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Frontiers in Optics

Spectrum measurement of the cavity-QED microlaser: Deviation from the Schawlow-Townes linewidth

Hyun-Gue Hong, Wontaek Seo, Moonjoo Lee, Younghoon Song, Wonshik Choi, Christopher M. Fang-Yen, Ramachandra Rao Dasari, Michael S. Feld, Jai-Hyung Lee, Kyungwon An

Published Jan 1, 2010• 0 citations
Publisher

Abstract

We report the measurement of the cavity-QED microlaser linewidth near threshold by using the photon-counting-based second-order correlation spectroscopy. The abrupt rise-and-fall of the linewidth near thereshold is observed.
DOI
10.1364/ls.2010.lwd4

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Summary form only given. We have investigated the wave properties manifested in the first-order coherence or the spectrum of the cavity-QED microlaser by using a high-sensitive photon-counting-based heterodyne spectroscopy. The emission spectrum exhibited a linewidth broadening due to the quantum frequency pulling effect, namely the frequency pulling dependent on the intracavity atom- and photon-number intrinsic distributions, in the presence of nonzero atom-cavity detuning. We also observed a large per-atom frequency pulling (2.1 kHz maximally), enhanced by the strong atom-cavity coupling. Moreover, a stepwise change in the frequency pulling, associated with the coherent Rabi oscillations, was observed with the increased pumping over multiple thresholds.
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We study the linewidth of a one-photon laser through the approach of the two-time correlation function. Our numerical calculations cover the regions of below, near, and above threshold. Well below threshold, the laser linewidth decreases linearly with increasing linear gain. Near and above threshold, in comparison with the approximate linewidth given by the phase-diffusion coefficient at the mean photon number (which is inversely proportional to the laser intensity---the Schawlow-Townes formula), we find that the laser intensity fluctuations increase the laser linewidth and that the Schawlow-Townes formula is valid only when the mean photon number is large and the photon-number variance is much smaller than the square of the mean photon number. Near threshold, the laser linewidth can be twice as large as that given by the phase-diffusion coefficient at the mean photon number (i.e., twice as large as that given by the Schawlow-Townes formula).
We report the first experimental observation of the cavity-QED microlaser spectrum, specifically the unconventional frequency pulling brought by a strong atom-cavity coupling at off resonance. The pulling is enhanced quadratically by the atom-cavity coupling to result in a sensitive response to the number of pumping atoms (2.1 kHz per atom maximally). Periodic variation of the pulling due to the coherent Rabi oscillation is also observed as the number of pumping atoms is increased across multiple thresholds.
The spectrum of the cavity-QED microlaser or micromaser is expected to show distinctive features of the coherent light-matter interaction, which are obscured in the conventional Schawlow-Townes linewidth theory. However, the spectral studies have been limited to resonant atom-cavity interaction so far. Here we consider the dispersive interaction in the off-resonance case, from which we uncover a quantum frequency pulling effect in the microlaser or micromaser spectrum. We present a quantum theory of the spectrum which introduces the notion of a frequency-pulling distribution associated with the photon number. In contrast to the conventional laser, periodic variation of the mean frequency pulling is observed with increasing pump parameter and it is attributed to the strong atom-cavity coupling. The pulling distribution gives rise to a spectral broadening, which can be dominant over the nondispersive broadening addressed in the previous works. We also developed a corresponding semiclassical theory and discuss how the introduction of the frequency shift fits in with the extended quantum theory.