Publisher Summary This chapter describes the coherent population trapping in laser spectroscopy. Coherent population trapping may be also described as the pumping of the atomic system in a particular state, the coherent superposition of the atomic states, which is a nonabsorbing state. The exciting radiation creates an atomic coherence, such that the atom's evolution is prepared exactly out of phase with the incoming radiation and no absorption takes place. The chapter discusses the basic properties of an atomic system prepared with the coherent population-trapping superposition of states and outlines experimental observations concerned with the establishment of coherent trapping in different discrete systems. The chapter also discusses the theoretical and experimental aspects of trapping that involve states of the continuum and reviews the theoretical and experimental features associated with coherent population trapping in laser cooling, adiabatic transfer, lasing without inversion, pulse matching, and photon statistics. The theoretical aspect of coherent population trapping created by spontaneous emission is also discussed in the chapter.

Coherent Population Trapping in Laser Spectroscopy

The subject of electromagnetically induced transparency (EIT) is reviewed in this paper. Emphasis is placed on the experimental work reported in this field since 1990. Theoretical work is also covered, although it is not intended to review all the very numerous recent theoretical treatments on this topic. The basic physical ideas behind EIT are elucidated. The relation of EIT to other processes involving laser-induced atomic coherence, such as coherent population trapping, coherent adiabatic population transfer and lasing without inversion, is discussed. Experimental work is described covering the following topics: EIT with pulsed and continuous-wave sources, lasing without inversion, pulse propagation in a laser dressed medium and EIT in nonlinear optical processes. A full set of references and a bibliography are included.

Electromagnetically induced transparency

This review paper is devoted to amplification and lasing without population inversion involving atomic transitions in gas media. We start by discussing the main motivation in inversionless lasing research, namely, the generation of short-wavelength laser light. Then, we review the basic physics of inversionless lasing in two-level and, eventually, in three- and multilevel atomic configurations. Finally, we summarize the current state of the art of LWI experiments and indicate the main difficulties with respect to short-wavelength laser generation.

https://iopscience.iop.org/article/10.1088/1464-4266/2/3/201/pdf

Lasing without inversion

https://www.physik.uni-kl.de/agfleischhauer/dokuwiki/lib/exe/fetch.php?media=publications:optcomm-134-431-1997.pdf

White-light cavities, atomic phase coherence, and gravitational wave detectors

The historical development that led to the modern concept of amplification and losing without inversion is reviewed. The two basic mechanisms for amplification without inversion and their theoretical formulation are discussed. Finally, the three experiments that confirm the occurrence of these effects are described.

Lasing without inversion: A useful concept?

We report the observation of light amplification without population inversion. A linear superposition of an m=1 and an m=-1 magnetic substate is populated coherently. Absorption from this superposition to a higher state is canceled by interference of channels, a so-called dark resonance. An inverse transition is used for amplification of light. The temporal behavior of the amplified pulse and the gain are studied as a function of the energy splitting due to a magnetic field. In a vapor of cadmium-112 gain factors of up to 4.3 are observed

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Experimental demonstration of light amplification without population inversion.

The hyperfine structure sub-levels of the 5s5p3P1 state of cadmium are populated coherently with a 35 ps light pulse. The coherence is detected in a second excitation step resulting in a modulation of the ionization probability. A third light pulse ionizes the atom. Differences in the H.F.S. of the odd isotopes are used to selectively enhance the ionization of one isotope. In a magnetic field the Zeeman splitting of the hyperfine structure causes an additional modulation. Measurements and a theoretical interpretation are given.

Isotope selective photoionization of cadmium by coherent excitation of the hyperfine structure of the 5s5p 3 P 1 level

Coherence between the magnetic sub-levels of the 5s5p3P1 level of cadmium is induced by excitation. A second excitation step and photo-ionization are used to detect the coherence and perturbations by the nuclear angular momentum. The ionization probability of even isotopes is suppressed with respect to the odd isotopes by a factor 59.

Coherent excitation and isotope selective photoionization of cadmium

The radiative lifetimes of the levels in the 3d94s(1D)4p configuration of Cu I are measured. The levels are excited from the metastable 3d94s22D3/2,5/2 levels. The metastable Cu atoms are generated in a pulsed hollow cathode discharge. The levels investigated are populated with a 35-ps laser pulse at wavelengths around 220 nm. The laser induced fluorescence signal is detected. The lifetime of the 3d94s(3D)4p4D1/2 level is also determined by direct excitation from the ground state. A comparison with calculated literature values is given.

Lifetime measurements of the 3d94s(1D) 4p configuration of Cu I

Quantum beats of magnetic and hyperfine structure sub-states can be used to selectively photoionize isotopes. It is investigated under what conditions of the atomic and laser system this mechanism can be operated effectively. An expression for the final signal is obtained by:i) adding the transition amplitudes numerically;ii) carrying out summations over the transition amplitudes analytically by means of angular momentum algebra. Advantages and limitations of the two approaches are compared. A list of conditions for this type of experiment is given. Experiments on Cd are used as an example.

W. E. van der Veer

Papers

Experimental demonstration of light amplification without population inversion.

Isotope selective photoionization of cadmium by coherent excitation of the hyperfine structure of the 5s5p 3 P 1 level

Coherent excitation and isotope selective photoionization of cadmium

Lifetime measurements of the 3d94s(1D) 4p configuration of Cu I

Laser isotope separation via coherent excitation of Zeeman or hyperfine levels: what is required ?