Coherent subterahertz radiation from femtosecond infrared filaments in air

Linear and Nonlinear Optics of Broad-Band Laser Pulses: Diffraction

The typical spectrally limited laser pulse in the near-infrared region is narrow-band up to 40-50 fs. Its spectral width Δk is much smaller than the carrying wavenumber k0 (Δk ≪ k0) . For such kinds of pulses, on distances of a few diffraction lengths, the diffraction is of a Fresnel's type and their evolution can be described correctly in the frame of the well-known paraxial evolution equation. The technology established in 1985 of amplification through chirping of laser pulses triggered remarkable progress in laser optics along with the construction of femtosecond (fs) laser facilities producing high intensity fields of the order of 1015-1021 W/cm2. However, the duration of the pulse was quickly shortened from picoseconds down to 5-6 fs, which have a broad-band nature (Δk ∼ k0). The linear and nonlinear propagation dynamics of broad-band pulses is quite different form their narrow-band counterparts. Here, we review the appropriate theoretical approach to study the evolution of the pulse. Moreover, we shed light on the different diffraction regimes inherent to both narrow-band and broad-band laser pulses and compare them to unveil the main differences. Using this very method, in subsequent papers, we will investigate the influence of the dispersion and nonlinearity on the laser pulse propagation in isotropic media.

Exceeding the Boundaries of the Paraxial Spatiotemporal Nonlinear Optics and Filamentation for Ultrashort Laser Pulses

An impressive phenomenon of observed plasma instability and conical emission under the propagation of ultrashort laser pulses in the air is reported. The discussed novel findings demonstrating nonlinear effects are incapable to be explained in the standard spatiotemporal paraxial optics. Three main mechanisms are investigated. The first one is related to the nonlinear nonparaxial mechanisms for waveguiding of femtosecond pulses, and the second one considers the mechanism of single filament formation at weak ionization. The third mechanism demonstrates a new physical effect leading to collision ionization with intensities in the range of 10¹⁰-10¹¹ W/cm². Furthermore, a new ionization regime of instability is suggested at intensities below the critical thresholds for multiphoton and tunnel ionization. The experimental results and findings are supported by theoretical analyses and numerical simulations.

Terahertz emission pattern from a single-color filament plasma

We study the angular distribution of different spectral components of the terahertz emission from a single-color laser filament plasma. The opening angle of a terahertz cone is experimentally demonstrated to be proportional to the inverse square root of both plasma channel length and terahertz frequency in the non-linear focusing mode, whereas in the case of linear focusing this dependence breaks down. We also experimentally show that any conclusions of terahertz radiation spectral composition require the angle range from which it is collected to be specified.

Laser filaments as pulsed antennas

Secondary radiation emission of laser-induced filaments is revisited from a perspective of transient antenna radiation. Solutions for transient-antenna radiation fields are shown to provide an accurate description of the spectral and polarization properties, radiation patterns, and the angular dispersion of terahertz and microwave radiation emitted by laser filaments. Time-domain pulsed-antenna analysis offers a physically clear explanation for the bandwidth of this radiation, relating the low-frequency cutoff in its spectrum to the filament length, thus explaining efficient microwave generation in laser filamentation experiments.

Enhance terahertz radiation and its polarization- control with two paralleled filaments pumped by two-color femtosecond laser fields

We present experimentally an obvious enhancement of the terahertz (THz) radiation with two paralleled filaments pumped by two-color laser fields for a full use of a high laser power, compared with single filament. By mapping the 3-dimensional electric trajectories of generated THz fields with a (111) ZnTe crystal, we observe that the total THz polarization from two filaments can be manipulated by varying the time delay between the two orthogonally polarized pumps, which agrees well with the simulations under the photocurrent model. Notably, the power and spectrum of the THz field almost keep unchanged while manipulating the ellipticity of the THz polarization, which is important for a polarization-controllable THz source.

