Femtosecond Enhancement Cavities in the Nonlinear Regime

We combine high-finesse optical resonators and spatial-spectral interferometry to a highly phase-sensitive investigation technique for nonlinear light-matter interactions. We experimentally validate an ab initio model for the nonlinear response of a resonator housing a gas target, permitting the global optimization of intracavity conversion processes like high-order harmonic generation. We predict the feasibility of driving intracavity high-order harmonic generation far beyond intensity limitations observed in state-of-the-art systems by exploiting the intracavity nonlinearity to compress the pulses in time.

Figure 1

Experimental setup: laser system with optional fiber broadening and compression, comb offset frequency (${\omega }_{\mathrm{ceo}}$) detection unit, enhancement cavity ($1/e^2$ focus radius: $25\mu \mathrm{m}$), active stabilization of the center frequency to the cavity resonance, and imaging spectrometer. The offset frequency of the laser is adjustable and free running. Insets: autocorrelation trace of laser and intracavity pulse for compressed input pulses and interferogram showing distinct phase jumps. HWP: half-wave plate, PBS: polarizing beam splitter.

Figure 2

Validation of the nonlinear cavity model in a standard-approach cavity. (a) Measured power spectral densities (PSD, upper panel) and acquired phases (lower panel) of the intracavity pulse along with simulation results. Left to right: input pulse duration, 640 fs (upchirped); 180 fs (Fourier limited); 30 fs (fiber broadened). Without ionization, the spectrum is symmetrically enhanced around the central wavelength. (b) Intracavity peak intensity as a function of input peak power for 640-fs (left) and for 30-fs pulses (right). The outlier in the right panel is due to an incorrectly set offset frequency of the comb. Gas density, ${9\times 10}^{18}{\mathrm{cm}}^{-3}$; interaction length, $180\mu \mathrm{m}$; finesse, 1190 (narrowband case) and 950 (broadband case).

Figure 3

(a) Example of a tailored input coupler transmission for best peak power enhancement at ${8\times 10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ peak intensity and corresponding intracavity and input spectrum (right $y$ axis). Note the reduced width of the input spectrum. (b) The same input spectrum enhanced in a standard cavity requires a 10 times more powerful laser to reach the same peak intensity at 5 times longer intracavity pulses. Xe gas target length, $400\mu \mathrm{m}$; particle density, ${2.5\times 10}^{18}{\mathrm{cm}}^{-3}$.