High-Order Harmonic Generation up to 250 eV from Highly Ionized Argon

We demonstrate the generation of very high-order harmonics, up to 250 eV, using argon gas. This extends by 100 eV the highest harmonics previously observed using Ar and exceeds the energies observed using any other medium besides helium. This advance is made possible by using a waveguide geometry to limit plasma-induced laser beam defocusing, making it possible to generate high harmonics from Ar ions. This work shows that high harmonic emission from ions can extend laser-based coherent up-conversion into the soft x-ray region of the spectrum.

Figure 1

(a) Harmonic emission from a straight $150\mu \mathrm{m}$ inner diameter, 2.5 cm long fiber filled with low-pressure Ar (7 Torr), driven by an 800 nm laser with an 18 fs pulse duration and a peak intensity of $9\times {10}^{14}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$. (b) Harmonic emission at a higher peak laser intensity of $1.3\times {10}^{15}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, using a 0.25 mm period modulated fiber.

Figure 2

(a) Calculated ionization levels in argon for an 18 fs laser pulse at a peak laser intensity of $1.3\times {10}^{15}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, using ADK rates. Laser pulse (black dashed curve); Ar (red); ${\mathrm{A}\mathrm{r}}^{+}$ (green); ${\mathrm{A}\mathrm{r}}^{++}$ (blue). The right axis shows the predicted HHG cutoff energy for the laser intensity profile, calculated from the cutoff rule $h{\nu }_{\max }=I_p+3.17U_p$. (b) Calculated ionization rates of argon species, under the same conditions.

Figure 3

Calculated harmonic emission from neutral (red) and singly ionized Ar (blue) for an 18 fs laser pulse centered at 800 nm, at a laser intensity of $1.3\times {10}^{15}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$. The ion spectrum has not been corrected for the dynamically varying populations.