Symmetry breaking and strong persistent plasma currents via resonant destabilization of atoms

The ionization rate of an atom in a strong optical field can be resonantly enhanced by the presence of long-living atomic levels (so called Freeman resonances). This process is most prominent in the multiphoton ionization regime meaning that ionization event takes many optical cycles. Nevertheless, here we show that these resonances can lead to fast subcycle-scale plasma buildup at the resonant values of the intensity in the pump pulse. The fast buildup can break the cycletocycle symmetry of the ionization process, resulting in generation of persistent macroscopic plasma currents which remain after the end of the pulse. This, in turn, gives rise to a broadband radiation of unusual spectral structure forming a comb from terahertz (THz) to visible. This radiation contains fingerprints of the attosecond electronic dynamics in Rydberg states during ionization

Nonlinear transparency window for ultraintense femtosecond laser pulses in the atmosphere

We have found the optimum range of driver wavelengths for mid-infrared ultraintense femtosecond pulses undergoing filamentation in atmospheric air. This wavelength range between 3.1 and 3.5μm forms a nonlinear transparency window identified through a diligent scan of pulse central wavelengths in the range 2.2–4.7μm with a best resolution of 5 nm. Each of 123 wavelengths scanned corresponds to the solution of the full three-dimensional + time pulse propagation and filamentation problem on a 7–19 m path in air. Due to the discovered universal asymmetric character of the nonlinearly enhanced linear absorption in the vicinity of atmospheric molecular band, the optimum driver wavelength belongs to the long-wavelength side of the band.Russian Science Foundation [18-12-00422]; National Key Research and Development Program [2018YFB0504400]; 111 Project [B16027]; U.S. AFOSR under MURI [FA9550-16-1-0013]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Propagation equation for tight-focusing by a parabolic mirror

International audiencePart of the chain in petawatt laser systems may involve extreme focusing conditions for which nonparaxial and vectorial effects have high impact on the propagation of radiation. We investigate the possibility of using propagation equations to simulate numerically the focal spot under these conditions. We derive a unidirectional propagation equation for the Hertz vector, describing linear and nonlinear propagation under situations where nonparaxial diffraction and vectorial effects become significant. By comparing our simulations to the results of vector diffraction integrals in the case of linear tight-focusing by a parabolic mirror, we establish a practical criterion for the critical f -number below which initializing a propagation equation with a parabolic input phase becomes inaccurate. We propose a method to find suitable input conditions for propagation equations beyond this limit. Extreme focusing conditions are shown to be modeled accurately by means of numerical simulations of the unidirectional Hertz-vector propagation equation initialized with suitable input conditions

Scaling Law of THz Yield from Two-Color Femtosecond Filament for Fixed Pump Power

In 3D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μm and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μm depending on the focusing conditions

Scaling Law of THz Yield from Two-Color Femtosecond Filament for Fixed Pump Power

In 3D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μm and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μm depending on the focusing conditions

Supercontinuum of a 3.9μm filament in air: Formation of a two-octave plateau and nonlinearly enhanced linear absorption

Through numerical simulations we reveal the scenario of 3.9-mu m filament spectrum enrichment in the atmosphere in the cases of linear and circular polarization of the incident pulse. The discrete spectrum of odd harmonics transforms into the two-octave plateau in the case of linear polarization. In contrast, in the case of circular polarization of the incident pulse, the harmonic-free flat supercontinuum appears with the plasma onset, reaching the tenth harmonic of the input radiation. We identify the energy balance specific to the filamentation near 4 mu m: the absorption on CO2 lines in the atmosphere is accelerated by the self-phase modulation in the Kerr nonlinearity early before the plasma channel is formed. This nonlinearly enhanced linear absorption overwhelms the plasma losses and conversion of the input pulse energy to the higher harmonics as well as the plateau.Authors retain "The right to use all or part of the Article, including the APS-prepared version without revision or modification, on the author(s)’ web home page or employer’s website and to make copies of all or part of the Article, including the APS-prepared version without revision or modification, for the author(s)’ and/or the employer’s use for educational or research purposes."This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Symmetry Breaking and Strong Persistent Plasma Currents via Resonant Destabilization of Atoms

