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Annual Meeting Optical Society of America

Phase-sensitive multiphoton ionization

Ce Chen, Yi-Yian Yin, Daniel S. Elliott

Published Jan 1, 1990• 0 citations
Publisher

Abstract

We have demonstrated an interference between multiphoton ionization processes that results in a phase-sensitive enhancement or inhibition of the net ionization rate. We focus two laser beams, either of which can induce an atomic transition between the same two states, into a cell containing atomic mercury. One of the fields is at a wavelength of 185 nm and can linearly induce the 6s 1S0 → 6p 1P1 transition in mercury. The other field is at a wavelength precisely three times as long, and it can excite the 6p resonance by a three-photon process. The absorption of two additional visible photons ionizes the mercury. In the presence of both fields, the transition amplitudes interfere, and their relative phases become very important. Their phases can be controlled by controlling the phases of the optical fields inducing the transitions. Coherence between the two fields, vital for the interference, is ensured by generating one field from the other using a nonlinear third-harmonic process. We have also demonstrated the effect of spatial phase variations between the two fields on the local photo-ionization rate.

DOI
10.1364/oam.1990.tue7

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We have demonstrated an interference between multiphoton ionization processes that results in a phase-sensitive enhancement or inhibition of the net ionization rate. We focus two laser beams, either of which can induce an atomic transition between the same two states, into a cell containing atomic mercury. One of the fields is at a wavelength of 185 nm and can linearly induce the 6s 1S0 → 6p 1P1 transition in mercury. The other field is at a wavelength precisely three times as long, and it can excite the 6p resonance by a three-photon process. The absorption of two additional visible photons ionizes the mercury. In the presence of both fields, the transition amplitudes interfere, and their relative phases become very important. Their phases can be controlled by controlling the phases of the optical fields inducing the transitions. Coherence between the two fields, vital for the interference, is ensured by generating one field from the other using a nonlinear third-harmonic process. We have also demonstrated the effect of spatial phase variations between the two fields on the local photo-ionization rate.
This thesis reports an experimental study of an interference phenomenon between one-photon and three-photon 6s$\sp1$S$\sb0$-6p$\sp1$P$\sb1$ transition amplitudes in a vapor of atomic mercury. The interference was observed by monitoring the multiphoton ionization signal. Using a three-cell design we were able to separate the third-harmonic generation process and the interfering absorption processes such that we could externally control the relative phase between the two transition amplitudes and directly study the interference phenomena. We observed that the multiphoton ionization signal, which is a measure of the net transition moment, varied sinusoidally as the relative phase increased. Using an eight-electrode design to collect the electrons ionized at different positions along the beam axis around the focal region, we measured the phase shift of the interference due to the π\piπ phase shift of a focused Gaussian beam propagating through the focal region. The experimental setup and the experimental procedure are discussed in detail. This new method can be used in the study of many phase sensitive processes including potential applications in coherent control of chemical reactions.
We report evidence of an interference process between two bound-free atomic transitions induced by laser fields. Sodium atoms, initially prepared in the 3P1/2{3\mathrm{P}}_{1\mathrm{/}2}3P1/2​ excited state, are ionized by a nonresonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The experiment has provided a fringe visibility as high as 42%.
We have investigated a two-color coherent-control process at high densities of the absorbing medium. Irradiating a vapor of atomic mercury with a coherent field, with one component nearly resonant with the three-photon 6s1{\mathrm{}}^{1}1S0{\mathit{S}}_{0}S0​\ensuremath{\rightarrow}6p1{\mathrm{}}^{1}1P1o{\mathit{P}}_{1}^{\mathit{o}}P1o​ transition, and the other component the third harmonic of the first, we observe the interference between the two excitation pathways for this transition. Measurements of this interaction as the density of the mercury vapor is increased show evidence of nonlinear coupling (phase and amplitude locking) of the two field components with each other. This is manifested as a decrease in the magnitude of the photoionization signal, as well as a decrease of the depth of modulation of the photoionization signal. \textcopyright{} 1996 The American Physical Society.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
We have used interfering multiphoton ionization processes to measure changes in the relative phase between two optical laser beams. In this work, the phase variation is due to the \ensuremath{\pi} phase shift of a focused Gaussian beam as it propagates through the focal region. An array of linear collection electrodes is used to measure the multiphoton ionization rate as a function of the distance from the beam waist when two laser fields are resonant with an atomic transition, one through a linear process, the other through a three-photon process.
The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment.