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ABSTRACT: We demonstrate that the ratio of group to phase velocity has a simple relationship to the orientation of the electromagnetic field. In nondispersive materials, opposite group and phase velocity corresponds to fields that are mostly oriented in the propagation direction. More generally, this relationship (including the case of dispersive and negative-index materials) offers a perspective on the phenomena of backward waves and left-handed media. As an application of this relationship, we demonstrate and explain an irrecoverable failure of perfectly matched layer absorbing boundaries in computer simulations for constant cross-section waveguides with backward-wave modes and suggest an alternative in the form of adiabatic isotropic absorbers.
Physical Review E 07/2009; 79(6 Pt 2):065601. · 2.26 Impact Factor
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ABSTRACT: We propose a two dimensional (2D) photonic crystal (PhC) structure that supports super-collimation over a large frequency range (over 4 times that of a traditional square lattice of holes). We theoretically and numerically investigate the collimation mechanism in our 2D structure, in comparison to that of two other frequently used related PhC structures. We also point out the potential importance of our proposed structure in the design of super-collimation-based devices for both monochromatic and polychromatic light.
Optics Express 06/2009; 17(10):8109-18. · 3.59 Impact Factor
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ABSTRACT: We present a longitudinally uniform slow light waveguide, which consists of transverse periodic dielectric strips with a few defect layers. The parameters of the proposed waveguide can be tuned to have two anomalous dispersion curves with extreme points which have zero-group-velocity frequencies at nonzero wave vector in uniform-index direction. The group velocity and group velocity dispersion of the two photonic bands are analyzed, and the propagation of the slow mode with a given bandwidth is demonstrated theoretically.
Applied Physics Letters 12/2008; 93(24):241111-241111-3. · 3.84 Impact Factor
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ABSTRACT: We develop a coupled mode theory (CMT) model of the behavior of a polarization source in a general photonic structure, and obtain an analytical expression for the resulting generated electric field; loss, gain and/or nonlinearities can also be modeled. Based on this treatment, we investigate the criteria needed to achieve an enhancement in various nonlinear effects, and to produce efficient sources of terahertz radiation, in particular. Our results agree well with exact finite-difference time-domain (FDTD) results. Therefore, this approach can also in certain circumstances be used as a potential substitute for the more numerically intensive FDTD method.
Optics Express 09/2008; 16(17):12523-37. · 3.59 Impact Factor
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ABSTRACT: We present a method of computing Casimir forces for arbitrary geometries, with any desired accuracy, that can directly exploit the efficiency of standard numerical-electromagnetism techniques. Using the simplest possible finite-difference implementation of this approach, we obtain both agreement with past results for cylinder-plate geometries, and also present results for new geometries. In particular, we examine a pistonlike problem involving two dielectric and metallic squares sliding between two metallic walls, in two and three dimensions, respectively, and demonstrate nonadditive and nonmonotonic changes in the force due to these lateral walls.
Physical Review Letters 09/2007; 99(8):080401. · 7.37 Impact Factor
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ABSTRACT: We describe intricate cavity mode structures, that are possible in waveguide devices with two or more guided modes. The main element is interference between the scattered fields of two modes at the facets, resulting in multipole or mode cancelations. Therefore, strong coupling between the modes, such as around zero group velocity points, is advantageous to obtain high quality factors. We discuss the mechanism in three different settings: a cylindrical structure with and without negative group velocity mode, and a surface plasmon device. A general semi-analytical expression for the cavity parameters describes the phenomenon, and it is validated with extensive numerical calculations.
