Destructive Interference during High Harmonic Generation in Mixed Gases

We report on the first experimental evidence of the destructive and constructive interference of high harmonics generated in a mixed gas of He and Ne, which facilitates the coherent control of high harmonic generation. Theoretically, we develop an analytical model of high harmonic generation in mixed gases and succeed in reproducing the experimental results and deriving the optimization conditions for the process. The observed interference modulation is attributed to the difference between the phases of the intrinsically chirped harmonic pulses from He and Ne, which leads to a novel method for broadband measurement of the harmonic phases and for observing the underlying attosecond electron dynamics.

Figure 1

An illustration of two quantum paths of electrons that contribute the HHG in a mixed gas. The variation on quantum path $\delta \mathit{r}$ generates the phase difference of harmonics. Notice that $\Delta I_p$ expresses the difference in energy between the two potentials.

Figure 2

(a) The harmonic spectra from a 13.5 Torr He gas (dashed line), a 1.5 Torr Ne gas (dashed-dotted line), and the mixture of them (solid line). Inset shows the enlarged illustration of the spectra, where DI takes place. (b) The experimentally determined relative phases of harmonics and the corresponding excursion time ${\tau }_1$ as a function of harmonic order (squares). The error bars are about the sizes of the symbols for the experimental data. Theoretically obtained curve are also shown by the solid line.

Figure 3

(a) Integrated intensity of 59th harmonics from a He-Ne gas ($r_{\mathrm{Ne}}=10\%$) as a function of total pressure $p_{\mathrm{tot}}$. (b) Harmonic spectra from the optimized He-Ne gas ($r_{\mathrm{Ne}}=35\%$ and $p_{\mathrm{tot}}=14.0\mathrm{Torr}$; solid line) and the optimized pure Ne gas ($p_{\mathrm{Ne}}=9.1\mathrm{Torr}$; dashed line).