Abstract Slow and fast light are light pulses moving at group velocities slower or faster than the speed of light in vacuum. Current methods to achieve slow and fast light are based on strong dispersion attainable in resonant systems. By defining slow and fast light based on the arrival of the expectation value of the pulse in time, an alternative route for achieving slow and fast light is experimentally demonstrated where no dispersion engineering is needed and thus can be applicable to any medium and system where light is propagating. The method is based on the phenomenon of superoscillations and entails a weak measurement of the arrival time of a light pulse. The findings can be useful for applications in telecommunication, interferometry, and laser radars.
Abstract Slow and fast light are light pulses moving at group velocities slower or faster than the speed of light in vacuum. Current methods to achieve slow and fast light are based on strong dispersion attainable in resonant systems. By defining slow and fast light based on the arrival of the expectation value of the pulse in time, an alternative route for achieving slow and fast light is experimentally demonstrated where no dispersion engineering is needed and thus can be applicable to any medium and system where light is propagating. The method is based on the phenomenon of superoscillations and entails a weak measurement of the arrival time of a light pulse. The findings can be useful for applications in telecommunication, interferometry, and laser radars.
Fast or slow light propagation means that the group velocity of a quasi-monochromatic light pulse propagating in a resonant medium can be significantly faster or slower than the light speed in a vacuum. Since the end of the last century and the beginning of this century, studies on this subject have attracted a great deal of interest. Remarkable progress has been achieved, based on various nonlinear optical interactions in resonant media. The fundamental research on this subject can enrich our understanding of special relativity and causality in physics. On the other hand, those mechanisms and techniques developed for fast and slow light propagation have provided the potential for many future applications, such as optical buffers and variable delay lines for optical networks, optical computing, optical telecommunications, optical data storage, and signal processing.
In early 1999, a news article in the prestigious journal Nature led off with the announcement, "An experiment with atoms at nanokelvin temperatures has produced the remarkable observation of light pulses traveling at velocities of only 17 m/s." The review continued with the understatement, "Observation of light pulses propagating at a speed no faster than a swiftly moving bicycle...comes as a surprise." These findings (and their review) marked the beginning of the current wave of interest in the field that has come to be called "slow light.â" When we refer to "the speed of light," we typically mean c, the phase velocity of light in a vacuum, or the speed of propagation of the phase fronts of monochromatic light. The phase fronts travel more slowly through a material, propagating at the speed c/n, where n is the index of refraction of the medium. However, this ordinary slowing of the phase velocity is not slow light. "Slow light" and "fast light" refer to changes in the group velocity of light in a medium. A pulse of light can be decomposed mathematically into a group of monochromatic waves at slightly different frequencies, as in Fig. 19.1. In a dispersive material, these monochromatic waves travel at different speeds. When one views the propagation of the pulse as a whole, its apparent velocity depends on the extent of the spread of individual monochromatic-wave velocities. Each monochromatic wave travels at its own phase velocity, while the pulse travels at the group velocity. Of course, the group velocity of a pulse of light is not a new concept. The field of slow and fast light has drawn on theory and developments from the work of Sommerfeld and Brillouin from 1907 to 1914, experiments with early laser amplifiers in 1966, and other work done through the end of the 20th century. (For more of the history behind slow and fast light, see Refs. 8 and 9.)
