Generation of Coherent Soft-X-Ray Radiation Extending Far Beyond the Titanium $L$ Edge

Coherent soft-x-ray radiation up to photon energies of 700 eV is obtained by focusing several-mJ, 10-fs near infrared laser pulses into a He gas jet. The observed nearly constant photon yield over several hundred eVs may be attributed to nonadiabatic self-phase matching, originating from a substantial ionization within a fraction of the optical cycle of the driving laser pulse.

Figure 1

Measured spectrum of coherent extreme-ultraviolet and soft-x-ray radiation emitted by a gas of He atoms (black line) ionized by 10-fs, 780-nm laser pulses having a peak intensity of $8\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. The thin and thick gray lines depict the background measured without gas flow and the scaling of the soft-x-ray production yield with photon energy as estimated on the basis of the adiabatic approximation $I_N\sim ({\chi }^{(1)}{I}_1{)}^N{sin}^2(\pi l/2l_c)/(\pi l/2l_c{)}^2$ [22], where $N$, $l$, and $l_c$ are the harmonic order, interaction length, and coherence length, respectively. ${\chi }^{(1)}{I}_1$ was chosen to fit the low energy part to the measurement. Inset: High-order harmonic lines in the low-energy, extreme-ultraviolet part of the emitted spectrum.

Figure 2

Measured coherent extreme-ultraviolet and soft-x-ray spectrum from He (produced under the experimental conditions summarized in the caption of Fig. 1) filtered by a 100-nm-thick aluminum foil (0–200 eV range of the spectrum) or a 300-nm-thick titanium filter carried by a 200-nm parylene foil (200–500 eV range). Spectral transmission curves of the Al (thick gray line) and $\mathrm{Ti}+\mathrm{\text{parylene}}$ (thick black line) filters are also shown.

Figure 3

Soft-x-ray spectra recorded at three different backing pressures: 150 mbar (thin line), 250 mbar (thick line), and 400 mbar (thin gray line).