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Journal of the Optical Society of America B

Steady-state resonance fluorescence spectrum of a two-level atom in a cavity

Helen S. Freedhoff, Tran Quang

Published Aug 1, 1993• 25 citations
Publisher

Abstract

We calculate the steady-state spectrum of resonance fluorescence from a two-level atom coupled to a single cavity mode and driven by a strong, in general off-resonance, external field. In the strong-coupling limit, each Mollow triplet component is split into a multiplet, whose detailed structure depends on the atom–cavity coupling, the cavity and spontaneous emission decay constants, and the photon-number distribution of the cavity mode. The dressed states for this system are identified, and the spectral features are interpreted in terms of transitions among these dressed states. In particular, we find that the central component of the central multiplet can have a strongly subnatural linewidth.

DOI
10.1364/josab.10.001337

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We calculate the steady-state spectrum of resonance fluorescence from a two-level atom coupled to a single cavity mode and driven by a strong, in general off-resonance, external field. In the strong-coupling limit, each Mollow triplet component is split into a multiplet, whose detailed structure depends on the atom–cavity coupling, the cavity and spontaneous emission decay constants, and the photon-number distribution of the cavity mode. The dressed states for this system are identified, and the spectral features are interpreted in terms of transitions among these dressed states. In particular, we find that the central component of the central multiplet can have a strongly subnatural linewidth.
We investigate properties of a two-level atom coupled to a single cavity mode, and driven by a strong, in general off-resonance, external field. The cavity field is tuned to resonance with a Mollow sideband. We show that the steady-state atomic population may be highly (nearly totally) inverted, a phenomenon forbidden in semiclassical theory, and that a strong enhancement of the resonance fluorescence results. This system is suggested as an alternative approach to create the atomic inversion necessary for laser or maser generation without using additional atomic levels. We also investigate properties of the cavity field, derive an analytical expression for the photon number distribution in the good cavity or strong-coupling limit, and discuss the conditions under which this system acts as a single-atom dressed-state laser.
The standard theory of resonance fluorescence is a weak-coupling theory.1 If n ¯ is the mean photon number for the driving field, g is the atom field coupling constant, and γ is the Einstein A coefficient, formally the standard theory of resonance fluorescence assumes the limit n ¯ → ∞ , g/ γ → 0, with the ratio of Rabi frequency to atomic linewidth 2 n ¯ n g / γ finite. The limit g/γ → 0 allows the variation of the Rabi frequency 2 n g across the photon number distribution of the driving field to be neglected. By placing the atom inside an optical cavity the ratio g/γ can be changed. In this paper we calculate spectra for intracavity resonance fluorescence in the strong-coupling limit g/γ ≫ 1, g/κ ≫ 1, where κ is the cavity linewidth. We consider a single two-level atom interacting on resonance with a single cavity mode driven by a resonant coherent field. The cavity subtends a small solid angle at the atom, so that the spontaneous emission into free-space modes is not negligible. We calculate the spectrum of the light transmitted by the cavity (the transmitted spectrum) and the spectrum of the light radiated by the atom into free space (the fluorescent spectrum). The transmitted and fluorescent spectra show fundamental differences from the (weak-coupling) spectrum of free-space resonance fluorescence.
We experimentally demonstrate the resonance fluorescence of a two-level artificial atom strongly coupled to a single-mode cavity field. The effect was theoretically predicted thirty years ago by Savage [Phys. Rev. Lett. 63, 1376 (1989)]. The system consists of a superconducting qubit circuit and a one-dimensional transmission line resonator. In addition, a one-dimensional transmission line strongly coupled to the atom serves as an open space. The effect takes place, when a microwave field is applied to the cavity, which in turn is resonantly coupled to the atom. The fluorescence spectrum is measured via the emission into the transmission line. We find that the central peak is determined by the atom spontaneous emission to the open space and the widths of side peaks are largely determined by the coherent interaction between the atom and the cavity, that is, the fluorescence spectrum here is very different from that of the Mollow triplet. We also derive analytical form for the spectrum. Our experimental results agree well with theoretical calculations.
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.
This study concerns the shape of the resonance fluorescence spectrum emitted by a V-type three-level atom driven by two coherent fields which are resonant with the transitions connecting the ground state and the second and third level, respectively. We are especially interested in monitoring the spontaneous emission from the top level to the ground state. If the field connecting levels 1 and 2 is absent or small, the spontaneous emission from level 3 is characterized by the standard Mollow spectrum. If instead the 1-2 transition is saturated, the spectrum of resonance fluorescence from the top level is characterized by the following signatures: (i) the appearance of sidebands due to Rabi splitting effects, (ii) a reduction of its integrated area because of coherent population trapping1, and (iii) a significant reduction of its linewidth if the natural lifetime of level 2 is longer than level 3. The latter effect is especially interesting and rather unexpected, as the decay properties of an optical transition are being transferred to another by controlling the relative magnitudes of the driving fields. We have carried out our calculations with the help of the regression theorem and also developed a simple physical interpretation in terms of appropriate dressed states2.