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We present experimental results on the sub-Doppler Rydberg spectroscopy ofpotassium in a hot cell and cold atoms, performed with two counter-propagatinglaser beams of 405 nm and 980 nm in the inverted ladder-type system(4S1/2-5P3/2-nS1/2 and nD3/2;5/2). Such an inverted ladder-type scheme ispredicted to be without sub-Doppler electromagnetically induced transparency(EIT) feature in a thermal ensemble under the weak-probe approximation.Instead, we utilized a strong probe field and successfully observed atransparency window with a width narrower than 50~MHz. Our all-order numericalsimulation is in satisfactory agreement with the experimental results. Thisnarrow linewidth allows us to measure the energy levels of the Rydberg levelsfrom $n$=20-70 with improved accuracy. The deduced ionization energy agreeswith the previous measurements. Furthermore, the two-photon Rydberg excitationscheme was applied to the cold ensembles to study the ground-state atomspopulation decrease in the MOT for various Rydberg states. Our experimentalobservations suggested two distinct regimes of the trap losses under differentprobe detuning conditions. While the far off-resonance case (\delta p>>0) canbe described by the picture of dressed atom, the on-resonance case (\delta p~0)reveals more interesting results. The higher Rydberg states suffer larger traploss. Besides, even with similar level energies, the excitation to nD statesresult in faster escape of the ground-state atom from trap than nearby nSstates.

Inverted Ladder Type Optical Excitation of Potassium Rydberg States with Hot and Cold Ensembles