Spontaneous mode splitting in optically pumped molecular lasers due to the ac Stark effect

A new kind of spontaneous mode splitting leading to an oscillatory instability in emission is predicted for optically pumped molecular lasers, resulting from pump-field-induced Rabi splitting of the emission line shape. General conditions are presented for the occurrence of mode splitting in homogeneously broadened resonantly pumped systems. Results of detailed analysis pertaining to specific mid- and far-infrared lasing transitions in ${\mathrm{NH}}_3$ show that such effects could occur under typical experimental conditions.