Search

The transmission of near infrared femtosecond laser pulses (wavelength of 800 nm) and supercontinuum radiation (300–750 nm) generated by them through a 50-nm gold film immersed in water has been experimentally studied at the power of focused laser radiation in the range of 1–10 GW. An increase in the excitation intensity is accompanied by the blueshift of the edge of the extinction band of the film, which is attributed to the filling of electronic states of the s band with a low density of states caused by two-photon interband transitions from d bands with a high density of states.

The transmission of femtosecond laser pulses of the near-infrared range (wavelength of 1030 nm) through nanocrystalline gold films with a variable thickness (15–90 nm) has been experimentally studied for various intensities of laser radiation in the range of 1–10 TW/cm2. A dip in the transmission coefficient at the wave-length of pump pulses at moderate radiation intensities is due to two-photon transitions from d bands with a high density of states to s and p bands with a low density of states and their saturation at higher intensities. The cross section is estimated for two-photon absorption in gold films enhanced by plasmon resonance of gold nanocrystallites at the wavelength of the second harmonic of exciting radiation, which is compared to values for gold films and various plasmon nanostructures known for other spectral ranges.

Results concerning the controllable ablation of nano-layered thin films (NLTF) by femtosecond laser pulses are presented. Investigated samples were titanium-aluminum bilayers, deposited on a silicon substrate, with the top titanium or aluminum layer of variable thickness on the surface. Irradiation was done in ambient air with single femtosecond laser pulses under standard laboratory conditions. The samples were analyzed by complementary methods of optical and scanning electron microscopy and optical profilometry, exhibiting laser-fluence-dependent ablative removal either of the top layer or the entire bilayer or even partial ablation of the underlying silicon substrate. The removal (spallation) threshold fluences for the topmost layer are scalable versus its thickness almost irrespectively of its material, being rather selective for the Ti-coated samples and much less selective for the Al-coated samples. The removal of the entire bilayers was found to be strongly influenced by electronic properties of the underlying metallic layer, dictating the NLTF-Si adhesion, heat conduction, and capacity in the NLTFs toward the NLTF-Si interface and beyond, as well as by their thermophysical characteristics, e.g., almost twice higher melting temperature and enthalpy for Ti. As a result, precise fs-laser machining of the entire NLTFs is pronounced and selective for the samples with the fusible Al at the low-adhesion Al-Si interfaces, compared with the incomplete NLTF removal from the high-adhesion and refractory Ti-Si interfaces.

The dependences of the depth of single-pulse ablation of the steel surface on the energy density of ultrashort laser pulses (0.3–12 ps) with a wavelength of 515 nm have the form different from the usual logarithmic one for linear absorption of radiation. Since the density of states of free conduction electrons in the s band is low, and the d band of strongly bound electrons with a high density of states is located around the Fermi level, these dependences have been approximated within the two-photon absorption model. The two-photon absorption coefficient increases almost linearly with the duration of laser pulses, which indicates the saturation of absorption and allows estimating the corresponding parameters of the electronic subsystem. © 2019, Pleiades Publishing, Inc.

We have found the time dependence of the ablation depth of aluminium irradiated by a femtosecond laser pulse. It is shown to what extent an increase in the radiation energy flux density leads to an increase in the quasi-stationary value of the ablation depth. By reducing the aluminium film thickness down to one hundred nanometres and less, the ablation depth significantly increases. At the same time, the quasi-stationary value of the ablation depth of a thin film is obtained due to the removal of heat from the focal spot region.

It is shown that an experimental decrease in the reflection of a probe femtosecond pulse from an aluminum film heated by a higher-power femtosecond pulse can be quantitatively described taking into account the inhomogeneous distribution of the laser pulse field in the film and the evolution of the electron and lattice temperature during absorption of the heating inhomogeneous field. Analysis of the electron temperature evolution on the heated film surface combined with modern concepts about the influence of a surface volume charge on thermal emission gave the relation between the amount of emitted electrons and experimental data on the heating of the aluminum film by the femtosecond pulse.

A nanoscale chaotic relief structure appears as a result of subthreshold single-shot femtosecond laser ablation of gold films in the regimes of fabrication of microbumps and nanospikes, but only for a relatively thick film. The observed nanoablation tendency versus film thickness makes it possible to suppose the existence of a sub-surface temperature maximum in thicker gold films and its absence within thinner film, which results from competing evaporative cooling and electronic heat conduction, as demonstrated by numerical simulations of the thermal dynamics.

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. (nanostructures)

Nonlinear currents slowly varying in time are found in the skin layer of a metal irradiated by short laser pulses. The low-frequency field generated by the nonlinear currents in metal and vacuum is studied. The spectral composition, energy and shape of the low-frequency radiation pulse are described. (nonlinear optical phenomena)

Heating of electrons and lattice of aluminum caused by interaction with a femtosecond laser pulse is described quantitatively. Coefficients of absorption of s- and p-polarized pulses are calculated. Frequencies of electron-electron collisions in aluminum are determined from comparison of experimentally measured and calculated coefficients of absorption.

The effects of rapid nonuniform heating of electrons on reflection of a p-polarised pulse from the metal are quantitatively described. Under conditions of normal and high-frequency skin effects, the absorption coefficient and phase shift of the reflected wave are calculated numerically upon irradiation of a gold target.

We describe quantitatively how fast inhomogeneous heating of electrons influences the complex reflection coefficient of the s-polarized light wave. When the collision frequency of heated electrons is of the order of the field frequency, we observe a significant difference in values of the absorption coefficient and the phase shift of the reflected wave and the ones given by previous theoretical considerations.

