line projections of the initial energy distribution, from which section images are obtained by applying the inverse Radon transform. By using data from few sections, a significant reduction of artifacts related to the imperfections of the sensor is demonstrated both in simulations and in phantom experiments.

Generation, transport, and focusing of fast electrons in nanofilaments of a target irradiated by a short laser pulse with ultrarelativistic intensity

Absorption of an intense short laser pulse by targets consisting of a bungle of carbon nanofilaments either parallel or converging to the vertex of a cone has been numerically simulated. Such targets efficiently generate a flux of relativistic electrons propagating along filaments and following their bends. The focusing of fluxes from several filaments to one flux makes it possible to reduce the transverse size of an electron bunch as compared to the diameter of the laser beam. The energy flux density of electrons in such a bunch propagating along one nanofilament is several times higher than the energy flux density of the laser pulse.

Photoacoustic measurements of UV laser Pulse (266 nm) absorption in mixtures of water vapor with nitrogen

Results of photoacoustic measurements of absorption of 266 nm-laser pulses (the fourth harmonics of a YAG laser) by mixtures of water vapor and nitrogen, depending on the radiation intensity (0.5–10 MW cm−2) and H2O partial pressure (0–10 mbar), are presented. It is shown that the linear absorption (in the given range of intensities) increases with H2O partial pressure in a range 0–5 mbar and remains almost stable in a range 5–10 mbar exceeding the absorption in the pure nitrogen only by two orders of magnitude.

Successive laser-induced breakdowns in atmospheric pressure air and premixed ethane–air mixtures

Two successive focused laser pulses are employed to experimentally simulate laser-induced breakdown plasmas at high repetition rates. We find that energy absorption of the second laser pulse by the plasma produced by the first laser pulse is enhanced slightly when the time interval between the pulses is shorter than several tens of nanoseconds but falls to almost zero when the time interval is between a few hundreds of nanoseconds and several tens of microseconds. This behavior is attributed to gas heating by the first breakdown event. In premixed ethane–air mixtures, we identify another strong reduction in the second laser pulse absorption when this pulse coincides with the heat released by combustion, typically milliseconds after the first laser pulse. The fuel–air equivalence ratio (ϕ) and base flow speed are also varied in this study. The results show that the window of reduced absorption coinciding with heat release due to combustion is narrowed when the base flow speed is increased, and also under fuel lean and fuel rich conditions. These results suggest that the use of pulsed high frequency laser breakdowns for premixed combustion stabilization is optimized when laser pulse repetition rates below a certain frequency (e.g., 500Hz at the conditions that ϕ is 1 and the base flow speed is 4.9m/s) to maximize laser energy coupling and for improved anchoring of the flame base.

Nonlinear contrast enhancement in photoacoustic molecular imaging with gold nanosphere encapsulated nanoemulsions.

A composite contrast agent, a nanoemulsion bead with assembled gold nanospheres at the interface, is proposed to improve the specific contrast of photoacoustic molecular imaging. A phase transition in the bead's core is induced by absorption of a nanosecond laser pulse with a fairly low laser fluence (∼3.5 mJ/cm2), creating a transient microbubble through dramatically enhanced thermal expansion. This generates nonlinear photoacoustic signals with more than 10 times larger amplitude compared to that of a linear agent with the same optical absorption. By applying a differential scheme similar to ultrasound pulse inversion, more than 40 dB contrast enhancement is demonstrated with suppression of background signals.

Формирование ударной волны в металле в процессе поглощения короткого импульса лазерного излучения

Формирование ударной волны в металле в процессе поглощения короткого импульса лазерного излучения

Modification of a-Si: H Films via Non-linear Femtosecond Laser Pulse Absorption

The modification of a-Si:H via non-linear femtosecond laser pulse absorption was studied and the characteristic thresholds for hydrogen diffusion/effusion, crystallization and material ablation were determined. To consider the impact of the hydrogen content on laser materials processing, a-Si:H was deposited at different temperatures (25°C, 200°C, 520°C) resulting in different hydrogen contents (30%, 13%, and <1%). Essential information for device applications such as the degree of crystallization and the Si-H dissociation are obtained from micro-Raman spectroscopy of the laser treated areas. The prospects of a flexible non-linear fs laser material processing of a-Si:H for bulk and surface modification will be discussed.

Simulation of the absorption of a femtosecond laser pulse in crystalline silicon

The effect of the nonlinearity of the absorptivity and absorption coefficient on the process of the intense photoexcitation of silicon is studied on the basis of a model of the two-photon excitation of a semiconductor with consideration for external emission. Correlation between the results of simulation of the absorption of a femtosecond laser pulse in single-crystal silicon and experimental data under conditions of the femtosecond excitation of surface plasmon polaritons makes it possible to refine the mechanism of changes in the absorptivity and to make an inference regarding the necessity of considering these changes when assessing the conditions of the laser treatment of semiconductors. Avenues for further improvement of the theoretical model are discussed.

Formation of periodic disruptions induced by heat accumulation of femtosecond laser pulses

The absorption and heat accumulation of successive ultrashort laser pulses in fused silica leads to melting of the material. We analyze the structure and formation of disruptions that occur within the trace of the molten material. We employed focused ion beam (FIB) milling to reveal the inner structure of these disruptions. The disruptions consist of several small voids which form a large cavity with a diameter of several tens of micrometer. Based on the observations, we suggest a model explaining the formation of these disruptions as a results of a fast quenching process of the molten material after the laser irradiation has stopped. In addition, we analyzed the periodic and non-periodic formation of disruptions. The processing parameters strongly influence the formation of disruptions.