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.