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Nanoscale periodical chemical composition modification of different two-component compound materials GaAs, CdTe, W0.5C and TiC was realized for the first time through multi-shot femtosecond laser irradiation of their surfaces. The resulting surface modification represents periodical nanoscale ripples with different chemical composition in valleys and ridges. The underlying mechanism is related to a sequence of elementary processes, such as laser excitation surface plasmon-polariton, their interference with the incident laser radiation, inhomogeneous surface heating and incongruent cavitation-like material removal. The latter mechanism is responsible for higher evaporation rate of the components with higher volatility, resulting in growth of surface concentration of low-volatility components.

Use of inorganic oxides as transport layer material is a promising way to increase the efficiency of perovskite solar cells. Results of the studies of the influence of the gas mix composition in the plasma discharge used during magnetron sputtering on the optical, electrical, and structural parameters of deposited thin nickel oxide films are reported. Addition of oxygen or nitrogen to pure argon atmosphere (up to 30 vol %) was shown to change the growth rate (1.2–2.3 nm/min), resistivity of the samples (8.5–208 Ω cm), material band gap (2.85–3.43 eV), and the spectral dependence of the extinction coefficient, while the structural and morphological parameters of synthesized thin films were not affected. The lowest extinction coefficients were found in films deposited in pure argon atmosphere, which determines the capabilities of their usage in photovoltaic converters based on perovskite compounds.

Local optical heating and Raman nanothermometry based on resonant silicon particles provide a new promising platform for a number of key nanophotonics applications associated with thermally induced processes at the nano- and microscale. In this work, the crystallization of amorphous silicon nanodisks with optical resonances caused by local optical heating has been studied. The crystallization process is controlled by Raman microspectroscopy. The crystallization temperature of a single nanodisk of about 900 K has been determined under the action of a strongly focused cw laser beam. As a result, an annealed resonant silicon nanoparticle has allowed controlled and reversible heating in the temperature range of 300–1000 K with the possibility of mapping the heating region with submicron spatial resolution.

Organic–inorganic perovskites with a mixed anion composition are widely used in solar cells, light-emitting diodes, and nanophotonic structures. Light nanosources based on resonant perovskite nanoparticles are of particular interest. However, perovskites with such a composition demonstrate the light-induced segregation of anions, which leads to a reversible dynamic rearrangement of the optical properties of a material and photoluminescence spectra. In this work, the photoinduced process of change in optical properties in resonant hybrid perovskite nanoparticles with a mixed anion composition (MAPbBr1.5I1.5, where MA = NH3CH 3 + ) has been studied. Comparison with a similar process in a perovskite thin film with a similar composition has shown that the photoinduced migration of halogen ions in a nanoparticle occurs cyclically. This is due to the competition of two processes: the concentration of ions near the boundaries of the particle and migration caused by the gradient of the density of light-generated electron–hole pairs. This effect in resonant nanoparticles makes it possible to obtain optically tunable nanoantennas.

Effect of femtosecond laser processing on mechanical properties of plates made of submicrocrystalline VT1-0 titanium alloy is studied using active deformation and fatigue testing involving cantilever bending.

Multishot exposure of nickel and iron surfaces by loosely focused femtosecond laser pulses in isopropyl alcohol resulted in quasi-regular arrays of submicron spikes and other intermediate self-organized structures. Such self-organized surface relief is considered to be formed via hydrodynamic instability of molten metallic surfaces in their contact with a near-critical alcohol vapor layer with the pressure of about 5 MPa.

Single-shot thresholds of surface ablation of aluminum and silicon via spallative ablation by infrared (IR) and visible ultrashort laser pulses of variable width τlas (0.2–12 ps) have been measured by optical microscopy. For increasing laser pulse width τlas < 3 ps, a drastic (threefold) drop of the ablation threshold of aluminum has been observed for visible pulses compared to an almost negligible threshold variation for IR pulses. In contrast, the ablation threshold in silicon increases threefold with increasing τlas for IR pulses, while the corresponding thresholds for visible pulses remained almost constant. In aluminum, such a width-dependent decrease in ablation thresholds has been related to strongly diminished temperature gradients for pulse widths exceeding the characteristic electron-phonon thermalization time. In silicon, the observed increase in ablation thresholds has been ascribed to two-photon IR excitation, while in the visible range linear absorption of the material results in almost constant thresholds.