Coherently enhanced microwave pulses from midinfrared-driven laser plasmas

Ultrafast ionization of a gas medium driven by ultrashort midinfrared laser pulses provides a source of bright ultrabroadband radiation whose spectrum spans across the entire microwave band, reaching for the sub-gigahertz range. We combine multiple, mutually complementary detection techniques to provide an accurate polarization-resolved characterization of this broadband output as a function of the gas pressure. At low gas pressures, the lowest-frequency part of this output is found to exhibit a drastic enhancement as this field builds up its coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. This behavior of the intensity, coherence, and polarization of the microwave output is shown to be consistent with Cherenkov-type radiation by ponderomotively driven plasma currents.

Gas pressure dependence of microwave pulses generated by laser-produced filament plasmas

The plasma arising due to the propagation of a filamenting ultrafast laser pulse in air contains currents driven by the pulse that generate radiated electromagnetic fields. We report absolutely calibrated measurements of the frequency spectrum of microwaves radiated by the filament plasma from 2-40 GHz. The emission pattern of the electric field spectrum is mapped as a function of air pressure from atmosphere to 0.5 Torr. For fixed laser pulse energy, duration, and focal geometry, we observe that decreasing the air pressure by a factor of approximately 10³ increases the amplitude of the electric field waveform by a factor of about 40. As the air pressure decreases, the lower frequency components (<10  GHz) increase in amplitude faster than those at higher frequencies (>20  GHz). To the best of our knowledge, this behavior has not been observed before, is not predicted by existing theory, and implies the existence of a radiation mechanism in the plasma distinct from that which emits at terahertz frequencies.

Avalanche parametric conversion and white spectrum generation from infrared femtosecond pulses in glasses

The observation of discrete lines in the white spectrum at the initial stage of filamentation of powerful femtosecond laser pulses, propagating in silica glasses, as well as the filamentation without plasma channels observed in the experiments in air, pushed us to look for other nonlinear mechanisms for describing these effects. In this paper, we present a new parametric conversion mechanism for asymmetric spectrum broadening of femtosecond laser pulses towards higher frequencies in isotropic media. This mechanism includes cascade generation with THz spectral shift for solids and GHz shift for gases. The process works simultaneously with the four-photon parametric wave mixing. The theoretical model proposed agrees well with the experimental data.

Third-harmonic generation and scattering in combustion flames using a femtosecond laser filament

Coherent radiation in the ultraviolent (UV) range has high potential applicability to the diagnosis of the formation processes of soot in combustion because of the high scattering efficiency in the UV wavelength region, even though the UV light is lost largely by the absorption within the combustion flames. We show that the third harmonic (TH) of a Ti:sapphire 800 nm femtosecond laser is generated in a laser-induced filament in a combustion flame and that the conversion efficiency of the TH varies sensitively by the ellipticity of the driver laser pulse but does not vary so much by the choice of alkanol species introduced as fuel for the combustion flames. We also find that the TH recorded from the side direction of the filament is the Rayleigh scattering of the TH by soot nanoparticles within the flame and that the intensity of the TH varies depending on the fuel species as well as on the position of the laser filament within the flame. Our results show that a remote and in situ measurement of distributions of soot nanoparticles in a combustion flame can be achieved by Rayleigh scattering spectroscopy of the TH generated by a femtosecond-laser-induced filament in the combustion flame.

Linearity of charge measurement in laser filaments

We evaluate the linearity of three electric measurement techniques of the initial electron density in laser filaments by comparing their results for a pair of filaments and for the sum of each individual filament. The conductivity measured between two plane electrodes in a longitudinal configuration is linear within 2 % provided the electric field is kept below 100 kV/m. Furthermore, simulations show that the signal behaves like the amount of generated free electrons. The slow ionic current measured with plane electrodes in a parallel configuration is representative of the ionic charge available in the filament, after several μs, when the free electrons have recombined. It is linear within 2 % with the amount of ions and is insensitive to misalignment. Finally, the fast polarization signal in the same configuration deviates from linearity by up to 80 % and can only be considered as a semi-qualitative indication of the presence of charges, e.g., to characterize the filament length.

