Optical Nonreciprocity of Cold Atom Bragg Mirrors in Motion

Abstract

Reciprocity is fundamental to light transport and is a concept that holds also in rather complex systems. Yet, reciprocity can be switched off even in linear, isotropic and passive media by setting the material structure into motion. In highly dispersive multilayers this leads to a fairly large forwardbackward asymmetry in the pulse transmission. Moreover, in multilevel systems, this transport phenomenon can be all-optically enhanced. For atomic multilayer structures made of three-level cold 87 Rb atoms, for instance, forward-backward transmission contrast around 95% can be obtained already at atomic speeds in the meter per second range. The scheme we illustrate may open up avenues for optical isolation that were not previously accessible.PACS numbers: 42.50. Wk, 42.70.Qs, 42.50.Gy, 37.10.Vz Much attention has been devoted to the development of advanced materials and composite systems to achieve optical functionalities not readily available in natural media. Such optical metamaterials can be engineered to stretch the rules that govern light propagation and lightmatter interaction, potentially seeding a new paradigm in all-optical, optoelectronic and optomechanical devices. Photonic crystals and negatively refracting media are prominent instances of man-made systems the optical properties of which can be tailored to a great extent. Nevertheless, some tasks are more difficult than others. Already in the familiar process of linear reflection and transmission of light [1] it is in general hard to achieve non-reciprocity. In particular, multilayer photonic structures made with linear isotropic media with dissipation and/or gain may exhibit reflection non-reciprocity, i.e., an unbalance between the forward and backward reflectivities. There are even cases in which one of them can be made negligible and the other one can increase without limit with the sample thickness [2]; this occurs in the so called P T -symmetric media which exhibit a variety of peculiar optical properties [3,4]. Yet, it is not possible to achieve a non-reciprocal transmissivity in such linear and passive systems. Transmission reciprocity is almost ubiquitous in optics [1,5].Non-reciprocal transmission is however rather desirable for information processing and crucial to the development of optical-based functional components in photonics. In much the same way in which electrical nonreciprocity has been realized through diodes, devising an optical diode is challenging, even in theory. Ideally, an optical diode would allow total light transmission over a bundle of wavelengths in one direction, providing total isolation in the reverse direction. Previous * s.horsley@exeter.ac.uk work on non-reciprocal transmission has been based on either magneto-optical effects [6][7][8] or non linear processes [9,10]. Other mechanisms have also been explored [11][12][13] and realized experimentally [14], including more involved diode designs based on two-dimensional square-lattice photonic crystals [15] or non-symmetric photonic crystal gratings exhibiting anomal...

Non-Hermitian Degeneracies and Unidirectional Reflectionless Atomic Lattices

“…Such proposals have obvious advantages of real-time all-optical reconfigurable capabilities and implicit disadvantages of intractable field modulations and considerable symmetry errors. Large optical nonreciprocities may also be achieved by exploiting the asymmetric Doppler shift in moving atomic Bragg mirrors [20], and proofof-principle experiments have been carried out [21].…”

“…Such proposals have obvious advantages of real-time all-optical reconfigurable capabilities and implicit disadvantages of intractable field modulations and considerable symmetry errors. Large optical nonreciprocities may also be achieved by exploiting the asymmetric Doppler shift in moving atomic Bragg mirrors [20], and proofof-principle experiments have been carried out [21].The great interest in PT-symmetric complex media stemmed, however, from the non-Hermitian extensions of quantum mechanics and quantum field theories [22,23], and it is perhaps worth going back to the essential nonHermitian behavior of light transport to get a broader picture on reciprocity violations and unidirectional reflectionlessness. Take, e.g., a typical one-dimensional (1D) light scattering process as shown in Fig.…”

Revisiting the Bragg reflector to illustrate modern developments in optics

“…4), the nonreciprocity of this system as determined by DT can be in the tens of percent for velocities of meters per second. 27 For the same velocity regime, an effective optical diode would require only an order of magnitude increase in the dispersion. …”

Parity-time-antisymmetric atomic lattices without gain

“…However, they cannot be directly used to implement unidirectional light transport and its manipulation [8,9], an essential and more difficult task than others in all-optical networks. In this respect, only in recent years significant progress has been made by considering moving photonic crystals of driven atoms [10,11] and optical materials with parity-time (P T ) symmetry [12,13]. As compared to traditional photonic crystals, P T -symmetric materials are periodically modulated not only in terms of the real part n but also the imaginary part n of the refractive index n, exhibiting a delicate balance of gain and loss alternately along the modulation direction.…”