Continuously tunable sum-frequency generation involving sodium Rydberg states

Broadly tunable sum-frequency generation has been observed in a vapor of atomic sodium in the presence of a dc electric field. This field induces a \chi^{(2)} nonlinearity which is resonantly enhanced when the sum frequency corresponds to the energy separation between the ground state and an atomic Rydberg state. In a vapor of number density 4 \times 10^{14} cm -3 , we obtain an energy conversion efficiency as large as 3 \times 10^{-4} and a \chi^{(2)} as large as 1.2 \times 10^{-8} ESU. We have also observed sum-frequency generation in the absence of an applied dc field, and we relate these observations to mechanisms that have been proposed to explain this effect.