arXiv subjects

John D. Joannopoulos

Publications and source records attributed to John D. Joannopoulos.

At least 73 records · Page 4Linked to original sources

arXiv2014

Structural Colors from Fano Resonances

Structural coloration is an interference phenomenon where colors emerge when visible light interacts with nanoscopically structured material, and has recently become a most interesting scientific and engineering topic. However, current structural color generation mechanisms either require thick (compared to the wavelength) structures or lack dynamic tunability. This report proposes a new structural color generation mechanism, that produces colors by the Fano resonance effect on thin photonic crystal slab. We experimentally realize the proposed idea by fabricating the samples that show resonance-induced colors with weak dependence on the viewing angle. Finally, we show that the resonance-induced colors can be dynamically tuned by stretching the photonic crystal slab fabricated on an elastic substrate.

physics.optics↗

arXiv2013

Larger-area single-mode photonic crystal surface-emitting lasers enabled by an accidental Dirac point

By altering the lattice geometry of the photonic crystal surface-emitting lasers (PCSELs), we tune the regular lasing band edges of quadratic dispersions to form a single accidental Dirac point of linear dispersion at the Brillouin zone center. This not only increases the mode spacing by orders of magnitude, but also eliminates the distributed in-plane feedback to enable single-mode PCSELs of substantially larger area thus substantially higher output power. The advantages of using accidental Dirac cones are systematically evaluated through two-dimensional in-plane calculations and confirmed by three-dimensional simulations of photonic crystal slab devices.

physics.optics↗

arXiv2013

Fundamental limits to nanoparticle extinction

We show that there are shape-independent upper bounds to the extinction cross section per unit volume of randomly oriented nanoparticles, given only material permittivity. Underlying the limits are restrictive sum rules that constrain the distribution of quasistatic eigenvalues. Surprisingly, optimally-designed spheroids, with only a single quasistatic degree of freedom, reach the upper bounds for four permittivity values. Away from these permittivities, we demonstrate computationally-optimized structures that surpass spheroids and approach the fundamental limits.

physics.optics↗

arXiv2013

Comment on "A self-assembled three-dimensional cloak in the visible" in Scientific Reports 3, 2328

Mühlig et. al. propose and fabricate a "cloak" comprised of nano-particles on the surface of a sub-wavelength silica sphere. However, the coating only reduces the scattered fields. This is achieved by increased absorption, such that total extinction increases at all wavelengths. An object creating a large shadow is generally not considered to be cloaked; functionally, in contrast to the relatively few structures that can reduce total extinction, there are many that can reduce scattering alone.

physics.optics↗

arXiv2013

Comment on "Linear and passive silicon optical isolator" in Scientific Reports 2, 674

Wang et al. [1] demonstrated different power transmission coefficients for forward and backward propagation in simulation and experiment. From such a demonstration, the central claim of their paper is that "the spatial inversion symmetry breaking diode can construct an optical isolator in no conflict with any reciprocal principle". Their claim contradicts the Lorentz reciprocity theorem, from which it is well known that one cannot construct an isolator this way.

physics.optics↗

arXiv2012

Anomalous near-field heat transfer between a cylinder and a perforated surface

We predict that the radiative heat-transfer rate between a cylinder and a perforated surface depends non-monotonically on their separation. This anomalous behavior, which arises due to near-field effects, is explained using a heuristic model based on the interaction of a dipole with a plate. We show that nonmonotonicity depends not only on geometry and temperature but also on material dispersion - for micron and submicron objects, nonmonotonicity is present in polar dielectrics but absent in metals with small skin depths.

cond-mat.mtrl-sci↗

arXiv2012

Weyl points and line nodes in gapless gyroid photonic crystals

Weyl points and line nodes are three-dimensional linear point- and line-degeneracies between two bands. In contrast to Dirac points, which are their two-dimensional analogues, Weyl points are stable in the momentum space and the associated surface states are predicted to be topologically non-trivial. However, Weyl points are yet to be discovered in nature. Here, we report photonic crystals, based on the double-gyroid structures, exhibiting frequency-isolated Weyl points with intricate phase diagrams. The surface states associated with the non-zero Chern numbers are demonstrated. Line nodes are also found in similar geometries; the associated surface states are shown to be flat bands. Our results are readily experimentally realizable at both microwave and optical frequencies.

cond-mat.mes-hall↗

arXiv2012

Near-field thermal radiation transfer controlled by plasmons in graphene

It is shown that thermally excited plasmon-polariton modes can strongly mediate, enhance and \emph{tune} the near-field radiation transfer between two closely separated graphene sheets. The dependence of near-field heat exchange on doping and electron relaxation time is analyzed in the near infra-red within the framework of fluctuational electrodynamics. The dominant contribution to heat transfer can be controlled to arise from either interband or intraband processes. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement (e.g. $10^2$ to $10^3$ for separations between $0.1μm$ to $10nm$) over the Stefan-Boltzmann law, known as the far field limit. Strong, tunable, near-field transfer offers the promise of an externally controllable thermal switch as well as a novel hybrid graphene-graphene thermoelectric/thermophotovoltaic energy conversion platform.

cond-mat.mes-hall↗

arXiv2011

Frequency-selective near-field enhancement of radiative heat transfer via photonic-crystal slabs: a general computational approach for arbitrary geometries and materials

