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The low-frequency vibrational modes of the series of merocyanines (malononitrile derivative) have been investigated by pulsed terahertz spectroscopy. The terahertz absorption spectra are shown to contain both intermolecular and intramolecular vibrational modes in the range of 0.15–3.45 THz (5–115 cm−1). An unambiguous correlation is established between the purposeful modification of the molecular structure of merocyanine dyes and the change in their terahertz absorption spectra.

The effect of the appearance of band gaps in the energy spectra of terahertz surface plasmons has been experimentally observed and investigated. The band gaps are formed due to the interference interaction of the surface plasmons excited by pulsed terahertz radiation on metallic diffraction gratings. It has been shown that the experimental dispersion curves of terahertz plasmons are in good agreement with the dispersion curves obtained in the numerical simulation.

We carried out an experimental study of the use of nonlinear crystals ZnTe, GaP, GaSe, DAST, and LiNbO3 and semiconductors GaAs and InAs for the rectification of femtosecond optical pulses. This conversion produces broadband terahertz pulses consisting of a single period of the field oscillation. Here, we discuss features of different types of the terahertz converters, their efficiency, and spectral range.

We study experimentally the absorption and refraction spectra for a number of amino acids, proteins and nucleic acids in the frequency range 0.1–3.5 THz. The characteristic absorption lines are revealed. The spectroscopy of water solutions of proteins and nucleic acids is carried out under conditions of total internal refraction. This method can be used for studies of the functional properties of biomolecules and the dynamics of their interaction.

Terahertz absorption spectra and dispersion of biologically important substances such as sugar, water, hemoglobin, lipids and tissues are studied. The characteristic absorption lines in the frequency range of a terahertz spectrometer (0.1—3.5 THz) are found. The refraction indices and absorption coefficients of human tooth enamel and dentine are measured. The method of terahertz phase reflection spectroscopy is developed for strongly absorbing substances. Simple and reliable methods of time-resolved terahertz spectroscopy are developed.

We develop the theory of generation of terahertz Cerenkov radiation in a homogeneous nonlinear optical crystal if the amplitude and phase fronts of the excitation pulse do not coincide. It is shown that in the case of the nonlinear optical interaction, the use of laser pulses with oblique amplitude front makes it possible to increase the efficiency of generation of terahertz radiation. In contrast to the case where the amplitude and wave fronts coincide, radiation propagates in a strictly determined direction specified by the amplitude-front plane. It is established that the maximum generation efficiency is reached if the angle of inclination of the amplitude front coincides with that of the terahertz-radiation wave vector.

The second harmonic generation (SHG) by the femtosecond pulses in a nematic liquid crystal has been experimentally studied. It was found that the SHG by a light beam occurs in the bulk of the liquid crystal sample, is accompanied by a luminescence, and is only observed in the presence of the light-induced director reorientation.

Spatio-spectral distribution of THz radiation generated by two-color femtosecond laser breakdown in air is investigated theoretically. The theoretical model is based on the fast oscillating light field propagation and self-consistent free electron generation process. We find that the THz emission spectrum has both the low-frequency component related to the transient photocurrent with the maximum spectral intensity at ~1 THz, and the high-frequency component at ~10 THz related to the nonlinear response of bound electrons.

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The present paper analyses the main mechanisms and properties of terahertz radiation generated by intense femtosecond laser pulses from gas and nanosize gas clusters. The possibilities of simultaneous generation terahertz and X-ray radiations are discussed and demonstrated experimentally. It was shown that optimal conditions for effective generation of terahertz and X-ray radiation are different. That makes possible to control the magnitudes of terahertz and X-ray signals simultaneously generated from gas nano-cluster jet. Controlling could be provided by means: by varying of time delay between laser pulse and cluster jet formation moment, or by varying of chirp parameter of laser pulses.

Studies of a rat blood and skin were carried out using THz-TDs. The transmission and the attenuated total internal reflection geometries have been used for measuring the dielectric properties of the water solutions and biological samples with high glucose concentration. We discuss the reasons for the observed THz sensitivity to diabetes mellitus in the context of changes in the bounded water caused by glucose.

We present a comprehensive experimental and numerical study of spectral, spatial and polarization features of the widely used THz source — plasma of optical breakdown by a two-color femtosecond laser. We show experimentally presence of high-frequency component of THz radiation related to response of neutral gas molecules. Taking into account evolution of phase and polarization of optical pulses propagating through plasma allows to account for the experimentally observed polarization and spectrum of the emitted pulse.