Probing the effective length of plasma inside a filament

We present a novel method based on plasma-guided corona discharges to probe the plasma density longitudinal distribution, which is particularly good for the weakly ionized plasmas (~10¹⁴ cm^-3). With this method, plasma density longitudinal distribution inside both a weakly ionized plasma and a filament were characterized. When a high voltage electric field was applied onto a plasma channel, the original ionization created by a laser pulse would be enhanced and streamer coronas formed along the channel. By measuring the fluorescence of enhanced ionization, in particular, on both ends of a filament, the weak otherwise invisible plasma regions created by the laser pulse were identified. The observed plasma guided coronas were qualitatively understood by solving a 3D Maxwell equation through finite element analysis. The technique paves a new way to probe low density plasma and to precisely measure the effective length of plasma inside a filament.

Simple method to enhance terahertz radiation from femtosecond laser filament array with a step phase plate

In this work, we experimentally demonstrate a 200% enhancement of terahertz (THz) wave amplitude generated by femtosecond laser filamentation in air. The experimental setup simply uses a semicircular phase plate to generate two parallel filaments. Temporally overlapped THz pulses from two filaments coherently add up, giving rise to significant enhancement of the THz pulse amplitude. It has been foreseen that further enhancement would be achieved if the design of phase plates could be optimized to generate a filament array. This simple method makes full use of the laser energy and could potentially open a new approach to remotely enhance the THz emission in air.

Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses

We experimentally demonstrate ultrafast dynamic of generation of the 337-nm nitrogen laser by injecting an external seed pulse into a femtosecond laser filament pumped by a circularly polarized laser pulse. In the pump-probe scheme, it is revealed that the population inversion between the C(3)Π(u) and B(3)Π(g) states of N(2) for the free-space 337-nm laser is firstly built up on the timescale of several picoseconds, followed by a relatively slow decay on the timescale of tens of picoseconds, depending on the nitrogen gas pressure. By measuring the intensities of 337-nm signal from nitrogen gas mixed with different concentrations of oxygen gas, it is also found that oxygen molecules have a significant quenching effect on the nitrogen laser signal. Our experimental observations agree with the picture of electron-impact excitation.

Role of resonance absorption in terahertz radiation generation from solid targets

The interaction of 100-fs laser pulses with solid targets at laser intensities 10(16)-10(18)W/cm(2) has been investigated experimentally by simultaneous measurements of terahertz (THz) and second harmonic signals. THz yield at the front side of the target, which rises from the self-organized transient electron currents along the target surface, is found scaling linearly with the laser intensity basically. Measurements of specularly reflected light spectrum show clear evidence of resonance absorption. The positive effects of resonance absorption on surface current and THz radiation generation have been confirmed by two-dimensional (2D) particle-in-cell (PIC) simulations and angular-dependent experiments, respectively.

Dissociative recombination in ultraviolet filamentary plasma gratings

We investigated collisions of nitrogen and argon gas mixture with energetic electrons accelerated by Bragg incident intense infrared femtosecond laser pulses in ultraviolet filamentary plasma gratings. Significant decrease of fluorescence spectra of argon atoms were observed when a small amount of nitrogen gas was mixed with argon gas that facilitated observable argon-nitrogen collisions. We experimentally measured the fluorescence emission from the argon and nitrogen gas mixture under different driving pulse energies, the fluorescence decay dynamics after the impact excitation, as well as the fluorescence intensity dependence on the nitrogen and argon pressures. The experimental measurements were based on the electron acceleration and its subsequent impact with the gas mixture in the filamentary plasma gratings, which was essential for the observation of the dominant dissociative recombination in the gas mixture.

