Competition of Concentration Narrowing and Power Broadening of a Dark Resonance in a Ladder System of Rubidium Atoms: Sp
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TROSCOPY AND PHYSICS OF ATOMS AND MOLECULES
Competition of Concentration Narrowing and Power Broadening of a Dark Resonance in a Ladder System of Rubidium Atoms: Specificities of Its Occurrence in Thin Spectroscopic Cells A. Sargsyana, Ch. S. Adamsb, T. A. Vartanyanc, *, and D. Sarkisyana a Institute b
for Physical Research, National Academy of Sciences of Armenia, 0203 Ashtarak, Armenia Joint Quantum Centre (Durham–Newcastle), Department of Physics, Durham University, South Road, Durham, DH1 3LE, United Kingdom c ITMO University, St. Petersburg, 197101 Russia *e-mail: [email protected] Received June 3, 2020; revised June 3, 2020; accepted June 23, 2020
Abstract—The effect of electromagnetically induced transparency (EIT) in a three-level ladder system 5S1/2‒5P3/2 –5D5/2 of Rb atoms is investigated. The effect of spectral narrowing of a dark resonance (DR) as a function of atomic vapor density and thickness L of a spectroscopic cell containing Rb vapor is demonstrated. Thickness L was varied in the range of 390 nm to 4 mm, while atomic density N was increased up to ~1016 cm–3; in the process, high-intensity coupling radiation and weak probe radiation were used. The strongest effect, namely, a 22-fold spectral narrowing of the DR, was achieved in a cell of thickness L = 4 mm. The effect of spectral narrowing was decreasing with decrease in thickness L: a 2.4-fold spectral narrowing of the DR was observed at L = 2 μm. Spectral narrowing was nearly absent at L = 0.8 μm, and spectral broadening of the DR was observed with further decrease in the cell thickness down to L = 0.4 μm. A close to 100% DR contrast was achieved in nearly all cases at moderate atomic densities and high intensities of the coupling radiation. An interpretation of the effect of spectral narrowing and broadening of the DR is proposed. Keywords: Electromagnetically induced transparency, ladder type system, dark resonances, Rb atoms, Doppler broadening DOI: 10.1134/S0030400X20100227
1. INTRODUCTION Specific feature of the effect of electromagnetically induced transparency [1–4] consists in the possibility of formation of a spectrally narrow dark optical resonance (DR) in both a Λ-system and a ladder system (the Ξ-system) of levels even at small thickness of atomic-vapor column of L ~ 1 μm [5]. This fact is important for practical applications, because it allows not only dramatically reducing the size of atomic cells used for formation of the DR [6, 7] but also considerably increasing the intensity of laser radiation by using lenses with a short focal distance. The possibility of tight focusing becomes particularly important when using high-lying Rydberg levels in which case the probability of the second (upper) transition is low [8, 9], as well as in the case of two-photon absorption [10]. As a rule, a Λ-system consisting of two long-lived lower levels (initial and final) and one high-lying level is used for formation of a DR. Under certain conditions, this allows forming ultra-narrow (with a width of several Hz) dark optical resonances [11,
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