The group velocity at which light pulses propagate through a dispersive material system is very different from the vacuum speed of light c, One refers to light as being "slow" for v g << c (Boyd & Gauthier, 2009) or "fast" for v g > c or v g <0 (Stenner et al, 2003 ).For v g <0, the pulse envelope appears to travel backward in the material (Gehring et al, 2006), and hence it is sometimes referred to as "backward light."The subject of slow light has caused keen interest in the past decade or more, and it is possible to control the group velocity of light pulses in the dispersive materials.Interest in slow and fast light dates back to the early days of the 20th century.Sommerfeld and Brillouin (Sommerfeld & Brillouin, 1960) were intrigued by the fact that theory predicts that v g can exceed c, which leads to apparent inconsistencies with Einstein's special theory of relativity.Experimental investigations of extreme propagation velocities were performed soon after the invention of the laser (Faxvog and et al, 1970).In 1999, Harris's group research work greatly stimulated researchers' interests, which showed that light could be slowed down to 17m/s.The result was obtained in ultra cold atom clouds with the use of electromagnetically induced transparency (EIT), which induces transparency in a material while allowing it to retain strong linear and nonlinear optical properties (Hau et al, 1999).Slow light can also be obtained through the use of the optical response of hot atomic vapors (Philips et al, 2001).These early research works require hard conditions and the slow light cannot operate in room temperature.Recently, researchers found ways to realize slow light operating in room temperature and solid-state materials, which are more suited for many practical applications, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.In this chapter, we describe some of the physical mechanisms that can be used to induce slow and fast light effects in room-temperature solids (Bigelow et al, 2003) and some of the exotic propagation effects that can thereby be observed.We also survey some applications of slow and fast light within the fields of quantum electronics and photonics. Fundamentals of slow and fast lightSlow and fast light refer to the group velocity of a light wave.The group velocity is the velocity most closely related to the velocity at which the peak of a light pulse moves through an optical dispersive material (Milonni, 2005), and is given by the standard result www.intechopen.comRecent Progress in Optical Fiber Research 82where n is the refractive (phase) index and ω is the angular frequency of the carrier wave of the light field.One refers to light as being slow light for v g << c, fast light for for v g > c, and backwards light for v g <0 or v g is negative.Extreme values of the group velocity invariably rely on the dominance of the second contribution to the group index of Equation ( 1).This contribution of course results from the frequency dependence of the refractive index, and for this reason extreme values of the group velocity are usually associated with the resonant or near-resonant response of material systems.According to this theory, slow light is expected in the wings of an absorption line and fast light is expected near line center, and the spatial dispersion, that is, the non-locality in space of the medium response, is another mechanism that can lead to slow light, as has been predicted (Kocharovskaya et al,2001) and observed (Strekalov et al, 2001).We catalogue the main methods to realize slow light in room temperature solid, namely slow light via stimulated Brillouin scattering(SBS), slow light via coherent population oscillations (CPO), tunable time delays based on group velocity dispersion or conversion/ dispersion(C/D), slow light in fiber Bragg gratings and so on.We will describe CPO and SBS slow light in more details.
Slow light is light that travels at unusual, extreme group velocities—sometimes as slow as walking speed or slower. Light waves can be described by many velocities, but for a narrow-band pulse of light (a carrier frequency modulated by an envelope), the group velocity is the speed of the pulse envelope. The term slow light also encompasses other exotic group velocities due to similar techniques, including fast light, stopped light, and backwards light. The science of slow light has been established over the past several years, and research attention is now turning to potential applications of slow light. One application of slow light is as an all-optical true-time delay. Two slow light methods in optical fibers, stimulated Brillouin scattering (SBS) and dispersive delay, are used to provide a controllable pulse delay in a prototype slow-light phased-array laser radar, called SLIDAR. These slow light methods compensate the group delay mismatch of pulses of 6 ns duration while the phased array is steered in two dimensions. A phase control system is described that maintains phase lock among three signal channels and a reference channel, each containing 2.2 km of optical fiber, while accommodating the demands of the slow light techniques. Residual phase error is kept below π /5 radians (1/10 wave) RMS. Slow light can also enhance the spectral sensitivity of spectrometers and interferometers. A design for a slow-light-enhanced nanophotonic spectrometer is presented. One important use of spectrometers is to detect specific chemicals, and I describe an approach to multivariate optical computation, which can be used for automatic chemical spectrum recognition. This technique, which could one day be implemented with a slow-light-enhanced spectrometer as a single-chip chemical detection platform, is explored experimentally in the visible spectrum using a spatial light modulator.