Generation of terahertz (THz) radiation was observed in epitaxial VO 2 films grown on R- and C-cut sapphire substrates above and below the metal-insulator phase transition temperature. Polarization analysis of the emitted THz radiation reveals strong in-plane anisotropy of the conductive phase of VO 2 which is not observed for insulating phase, generation efficiency increases up to 30 times after phase transition. Properties of generated THz radiation in VO 2 are defined by the displacement photocurrent at the film-air and film-substrate interfaces.

The papers in this special section were presented at the 16th International Conference on Laser Optics, Saint Petersburg, Russia, June 30th–July 4th 2014, and the International Conference on Advanced Laser Technologies (ALT 2014), held in Cassis, France, from October 6th–10th, 2014.

We report here experiments on surface plasmon excitation and propagation along corrugated and smooth aluminum surface in the terahertz frequency range. Narrowband plasmon excitation by a subpicosecond terahertz pulse is shown to be a transient process and plasmon propagation sufficiently changes its measured time profile. Plasmon propagation during its excitation and detection changes measured signal. We suggest to use parameters T (plasmon duration) and τ (plasmon lifetime) to describe the narrowband THz plasmon pulse. Plasmon duration and lifetime were defined and plasmon propagation lengths on smooth and corrugated surface were measured. Plasmon propagation length on flat surface turned out to be much smaller than it is predicted by the Drude model.

The topical problem of measuring the time dependences of the femtosecond pulse instantaneous spectral intensity using the Fourier-Gabor transformation is considered. Computer simulation shows that the width of the window in which the spectral intensity is constant and the number of these windows “along” the pulse where the spectral intensity does not depend on the window width may vary in wide limits. The width of the window and its travel along the signal are optimized without sacrifice of the measurement accuracy. The efficiency of the method is demonstrated in a physical experiment with terahertz pulses acting on L-cystine amino acid.

Photoinduced changes of transmittance and refractive index of 5CB nematic liquid crystal doped with anthraquinone dye under the action of femtosecond laser pulses are studied. The processes are considered in terms of strong interaction between liquid crystal matrix and dye molecules that is demonstrated by the example of fluorescence spectra. Spatial profile transformation of the femtosecond laser beam

Orienting influence of femtosecond laser pulses on nematic liquid crystals has been studied. We have shown experimentally that in both a nematic liquid crystal doped with anthraquinone dye D4 and nematic matrix “mixture A” (consisting of azoxymolecules) the efficiency of the liquid-crystal director reorientation under the influence of femtosecond pulses is less than for continuous wave radiation. The mechanism responsible for this difference is associated with light-induced variation of the anchoring conditions.

The generation of terahertz (THz) pulses by the optical rectification of femtosecond laser pulses in a ZnTe crystal is studied. A substantial decrease in the THz radiation power was observed upon tight focusing of laser radiation into the crystal. It is shown that the consideration of competing two-photon absorption and second-harmonic generation processes proceeding simultaneously with optical rectification cannot explain this effect even qualitatively. It is assumed that the observed decrease in the THz radiation power is caused by a decrease in the size of a source of nonlinear polarisation. The conditions are found for the most efficient generation of THz radiation in the ZnTe crystal.

We show the spectrochronography as an efficient tool to extend the THz time domain technique. Assuming that all the information is contained in the time-dependent electric field we apply a windowed transfer technique to measure the instantaneous spectra evolution. As an example, surface electromagnetic waves on the metal grating are considered. Dynamics of surface wave spectral component is measured.

The efficiency of excitation of surface plasmons upon noncollinear light scattering from a metal diffraction grating is studied. It is shown that this efficiency strongly depends on the grating profile and the azimuthal angle of rotation. The relation between the spatial configuration of the electromagnetic field near the grating—vacuum interface and the possibility of excitation of plasmons is found. Taking into account different conditions for plasmon excitation, the peculiarities of experimental angular dependences of specular reflection are explained.

In this letter we propose an approach to obtain directive radiation from wire lasers with subwavelength transverse dimensions and length much larger than the radiation wavelength (wire lasers) based on spatial filtering of their radiation using a combination of a spherical lens and a diaphragm. Theoretical modeling based on the antenna model for wire lasers shows that a directive beam with the uniform phase front can be formed when the diaphragm separates the maximum of the image field of the laser created by the lens. We demonstrate spatial filtering of wire laser radiation experimentally using a terahertz quantum cascade laser.

Terahertz time-domain spectroscopy has been used for measuring of bovine serum albumin and glucose solutions response. The transmission and the attenuated total internal reflection geometries have been combined for analyzing the dielectric properties of aqueous solutions spectra at 0.07-3.2 THz.

The results of the study of THz imaging of soft (skin) and hard tissues (teeth and nail sections) are presented. THz images of tissue specimens and the results of various imageprocessing technologies are discussed.