The ionization rate of an atom in a strong optical field can be resonantly enhanced by the presence of long-living atomic levels (so-called Freeman resonances). This process is most prominent in the multiphoton ionization regime, meaning that the ionization event takes many optical cycles. Nevertheless, here, we show that these resonances can lead to rapid subcycle-scale plasma buildup at the resonant values of the intensity in the pump pulse. The fast buildup can break the cycle-to-cycle symmetry of the ionization process, resulting in the generation of persistent macroscopic plasma currents which remain after the end of the pulse. This, in turn, gives rise to a broadband radiation of unusual spectral structure, forming a comb from terahertz to visible. This radiation contains fingerprints of the attosecond electron dynamics in Rydberg states during ionization

Role of phase matching in pulsed second-harmonic generation: Walk-off and phase-locked twin pulses in negative-index media

The present investigation is concerned with the study of pulsed second-harmonic generation under conditions of phase and group velocity mismatch, and generally low conversion efficiencies and pump intensities. In positive-index, nonmetallic materials, we generally find qualitative agreement with previous reports regarding the presence of a double-peaked second harmonic signal, which comprises a pulse that walks off and propagates at the nominal group velocity one expects at the second-harmonic frequency, and a second pulse that is "captured" and propagates under the pump pulse. We find that the origin of the double-peaked structure resides in a phase-locking mechanism that characterizes not only second-harmonic generation, but also chi((3)) processes and third-harmonic generation. The phase-locking mechanism that we describe occurs for arbitrarily small pump intensities, and so it is not a soliton effect, which usually relies on a threshold mechanism, although multicolor solitons display similar phase locking characteristics. Thus, in second harmonic generation a phase-matched component is always generated, even under conditions of material phase mismatch: This component is anomalous, because the material does not allow energy exchange between the pump and the second-harmonic beam. On the other hand, if the material is phase matched, phase locking and phase matching are indistinguishable, and the conversion process becomes efficient. We also report a similar phase-locking phenomenon in negative index materials. A spectral analysis of the pump and the generated signals reveals that the phase-locking phenomenon causes the forward moving, phase-locked second-harmonic pulse to experience the same negative index as the pump pulse, even though the index of refraction at the second-harmonic frequency is positive. Our analysis further shows that the reflected second-harmonic pulse generated at the interface and the forward-moving, phase-locked pulse appear to be part of the same pulse initially generated at the surface, part of which is immediately back-reflected, while the rest becomes trapped and dragged along by the pump pulse. These pulses thus constitute twin pulses generated at the interface, having the same negative wave vector, but propagating in opposite directions. Almost any break of the longitudinal symmetry, even an exceedingly small chi((2)) discontinuity, releases the trapped pulse which then propagates in the backward direction. These dynamics are indicative of very rich and intricate interactions that characterize ultrashort pulse propagation phenomena

Low-Frequency Content of THz Emission from Two-Color Femtosecond Filament

We experimentally investigate the low-frequency (below 1 THz) spectral content of broadband terahertz (THz) emission from two-color femtosecond filament formed by the 2.7-mJ, 40-fs, 800+400-nm pulse focused into air. For incoherent detection, we screened the Golay cell by the bandpass filters and measured the THz angular distributions at the selected frequencies ν=0.5, 1, 2 and 3 THz. The measured distributions of THz fluence were integrated over the forward hemisphere taking into account the transmittance of the filters, thus providing the estimation of spectral power at the frequencies studied. The spectral power decreases monotonically with the frequency increasing from 0.5 to 3 THz, thus showing that the maximum of THz spectrum is attained at ν≤0.5 THz. The THz waveform measured by electro-optical sampling (EOS) based on ZnTe crystal and transformed into the spectral domain shows that there exists the local maximum of the THz spectral power at ν≈1 THz. This disagrees with monotonic decrease of THz spectral power obtained from the filter-based measurements. We have introduced the correction to the spectral power reconstructed from EOS measurements. This correction takes into account different focal spot size for different THz frequencies contained in the broadband electromagnetic pulse. The corrected EOS spectral power is in semi-quantitative agreement with the one measured by a set of filters