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arXiv:1804.01541 · Optics Express

Faster light with competing absorption and gain

Jon D. Swaim, Ryan T. Glasser

Published Apr 11, 2018• 3 citations
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Abstract

We experimentally investigate the propagation of optical pulses through a fast-light medium with competing absorption and gain. The combination of strong absorption and optical amplification in a potassium-based four-wave mixing process results in pulse peak advancements up to 88% of the input pulse width, more than 35× that which is achievable without competing absorption. We show that the enhancement occurs even when the total gain of the four-wave mixer is unity, thereby rendering the medium transparent. By varying the pulse width, we observe a transition between fast and slow light, and show that fast light is optimized for large pulse widths.

DOI
10.1364/oe.26.010643

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We experimentally investigate the propagation of optical pulses through a fast-light medium with competing absorption and gain. The combination of strong absorption and optical amplification in a potassium-based four-wave mixing process results in pulse peak advancements up to 88% of the input pulse width, more than 35× that which is achievable without competing absorption. We show that the enhancement occurs even when the total gain of the four-wave mixer is unity, thereby rendering the medium transparent. By varying the pulse width, we observe a transition between fast and slow light, and show that fast light is optimized for large pulse widths.
We have investigated experimentally and theoretically four-wave mixing (FWM) in hot potassium vapor as a medium for producing slow and store light. Two pump and a single probe photon were mixed in double-Λ system, characterized by parametric amplification of coupled co-propagating probe and conjugate beams. We have observed very high gains of probe and conjugate, up to 60 and 80 respectively, at two photon Raman resonance, under large one photon detuning, and high K-cell temperature. Using an amplifying medium to alleviate the absorption and distortion of propagating pulses are important for slow-light propagation. We observed ultraslow propagation of probe and conjugate pulses under conditions when parametric amplification of twin beams is well below their maximum values. A fractional delays of close to 5 at a gain ∼1.5, for the probe pulse of 20 ns, have been achieved simultaneously for both probe and accompanying conjugate pulse. FWM mixing process under conditions of the experiment results in broadening of twin beams, close to observed delays.
Manipulating the group velocity of light has resulted in many advances, including slowing light to a car’s pace, stopping light and even forcing pulses to propagate faster than the speed of light in a vacuum. While the group velocity is equal to or less than the speed of light in a vacuum (c) when propagating through a medium exhibiting normal dispersion, anomalous dispersion allows the group velocity to be greater than c, or even negative. Strangely, the peak of a pulse propagating with a negative group velocity through a medium appears to exit the medium before entering it. This is a result of interference from spectral components of a pulse propagating with different phase velocities in the medium.
We experimentally demonstrate the generation of squeezed, bright twin beams that arise due to competing gain and absorption, in a medium that is overall transparent. To accomplish this, we make use of a nondegenerate four-wave-mixing process in warm potassium vapor such that one of the twin beams experiences strong absorption. At room temperature and above, due to Doppler broadening and smaller frequency detunings compared to other schemes, the ground-state hyperfine splittings used in the present double-$\mathrm{\ensuremath{\Lambda}}$ setup are completely overlapped. We show that despite the resulting significant asymmetric absorption of the twin beams, quantum correlations may still be generated. Our results in this regime demonstrate that the simplified model of gain, followed by loss, is insufficient to describe the amount of quantum correlation resulting from the process.
We present an analysis of the propagation of light pulses through materials possessing extreme values of the group velocity. We begin with an analysis of the behaviour that occurs upon propagation through materials possessing simple Lorentzian gain or absorption lines or materials possessing sharp dips in gain or absorption features. We also describe how more complicated lineshapes can be used to tailor the dispersion of a slow-light system. We furthermore present an analysis of fundamental limitations to how many pulse widths a pulse of light can be delayed or advanced. We show how these mechanisms lead to different limits for slow and fast propagation media.