INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF OPTICS B: QUANTUM AND SEMICLASSICAL OPTICS J. Opt. B: Quantum Semiclass. Opt. 7 (2005) 109–118 doi:10.1088/1464-4266/7/4/004 Optical pumping and electromagnetically induced transparency in a lithium vapour FMagnus1,ALBoatwright2,AFlodin3 and R C Shiell4 Department of Physics, University of Sussex, Falmer, Brighton BN1 9QH, UK E-mail: [email protected] Received 20 November 2004, accepted for publication 7 February 2005 Published 9 March 2005 Online at stacks.iop.org/JOptB/7/109 Abstract We report the first study of electromagnetically induced transparency (EIT) on the D1 and D2 lines of 7Li in a vapour cell. The effect of different polarizations, background gas pressures and experimental designs on the dark resonance are examined. It is found that EIT is more prominent on the D1 line than on the D2 line and is present at the D2 transition under incident orthogonal linearly polarized fields and parallel circularly polarized fields, butabsent for parallel linearly polarized fields and for orthogonal circularly polarized fields. It is shown that the contrast of the dark resonances can be affected by the presence of an inert background gas. An analysis of a further nine resonances observed at the D1 transition and three resonances at the D2 transition is presented. Keywords: electromagnetically induced transparency, coherent population trapping, optical pumping, lambda systems, lithium (Some figures in this article are in colour only in the electronic version) 1. Introduction trapped, and so this phenomenon is called coherent population trapping, and where the transmission of the probe beam There has recently been increasing interest in the role that non- through the medium is observed to increase under these linear optical phenomena play within atoms and molecules [1]. conditions, this phenomenon is called electromagnetically These can be clearly demonstrated when two light fields are induced transparency (EIT). Applications of this process incident on an atom or molecule with three internal levels include lasing without inversion [2], the slowing down of light where effects such as saturation and optical pumping of a signals in dilute media by many orders of magnitude [3, 4], particular velocity class by a strong ‘pump’ laser beam can and the measurement of transition dipole moments (see [5] for be detected through the increased transmission or absorption such a measurement in the Li2 molecule). of a weak ‘probe’ laser beam that interrogates the same The theory that describes saturation, optical pumping velocity class of atoms. The presence of the two laser andcoherent population trapping for three-level systems that beams can also create quantum coherences within the atoms can interact with a bichromatic light field for a long time wherebyattwo-photon resonance an ‘uncoupled’ state exists is well understood. A typical three-level lambda system is such that the probability amplitudes for optical excitation shown in figure 1. We shall label the energy of the ith bare from each of its components due to the two light fields state (i.e. the energy in the absence of the laser fields) by exactly cancel each other. The population in this state is Ei and take the energy separation between states |i and | j − = ω 1 Present address: Blackett Laboratory, Imperial College of Science, to be Ei E j h¯ ij.Theatoms are irradiated by two Technology and Medicine, Prince Consort Road, London SW7 2BZ, UK. monochromatic laser fields (which are treated semiclassically) 2 Presentaddress: School of Chemistry,University of Nottingham,University of amplitude ε1 and ε2 andangular frequency ω1 and ω2.We Park,Nottingham NG7 2RD, UK. assume that the frequency of laser 1 is near resonant with the 3 Present address: Department of Physics, Uppsala University, SE-751 21 transition frequency ω ,thatlaser 2 is near resonant with ω , Uppsala, Sweden. 12 23 4 Author to whom any correspondence should be addressed. Present address: andthat the laser polarizations are such as to couple the lasers to Physics Department, Trent University, 1600 West Bank Drive, Peterborough, their respective transitions. It isconvenient to define the (small) ON, K9J 7B8,Canada. detunings of the lasers from resonance as 1 = ω1 − ω12 1464-4266/05/040109+10$30.00 © 2005 IOP Publishing Ltd Printed in the UK 109 FMagnus et al which was attributed to the leaky nature of the single system present, and the dark resonances at the |Fg = 1→|Fe = 1 and the |Fg = 2→|Fe = 2 transitions demonstrated very similar amplitudes to each other. The dark resonance at the |Fg = 2→|Fe = 1 transition had the greatest contrast; this wasbecause this system was least ‘leaky’ (within five cycles 2 of spontaneous decay from the P1/2 state, only one of these would be to |Fg = 1, outside the system). Studies on the effect of hyperfine structure in EIT observed in cascade systems have included those of McGloin et al [10] and Badger et al Figure 1. The energy level configuration. [11]. In both these studies there was a significant dependence of the strength of the two-photon resonance peak upon the → → and 2 = ω2 − ω12.The two-photon resonance, or Raman incident laser polarization at the 5s 5p 5d transitions in resonance, condition then corresponds to 1 = 2. rubidium. In the former study, the ratios of the observed peak In particular, this model is a useful one with which to heights agreed with those calculated from transition dipole describe the effect of a bichromatic light field on the D matrix calculations. 