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Observation and Origin of Multi-THz Quantum Beats in Superfluorescent Emission from Argon

Noa Nambu, Zan Nie, Kenneth A. Marsh, C. Kumar N. Patel, C. Joshi

Published Jan 1, 2025• 0 citations
Publisher

Abstract

Observation of quantum beats in lasing signals in argon atoms pumped by a 3-photon resonance with an intense 260.5 nm laser were used to observe excitation of a “3ω-induced” state mediated by phase-matched third-harmonic photons.
DOI
10.1364/cleo_at.2025.jps200_191

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We report observation of multi-THz quantum beats in the amplitude of superfluorescence in pure argon gas, using a time delayed 780-nm probe beam that interrogates the relevant population densities. The superfluorescence is induced by three-photon absorption of an ultrashort 260.5-nm laser pulse that creates coherent and coupled populations in multiple closely spaced 3d3d3d and 5s5s5s Ar levels. Argon pressure and pump laser focusing parameters are seen to modify the frequency of the quantum beats, which is attributed to the absorption of the self-generated, nonresonant third harmonic of the pump from Ar near the zero of the real part of the overlapping linear susceptibilities of the closely spaced 3d3d3d and 5s5s5s excited states. The self-absorption of the phase-matched THG photons creates a coherent population of atoms which acts like an atomic excited state ($3\ensuremath{\omega}$-induced state). We have also observed a new two-step superfluorescent cascade that results from the population redistribution enabled by the 780-nm probe pulse between the $3{d}^{\ensuremath{'}}{[5/2]}_{3}$ and the $3\ensuremath{\omega}$-induced state and gives rise to a previously unknown transition that emits on 1587 nm.
In femtosecond degenerate four-wave mixing experiments in sodium vapor, quantum beats are observed at the wavelength of the third harmonic when the fundamental beams are three-photon resonant. The period of beating, which corresponds to fine structure energy-level splitting, does not change with the atomic number density, buffer gas pressure, or laser intensity. Surprisingly, the third-harmonic signal is observed only at negative time delays for the transient grating pulse sequence. We relate this to the well-known interference effect between excitation pathways involving the fundamental and the third-harmonic fields, which leaves atoms virtually unexcited and prevents formation of population grating.