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Physical Review A

Observation of atomic coherence in laser-excited uranium vapor

D. B. Scarl, Lloyd A. Hackel, Michael Austin Johnson, Michael C. Rushford

Published Aug 1, 1981• 9 citations
Publisher

Abstract

For the pair of $^{238}\mathrm{U}$ levels at 3868 and 26 558 ${\mathrm{cm}}^{\ensuremath{-}1}$0, the upper-state population was measured as a function of the irradiance of a resonant exciting pulse of fixed duration. The observed damped Rabi flopping showed that the two-level system was partially coherently excited. The results were compared with the prediction of an exact solution of the time-dependent Schr\"odinger equation for two-level systems excited by a detuned rectangular pulse. The Schr\"odinger equation solution, after averaging over laser irradiance fluctuations, agrees well with the data.

DOI
10.1103/physreva.24.883

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A coherent superposition state of 5S and 5D levels of Rb vapor, when it was excited by two-photon resonant transition between these levels, was observed by applying an interrogation pulse which was non-resonant to the system. The emphasis is laid on this coherence in the atomic excitation, instead of just the population inversion, in explaining the origin of various strong collimated emissions observed by many authors. Saturation of the off diagonal density matrix element between 5S and 5D levels was observed at higher excitation level due to the population change. An analysis of the saturation is given using a simple model for the coherence of the dye laser beam.

Journal of the Physical Society of Japan•Nov 1, 1974•17 citations
For the pair of 238U^{238}\mathrm{U}238U levels at 3868 and 26 558 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}cm\ensuremath−10, the upper-state population was measured as a function of the irradiance of a resonant exciting pulse of fixed duration. The observed damped Rabi flopping showed that the two-level system was partially coherently excited. The results were compared with the prediction of an exact solution of the time-dependent Schr"odinger equation for two-level systems excited by a detuned rectangular pulse. The Schr"odinger equation solution, after averaging over laser irradiance fluctuations, agrees well with the data.
The authors investigate resonant two-step laser excitation in a cascade three-level system using an atomic-uranium beam. Both single-photon and two-photon processes are studied by varying the time delay between the two different excitation pulses. The experimental data are compared to the results of simple theoretical models. The calculations include the effects of the magnetic sublevels. The major theoretical prediction of a two-photon resonance is verified experimentally by using simultaneous irradiation from two lasers. The agreement between theory and experiment is found to be satisfactory.
Laser excitation of a two-level atom is analytically studied for arbitrary pulse shape, area, and energy. The ionization mechanism is modeled as a homogeneous width $\ensuremath{\gamma}$ of the atomic excited state. A transformation of the optical Bloch equations allows the dynamics to be completely described by a single equation for the area of the pulse at time ttt, $\ensuremath{\Phi}(t)$. Analytic solutions for the total bound population for a wide variety of pulse shapes can be found in two regions: $\ensuremath{\Phi}(t)\ensuremath{\approx}sin\ensuremath{\Phi}(t)$ and $\ensuremath{\Omega}(t)>\ensuremath{\gamma}$, where $\ensuremath{\Omega}(t)$ is the resonance Rabi frequency. The generalization of the two-level atom results to the NNN-level atom is discussed.
Two methods have been used to measure the oscillator strength of the transition between the ground and 17,992 cm level in XUYb. The first technique involves exciting the transition with a laser pulse that is nearly time-bandwidth limited, of uniform intensity, and has a reproducible shape from shot to shot. The population left in the excited state after the pulse varies sinusoidally with a period that depends on the integral over time of the electric field amplitude and the transition oscillator strength. These are the Rabi oscillations that are predicted by application of the Schrodinger equation to the two-level atom. The second method involves observation of the polarization rotation of a set of degenerate sublevels brought about by a light-shift laser. One sublevel (m/sub j/ = 0) of the J = 1 level at 17,992 cm is populated by a linearly polarized laser. A second copropagating light-shift laser, which is linearly polarized at an angle to the first laser, is tuned between 7.5 and 30 GHz off-resonance with the transition. The light-shift laser causes population to be promoted into the m/sub j/ = +-1 levels through the virtual J = 0, m/sub j/ = 0 level. Two linearly polarized photoionizing lasers photoionize the population only from the m/sub j/ = +-1 levels. The photoion signal oscillates cosinusoidally with a period that depends only on the integrated pulse intensity, the laser detuning, and the transition oscillator strength. Finally, polarization selectivity has been shown experimentally to allow selective photoionization of the odd isotopes of ytterbium using broadband lasers.
In this research study, we derive the exact solutions of the Bloch equations describing the dynamics of a two-level atom with dephasing. In the two-level atom, a strong laser pump couples a ground state to an upper excited state with a time-dependent Rabi-frequency. The exact solutions are given for the atomic population inversion and the real and imaginary parts of the coherence while the input pulse is an asymmetric hyperbolic cosine form. Additionally, the system is under a chirped detuning. The method of solving the Bloch equations analytically is a very tedious part of the research, and as far as we know, there are few exact solutions available in this field. Hence, our solutions might be of great interest to various research areas, including nuclear magnetic resonance, where analytical solutions to the Bloch equations play a major role in the study of the information on the state of the medium as determined by the NMR signals.
We have used two methods to measure the oscillator strength of the transition between the ground and 17992 cm-1 level in 174Yb. The first technique involves exciting the transition with a laser pulse that is nearly time-bandwidth limited, of uniform intensity, and has a reproducible shape from shot to shot. The population left in the excited state after the pulse varies sinusoidally with a period that depends on the integral over time of the electric field amplitude and the transition oscillator strength. These are the Rabi oscillations that are predicted by application of the Schrodinger equation to the two-level atom. The excited-state population is probed using a two-step photoionization to the continuum. The electric field amplitude is determined from the temporal profile and the intensity of the laser pulse. The second method involves observation of the polarization rotation of a set of degenerate sublevels brought about by a time-bandwidth limited light-shift laser. One sublevel (mj = 0) of the J = 1 level at 17992 cm-1 is populated by a linearly polarized laser. A second copropagating light-shift laser, which is linearly polarized at an angle to the first laser, is tuned between 7.5 and 30 GHz off-resonance with the transition. The light-shift laser causes population to be promoted into the mi = ±1 levels by a virtual transition through the ground J = 0, = 0 level. Two linearly polarized photoionizing lasers photoionize the population only from the mi = ±1 levels. The photoion signal oscillates sinusoidally with a period that depends only on the integrated pulse intensity, the laser detuning, and the transition oscillator strength.
A coherent superposition state of 5S and 5D levels of Rb vapor, when it was excited by two-photon resonant transition between these levels, was observed by applying an interrogation pulse which was non-resonant to the system. The emphasis is laid on this coherence in the atomic excitation, instead of just the population inversion, in explaining the origin of various strong collimated emissions observed by many authors. Saturation of the off diagonal density matrix element between 5S and 5D levels was observed at higher excitation level due to the population change. An analysis of the saturation is given using a simple model for the coherence of the dye laser beam.