Optics Express 06/2007; 15(10):6268-78. · 3.59 Impact Factor
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ABSTRACT: We describe a numerical method to compute Casimir forces in arbitrary geometries, for arbitrary dielectric and metallic materials, with arbitrary accuracy (given sufficient computational resources). Our approach, based on well-established integration of the mean stress tensor evaluated via the fluctuation-dissipation theorem, is designed to directly exploit fast methods developed for classical computational electromagnetism, since it only involves repeated evaluation of the Green's function for imaginary frequencies (equivalently, real frequencies in imaginary time). We develop the approach by systematically examining various formulations of Casimir forces from the previous decades and evaluating them according to their suitability for numerical computation. We illustrate our approach with a simple finite-difference frequency-domain implementation, test it for known geometries such as a cylinder and a plate, and apply it to new geometries. In particular, we show that a piston-like geometry of two squares sliding between metal walls, in both two and three dimensions with both perfect and realistic metallic materials, exhibits a surprising non-monotonic ``lateral'' force from the walls. Comment: Published in Physical Review A, vol. 76, page 032106 (2007)
05/2007;
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Ofer Shapira,
Ken Kuriki,
Nicholas D Orf,
Ayman F Abouraddy,
Gilles Benoit,
Jean F Viens,
Alejandro Rodriguez, Mihai Ibanescu,
John D Joannopoulos,
Yoel Fink,
Megan M Brewster
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ABSTRACT: All fiber lasers to date emit radiation only along the fiber axis. Here a fiber that exhibits laser emission that is radially directed from its circumferential surface is demonstrated. A unique and controlled azimuthally anisotropic optical wave front results from the interplay between a cylindrical photonic bandgap fiber resonator, anisotropic organic dye gain, and a linearly polarized axial pump. Low threshold (86nJ) lasing at nine different wavelengths is demonstrated throughout the visible and near-infrared spectra. We also report the experimental realization of unprecedented layer thicknesses of 29.5 nm maintained throughout meter-long fibers. Such a device may have interesting medical applications ranging from photodynamic therapy to in vivo molecular imaging, as well as textile fabric displays.
Optics Express 06/2006; 14(9):3929-35. · 3.59 Impact Factor
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ABSTRACT: We study the radiation emitted by an oscillating dipole moving with a constant velocity in a photonic crystal, and analyze the effects that arise in the presence of a photonic band gap. It is demonstrated through numerical simulations that the radiation strength may be enhanced or inhibited according to the photonic band structure, and anomalous effects in the sign and magnitude of the Doppler shifts are possible, both outside and inside the gap. We suggest that this effect could be used to identify the physical origin of the backward waves in recent metamaterials.
Physical Review Letters 03/2006; 96(4):043903. · 7.37 Impact Factor
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ABSTRACT: Diffraction, a fundamental process in wave physics, leads to spreading of the optical beams as they propagate. However, new photonic crystal (PhC) meta-materials can be nano-engineered to generate extreme anisotropy, resulting in apparent propagation of light without diffraction. This surprising phenomenon, called supercollimation, effectively freezes the spatial width of a light beam inside a PhC, observed over a few isotropic diffraction-lengths. However, using such experiments to predict the behaviour for longer propagation lengths is difficult, as a tiny error in a measured width can extrapolate to order unity uncertainty in the width at distances over hundreds of diffraction-lengths. Here, supercollimation is demonstrated in a macroscopic PhC system over centimetre-scale distances, retaining spatial width confinement without the need for waveguides or nonlinearities. Through quantitative studies of the beam evolution in a two-dimensional PhC, we find that supercollimation possesses unexpected but inherent robustness with respect to short-scale disorder such as fabrication roughness, enabling supercollimation over 600 isotropic diffraction-lengths. The effects of disorder are identified through experiments and understood through rigorous simulations. In addition, a supercollimation steering capability is proposed.
Nature Material 03/2006; 5(2):93-6. · 32.84 Impact Factor
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ABSTRACT: We show that a saddle point Van Hove singularity in a band adjacent to a photonic crystal band gap can lead to situations which defy the conventional wisdom that the strongest band-gap confinement is found at frequencies near the midgap. As an example, we present a two-dimensional square photonic crystal waveguide where the strongest confinement is close to the band edge. The underlying mechanism can also apply to any system that is described by a band structure with a gap. In general, the saddle point favors the appearance of a very flat band, which in turn results in an enhanced confinement at band-gap frequencies immediately above or below the flat band.