Optical Spectroscopy Using Gas-Phase Femtosecond Laser Filamentation

Femtosecond laser filamentation occurs as a dynamic balance between the self-focusing and plasma defocusing of a laser pulse to produce ultrashort radiation as brief as a few optical cycles. This unique source has many properties that make it attractive as a nonlinear optical tool for spectroscopy, such as propagation at high intensities over extended distances, self-shortening, white-light generation, and the formation of an underdense plasma. The plasma channel that constitutes a single filament and whose position in space can be controlled by its input parameters can span meters-long distances, whereas multifilamentation of a laser beam can be sustained up to hundreds of meters in the atmosphere. In this review, we briefly summarize the current understanding and use of laser filaments for spectroscopic investigations of molecules. A theoretical framework of filamentation is presented, along with recent experimental evidence supporting the established understanding of filamentation. Investigations carried out on vibrational and rotational spectroscopy, filament-induced breakdown, fluorescence spectroscopy, and backward lasing are discussed.

Quantum theory of terahertz emission due to ultrashort pulse ionization of gases

A microscopic model is developed to analyze terahertz (THz) emission after ultrashort one- and two-color laser-pulse excitations of an atomic gas. Optical Bloch equations are derived to describe the pulse-induced ionization in the many-atom system including the Coulombic scattering of the ionized electrons. The model captures the continuous transition between the tunneling and the multiphoton ionization regimes. Numerical evaluations for a wide range of pulse configurations identify optimized excitation conditions for strong THz emission.

Kilometer range filamentation

We demonstrate for the first time the possibility to generate long plasma channels up to a distance of 1 km, using the terawatt femtosecond T&T laser facility. The plasma density was optimized by adjusting the chirp, the focusing and beam diameter. The interaction of filaments with transparent and opaque targets was studied.

Impact excitation of neon atoms by heated seed electrons in filamentary plasma gratings

We demonstrate impact ionization and dissociative recombination of neon (Ne) atoms by means of seeded-electron heating and subsequent electron-atom collisions in an ultraviolet plasma grating, allowing for a substantial fraction of the neutral Ne atomic population to reside in high-lying excited states. A buffer gas with relatively low ionization potential (nitrogen or argon) was used to provide high-density seed electrons. A three-step excitation model is verified by the fluorescence emission from the impact excitation of Ne atoms.

Conical emission from laser filaments and higher-order Kerr effect in air

We numerically investigate the conical emission (CE) from ultrashort laser filaments, both considering and disregarding the higher-order Kerr effect (HOKE). While the consideration of HOKE has almost no influence on the predicted CE from collimated beams, differences arise for tightly focused beams. This difference is attributed to the different relative contributions of the nonlinear focus and of the modulational instability over the whole filament length.

Gradient enhanced third harmonic generation in a femtosecond filament

The third harmonic generated during femtosecond filamentation in air is studied. By establishing a gradient from atmospheric pressure to vacuum conditions, we truncate the filament abruptly at defined positions. The introduction of the pressure gradient leads to an enhancement of the generated third harmonic radiation by 3 orders of magnitude. This effect is attributed to an improved on-axis phase-matching condition. We investigate the spectral shape and the conversion efficiency of the third harmonic during the propagation in the filament.

Tracking spectral shapes and temporal dynamics along a femtosecond filament

The spectral evolution of a high-intensity light channel formed by filamentation is investigated in a detailed experimental study. We also track the spatio-temporal dynamics by high-order harmonic generation along the filament. Both the spectral and temporal diagnostics are performed as a function of propagation distance, by extracting the light pulses directly from the hot filament core into vacuum via pinholes that terminate the nonlinear propagation. We compare the measured spectral shapes to simulations and analyze numerically the temporal dynamics inside the filament.

Effects of laser-plasma interactions on terahertz radiation from solid targets irradiated by ultrashort intense laser pulses

Interactions of 100-fs laser pulses with solid targets at intensities of 10(18) W/cm(2) and resultant terahertz (THz) radiation are studied under different laser contrast ratio conditions. THz emission is measured in the specular reflection direction, which appears to decrease as the laser contrast ratio varies from 10(-8) to 10(-6). Correspondingly, the frequency spectra of the reflected light are observed changing from second harmonic dominant, three-halves harmonic dominant, to vanishing of both harmonics. Two-dimensional particle-in-cell simulation also suggests that this observation is correlated with the plasma density scale length change. The results demonstrate that the THz emission is closely related to the laser-plasma interaction processes. The emission is strong when resonance absorption is a key feature of the interaction, and becomes much weaker when parametric instabilities dominate.

