We describe propagation of light in a gas with periodic density modulation, demonstrating photonic-crystal-like refraction with negative refraction angles. We address the role of poorly defined boundaries and damping and derive an optical analog of the quantum adiabatic theorem. For Cs atoms in an optical lattice, we show that relying on semiadiabatic propagation one can excite and spatially split positively and negatively refracting modes at experimentally available gas densities.
We describe propagation of light in a gas with periodic density modulation, demonstrating photonic-crystal-like refraction with negative refraction angles. We address the role of poorly defined boundaries and damping and derive an optical analog of the quantum adiabatic theorem. For Cs atoms in an optical lattice, we show that relying on semiadiabatic propagation one can excite and spatially split positively and negatively refracting modes at experimentally available gas densities.
The quest to manipulate light propagation in ways not possible with natural media has driven the development of artificially structured metamaterials. One of the most striking effects is negative refraction, where the light beam deflects away from the boundary normal. However, due to material characteristics, the applications of this phenomenon, such as lensing that surpasses the diffraction limit, have been constrained. Here, we demonstrate negative refraction of light in an atomic medium without the use of artificial metamaterials, employing essentially exact simulations of light propagation. High transmission negative refraction is achieved in atomic arrays for different level structures and lattice constants, within the scope of currently realised experimental systems. We introduce an intuitive description of negative refraction based on collective excitation bands, whose transverse group velocities are antiparallel to the excitation quasi-momenta. We also illustrate how this phenomenon is robust to lattice imperfections and can be significantly enhanced through subradiance.
We consider a physical scheme for the realization of optical lattices that have negative refracting under certain conditions. The lattice is formed by clusters of five-level closed-loop chiral atoms assumed to have a Gaussian density distribution in dipole traps along one or two dimensions. With a suitable combination of strong standing wave control fields, spatial symmetry of atoms is achieved. Besides, the dispersion properties of an atomic lattice can be modified conveniently by adjusting various parameters related to the atoms or the lattice. We show that negative-like refraction is not specific to the atoms themselves, but also emerges due to the periodicity of the lattice. We further reveal that the refraction is negative over specific regions on the lattice and strongly depends on the number of atoms trapped along a particular direction. Our model is effectively controllable remotely and may have applications in negative refraction devices and optical information processing.
Using the technique of electromagnetically induced transparency, three photonic bandgaps can be established and manipulated at any time due to the refraction modulated periodically by the one-dimensional optical lattice in a tripod atomic system which is trapped in a one-dimensional optical lattice with a Gaussian density distribution. Using the density-matrix equations to describe the interaction between laser and atoms and the transfer-matrix equation to describe the scattering of light waves in periodic media, we can obtain the steady reflection and transmission spectra. It can be found that the position and width as well as the reflectivity of the photonic band-gap could be tuned by changing the detunings and intensities of the coupling fields and the geometric Bragg detuning.
Recently, strongly modulated photonic crystals, fabricated by the state-of-the-art semiconductor nanofabrication process, have realized various novel optical properties.This paper describes the way in which they differ from other optical media, and clarifies what they can do.In particular, three important issues are considered: light confinement, frequency dispersion and spatial dispersion.First, I describe the latest status and impact of ultra-strong light confinement in a wavelength-cubic volume achieved in photonic crystals.Second, the extreme reduction in the speed of light is reported, which was achieved as a result of frequency dispersion management.Third, strange negative refraction in photonic crystals is introduced, which results from their unique spatial dispersion, and it is clarified how this leads to perfect imaging.The last two sections are devoted to applications of these novel properties.First, I report the fact that strong light confinement and huge light-matter interaction enhancement make strongly modulated photonic crystals promising for on-chip all-optical processing, and present several examples including all-optical switches/memories and optical logics.As a second application, it is shown that the strong light confinement and slow light in strongly modulated photonic crystals enable the adiabatic tuning of light, which leads to various novel ways of controlling light, such as adiabatic frequency conversion, efficient optomechanics systems, photon memories and photons pinning.
Introduction The propagation of light in periodically ordered photonic crystals bears a strong analogy to the wave propagation of conduction electrons in atomic crystals. The development of Bloch modes and dispersion are determined by the interference of waves that are diffracted by lattice planes [19, 52]. The periodicity commensurate with the wavelength (a ≈ λ/2) gives rise to Bragg diffraction that is associated with frequency windows for which waves are forbidden to propagate in a particular direction. Such stop gaps have long been known to arise for electromagnetic waves, notably for X-rays in atomic crystals [214]. At optical frequencies, the stop gaps that occur in one-dimensional periodic structures, known as Bragg stacks, are widely used in optical laboratories as broadband highly reflecting dielectric mirrors [556]. The distinguishing feature of three-dimensional (3D) photonic crystals is that a common stop gap can be achieved for all directions and for all polarizations simultaneously: the widely pursued 3D photonic band gap . Historically a 3D band gap has never been considered for the propagation of X-rays in periodic media, since a gap requires a high refractive index contrast of order unity, whereas the refractive index varies by less then 10 −4 in the X-ray range. At frequencies inside the 3D photonic band gap, the density of optical states (DOS) completely vanishes. As the density of states can also be interpreted as the density of vacuum fluctuations [334], a 3D band gap thus serves as an effective shield to these fluctuations. The total absence of optical modes in a photonic band gap has implications beyond classical optics that break the analogy between the behavior of light in photonic crystals and the behavior of electrons in atomic crystals. 3D photonic band gap crystals play an important role in cavity quantum electrodynamics (cQED) [64, 549], where they offer at least five prospects for new physics. Firstly, probably the most eagerly pursued phenomenon is the complete inhibition of spontaneous emission: an excited quantum emitter – such as an atom, molecule, or quantum dot – embedded in a crystal with its transition frequency tuned to within the 3D band gap remains forever excited since it cannot decay to the ground state by emitting a photon.
In this paper the effects of coupling a non‐resonant optical lattice to a gas is studied. When the lattice induces a periodic modulation of the gas density, strong Bragg diffraction of light can occur. If a significant fraction of the light used to make the periodic structure is diffracted it can potentially limit the achievable intensities inside the lattice limiting the ability to manipulate the gas with the light. We calculate the evolution of the light fields by finding solutions to the wave equation with a periodic refractive index induced by the lattice. The local gas density perturbation induced by the lattice is determined by solution of the Boltzmann kinetic equation in the collisional regime. This allows us to predict the limits of gas transport within optical lattice. We discuss the maximum energy and momentum transfer to the gas from the lattice.