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Coherent Control of Laser Field and Spectroscopy in Dense Atomic Vapor

Hebin Li

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

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.

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•Feb 24, 2016•0 citations
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.
Spectroscopic properties of coherently prepared, optically dense atomic media are studied experimentally and analyzed theoretically. It is shown that in such media the power broadening of the resonances can be substantially reduced. A density-dependent spectral narrowing of the electromagnetically induced transparency (EIT) window and novel, even narrower, resonances superimposed on the EIT line are observed in dense Rb vapor. A nonlinear two-photon spectroscopic technique based on coherent atomic media and combining high resolution with a large signal-to-noise ratio seems feasible.
We have investigated the nonlinear selective reflection of a laser beam (λ = 780 nm) from a high density rubidium vapor. The laser frequency was tuned over atomic transitions and coherent optical excitation of the atomic vapor was realized. The observed narrowing of reflection spectral profile is associated with a competition of two nonlinear processes: optical field induced coherent broadening (Rabi broadening) and optical saturation induced decreasing of dipole–dipole broadening of atomic transitions. For more complete understanding the spectral narrowing we suggest to perform a set of new experiments.
The coherent control of atomic energy level population is important for several areas physics and engineering including fluorescent spectroscopy, resonantly enhanced multiphoton ionization, harmonic generation, remote sensing and lasing. At low intensities where the Stark shift can be neglected, coherent control mechanisms usually exploit resonances created from stationary energy states dressed by integer multiples of the photon energy. However, in the strong field limit where large laser pulse intensities lead to significant Stark shifts, the resonant conditions are dynamic both in time and space and will likely not be satisfied for the entire laser pulse or beam.\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. We also make a direct measurement of the ponderomotive shift of the continuum. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.\n\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.
Selective resonant excitations can be realized with a shaped ultrashort pulse in an optically dense collection of multilevel individual atoms, where the near dipole-dipole interactions induce intrinsic frequency shifts of the atomic resonance. A proper transform-limited pulse and modulation of excitation pulses can be controlled to establish constructive or destructive quantum interference between different excitation pathways towards a desired selective resonant excitation, even driven by pulses with broadband spectral components.
In this work quantum coherence effects in dense rubidium atomic vapour are investigated. Prepared in a laser driven atomic system, coherent effects, like the well-known electromagnetically induced transparency, have a remarkable ability to significantly alter optical properties of the used medium. Electromagnetically induced transparency and related phenomena like slow and stored light can be explained in terms of dark state polaritons, representing quasiparticles consisting of light and matter components. In this thesis, experiments aiming to the concept of quasiparticles are described. Furthermore, in this thesis the concept of resonance retrieval of a stored coherence is investigated, a concept that might prove useful in metrology. In this thesis, the focus is set on the experimental study of the particle-like nature of dark state polaritons. Therefore, a radiofrequency spectroscopy is performed on stationary and moving polaritons with an internal level structure. To this end, an experimental setup is described that allows for the existence of two state dark polaritons in a tripod level scheme. The internal level structure of these polaritons is then probed by a radiofrequency field and a transfer of population between two dark states is observed. The measured population transfer, though with a limited contrast, strongly indicates that dark state polaritons can be considered as quasiparticles with an internal level structure. Since the dark state polaritons exhibit features of an effective magnetic dipole moment, the existence of the Aharonov-Casher phase effect for these quasiparticles is investigated experimentally. It is predicted by a known Aharonov-Casher effect that taking two particles with a finite magnetic dipole moment around a line of charge causes an accumulated phase difference between them. In this work, a detection system with a phase sensitivity of 10-5 rad is described, which sets the upper boundary on the experimentally detectable Aharonov-Casher phase for the hot rubidium gas system. In the last part of this thesis experiments on storage and resonant retrieval of an atomic coherence are described. To this end, an RF field creates an atomic coherence which is stored and subsequently read out with an optical field. The retrieved field is shown to be frequency locked to the optical field. The difference frequency is, within the limits of experimental sensitivity, at the atomic resonance frequency independently on the amplitude and frequency variations of the radiofrequency excitation field.