Continuous resonant four-wave mixing in double-Λlevel configurations of Na_2

Efficient continuous resonant frequency mixing omega(4) = omega(1) - omega(2) + omega(3) in Na(2) has been realized. A bichromatic field (lambda(1) = 488 nm, lambda(2) = 525 nm), generated by an Ar(+)-laser-pumped Na(2) Raman laser, and radiation at lambda(3) = 655 nm from a dye laser interact resonantly with corresponding transitions X(1)Sigma(g)(+)(upsilon = 3, J = 43) ? B(1)Pi(u)(6, 43) ? X(1)Sigma(g)(+)(13, 43) ? A(1)Sigma(g)(+)(24, 44) in a test Na(2) heat pipe. For input powers of 200, 25, and 400 mW an output beam of as much as 0.2 mW at lambda(4) = 599 nm has been observed. Measured parameter dependences indicate an influence of interference effects. This is directly related to the discussion of lasing without inversion.