Extreme Nonlinear Optics in a Femtosecond Enhancement Cavity

Intrinsic to the process of high-order harmonic generation is the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse. Inside a high-finesse cavity where the driver pulse and gas medium are reused, this can lead to optical bistability of the cavity-plasma system, accumulated self-phase modulation of the intracavity pulse, and coupling to higher-order cavity modes. We present an experimental and theoretical study of these effects and discuss their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency combs.

Figure 1

(a) Intracavity power, (b) PDH error signal, and (c) 13th harmonic power for a comb-cavity detuning scan with 40-W pump power. The solid colored lines are experimental data, and the black dashed lines are model results. The solid gray lines illustrate the intracavity power and error signal in the absence of gas.

Figure 2

(a) Measured intracavity spectra with (red) and without (gray) gas for offset $\Delta \approx 40\mathrm{kHz}$ and 38 W pump power. The intracavity power is reduced by 11%. (b) The high power solution under similar conditions. (c) Corresponding time domain $|\mathcal{E}(t)|$ and $\eta (t)$ (black dashed line, right scale). Inset: Intracavity power vs time with (red curve) and without (blue curve) the lock offset $\Delta$. With $\Delta =0$, the system oscillates on the $\sim 20\mu \mathrm{s}$ time scales characteristic of the servo loop.

Figure 3

rf spectra of the intracavity power signal with 1 MHz resolution bandwidth. The beat frequencies are labeled with the higher-order modes that have the corresponding frequency offset from the fundamental ${\mathrm{TEM}}_{00}$.