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Optics and Spectroscopy

Interaction of Phase-Modulated Femtosecond Pulses with an Optically Dense Quasi-Resonant Medium of Rubidium Vapors

Sergei N Bagaev, A. A. Preobrazhenskaya, Nikolai A. Timofeev, A. A. Pastor, Igor B. Mekhov, Чехонин Игорь Анатольевич, Pavel Yu. Serdobintsev, V. S. Egorov, M A Chekhonin, Anastasiia Mashko

Published Nov 1, 2018• 2 citations
Publisher

Abstract

Abstract For the first time, it is demonstrated that the magnitude and sign of the effect of “spectral condensation” of a laser pulse at the resonant-transition frequency of a dense medium can be controlled by changing the driving-pulse parameters (chirp, pulse width, and pulse amplitude). In the process of this, importantly, the driving-pulse energy and spectrum remain unchanged. Direct time-resolved measurements revealed an oscillatory character of the induced superradiance of rubidium vapors representing a long train of decaying short pulses. The width and repetition rate of the pulses in the train are determined by atomic density N0 of the medium, while the width of an entire superradiance pulse (10 ps) is considerably larger than that of the driving laser pulse (50 fs).

DOI
10.1134/s0030400x1811005x

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Coherent effects are studied in a dense atomic vapor driven by laser fields. With\noptical properties dramatically modified by these effects, the medium can be used\nto manipulate some of the properties of laser field. Our experiments demonstrate\nthe coherent control over transmission, spatial distribution and noise feature of the\nlaser field interacting with coherent media. The results have potential applications\nin the field such as precision metrology, precision spectroscopy, optical imaging and\nlithography.\nWe develop an experiment to investigate the atomic excitation by few-cycle radio\nfrequency (RF) pulses interacting with Zeeman sublevels. The system provides the\nflexibility to fully control all parameters of RF pulses. Such a flexibility can not be\nachieved in optical domain. Based on this system, experiments can be conducted to\nsimulate processes in ultra-short laser physics. In particular, we study the carrier-envelope\neffect of few-cycle pulses and the strong off-resonant excitation by short\npulses.\nWe also discuss the selective reflection spectrum on a highly dense atomic vapor\nin which the dipole-dipole interaction can not be neglected. The spectrum broadening\ndue to dipole-dipole interaction is much broader than the Doppler broadening. Our\nexperiments show that the excitation by a pump laser can reduce the dipole-dipole\ninteraction, thus reduce the broadening and improve the spectral resolution. The\nexcitation dependence is studied at various atomic densities.

•Jan 1, 2010•0 citations
Abstract For the first time, it is demonstrated that the magnitude and sign of the effect of “spectral condensation” of a laser pulse at the resonant-transition frequency of a dense medium can be controlled by changing the driving-pulse parameters (chirp, pulse width, and pulse amplitude). In the process of this, importantly, the driving-pulse energy and spectrum remain unchanged. Direct time-resolved measurements revealed an oscillatory character of the induced superradiance of rubidium vapors representing a long train of decaying short pulses. The width and repetition rate of the pulses in the train are determined by atomic density N0 of the medium, while the width of an entire superradiance pulse (10 ps) is considerably larger than that of the driving laser pulse (50 fs).
It is known that ultrashort pulse coherent propagation through the resonant absorbing medium leads to the pulse splitting and pulse train generation. First of these pulses is the input pulse and the following pulses are resonant medium response. It is also known that such a propagation mode leads to the «spectrum condensation» effect, at which broad-band input femtosecond laser emission, as it leaves opically-dense medium, centralized in the vicinity of the resonance absorption lines. It's interesting to clear up the dependence of a condensation effect on an input femtosecond pulse phase modulation value (chirp).
We present an experiment in which an ultrashort pulse train propagates resonantly through anoptically dense vapor of atomic rubidium. The sequence obtained from a Fabry-Perot interferometer comprises nearly 10 regularly time-delayed and mode-locked pulses. We show that a sequence with phase shift phi = 0[2pi] between two successive pulses propagates with important temporal distortion, whereas a sequence with phi = pi[2pi] experiences few propagation effects, thus leading for the first time to our knowledge to the possibility of phase control of dispersion effects for an ultrashort pulse train.
We introduce a new scheme for generating short-wavelength coherent broadband radiation with well-controlled spectral characteristics. It is based on shaping long-wavelength femtosecond pulse to coherently control atomic resonance-mediated (2+1) three-photon excitation to a broad far-from-resonance continuum. Here, the spectrum (central frequency and bandwidth) of deep-ultraviolet coherent broadband radiation generated in Na vapor is experimentally controlled by tuning the linear chirp we apply to the driving phase-shaped near-infrared femtosecond pulse. This is a first step in implementing the full scheme for producing shaped femtosecond pulses at wavelengths down-to the vacuum-ultraviolet range.
A sequence of two femtosecond coherent pulses--a strong pi-polarized pulse and a weak sigma-polarized pulse--excite the S1/2-P1/2 transition of atomic rubidium in an optically dense vapor. The sigma pulse induces transitions between the adiabatic states with a coupling strength that is different for identically and oppositely light-shifted coupled states, and that can be modified by tuning the relative phase between the pulses. An efficient control of the medium gain for the sigma pulse is experimentally demonstrated. It is shown to be the result of interference between the absorption and the stimulated emission paths for sigma photons.
Photoassociation of ultracold rubidium atoms with femtosecond laser pulses is studied theoretically. The spectrum of the pulses is cut off in order to suppress pulse amplitude at and close to the atomic resonance frequency. This leads to long tails of the laser pulse as a function of time giving rise to coherent transients in the photoassociation dynamics. They are studied as a function of cut-off position and chirp of the pulse. Molecule formation in the electronically excited state is attributed to off-resonant excitation in the strong-field regime.
Coherent effects are studied in a dense atomic vapor driven by laser fields. With\noptical properties dramatically modified by these effects, the medium can be used\nto manipulate some of the properties of laser field. Our experiments demonstrate\nthe coherent control over transmission, spatial distribution and noise feature of the\nlaser field interacting with coherent media. The results have potential applications\nin the field such as precision metrology, precision spectroscopy, optical imaging and\nlithography.\nWe develop an experiment to investigate the atomic excitation by few-cycle radio\nfrequency (RF) pulses interacting with Zeeman sublevels. The system provides the\nflexibility to fully control all parameters of RF pulses. Such a flexibility can not be\nachieved in optical domain. Based on this system, experiments can be conducted to\nsimulate processes in ultra-short laser physics. In particular, we study the carrier-envelope\neffect of few-cycle pulses and the strong off-resonant excitation by short\npulses.\nWe also discuss the selective reflection spectrum on a highly dense atomic vapor\nin which the dipole-dipole interaction can not be neglected. The spectrum broadening\ndue to dipole-dipole interaction is much broader than the Doppler broadening. Our\nexperiments show that the excitation by a pump laser can reduce the dipole-dipole\ninteraction, thus reduce the broadening and improve the spectral resolution. The\nexcitation dependence is studied at various atomic densities.