Generation of terahertz radiation upon the optical breakdown of a gas

Terahertz radiation of plasma oscillations excited upon the optical (axicon) breakdown of a gas in the presence of external fields of other frequency ranges has been analyzed. It has been shown that the spectra and intensity of oscillations and radiation generated by them depend strongly on the character of the spatiotemporal evolution of the formed plasma. The intensity is maximal upon the rapid formation of the plasma with a sharp boundary and decreases to very low values for objects with a smooth density profile. New schemes and regimes of break-down have been proposed, which make possible, according to preliminary estimates, an emitted energy of about 10 mJ with moderate intensities of ionizing laser radiation.     

On the generation of terahertz radiation by a phase-modulated optical pulse

The problem of terahertz radiation generation in a quadratic nonlinear medium by a nanosecond phase-modulated optical pulse possessing the properties of supercontinuum has been investigated. The possibility of generating both broadband and quasi-monochromatic terahertz signals has been demonstrated. The conditions have been revealed under which the generation efficiency of a phase-modulated nanosecond optical pulse is greater than that of a spectrally confined femtosecond signal.     

Generation of ultrabroadband terahertz radiation under optical breakdown of air by two femtosecond pulses of different frequencies

Experimental results on the generation of terahertz (THz) electromagnetic radiation under ionization of air by femtosecond pulses at fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. The mean power of the generated THz radiation as a function of the time delay between two pulses of different frequencies is found to be quasi-periodical. Theoretically, we show that efficient generation of THz radiation is governed by the inertial part of the nonlinear response of the medium, which is determined by the dynamics of population of highly excited states with subsequent emission of electrons.     

3-Dimensional simulation and footprint of optical breakdown in dielectrics induced by femtosecond laser pulse

In this paper dynamics of optical breakdown process in dielectrics induced by femtosecond laser pulses have been simulated numerically. Using rate equations, the dynamics of free electron density in the focusing region during a few laser pulse duration time was studied numerically. Sources of free electron production as well as sinks of electron reduction in the interacting region were considered in the calculations. In the simulation, the propagation of the laser beam through the focusing volume was also taken into account and the footprint of the breakdown process (as an estimation for medium bulk damage) was simulated 3-dimensionally. The temporal and spatial evolution of free electron density has been used for simulating the 3-dimensional footprint of optical breakdown region. The results show that the dynamics of the breakdown footprint (and volume) strongly depends on the laser and medium characteristics.     

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A theoretical model developed for the terahertzradiation generation includes the nonlinear interaction of a femtosecond pulse with an ionized gas and the excitation of transverse oscillations of the plasma column. The results of the theoretical analysis are compared with the experimental data.     

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

JETP Letters - Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A...     

Coherent hole-burnings induced by a bichromatic laser field

In a Doppler-broadened three-level Λ-type system driven simultaneously by a coupling laser and two equal-amplitude saturating laser fields with a frequency difference 2δ, the absorption spectrum of a weak probe laser exhibits multiple deep spectral holes through coherent hole-burning CHB with controllable numbers, widths, depths, and positions. More significant, changing δ or lasers directions, CHBs can degenerate into narrower and deeper spectral holes where the slope of the refractive index is very steep. The multiple narrow spectral holes in a single absorption profile are expected to have potential applications in high density storage, optical information processes, and slow-light.     

Generation of terahertz radiation by optical rectification in presence of resonance impurities

We demonstrate the possibility of an efficient generation of terahertz radiation under optical rectification in a uniaxial crystal containing impurities with resonance two-level quantum transitions. The group velocity of an optical pulse entering the regime of self-induced transparency decreases to the value of phase velocity of the terahertz radiation, which results in phase matching of long- and short-wavelength waves. The terahertz generation is accompanied by strong self-modulation of the optical pulse. Two-component soliton-like structures can be formed at long distances accompanied by spectral red shift of the optical component.     

Generation of terahertz radiation in the regime of resonant-nonresonant optical rectification

The generation of broadband terahertz radiation in quadratically nonlinear crystals containing additional resonant impurities having a permanent dipole moment has been theoretically analyzed. It has been shown that the contributions to the intensity of the generated radiation from resonance and nonresonance mechanisms of optical rectification are nonadditive. In semiconductor nanostructures and systems of organic molecules, the energy efficiency of generation due to the resonance mechanism can reach about 0.001, which is much higher than the corresponding efficiencies of the currently widespread tilted front technique. The spectra of the input optical pulses exhibit a noticeable red shift due to the resonance-nonresonance rectification.     

Generation of high peak power excimer laser radiation by pulse shortening

Techniques for shortening the pulses of a commercial oscillator-amplifier excimer laser have been investigated. By a combination of a H₂-Raman cell and a saturable absorber dye jet the oscillator pulses are shortened from about 25 ns to typically 1.5 ns. Upon amplification pulses around 3.5 ns with peak powers of more than 100 MW for KrF (248 nm) and 25 MW for XeCl (308 nm) are obtained.     

Efficient generation of terahertz radiation by the method of optical rectification of terawatt laser pulses

Factors that lead to a decrease in the width and the mean frequency of the spectrum of terahertz (THz) radiation that were observed in recent experiments on optical rectification of powerful (∼0.5 TW) femtosecond laser pulses with a tilted amplitude front in a lithium niobate crystal are considered. A simple method is proposed that allows one to avoid the above transformation of the spectrum and to ensure a high efficiency of generation of THz radiation when laser pulses with a power of several TW are used.     

