Experimental scheme. The pump beam branch contains a specific arrangement for a careful control of the pump beam attenuation and polarization and a second-harmonic stage (see Appendix A for further details). Inset: detail of the geometry of the aperture and beams and of the interaction volume; represents the length of the interaction region between the terahertz pulse and the plasma.
Potential curves of and with the most probable multiphoton excitation and ionization process at 405 nm denoted by the arrows. The right-hand scale is normalized by the energy of 405-nm photons, i.e., it indicates the number of blue photons needed for a particular transition. The full arrow indicates the first resonant step and the dashed arrow indicates the second step involving the photoionization of the oxygen molecule.
Schematic picture of the influence of the laser electric field on the Coulomb potential due to the positively charged core felt by the electron in the static approximation. The maximum available values of the electric field for both experimental wavelengths were assumed in the plot.
Upper panel: measured ratio between the transient terahertz signal for circularly and linearly polarized pumps. Lower panel: Compilation of results showing the transient terahertz signal and free-electron density vs pump intensity for the linearly polarized pump. For low the transient wave forms scale with the pump intensity without significant reshaping. In this regime the free-electron density is proportional to ; both vertical scales are relevant for such measurements (represented by the stars and crosses in the plot). For higher transient signal a significant reshaping of the transient terahertz wave form is observed: only one vertical scale then can be used for a given point as shown by the arrows (filled symbols: only left-hand scale is relevant; open symbols: only right-hand scale is relevant).
Pump-probe scans: maximum field amplitude of the transient signal normalized by the maximum field amplitude of the reference wave form vs pump-probe delay.
Transient field transmittance at low pump intensity and the best fit using Drude model; pump wavelength: 405 nm; pump intensity: ; pump-probe delay: . Inset: corresponding equilibrium and transient wave forms.
Dielectric spectra of electronic plasma in photoionized oxygen calculated from transient wave forms using Eq. (3) and the fits by the Drude-Lorentz model (10) for several pump pulse intensities. Optical pump at 405 nm, linearly polarized, . Inset: momentum relaxation time extracted from the fits.
Transient dielectric spectra obtained for and two different pump-probe delays and and their fits using the Drude-Lorentz model (10) superposed with an underdamped harmonic oscillator at and at . See the text for details. Inset: Imaginary part of the permittivity for several pump fluences at .
Transient field transmittance spectra at for the pump wavelength of 405 nm and for three significantly different pump-probe delays .
Transformation of the pump beam in view of the intensity scaling for the two polarizations used. Pumping with (a) 405 nm, linear or circular polarizations; (b) 810 nm, linear polarization; (c) 810 nm, linear or circular polarizations. : half-wave plate, SHG: second-harmonic generation, RP:tunable retardation plate, BP: Brewster plate, GF: thin gray filter, FM: flipping mirror, and PM: power meter.
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