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Optics Letters

Observation of a critical concentration in laser-induced transparency and multiphoton excitation and ionization in rubidium

L. Deng, D.Z. Lee, W. R. Garrett, M. G. Payne

Published Jul 1, 1996• 11 citations
Publisher

Abstract

We report the behavior of Autler-Townes splitting and production of a four-wave mixing (FWM) field in rubidium in the context of laser-induced transparency. Gain saturation of the FWM and simultaneous suppression of Autler-Townes splitting above a critical concentration are interpreted in terms of the odd-photon destructive interference effect. The results demonstrate that, when multimode lasers are used, odd-photon destructive interference significantly limits the high-efficiency and high-intensity FWM generation promised by early studies of laser-induced transparency.

DOI
10.1364/ol.21.000928

Related papers

Effect of the odd-photon destructive interference on laser-induced transparency and multiphoton excitation and ionization in rubidium

L. Deng, W. R. Garrett, M. G. Payne +1 more

We report experimental results on two-color multiphoton ionization and four-wave-mixing production under conditions where one form of laser-induced transparency (LIT) occurs. Under the conditions of LIT, multiphoton-ionization line shapes obtained by tuning one laser through a two-photon resonance exhibit a pronounced Autler-Townes splitting at low concentrations, when a second laser couples the two-photon state to a third level. In this concentration region we observed a four-wave-mixing field that increased quadratically with concentration. As the concentration is increased the Autler-Townes splitting starts to decrease, and disappears completely as a critical concentration is reached. Simultaneously, the four-wave-mixing field intensity starts to level off and finally becomes concentration independent. These observations are explained in terms of odd-photon destructive interference between different excitation pathways. In rubidium, we demonstrate that above concentrations of n\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ the four-wave-mixing field is concentration independent and the multiphoton-ionization line shape shows no Autler-Townes splitting, and the width of the line shape is determined by the laser bandwidth. The results presented here demonstrate that destructive interference significantly limits the high-efficiency and high-intensity nonlinear optical generation promised in early studies on LIT, at least for the multimode laser system used in the present experiment. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Nov 1, 1996•8 citations

Evidence of Autler–Townes splitting in high-order nonlinear processes

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Interference effect and Autler-Townes splitting in coherent blue light generated by four-wave mixing

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An unexpected interference effect between four-wave mixing excitation routes in rubidium vapor is observed using a combination of a cw diode laser and a 1-GHz femtosecond pulse train. The generated coherent blue light is analyzed by scanning the diode frequency and the repetition rate of the frequency comb. In both cases, the cw laser induces an Autler-Townes splitting, and a large separation is observed in the doublet structure due to the configuration of copropagating fields. Remarkably, by scanning the repetition rate, a narrow peak appears as a signature of the interfering pathways. A nonperturbative treatment of the Bloch equations achieves good agreement with the experimental data and demonstrates that the interfering effect appears only when both lasers are resonant with one- and two-photon transitions for the same atomic velocity group.

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We investigate a four-state system interacting with long and short laser pulses in a weak probe beam approximation. We show that when all lasers are tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM) field is strongly absorbed. The part that is not absorbed has the exact intensity required to destructively interfere with the excitation pathway involved in producing the FWM state. We show that with this three-photon destructive interference, the conversion efficiency can still be as high as 25%. Contrary to common belief, our calculation shows that this process, where an ideal one-photon electromagnetically induced transparency is established, is not most suitable for high-efficiency conversion. With appropriate phase matching and propagation distance, and when the three-photon destructive interference does not occur, we show that the photon flux conversion efficiency is independent of probe intensity and can be close to 100%. In addition, we show clearly that the conversion efficiency is not determined by the maximum atomic coherence between two lower excited states, as commonly believed. It is the combination of phase matching and constructive interference involving the two terms arising in producing the mixing wave that is the key element for the optimized FWM generation. Indeed, in this scheme no appreciable excited state is produced, so that the atomic coherence between states |0〉 and |2〉 is always very small.

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Efficient multiwave mixing in the ultraslow propagation regime and the role of multiphoton quantum destructive interference

Ying Wu, M. G. Payne, E. W. Hagley +1 more

We analyze a lifetime-broadened four-state four-wave-mixing (FWM) scheme in the ultraslow propagation regime and show that the generated FWM field can acquire the same group velocity and pulse shape as those of an ultraslow pump field. We show that a new type of induced transparency resulted from multiphoton destructive interference that significantly reduced the pump field loss. Such induced transparency based on multphoton destructive interference may have important applications in other nonlinear optical processes.

