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Canadian Journal of Physics

Absorption spectroscopy of trapped rubidium atoms

S. Cauchi, Andrejs Vorozcovs, M. Weel, Scott Beattie, O Gagnon, Anantharaman Kumarakrishnan

Published Nov 1, 2004• 10 citations
Publisher

Abstract

We determine the absolute density of a sample of laser-cooled atoms in a two-level system by recording the absorption spectrum of the 85Rb 5S1/2 (F = 3, mf = 3) → 5P3/2 (F′ = 4, m′f = 4) transition. Trapped atoms were prepared in the (F = 3, mf = 3) ground state through optical-pumping techniques. We compare our results with an independent measure of the density that relies on a direct measurement of the number of atoms and size of the atomic sample. We also study the contributions of power broadening, laser line width, and Doppler broadening to the measured absorption spectrum. Our studies suggest that the natural line width (~6 MHz) can be measured to a precision of less than ~50 kHz if the laser line width is measured in real-time with a high-finesse Fabry–Perot cavity. PACS Nos.: 32.70.Cs, 32.70.Jz, 32.80.Pj, 42.62.Fi, 32.70.–n, 32.30.–r

DOI
10.1139/p04-055

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We determine the absolute density of a sample of laser-cooled atoms in a two-level system by recording the absorption spectrum of the 85Rb 5S1/2 (F = 3, mf = 3) → 5P3/2 (F′ = 4, m′f = 4) transition. Trapped atoms were prepared in the (F = 3, mf = 3) ground state through optical-pumping techniques. We compare our results with an independent measure of the density that relies on a direct measurement of the number of atoms and size of the atomic sample. We also study the contributions of power broadening, laser line width, and Doppler broadening to the measured absorption spectrum. Our studies suggest that the natural line width (~6 MHz) can be measured to a precision of less than ~50 kHz if the laser line width is measured in real-time with a high-finesse Fabry–Perot cavity. PACS Nos.: 32.70.Cs, 32.70.Jz, 32.80.Pj, 42.62.Fi, 32.70.–n, 32.30.–r
An ongoing spectroscopic investigation of atomic rubidium utilizes a two-photon, single-laser excitation process. Transitions accessible with our tunable laser include 5P1/2 (F ′ ) ← 5S1/2 (F) and 5P3/2 (F ′ ) ← 5S1/2 (F). The laser is split into a pump and probe beam to allow for Doppler-free measurements of transitions between hyperfine levels. The pump and probe beams are overlapped in a counter-propagating geometry and the laser frequency scans over a transition. Absorption, fluorescence and polarization spectroscopy techniques are applied to this basic experimental setup. The temperature of the vapor cell and the power of the pump and probe beams have been varied to explore line broadening effects and signal-to-noise of each technique. This humble setup will hopefully grow into a more robust experimental arrangement in which double resonance, two-laser excitations are used to explore hyperfine state changing collisions between rubidium atoms and noble gas atoms. Rb-noble gas collisions can transfer population between hyperfine levels, such as 5P3/2 (F ′ = 3) Collision ←− 5P3/2 (F ′ = 2), and the probe beam couples 7S1/2 (F ′′ = 2) ← 5P3/2 (F ′ = 3). Polarization spectroscopy signal depends on the rate of population transfer due to the collision as well as maintaining the orientation created by the pump laser. Fluorescence spectroscopy relies only on transfer of population due to the collision. Comparison of these techniques yields information regarding the change of the magnetic sublevels, mF , during hyperfine state changing collisions.
Spectroscopy of the rubidium 5P3/2 yields 4D5/2 transition near 1.529 micrometers has been performed using a single-longitudinal-mode erbium-doped fiber laser. Rubidium atoms were laser-cooled and confined in a vapor-cell Zeeman optical trap. This produced a dense sample of cold atoms and reduce the Doppler broadening of the transition to less than the natural linewidth. Transition linewidths of 10 MHz were observed on the 5P3/2 yields 4D5/2 transition and the fiber laser was actively stabilized to the 5P3/2, F equals 3 yields 4D5/2, F' equals 3 line of 87Rb.