We present a quantitative description of the s-polarized second-harmonic generation produced by mixing of the electron-heating pump wave and a weak s-polarized probe wave, taking into account inhomogeneous heating of electrons that affects the field distribution in metals. We reveal the effect of nonlinear second-harmonic pulse shortening.

It is shown that when an s-polarised wave is mixed with a wave of the same frequency, which is incident along the normal to the metal surface, three frequency-doubled waves are generated. Two of them are p-polarised and one is s-polarised. The effect of the electron and lattice temperature evolution on the SHG efficiency is described quantitatively. An anomalously strong change in the generation efficiency of an s-polarised harmonic is found during temperature variations, which leads to a noticeable change in the electron collision frequency.

Nonlinear interaction of the powerful terahertz radiation pulse with metal nanofilm is investigated. Quantitative analysis of the nonlinear interaction is illustrated by an example of an aluminum film of a thickness of several tens of nanometers. Rapid heating of the conduction electrons and the lattice causes significant rise of the effective electron collision frequency. This implies a noticeable change in reflectivity and an increase of transmissivity by an order of magnitude.

We study experimentally and numerically the increase in ultrashort laser pulse transmissivity through thin silver films caused by the heating of electrons. Low to moderate energy femtosecond laser pulse transmission measurements through 40–125 nm thickness silver films were carried out. We compare the experimental data with the values of transmitted fraction of energy obtained by solving the equations for the field together with the two-temperature model. The measured values were fitted with sufficient accuracy by varying the electron-electron collision frequency whose exact values are usually poorly known. Since transmissivity experiences more pronounced changes with the increase in temperature compared to reflectivity, we suggest this technique for studying the properties of nonequilibrium metals.We study experimentally and numerically the increase in ultrashort laser pulse transmissivity through thin silver films caused by the heating of electrons. Low to moderate energy femtosecond laser pulse transmission measurements through 40–125 nm thickness silver films were carried out. We compare the experimental data with the values of transmitted fraction of energy obtained by solving the equations for the field together with the two-temperature model. The measured values were fitted with sufficient accuracy by varying the electron-electron collision frequency whose exact values are usually poorly known. Since transmissivity experiences more pronounced changes with the increase in temperature compared to reflectivity, we suggest this technique for studying the properties of nonequilibrium metals.

The weak nonlinear response of a metal to the action of low-intensity laser radiation was studied under the normal skin-effect conditions. We have shown that temperature variations with a double frequency emerging during the electron heating lead to the generation of the third harmonic of the fundamental wave. The density of the radiation flux at the tripled frequency was calculated. By measuring this flux one can determine the frequency of the electron-electron collisions with the umklapp of the quasimomentum.

General features of the absorption and reflection of a test wave by a nonequilibrium plasma produced in the tunneling ionization of atoms of a matter by a circularly polarized laser pulse are described. Because of the highly anisotropic distribution of photoelectrons, the optical properties of a nonequilibrium plasma differ considerably from those of a plasma with a Maxwellian electron velocity distribution. Physically, an anomalous behavior of the absorption coefficient and of the phase shift stems from the fact that electron kinetics in the skin layer is modified by the alternating magnetic field of the test wave.

We present a time-domain solution of the equations describing low-frequency radiation generation when a metal surface is irradiated by a non-damaging femtosecond optical pulse of s-polarized radiation. Ponderomotive force, drag current, and temperature gradient along the surface are found to be sources of the terahertz radiation. A comparison of these sources under typical experimental conditions reveals that both the drag current and the thermal gradient give similar contribution to the terahertz signal. The ponderomotive force effect is several times less than these two and exists only during the incident pulse.

It is shown that when an s-polarized wave is mixed with a normally incident wave of the same frequency, it is possible to generate an s-polarized second harmonic wave in the metal skin-layer. The effect of the electron and lattice temperature evolution on the SHG efficiency is quantitatively described. An anomalously strong change in the generation efficiency of this harmoni c is found due to electron heating. Keywords: second harmonic generation, wave mixing, femtosecond pulse, heating of electrons. 1.INTRODUCTION Many papers are devoted to investigation of the SHG by electrons in the metal (see, for example, [1-8]). In metals, harmonics are generated in a relatively narrow skin-layer; so the conversion coefficient of radiation with the frequency & into radiation with the frequency 2Z is rather small. The simplest description by using the perturbation theory gives radiation flux density of the second harmonic pr oportional to the square of the radiation flux density at the fundamental frequency. Thus to achieve a noticeable generation effect it is desirabl e to use radiation with a rather large intensity, which does not lead, however, to the melting of the metal surface. This radiation can be obtained by using of femtosecond laser pulses. For ultrashort laser pulses already at a moderate incident radiation flux density the SHG is accompanied by the electron heatin g leading to a change in the generation e fficiency. The necessity to account for the electron and lattice heating during the SHG was demonstrated in papers [5-9]. The metal heating is accompanied by a noticeable increase in the elec tron collision frequency, which can be comparable with the radiation frequency and even exceed it. The theoretical description of the SHG suitable at relatively large electron collision frequencies was performed in [9]. It is based on the simultaneous solution of Maxwell’s equations, equation for the electron average velocity as well as equations for th e electron and lattice temperatures. We develop the approach of paper [9] by presenting the SHG theory based on the same equations under conditions of mixing of an s-polarized wave and a normally in cident heating wave of the same frequency. We show that when these waves are mixed, three waves with the frequency