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 multiplied by 10**1**4 cm** minus **3, we obtain an energy conversion efficiency as large as 3 multiplied by 10** minus **4 and a chi **(**2**) as large as 1. 2 multiplied by 10** minus **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 are proposed to explain this effect.