  We demonstrate the use of multiple atomic-level Rydberg-atom schemes for
continuous frequency detection of radio frequency (RF) fields. Resonant
detection of RF fields by electromagnetically-induced transparency and
Autler-Townes (AT) in Rydberg atoms is typically limited to frequencies within
the narrow bandwidth of a Rydberg transition. By applying a second field
resonant with an adjacent Rydberg transition, far-detuned fields can be
detected through a two-photon resonance AT splitting. This two-photon AT
splitting method is several orders of magnitude more sensitive than
off-resonant detection using the Stark shift. We present the results of various
experimental configurations and a theoretical analysis to illustrate the
effectiveness of this multiple level scheme. These results show that this
approach allows for the detection of frequencies in continuous band between
resonances with adjacent Rydberg states.
