Vyacheslav A. Trofimov

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.

An Effective Method for Substance Detection Using the Broad Spectrum THz Signal: A "Terahertz Nose"

We propose an effective method for the detection and identification of dangerous substances by using the broadband THz pulse. This pulse excites, for example, many vibrational or rotational energy levels of molecules simultaneously. By analyzing the time-dependent spectrum of the THz pulse transmitted through or reflected from a substance, we follow the average response spectrum dynamics. Comparing the absorption and emission spectrum dynamics of a substance under analysis with the corresponding data for a standard substance, one can detect and identify the substance under real conditions taking into account the influence of packing material, water vapor and substance surface. For quality assessment of the standard substance detection in the signal under analysis, we propose time-dependent integral correlation criteria. Restrictions of usually used detection and identification methods, based on a comparison between the absorption frequencies of a substance under analysis and a standard substance, are demonstrated using a physical experiment with paper napkins.

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.

New method for analysis of temporal dynamics of medium spectrum under the action of terahertz pulse

The new method of measurement of instantaneous spectral amplitudes for requiring frequencies under the action of terahertz pulse is proposed. It allows one to investigate the frequency dynamics by using the integral measurements of the optical signal.

Possible way for increasing the quality of imaging from THz passive device

Using the passive THz imaging system developed by the CNU-THz laboratory, we capture the passive THz image of human body with forbidden objects hidden under opaque clothes. We demonstrate the possibility of significant improving the quality of the image. Our approach bases on the application of spatial filters, developed by us for computer treatment of passive THz imaging. The THz imaging system is constructed with accordance to well known passive THz imaging principles and to the THz quasi-optical theory. It contains a scanning mechanism, which has a detector approximately with 1200μm central wavelength, a data acquisition card and a microcomputer. To get a clear imaging of object we apply a sequence of the spatial filters to the image and spectral transforms of the image. The treatment of imaging from the passive THz device is made by computer code. The performance time of treatment of the image, containing about 5000 pixels, is less than 0.1 second. To illustrate the efficiency of developed approach we detect the liquid explosive, knife, pistol and metal plate hidden under opaque clothes. The results obtained demonstrate the high efficiency of our approach for the detection and recognition of the hidden objects and are very promising for the real security application.

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.

Mode of propagation of optical radiation with self-similar pulse shape in layered medium with nonlinear absorption

We investigate the mode of laser pulse propagation as in a homogeneous medium so in layered medium with the twophoton absorption at which the shape of pulse is self-similar along some distance of the propagation. Finding of the laser pulse shape with such property is based on the solution of nonlinear eigenfunction problem for Schrödinger equation with nonlinear absorption. Under certain conditions this eigenfunction gives us the pulse shape with requiring properties. This eigenfunction is considered as a spatial distribution of function for the nonlinear Schrödinger equation which contains the term describing the two-photon absorption. We have found out that the self-similar shape of pulse in a medium with the two-photon absorption is similar to the laser soliton at its propagation in a medium with Kerr nonlinearity. Nevertheless, the duration of self-similar pulse propagating in the homogeneous medium is less in comparison with the soliton duration for Kerr medium. The other difference between self-similar shape of pulse and soliton concludes in existence of mode of the self-similar shape of pulse only on limited distance of the optical pulse propagation. We show the reason of this evolution of the laser pulse. We see some applications of such mode of the laser pulse propagation. First, it is important for the laser pulse propagation in an active medium: obviously, the self-similar mode of laser pulse propagation may take place. Second, at a formation of TW or PW laser pulse with wide aperture on the base of non-linear compression of laser pulse in glass the two-photon (or three-photon) absorption occurs. Hence, the self-similar shape of pulse plays an essential role for practical problems.

All-optical switching using controlled formation of large volume three-dimensional optical matter

We demonstrated the creation of large volume three-dimensional optical matter by optically trapping polystyrene spheres in a capillary and the resulting switching operation. The formation of optical matter was confirmed by examining the diffraction pattern of the trap region. Optical switching with an extinction ratio as large as ∼20dB was realized. From the dynamics of the optical matter, it was found that the transition from a disordered state to an ordered one appeared to be quite fast while the recovery of the system to the disordered state took a much longer time.