Tatiana M. Lysak

High spatial frequency periodic structures induced on metal surface by femtosecond laser pulses

The high spatial frequency periodic structures induced on metal surface by femtosecond laser pulses was investigated experimentally and numerically. It is suggested that the redistribution of the electric field on metal surface caused by the initially formed low spatial frequency periodic structures plays a crucial role in the creation of high spatial frequency periodic structures. The field intensity which is initially localized in the grooves becomes concentrated on the ridges in between the grooves when the depth of the grooves exceeds a critical value, leading to the ablation of the ridges in between the grooves and the formation of high spatial frequency periodic structures. The proposed formation process is supported by both the numerical simulations based on the finite-difference time-domain technique and the experimental results obtained on some metals such as stainless steel and nickel.

Colorizing silicon surface with regular nanohole arrays induced by femtosecond laser pulses

We report on the formation of one- and two-dimensional (1D and 2D) nanohole arrays on the surface of a silicon wafer by scanning with a femtosecond laser with appropriate power and speed. The underlying physical mechanism is revealed by numerical simulation based on the finite-difference time-domain technique. It is found that the length and depth of the initially formed gratings (or ripples) plays a crucial role in the generation of 1D or 2D nanohole arrays. The silicon surface decorated with such nanohole arrays can exhibit vivid structural colors through efficiently diffracting white light.

Size dependent competition between second harmonic generation and two-photon luminescence observed in gold nanoparticles

We investigate systematically the competition between the second harmonic generation (SHG) and two-photon-induced luminescence (TPL) that are simultaneously present in Au nanoparticles excited by using a femtosecond (fs) laser. For a large-sized (length ~ 800 nm, diameter ~ 200 nm) Au nanorod, the SHG appears to be much stronger than the TPL. However, the situation is completely reversed when the Au nanorod is fragmented into many Au nanoparticles by the fs laser. In sharp contrast, only the TPL is observed in small-sized (length ~ 40 nm, diameter ~ 10 nm) Au nanorods. When a number of the small-sized Au nanorods are optically trapped and fused into a large-sized Au cluster by focused fs laser light, the strong TPL is reduced while the weak SHG increases significantly. In both cases, the morphology change is characterized by scanning electron microscope. In addition, the modification of the scattering and absorption cross sections due to the morphology change is calculated by using the discrete dipole approximation method. It is revealed that SHG is dominant in the case when the scattering is much larger than the absorption. When the absorption becomes comparable to or larger than the scattering, the TPL increases dramatically and will eventually become dominant. Since the relative strengths of scattering and absorption depend strongly on the size of the Au nanoparticles, the competition between SHG and TPL is found to be size dependent.

Parameter control of optical soliton in one‐dimensional photonic crystal

Abstract The paper deals with finding of soliton solution for Schrodinger equation with periodic linear and nonlinear properties of medium in 1D case. Such structure is named as photonic crystal. To find soliton solution the corresponding problem for finding of eigenfunctions and eigenvalues is formulated. Iterative process is proposed for solution of this problem. Using the technique of continuation on parameter we investigate a dependence of soliton location on its maximum intensity, on ratio between light frequency and frequency of structure, on ratio between dielectric permittivity of alternating linear and nonlinear layers and on position between centre of initial distribution of eigenfunction and center of considered photonic structure area. The results of this paper confirm the features of soliton self‐formation investigated early in our papers [37, 38, 39, 40, 41, 42], in which one considered a propagation of femtosecond laser pulse through nonlinear layered structure.

Role of interfering optical fields in the trapping and melting of gold nanorods and related clusters

We investigate the simultaneous trapping and melting of a large number of gold (Au) nanorods by using a single focused laser beam at 800 nm which is in resonance with the longitudinal surface plasmon resonance of Au nanorods. The trapping and melting processes were monitored by the two-photon luminescence of Au nanorods. A multi-ring-shaped pattern was observed in the steady state of the trapping process. In addition, optical trapping of clusters of Au nanorods in the orbits circling the focus was observed. The morphology of the structure after trapping and melting of Au nanorods was characterized by scanning electron microscope. It was revealed that Au nanorods were selectively melted in the trapping region. While Au nanorods distributed in the dark rings were completely melted, those located in the bright rings remain unmelted. The multi-ring-shaped pattern formed by the interference between the incident light and the scattered light plays an important role in the trapping and melting of Au nanorods.

