The present paper studies the generation mechanism of terahertz (THz) radiation from tightly focused femtosecond laser pulses in a gas medium. We measured the angular radiation pattern under different focusing conditions and observed that, with the deepening of focus, the angular radiation pattern changes and optical-to-THz conversion efficiency increases. The analysis of the observed phenomena led to the assumption that the dipole radiation prevails in most cases despite the existing conception regarding the dominating role of the quadrupole mechanism of radiation. Based on these assumptions, the transient photocurrent theory of the phenomenon presented in this paper was developed by us and used for the numerical fit of the experimental data.

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 investigate and compare the features of the THz generation process caused by an optical breakdown of air both in a “filament” and “micro-plasma” regimes. We explore efficiency, a spatial and spectral distributions of terahertz radiation in each case.

We have studied for the first time the simultaneous generation of terahertz and X-ray radiation from noble gas based nano-cluster supersonic jet under “one-” and “two-color” (fundamental and its second harmonic) excitation with high-power femtosecond laser pulses. It was shown that optimal conditions for effective generation of terahertz and X-ray radiation are different. That makes possible efficiently control the magnitudes of terahertz and X-ray signals simultaneously generated from gas nano-cluster jet. Controlling could be provided by different means: by varying of time delay between laser pulse and cluster jet formation moment, or by varying of chirp parameter of laser pulses. Also we shown that efficiency of THz generation from cluster jet could be effectively controlled by varying of ratio between clustered and buffer (carrier) gases in mixture. Angular distribution of terahertz beam generated near by optical axis under “two-color” laser excitation conditions has been measured as well.

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.

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.

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.

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.

Chymotrypsin activity in nonaqueous solvents increases in the presence of crown-ethers, presumably, due to the interaction of crown-ether molecules with surface amino-groups of protein. FTIR and THz-TDS spectroscopic techniques are used to study the interaction of crown-ether with chymotrypsin and protonated tris(hydroxymethyl)aminomethane, which serves as a model of the amino-groups of protein. Spectra measured at different relative molar concentrations indicate the interaction of the components. The spectral changes and variations in the absolute values of the absorption coefficient are discussed. The similarity of the spectral changes is demonstrated for protein-crown–ether and tris-crown–ether samples.

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.

In this letter 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.

A theory is developed for calculating the spectrum and the shape of a terahertz wave packet from the temporal profile of the energy of the second harmonic of the laser field generated during nonlinear interaction of laser and terahertz pulses in an optical-breakdown plasma. The spectral and temporal characteristics of the second-harmonic envelope and a terahertz pulse are shown to coincide only for short laser pulses. For long laser pulses, the second-harmonic spectral line shifts to the red and its temporal profile is determined by the time integral of the electric field of terahertz radiation.

The effect of multiphoton and tunnel ionization on the generation of terahertz radiation for optical breakdown in the focus of femtosecond bichromatic laser pulses is discussed. (C) 2012 Optical Society of America

The probability of multiphoton ionization of atoms in a laser radiation field containing an additional second harmonic is calculated by the imaginary time method [9, 10]. The conditions are found when the second-harmonic contribution to the ionization of atoms dominates over that of the first harmonic. It is shown that the average momentum of photoelectrons ejected from atoms depends on the phase shift between the first and second harmonics and their mutual polarization. The obtained asymptotic expressions can be used for the qualitative explanation of experiments on generation of terahertz radiation from the optical breakdown region in gas in the focus of a femtosecond laser.

We develop a theoretical model for generation of second harmonics of a laser pulse in a low-density collisionless plasma through interaction of laser radiation with a THz pulse and applied external DC field. We obtain the expression for intensity of 2ω radiation which depends on the integral of THz field strength over duration of the probe pulse.

The interaction between high intensity ultrashort laser pulses and the atomic gas in the ionization-free regime leads to the emission of coherent, short pulse radiation at terahertz frequency. In this work we discuss a model for this effect and its experimental realization.

We critically revise the theory of terahertz emission from a plasma filament induced in a gas media by one or two focusd femtosecond laser pulses. We distinguish a radiation pressure force (RPF) from a ponderomotive force (PF), discuss conditions for one of these forces to be the dominating contribution to the terahertz emission, and also show that the angular distribution of the emitted power critically depends on which of the two forces dominates in a particular experiment. We show that the experimentally observed periodic dependence of the emitted terahertz power on the gas pressure reveals the dominating role of the RPF over the PF, whereas the angular diagram of the emission allows us to determine the predominant direction of the force. We also emphasize that the terahertz emission originated by a transient photocurrent exhibits a different dependency from the phase difference between the first and the second harmonics of the optic laser field, which generally enables the experimental detection of the prevailing mechanism of the terahertz emission from the plasma filament. (C) 2009 Optical Society of America