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Narrow-line absorption at 689 nm in an ultracold strontium gas

Fachao Hu, Canzhu Tan, Yuhai Jiang, Matthias Weidemüller, Bing Zhu

Published Jan 27, 2021• 1 citation
OpenAlex

Abstract

We analyse the spectrum on the narrow-line transition \narrow at 689 nm in an ultracold gas of $^{88}$Sr via absorption imaging. In the low saturation regime, the Doppler effect dominates in the observed spectrum giving rise to a symmetric Voigt profile. The atomic temperature and atom number can accurately be deduced from these low-saturation imaging data. At high saturation, the absorption profile becomes asymmetric due to the photon-recoil shift, which is of the same order as the natural line width. The line shape can be described by an extension of the optical Bloch equations including the photon recoil. A lensing effect of the atomic cloud induced by the dispersion of the atoms is also observed at higher atomic densities in both the low and strong saturation regimes.

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We analyse the spectrum on the narrow-line transition \narrow at 689 nm in an ultracold gas of 88^{88}88Sr via absorption imaging. In the low saturation regime, the Doppler effect dominates in the observed spectrum giving rise to a symmetric Voigt profile. The atomic temperature and atom number can accurately be deduced from these low-saturation imaging data. At high saturation, the absorption profile becomes asymmetric due to the photon-recoil shift, which is of the same order as the natural line width. The line shape can be described by an extension of the optical Bloch equations including the photon recoil. A lensing effect of the atomic cloud induced by the dispersion of the atoms is also observed at higher atomic densities in both the low and strong saturation regimes.
We report on observing photon recoil effects in the absorption of a single monochromatic light at 689 nm through an ultracold 88Sr gas, where the recoil frequency is comparable to natural linewidth of the narrow-line transition 5s2 1S0–5s5p 3P1 in strontium. In the regime of high-saturation, the absorption profile becomes asymmetric due to the photon-recoil shift, which is of the same order as the natural linewidth. The lineshape is described by an extension of the optical Bloch equations including the momentum transfers to atoms during emission and absorption of photons. Our work reveals the photon recoil effects in a simplest single-beam absorption setting, which is of significant relevance to other applications such as saturation spectroscopy, Ramsey interferometry, and absorption imaging.
Ultracold neutral plasmas are formed by photoionizing laser‐cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1–1000 K and the ion temperature is around 1 K. The density can be as high as 1010 cm−3. Fundamental interest stems from the possibility of creating strongly‐coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+ 2S1/2 → 2P1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time‐scale. The Doppler‐broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.
The spectral lineshapes of the Sr (51P1–51P0) resonance transition perturbed by rare gases have been examined with a high-resolution laser-induced fluorescence method. In He and Ne, the collisional broadened spectral profiles are found to be symmetrical over the entire studied pressure range (0–600 Torr). The resulting line shapes are accurately represented by a simple Voigt profile expression. The Ar, Kr, and Xe perturbing gases, however, produce anomalous asymmetrical line broadening. To our knowledge, this represents the first measurement of asymmetrical broadening in the near wings of the emission profiles of the Sr-rare gas systems. Quantitative comparisons of the measured and theoretical line asymmetries suggest that atomic interactions in the Sr-rare gas systems are not entirely due to van der Waals (R-6)-type interactions. The line asymmetries could be caused by the correlations between thermal motions of the Sr atoms and collisional interactions.
Summary form only given. All-optical production of quantum degenerate atomic gases has been one of the long-standing targets in laser cooling. We report a novel approach towards this direction based on a narrow-line laser cooling for strontium atoms. By employing two transitions with markedly different dipole moments, i.e., the allowed /sup 1/S/sub 0/-/sup 1/P/sub 1/ (/spl lambda/=461 nm, /spl gamma/=2/spl pi//spl times/32 MHz) and the spin-forbidden /sup 1/S/sub 0/-/sup 3/P/sub 1/ (/spl lambda/=689 nm, /spl gamma/=2/spl pi//spl times/7.6 kHz) transition, thermal strontium atoms were Doppler-cooled down to 400 nK or the photon recoil temperature, in a magneto-optical trap (MOT). Presents the corresponding energy levels for cooling and trapping. The use of a narrow spin-forbidden transition successfully reduced the radiation trapping effects at a high atom density over 10/sup 12//cm/sup 3/, thus enabling us to attain the hitherto high phase space density of 0.01 in a MOT.
In the presence of an external static magnetic field, an atomic gas becomes optically active, showing magneto-optical rotation. In the saturated regime, the coherences among the excited substates give a nonlinear contribution to the rotation of the light polarization. In contrast with the linear magneto-optical rotation, the nonlinear counterpart is insensitive to Doppler broadening. By varying the temperature of a cold strontium gas, we observe both regimes by driving the $J=0\ensuremath{\rightarrow}J=1$ transition on the intercombination line. For this narrow transition, the sensitivity to the static magnetic field is typically three orders of magnitude larger than for a standard broad alkali-metal transition.