High Harmonic Frequency Combs for High Resolution Spectroscopy

We generated a series of harmonics in a xenon gas jet inside a cavity seeded by pulses from a Ti:sapphire mode-locked laser with a repetition rate of 10.8 MHz. Harmonics up to 19th order at 43 nm were observed with plateau harmonics at the $\mu \mathrm{W}$ power level. An elaborate dispersion compensation scheme and the use of a moderate repetition rate allowed for this significant improvement in output power of the plateau harmonics of 4 orders of magnitude over previous results. With this power level and repetition rate, high-resolution spectroscopy in the extreme ultraviolet region becomes conceivable. An interesting target would be the $1S\mathrm{\text{−}}2S$ transition in hydrogenlike ${\mathrm{He}}^{+}$ at 60 nm.

Figure 1

Schematic of the setup as described in the text. The $f$-to-$2f$ interferometer [4] is used to measure intracavity dispersion [22]. The reflection from the cavity is dispersed by a grating to generate an error signal for locking the comb at a specific wavelength.

Figure 2

Left: The spectrum of the seed laser (solid black curve) together with the intracavity spectrum of a compressed (dashed blue curve) and highly chirped pulse (dotted red curve). Right: Normalizing the intracavity spectra to the seed spectrum and the laser power measured in front of the cavity yields the spectrally resolved power enhancement $U_n$ for compressed (dashed blue curve) and chirped (dotted red curve) pulses. Because of the intensity dependence of $\Delta \varphi (\omega )$, the enhancement varies more strongly for shorter pulses.

Figure 3

(a) Spectrum of the outcoupled XUV generated in Xe. Higher harmonics ($>\mathrm{H}15$) are measured with an Al filter to suppress stray light. The spectrally integrated powers for the individual harmonics are given in the plot. Compared to previously reported powers, this represents an improvement of ${10}^4$. (b) The single pass conversion efficiency is derived from the intracavity infrared power for each order of harmonics. The extracted power for H3 is measured with a Si photodiode based power meter.