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Canadian Journal of Physics

Interference phenomena in two-photon transitions caused by laser-induced ac-Stark effect

A.F. Linskens, N. Dam, J. Reuß, Boris G. Sartakov

Published Nov 1, 1994• 4 citations
Publisher

Abstract

A system is described in which a real molecular state is tuned into and out of resonance with the intermediate level of a two-photon transition, by the laser-induced ac-Stark effect. Two narrow-band CO2 wave-guide lasers are utilized to induce rovibrational transitions in the ν3 ladder of SF6. A weak probe laser monitors the absorption for the [Formula: see text] two-photon transition. Simultaneously, a strong pump laser is in resonance with the [Formula: see text] one-photon transition. The ac-Stark splitting is observed and quantum interferences between two-photon amplitudes involving different intermediate states occur. It is demonstrated that destructive interference allows suppression of the two-photon excitation for an appropriate pump laser power. Simulations based on the Dressed State Picture and the optical Bloch equations show a satisfactory agreement with the experiment.

DOI
10.1139/p94-163

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Physical Review A•Jul 1, 1995•11 citations
A system is described in which a real molecular state is tuned into and out of resonance with the intermediate level of a two-photon transition, by the laser-induced ac-Stark effect. Two narrow-band CO2 wave-guide lasers are utilized to induce rovibrational transitions in the ν3 ladder of SF6. A weak probe laser monitors the absorption for the [Formula: see text] two-photon transition. Simultaneously, a strong pump laser is in resonance with the [Formula: see text] one-photon transition. The ac-Stark splitting is observed and quantum interferences between two-photon amplitudes involving different intermediate states occur. It is demonstrated that destructive interference allows suppression of the two-photon excitation for an appropriate pump laser power. Simulations based on the Dressed State Picture and the optical Bloch equations show a satisfactory agreement with the experiment.
We describe ac Stark shift production and measurement in a two-laser mode where a first laser is tuned through a two-photon resonance between the ground state and an excited state in order to record a line shape for the corresponding resonantly enhanced multiphoton ionization. A second laser is tuned to a fixed wavelength which is close to a resonance between the upper state in the two-photon transition and a third state which has dipole-allowed transitions back to the ground state. At low concentrations the second laser leads to very large ac Stark shifts in the two-photon excitation and in the related resonantly enhanced multiphoton ionization. However, at high concentrations a destructive interference occurs involving the three-photon coupling between the third state and the ground state and the one-photon coupling due to the four-wave-mixing field at frequency 2${\mathrm{\ensuremath{\omega}}}_{\mathit{L}1}$\ifmmode\pm\else\textpm\fi{}${\mathrm{\ensuremath{\omega}}}_{\mathit{L}2}$. This interference leads to a pressure-dependent suppression of the amplitude for the third state. When this destructive interference occurs, the ac Stark shift due to the second laser undergoes a corresponding pressure-dependent suppression. The criteria for the pressure to be high enough to completely suppress the ac Stark shift is that the real part of phase mismatch \ensuremath{\Vert}\ensuremath{\Delta}k\ensuremath{\Vert} at the angular frequency 2${\mathrm{\ensuremath{\omega}}}_{\mathit{L}1}$\ifmmode\pm\else\textpm\fi{}${\mathrm{\ensuremath{\omega}}}_{\mathit{L}2}$ and (or) the absorption coefficient \ensuremath{\beta} at the same frequency should be very large, so that with unfocused focused beams \ensuremath{\Vert}\ensuremath{\Delta}k\ensuremath{\Vert}L>5[(${\mathrm{\ensuremath{\Delta}}}_{\mathit{s}}^{(\mathrm{max})}$\ensuremath{\tau}]1/2{]}^{1/2}]1/2, where L is the thickness of gas penetrated before the shift is measured, ${\mathrm{\ensuremath{\Delta}}}_{\mathit{s}}^{(\mathrm{max})}$ is the maximum ac Stark shift at very low pressure, and \ensuremath{\tau} is the pulse length of the lasers.At positions and concentrations such that 1\ensuremath{\Vert}\ensuremath{\Delta}k\ensuremath{\Vert}L\ensuremath{\le}2[(${\mathrm{\ensuremath{\Delta}}}_{\mathit{s}}^{(\mathrm{max})}$\ensuremath{\tau})]1/2{]}^{1/2}]1/2 the ac Stark shift is present but the two-photon excitation is suppressed due to a destructive interference between excitation by two photons from the first laser and by two-photon excitation between a four-wave-mixing photon and a photon from the second laser.
