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arXiv:1806.06510

Cold Atom targets in the femtosecond laser field

Renyuan Li, Junyang Yuan, Xinya Hou, Shuai Zhang, Zhiyuan Zhu, Yixuan Ma, Qi Gao, Zhongyang Wang, Tianyi Yan, Matthias Weidemüller, Xincheng Wang, Yizhu Zhang, Yuhai Jiang

Published Jun 18, 2018• 0 citations
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Abstract

We study photoionization of cold rubidium atoms in a strong infrared laser field using a magneto-optical trap (MOT) recoil ion momentum spectrometer. Three types of cold rubidium target are provided, operating in two-dimension (2D) MOT, 2D molasses, and 3D MOT with densities in the orders of $10^7$ atoms/cm$^3$, $10^8$ atoms/cm$^3$, and $10^9$ atoms/cm$^3$, respectively. The density profile and the temperature of 3D MOT are characterized using the absorption imaging and photoionization. The momentum distributions of Rb$^+$ created by absorption of two- or three-photon illuminate a dipole-like double-peak structure, in good agreement with the results in the strong field approximation. The yielding momentum resolution of $0.12 \pm 0.03$ a.u. is achieved in comparison with theoretical calculations, exhibiting the great prospects for the study of electron correlations in alkali metal atoms through interaction with strong laser pulses.