Terahertz radiation from a laser plasma filament

By the use of two-dimensional particle-in-cell simulations, we clarify the terahertz (THz) radiation mechanism from a plasma filament formed by an intense femtosecond laser pulse. The nonuniform plasma density of the filament leads to a net radiating current for THz radiation. This current is mainly located within the pulse and the first cycle of the wakefield. As the laser pulse propagates, a single-cycle and radially polarized THz pulse is constructively built up forward. The single-cycle shape is mainly due to radiation damping effect.

Optimization of few-cycle pulse generation: spatial size, mode quality and focal volume effects in filamentation based pulse compression

We demonstrate the key role played by the spatial characteristics and focusing conditions of a femtosecond multi-cycle laser pulse in optimization of filament output for the purpose of obtaining compressed light pulses in the few-cycle regime. We find that for a given beam profile and focal parameters, driving the filament with energy above a certain limiting value can negatively impact pulse compression. However, for a given energy, a smaller and cleaner input beam mode obtained by using a hard aperture can substantially improve the pulse compression ability. In addition, we show that a larger focal volume can assist in creation of a shorter output pulse.

Plasma waveguide array induced by filament interaction

We demonstrate that interference-assisted coalescence of two noncollinearly overlapped filaments creates a wavelength-scale periodic plasma density modulation to guide the input pulses equivalently as a photonic crystal plasma waveguide. The periodic self-channeling is evidenced by the direct observation of the filament coalescence, which reveals wavelength-scale spatial widths and periodicity dependent on the crossing angles and intensity ratios between the incident filaments.

Transverse-mode dependence of femtosecond filamentation

We theoretically investigate the transverse-mode dependence of femtosecond filamentation in Ar gas. Three different transverse modes, Bessel, Gaussian, and Laguerre modes, are considered for incident laser pulses. By solving the extended nonlinear Schrödinger equation coupled with the electron density equation, we find that the lengths of the filament and the plasma channel induced by the Bessel incident beam is much longer than the other transverse modes with the same peak intensity, pulse duration, and beam diameter. Moreover we find that the temporal profile of the pulse with the Bessel incident mode is nearly undistorted during the propagation. Since the pulse energy that the Bessel beam can carry is more than one order of magnitude larger than the other modes for the same peak intensity, pulse duration, and beam diameter, the Bessel beam can be a very powerful tool in ultrafast nonlinear optics involving propagation in a Kerr medium.

Maker fringes in the Terahertz radiation produced by a 2-color laser field in air

The terahertz radiation produced by a 2-color femtosecond laser scheme strongly saturates and develops an oscillatory behavior with increasing power of the driving femtosecond laser pulses. This is explained by the formation of a plasma channel due to filamentation. Due to dispersion inside the filament and the Gouy phase shift, the phase difference between the 800 nm and 400 nm pulses varies along this plasma emitter. As a result, the local THz radiations generated along the filament interfere destructively or constructively, which manifests itself in the form of Maker fringes.

Impact-ionization cooling in laser-induced plasma filaments

The ionization rates and subsequent electron dynamics for laser-induced plasma channels are measured for the noble gas series He, Ne, Ar, Kr, and Xe at 1.0 atm. The cw fluorescence emission increases superlinearly in the series from He to Xe in agreement with Ammosov-Delone-Krainov tunnel ionization calculations. The electron temperature after laser-induced plasma formation, measured by four-wave mixing, evolves from >20 eV to <1 eV kinetic energies with time constants ranging from 1 ns for He to 100 ps for Xe in agreement with an impact-ionization cooling model.

