Abstract

We have investigated high harmonics generated by a 25 fs, 810 nm Tirsapphire laser¹ in various noble gases. From the heavier noble gases, we have seen harmonics with photon energies 35% higher than those reported previously.² In argon, harmonics orders up to the 61st are visible, as shown in Figure 1. The lowest three harmonic orders seen are artificially attenuated by the spectrometer grating efficiency. In krypton, harmonics up to the 41st were observed, while in xenon, harmonics up to the 29th can be seen. If Barrier Suppression Ionization (BSI) is assumed to determine the intensity up to which the atom can be exposed (i.e. Ar, Kr, Xe ionize respectively at 2.5×10¹⁴ W/cm², 1.5×10¹⁴ W/cm², and 8.6×10¹³ W/cm²), these harmonic photon energies far exceed the theoretical limit⁴ of I_p +3.2U_p, where I_p is the atomic ionization potential and U_p is the laser ponderomotive potential. In fact, under the BSI assumption, the harmonic photon energies exceed I_p + 5U_p for each gas.

© 1995 Optical Society of America