Second harmonic generation by femtosecond pulses under the condition of nonzero amplitude of the double-frequency wave

Second-harmonic generation by femtosecond pulses in a medium with simultaneous action of the second-and third-order nonlinearities under the condition of group matching of interacting waves is analyzed. The problem is solved analytically within the long-pulse approximation, and possible generation modes are identified. In particular, it is shown that the presence of even a small second-harmonic signal (with the intensity comprising no more than 1% of that of the main wave) enables the maximal efficiency of energy conversion to be attained well before the intensities of interacting waves are increased by more than one order of magnitude due to the compression of pulses. This result is practically significant since it allows one to avoid the optical breakdown of the medium.

Acceleration of light and soliton formation due to nonlinear absorption of femtosecond laser pulse energy by a medium containing nanorods

We investigate femtosecond pulse propagation in a medium containing nanorods, taking into account the dependence of two-photon absorption from the aspect ratio of nanorods. Using density matrix formalism, we derive a set of equations describing this process. We consider two cases: formation of a pure absorption grating and formation of both absorption and refraction gratings. Under weak absorption we discovered the acceleration of light or slowing of light (fast light or slow light) in comparison with light propagation in a linear medium. Using spatial-temporal analogy, one can see that this phenomenon is similar to the displacement of the laser beam center in the moving medium under thermal response or under thermal response and evaporation of clouds and fogs if the direction of their motion is perpendicular to the direction of the incident laser beam. We also found out the soliton formation under small nonlinear absorption on the dispersion length.

Assembling of three-dimensional crystals by optical depletion force induced by a single focused laser beam

We propose and demonstrate a method to achieve large effective Soret coefficient in colloids by suitably mixing two different particles, e.g., silica beads and Fe3O4 nanoparticles. It is shown that the thermophoretic motion of Fe3O4 nanoparticles out of the heating region results in a large nonequlibrium depletion force for silica beads. Consequently, silica beads are driven quickly to the heating region, forming a three-dimensional crystal with few defects and dislocations. The binding of silica beads is so tight that a colloidal photonic crystal can be achieved after the complete evaporation of solvent, water. Thus, for fabrication of defect free colloidal PCs, periodic structures for molecular sieves, among others, the proposed technique could be a low cost alternative. In addition as we use biocompatible materials, this technique could be a tool for biophysics studies where the potential of large effective Soret coefficient could be useful.We proposed a method to assemble microspheres into a three-dimensional crystal by utilizing the giant nonequilibrium depletion force produced by nanoparticles. Such assembling was demonstrated in a colloid formed by suitably mixing silica microspheres and magnetic nanoparticles. The giant nonequilibrium depletion force was generated by quickly driving magnetic nanoparticles out of the focusing region of a laser light through both optical force and thermophoresis. The thermophoretic binding of silica beads is so tight that a colloidal photonic crystal can be achieved after complete evaporation of solvent. This technique could be employed for fabrication of colloidal photonic crystals and molecular sieves.

Asymmetric femtosecond laser ablation of silicon surface governed by the evolution of surface nanostructures

The femtosecond laser ablation of silicon surface near the ablation threshold was investigated and the preferential ablation along different directions was observed in different stages. It was found that the ripples formed in the initial stage facilitate the ablation along the direction perpendicular to the ripples, leading to the formation of an elliptical ablation area. With increasing length and depth of the ripples, however, nanohole arrays formed in the ripples will modify the distribution of electric field which benefits the ablation along the direction parallel to the ripples. Consequently, the ablation area is gradually changed to a circular one after irradiating sufficient number of pulses.

Influence of Transverse Perturbation of Soliton Propagation Direction on Laser Radiation Evolution along the Layered Medium

We analyze a stability of optical soliton, which is propagated along the nonlinear layered structure (photonic crystal), with respect to perturbation of propagation direction. Soliton is located over a number of layers. Profile of soliton is found as a solution of corresponding eigenfunction problem for nonlinear Schrödinger equation with periodic coefficients. The stability of optical soliton is investigated for transverse perturbations with respect to propagation direction on the base of computer simulation. Three different types of soliton evolution depending on the amplitude of transverse perturbation are discussed.