George M. Gehring

Fiber-Based Slow-Light Technologies

A review of current fiber-based technologies capable of producing slow-light effects is presented, with emphasis on the applicability of these technologies to telecommunications. We begin with a review of the basic concepts of phase velocity, group velocity, and group delay. We then present a survey of some of the figures of merit used to quantify the engineering properties of slow-light systems. We also present a description of several of the physical processes that are commonly used to induce a slow-light effect. Finally, a review of some recent advances in this field is presented.

Reducing pulse distortion in fast-light pulse propagation through an erbium-doped fiber amplifier

When a pulse superposed on a cw background propagates through an erbium-doped fiber amplifier with a negative group velocity, either pulse broadening or pulse compression can be observed. These effects can be explained in terms of two competing mechanisms: gain recovery and pulse spectrum broadening. The distortion of the pulse shape caused by these effects depends on input pulse width, pump power, and background-to-pulse power ratio. With the proper choice of these three parameters, we can obtain significant pulse advancement with minimal pulse distortion.

Simulating Quantum-Mechanical Barrier Tunneling Phenomena with a Nematic-Liquid-Crystal-Filled Double-Prism Structure

We present an electrically-controlled nematic-liquid-crystal-filled double-prism structure that can be used to simulate quantum-mechanical tunneling through a barrier of variable height. Measurements of time delay in reflection from this structure, taken with femtosecond resolution using entangled photon pairs in a Hong-Ou-Mandel interferometer, are compared to theoretical predictions. We show that the Goos-Hänchen contribution to the tunneling delay is unmeasurable in this geometry. Our research contributes to the understanding of quantum-mechanical barrier tunneling times, and can lead to the fabrication of optical analogues to the tunnel junction and other photonic devices.

Slow and fast light propagation in erbium-doped fiber

We study propagation of light pulses and modulation in an Er-doped fiber in the regimes of anomalously slow and superluminal group velocities. The pulses experience either delay or advancement depending on the pump power.

Single-photon experiments with liquid crystals for quantum science and quantum engineering applications

We present here our results on using liquid crystals in experiments with nonclassical light sources: (1) single-photon sources exhibiting antibunching (separation of all photons in time), which are key components for secure quantum communication systems, and (2) entangled photon source with photons exhibiting quantum interference in a Hong-Ou- Mandel interferometer. In the first part, cholesteric liquid crystal hosts were used to create definite circular polarization of antibunched photons emitted by nanocrystal quantum dots. If the photon has unknown polarization, filtering it through a polarizer to produce the desired polarization for quantum key distribution with bits based on polarization states of photons will reduce by half the efficiency of a quantum cryptography system. In the first part, we also provide our results on observation of a circular polarized microcavity resonance in nanocrystal quantum dot fluorescence in a 1-D chiral photonic bandgap cholesteric liquid crystal microcavity. In the second part of this paper with indistinguishable, time-entangled photons, we demonstrate our experimental results on simulating quantum-mechanical barrier tunnelling phenomena. A Hong-Ou-Mandel dip (quantum interference effect) is shifted when a phase change was introduced on the way of one of entangled photons in pair (one arm of the interferometer) by inserting in this arm an electrically controlled planar-aligned nematic liquid crystal layer between two prisms in the conditions close to a frustrated total internal reflection. By applying different AC-voltages to the planar-aligned nematic layer and changing its refractive index, we can obtain various conditions for incident photon propagation – from total reflection to total transmission. Measuring changes of tunnelling times of photon through this structure with femtosecond resolution permitted us to answer some unresolved questions in quantum-mechanical barrier tunnelling phenomena.

Time-domain Measurements of Single-Photon Tunneling Phenomena

Using a Hong-Ou-Mandel interferometer, we have measured single-photon time delays in frustrated total internal reflection in a liquid-crystal-filled double-prism structure and in transmission through chiral photonic band gap liquid crystal structures with sub-fs resolution.

Single-Photon Measurement of the Hartman Effect in Frustrated Total Internal Reflection

Using fourth-order interference, we have measured the single-photon tunneling delay in frustrated total internal reflection. The results are explained in terms of a dwell-time interpretation of the Hartman effect.

Phased-Array Laser Radar System Based on Slow Light

We describe an electronically steerable laser-radar system based on a sparse phased array and the use of slow-light methods to ensure synchronization of the light leaving each subaperture.

Pulse broadening or compression in fast-light pulse propagation through an erbium-doped fiber amplifier

Pulse broadening or compression in an Er3+-doped fiber amplifier is observed, and explained by gain recovery and pulse spectrum broadening effects. Maximal pulse advancement and minimal pulse distortion are obtained by optimizing these competing effects.

Reducing pulse distortion in fast light pulse propagation by Pulse-on-Background method through an erbium-doped fiber amplifier

A novel method (pulse-on-background) to reduce pulse distortion in fast light pulse propagation through an erbium-doped fiber amplifier by adding a background of appropriate power is suggested. The pulse-on-background method depends on pulse width, pump power, pulse power and background-to-pulse power ratio.