CONTRAST SATURATION RESONANCES IN THE ... - Springer Link

Journal of Russian Laser Research, Volume 34, Number 4, July, 2013

CONTRAST SATURATION RESONANCES IN THE ABSORPTION BAND OF RUBIDIUM MOLECULES Vladimir A. Sautenkov,1,2 ∗ Sergey A. Saakyan,1 Alexander M. Akulshin,1,3 Mikhail A. Gubin,2 Vladimir N. Kuliasov,4 and Boris B. Zelener1,5

1 Joint 2 P.

Institute for High Temperatures, Russian Academy of Sciences Izhorskaya Street 13, Bd. 2, Moscow 125412, Russia N. Lebedev Physical Institute, Russian Academy of Sciences Leninskii Prospect 53, Moscow 119991, Russia 3 Center

for Atom Optics and Ultrafast Spectroscopy Swinburne University of Technology PO Box 218, HAWTHORN VIC 3122, Melbourne, Australia 4 S.

I. Vavilov State Optical Institute Birzhevaya Line 12, St. Petersburg 199034, Russia 5 National

Research Nuclear University “MEPhI” Kashirskoye Chaussee 31, Moscow 115409, Russia ∗ Corresponding

author e-mail:

vsautenkov @ gmail.com

Abstract We study Doppler-free saturation resonances in the absorption band B1 Πu – X1 Σ+ g of rubidium diatomic molecules in the frequency range near the D2 line of lithium atoms (671 nm). We observe contrast saturation resonances and record a variation in the laser light transmission of 4% due to optical saturation. The large optical nonlinearities in the molecular diatomic gas can be used for investigating the four-wave mixing and other nonlinear effects.

Keywords: alkali diatomic molecules, Doppler-free saturation resonances, nonlinear optical effects.

1.

Introduction

A high-density alkali vapor is a thermodynamic mixture of two components: mainly atomic gas and a relatively small amount of molecular gas [1]. The nonlinear optical response of a dense atomic gas shows nontrivial behavior like optically induced spectral narrowing [2]. Also alkali molecules can be used as a resonance medium for investigating different nonlinear optical effects. Strong absorption bands B1 Πu – X1 Σ+ g of the alkali molecules cover visible and near infrared spectral regions [3]. For example, the absorption bands B1 Πu – X1 Σ+ g of cesium and rubidium molecules overlap working wavelengths of commercially available diode lasers. It appears that cesium and rubidium molecules are very attractive for experiments with tunable diode lasers. Saturation Doppler-free spectroscopy of absorption bands was successfully performed with cesium molecules, and narrow absorption and polarization resonances were Manuscript submitted by the authors in English on June 7, 2013. c 1071-2836/13/3404-0375 2013 Springer Science+Business Media New York

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Journal of Russian Laser Research

Volume 34, Number 4, July, 2013

observed in [4–7]. However, for the rubidium molecular band B1 Πu – X1 Σ+ g , which covers the spectral region 640–720 nm, the observations reported in [8–12] are limited by the Doppler-broadened absorption and fluorescence lines. In this paper, we report the first observation of saturation Doppler-free resonances in the absorption band B1 Πu – X1 Σ+ g of rubidium diatomic molecule. Our goals are the demonstration of a large nonlinear variation of molecular absorption and absolute precise frequency addressing.

2.