Physical Review Letters 02/2006; 96(3):033904. · 7.37 Impact Factor
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ABSTRACT: Forces arising from overlap between the guided waves of parallel, microphotonic waveguides are calculated. Both attractive and repulsive forces, determined by the choice of relative input phase, are found. Using realistic parameters for a silicon-on-insulator material system, we estimate that the forces are large enough to cause observable displacements. Our results illustrate the potential for a broader class of optically tunable microphotonic devices and microstructured artificial materials.
Optics Letters 12/2005; 30(22):3042-4. · 3.40 Impact Factor
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ABSTRACT: We have calculated the optically-induced force between coupled high-Q whispering gallery modes of microsphere resonators. Attractive and repulsive forces are found, depending whether the bi-sphere mode is symmetric or antisymmetric. The magnitude of the force is linearly proportional to the total power in the spheres and consequently linearly enhanced by Q. Forces on the order of 100 nN are found for Q=108, large enough to cause displacements in the range of 1mum when the sphere is attached to a fiber stem with spring constant 0.004 N/m.
Optics Express 11/2005; 13(20):8286-95. · 3.59 Impact Factor
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ABSTRACT: A class of axially uniform waveguides is introduced, employing a new mechanism to guide light inside a low-index dielectric material without the use of photonic band gap, and simultaneously exhibiting subwavelength modal size and very slow group velocity over an unusually large frequency bandwidth. Their basis is the presence of plasmonic modes on the interfaces between dielectric regions and the flat unpatterned surface of a bulk metallic substrate. These novel waveguides allow for easy broadband coupling and exhibit absorption losses limited only by the intrinsic loss of the metal.
Physical Review Letters 09/2005; 95(6):063901. · 7.37 Impact Factor
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ABSTRACT: We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalous uniform waveguide mode that has zero group velocity at a nonzero wave vector. In a short piece of a uniform waveguide with a specially designed cross section, light is confined longitudinally by small group-velocity propagation and transversely by a reflective cladding. The quality factor Q is greatly enhanced by the small group velocity for a set of cavity lengths that are separated by approximately pi/k0, where k0 is the longitudinal wave vector for which the group velocity is zero.
Optics Letters 04/2005; 30(5):552-4. · 3.40 Impact Factor
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Photonic Crystal Materials and Devices III. 01/2005; 5733:152.
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ABSTRACT: We predict that reversed and anomalous non-relativistic Doppler shifts can be observed under some circumstances when light reflects from a shock wave front propagating through a photonic crystal, or material with a periodic modulation of the dielectric. This theoretical prediction is generalizable and applies to wave-like excitations in a variety of periodic media. An experimental observation of this effect has recently been made (Seddon, N. and Bearpark, T. Science, 302 (2003) 1537) and we provide a brief discussion of this experiment.
Journal of Computer-Aided Materials Design 12/2004; 12(1):1-15. · 1.30 Impact Factor
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Science 09/2004; 305(5685):778; author reply 778. · 31.20 Impact Factor
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ABSTRACT: We study the radiation pressure on the surface of a waveguide formed by omnidirectionally reflecting mirrors. In the absence of losses, the pressure goes to infinity as the distance between the mirrors is reduced to the cutoff separation of the waveguide mode. This divergence at constant power input is due to the reduction of the modal group velocity to zero, which results in the magnification of the electromagnetic field. Our structure suggests a promising alternative, microscale system for observing the variety of classical and quantum-optical effects associated with radiation pressure in Fabry–Perot cavities.
Applied Physics Letters 08/2004; 85(9):1466-1468. · 3.84 Impact Factor
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ABSTRACT: We present detailed theoretical and numerical analysis of certain novel non-linear optical phenomena enabled by photonic bandgap fibers. In particular, we demonstrate the feasibility of optical bistability in an axially modulated nonlinear photonic bandgap fiber through analytical theory and detailed numerical experiments. At 1.55microm carrier wavelength, the in-fiber devices we propose can operate with only a few tens of mW of power, have a nearly instantaneous response and recovery time, and be shorter than 100microm. Furthermore, we predict existence of gap-like solitons (which have thus-far been described only in axially periodic systems) in axially uniform photonic bandgap fibers.
Optics Express 05/2004; 12(8):1518-27. · 3.59 Impact Factor
