Arthur Dogariu

Gain-assisted superluminal light propagation

Einsteins theory of special relativity and the principle of causality imply that the speed of any moving object cannot exceed that of light in a vacuum (c). Nevertheless, there exist various proposals for observing faster-than- c propagation of light pulses, using anomalous dispersion near an absorption line, nonlinear and linear gain lines, or tunnelling barriers. However, in all previous experimental demonstrations, the light pulses experienced either very large absorption or severe reshaping, resulting in controversies over the interpretation. Here we use gain-assisted linear anomalous dispersion to demonstrate superluminal light propagation in atomic caesium gas. The group velocity of a laser pulse in this region exceeds c and can even become negative, while the shape of the pulse is preserved. We measure a group-velocity index of ng = -310(±5); in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. The observed superluminal light pulse propagation is not at odds with causality, being a direct consequence of classical interference between its different frequency components in an anomalous dispersion region.

Transparent anomalous dispersion and superluminal light-pulse propagation at a negative group velocity

Anomalous dispersion cannot occur in a transparent passive medium where electromagnetic radiation is being absorbed at all frequencies, as pointed out by Landau and Lifshitz. Here we show, both theoretically and experimentally, that transparent linear anomalous dispersion can occur when a gain doublet is present. Therefore, a superluminal light pulse propagation can be observed even at a negative group velocity through a transparent medium with almost no pulse distortion. Consequently, a {\it negative transit time} is experimentally observed resulting in the peak of the incident light pulse to exit the medium even before entering it. This counterintuitive effect is a direct result of the {\it rephasing} process owing to the wave nature of light and is not at odds with either causality or Einsteins theory of special relativity.

High-Gain Backward Lasing in Air

Focused ultraviolet light creates a distant “lasing spark” in air that can then be used for remote detection. The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.

Signal velocity, causality, and quantum noise in superluminal light pulse propagation

We consider pulse propagation in a linear anomalously dispersive medium where the group velocity exceeds the speed of light in vacuum ( c) or even becomes negative. A signal velocity is defined operationally based on the optical signal-to-noise ratio, and is computed for cases appropriate to the recent experiment where such a negative group velocity was observed. It is found that quantum fluctuations limit the signal velocity to values less than c.

Single-shot detection of bacterial endospores via coherent Raman spectroscopy

Recent advances in coherent Raman spectroscopy hold exciting promise for many potential applications. For example, a technique, mitigating the nonresonant four-wave-mixing noise while maximizing the Raman-resonant signal, has been developed and applied to the problem of real-time detection of bacterial endospores. After a brief review of the technique essentials, we show how extensions of our earlier experimental work [Pestov D, et al. (2007) Science 316:265–268] yield single-shot identification of a small sample of Bacillus subtilis endospores (≈104 spores). The results convey the utility of the technique and its potential for “on-the-fly” detection of biohazards, such as Bacillus anthracis. The application of optimized coherent anti-Stokes Raman scattering scheme to problems requiring chemical specificity and short signal acquisition times is demonstrated.

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air

Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.

Low-threshold amplified spontaneous emission in blends of conjugated polymers

Low thresholds (∼500 W/cm2) for amplified spontaneous emission(ASE) are reported in films of soluble poly(paraphenylene vinylene)-based conjugated polymer blends. Efficient Forster energy transfer from the absorbing host polymer to the emitting guest polymer is observed. Emission in the blends originates predominantly from the guest polymer. The large spectral shift between the absorption and emission wavelengths lowers the self-absorption losses and results in low ASE thresholds. Initial results show an enhancement in photoluminescencequantum efficiency of the blends.

1.05Pb/s Transmission with 109b/s/Hz Spectral Efficiency using Hybrid Single- and Few-Mode Cores

We demonstrated 1.05-Pb/s transmission over 3km of multicore fiber with spectral efficiency of 109b/s/Hz, using twelve single-mode cores carrying DP-32QAM-OFDM signals and two few-mode cores carrying DP-QPSK in their LP01 and two LP11 modes.

Time-resolved Förster energy transfer in polymer blends

Abstract Sub-picosecond spectroscopy and pump–probe experiments show Forster energy transfer in blends from larger gap (blue or green-emitting) host polymers poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV) or poly[2-(meta-2′-ethylhexoxyphenyl)-1,4-phenylenevinylene) (m-EHOP-PPV) to the smaller gap, red-emitting guest polymer poly(2,5-bis(2′-ethylhexoxy)-1,4-phenylenevinylene) (BEH-PPV). The dynamics of the stimulated emission (SE) and photoinduced absorption (PA) of the blends indicate that 10–20 ps are required for complete energy transfer. Quantitative measurements of energy transfer rates give a Forster interaction range of 3–4 nm, 1.4 times longer than the theoretical values as calculated from the spectral overlap. We attribute this difference to delocalization of the excited state. Insufficient spectral overlap between the emission of the host and absorption of the guest is shown to be the cause for the absence of energy transfer in a blend with poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene) (BCHA-PPV) as the guest polymer.

Temporal behavior of cold atmospheric plasma jet

Temporally resolved evolution of parameters in atmospheric plasma jet is studied by means of microwave scattering, fast photographing, and measuring of jet currents. It is observed that streamer (“plasma bullet”) propagating along with gas flow is generated immediately after the breakdown. It is demonstrated that an afterglow plasma column remains on the way of streamer passing. Lifetime of the afterglow plasma column is 3–5 μs, which is longer than that of the streamer.