Theoretical analysis and simulations of strong terahertz radiation from the interaction of ultrashort laser pulses with gases

Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both by theoretical analysis and particle-in-cell (PIC) simulations. A one-dimensional THz generation model based on the transient ionization electric current mechanism is given, which explains the results of one-dimensional PIC simulations. At the same time the relation between the final THz field and the initial transient ionization current is shown. One- and two-dimensional simulations show that for the THz generation the contribution of the electric current due to ionization is much larger than the one driven by the usual ponderomotive force. Ionization current generated by different laser pulses and gases is also studied numerically. Based on the numerical results we explain the scaling laws for THz emission observed in the recent experiments performed by Xie et al. [Phys. Rev. Lett. 96, 075005 (2006)]. We also study the effective parameter region for the carrier envelop phase measurement by the use of THz generation.

Perturbative and non-perturbative aspects of optical filamentation in bulk dielectric media

The field of optical filament formation from initial ultrashort laser pulses in bulk dielectric media has now reached a high state of maturity, and has been studied in all three phases of matter, including long distance propagation in air, also termed light string propagation, water, and glass. From the earliest studies of light string propagation in air it was observed that conical emission, namely colored light emission off-axis from the filament, was a byproduct that accompanied the filamentation process. Since then several other byproducts accompanying optical filamentation have been studied, namely, white light or supercontinuum (SC) generation, third-harmonic (TH) generation, and X- and O-waves. Our goal in this paper is to review the theory and simulation of the byproducts accompanying optical filamentation, and to show that a unified approach is possible. Employing the angularly resolved spectrum, or K -Omega spectrum, a notion that has been used to great effect in the area of nonlinear conical waves, we demonstrate that a unified approach to the byproducts accompanying optical filamentation can be achieved using the twin notions of the Effective Three-Wave-Mixing (ETWM) picture of wave-mixing in the presence of filaments, which determines the locus of phase-matched wave generation in the angularly resolved spectrum, and the first-Born approximation to determine the profile of the angularly resolved spectrum. We summarize results of previous works and show that unlike the essentially non-perturbative core of the filament, several byproducts of filamentation can be treated as perturbative effects that have negligible feed-back effects on the filament itself. This should be of great utility for future studies of optimization of the yield of a given byproduct.

Powerful terahertz emission from laser wakefields in inhomogeneous magnetized plasmas

Powerful coherent terahertz (THz) pulses with a broad spectrum (0.1-3 THz) can be produced from a laser-driven wakefield through linear mode conversion in inhomogeneous magnetized plasmas with the maximum plasma density of 10(17) cm(-3). This occurs when a laser pulse, with an optimized duration about 300 fs, is incident either normally or obliquely to the density gradient of inhomogeneous magnetized plasmas. The external dc magnetic field has a magnitude of a few tesla. By changing the strength and direction of the magnetic field, one can enhance or suppress the THz emission. The maximum energy conversion efficiency in the magnetized plasmas can be double that in the unmagnetized plasmas. Such wakefield emission can be a powerful THz source at the MW level and capable of affording field strength of a few MV/cm, suitable for THz nonlinear physics. Because these THz emissions are always with a positive chirp, with a proper dispersion compression, single-cycle THz pulses can be generated with higher peak powers and field strengths.

Detection of broadband terahertz waves with a laser-induced plasma in gases

We report the experimental results and theoretical analysis of broadband detection of terahertz (THz) waves via electric-field-induced second-harmonic generation in laser-induced air plasma with ultrashort laser pulses. By introducing the second-harmonic component of the white light in the laser-induced plasma as a local oscillator, coherent detection of broadband THz waves with ambient air is demonstrated for the first time. Our results show that, depending on the probe intensity, detection of THz waves in air can be categorized as incoherent, hybrid, and coherent detection. Coherent detection is achieved only when the tunnel ionization process dominates in gases.

Long-range filamentary propagation of subpicosecond ultraviolet laser pulses in fused silica

We report on what is to our knowledge the first observation of subpicosecond ultraviolet laser pulse filamentation in transparent solid materials. Robust filaments were created in fused silica and observed over distances that exceed 3 cm in length. Intensities as high as 10(13) W/cm2 were found to be transported in the filamented beam without material damage in the bulk of the fused-silica sample.

