Optics
When light passes through an optical material, its speed is reduced by the refractive index of that material. Under exceptional circumstances, light can be slowed to a walking pace or even stopped momentarily. Exploring approaches for practical applications, Tsakmakidis et al. review how the speed of light can be controlled using designed materials and fabricated structures. They show how the combination of slow light and nanotechnology gives rise to a number of effects of interest in signal processing and optoelectronic communication.
Science , this issue p. [eaan5196][1]
[1]: /lookup/doi/10.1126/science.aan5196
Optics
When light passes through an optical material, its speed is reduced by the refractive index of that material. Under exceptional circumstances, light can be slowed to a walking pace or even stopped momentarily. Exploring approaches for practical applications, Tsakmakidis et al. review how the speed of light can be controlled using designed materials and fabricated structures. They show how the combination of slow light and nanotechnology gives rise to a number of effects of interest in signal processing and optoelectronic communication.
The control of the group velocity of optical signals has been widely demonstrated using various optical phenomena and different media. Slow and fast light is achieved in a controllable manner by modifying the refractive index dispersion through light induced modifications of the material or the structural properties of the medium. The potential for a continuous tuning of the phase or time delay of broadband microwave signals modulating the optical carrier opens previously unexplored horizons. The talk will review the recent advancements and the future directions of slow and fast light research with an emphasis on microwave-photonics applications.
Optics
We are taught that the speed of light in free space is one of the universal physical constants: c. Giovannini et al. now show that there are certain conditions under which such certainty can be broken (see the Perspective by Sambles). Adding spatial structure to an optical beam of single photons reduced the speed of light. The magnitude of the decrease depended on the complexity of the structure imprinted onto the photons.
Science , this issue p. [857][1]; see also p. [828][2]
[1]: /lookup/doi/10.1126/science.aaa3035
[2]: /lookup/doi/10.1126/science.aaa6931
Research into slow light began theoretically in 1880 with the paper [1] of H A Lorentz, who is best known for his work on relativity and the speed of light. Experimental work started some 60 years later with the work of S L McCall and E L Hahn [2] who explored non-linear self-induced transparency in ruby. This field of research has burgeoned in the last 10 years, starting with the work of L Vestergaard Hau and coworkers on slow light via electromagnetically induced transparency in a Bose–Einstein condensate [3]. Many groups are now able to slow light down to a few metres per second or even stop the motion of light entirely [4]. Today, slow light – or more often `slow and fast light' – has become its own vibrant field with a strongly increasing number of publications. In broad scope, slow light research can be categorized in terms of the sort of physical mechanism used to slow down the light. One sort of slow light makes use of material dispersion. This dispersion can be the natural dispersion of the ordinary refractive index or can be the frequency dependence of some nonlinear optical process, such as electromagnetically induced transparency, coherent population oscillations, stimulated light scattering, or four-wave mixing processes. The second sort of slow light makes use of the wavelength dependence of artificially structured materials, such as photonic crystals, optical waveguides, and collections of microresonators. Material systems in which slow light has been observed include metal vapours, rare-earth-doped materials, Raman and Brillioun gain media, photonic crystals, microresonators and, more recently, metamaterials. A common feature of all of these schemes is the presence of a sharp single resonance or multiple resonances produced by an atomic transition, a resonance in a photonic structure, or in a nonlinear optical process. Current applications of slow light include a series of attractive topics in optical information processing, such as optical data storage, optical memories, quantum information devices, and optical communication systems in which the use of slow light will allow all-optical processing with less wasted heat. To implement these applications, devices such as buffers, memories, interferometers and switches that utilize slow light need to be developed. Future challenges include the need for improved coupling of light into slow light modes, overcoming propagation losses, and mitigating the influence of large dispersion of the group velocity. The collection of papers in this special issue of Journal of Optics features a broad spectrum of articles that highlight actual developments in many of the material types and schemes described above. It represents therefore an excellent up to date snapshot of the current state of the field of slow light research. References [1] Lorentz H A 1880 Uber die Beziehung zwischen der Fortpflanzung des Lichtes und der Körperdichte Wiedemann Ann. 9 641–64 [2] McCall S L and Hahn E L 1967 Self-induced transparency by pulsed coherent light Phys. Rev. Lett. 18 908–11 [3] Vestergaard Hau L, Harris S E, Dutton Z and Behroozi C H 1999 Nature 397 594 [4] Philips D F, Fleischhauer A, Mair A, Walsworth R L and Lukin M D 2001 Storage of light in atomic vapor Phys. Rev. Lett. 86 783–6
In this paper we review our recent works in developing photonic crystals and nanophotonic structures for the control of light. We show that dynamic crystal structures, in which the refractive index of the material is modulated by a small extent while a photon pulse is in the crystal, can be used to stop, store and even time-reverse light pulses [ 1][ 2][ 3]. We also show that by incorporating magneto-optical effects into the crystal structures, the reciprocity of light can be broken, resulting in uni-directional devices that are extremely compact [ 4]. And finally, by exploiting sub-wavelength electromagnetic resonances in meta structures, new meta-materials can be created. These materials feature frequency-independent refractive index that is controlled by geometry and can be arbitrarily large [ 5]. Such materials can be of importance for integrated optics and image sensing applications.
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.
Recently there has been a considerable interest in metamaterial waveguide structures capable of dramatically slowing down or, even, completely stopping light. Here, we shall explain in some detail the working principle behind the deceleration and/or stopping of light in metamaterial structures, and review the various, metamaterial-enabled, methods that have been proposed thus far towards achieving such a goal. Further, we will concisely describe how one can construct zero-loss metamaterials over a continuous and broad (but not infinite) range of frequencies, which is an essential prerequisite for any slow-light system. Moreover, it will be explained that inside such waveguide structures light can in principle be stopped (zero group velocity, vg = 0) even in the presence of losses. By nature, metamaterialenabled schemes for stopping/storing light invoke solid-state materials and, as such, are not subject to low-temperature or atomic coherence limitations. Furthermore, these methods simultaneously allow for broad bandwidth operation, since they do not rely on group index resonances; large delay-bandwidth products, since a wave packet can, in principle, be completely stopped and buffered indefinitely; and (for the case, in particular, where a negative-index metamaterial is used) high, almost 100%, in/out-coupling efficiencies. Thus, we conclude that these methods for trapping photons, which can be realised using existing technology, could open the way to a multitude of hybrid optoelectronic devices to be used in 'quantum information' processing, communication networks and signal processors and may conceivably herald a new realm of combined metamaterials and slow light research.