Experimental observation of the dependence of Autler-Townes splitting on the probe and coupling laser wave-number ratio in Doppler-broadened open molecular cascade systems

We have investigated the effects of inhomogeneous Doppler line broadening on the Autler-Townes (AT) splitting in a three-level open molecular cascade system. For moderate Rabi frequencies in the range of 300-600 MHz, the fluorescence line shape from the uppermost level 3 in this system depends strongly on the wave-number ratio of the two laser fields. However, the fluorescence spectrum of the intermediate level 2 appears as expected. We provide a description of the conditions for optimally resolved AT splitting in terms of the ratio of the probe and coupling laser wave numbers, and the laser propagation geometry based on our theoretical analysis of the Doppler integral. The low threshold Rabi frequency region for Autler-Townes splitting is important for high-resolution Autler-Townes spectroscopy since the laser E-field amplitudes are modest in continuous-wave laser experiments.