Photoluminescence and terahertz emission from femtosecond laser-induced plasma channels

Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.

Light filaments without self-channeling

The propagation of intense 200 fs pulses in water reveals light filaments not sustained by static balance between Kerr-induced self-focusing and plasma-induced defocusing. Numerical calculations outline the occurrence of a possible scenario where filaments appear because of spontaneous reshaping of the Gaussian input beam into a conical wave, driven by the requirement of maximum localization, maximum stationarity, and minimum nonlinear losses.

Ultrashort laser pulses and electromagnetic pulse generation in air and on dielectric surfaces

Intense, ultrashort laser pulses propagating in the atmosphere have been observed to emit sub-THz electromagnetic pulses (EMPS). The purpose of this paper is to analyze EMP generation from the interaction of ultrashort laser pulses with air and with dielectric surfaces and to determine the efficiency of conversion of laser energy to EMP energy. In our self-consistent model the laser pulse partially ionizes the medium, forms a plasma filament, and through the ponderomotive forces associated with the laser pulse, drives plasma currents which are the source of the EMP. The propagating laser pulse evolves under the influence of diffraction, Kerr focusing, plasma defocusing, and energy depletion due to electron collisions and ionization. Collective effects and recombination processes are also included in the model. The duration of the EMP in air, at a fixed point, is found to be a few hundred femtoseconds, i.e., on the order of the laser pulse duration plus the electron collision time. For steady state laser pulse propagation the flux of EMP energy is nonradiative and axially directed. Radiative EMP energy is present only for nonsteady state or transient laser pulse propagation. The analysis also considers the generation of EMP on the surface of a dielectric on which an ultrashort laser pulse is incident. For typical laser parameters, the power and energy conversion efficiency from laser radiation to EMP radiation in both air and from dielectric surfaces is found to be extremely small, < 10(-8). Results of full-scale, self-consistent, numerical simulations of atmospheric and dielectric surface EMP generation are presented. A recent experiment on atmospheric EMP generation is also simulated.

Kilometer-range nonlinear propagation of femtosecond laser pulses

Ultrashort, high-power laser pulses propagating vertically in the atmosphere have been observed over more than 20 km using an imaging 2-m astronomical telescope. This direct observation in several wavelength bands shows indications for filament formation at distances as far as 2 km in the atmosphere. Moreover, the beam divergence at 5 km altitude is smaller than expected, bearing evidence for whole-beam parallelization about the nonlinear focus. We discuss implications for white-light Lidar applications.

Terahertz radiation from the vacuum-plasma interface driven by ultrashort intense laser pulses

Coherent terahertz (THz) emission from the vacuum-plasma interface induced through laser wake-field excitation has been investigated by particle-in-cell simulations. The emission frequency appears around tau(-1)(L), where tau(L) is the laser pulse duration, even though the plasma density is distributed inhomogeneously near the interface. The emission amplitude, which is zero on the propagation axis of the incident pulse, increases transversely until reaching the maximum amplitude at the beam edge of the incident pulse and then decays transversely. The emission power scales like P approximately 10(8) x a(4)(0) W, where a(0) is the normalized field amplitude of the laser pulse. For an incident pulse of a few tens of femtoseconds at the forced intensity of 3 x 10(17) W/cm(2), it can generate THz radiation with a power of a few MW and with an energy of several microJ/pulse.

Sonographic probing of laser filaments in air

The acoustic wave emitted from the plasma channel associated with a filament induced by a femtosecond laser pulse in air was detected with a microphone. This sonographic detection provides a new method to determine the length and the spatial profile of the free-electron density of a filament. The acoustic wave is emitted owing to the expansion of the gas in the filament, which is heated through collisions with high-energy photoelectrons generated by multiphoton ionization. Compared with other methods, the acoustic detection is simpler, more sensitive, and with higher spatial resolution, making it suitable for field measurements over kilometer-range distances or laboratory-scale studies on the fine structure of a filament.