Elena B. Cherepetskaya

Emission of terahertz pulses from vanadium dioxide films undergoing metal-insulator phase transition

This paper describes research on the optics of functional materials, which can change their dielectric properties according to their function. Vanadium dioxide is a good example of such a material where the insulator-to-metal phase transition offers the possibility to control dielectric properties and to use them as a triggering element for photonic applications in the wide spectral range from optical to terahertz frequencies. We observed emission of terahertz (THz) radiation from VO2 films in insulating and conductive phase states under femtosecond pulse irradiation. We found that the efficiency of THz emission increases up to 30 times after the insulator-to-metal phase transition. This process occurs in thin films while it is fundamentally forbidden in the bulk material, and polarization analysis of the emitted radiation reveals the crucial importance of interface contributions. The properties of the THz radiation emitted by VO2 are determined by displacement photocurrents at the VO2–air and VO2–substrate interfaces induced by the incident laser light. In each phase state the contributions of the two boundaries are different. Properties of the effective dielectric susceptibility χ(2) tensor for the insulating phase were defined. In demonstrating the conversion of optical into THz radiation in VO2 films, we found that fundamental symmetry restrictions are not applicable to problems of nonlinear optics of thin films.

Laser optoacoustic method for quantitative nondestructive evaluation of the subsurface damage depth in ground silicon wafers

This paper is a report on the novel laser optoacoustic method for nondestructive evaluation of the depth of the subsurface damage in ground single-crystal silicon wafers. It is based on different mechanisms of laser excitation of ultrasound by absorption of Q-switched Nd:YAG laser pulses at the fundamental wavelength: the concentration-deformation mechanism in the undamaged single-crystal silicon and the thermoelastic one in the subsurface damaged layer. Due to the uniform heating of the whole damaged layer during the laser pulse action the amplitude of the compression phase of the laser-induced ultrasonic signal is proportional to the damaged depth. The rarefaction phase of this signal arises by absorption of the remaining laser energy in the single-crystal silicon beneath the damaged layer. The empirical relation between the depth of the subsurface damage and the ratio of the amplitudes of compression and rarefaction phases of the laser-induced ultrasonic signal can be fitted by a linear function within the depth variation and the corresponding spread of the signal amplitudes. The proposed method attracts some interest for in situ control of the solid surface condition that is important in different tasks of linear and nonlinear optics.

Measuring the dependence of the local Young’s modulus on the porosity of isotropic composite materials by a pulsed acoustic method using a laser source of ultrasound

A laser optoacoustic method for analyzing the effect of porosity on the local isotropic Young’s modulus of composite materials was proposed and experimentally implemented. Using as an example samples of a metal matrix composite based on silumin with reinforcing microparticles of silicon carbide, SiC, in various concentrations, it is shown that to provide an effective increase in Young’s modulus with increasing concentration of SiC, the porosity of the final sample should not exceed 2%.

Ultrashort elliptically polarized laser pulse interaction with helical photonic metamaterial

Using the finite-difference time-domain (FDTD) method we have numerically investigated the transmission and reflection of both long and ultrashort elliptically polarized light pulses in periodic metamaterial made of polymer. In the first time we have analyzed the polarization evolution in the hodograph of the transmitted long pulses, and we demonstrated the behavior of the electric field in transmitted ultrashort pulses. The mechanisms of light-matter interaction in terms of the electromagnetic energy oscillation in polymeric metamaterial are shown. We studied the influence of all the parameters of metamaterial unit cell (a helix) on the transmission and reflection. Particularly, the increase of the amount of the helix cycles broadens the polarization-selective frequency range for the transmitted light.

Laser optoacoustic tomography for the study of femtosecond laser filaments in air

We propose to use optoacoustic tomography to study the characteristics of femtosecond laser filamentation in air and condensed matter. The high spatial resolution of the proposed system, which consists of an array of broadband megahertz piezoelectric elements, ensures its effectiveness, despite the attenuation of ultrasonic waves in air.

Laser-induced ultrasonic imaging for measurements of solid surfaces in optically opaque liquids [Invited]

The paper describes a novel laser ultrasonic profilometry method which uses pulsed laser radiation for imaging of the surface profile of solid objects in optically opaque liquids by scattering of ultrasonic waves. Algorithms for the construction of laser ultrasonic images and for profile segmentation are presented. An experimental setup for profile measurements is described. It allows reconstructing of laser ultrasonic images with a frame rate of 10 Hz and performing an automated 3D scanning of samples. The results of the experimental testing of laser ultrasonic profilometry on duralumin samples are presented. The approximation error of duralumin cylinder surface profile measurements in water is 15 μm. The results are compared to those obtained by x-ray tomography.

On the use of an optoacoustic and laser ultrasonic imaging system for assessing peripheral intravenous access

We describe a universal system for research in combined real-time optoacoustic (OA) and laser-ultrasonic (LU) imaging. The results of its testing on the task of needle insertion into the blood vessel model diagnostics are presented. In OA mode, where laser light is absorbed directly in the sample, the contents of blood vessel model is clearly visible. In LU mode, where the short ultrasonic probe pulse scattered on the sample is detected, the needle is clearly visible. The developed solution combining OA and LU imaging modalities due to the common detection system allowed real-time diagnostics of the position of medical needles (0.63 mm and 0.7 mm in diameter) inside blood vessel models (1.6 mm and 2.4 mm in diameter). Frame rate was 10 Hz. High longitudinal spatial resolution of the system − 0.1 mm − allows distinguishing the two walls of the vessel model and the position of the needle inside.

Second harmonic generation in isotropic chiral medium with nonlocality of nonlinear optical response by heterogeneously polarized pulsed beams

The second harmonic generation in the bulk of an isotropic chiral liquid with spatial or temporal nonlocality of its quadratic response is studied analytically. The fundamental pulsed beam has Gaussian time envelope and carries a polarization singularity at its axis. The influence of the topological charge and the handedness of the polarization singularity on the polarization state of the signal beam is revealed.

Laser optoacoustic diagnostics of femtosecond filaments in air using wideband piezoelectric transducers

New opportunities in ultrasound diagnostics of femtosecond laser filaments with wideband piezoelectric transducers are considered. Transverse spatial resolution better than 100 microns is demonstrated in the single and regular multiple filamentation regime making path toward 3D filament tomography. The simple analytical model of the cylindrical acoustic source fitted well with the experimental data.

Study of Effects of Weathering Factors on Internal Structure of Rocks by Laser Ultrasonic Spectroscopy

The paper addresses the use of laser ultrasonic structuroscopy to study how weathering affects the internal structure of rocks used for facing buildings. For 1,250 hours rock samples were subjected to 150 cycles of freezing (at-20°C) and thawing in water (at +20°C) to determine their frost resistance. Also, moistened every 30 minutes, rock samples were exposed to thermal and ultraviolet radiation for 480 hours to determine their weather resistance. The frequency-dependent phase velocity and attenuation coefficient of longitudinal ultrasonic pulses in the samples were measured. It is found that the rock samples are most seriously damaged when exposed to sharp changes in temperature. As a result of freeze-thaw processes, the velocity of elastic waves decreases by 10% on average, and the attenuation coefficient increases by a factor of 1.5 in the range of 300kHz-500kHz and more than 3 times in the range of 1.0MHz-1.5MHz. The coefficient of the relative power of “structural” noise (K parameter) is introduced to characterize the degree of degradation of rock samples. The parameter K is defined as the ratio of the power of noise component in the spectrum of scattered waves to the power of reference signal. It is shown that the parameter K increases almost by a factor of 10 as a result of various weathering processes.