Double pulse UV laser induced breakdown spectroscopy of stainless steel

Results of experimental investigations of 304 austenitic stainless steel (ASS) ultraviolet spectral range by single and double pulse laser induced breakdown spectroscopy (LIBS) at atmospheric pressure are reported. Various parameters, such as laser energy, placement of the laser beam focus with respect to the surface of illumi-nation, and collinear double laser pulse delay were used as variables. This study contributes to a better under-standing of the LIBS plasma dynamics by observing the temporal evolution of various emission lines. Temperature measurements were made by the Boltzmann diagram method using singly ionized Fe lines, and electron densities were found from Stark broadening. The temporal behaviors of these parameters were also estimated. It was found that the electron temperature for double pulses is higher than that for single pulse of the same total energy. For double pulse LIBS, the iron line emission intensities are enhanced and the analytical performance is improved. For instance, the intensity of iron line Fe I 275.01 nm was a factor of about 300 times higher if a double pulse of 2 × 20 mJ was used instead of a single pulse of 40 mJ when focusing the beam 4.7 mm behind the target surface. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 79, No. 5, pp. 654–660, 2006.     

Generation of sub-mJ terahertz pulses by optical rectification

Recent theoretical calculations predicted an order-of-magnitude increase in the efficiency of terahertz pulse generation by optical rectification in lithium niobate when 500 fs long pump pulses are used, rather than the commonly used ~100 fs pulses. Even by using longer than optimal pump pulses of 1.3 ps duration, 2.5× higher THz pulse energy (125 μJ) was measured with 2.5× higher pump-to-THz energy conversion efficiency (0.25%) than reported previously with shorter pulses. These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.     

Generation of terahertz radiation during reflection of femtosecond laser pulses from a metal surface

The features of terahertz radiation generated upon oblique incidence of a femtosecond laser pulse on a metal surface are investigated theoretically. We propose a Cherenkov-type generation mechanism associated with excitation of low-frequency surface currents by a p-polarized optical field. The nonlinear surface currents and corresponding electrodynamic characteristics of low-frequency radiation are calculated analytically and by numerical simulation using the hydrodynamic model. The features of Cherenkov generation of terahertz radiation are analyzed, and techniques are proposed for increasing efficiency.     

Specific features of the acoustic emission upon the optical breakdown in liquid induced by the radiation of Nd:YAG laser

The acoustic emission from the zone of the optical breakdown in liquid is experimentally studied. The spectral characteristics and energy of the acoustic wave that is generated in liquid due to expansion of the plasma formation initiated by the optical breakdown at a wavelength of 532 nm are analyzed. Two spectral peaks that characterize the acoustic emission and the low-frequency shift of the low-frequency peak owing to an increase in the laser pulse energy are demonstrated. In general, the linear dependence of the acoustic pressure on the laser pulse energy is observed. The acoustic data can be used to reconstruct function R(t) that is in agreement with dependences R(t) resulting from the optical data. This circumstance is important for the study of breakdown in opaque media.     

Dynamics of molecular alignment steered by a few-cycle terahertz laser pulse

The field-free alignment of molecule ClCN is investigated by using a terahertz few-cycle pulse (THz FCP) based on the time-dependent density matrix theory. It is shown that a high degree of molecular alignment can be obtained by changing the matching number of the THz FCPs in the adiabatic regime and the non-adiabatic regime. The matching number can affect both the maximum value of the alignment and the time at which it is achieved. It is also found that a higher degree of alignment can be achieved by using the THz FCP at lower intensity and there exists an optimal threshold of molecular alignment with the increase of the field amplitude. Also found is the frequency sensitive region in which the degree of maximum alignment can be enhanced greatly by modulating the center frequencies of different THz FCPs. The investigation demonstrates that comparing with a THz single-cycle pulse, a better result of the field-free alignment can be created by a THz FCP at a constant rotational temperature of molecule.     

Generating terahertz radiation by means of optical rectification

Various means of raising the efficiency of generating wideband terahertz radiation on the basis of optical rectification are analyzed.     

Generation of nanosecond terahertz pulses by the optical rectification method

The possibility of the generation of quasi-cw terahertz radiation by the optical rectification method for broad-band Fourier unlimited nanosecond laser pulses has been experimentally demonstrated. The broadband radiation of a LiF dye-center laser is used as a pump source of a nonlinear optical oscillator. The energy efficiency of terahertz optical frequency conversion in a periodically polarized lithium niobate crystal is 4 × 10⁻⁹ at a pump power density of 7 MW/cm².     

Generation of a single-cycle optical pulse

We make use of coherent control of four-wave mixing to the ultraviolet as a diagnostic and describe the generation of a periodic optical waveform where the spectrum is sufficiently broad that the envelope is approximately a single-cycle in length, and where the temporal shape of this envelope may be synthesized by varying the coefficients of a Fourier series. Specifically, using seven sidebands, we report the generation of a train of single-cycle optical pulses with a pulse width of 1.6 fs, a pulse separation of 11 fs, and a peak power of 1 MW.     

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.