The current state of research in the field of dielectric nanophotonics has been reviewed. “Dielectric nanophotonics” is considered to mean the field of science that studies the interaction of light with nanostructures composed of dielectric nanoparticles with a high value of the refractive index (high-index). These nanostructures allow to control not only the electric but also the magnetic component of light at the nanoscale. Optical properties of high-index dielectric nanoparticles are described; studies devoted to the development of dielectric nanophotonics devices, such as dielectric discrete waveguides, dielectric nanoantennas, and oligomers have been reviewed, and various methods of their preparation have been discussed.

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.

Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding transient surface plasmon-polariton (SPP) dispersion curves of the photo-excited material were simulated as a function of the electron-hole plasma density, using the derived optical dielectric function model, and directly mapped at several laser photon energies, measuring spatial periods of the corresponding SPP-mediated surface relief nanogratings. The unusual spectral dynamics of the surface plasmon resonance, initially increasing with the increase in the electron-hole plasma density but damped at high interband absorption losses induced by the high-density electron-hole plasma through instantaneous bandgap renormalization, was envisioned through the multi-color mapping.

The electromagnetic field interference distribution on the aluminum surface during formation of laser-induced periodic surface structures (ripples) under femtosecond laser irradiation is studied. The nonlinear dependence of optical feedback on the geometric parameters of the ripples is shown to play a key role in nonlinear evolution of the relief with increasing number of pulses. The strongest optical feedback is observed for periods of ripples in the range of Λ = 0.65λ–0.75λ at relief modulation of h = 0.15λ–0.2λ, where λ is the laser wavelength. On the basis of the developed approach, we explain why the frequently observed femtosecond laser-induced ripples have similar periods on materials with a high value of the imaginary part and strongly negative real part of permittivity.

Emission of erosive plasma has been observed during electric probe and optical emission spectral measurements of plumes produced by single-shot femtosecond laser ablation of optical-quality surfaces of various materials—copper, titanium, and silicon—at laser fluences well below the corresponding thermal ablation thresholds, replacing presumably electron emission at lower fluences. The onset of erosive plasma correlates on the fluence scale with saturation of dependences of self-reflectivity of the pumping femtosecond laser pulses, reflecting the “freezing” of electron dynamics (variation of electron density or temperature) during the pumping pulses, despite the monotonically increasing laser fluences.

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.

A two-dimensional square transmission diffraction grating consisting of micron holes in a gold film with a period of about 6 μm exhibits surface enhanced infrared absorption of rhodamine 6G in the range of 1400–1600 cm−1 near the edge of the lower allowed band with a gain of about 10.

Thermocavitation instability of a molten layer on a silicon surface was experimentally revealed in the form of a microscale surface crown-like feature produced by multiple infrared or visible femtosecond laser pulses near the spallation threshold fluence. The number of crown spikes varied versus the crown perimeter, monotonically increasing with increasing laser shot number. The instability dynamics was described in terms of the intermediate crown structures (the spike number) using the proposed thermocavitation model based on the Kuramoto-Sivashinsky hydrodynamic equation.

Inhomogeneous hyperdoping of a 100-nm-thick silicon surface layer with sulfur atoms at concentrations above 2 × 1021 cm−3 was obtained via its femtosecond laser ablation in a sulfur-containing organic solvent. Infrared transmission spectroscopy reveals distinct interband absorption peaks of donor sulfur states, which are absent in the initial crystalline silicon, and a broad absorption band of free carriers with a concentration of ∼1018 cm−3. The rather low free-carrier concentration is related to equilibrium room-temperature ionization of localized donor sulfur states, preserving their nondegenerate character owing to the strong electronion binding in the donor states.

A mechanism of the formation of a nanotip with a nanoparticle at its top that appears in a thin metal film irradiated by a single femtosecond laser pulse has been studied experimentally and theoretically. It has been found that the nanotip appears owing to a melt flow and a nanojet formation, which is cooled and solidified. Within a proposed hydrodynamic model, the development of thermocapillary instability in the melted film is treated with the use of the Kuramoto-Sivashinsky-type hydrodynamic equation. The simulation shows that the nanojet nucleates in the form of a nanopeak in a pit on the top of a microbump (linear stage) and, then, grows in a nonlinear (explosive) regime of an increase in thermocapillary instability in good agreement with experimental data.

