PHYSICAL REVIEW A 84, 052511 (2011) Observation of Doppler-free electromagnetically induced transparency in atoms selected optically with specific velocity Hoon Yu, Kwan Su Kim, Jung Dong Kim, Hyun Kyung Lee, and Jung Bog Kim * Department of Physics Education, Korea National University of Education, Chung-Buk 363-791, Korea (Received 3 July 2011; published 18 November 2011) We observed an electromagnetically induced transparency signal in a four-level system with optically selected rubidium atoms at specific velocities in a room-temperature vaporized cell. Since the atoms behave like cold atoms in the selected atomic view, the observed signals coincide with a trapped atomic system. According to this result, we can observe Doppler-free signals, which correspond from 1.2 to 1.0 K in a Doppler-broadened medium. And the selected atoms have velocity components of ±(131 ± 3) MHz per wave number. Our experimental results can provide insight for research in cold media. DOI: 10.1103/PhysRevA.84.052511 PACS number(s): 32.10.Fn, 32.80.Qk, 32.80.Xx, 32.80.Wr I. INTRODUCTION Coherent effects based on atomic energy levels have attracted considerable attention in recent years. Coherent population trapping (CPT) [1,2], electromagnetically induced transparency (EIT) [3–9], and lasing without inversion occur by coherent interaction of atoms and laser beams. In a three- level system, λ-type EIT, which can be observed when two ground states are coherently coupled by two resonant coherent fields where the excited state is common, is a representative phenomenon. In this case, a weak probe laser field is seldom absorbed even on resonance. Since EIT provides a narrower spectral width than the natural linewidth, the atomic medium turns out to be very highly normal dispersive. Many studies have been carried out using EIT in applications, such as slow light, light storage, optical switching, quantum information processing, magnetometry, and atomic frequency references [3–14]. EIT signals also can be observed in various other energy-level schemes, namely, the so-called ladder, V-, Y-, or N-type systems [5–7,15–20]. As with rising laser-cooling technologies, which are used to enhance nonlinear effects by increasing the number of interacting atoms, EIT also has been reported in magneto- optically trapped or even coherent atomic media, such as a Bose-Einstein condensate [21–23]. Despite many outstanding achievements with cold atoms, there are some difficulties in preparing such cold media. If the observation of coherent effects, such as cold atoms in thermal atoms, is feasible, these results can provide experimental information to anticipate and to design an experimental scheme. Many studies reported recording Doppler-free spectra in Doppler media by using counterpropagation of both the pump beam and the probe beam since saturated spectroscopy was reported [24,25]. Variation in the population of atomic states by using two counterpropagating laser beams, known as optical pumping, can be observed when two beams seem to be resonant with the atomic transition in the atomic frame. Therefore, this technique is used to observe the Doppler-free spectrum. For example, velocity-selective absorption (VSA) is related to an atomic group having a specific velocity component in the direction of the beam’s propagation. Since * jbkim@knue.ac.kr only specific velocity components involving zero interact resonantly with laser fields, optical pumping can be used to observe a Doppler-free spectrum in a Doppler-broadened medium. Appropriately using this experimental technique, it is possible to observe coherent effects, such as a cold- atom medium in a thermal atom medium. In this paper, we describe experimental results in the atomic vapor cell at room temperature, which are very similar to EIT signals obtained in cold atoms. II. EXPERIMENTAL SETUP The experimental setup is shown in Fig. 1. We use three independent external cavity diode lasers with no phase matching. Their linewidths are less then 1 MHz; however, they will become a little broader because of frequency modulation for locking the laser frequency to the atomic transition line. Co-propagating pumping and coupling beams are overlapped by about 1 ◦ with the probe beam, which counterpropagates in the Rb vapor cell, which is 5-cm long and 2.54 cm in diameter without coating for antirelaxation. All laser fields are linearly polarized where the probe field is perpendicular to coupling and pumping fields have the same polarization. Coupling and pumping beams are overlapped by a nonpolarized beam splitter [(BS) in Fig. 1] and pass through a 3.2-mm aperture, while the probe beam passes through a 0.9-mm aperture. The power of each beam is controlled by a combination of polarization BSs and HWPs. The air-conditioned room temperature is maintained at 293 K. We compensate the terrestrial magnetic field by shielding the perimeter of the cell with mu metal. We consider the six-level hyperfine states of a 87 Rb D 2 transition in Fig. 2. For 87 Rb, the splitting of lower levels (5S 1/2 , F = 1,2) is 6834 MHz, while the upper levels (5P 3/2 , F ′ = 0–3) are split by 71, 157, and 267 MHz, respectively. In this paper, the resonant transition frequencies between the hyperfine ground state (F = 1,2) and the excited state (F ′ = 0–3) are denoted by ω FF ′ . Here, ω b , ω c , and ω p represent the frequencies of the probe, coupling, and pumping beams, respectively. Detuning of each laser beam from ω 12 is represented as  b ,  c , and  p , respectively. Figure 2 shows predictable configurations of the experi- mental condition when ω b and ω c are locked on the crossover between ω 23 and ω 22 of 87 Rb atoms, while the pumping (ω p ) 052511-1 1050-2947/2011/84(5)/052511(5) ©2011 American Physical Society