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Physical Review A

Half-cycle microwave stabilization of rubidium Stark atoms

C. Wesdorp, F. Robicheaux, Lambertus D. Noordam

Published Dec 1, 1999• 4 citations
Publisher

Abstract

We report on the observation of stabilization of laser-excited rubidium Stark atoms above the classical ionization limit with the help of short (9 ns) unipolar electric-field pulses. Dipping the field down from 40 to 0 V/cm and back to 40 V/cm, 30 ns after photoexcitation into the continuum, yields a large bound-state population. The results suggest an additional mechanism of the recombination as seen in the THz experiment by Bensky, Campbell, and Jones, [Phys. Rev. Lett. 81, 3112 (1998)].

DOI
10.1103/physreva.60.5122

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Based on the results of a previous communication [Dimitrovski et al., Phys. Rev. Lett. 93, 083003 (2004)], we study ionization and excitation of a hydrogenic atom from the ground and first excited states in short electric field pulses of several cycles. A process of ionization and recombination which occurs periodically in time is identified, for both small and extremely large peak electric field strengths. In the limit of large electric peak fields closed-form analytic expressions for the population of the initial state after single- and few-cycle pulses are derived. These formulas, strictly valid for asymptotically large momentum transfer from the field, give excellent agreement with fully numerical calculations for all momentum transfers.

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We report on the observation of stabilization of laser-excited rubidium Stark atoms above the classical ionization limit with the help of short (9 ns) unipolar electric-field pulses. Dipping the field down from 40 to 0 V/cm and back to 40 V/cm, 30 ns after photoexcitation into the continuum, yields a large bound-state population. The results suggest an additional mechanism of the recombination as seen in the THz experiment by Bensky, Campbell, and Jones, [Phys. Rev. Lett. 81, 3112 (1998)].
Excitation from ns\mathrm{ns}ns, np\mathrm{np}np, and nd\mathrm{nd}nd Rydberg states of rubidium to the adjacent manifolds by electrical pulses of 140 and 9 ns duration up to 200 V/cm is studied. These pulses correspond to half cycles of microwave fields of 2.6 GHz and 40 MHz. The observed population transfer (up to 40%) is not monotonically increasing with the field strength. For the 140 ps pulse local minima are found due to the absence of avoided crossings of appropriate size at the peak of the pulse. For the long pulses Stueckelberg oscillations are observed in the transfer yield.
Ionization of Rydberg atoms by short, half-cycle electrical pulses is investigated experimentally in the intermediate-pulse duration regime where the ionization threshold fields are determined both by the slew rate of the ionizing pulse and its duration. Electrical pulses with durations ranging from 140 ps to 9 ns and amplitudes up to are applied to rubidium and lithium atoms prepared in high Rydberg states by laser excitation (n ranges from 42 to 82). The amplitude of the 140 ps pulse required for ionization of the Rb and Li Rydberg population is found to be a factor of two higher than the expected amplitude based on diabatic field ionization and recent theoretical calculations by Reinhold et al (1993 26 L659). Using the longer pulses we studied the transition from the diabatic/adabatic ionization towards ionization in the impulse-kick regime.
We have measured the microwave ionization threshold electric fields for ns-state Na Rydberg atoms from n=24--33 as a function of the duration of the ionizing microwave pulse. Pulses as short as 2.8 ns were used, containing fewer than 25 cycles of 8.55-GHz radiation. While the fields required to produce the onset of ionization were independent of pulse duration and exhibited the ${\mathit{n}}^{\mathrm{\ensuremath{-}}5}$ scaling characteristic of ionization by diffusion through higher-energy states, the fields required to produce nearly complete ionization increased dramatically as the pulse was shortened. Additional measurements of the populations of bound states after the shortest microwave pulses showed that up to \ensuremath{\sim}10% of the atoms were excited to states above the initially populated ns state and the adjacent (n-1) manifold of higher angular momentum states, while most either ionized or remained in these states. From this observation we conclude that the rate-limiting process for ionization is the transition from the initially populated ns states or the adjacent manifold to the next higher manifold, not the diffusion through higher-lying states.
An amplitude-modulated near-infrared laser pulse synchronized to a 14-GHz microwave field is used to excite atoms to the vicinity of the ionization limit at specific phases of the microwave field. When the laser is tuned above and below the ionization limit, phase-dependent modulation is observed in the recombination and ionization, respectively. The phase-dependent modulation is 10% of the total excitation, far greater than the 0.1% modulation observed with single-picosecond laser pulse excitation, and in agreement with calculations based on coherent excitation over several microwave cycles [Phys. Rev. Lett. 95, 013001 (2005)].
The recent development of laser cooling and trapping techniques has made possible the controlled realization of dense and cold atomic samples, thus opening the way for spectroscopic investigations in the low and ultra-low temperature regimes not accessible with conventional techniques. In our lab, an apparatus for laser cooling and Bose-Einstein condensation of rubidium atoms is in operation, producing a sample large enough to perform reliable investigations by absorption or emission techniques. Moving from previous photoionization experiments of laser-cooled atoms in a magneto-optical trap irradiated by cw-laser radiation [1] and stimulated by the interest grown recently on ultra-cold plasma [2], we have started a series of experiments aimed at investigating the interaction of intense laser pulses with the ultra-cold neutral sample, boty aboeand below the condensation temperature. Radiation from an excimer-pumped dye laser was used at a wavelength close to 594 nm, corresponding to the ionization threshold of rubidium for two photon non-resonant absorption from the ground state, with intensities in the tens of MW/cm^2 range and a pulse duration of 16 ns. Preliminary results, attained through the dynamical analysis (atom number, temperature) of the cold atom sample after the arrival of the laser pulse, suggest the occurrence of different processes, initiated by multi-photon ionization, involving collisions between cold electrons and ultra-cold neutrals, recombination at low temperatures and collective processes in the weak and cold plasma. Interpretation of those results demonstrates that this class of experiments, based on a variety of topics typical of atomic, plasma and statistical physics, can open the way for novel and intriguing investigations of laser-matter interaction. Further experimental developments will include duplication of the laser pulse frequency and one photon ionization of ground state rubidium laser-cooled atoms. [1] O. Maragò et al., Phys. Rev. A 57 R4110 (1998); E., Arimondo et al., Appl. Surf. Sci. 154-155 527 (2000); F. Fuso et al., Opt. Commun. 173 223 (2000). [2] T.C. Killian et al., Phys. Rev. Lett. 83 4776 (1999); S. Kulin et al., Phys. Rev. Lett. 85 318 (2000).
Based on the results of a previous communication [Dimitrovski et al., Phys. Rev. Lett. 93, 083003 (2004)], we study ionization and excitation of a hydrogenic atom from the ground and first excited states in short electric field pulses of several cycles. A process of ionization and recombination which occurs periodically in time is identified, for both small and extremely large peak electric field strengths. In the limit of large electric peak fields closed-form analytic expressions for the population of the initial state after single- and few-cycle pulses are derived. These formulas, strictly valid for asymptotically large momentum transfer from the field, give excellent agreement with fully numerical calculations for all momentum transfers.