Fig. 1. Schematic of the experimental setup. Note that the beams are all vertically polarized. The Brewster windows of the gas-filled cell are tilted vertically.

Fig. 2. (a) Measured reflectivity and (b) group delay of the band-stop mirror.

Fig. 3. Relative group delays of 1-m-long air (black line), 0.5-mm-thick fused silica (red line) and 0.7-mm-thick $\text{CaF}_{2}$ (blue dashed line).

Fig. 4. Results of SHG-FROG measurement. (a), (b) Measured and reconstructed FROG traces, respectively. (c) Retrieved temporal intensity profile (measured: 6.5 fs, transform-limited case: 4.6 fs), (d) Intensity spectrum measured by a spectrometer (black line), retrieved spectrum (red line), and retrieved spectral phase (blue line).

Fig. 5. Beam profiles of the compressed pulse (a) before and (b) after focusing with a concave mirror ($f=300$ mm).

Fig. 6. Results of the spectral stability measurement. (a) Averaged spectrum of 1000 single-shot spectra. (b) Standard deviation of each spectral component.