Experimental Setup and the Results

A simplified scheme of the experimental setup is shown in Fig. 1. An extended cavity diode laser (ECDL) served as a source of tunable coherent emission (line width less 1 MHz) [13]. We study the rubidium vapor (the natural abundance of 85 Rb and 87 Rb) at a temperature near 570 K. The vapor pressure is defined by the temperature of the cell cold spot [14]. The ratio of the molecular number density and atomic number density is of the order of 10−2 [1]. To increase the lifetime of the cell, we use a high-temperature sealed cell consisting of a sapphire cylindrical body and YAG garnet windows [15]. The cell length L = 7 cm. In the experiment, in order to detect the Doppler-free saturation resonances, a linear polarized counter-propagating pump and probe beams with diameter 0.2 cm are sent to the rubidium cell. The laser beams cross at a small angle (≈10−2 rad) Fig. 1. Experimental setup consisting of the extended in the rubidium cell. cavity diode laser (ECDL), the cell with rubidium vapor In our measurements, the saturation resonances (Rb cell), the cell with lithium vapor (Li cell), the poin a gas of lithium atoms are chosen as frequency larization beam splitter (PBS), the mirror with 100% rereferences. A part of the laser emission is sent to the flectivity (M), the quarter-wave retardation plate (λ/4), lithium cell. The beam power is near 2 mW. The the silicon photodiode (PD), and the digital sampling active region of an “open” quartz cell (a cell with oscilloscope (DSO). cold windows) with a small amount of the metal lithium is heated up to 670 K. The saturation resonances in the lithium vapor are observed by using counter-propagating beams with orthogonal linear polarization. Also the frequency scale is checked using a confocal cavity (free range 1.5 GHz, finesse 200). The absorption spectra of rubidium molecules in the vicinity of the 7 Li D2 line (671 nm) are shown in Fig. 2. Note that the direct signals from photodiodes are presented. The linear absorption spectrum (curve a) is recorded only with a weak probe beam. The saturated absorption spectrum (curve b) is recorded with two counter-propagating beams. The powers of the pump and probe beams are 20 and 0.4 mW, respectively. The frequency in Fig. 2 is defined as detuning from the cycling transition 6 S1/2 (F=2) – 6 S1/2 (F’=3) in the 7 Li atoms. The frequency of the cycling transition S1/2 (F=2) – 6 S1/2 (F’=3) is indicated by the arrow. On curve b, one can see narrow structures on the Doppler-broadened background. After frequency calibration, these narrow saturation resonances can serve as frequency references. For example, the selected resonances β1 and β2 are close to the hyperfine

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Fig. 2. Nonsaturated absorption of rubidium Fig. 3. Doppler-free saturation resonances on rubidium molecules (a), saturated absorption of rubidium molecular transitions. molecules (b), and saturated absorption of 7 Li atoms (c).

components of D2 line of 7 Li atoms. These resonances can be used for frequency lock of tunable lasers in experiments with lithium atoms where fixed detuning of the laser frequencies required. This approach can help to simplify experimental arrangements for optical cooling and trapping lithium atoms [16]. Several expensive optical components like the lithium cell and AOM’s can be replaced by the rubidium cell and simple electronics. In Fig. 2, by α we indicate the most contrast Doppler-free resonance on curve b. The width 45 MHz of the resonance is a combination of the natural width and the field broadening [7]. The saturation resonance α on the Doppler-broadened background is shown in Fig. 3. We note that the observed optical saturation of molecular absorption is quite strong as compared with the previous experiments [4–7]. It is interesting to estimate the contrast of the saturation resonance. The transmission in a resonance medium is described by the Beer–Lambert law T = exp(−k(ω)L),

(1)

where k(ω) is the attenuation coefficient, ω is the frequency of the probe-laser beam, and L is the thickness of the resonance medium. In Fig. 3, the maximum absorption of the rubidium molecular gas (near the resonance α) kmax = 0.027 cm−1 . The relative reduction of absorption due to the optical saturation is expressed as (Δksat /kmax ) · 100% ≈ 25%. The variations in the transmission due to the optical saturation ΔTsat =0.04. The estimated optical nonlinearity makes possible efficient resonance four-wave mixing [17] in rubidium molecules.

3.

Summary

We observed the contrast saturation resonances in rubidium diatomic molecules. In [7], optical saturation of the cesium molecules was attributed to the velocity-selective optical pumping of the ground-state

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structure. The long lifetime of the nonequilibrium ground-state population helps to reduce the saturation intensity. The next step in our research is to study the ground-state population lifetime and decoherence lifetime in the alkali molecular gas. The broad and dense molecular spectra allow one to observe groundstate coherent effects like EIT and light storage on many wavelengths in the visible and near infrared ranges.

Acknowledgments The authors thank V. V. Vassiliev, V. L. Velichansky, and B. V. Zelener for help and useful discussions. This work was supported by the Ministry of Education and Science of the Russian Federation under Grants FCP Nos. 8679, 8513, and 8364.

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