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Journal of Physics B Atomic Molecular and Optical Physics

Enhancing the probability of three-photon absorption in iodine through pulse shaping

Nicholas T. Form, Benjamin J. Whitaker, C. Meier

Published Mar 26, 2008• 17 citations
Publisher

Abstract

We demonstrate experimentally that several classes of phase functions are capable of enhancing the amount of three-photon fluorescence from molecular iodine by at least a factor 3 with respect to an unshaped pulse. The investigated functions include third-order Taylor expansions, π-steps and highly structured forms identified by an evolution strategy. The mechanism of the observed control is investigated using wavepacket simulations and found to be due to a time-delay in the second resonance of the excitation ladder.

DOI
10.1088/0953-4075/41/7/074011

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We demonstrate experimentally that several classes of phase functions are capable of enhancing the amount of three-photon fluorescence from molecular iodine by at least a factor 3 with respect to an unshaped pulse. The investigated functions include third-order Taylor expansions, π-steps and highly structured forms identified by an evolution strategy. The mechanism of the observed control is investigated using wavepacket simulations and found to be due to a time-delay in the second resonance of the excitation ladder.
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We study in detail coherent phase control of femtosecond resonance-mediated (2+1)(2+1)(2+1) three-photon absorption and its dependence on the spectral bandwidth of the excitation pulse. The regime is the weak-field regime of third perturbative order. The corresponding interference mechanism involves a group of three-photon excitation pathways that are on resonance with the intermediate state and a group of three-photon excitation pathways that are near resonant with it. The model system of the study is atomic sodium (Na), for which experimental and numerical-theoretical results are obtained. Prominent among the results is our finding that with simple proper pulse shaping an increase in the excitation bandwidth leads to a corresponding increase in the enhancement of the three-photon absorption over the absorption induced by the (unshaped) transform-limited pulse. For example, here, a 40 nm bandwidth leads to an order-of-magnitude enhancement over the transform-limited absorption.
Femtosecond coherent phase control of resonance mediated (2+1)(2+1)(2+1) three-photon absorption is studied both theoretically and experimentally. The regime is perturbative of third order. The photoexcitation coherently combines elements of both nonresonant and resonance-mediated multiphoton transitions. By proper simple pulse shaping the three-photon absorption in Na is effectively controlled experimentally, enhanced up to $\ensuremath{\sim}300%$ of the absorption induced by the transform-limited pulse. It is achieved by phase manipulating intra- and intergroup interferences involving two groups of three-photon excitation pathways: (i) on resonance and (ii) near resonance with the intermediate resonance state accessed by nonresonant two-photon transition.
In this paper, we study theoretically and experimentally the coherent control of non-resonant two-photon transition in a molecular system (Perylene dissolved in chloroform solution) by shaping the femtosecond pulses with simple phase patterns (cosinusoidal and π phase step-function shape). The control efficiency of the two-photon transition probability is correlated with both the laser field and the molecular absorption bandwidth. Our results demonstrate that, the two-photon transition probability in a molecular system can be reduced but not completely eliminated by manipulating the laser field, and the control efficiency is minimal when the molecular absorption bandwidth is larger than twice the laser spectral bandwidth.
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