Optics Letters•Oct 1, 2004•111 citations
We report the behavior of Autler-Townes splitting and production of a four-wave mixing (FWM) field in rubidium in the context of laser-induced transparency. Gain saturation of the FWM and simultaneous suppression of Autler-Townes splitting above a critical concentration are interpreted in terms of the odd-photon destructive interference effect. The results demonstrate that, when multimode lasers are used, odd-photon destructive interference significantly limits the high-efficiency and high-intensity FWM generation promised by early studies of laser-induced transparency.
We report experimental results on two-color multiphoton ionization and four-wave-mixing production under conditions where one form of laser-induced transparency (LIT) occurs. Under the conditions of LIT, multiphoton-ionization line shapes obtained by tuning one laser through a two-photon resonance exhibit a pronounced Autler-Townes splitting at low concentrations, when a second laser couples the two-photon state to a third level. In this concentration region we observed a four-wave-mixing field that increased quadratically with concentration. As the concentration is increased the Autler-Townes splitting starts to decrease, and disappears completely as a critical concentration is reached. Simultaneously, the four-wave-mixing field intensity starts to level off and finally becomes concentration independent. These observations are explained in terms of odd-photon destructive interference between different excitation pathways. In rubidium, we demonstrate that above concentrations of n\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ the four-wave-mixing field is concentration independent and the multiphoton-ionization line shape shows no Autler-Townes splitting, and the width of the line shape is determined by the laser bandwidth. The results presented here demonstrate that destructive interference significantly limits the high-efficiency and high-intensity nonlinear optical generation promised in early studies on LIT, at least for the multimode laser system used in the present experiment. \textcopyright{} 1996 The American Physical Society.
We demonstrate Autler-Townes (AT) splitting of four-wave mixing in an electromagnetically induced transparency window, which results from the destructive interference between a three-photon process and a five-photon process. The primary and secondary AT splittings are achieved via induced atomic coherence in a four-level Y-type atomic system. Theoretical calculations fit well with the experimentally measured results. Such controlled multichannel splitting of nonlinear optical signals can have potential applications in optical communication and quantum information processing.
We present a theory for two- and three-photon excitation, optical shifting, and four-wave mixing when a first laser is tuned onto, or near, a two-photon resonance and a second much more intense laser is tuned near or on resonance between the two-photon resonance and a second excited state. When the second excited state has a dipole-allowed transition back to the ground state and the concentration is sufficiently high, a destructive interference is produced between three-photon coupling of the ground state and the second excited state and one-photon coupling between the same states by the internally generated four-wave mixing field. This interference leads to several striking effects. For instance, as the onset of the interference occurs, the optical shifts in the two-photon resonance excitation line shape become smaller in copropagating geometry so that the line shapes for multiphoton ionization enhanced by the two-photon resonance eventually become unaffected by the second laser. In the same range of concentrations the four-wave mixing field evolves to a concentration-independent intensity. With counterpropagating laser beams the line shape exhibits normal optical shifts like those observed for both copropagating and counterpropagating laser beams at very low concentrations. The theoretical work presented here extends our earlier works by including the effect of laser bandwidth and by removing the restriction of having the second laser be tuned far from three-photon resonance. In this way we have now included, as a special case, the effect of both laser bandwidth and interference on laser-induced transparency. Unlike other effects related to odd-photon destructive interference, the effect of a broad bandwidth is to bring about the predicted effects at much lower concentrations. Studies in rubidium show good agreement between theory and experiment for both ionization line shapes and four-wave mixing intensity as a function of concentration.
An unexpected interference effect between four-wave mixing excitation routes in rubidium vapor is observed using a combination of a cw diode laser and a 1-GHz femtosecond pulse train. The generated coherent blue light is analyzed by scanning the diode frequency and the repetition rate of the frequency comb. In both cases, the cw laser induces an Autler-Townes splitting, and a large separation is observed in the doublet structure due to the configuration of copropagating fields. Remarkably, by scanning the repetition rate, a narrow peak appears as a signature of the interfering pathways. A nonperturbative treatment of the Bloch equations achieves good agreement with the experimental data and demonstrates that the interfering effect appears only when both lasers are resonant with one- and two-photon transitions for the same atomic velocity group.
We investigate a four-state system interacting with long and short laser pulses in a weak probe beam approximation. We show that when all lasers are tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM) field is strongly absorbed. The part that is not absorbed has the exact intensity required to destructively interfere with the excitation pathway involved in producing the FWM state. We show that with this three-photon destructive interference, the conversion efficiency can still be as high as 25%. Contrary to common belief, our calculation shows that this process, where an ideal one-photon electromagnetically induced transparency is established, is not most suitable for high-efficiency conversion. With appropriate phase matching and propagation distance, and when the three-photon destructive interference does not occur, we show that the photon flux conversion efficiency is independent of probe intensity and can be close to 100%. In addition, we show clearly that the conversion efficiency is not determined by the maximum atomic coherence between two lower excited states, as commonly believed. It is the combination of phase matching and constructive interference involving the two terms arising in producing the mixing wave that is the key element for the optimized FWM generation. Indeed, in this scheme no appreciable excited state is produced, so that the atomic coherence between states |0〉 and |2〉 is always very small.
We analyze a lifetime-broadened four-state four-wave-mixing (FWM) scheme in the ultraslow propagation regime and show that the generated FWM field can acquire the same group velocity and pulse shape as those of an ultraslow pump field. We show that a new type of induced transparency resulted from multiphoton destructive interference that significantly reduced the pump field loss. Such induced transparency based on multphoton destructive interference may have important applications in other nonlinear optical processes.