2 transitions of the alkali atoms. For these atoms the S1/2 This work presents some results due to two incident laser electronic ground state is split into two levels due to hyperfine beams on a vapour of lithium atoms at both the 7Li D1 and interactions and a fine-structure component of the upper 2P D2 transitions, in the first study on this species. Lithium term is split into hyperfine components, one member of which is of general interest in atomic physics; in addition to being plays the role of state |2 in this model. In reality, however, asystem in which the effects of closely-spaced hyperfine the actual levels within the alkali atoms are more complicated states can be studied, it is the simplest of the alkalis, for than figure 1 would indicate—each hyperfine level comprises which very accurate atomic structure calculations can be several degenerate (in the absence of any external fields) MF performed [12]. A theoretical analysis of the behaviour of 2 states and, for the P3/2 levels of both lithium and potassium, dark resonances for atoms with upper-level structure in a high- the small hyperfine splitting (A =−3.06 MHz for 7Li and pressure buffer gas has been developed by Taˇıchenachev et al A = 6.06 MHz for 39K, for example [6]) raises the interesting [13], where the assumptions used correspond to buffer gas possibility of investigating the effect of very dense excited pressures significantly greater than used here. state hyperfine structure on the formation of trapping states at This paper is presented as follows. In section 2 we the D2 transitions. This structure can be expected to result in describe the experiment in which various configurations of significant deviations from theoretical predictions based solely laser polarization and beam geometry were used, in section 3 on an analysis of the simple system shown in figure 1. In this we present the spectra obtained by detecting the transmission paper we report and compare the presence (and absence) of of either the probe laser, or the pump and the probe EIT at the D1 and D2 transitions of lithium atoms in a vapour simultaneously, and in section 4 we present an analysis of the cell for the first time, and the effect of different experimental results and discuss their significance. conditions upon the spectral features. Previous experimental studies on the effects of hyperfine 2. Experimental details structure on EIT have investigated the spectroscopy of cesium, The lithium vapour cell consists of a stainless steel tube of rubidium and sodium in both and cascade configurations. 3/4inchexternal diameter with glass windows at each end Stahler¨ et al [7] compared the visibility of the EIT peak and a single core heating element (Thermocoax) wrapped 85 2 at the D1 transitions in Rb (A( P1/2) = 121 MHz) and around the centre of the tube. The cable was wound in 2 the D2 transitions (A( P1/2) = 25 MHz), and observed a abifilar arrangement with an equal number of windings in significant increaseincontrast, and decrease in resonance both directions to reduce any induced magnetic field. A width, for the D1 system. Using a small magnetic field thermocouple is held near the centre of the tube and the tube is Nagel et al [8] have recorded the amplitude and FWHM of rigidly held at each end by water-cooled aluminium plates. | = , = the coherence formed between the states Fg 3 MFg 0 This ensures that lithium vapour does not reach and react | = , = and Fg 4 MFg 0 at the cesium D2 transition with with the glass windows, but necessarily means that the atomic | = , = | = circularly polarized light (i.e. with Fe 3 MFe 1 or Fe vapour is not in thermodynamic equilibrium. The cell has two , = | 4 MFe 1 playing the role of state 2 in this system). They gas inlets/outlets; one is connected to a capsule pressure gauge observed that as the carrier frequency and its 9.2 GHz sideband (Edwards CG3) and the other to a pumping station consisting 2 were simultaneously scanned across the P3/2 manifold and of a diffusion pump and a rotary backing pump. The cell can be detuned away from either of the upper states of the system, pumped down to approximately 10−4 mbar and could be filled the amplitude of the dark resonance decreased.