In this Letter, we introduce a new method of estimation of the terahertz (THz) field amplitude. This method uses second-harmonic generation (SHG) in the presence of THz and DC fields in gaseous media. We take into account contributions from both nonionized molecules and free plasma electrons to the nonlinear process of SHG. We analyze the applicability of this method of detection to obtaining correct information on the waveform and amplitude of broadband THz pulses.

Different ways for monitoring the dehydration of tissues affected by hyperosmotic agents have been comparatively analyzed to increase the THz transparency of biological tissues. The data obtained with an original THz laser spectrometer, a Nicolet 6700 Fourier spectrometer, and a Callegari Soft Plus system for skin diagnostics are in good agreement. The corresponding responses of biological tissues (in the form of THz transmittance, reflectance, absorption coefficient, and hydration coefficient) to the effect of biologically compatible hyperosmotic agents have been studied.

In the paper we present the results of study of rat skin and rat subcutaneous tumor under the action of dehydrating agents in terahertz (THz) range (15-30 THz). Frustrated Total Internal Reflection (FTIR) spectra were obtained with infrared Fourier spectrometer Nicolet 6700 and then they were recalculated in the transmittance spectra with Omnic software. Experiments were carried out with healthy and xenografted tumor in skin tissue in vitro. As the dehydrating agents 100% glycerol, 40%-water glucose solution, PEG-600, and propylene glycol were used. To determine the effect of the optical clearing agent (OCA), the alterations of terahertz transmittance for the samples were analyzed. The results have shown that PEG-600 and 40%-glucose water solution are the most effective dehydrating agent. The transmittance of healthy skin after PEG-600 application increased approximately by 6% and the transmittance of tumor tissue after PEG- 600 and 40%-glucose water solution application increased approximately by 8%. Obtained data can be useful for further application of terahertz radiation for tumor diagnostics.

We describe a compact dual-wavelength (1.047 and 1.053 μm) diode end-pumped Q-switched Nd:YLE laser source which has a number of applications in demand. In order to achieve its dual-wavelength operation it is suggested for the first time to use essentially nonmonotonous dependences of the threshold pump powers at these wavelengths on the cavity length in the region of the cavity semiconfocal configuration under a radius of the pump beam smaller than the radius of the zero Gaussian mode. Here we demonstrate one of the most interesting applications for this laser: difference frequency generation in a GaSe crystal at a frequency of 1.64 THz. A superconducting hot-electron bolometer is used to detect the THz power generated and to measure its pulse characteristics.

A new property of a one-dimensional periodic structure — amplification of the sum-frequency signal arising under the simultaneous action of two laser pulses on this structure with radiation frequencies corresponding to the edges of the fixed Bragg band gap — is experimentally observed and described.

A theory of second-harmonic generation in spatially periodic media is developed. This theory takes account of distortions of the atomic wave functions in the field of an external electromagnetic wave and also of the spatial dispersion. The results of experiments on second-harmonic generation in one-dimensional periodic media are discussed. The angular and polarisation dependences of the reflected second harmonic are calculated. A comparison of the experimental results with the theoretical calculations shows that they are in good agreement.

The electromagnetic irradiation of Terahertz (THz) frequency could be obtained by means of optical rectification, using the media with non-linear properties. We demonstrated that the semiconductor crystal (GaAs) with heavy-ion induced defects (irradiation by Kr ions with the energy of 90 MeV up to a fluence of 10 10 cm - 2 ) could be used for this process. Laser ( λ = 795 nm ) pulse signal ( τ = 100 fs ) was transformed into a signal of THz range by means of difference generation. The spectrum of the THz waveform was investigated and high efficiency of this process in the range 1–3 THz was shown. The calculation of non-linear susceptibility demonstrated the dependence of this parameter on carrier concentration and temperature.

Efficient visible-range photoluminescence with photon energy higher than the photon energy of the exciting radiation is observed in nanostructures of porous silicon subjected to heat treatment in vacuum. The photoluminescence intensity is found to be virtually identical for cw and femtosecond excitation by Ti:sapphire laser radiation with the same average power. The results can be explained by a two-step cascade photoluminescence excitation process in which optical passivation of defects of the dangling silicon bond type occurs.

The spectral dependence of the efficiency of generation of the forbidden (in the electric-dipole approximation) second harmonic (FSH) and its energy on the energy of the excitation pulses is investigated experimentally in a highly disperse suspension of purple membranes containing bacteriorhodopsin (BR) under excitation by femtosecond laser pulses into the single-photon absorption band of BR. The experimental data for the case of resonance excitation attest to an interference character of the interaction of optical nonlinearities of different orders in the process leading to the formation of the FSH signal.

