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Journal of Modern Optics

Switching Between Rayleigh-like and Lorentzian Lineshapes of the Dispersion in Driven Two-level Atoms

Carsten Szymanowski, Christoph H. Keitel, Bryan J. Dalton, P. L. Knight

Published May 1, 1995• 24 citations
Publisher

Abstract

We investigate the structure of the dispersion and absorption lineshapes for driven two-level systems against a weak probing laser field. It is shown that the shape of both curves depends critically on the detuning of the driving laser to the atomic resonance frequency and can be transformed continuously and oppositely between Rayleigh (or ‘dispersion-like’) and Lorentzian (or ‘absorption-like’) lineshapes. An analysis in the dressed state basis explains these features via the different influences of coherences and populations of the corresponding dressed states.

DOI
10.1080/09500349514550891

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Narrow Resonances with Excitation of Finite Bandwidth Field

Peng Zhou, Mao-Fa Fang, Qing-Ping Zhou +1 more

The effect of the laser linewidth on the resonance fluorescence spectrum of a two-level atom is revisited. The novel spectral features, such as hole-burning and dispersive profiles at line centre of the fluorescence spectrum are predicted when the laser linewidth is much greater than its intensity. The unique features result from quantum interference between different dressed-state transition channels. c # 1999 Elsevier Science B.V. PACS : 42.50.Hz; 32.80.-t Keywords: Resonance fluorescence; Laser linewidth; Quantum interference The study of resonance fluorescence spectra has provided much fundamental insight into the subject of atom1light interactions. It is well known that for weak laser field excitation, the spectrum exhibits a Lorentzian lineshape, whereas it develops into the Mollow triplet for strong field excitation [1]. The latter is a direct signature of stimulated emissions and absorptions of the atom during the time interval for one spontaneous decay. Recently, considera...

