Almost twenty years ago, light was slowed down to less than 10^{-7} of its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden turn-off of the coupling laser, a slow light pulse can be imprinted on cold atoms such that it can be read out and converted into a photon again. In this process, the light is stopped by absorbing it and storing its shape within the atomic ensemble. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides, where the group speed vanishes. Here, we extend the phenomenon of stopped light to the new field of parity-time (PT) symmetric systems. We show that zero group speed in PT symmetric optical waveguides can be achieved if the system is prepared at an exceptional point, where two optical modes coalesce. This effect can be tuned for optical pulses in a wide range of frequencies and bandwidths, as we demonstrate in a system of coupled waveguides with gain and loss.
Almost twenty years ago, light was slowed down to less than 10^{-7} of its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden turn-off of the coupling laser, a slow light pulse can be imprinted on cold atoms such that it can be read out and converted into a photon again. In this process, the light is stopped by absorbing it and storing its shape within the atomic ensemble. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides, where the group speed vanishes. Here, we extend the phenomenon of stopped light to the new field of parity-time (PT) symmetric systems. We show that zero group speed in PT symmetric optical waveguides can be achieved if the system is prepared at an exceptional point, where two optical modes coalesce. This effect can be tuned for optical pulses in a wide range of frequencies and bandwidths, as we demonstrate in a system of coupled waveguides with gain and loss.
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.
I n 1998, laser pulses were slowed [1] in a Bose-Einstein condensate (BEe) [2] ofsodium to only 17 m/s, more than seven orders of magnitude lower than the speed of light in vacuum. Associated with the dramatic reduction factor for the light speed was a spatial compression of the pulses by the same large factor. A light pulse, which was more than 1 km long in vacuum, was compressed to a size of -50 flm, and at that point was completely contained within the condensate [1]. This allowed the light-slowing experiments to be brought to their ultimate extreme [3]: in the summer of 2000, light pulses were completely stopped, stored, and subsequently revived in an atomic medium, with millisecond storage times [4]. The initial ultra-slow light experiments spurred a flurry of slow light investigations, and slow or partially stopped light has now been observed in limited geometries in warm rubidium vapours [5-7], liquid-nitrogen cooled crystals [8], and recently in room temperature crystals [9]. Here, we begin with a discussion ofultra-slow and stopped light. We describe how cold atoms and Bose-Einstein condensates have been manipulated to generate media with extreme optical properties. While the initial experiments concentrated on the light propagation, we have recently begun a number of investigations of the effects that slow light has on the medium in which it propagates. Effects are profound because both the velocity and length scales associated with propagating light pulses have been brought down to match the characteristic velocity and length scales of the medium. With the most recent extension, the light roadblock [10], we have compressed light pulses to a length ofonly 2 f!ill. Here, we describe the use of ultra-compressed light pulses to probe superfluidity and the creation of quantized vortices in BEes through formation of'superfluid shock. waves'. We also present the observation of an ultra-slow-light-based, pulsed atom laser. Furthermore, we demonstrate the use of slow and stopped light for manipulation of optical information, in particular in Bose-Einstein condensates that allow for phase coherent processing of three-dimensional, compressed patterns of stored optical information.
One path to quantum information processing involves light-matter interaction, such as light in a photonic-crystal waveguide coupled to a cloud of ultracold alkali atoms. However, to actually implement such a system, it is crucial to increase the coupling strength beyond what is presently achievable. The authors explain the design requirements to achieve large coupling by decreasing the group velocity of an edge mode in a hybrid-clad waveguide, allowing photons of ``slow light'' to interact with trapped cold atoms in a robust, chip-integrated setting.
We introduce a new all-optical mechanism that can compress the bandwidth of light pulses to absolute zero, and bring them to a complete stop. The mechanism can be realized in a system consisting of a waveguide side coupled to tunable resonators, which generates a photonic band structure that represents a classical analogue of the electromagnetically induced transparency. The same system can also achieve a time-reversal operation. We demonstrate the operation of such a system by finite-difference time-domain simulations of an implementation in photonic crystals.
Ability to tune the group velocity of a light pulse is of great importance for optical communication applications and realization of quantum information processing. Tunability of group velocity can be achieved by using either optical or electronic resonances. Tunability of an optical resonance depends on the change in refractive index of the cavity material. However, since electro-optical coefficients of non-engineered materials are quite small, the tuning range of optical resonances by electric field is narrow. This makes tuning by electric field impractical for most applications. Quantum dot (QD) coupled to a photonic crystal cavity is a useful hybrid system exhibiting nonlinear features. In this work, we analyze the use of quantum dot - optical cavity hybrid systems to engineer nonlinear waveguides susceptible to electric fields. We start by theoretically analyzing the optical pulse propagation at low-photon number excitation limit in a periodically arranged strongly coupled quantum dot - photonic crystal system. A one dimensional periodic array of evanescently coupled photonic cavities (coupled resonator optical waveguides, CROWs) containing non-interacting quantum dots allows us to tune the group velocity and the bandwidth of the pulse by adjusting the cavity/QD coupling. Tunable group velocity can be achieved by applying an external electric field which will result in a significant decrease in the cavity/QD coupling because of DC Stark effect. We also show that, using this approach, light pulses can be slowed down or stored by compressing the pulse bandwidth adiabatically and reversibly. Adiabatic bandwidth compression can be achieved by slowly decreasing the coupling strength when the light pulse is inside the coupled resonator optical waveguide. The energy splitting and the coupling constant after applying electric field is calculated by using perturbation theory for two level systems. With our approach, nonlinear materials highly susceptible to electric fields can be engineered in low-excitation regime.
We present a general analysis for the criteria to stop and store light coherently. We show that a light pulse can be stopped in any physical system, provided that (i) the system bandwidth can be compressed to zero; (ii) the system has sufficient degrees of freedom to accommodate the pulse, and the bandwidth compression occurs while the pulse is in the system; and (iii) the bandwidth compression is done reversibly in an adiabatic fashion that preserves the phase space and the information in the original photon pulse during the entire duration of the stopping process. Based upon this general criterion, we present a brief discussion of stopping-light schemes using atomic resonances, and a detailed analysis of the all-optical scheme that we recently proposed. We show that the all-optical scheme can achieve arbitrarily small group velocities for large bandwidth pulses, and opens up new opportunities in both fundamental sciences and technological applications.