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We study photoionization of cold rubidium atoms in a strong infrared laser field using a magneto-optical trap (MOT) recoil ion momentum spectrometer. Three types of cold rubidium target are provided, operating in two-dimension (2D) MOT, 2D molasses, and 3D MOT with densities in the orders of 10710^7107 atoms/cm3^33, 10810^8108 atoms/cm3^33, and 10910^9109 atoms/cm3^33, respectively. The density profile and the temperature of 3D MOT are characterized using the absorption imaging and photoionization. The momentum distributions of Rb+^++ created by absorption of two- or three-photon illuminate a dipole-like double-peak structure, in good agreement with the results in the strong field approximation. The yielding momentum resolution of 0.12±0.030.12 \pm 0.030.12±0.03 a.u. is achieved in comparison with theoretical calculations, exhibiting the great prospects for the study of electron correlations in alkali metal atoms through interaction with strong laser pulses.
We report on a compact and transportable apparatus that consists of a cold atomic target at the center of a high resolution recoil ion momentum spectrometer. Cold rubidium atoms serve as a target which can be operated in three different modes: in continuous mode, consisting of a cold atom beam generated by a two-dimensional magneto-optical trap, in normal mode in which the atoms from the beam are trapped in a three-dimensional magneto-optical trap (3D MOT), and in high density mode in which the 3D MOT is operated in dark spontaneous optical trap configuration. The targets are characterized using photoionization.
Photoionization of rubidium atoms cooled in a magneto-optical trap, characterized by the coexistence of the ground 5S1/25{S}_{1/2}5S1/2​ and excited 5P3/25{P}_{3/2}5P3/2​ states, is investigated experimentally and theoretically with the 400-nm femtosecond laser pulses at intensities of $I=(3\ifmmode\times\else\texttimes\fi{}{10}^{9})\text{--}(4.5\ifmmode\times\else\texttimes\fi{}{10}^{12})$ W/cm2\mathrm{W}/{\mathrm{cm}}^{2}W/cm2. The recoil-ion momentum distribution (RIMD) of Rb+{\mathrm{Rb}}^{+}Rb+ exhibits rich ringlike structures and their energies correspond to one-photon ionization of the 5P3/25{P}_{3/2}5P3/2​ state and two-photon and three-photon ionizations of the 5S1/25{S}_{1/2}5S1/2​ state, respectively. With increasing III, the dips near zero momentum (NZM) in the experimental RIMDs become shallow dramatically and their peaked Rb+{\mathrm{Rb}}^{+}Rb+ momenta ionized from the 5P3/25{P}_{3/2}5P3/2​ state move obviously toward zero while the peaks from the 5S1/25{S}_{1/2}5S1/2​ state do not shift. In addition, the ion-yield ratio of the 5S1/25{S}_{1/2}5S1/2​ state to the 5P3/25{P}_{3/2}5P3/2​ state varies from III to I1.5{I}^{1.5}I1.5 as III increases. These features indicate a transition from perturbative ionization to strongly perturbative ionization for the 5P3/25{P}_{3/2}5P3/2​ state. Numerical simulations by solving the time-dependent Schr\"odinger equation (TDSE) can qualitatively explain the measurements of the RIMD, photoion angular distributions, and ion-yield ratio. However, some discrepancies still exist, especially for the NZM dip, which could stem from the electron-electron correlation that is neglected in the present TDSE simulations since we have adopted the single-active-electron approximation.
Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.
We developed a magneto-optical trap reaction microscope (MOTREMI) for strontium atoms by combining multiparticle coincident detection with the laser cooling technique. The present compact injection system enables the production of cold Sr atoms in three modes of two-dimensional (2D) magneto-optical trap (MOT) beam, molasses, and three-dimensional (3D) MOT, providing adjustable densities and ratios of the ground state 5s25{s}^{2}5s2 (1S0^{1}S_{0}1S0​) and the excited states 5s5p1P15s5p{\phantom{\rule{4pt}{0ex}}}^{1}{P}_{1}5s5p1P1​ and 3PJ^{3}P_{J}3PJ​. The target profiles including temperature, density, and size of the 3D MOT as well as the 2D MOT beam cold atomic flux were characterized in detail. Using the present setup, we investigate the single photoionization of Sr atoms with molasses through few-photons absorption in 800 nm, where Sr+{\mathrm{Sr}}^{+}Sr+ and ${e}^{\ensuremath{-}}$ are detected in coincidence and most of the ionization channels are identified. The momentum resolution for coincident Sr+{\mathrm{Sr}}^{+}Sr+ and ${e}^{\ensuremath{-}}$ along time-of-flight direction are achieved up to 0.12 a.u. and 0.02 a.u., respectively. The spectra of photoelectron ionized from the ground state as well as several excited states show distinct structures manifesting prominent contributions from multiphoton absorption. Detecting complete vector momenta of electrons and recoil ions in coincidence paves a way for further investigation of two-electron correlation dynamics and multielectron effects with alkaline-earth atoms.
Abstract We propose a method to characterize the features of a cold strontium cloud in a magneto-optical trap (MOT) through the photoionization of cold Sr atoms in a custom-designed reaction microscope. Sr atoms in the dark state of 5s5p3P2 populated via the cascade transition 5s5p1P1 → 5s4d1D2 → 5s5p3P2 accumulate a significant fraction, giving a long lifetime of 520 s. These atoms in the dark state are subsequently trapped by the gradient magnetic field of the MOT. By scanning the Sr+ momentum distributions ionized with an 800 nm infrared femtosecond laser, we are able to outline the size of ∼0.55 mm in radius and the temperature of ∼0.40 mK for the dark-state atoms, which is significantly cooler than the MOT temperature of 3.3 mK trapped in the 461 nm. The size of MOT exhibits an oblate spheroidal distribution with a radius of approximately 0.35 mm and 0.55 mm, extracted with momenta of photoion and absorption imaging, respectively. The results using the photoion momenta are consistent with the expected results from absorption imaging, which confirms the method’s reliability. The advantage of this method is the ability to simultaneously characterize the distribution information of atoms in different initial states within the cold atomic cloud.
We report an experimental demonstration of optical two-dimensional coherent spectroscopy (2DCS) in cold atoms. The experiment integrates a collinear 2DCS setup with a magneto-optical trap (MOT), in which cold rubidium (Rb) atoms are prepared at a temperature of approximately 200 µK and a number density of 10 10 cm −3 . With a sequence of femtosecond laser pulses, we first obtain one-dimensional second- and fourth-order nonlinear signals and then acquire both one-quantum and zero-quantum 2D spectra of cold Rb atoms. The capability of performing optical 2DCS in cold atoms is an important step toward optical 2DCS study of many-body physics in cold atoms and ultimately in atom arrays and trapped ions. Optical 2DCS in cold atoms/molecules can also be a new avenue to probe chemical reaction dynamics in cold molecules.