Role of atomic coherence effects in four-wave mixing using autoionizing resonances

The generation of tunable, nanosecond-pulsed extreme-ultraviolet (EUV) light $(84.92\mathrm{}\mathrm{nm}$ to $84.97\mathrm{nm}$ $\sim 14.6\mathrm{eV})$ using four-wave mixing in optically deep krypton gas was investigated experimentally and theoretically. The sum-frequency mixing scheme generated light from two overlapping autoionizing resonances, ${17s}^{\prime }$ and ${15d}^{\prime }$. We investigated whether the laser-induced atomic coherence effect of electromagnetically induced transparency (EIT) could enhance the intensity of the EUV light generated. The maximum EUV output was seen from the region where the ${15d}^{\prime }$ autoionizing resonance has a transparency window due to Fano-type quantum interference. An analytical model using appropriate atomic parameters explains how background absorption can prevent EIT enhancement of EUV generation; and shows how a background Raman-like coupling term in the nonlinear susceptibility can well describe the measured EUV generation spectrum.