V. I. Emel’yanov

Inhomogeneous deformation of silicon surface layers probed by second-harmonic generation in reflection

Semiconductor crystals possessing inversion symmetry (Si, Ge) are known to have a rather weak second-order nonlinearity of the quadrupole type [ Phys. Rev. Lett.51, 1983 ( 1983)], since the electric-dipole contribution is forbidden by symmetry. We report the experimental observation of anomalously highly efficient second-harmonic generation (SGH) in reflection from the surface of Si under inhomogeneous deformation. This effect is believed to be due to an electric-dipole contribution to the second-order susceptibility induced in the near-surface layer by inhomogeneous mechanical stress. This fact is consistent with theoretical calculations based on the molecular sp3-orbital model. Experimentally we observed an increase in the second-harmonic intensity by more than 2 orders of magnitude and a modification of the second-harmonic intensity dependence on crystal orientation with respect to the surface normal in the case of ion-implanted, pulsed-laser-annealed Si (111) samples. A similar effect was observed with thermally oxidized Si wafers and silicide-on-cSi structures. These results demonstrate the sensitivity of SHG in reflection to the presence of inhomogeneous stress in Si surface layers, which enables one to use SHG for nondestructive monitoring of stress in semiconductor structures.

Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

One-dimensional quasiperiodic structures whose period is much smaller than the wavelength of exciting radiation have been obtained on a titanium surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densities. As the radiation energy density increases, the one-dimensional surface nanorelief oriented perpendicularly to the radiation polarization evolves from quasiperiodic ablation nanogrooves to regular lattices with subwave periods (100–400 nm). In contrast to the preceding works for various metals, the period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface electromagnetic wave excited by this radiation, because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases with an increase in the electron excitation of the material.

Phase matched second harmonic generation from nanostructured metallic surfaces

Planar structures containing oriented and ordered metallic nanoparticles with shapes lacking an inversion centre can act as a nonlinear medium for generation of second harmonic optical radiation by a process whose directional features resemble those of phase matched second harmonic generation (SHG) in bulk media. The nonlinearity of the metallic patterns stems from the asymmetric modulation of the local field inside nanoparticle by electron oscillations and is deeply rooted in the nanostructured nature of the system. The SHG efficiency is inversely proportional to the second power of the nanoparticle size.

Optical properties of closely packed nanoparticle films: spheroids and nanoshells

We have developed an effective medium theory for the optical properties of nanoparticle films by considering the exact local fields of spheroidal nanoparticles on a substrate. The model allows the calculation of reflectivity, transmission and absorption of nanoparticle films for a wide range of filling factors, nanoparticle aspect ratios and substrate dielectric characteristics. It is suitable for many applications as it can treat films of homogeneous or binary core-shell nanoparticles.

Optical control of gallium nanoparticle growth

The study of metallic nanoparticles has a long tradition in linear and nonlinear optics [1], with current emphasis on the ultrafast dynamics, size, shape and collective effects in their optical response [2-6]. Nanoparticles also represent the ultimate confined geometry:high surface-to-volume ratios lead to local field enhancements and a range of dramatic modifications of the materials properties and phase diagram [7-9]. Confined gallium has become a subject of special interest as the light-induced structural phase transition recently observed in gallium films [10, 11] has allowed for the demonstration of all-optical switching devices that operate at low laser power [12]. Spontaneous self-assembly has been the main approach to the preparation of nanoparticles (for a review see 13). Here we report that light can dramatically influence the nanoparticle self-assembly process: illumination of a substrate exposed to a beam of gallium atoms results in the formation of nanoparticles with a relatively narrow size distribution. Very low light intensities, below the threshold for thermally-induced evaporation, exert considerable control over nanoparticle formation through non-thermal atomic desorption induced by electronic excitation.We report that low-intensity light can dramatically influence and regulate the nanoparticle self-assembly process: Illumination of a substrate exposed to a beam of gallium atoms results in the formation of gallium nanoparticles with a relatively narrow size distribution. Very low light intensities, below the threshold for thermally induced evaporation, exert considerable control over nanoparticle formation.

Nanoscale hydrodynamic instability in a molten thin gold film induced by femtosecond laser ablation

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.

Nanoscale boiling during single-shot femtosecond laser ablation of thin gold films

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.

Electron gas compression and Coulomb explosion in the surface layer of a conductor heated by femtosecond laser pulse

The hypothesis is put forward on the basis of experimental data that strong inhomogeneous heating of the skin layer of conducting materials by a femtosecond pulse gives rise to a double electrical layer that is formed of a “surface” layer of positive ions and a thin (about 1 nm) “subsurface” layer of a superdense (1023–1025 cm−3) degenerate electron gas. The double layer breaks within one picosecond through the Coulomb explosion.

Formation of periodic surface ripples under the action of pulsed carbon dioxide laser radiation on fused silica

We report theoretical and experimental investigations of the formation of periodic surface ripples (PSR’s) on fused silica under normal incidence of pulsed linearly polarized light of a wavelength-tunable CO2 laser (λ = 10.6, 10.3, 9.6, and 9.3 μm). For the first time to our knowledge, the dielectric permittivity ℰ = ℰ′ + iℰ″ = (n + im)2 of silica under conditions of PSR formation has been measured. PSR’s are generated at all values of λ. It is shown that the inequality ℰ′ 1. A theoretical description of the phenomenon is based on a model, which considers silica under conditions of PSR formation to be a viscous liquid. The fundamental mechanisms of PSR formation are the generation of capillary waves in this liquid owing to vapor recoil pressure and nonuniform removal of the substance from the surface by evaporation. It is shown that good agreement between theoretical and experimental results can be achieved only if a complete theory of PSR is applied, including both electodynamic and thermal processes as well as the calculation of growth rates for capillary waves with allowance made for the values of ℰ′ and ℰ″, which were measured during the PSR formation. We have demonstrated that it is impossible to describe the phenomenon of PSR’s on the fused silica surface by using only the electrodynamic model.

Structural transitions in silicon induced by a femtosecond laser pulse: The role of an electron-hole plasma and phonon-phonon anharmonicity

It is shown by the methods of time-resolved self-reflection and linear reflection that irradiation of a silicon target by a 100-fs laser pulse induces successive structural transitions of the target material to new crystal and liquid metal phases, which can occur during the laser pulse or 0.1–103 ps after the pulse termination, depending on the excitation conditions. The thresholds of these structural transitions are determined, and “’soft” phonon modes involved in them are identified, which represent “hot” short-wavelength LA modes. The dynamics of the structural transitions in silicon in the time interval from 0.1 to 103 ps is described using the model of instability of phonon modes caused by an electron-hole plasma and intra-and intermode phonon-phonon anharmonic interactions