Modulation Transfer Spectroscopy of a four-level ladder system in atomic rubidium

We demonstrate Modulation Transfer Spectroscopy for a four-level ladder-configuration system in atomic rubidium that probes Rydberg states. Three lasers at wavelengths of 780 nm (probe), 1529 nm (dressing), and similar to 700 nm (coupling) drive the 5S(1/2)-> 5P(3/2)-> 4D(3/2)-> n(5/2) excitation path, with the first two lasers independently frequency-locked to their respective atomic resonances. To observe modulation transfer that involves the uppermost step, we apply frequency sidebands to the coupling laser with an electro-optic modulator before overlapping all three beams in a rubidium vapor cell. Within the cell, the probe beam efficiently acquires sidebands at the modulation frequency through a nonlinear mixing process that occurs when the coupling beam is tuned to atomic resonance. The transmitted probe beam is focused onto a fast photodiode; the phase of the resulting beat note exhibits a derivative-like response. We demonstrate that this response can be obtained with relatively low laser powers and use it as an error signal to limit the relative frequency drift of the coupling laser to similar to 30 kHz on the timescale of hours.

Automated summary

We demonstrate Modulation Transfer Spectroscopy for a four-level ladder-configuration system in atomic rubidium that probes Rydberg states. By applying frequency sidebands to the coupling laser, we observed modulation transfer involving the uppermost step. The experimental results show that the relative frequency drift of the coupling laser can be limited using the phase response of the spectroscopy, even with relatively low laser powers.