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Conference on Lasers and Electro-Optics

AC Stark splitting of a three-level system driven by two strong fields

C. Wei, Neil B. Manson

Published Nov 27, 2002• 0 citations
Publisher

Abstract

Summary form only given.It is well known that a strongly driven transition exhibits a doublet splitting when probed by a weak field, commonly known as AC Stark splitting. It can be observed either by probing the driven two-level itself (Mollow-type spectrum) or via a third level (Autler-Townes-type spectrum). These two type splittings can have very different characteristics, particularly when driven resonantly. The absorptive Mollow spectrum consists of two dispersion-like features, whereas for the Autler-Townes spectrum there is a simple doublet splitting. In this paper we report the observation of both Mollow and Autler-Townes spectrum simultaneously by applying two strong driving fields in a three-level configuration.

DOI
10.1109/iqec.1998.680243

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Summary form only given.It is well known that a strongly driven transition exhibits a doublet splitting when probed by a weak field, commonly known as AC Stark splitting. It can be observed either by probing the driven two-level itself (Mollow-type spectrum) or via a third level (Autler-Townes-type spectrum). These two type splittings can have very different characteristics, particularly when driven resonantly. The absorptive Mollow spectrum consists of two dispersion-like features, whereas for the Autler-Townes spectrum there is a simple doublet splitting. In this paper we report the observation of both Mollow and Autler-Townes spectrum simultaneously by applying two strong driving fields in a three-level configuration.
Abstract When a two-level system is driven on resonance by a strong incident field, its emission spectrum is characterized by the well-known Mollow triplet. If the absorption from the excited state, in this continuously driven two-level system, to a third, higher energy level, is probed by a weak field, the resulting absorption spectrum features the Autler-Townes doublet with two peaks separated by the Rabi frequency of the strong driving field. It has been shown that when the two-level system is instead driven by a periodic pulse sequence, the emission spectrum obtained has similarities with the Mollow triplet even though the system is only driven during the short application time of the pulses and is allowed to evolve freely between pulses. Here, we evaluate the absorption spectrum of the three-level system in the ladder/cascade configuration when the bottom two levels are driven by a periodic pulse sequence while the transition between the middle and the highest level is probed by a weak field. The absorption spectrum displays similarities with the Autler-Townes doublet with frequency separation between the main peaks defined by the inter-pulse delay. In addition, this spectrum shows little dependence on the pulse carrier frequency. These results demonstrate the capacity to modulate the absorption spectrum of a three-level system with experimentally achievable pulse protocols.
The effect of the Bloch–Siegert shift on a strongly driven transition is studied in a three-level double-resonance configuration and the result is presented in this paper. We show that when a resonantly driven transition is probed to a third level, the Bloch–Siegert shift leads to an asymmetric Autler–Townes doublet. An important conclusion is that the asymmetry depends only on the driving field intensity, in contrast to a previous study where it is reported that the asymmetry depends not only on the driving field intensity but also on the characteristics of the three-level system. Our result implies an alternative way of measuring the Bloch–Siegert shift.
We study the effects of driving a two-level atom by two intense field modes that have equal frequencies but are otherwise distinguishable; the intensity of one mode is also assumed to be greater than that of the other. We calculate first the dressed states of the system, and then its resonance fluorescence and Autler-Townes absorption spectra. We find that the energy spectrum of the doubly dressed atom consists of a ladder of doublet continua. These continua manifest themselves in the fluorescence spectrum, where they produce continua at the positions of the Mollow sideband frequencies ${\mathrm{\ensuremath{\omega}}}_{\mathrm{L}}$ \ifmmode\pm\else\textpm\fi{}2\ensuremath{\Omega} of the strong field, and in the Autler-Townes absorption spectrum, which becomes a two-continuum doublet.
In this paper we study the nonlinear behavior of an electromagnetically induced transparency (EIT) resonance subject to a coherent driving field. The EIT is associated with a $\ensuremath{\Lambda}$ three-level system where two hyperfine levels within an electronic ground state are coupled to a common excited state level by a coupling field and a probe field. In addition there is an radio-frequency (rf) field driving a hyperfine transition within the ground state. The paper contrasts two different situations. In one case the rf-driven transition shares a common level with the probed transition and in the second case it shares a common level with the coupled transition. In both cases the EIT resonance is split into a doublet and the characteristics of the EIT doublet are determined by the strength and frequency of the rf-driving field. The doublet splitting originates from the rf-field induced dynamic Stark effect and has close analogy with the Autler-Townes effect observed in three-level pump-probe spectroscopy study. The situation changes when the rf field is strong and the two cases are very different. One is analogous to two $\ensuremath{\Lambda}$ three-level systems with EIT resonance associated with each. The other corresponds to a doubly driven three-level system with rf-field-induced electromagnetically induced absorption resonance. The two situations are modeled using numerical solutions of the relevant equation of motion of density matrix. In addition a physical account of their behaviors is given in terms of a dressed state picture.
Calculations based on a full master equation approach have been made of the absorption spectrum for a certain three-level system in which two states mod 2), mod 1) are strongly driven by a near resonant monochromatic laser field whose Rabi frequency is large compared with the natural lifetime width of mod 2). A third state mod 3) is coupled to the nearby state mod 2) by a static electric field. As the static field coupling increases the spectrum in the neighbourhood of the laser frequency changes from the partly emissive, partly absorptive Mollow form to an absorptive spectrum with two strong peaks. Also, the spectrum in the neighbourhood of the upper level transition frequency changes from an Autler-Townes doublet to a spectrum where a new emissive line appears at the Rabi frequency. In terms of the dressed atom model, this transition corresponds to an intradoublet transition previously forbidden in two-level atoms, though predicted to occur in certain two-level molecular systems. The spectrum is interpreted in terms of a dressed atom model. In general terms not only do the dressed atom transition frequencies correlate well with the spectral features, but there is also good agreement between the heights and widths of the spectral features and those obtained from the full master equation approach. The importance of using the full dipole operator in the basic definitions of the spectral quantities is also demonstrated.