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Bulletin of the American Physical Society

Emission Spectra of Low-Z Atoms for Lasing Without Inversion in Helium-Like Atoms

James M. Mitrani, Hui Xia, S. Suckewer

Published Nov 16, 2011• 0 citations
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Lineshapes associated with the absorption and emission spectra of neutral atoms in liquid helium and cold helium gas are discussed. A neutral atom in liquid helium forms a microcavity due to a strong short-range repulsive force between the helium and impurity atoms, producing very broad and largely blueshifted absorption bands and relatively narrow emission lines with much smaller shifts. A broad and weak emission band observed by exciting the D2 line of Ag atoms in liquid helium has confirmed the hypothesis that quenching of D2 emission lines for heavy group 1a and 1b atoms are due to the formation of exciplex. In cold helium gas, sudden decrease of the emission lifetimes below 5.2 K by the excitation of the D2 line of Ag atoms is attributed to the condensation of He atoms on the metastable 2D5/2 state. Very narrow absorption lines of Eu innershell electron transitions have revealed that superfluid helium behaves as a static and highly homogeneous low temperature matrix.
Optical response of an active medium can substantially be modified when coherent superpositions of states are excited, that is, when systems display quantum coherence and interference. This has led to fascinating applications in atomic and molecular systems. Examples include coherent population trapping, lasing without inversion, electromagnetically induced transparency, cooperative spontaneous emission, and quantum entanglement. \n \nWe study quantum coherence effects in several quantum optical systems and find interesting applications. We show that quantum coherence can lead to transient Raman lasing and lasing without inversion in short wavelength spectral regions--extreme ultraviolet and x-ray--without the requirement of incoherent pumping. For example, we demonstrate transient Raman lasing at 58.4 nm in Helium atom and transient lasing without inversion at 6.1 nm in Helium-like Boron (triply-ionized Boron). We also investigate dynamical properties of a collective superradiant state prepared by absorption of a single photon when the size of the sample is larger than the radiation wavelength. We show that for large number of atoms such a state, to a good approximation, decays exponentially with a rate proportional to the number of atoms. We also find that the collective frequency shift resulting from repeated emission and reabsorption of short-lived virtual photons is proportional to the number of species in the sample. Furthermore, we examine how a position-dependent excitation phase affects the evolution of entanglement between two dipole-coupled qubits. It turns out that the coherence induced by position-dependent excitation phase slows down the otherwise fast decay of the two-qubit entanglement. We also show that it is possible to entangle two spatially separated and uncoupled qubits via interaction with correlated photons in a cavity quantum electrodynamics setup. Finally, we analyze how quantum coherence can be used to generate continuous-variable entanglement in quantum-beat lasers in steady state and propose possible implementation in quantum lithography.