We demonstrate efficient, pulsed, gas-phase, nonlinear frequency conversion in a quadruply resonant, double- $\Lambda$ system and, simultaneously, verify theoretical predictions of Rabi-frequency matching unique to absorbing nonlinear media. This system is used to up-convert ultraviolet light at 233 nm to the vacuum ultraviolet at 186 nm in atomic Pb vapor with small-signal conversion efficiencies exceeding $30\%$ and with modest atomic density-length (NL) products (scale ${10}^{14}{\mathrm{cm}}^{-2\mathrm{}}$) and optical power densities $(10–100\mathrm{kW}/{\mathrm{cm}}^2)$.

• Received 3 February 2000

DOI:https://doi.org/10.1103/PhysRevLett.84.5308