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Physical Review A

Vacuum-mediated multiphoton transitions

Jason R. Bochinski, Chung‐Chieh Yu, Thomas H. Loftus, THOMAS W. MOSSBERG

Published Apr 9, 2001• 13 citations
Publisher

Abstract

We have measured the emission spectra of two-level-like atoms driven by a spatially inhomogeneous bichromatic field comprised of one resonant and one off-resonant component. Under these excitation conditions, observed spectra consist of several narrow peaks appearing on top of a broad continuumlike structure. This experimental approach provides for selective observation of those spectral features having a Rabi-frequency-independent emission frequency. Such emission processes involve multiphoton transitions, with one leg being spontaneous, between dressed-state levels differing only in their photonic quantum numbers; i.e., between equivalent dressed-state-doublet sublevels. Related vacuum-mediated transitions have recently been invoked as an underlying mechanism of inversionless gain.

DOI
10.1103/physreva.63.051402

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We have measured the emission spectra of two-level-like atoms driven by a spatially inhomogeneous bichromatic field comprised of one resonant and one off-resonant component. Under these excitation conditions, observed spectra consist of several narrow peaks appearing on top of a broad continuumlike structure. This experimental approach provides for selective observation of those spectral features having a Rabi-frequency-independent emission frequency. Such emission processes involve multiphoton transitions, with one leg being spontaneous, between dressed-state levels differing only in their photonic quantum numbers; i.e., between equivalent dressed-state-doublet sublevels. Related vacuum-mediated transitions have recently been invoked as an underlying mechanism of inversionless gain.
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We have measured the emission spectrum of two-level-like Ba atoms driven by a continuous-wave bichromatic field containing a strong resonant component and a weaker component detuned from atomic resonance by the strong-field Rabi frequency. With the specified detuning, the weak field resonantly drives a transition of the atom--strong-field dressed states. Observed spectra show that each peak of the normal (single-driving-field) resonance fluorescence triplet is split into three subpeaks separated by one-half the weak-field Rabi frequency. Also seen is another triplet of peaks displaced from the atomic resonance by twice the strong-field Rabi frequency. Splitting of the normal triplet peaks can be explained through weak-field dressing of the strong-field dressed states. The origin of the additional triplet is less transparent. Comparison with theory is made.
We study the interaction of a two-level atom with two lasers of different frequencies and amplitudes: a strong laser of Rabi frequency $2{\ensuremath{\Omega}}_{1}$ on resonance with the atomic transition, and a weaker laser detuned by subharmonics $(2{\ensuremath{\Omega}}_{1}/n)$ of the Rabi frequency of the first. We find that under these conditions the second laser couples the dressed states created by the first in an nnn-photon process, resulting in ``doubly dressed'' states and in a ``multiphoton ac Stark'' effect. We calculate the eigenstates of the doubly dressed atom and their energies, and illustrate the role of this multiphoton ac Stark effect in its fluorescence, absorption, and Autler-Townes spectra.
We demonstrate coherent control of spontaneous emission from an indirectly coupled transition in a microwave driven four-level atomic system. The transition of concern is not directly coupled by any laser fields, while the ground state is coupled to another ground state by a microwave field. We show that the coupling of the microwave field produces interesting features such as double narrow lines in the emission spectrum. The heights, widths and positions of the emission peaks can be controlled by modifying the Rabi frequency and detuning of the microwave field. We discuss the spectra in the dressed states basis.
We present a comprehensive quantum electrodynamical analysis of the interaction between a continuum with photonic band gaps (PBGs) or frequency cut-off and an excited two-level atom, which can be either ‘bare’ or ‘dressed’ by coupling to a near-resonant field mode. A diversity of novel features in the atom and field dynamics is shown to arise from the non-Markovian character of radiative decay into such a continuum of modes. Firstly the excited atom is shown to evolve, by spontaneous decay, into a superposition of non-decaying single-photon dressed states, each having an energy in a different PBG, and a decaying component. This superposition is determined by the atomic resonance shift, induced by the spontaneously emitted photon, into or out of a PBG. The main novel feature exhibited by the decaying excited-state component is the occurrence of beats between the shifted atomic resonance frequency and the PBG cut-off frequencies, corresponding to a non-Lorentzian emission spectrum. Secondly the induced decay of a resonantly driven atom into such a continuum exhibits a cascade of transitions down the ladder of dressed states, which are labelled by decreasing photon numbers of the driving mode. Remarkably, this cascade is terminated at the dressed-state doublet, from which all subsequent transitions to lower doublets are forbidden because they fall within the PBG. This doublet then becomes an attractor state for the populations of higher-lying doublets. As a result, the photon-number distribution of the driving mode becomes strongly sub-Poissonian.
The interaction of a resonant light field with a quantum two-level system is of key interest both for fundamental quantum optics and quantum technological applications employing resonant excitation. While emission under resonant continuous-wave excitation has been well-studied, the more complex emission spectrum of dynamically dressed states, a quantum two-level system driven by resonant pulsed excitation, has so far been investigated in detail only theoretically. Here, we present the first experimental observation of the complete resonance fluorescence emission spectrum of a single quantum two-level system, in form of an excitonic transition in a semiconductor quantum dot, driven by finite Gaussian pulses. We observe multiple emerging sidebands as predicted by theory with an increase of their number and spectral detuning with excitation pulse intensity and a dependence of their spectral shape and intensity on the pulse length. Detuning-dependent measurements provide additional insights into the emission features. The experimental results are in excellent agreement with theoretical calculations of the emission spectra, corroborating our findings.