We study two-photon absorption in a three-level ladder atomic system driven by a pair of bichromatic fields of equal frequency differences. The high-frequency component of one bichromatic field and the low-frequency component of the other are on two-photon resonance. The transition probability is calculated by employing the method of harmonic expansion and matrix inversion. Unexpectedly, when the sums of the phases of the different pairs of field components on the two-photon resonance are equal to each other, two-photon absorption is dramatically suppressed and the atomic system becomes transparent against two-photon absorption. Physically, due to dynamical Stark splitting, the two-photon transitions induced by the different pairs of field components experience different dressed states with phase difference of $\ensuremath{\pi}$. As a result, destructive interference occurs between the two pathways and leads to the inhibition of two-photon absorption.
A three level ladder system is analyzed and the coherence of initially electric-dipole forbidden transition is calculated. Due to the presence of two laser fields the initially dipole forbidden transition becomes dynamically permitted due to ac Stark effect. It is shown that such transitions exhibit quantum-interference-related phenomena, such as electromagnetically induced transparency, gain without inversion and enhanced refractive index. Gain and dispersion characteristics of such transitions strongly depend upon the relative phase between the driving and the probe fields. Unlike allowed transitions, gain/absorption behavior of ac-Stark allowed transitions exhibit antisymmetric feature on the Rabi sidebands. It is found that absorption/gain spectra possess extremely narrow sub-natural resonances on these ac Stark allowed forbidden transitions. An interesting finding is simultaneous existence of gain and negative dispersion at Autler-Townes transition which may lead to both reduction of the group velocity and amplification of light.
We report on an experimental observation of quantum interference between two dressed states created by a coherent pumping laser in an electromagnetically induced transparency. In a \ensuremath{\Lambda}-type three-level atomic system in rubidium vapor, we reduce the Rabi frequency of the pumping laser in one arm down below the spontaneous decay rate of the common excited state and still observe a narrow dip with subnatural linewidth in the absorption curve of a probe beam in another arm. This clearly demonstrates that the absorption reduction at the low pumping intensity is mainly due to the interference between the two dressed states, not due to the ac--Stark-shift effect.
Since its inception, a primary focus of resonance ionization spectroscopy has been sensitive detection of atomic species in one form on another. But RIS has found many applications in spectroscopy where the nature of a resonant transition was of interest, and RIS provided a very useful tool for measuring a given feature. A large set of examples from this category can be found in studies of quantum interferences as they affect multiphoton-resonant transitions in gases and vapors. In many instances RIS has been used to show various effects that can be produced when two different optical paths simultaneously link a resonant transition in a gaseous system. We note that RIS has been used to show dramatic effects of wave mixing interferences on certain multi-photon driven resonant excitation processes. These include: (1) Complete destructive interference from a FWM field can reduce to zero the odd-photon-mediated excitation probability of a dipole allowed transition [1–4]; (2) For dual beam pumping the resonant excitation probability is restored but at a shifted frequency, [2]; (3) Separate resonant excitation profiles of a given transition can be produced under multi-beam pumping, all in conformance with theoretical predictions [3]; Quantum interference can suppress an ac Stark shift [4].
We report detailed experimental results in xenon that confirm theoretical predictions [Payne et al., Phys. Rev. A 48, 2334 (1993)] on the interference-based suppression of ac Stark shifting of even-photon resonances under circumstances where the shift is produced through strong coupling to a state that is also coupled back to the ground state. In the copropagating-beam configuration and in the pressure range of 0.5--250 Torr, the multiphoton ionization spectral line shape is unchanged regardless of the presence of the second laser field, i.e., the ac Stark shift introduced by the second laser is totally suppressed due to a complete destructive interference between two excitation pathways. In the counterpropagating-beam configuration, however, the ac Stark shift persists in the range of the pressure studied. By choosing a different set of energy levels where no four-wave-mixing field is permitted, we show that the very same ac Stark shift introduced by the second laser field persists even with copropagating-beam configuration, as predicted by theory.