Dark-State Resonances Observed On The D-2 Line Of Potassium

We report the first observation of sub-natural-width dark resonances of enhanced transparency on the D-2 line of potassium, contained in an optical cell with reduced (8mm) dimensions and buffered by 30Torr of Ne. The Potassium vapours are irradiated by a frequency modulated DFB diode laser light (with lambda = 766.7nm and 2MHz bandwidth), and are placed in orthogonal to the light magnetic field scanned around zero value. Very good signal-to-noise ratio and narrow (similar to 20kHz) dark resonances are observed and studied in unshielded laboratory environment. The dark resonance formation is related to coherent superposition of the ground-state Zeeman sublevels by respective components of the frequency-modulated light.