•Jan 1, 1999•2 citations
We investigate the structure of the dispersion and absorption lineshapes for driven two-level systems against a weak probing laser field. It is shown that the shape of both curves depends critically on the detuning of the driving laser to the atomic resonance frequency and can be transformed continuously and oppositely between Rayleigh (or ‘dispersion-like’) and Lorentzian (or ‘absorption-like’) lineshapes. An analysis in the dressed state basis explains these features via the different influences of coherences and populations of the corresponding dressed states.
The possibility of obtaining strong negative dispersion without absorption in the central resonance structure of the spectrum of a bichromatically driven two-level system is investigated. A nonlinear analysis is performed using the technique of continued fractions with respect to the dressedstate basis of a strong pumping laser. We discuss the transition between the two regimes of the monochromatic driving case with a weak probing laser and of nearly equally strong lasers in terms of the contributions from coherences and populations of these dressed states. In contrast with the strong dependence of the Rabi side bands on probe field intensity, we find a remarkable stability of the negative dispersion in the central resonance structure.
A cavity-modified master equation is derived for a coherently driven two-level atom coupled to a single-mode cavity in the bad cavity limit, in which the cavity frequency is tuned to either the center or one of the sidebands of the Mollow triplet. The atomic populations in both the bare- and dressed-state representations are analyzed in terms of the cavity-modified transition rates. In the bare-state basis, the role of the cavity may be interpreted as enhancing the stimulated absorption of the atom while suppressing the stimulated emission. The bare-state population may thus be inverted under appropriate conditions. The dressed-state inversion, however, originates from the enhancement of the atom-cavity interaction when the cavity is resonant with the atomic dressed-state transition. We show that two-phase quadratures of the atomic polarization decay at different rates. The decay of the in-phase (or out-of-phase) quadrature may be greatly inhibited as the driving intensity increases, depending on the cavity resonant frequency. The spectrum of the atomic fluorescence emitted out the side of the cavity is also studied. The spectral profiles are sensitive to the cavity frequency. When the cavity frequency is tuned to the center of the Mollow resonances, the fluorescence spectrum is symmetrical with three peaks whose linewidths and heights are intensity dependent. When the cavity frequency is tuned to one of the Mollow sidebands, however, it is asymmetric, and the central peak and the sideband on resonance with the cavity can be significantly suppressed for strong driving fields. All three spectral lines can be narrowed by increasing the Rabi frequency. The physics of these striking spectral features is explored in the dressed-state basis. We also investigate the probe absorption spectrum. When the cavity frequency is tuned to the center of the Mollow fluorescence triplet, the central component exhibits a Lorentzian line shape, while the side bands show the Rayleigh-wing line shape. Probe gain may occur at line center due to cavity-induced, bare-state population inversion. When the cavity is tuned to resonance with one sideband, only two sidebands, with Lorentzian profiles, dominate. Gain can occur at the sideband far off resonant with the cavity. The sideband gain is a consequence of an unbalanced dressed-state population distribution.
We examine the absorption of a weak probe beam by a laser driven V-type atom with a pair of closely lying excited levels, where both the driving and probe lasers interact simultaneously with the two transitions. The effects of quantum interference among decay channels on the absorption spectra are also investigated. We introduce dipole moments in the dressed-state representation and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. In the degenerate case, features similar to that of a driven two-level atomic system are found due to some dark transitions in the spontaneous emission and the fact that the probe beam only detects certain transitions. In the nondegenerate case, the absorption spectrum is strongly influenced by the degree of quantum interference, resulting in different line shapes for emission peaks, absorption peaks, and dispersionlike profiles. The effect of probe polarization on the absorption spectrum is also investigated.
We analyze the transient probe spectra that arise when two-level atoms in an atomic beam are excited by a ``pump'' laser field and are probed by a step-function weak field on the same atomic transition. The temporal relaxation of the probe spectra to their steady-state values is found to depend critically on the initial atom--pump-field dressed-state populations and coherences. The transient spectra may be composed of absorption-emission structure that is totally absent in the steady-state limit. A simple physical interpretation of the spectra is given in terms of a conventional dressed-atom picture. In addition, we study the effects of collisions on transient probe spectra and investigate the transient buildup of several collision-induced features in the spectra.
The coherent coupling of atomic levels under the action of driving electromagnetic fields is known to create unusual and interesting effects, such as the well-known population trapping phenomenon and subnatural line narrowing. In this paper we discuss two alternative configurations of three-level atoms, the cascade model and the inverted-V (lambda) model, in an effort to provide alternative options for experimental tests. In both cases we focus on the fluorescent emission spectrum of atom driven by two resonant fields and on the absorption spectrum of a weak probe passing through a gas in such a driven system. Subnatural line narrowing can be obtained from the cascade system but not from the lambda system. The lambda system, however, is characterized by strong population trapping so that it is not suitable for fluorescence studies (except out of resonance) because of its strongly reduced emission rate. The absorption spectrum of a weak probe displays gain features under high field conditions which suggest that useful amplification of a probe or laser-like emission should be possible over appropriate frequency ranges. We give a physical interpretation of the origin of the gain and explain why that in the case of a lambda system, but not a cascade system, steady-state population inversion can be obtained between both excited states relative to the lowest state of the atom. We also discuss the effect of Doppler broadening on the predicted emission and absorption lineshapes.
The effect of the laser linewidth on the resonance fluorescence spectrum of a two-level atom is revisited. The novel spectral features, such as hole-burning and dispersive profiles at line centre of the fluorescence spectrum are predicted when the laser linewidth is much greater than its intensity. The unique features result from quantum interference between different dressed-state transition channels. c # 1999 Elsevier Science B.V. PACS : 42.50.Hz; 32.80.-t Keywords: Resonance fluorescence; Laser linewidth; Quantum interference The study of resonance fluorescence spectra has provided much fundamental insight into the subject of atom1light interactions. It is well known that for weak laser field excitation, the spectrum exhibits a Lorentzian lineshape, whereas it develops into the Mollow triplet for strong field excitation [1]. The latter is a direct signature of stimulated emissions and absorptions of the atom during the time interval for one spontaneous decay. Recently, considera...