Atomic line emission and high-order harmonic generation in argon driven by 4-MHz sub-$\mu$J laser pulses

We report on the coexistence of atomic line emission (ALE) and high-order harmonic generation (HHG) from argon with experimental conditions bridging the multiphoton and tunnel ionization regimes. Driven by sub-$\mu$J femtosecond laser pulses in tight-focusing geometry, characteristic spectra of ALE from highly excited neutral argon as well as from singly ionized argon are detected in the presence of the harmonics. The results are discussed with respect to the electronic structure of argon and the phase-matching condition of the HHG process.

Figure 1

(a) Setup of experiments. (b) Spectra of the generated radiation from argon, measured for 7.5 s for spectra 1 and 2, and 60 s for spectrum 3. The three spectra are measured with different diffraction angles by which the grating is rotated. A smooth background was subtracted from the spectra. The order of the harmonics and the atomic lines (ALE) with their wavelength are indicated.

Figure 2

(a),(b) The diffraction patterns of generated vacuum-ultraviolet radiation for two different argon jet positions. In (a), the argon jet is positioned 30 $\mu$m closer toward the incident laser beam than in (b). (c) The averaged line profiles from (a), blue curve, and (b), red curve. A constant background in the line profile is subtracted. For clarity, the blue curve is vertically shifted. The exposure time for image (a) and (b) is 22 and 11 s, respectively, and a factor 2 should be taken into account when comparing their absolute intensity.

Figure 3

The relevant Grotrian diagrams of (a) a neutral argon atom [27, 28, 29] and (b) a singly ionized argon ion [28, 30] for the observed atomic emission lines. Excited states are shown with the energy (left axis) and emitted wavelength (right axis) in an optical transition to the ground states. Examples for dipole allowed transitions are indicated by the arrows labeled with the wavelength of emitted radiation. In (a), the ionization energy ($I_P$) for a singly charged ion is marked by the dashed line.