The second harmonic (SH) generation with a Ti:sapphire femtosecond laser system in a randomly oriented suspension of finely fragmented purple membranes of Halobacterium halobium has been studied experimentally. The existence of two interfering contributions to the SH signal due to second- and fourth-order nonlinear processes has been established. The phase and the value of the fourth-order electronic susceptibility (hyperpolarizability) relative to the second-order ones have been determined. The analysis of the experimental data shows the existence of a coherent contribution into the SH signal.

The generation of the second harmonic of femtosecond laser pulses in a bacteriorhodopsin solution has been experimentally studied for various wavelengths and polarization states of radiation at the fundamental frequency. The polarization properties of the effect are analyzed under various experimental conditions. The nature and properties of the signal are treated as the manifestation of a superposition of nonlinear optical effects of various orders (the second and the fourth). The second-order effects can have both an electric-dipole and a magnetic-dipole or electric-quadrupole character. In analyzing fourth-order processes, besides the direct electric-dipole contribution, the possibility of the participation of cascade processes at second-and third-order nonlinearities is also allowed.

An experimental investigation was made of nonlinear reflection (accompanied by second-harmonic generation) of focused beams, consisting of pulses of about 100 fs duration, from a profiled metal surface. A method for amplifying the nonlinear response (under conditions of generation of the reflected second harmonic) is proposed: it involves enhancement of the local field by a diffraction grating oriented in a special manner when a surface electromagnetic wave is excited.

We propose to measure the THz surface plasmon signal transmitted after a long distance propagation over a conductive sample in view of determining the THz properties of the sample material. We demonstrate this very sensitive method on a highly-doped silicon samples. The surface permittivity differs from the Drude model prediction.

Incorporation of small particles in polymer increases resulting refraction index. Waveguides form such material may be effectively coupled with photoconductive antennas. Refraction index as well as scattering losses of Si nano and micro particles in polystyrene matrix are experimentally studied. Fibers and structured waveguides are suggested.

We demonstrate in the experiment and simulations that interference between two contributions into the terahertz wave generation in a femtosecond filament, namely, the free electron photocurrent and the nonlinear polarization of neutrals, can be used for the femtosecond coherent control of the THz spectra.

The total energy of intermolecular hydrogen bonds in 17α-hydroxyprogesterone crystal (17HP) estimated on the basis of the DFT/B3LYP calculations is significantly higher than that in progesterone crystal.

Interference between the free electron photocurrent and the nonlinear polarization of neutrals, can be used for the femtosecond coherent control of the THz spectra and polarization in a femtosecond filament.

We discuss supercontinuum generation in crystalline potassium dihydrogen phosphate matrix with incorporated TiO2 nanocrystals in the field of femtosecond laser pulses. The observed efficiency enhancement to be five times more comparing with the nominally pure KDP crystal.

It is observed experimentally that the optical second-harmonic generation signal produced when femtosecond laser pulses are reflected from a metal surface increases more than 100-fold when a specially chosen periodic relief that gives quasiresonance excitation of a surface electro-magnetic wave is depoited on the surface.

We present the results of experimental study of terahertz (THz) generation in gas cluster beam excited by intense femtosecond laser pulses. Cluster beam was produced by partial condensation of pure Ar and mixtures CF2Cl2+He, Ar+He during their expansion through a conical nozzle into vacuum. There were used two excitation schemes in our experiments: single color and two color (fundamental frequency mixed with its second harmonic). We have studied how THz signal scales with various control parameters such as laser pulse duration, gas backing pressure and laser pulse energy. Simultaneously we measured intensity of X-Ray emission which originates from laser-cluster interaction. We found that in a single color scheme energy of THz pulses from Ar cluster beam strongly decreases in the region of minimum laser pulse duration while X-Ray power is maximal under these conditions. Both in single- and two color excitation regimes THz signal demonstrated growth without saturation with increasing of optical pulse energy up to its peak value of 25 mJ.

In this work we compare the terahertz time-domain and fourier-transform infrared spectra of tris and its mixtures with crown ether. We demonstrate the potential of all plasma-air based time-domain terahertz technique for the analysis of low-frequency vibrational spectra. The spectral features are interpreted using simulations of the crystalline structure and THz vibrational modes with the aid of the solid-state density functional theory.

Propagation of a broadband THz SP through a long dielectric slab of thickness from h<>λ is experimentally studied. Temporal and spectral shapes of the transmitted SP allows to estimate the optimal film thickness for SP confinement without much attenuation and dispersion.

Experimental and theoretical study of the mechanisms affecting the spectral form of the pulsed THz emission generated due to the interaction between a bichromatic laser field and a gaseous medium is presented. The influence of a photoionization process and Raman rotational molecular transitions on the THz spectrum is discussed. Typical spectral modifications of THz emission are tracked down and linked to the duration of the two-color laser pump pulse.