High-resolution hyperfine spectroscopy of excited states using electromagnetically-induced transparency

– We use the phenomenon of electromagnetically-induced transparency in a threelevel atomic system for hyperfine spectroscopy of upper states that are not directly coupled to the ground state. The three levels form a ladder system: the probe laser couples the ground state to the lower excited state, while the control laser couples the two upper states. As the frequency of the control laser is scanned, the probe absorption shows transparency peaks whenever the control laser is resonant with a hyperfine level of the upper state. As an illustration of the technique, we measure hyperfine structure in the 7S1/2 states of Rb and Rb, and obtain an improvement of more than an order of magnitude over previous values. The use of coherent-control techniques in three-level systems is now an important tool for modifying the absorption properties of a weak probe laser [1, 2, 3]. For example, in the phenomenon of electromagnetically induced transparency (EIT), an initially absorbing medium is made transparent to a probe beam when a strong control laser is switched on [4, 5]. EIT techniques have several practical applications in probe amplification [6], lasing without inversion [7], and suppression of spontaneous emission [3, 8, 9, 10]. Experimental observations of EIT have been mainly done using alkali atoms (such as Rb and Cs), where the transitions have strong oscillator strengths and can be accessed with low-cost tunable diode lasers. In this paper, we use the phenomenon of EIT in a novel application, namely high-resolution spectroscopy of hyperfine structure in excited states. The experiments are done in a ladder system, where the control laser drives the upper transition and the probe laser measures absorption on the lower transition. In normal EIT experiments, the frequency of the probe laser is scanned while the frequency of the control laser is kept fixed. By contrast, in our technique, it is the frequency of the control laser that is scanned while the probe laser remains locked on resonance. The probe-absorption signal then shows transparency peaks every time the control laser comes into resonance with a hyperfine level of the excited state. Measurement of hyperfine structure in excited states is important because these states are used in diverse experiments ranging from atomic signatures of parity non-conservation (PNC) (∗) E-mail: [email protected]