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Laser Physics Letters

A DFB diode laser for monitoring and repumping a barium metastable state

C. Gabbanini

Published Oct 9, 2006• 4 citations
Publisher

Abstract

A DFB diode laser at 1.500 μm has been applied to monitor absorption of the 5d6s 1D2 metastable state of barium in a hollow cathode lamp. The laser has been used to repump barium atoms from the metastable state in a thermal atomic beam. Suitable repumping schemes are discussed in order to implement efficient laser cooling of barium.
DOI
10.1002/lapl.200610080

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•Jan 1, 2006•4 citations
Laser cooling and trapping of heavy alkaline-earth element barium have been demonstrated for the first time ever. For any possible cycling transition in barium that could provide strong cooling forces, the excited state has a very large branching probability to metastable states. Additional lasers are thus needed to bring the atoms back in to the cooling cycle. In this work barium atoms were efficiently collected in a magneto-optical trap (MOT) by large optical forces from the strong 6s2 1S0 – 6s6p 1P1 transition. Trapping of barium has been achieved using up to seven lasers simultaneously. The properties of the barium MOT were characterized. The efficiency of capturing an atom from a thermal atomic beam into the MOT is 0.4(1) %. The developed scheme is very efficient for collecting atoms. Loss rate mechanisms from the trap were studied by observing the decay of the trap population. Typical lifetimes of the MOT cloud are on the order of one second and are limited mainly due to insufficient repumping of metastable states. The range of velocities from which barium atoms can be captured into the MOT is about 30 m/s. Different laser transitions were employed for repumping barium atoms from the 6s5d 3D1 and the 6s5d 3D2 states. They lead to similar trap populations and lifetimes. The temperature of the cold atomic barium cloud was determined to about 5 mK. Atomic properties of the 5d2 3F2 state were studied with trapped atoms, in particular it's lifetime was determined as 160(10) s. This work has shown a possible scheme to trap atoms with a leaky cooling cycle. Since atoms with such properties are the majority of the elements in the periodic table, the number of optically trapable elements can be significantly enlarged. Of particular high interest is radium, which is chemical homologue to barium. The interest in radium stems from it's high sensitivity to possible nuclear and electron permanent electric dipole moments (EDM's). An EDM violates parity as well as time reversal and is therefore of fundamental importance. Searches for such EDM's are among the main research goals of the recently commissioned TRImP facility at KVI.
Laser cooling and trapping of the heavy alkaline-earth-metal element barium has been achieved based on the strong $6{s}^{2}\text{ }{^{1}S}_{0}\text{\ensuremath{-}}6s6p\text{ }{^{1}P}_{1}$ transition. The excited state decays to a large fraction into metastable DDD states. Two schemes were implemented where three additional laser-driven transitions provide closed cooling cycles. This results in a fraction of 50(20)% of the trapped atoms in one of the metastable states. A total efficiency of $0.4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ for slowing a thermal atomic beam and capturing atoms into a magneto-optical trap was obtained. Trap lifetimes of more than 1.5 s were observed. They are shortened at high laser intensities by three photon photoionization losses. The developed scheme can be transferred to other systems with large leakage from the cooling transition, such as Ra which is interesting for sensitive fundamental symmetry research.
We present our recently experimental results on a DFB laser diode pumped Rubidium atomic frequency standard. Our experiment was based on a homemade lamp-pumped Rubidium frequency standard with the discharge lamp assembly removed. In the experiment, we first locked a DFB laser diode to the87Rb cycling transition (5S1/2, F=2→5P3/2, F=3) by means of saturation absorption locking technique. After that, we optimized the parameters carefully related to the clock performance, such as temperature of the physical package, amplitude of the modulation signal for the laser locking. The preliminary short-term stability of our clock is measured 5.25×10−12τ−1/2(1s<τ<100s).
The observation of dark-resonances in the two-electron atom barium and their influence on optical cooling is reported. In heavy alkali earth atoms, i.e. barium or radium, optical cooling can be achieved using n^1S_0-n^1P_1 transitions and optical repumping from the low lying n^1D_2 and n^3D_{1,2} states to which the atoms decay with a high branching ratio. The cooling and repumping transition have a common upper state. This leads to dark resonances and hence make optical cooling less inefficient. The experimental observations can be accurately modelled by the optical Bloch equations. Comparison with experimental results allows us to extract relevant parameters for effective laser cooling of barium.
Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p(3{(}^{3}(3P1{\mathit{P}}_{1}P1​) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s(1{(}^{1}(1D) and 5d6s(3{(}^{3}(3D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.
This thesis is written as a part of the TRImP programme. Rare and short-lived radioactive isotopes offer new possibilities for investigating fundamental interactions and symmetries. Radium, a radioactive alkaline earth element has been identified as a new candidate for searches for a permanent electric dipole moment in neutral atoms. The TRImP facility at KVI, Groningen, The Netherlands is being setup to produce and utilize these radioactive isotopes to study fundamental interactions and symmetries. We use atom traps for these studies. We presented the various stages for the production, separation and slowing of the short-lived radioactive isotopes of our interest at the TRImP facility. We need to expand laser cooling and trapping methods to heavy alkaline earth elements barium and radium. Stable barium is well suited as a precursor for radioactive radium, because of its similar level scheme and chemical properties. Because of the radioactivity of radium we have chosen to prepare the cooling and trapping with the homologue barium. Stable isotopes of barium were used in the present study. For spectroscopy, metastable singlet and triplet D-states were populated by optical methods rather than using the conventional method of atomic discharges. The singlet D-state is populated by driving the 1S0-1P1 transition. To populate the triplet D-states efficiently the branching ratio of the 3P1 level was exploited by optical pumping with the intercombination line using the diode lasers at 791.3 nm. Long term frequency stability of these lasers were achieved by referencing the frequency to a nearby transition in molecular iodine. We performed first direct optical spectroscopy of the 1D2-1P1(1500.4 nm), 3D1-1P1(1107.8 nm) and 3D2-1p1 (1130.6 nm) transitions respectively. The hyperfine structure of the 3D1 and 3D2 levels for the Ba-135,137 isotopes were optically resolved. In addition, isotope shifts of the 3D1-1P1 transition were measured. We report the experimental observation of coherent Raman resonances in barium. The relevant Rabi frequencies were extracted for the lambda system and estimated the transition rates, relevant for the laser cooling experiment. We have achieved the first slowing of a barium atomic beam. These results can be applied for atoms, in which a closed cooling cycle can only be found with several laser fields at different wavelengths.