An absorption cell containing low-pressure gas is placed inside a C${\mathrm{O}}_2$-${\mathrm{N}}_2$O infrared laser cavity and microwave resonances in the gas are detected through the off-resonant response of the infrared laser. The small variation in molecular population in certain rotational levels produced by a microwave pumping changes the bulk susceptibility of the gas through dispersion and thus changes the effective cavity length and the output power of the laser. This new mode of "double resonance" does not require exact coincidence between the laser lines and molecular transitions and is more widely applicable than the normal double-resonance method although its sensitivity is not as great. Characteristics of this method are theoretically studied and compared with experimental results obtained by using the simple molecules C${\mathrm{H}}_3$F, ${\mathrm{D}}_2$CO, ${\mathrm{D}}_2$O, and N${\mathrm{H}}_3$. Microwave resonances are observed through laser lines which are off from molecular infrared transitions by several ${\mathrm{cm}}^{-1\mathrm{}}$. Some unexpected asymmetry of the signals with respect to laser settings has been observed and discussed.

• Received 4 March 1982

DOI:https://doi.org/10.1103/PhysRevA.26.1494