Phase-matched high-order-harmonic generation in a gas-filled hollow fiber

Phase-matched high-order-harmonic generation of Ti:sapphire laser pulses of $\approx {10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ has been demonstrated using a gas-filled hollow fiber. The high-order-harmonic spectrum emitted from the fiber showed an intensity distribution significantly different from that in free space, indicating more than a hundred times enhancement around the 25th harmonic. A simple phase-matching calculation considering positive dispersion due to plasma and negative dispersion due to neutrals indicates that macroscopic phase matching achieved in the fiber is responsible for the strong enhancement.