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Optics Letters

Influence of propagation and external phase in sequential two-photon absorption of femtosecond pulses

H. G. Barros, Whualkuer Lozano B., S. S. Vianna, L. H. Acioli

Published Nov 15, 2005• 6 citations
Publisher

Abstract

We study the influence of shaping the phase of the optical field when time-delayed femtosecond pulse pairs excite a sequential two-photon transition in rubidium atoms. Propagation through this optically dense medium modifies the pulse profile. When an external phase is introduced into one of the pulses, partial cancellation of the two-photon absorption signal is observed for temporal delays much greater than the pulse duration. This behavior is qualitatively explained in terms of a negative group delay.

DOI
10.1364/ol.30.003081

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Observation and Symmetry-Based Coherent Control of Transient Two-Photon Absorption: The Bright Side of Dark Pulses

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We present and experimentally demonstrate for the first time the observation and femtosecond coherent control over the temporal evolution of a transient population that is excited via nonresonant two-photon absorption. Based on symmetry properties of the two-photon absorption process, the exciting femtosecond pulses are phase-shaped to photoinduce different evolutions of the transient excited population for a given final excited population. As a study case, we focus here on the attractive case of two-photon dark pulses that, although inducing zero final population (hence, the terminology of "dark pulses"), they induce a transient excited population during the pulse irradiation that can significantly deviate from zero. This nonzero transient population can be viewed as the bright side of such dark pulses. The symmetry-based coherent control is demonstrated first with dark pulses that we shape to induce transient excited population that at all times is kept below different target levels. Then, it is further demonstrated with pairs of dark pulses where one is rationally shaped to induce temporal evolution of the transient excited population that is the inverse of the evolution induced by the other. The work is conducted in the weak-field regime with the sodium atom as the model system. The approach developed here is general, conceptually simple, and very effective.

Physical Review Letters•Jul 23, 2014•6 citations
We study the influence of shaping the phase of the optical field when time-delayed femtosecond pulse pairs excite a sequential two-photon transition in rubidium atoms. Propagation through this optically dense medium modifies the pulse profile. When an external phase is introduced into one of the pulses, partial cancellation of the two-photon absorption signal is observed for temporal delays much greater than the pulse duration. This behavior is qualitatively explained in terms of a negative group delay.
Propagation of optical pulses in a resonantly absorbing medium is studied. Propagation time of nanosecond pulses was measured for the Rb D1{D}_{1}D1​ transition. At the center of two absorption lines, delay of the pulse peak which is about ten times as large as the pulse width was observed, where zero delay is defined for the propagation with the light velocity in vacuum. On the other hand, at the peak of an absorption line, negative delay was observed for large absorption, where the advance time is as large as 25% of the pulse width. Simulation including the effect of absorption and phase shift reproduced well the experimental results.
We demonstrate temporal coherent control in two-photon transitions. A pair of broadband (Δλ∼20nm), ultrashort (Δt∼100fs), collinear pulses with a variable time delay excites the rubidium atoms into the 5D state from the 5S ground state where the two-photon transitions are enhanced by the intermediate level 5P. The excited atoms radiate 5μm (5D–6P) and 420nm (6P–5S) light. As a result of tuning the wavelength of the input laser, a superfluorescence at 420nm exhibits different temporal behaviors. A switching from a beating at the frequency given by the difference between the sequential atomic transitions that involve the 5P3/2 intermediate level, to a quantum beating due to two different two-photon excitation paths, 5S→5P1/2→5D and 5S→5P3/2→5D is observed. Based on the simple atom-field interaction theory, an analytic solution, which qualitatively elucidates experimental results, is obtained.
Propagation of strong femtosecond hyper-Gaussian pulses in an organic molecular medium is studied by solving numerically the Maxwell-Bloch equations using an iterative predictor-corrector finite-difference time-domain technique. The carrier-wave frequency of the field is tuned in two-photon resonance with the molecular system simplified by a cascade three-level model. Strong two-photon absorption induced optical power limiting behavior is observed for the hyper-Gaussian pulses of different orders. For the ultrashort hyper-Gaussian pulses, it is found that the ”two-photon area” is no further the deciding quantity of the two-photon induced dynamics, and pulses with different temporal profiles will induce different two-photon absorption dynamics and optical limiting processes. With the same pulse duration and field amplitude, pulses of a lower order is found to have a larger input ”two-photon area” but a smaller output area, and therefore show a better optical limiting behavior. The population distribution and the generation of new fields during pulse propagation also depend on the shape of the incident field.
This thesis deals with the study of propagation effects experienced by weak light pulses as they propagate in atomic media driven by strong pulses. We explore both the ultra-short and long pulse regime and investigate the phenomena that arise at these different time scales. In the short pulse regime, a strongly driven two level atomic system presents transient light shifts, and non-adiabatic transitions occur between these adiabatic levels. We have studied a method to probe these light shifts in real time by propagating a weak probe through the medium. The light shifts enrich the spectrum of the probe and the probe gets shaped as a result. In this way a strongly driven two-level system can act as an active pulse shaper, and can introduce oscillations in the temporal profile of an ultra-short pulse at a time scale shorter than the pulse duration. We also show that by driving the system with two time delayed non-resonant strong fields, the non-adiabatic effects can be rendered phase dependent. This gives very sensitive phase control of the excited state population and can be used to formulate new techniques in interferometry. In the long pulse regime we present a new method of slowing light that can be realized in a double two-level system interacting with two orthogonally polarized light pulses that propagate along different axis. Spatio-temporal dependence of the total polarization induces a grating in the ground Zeeman coherence. The stronger of the two fields (the control field) is diffracted from this grating into the direction of the weak probe field compensating for the absorption of this latter field. A transparency window is thus created in the absorption spectrum of the probe leading to the slowing down of light. The transparency window exhibits characteristics similar to the one obtained by EIT (electromagnetic induced transparency) method. However, the important difference between our method and the traditional EIT method, is that ours doesn't rely on realizing dark state in the system. This may open the possibility of slowing down light in more complex atomic media. Moreover, when the linear absorptive response of the medium is cancelled in this manner, the nonlinear response becomes more important. In the situation where fields propagate in the same direction and have same frequency, two regimes have been investigated. For small optical depths, the effective susceptibility behaves as chi_lin exp[2 i phi] (with phi the phase difference between two fields). This renders phase control of the medium response, and the medium can be changed from an absorber to an amplifier, with normal or anomalous dispersion, by adjusting the relative phase phi.  In the regime of large optical thickness, phase saturation takes place and the effective susceptibility turns into chi_lin*, changing an absorber into an amplifier without effecting the dispersive response (chi_lin is linear susceptibility).
We present and experimentally demonstrate for the first time the observation and femtosecond coherent control over the temporal evolution of a transient population that is excited via nonresonant two-photon absorption. Based on symmetry properties of the two-photon absorption process, the exciting femtosecond pulses are phase-shaped to photoinduce different evolutions of the transient excited population for a given final excited population. As a study case, we focus here on the attractive case of two-photon dark pulses that, although inducing zero final population (hence, the terminology of "dark pulses"), they induce a transient excited population during the pulse irradiation that can significantly deviate from zero. This nonzero transient population can be viewed as the bright side of such dark pulses. The symmetry-based coherent control is demonstrated first with dark pulses that we shape to induce transient excited population that at all times is kept below different target levels. Then, it is further demonstrated with pairs of dark pulses where one is rationally shaped to induce temporal evolution of the transient excited population that is the inverse of the evolution induced by the other. The work is conducted in the weak-field regime with the sodium atom as the model system. The approach developed here is general, conceptually simple, and very effective.