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Applied Physics Letters

Observation of superluminal propagation at negative group velocity in C60 solution

Hao Wang, Yundong Zhang, Nan Wang, Wenlong Yan, He Tian, Wei Qiu, Ping Yuan

Published Mar 19, 2007• 18 citations
Publisher

Abstract

The authors observed the superluminal propagation at negative group velocity in C60 toluene solution firstly. The theoretical analysis was made based on density matrix model of superluminal propagation. The simulation result was well consistent with experimental measurement. The largest time advancement of 3.58ms was got at 5.6×10−4mol∕l of sample concentration; the corresponding group velocity is −0.28m∕s.

DOI
10.1063/1.2715035

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The authors observed the superluminal propagation at negative group velocity in C60 toluene solution firstly. The theoretical analysis was made based on density matrix model of superluminal propagation. The simulation result was well consistent with experimental measurement. The largest time advancement of 3.58ms was got at 5.6×10−4mol∕l of sample concentration; the corresponding group velocity is −0.28m∕s.
We demonstrated the superluminal propagation at negative group velocity using stimulated Brillouin scattering (SBS) in optical fibers. In the scheme, the Stokes wave is circulated back into the ring cavity for Brillouin lasing to enhance the peak loss. The experiment demonstrated the superluminal light propagation can be realized by increasing the input signal power. The largest time advancement of 221.2ns has been observed. Correspondingly, the negative group velocity is -0.15c and the group index is -6.636.
It says in the report1^11 by Wang et al. that a negative group velocity u=−c/310u=-c/310u=−c/310 is obtained and that a pulse advancement shift 62-ns is measured. The authors claim that the negative group velocity is associated with superluminal light propagation and that the pulse advancement is not at odds with causality or special relativity. However, it is shown here that their conclusions above are not true. Furthermore, I give some suggestion concerning a re-definition of group-velocity and a new explanation in special relativity of causality.
We report superluminal propagation in optical fibers using Brillouin lasing oscillation in a ring cavity. Negative group velocity propagation through a 10-m single mode fiber has been experimentally demonstrated. An advancement of 221.2 ns was observed before the input signal, which was achieved with a very high slope efficiency of 211.3 ns/dB. This indicates that this way is suitable for long-distance low-loss superluminal propagation via optical fibers. Correspondingly, the group velocity is -0.151c and the group index is -6.636-the highest group velocity ever reported for optical fibers.
We investigated the light propagation in an organic glass with phthalocyanine gallium (GaPc) dispersed in polymethyl methacrylate (PMMA) fabricated by injection molding. The GaPc/PMMA organic glass is a reverse saturation absorption medium. We observed the superluminal propagation in this organic glass by using the technique of coherent population oscillations at room temperature. The largest time advancement is 2.21 ms and the corresponding group velocity is −0.90 m/s at the wavelength of 532 nm and with the input power of 89 mW. The density matrix theory with four-level model was used to analyze the experimental data. The theoretical simulations showed that the time advancement not only depends on the modulation frequency but also on the lifetime of the first triplet-state of the GaPc. We compared the time advancements between the GaPc/PMMA organic glass and the C60/toluene solution. Although the value of the former is smaller, it is more suitable for practical applications such as optical information processing than the latter.
Summary form only given. We use gain-assisted linear anomalous dispersion to demonstrate superluminal light pulse propagation with a negative group velocity through a transparent atomic medium. We place two Raman gain peaks closely to obtain an essentially lossless anomalous dispersion region that results in a superluminal group velocity. The group velocity of a pulse in this region exceeds c and can even become negative,while the shape of the pulse is preserved. We measured a negative group velocity index of n/sub g/=-315(/spl plusmn/5). Experimentally, a light pulse propagating through the spin-polarized atomic vapor cell exits from it earlier than propagating through the same distance in vacuum by a time difference that is 315 times of the vacuum light propagation time L/c=0.2 ns.