Nanobumps and nanoholes have been formed in gold and silver films with various thicknesses on a dielectric substrate by strongly focused single nanosecond pulses of a Nd:YAG laser. An apertureless dielectric fiber probe and an aspherical lens with a numerical aperture of 0.5 were used to focus laser radiation into a diffraction-limited spot on the surface of gold and silver films, respectively. Atomic force and electron microscopy studies have demonstrated that the shape and dimension of nanostructures, as well as the threshold parameters of laser radiation for their formation, are determined by the thickness of a modified film (“size effect”) and by the duration of a laser pulse owing to the lateral heat conduction in films (nonlocal energy deposition effect). Mechanisms of the dynamic formation of such structures in metallic films by nanosecond laser pulses due to phase transformations of their material have been discussed.

Single-shot ablative spallation and fragmentation thresholds, as well as corresponding ablative crater depths, were measured on the surface of iron using optical interferometry, for different ultrashort laser pulse widths in the range τlas = 0.3–3.6 ps. The nonmonotonic dependence of these thresholds on τlas with the minimum near 1.2 ps (the characteristic electron-phonon relaxation time τep) represents transport and emission relaxation phenomena for nonthermalized and thermalized carriers, generated by sub- and picosecond laser pulses, respectively. Compared to rather slow spallative ablation, much faster—picosecond—fragmentation ablation of the iron surface through hydrodynamic expansion of its supercritical fluid ceased for τlas > τep as a result of evaporative cooling.

Nonlinear propagation and focusing within a metallic ring of intense (near or higher than metal ablation threshold) surface plasmon-polaritons excited by a femtosecond laser pulse was experimentally and theoretically studied. Fundamental possibility of local fluence increasing at focus of such plasmonic lens (the ring) by two orders of magnitude was shown. Formation of surface nanostructures as a result of the surface plasmon-polaritons focusing was experimentally observed.

The electron dynamics on the silicon surface during the pump ultrashort infrared laser pulse is studied by time-resolved optical microscopy and electron-emission measurements. It is found that the optical response of the material under the conditions where a dense electron-hole plasma is formed is determined by the renormalization of the band spectrum of the material rather than by intraband transitions of photoexcited carriers. Nonlinear Auger recombination in the plasma enhanced by the plasma-induced renormalization of the band gap and accompanied by the generation of hot charge carriers stimulates intense prompt emission of such carriers from the surface of the photoexcited material, whose work function decreases owing to the large plasma-induced renormalization of the energies of higher conduction bands.

Femtosecond laser nanostructuring at low fluences produces a onedimensional quasiperiodic grating of grooves on an aluminum surface with a period (≈0.5 μm) that is determined by the length of a surface elec� tromagnetic wave. The structure of the grooves of the surface nanograting is formed by regular nanopeaks fol� lowing with a period of about 200 nm. Some nanopeaks manifest craters at their tops. It is suggested that nanopeaks are formed due to the frozen nanoscale spallative ablation of a nanolayer of an aluminum melt in quasiperiodic regions corresponding to interference maxima of the laser radiation with the surface electro� magnetic wave. The periodicity of the appearance of nanopeaks along grooves is due to the previously pre� dicted mechanism of cavitation deformation of the melt surface in the process of macroscopic spallation ablation. However, in this case, cavitation is coherent (similar to a nearcritical spinodal decay) rather than spontaneous. DOI: 10.1134/S0021364011160065

Superstrong shock waves of multimegabar level generated during ablation of an aluminum surface by intense (

One-dimensional periodic nanostructures have been produced on the surface of an aluminum specimen using femtosecond laser pulses at wavelengths of 744 and 248 nm. The nanostructurization of the specimen has been conducted in water and in air in the preablation regime. We investigate the dependence that the surface topology has on the parameters of laser radiation (wavelength, fluence, and number of pulses), as well as on the medium in contact with the specimen surface. A calculation of the optical characteristics of aluminum as they depend on the electron temperature is performed that is good at describing the dependence that the reflection of the p-polarized infrared femtosecond pulses of pumping has on the fluence. Using these optical characteristics of the photoexcited aluminum within the interferential model, periods of the aluminum surface nanogratings are estimated which are in good agreement with the periods measured experimentally.

The propagation of focused femtosecond laser pulses with supercritical peak powers in air has been investigated by the methods of optical visualization, profilometry, and calorimetry. Laser pulses with supercritical powers create a bundle of submillimeter filaments with a diameter of about 5 µm ahead of the lens focus; the maximum number of filaments in the beam cross section and their length increase linearly and sublinearly, respectively, with the radiation peak power. The optical visualization and calorimetry indicate that the plasma channels of filaments are optical contrast (a plasma density of 1018–1019 cm−3), ensuring the refraction of laser radiation incident on them.