Laser spectroscopy experiments are reported on rubidium atoms by using two or three external cavity diode lasers to study multiple resonance transitions in Λ- and V-type systems as well as on (Λ+V)-type system. Electromagnetically induced transparency (EIT) peaks having sub-natural line-widths are found on the Doppler broadened transmission backgrounds with velocity selective enhanced absorption dips. In the presence of a pump laser, probe spectrum shows enhancement of EIT signal by tuning the control laser frequency when the Λ- and V-type EIT signals are made to overlap. Theoretical analysis is carried out by solving the optical Bloch equations for five-level atomic model under Λ configuration. With the numerically simulated spectra, variation of EIT transmission peak with ground state decay rate and excited state spontaneous decay rate are investigated. Effect of pump Rabi frequency on the transmission peak is also shown.
We propose and demonstrate pump-probe spectroscopy of rubidium absorption that reveals the sub-Doppler hyperfine structure of the ${}^{5}{\mathrm{S}}_{1/2}\ensuremath{\leftrightarrow}{}^{5}{\mathrm{P}}_{3/2}$ (D2{\mathrm{D}}_{2}D2​) transitions. The counterpropagating pump and probe lasers are independently tunable in frequency, with the probe operating at the single-photon level. The two-dimensional spectrum measured as the laser frequencies are scanned shows fluorescence, Doppler-broadened absorption dips, and sub-Doppler features. The detuning between the pump and probe lasers allows compensation of the Doppler shift for all atomic velocities in the room-temperature vapor, meaning we observe sub-Doppler features for all atoms in the beam. We detail a theoretical model of the system that incorporates fluorescence, saturation effects, and optical pumping and compare this with the measured spectrum, finding a mean absolute percentage error of 3.11%. This high level of agreement allows us to extract the number density and temperature of the Rb\mathrm{Rb}Rb vapor from the two-dimensional spectrum. In the future this technique could be used as an atomic frequency standard for calibrating and characterising a single-photon or low-light-level source.
The interaction of light with atoms and molecules is of fundamental interest in many branches of science. In atomic physics, this interaction can be used to cool and spatially confine (trap) atoms. These traps can be used as the starting point for other experiments, but the dynamics of the cooling and trapping processes is itself of interest. In order to better understand the physics of trapping atoms in an optical dipole force trap, we have conducted a series of spectroscopic measurements of ultracold rubidium atoms in such a trap. The trap was created at the focus of a Nd:YAG laser beam with wavelength 1064nm and nearly Gaussian spatial mode. For rubidium, the trap light is red-detuned and the atoms are confined to the highest intensity in the beam. In order to probe the atoms confined in the trap, we have performed spectroscopy on the 5S1/2 → 5P3/2 transition. Because the polarizability of the ground and excited states is not the same at the trap wavelength, the spectra exhibit both a shift and inhomogeneous broadening. We have investigated the spectra for both linear and circular polarized traps. We also studied the application of a second laser to couple the excited state (5P3/2) to another higher excited state (5D5/2) to reduce the inhomogeneous broadening of the 5S1/2 → 5P 3/2 transition. Finally, two-photon spectroscopy was performed on atoms in the dipole force trap and compared to spectra taken in a magneto optical trap (MOT). Autler-Townes splitting was clearly observed in the MOT and appears to have been observed in the dipole force trap also.
Here we report on measurements of the absolute absorption spectra of dense rubidium vapour on the D 1 line in the weak-probe regime for temperatures up to 170 °C and number densities up to 3 × 10 14 cm −3 . In such vapours, modifications to the homogeneous linewidth of optical transitions arise due to dipole–dipole interactions between identical atoms, in superpositions of the ground and excited states. Absolute absorption spectra were recorded with a deviation of 0.1% between experiment and a theory incorporating resonant dipole–dipole interactions. The manifestation of dipole–dipole interactions is a self-broadening contribution to the homogeneous linewidth, which grows linearly with number density of atoms. Analysis of the absolute absorption spectra allows us to ascertain the value of the self-broadening coefficient for the rubidium D 1 line: β/2π = (0.69 ± 0.04) × 10 −7 Hz cm 3 , in excellent agreement with the theoretical prediction.