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Physical Review A

Quantum-mechanical interference between optical transitions: The effect of laser intensity noise

James C. Camparo, P. Lambropoulos

Published Mar 1, 1999• 4 citations
Publisher

Abstract

In a previous publication [Phys. Rev. A 55, 552 (1997)] we considered the effect of laser phase fluctuations on three-photon--one-photon phase control of resonance-enhanced photoionization. Here we extend those studies by considering the role of laser intensity noise in addition to laser phase noise. While our results indicate that relative intensity fluctuations between the fundamental field and its third harmonic have a significant effect on control, the contrast between constructive and destructive interference is nonetheless two orders of magnitude, even under (reasonable) worst case situations. Consequently, neither laser intensity nor phase fluctuations appear to pose a serious impediment to the efficient phase control of atomic and molecular processes.

DOI
10.1103/physreva.59.2515

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In a previous publication [Phys. Rev. A 55, 552 (1997)] we considered the effect of laser phase fluctuations on three-photon--one-photon phase control of resonance-enhanced photoionization. Here we extend those studies by considering the role of laser intensity noise in addition to laser phase noise. While our results indicate that relative intensity fluctuations between the fundamental field and its third harmonic have a significant effect on control, the contrast between constructive and destructive interference is nonetheless two orders of magnitude, even under (reasonable) worst case situations. Consequently, neither laser intensity nor phase fluctuations appear to pose a serious impediment to the efficient phase control of atomic and molecular processes.
We consider three-photon--one-photon phase control of resonance-enhanced photonioization with a phase-diffusion field. As is well known, control is achieved because excitation via the fundamental field interferes with excitation via the third harmonic field, and the form of the interference (i.e., constructive or destructive) depends on the relative phase difference between the two fields. In the problem, the stochastic nature of the field influences control because the propagation constant of a dispersive medium depends on the field's fluctuating frequency. Here, we approach the influence of laser phase noise on control via (i) a physically intuitive zeroth-order approximation, (ii) first-order perturbation theory, and (iii) numerical simulation. Our results show that first-order perturbation theory is not attractive for the study of this problem, since it requires the evaluation of very high order correlation functions associated with the fundamental field"s fluctuating frequency. More importantly, however, numerical simulation demonstrates that highly efficient control can be attained even in the presence of very large linewidth fields.
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.
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%.
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.
In the weak-field limit, resonant absorption is viewed as a passive process: an optical field impinges on an atom, and within some cross-sectional area the atom has a high probability for absorbing the radiant energy. Absorption, however, is a dynamic process. Consequently, though a singlemode laser is highly monochromatic, the field's phase noise (i.e., quantum noise) generates fluctuations in the atom's absorption cross section. Laser phase noise (PM) thereby gives rise to absorption cross-section noise, and hence fluctuations in the medium's transmitted light intensity (AM). Following a brief overview of the PM-to-AM conversion process, we consider the role of collisions on PM-to-AM conversion efficiency in the weak-field regime. Specifically, the relative-intensity-noise of a diode laser, tuned to the Rb D1 transition, was measured after it passed through a rubidium/nitrogen vapor. Varying the nitrogen pressure, we found that rapid collisional dephasing decreased the efficiency of PM-to-AM conversion. Examining the rubidium hyperfine transition lineshape as a function of nitrogen pressure, we then found that pressure-broadening increased the transition's signal-to-noise ratio when limited by the PM-to-AM conversion process.
The effect of temporal coherent control on the two-photon resonant third-harmonic generation in the continuum of ionization of an atomic system is experimentally investigated by means of a pair of partially incoherent, identical laser pulses. The evidence of quantum interference in the third-harmonic generation, as a function of delay between pulses, is reported. The transition from a resonant to a nonresonant case is also shown. The results agree with the predictions of a recently published theoretical model showing that the onset of quantum interference appears to be related to the coherence time of the laser radiation that is much shorter than the pulse duration.