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arXiv:2107.02635 · Physical Review A

ac polarizability and photoionization-cross-section measurements in an optical lattice

Ryan Cardman, Xiaoxuan Han, Jamie MacLennan, Alisher Duspayev, Georg A. Raithel

Published Dec 8, 2021• 6 citations
arXivHTMLPDF

Abstract

We use double-resonant two-photon laser spectroscopy to measure the dynamic scalar polarizability of the rubidium $5{D}_{3/2}$ level, ${\ensuremath{\alpha}}_{5{D}_{3/2}}^{S}$, at a wavelength of $\ensuremath{\lambda}=1064$ nm. Since $\ensuremath{\lambda}$ is shorter than the photoionization (PI) limit of the Rb $5{D}_{3/2}$ level, this state undergoes significant broadening due to PI. The 1064-nm field is applied in the form of a deep optical lattice ($\ensuremath{\sim}{10}^{5}$ photon recoils) generated by an in-vacuum field-enhancement cavity. In our spectroscopic method, we use known dynamic polarizabilities to eliminate the need to measure the light intensity. Our method yields, in atomic units, ${\ensuremath{\alpha}}_{5{D}_{3/2}}^{S}=\ensuremath{-}524(17)$, in agreement with estimates. Additionally, we extract the $5{D}_{3/2}$ photoionization cross section $\ensuremath{\sigma}$ at 1064 nm from spectral linewidths; we find $\ensuremath{\sigma}=44(1)$ Mb.

DOI
10.1103/physreva.104.063304

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We use double-resonant two-photon laser spectroscopy to measure the dynamic scalar polarizability of the rubidium 5D3/25{D}_{3/2}5D3/2​ level, \ensuremathα5D3/2S{\ensuremath{\alpha}}_{5{D}_{3/2}}^{S}\ensuremathα5D3/2​S​, at a wavelength of \ensuremathλ=1064\ensuremath{\lambda}=1064\ensuremathλ=1064 nm. Since \ensuremathλ\ensuremath{\lambda}\ensuremathλ is shorter than the photoionization (PI) limit of the Rb 5D3/25{D}_{3/2}5D3/2​ level, this state undergoes significant broadening due to PI. The 1064-nm field is applied in the form of a deep optical lattice (\ensuremath∼105\ensuremath{\sim}{10}^{5}\ensuremath∼105 photon recoils) generated by an in-vacuum field-enhancement cavity. In our spectroscopic method, we use known dynamic polarizabilities to eliminate the need to measure the light intensity. Our method yields, in atomic units, \ensuremathα5D3/2S=\ensuremath−524(17){\ensuremath{\alpha}}_{5{D}_{3/2}}^{S}=\ensuremath{-}524(17)\ensuremathα5D3/2​S​=\ensuremath−524(17), in agreement with estimates. Additionally, we extract the 5D3/25{D}_{3/2}5D3/2​ photoionization cross section \ensuremathσ\ensuremath{\sigma}\ensuremathσ at 1064 nm from spectral linewidths; we find \ensuremathσ=44(1)\ensuremath{\sigma}=44(1)\ensuremathσ=44(1) Mb.
We employ doubly resonant two-photon excitation into the 74S74S74S Rydberg state to spectroscopically measure the dynamic scalar polarizability, ${\ensuremath{\alpha}}_{0}$, and tensor polarizability, ${\ensuremath{\alpha}}_{2}$, of rubidium 5P3/25{P}_{3/2}5P3/2​. A cavity-generated 1064-nm optical lattice allows us to reach intensities near $2\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{m}}^{2}$, where the atom field is larger than the hyperfine interaction, and magnetic sublevels are well resolved. In the evaluation of the data we use a self-referencing method that renders the polarizability measurement largely free from the intensity calibration of the laser light field. We obtain experimental values ${\ensuremath{\alpha}}_{0}=\ensuremath{-}1149(\ifmmode\pm\else\textpm\fi{}2.5%)$ and ${\ensuremath{\alpha}}_{2}=563(\ifmmode\pm\else\textpm\fi{}4.2%)$, in atomic units. Methods and results are supported by simulations. The results provide an experimental test of atomic-structure calculations used to determine systematic shifts in atomic clocks and to interpret fundamental-physics experiments.
We report on accurate measurements of the scalar ${\ensuremath{\alpha}}_{S}$ and tensor ${\ensuremath{\alpha}}_{T}$ polarizabilities of the 5D5D5D fine structure levels 5D3/25{D}_{3/2}5D3/2​ and 5D5/25{D}_{5/2}5D5/2​ in Rb. The measured values (in atomic units) ${\ensuremath{\alpha}}_{S}({5D}_{3/2})=18\phantom{\rule{0.16em}{0ex}}400(75)$, ${\ensuremath{\alpha}}_{T}({5D}_{3/2})=\ensuremath{-}750(30)$, ${\ensuremath{\alpha}}_{S}({5D}_{5/2})=18\phantom{\rule{0.16em}{0ex}}600(76)$ and ${\ensuremath{\alpha}}_{T}({5D}_{5/2})=\ensuremath{-}1440(60)$ show reasonable correspondence to previously published theoretical predictions, but are more accurate. We implemented laser excitation of the 5D5D5D level in a laser-cooled cloud of optically polarized Rb-87 atoms placed in a constant electric field.
We report a measurement of the dynamic (ac) scalar polarizability of the 5D3/2 state in 85Rb atoms at a laser wavelength of 1064 nm. Contrary to a recent measurement in Phys. Rev. A 104, 063304 (2021), the experiments are performed in a low-intensity regime in which the ac shift is less than the 5D3/2 state’s hyperfine structure, as utilized in numerous experiments with cold, trapped atoms. The extracted ac polarizability is α5D3/2=−499±59 a.u., within the uncertainty of the aforementioned previous result. The calibration of the 1064 nm light intensity, performed by analyzing light shifts of the D1 line, is the main source of uncertainty. Our results are useful for applications of the Rb 5D3/2 state in metrology, quantum sensing, and fundamental-physics research on Rydberg atoms and molecules.
In this dissertation, I explore several aspects of Rydberg physics with laser spectroscopy. The experiments center around the use of a very deep, cavity-generated, one-dimensional optical lattice in a cold atom sample. The optical lattice produces elongated clouds of dense, cold atoms via dipole trapping and is able to generate extreme light shifts in the atoms due to its high intensity. I describe experiments which may be divided into two main classes. In the first, I use the dipole trap to prepare a dense sample of atoms for photoassociation of Rb atom pairs into diatomic Rydberg molecules. The Rydberg molecules are formed by low-energy scattering of the Rydberg electron of one atom from the other atom, which is a ground-state atom. This novel binding mechanism also has relevance for chemical physics. I present binding energy measurements for eight different cases of Rydberg molecules. The binding energies reveal information about electron-atom collisions, such as their scattering lengths, and about the spin couplings present within the molecular system. In the second class of experiments, I introduce two proposed measurements in which the optical lattice is ramped to very high intensity during the laser excitation, creating AC Stark shifts in the transition frequencies. In one measurement, low-lying atomic states are probed. Measurements of the light shifts are expected to yield data on the dynamic polarizability and the photoionization cross section of the 5D3/2D_{3/2}D3/2​-state in rubidium in a 1064-nm light field. I show corresponding preliminary experimental data. The polarizability and photo-ionization cross-section measurements will provide tests of atomic structure theory, which is important both at a fundamental level and for applications such as atomic clocks. In another proposed measurement, Rydberg atoms in the lattice experience ponderomotive shifts and strong mixing of their angular-momentum states, allowing direct optical excitation of the lattice-mixed high-angular-momentum states. I describe the theoretical basis and experimental preparation for spectroscopically characterizing the lattice-mixed states. The ability to optically excite selected high-angular-momentum states without the use of static or RF fields may be useful for quantum information processing and simulation.
Two-photon ionization cross sections from the rubidium ground state have been measured with both a cw 421-nm laser and a combination of cw 421- and 1002-nm lasers. The measurements were performed within a high-vacuum magneto-optical trap while the trapping lasers were switched off, exploiting the long trap lifetime and also the trap laser confinement. The two-photon cross sections were determined for the blue laser near resonance with the 6P1/2 and 6P3/2 states and compared with the estimates of a theoretical model. In near resonance with the 6P states, large two-photon photoionization cross sections were measured.
The authors have used laser cooling and trapping techniques to investigate photoionization and cold collisions. With laser-trapped Rb, they have measured the photoionization cross section from the first excited (5P) level by observing the photoionization-induced loss rate of neutral atoms from the trap. This technique has the advantage that it directly measures the photoionization rate per atom. Knowing the ionizing laser intensity and the excited-state fraction, the measured loss rate gives the absolute cross section. Using this technique, the Rb 5P photoionization cross section at {approximately}400 nm has been determined with an uncertainty of 9%. The authors are currently attempting to extend this method to the 5D level. Using time-ordered pulses of diode-laser light (similar to the STIRAP technique), they have performed very efficient two-photon excitation of trapped Rb atoms to 5D. Finally, they will present results from a recent collaboration which combines measurements form conventional molecular spectroscopy (single photon and double resonance) with photoassociation collisions of ultracold Na atoms to yield a precise ({le}1 ppm) value for the dissociation energy of the X {Sigma}{sub g}{sub +} ground state of the Na{sub 2} molecule.