We demonstrate the possibility of achieving enhanced frequency-selective near-field radiative heat transfer between patterned (photonic crystal) slabs at designable frequencies and separations, exploiting a general numerical approach for computing heat transfer in arbitrary geometries and materials based on the finite-difference time-domain method. Our simulations reveal a tradeoff between selectivity and near-field enhancement as the slab--slab separation decreases, with the patterned heat transfer eventually reducing to the unpatterned result multiplied by a fill factor (described by a standard proximity approximation). We also find that heat transfer can be further enhanced at selective frequencies when the slabs are brought into a glide-symmetric configuration, a consequence of the degeneracies associated with the non-symmorphic symmetry group.

cond-mat.mtrl-sci↗

arXiv2011

Structural anisotropy and orientation-induced Casimir repulsion in fluids

In this work we theoretically consider the Casimir force between two periodic arrays of nanowires (both in vacuum, and on a substrate separated by a fluid) at separations comparable to the period. Specifically, we compute the dependence of the exact Casimir force between the arrays under both lateral translations and rotations. Although typically the force between such structures is well-characterized by the Proximity Force Approximation (PFA), we find that in the present case the microstructure modulates the force in a way qualitatively inconsistent with PFA. We find instead that effective-medium theory, in which the slabs are treated as homogeneous, anisotropic dielectrics, gives a surprisingly accurate picture of the force, down to separations of half the period. This includes a situation for identical, fluid-separated slabs in which the exact force changes sign with the orientation of the wire arrays, whereas PFA predicts attraction. We discuss the possibility of detecting these effects in experiments, concluding that this effect is strong enough to make detection possible in the near future.

quant-ph↗

arXiv2010

Microstructure Effects for Casimir Forces in Chiral Metamaterials

We examine a recent prediction for the chirality-dependence of the Casimir force in chiral metamaterials by numerical computation of the forces between the exact microstructures, rather than homogeneous approximations. We compute the exact force for a chiral bent-cross pattern, as well as forces for an idealized "omega"-particle medium in the dilute approximation and identify the effects of structural inhomogeneity (i.e. proximity forces and anisotropy). We find that these microstructure effects dominate the force for separations where chirality was predicted to have a strong influence. To get observations of chirality free from microstructure effects, one must go to large separations where the effect of chirality is at most $\sim10^{-4}$ of the total force.

quant-ph↗

arXiv2010

Achieving a Strongly Temperature-Dependent Casimir Effect

We propose a method of achieving large temperature sensitivity in the Casimir force that involves measuring the stable separation between dielectric objects immersed in fluid. We study the Casimir force between slabs and spheres using realistic material models, and find large > 2nm/K variations in their stable separations (hundreds of nanometers) near room temperature. In addition, we analyze the effects of Brownian motion on suspended objects, and show that the average separation is also sensitive to changes in temperature . Finally, this approach also leads to rich qualitative phenomena, such as irreversible transitions, from suspension to stiction, as the temperature is varied.

quant-ph↗

arXiv2009

Non-touching nanoparticle diclusters bound by repulsive and attractive Casimir forces

We present a scheme for obtaining stable Casimir suspension of dielectric nontouching objects immersed in a fluid, validated here in various geometries consisting of ethanol-separated dielectric spheres and semi-infinite slabs. Stability is induced by the dispersion properties of real dielectric (monolithic) materials. A consequence of this effect is the possibility of stable configurations (clusters) of compact objects, which we illustrate via a "molecular" two-sphere dicluster geometry consiting of two bound spheres levitated above a gold slab. Our calculations also reveal a strong interplay between material and geometric dispersion, and this is exemplified by the qualitatively different stability behavior observed in planar versus spherical geometries.

quant-ph↗

arXiv2009

Casimir forces in the time domain II: Applications

Our preceding paper introduced a method to compute Casimir forces in arbitrary geometries and for arbitrary materials that was based on a finite-difference time-domain (FDTD) scheme. In this manuscript, we focus on the efficient implementation of our method for geometries of practical interest and extend our previous proof-of-concept algorithm in one dimension to problems in two and three dimensions, introducing a number of new optimizations. We consider Casimir piston-like problems with nonmonotonic and monotonic force dependence on sidewall separation, both for previously solved geometries to validate our method and also for new geometries involving magnetic sidewalls and/or cylindrical pistons. We include realistic dielectric materials to calculate the force between suspended silicon waveguides or on a suspended membrane with periodic grooves, also demonstrating the application of PML absorbing boundaries and/or periodic boundaries. In addition we apply this method to a realizable three-dimensional system in which a silica sphere is stably suspended in a fluid above an indented metallic substrate. More generally, the method allows off-the-shelf FDTD software, already supporting a wide variety of materials (including dielectric, magnetic, and even anisotropic materials) and boundary conditions, to be exploited for the Casimir problem.

quant-ph↗

arXiv2009

Spherical cloaking using nonlinear transformations for improved segmentation into concentric isotropic coatings

Two novel classes of spherical invisibility cloaks based on nonlinear transformation have been studied. The cloaking characteristics are presented by segmenting the nonlinear transformation based spherical cloak into concentric isotropic homogeneous coatings. Detailed investigations of the optimal discretization (e.g., thickness control of each layer, nonlinear factor, etc.) are presented for both linear and nonlinear spherical cloaks and their effects on invisibility performance are also discussed. The cloaking properties and our choice of optimal segmentation are verified by the numerical simulation of not only near-field electric-field distribution but also the far-field radar cross section (RCS).

physics.optics↗

arXiv2009

Casimir forces in the time domain: I. Theory

We introduce a method to compute Casimir forces in arbitrary geometries and for arbitrary materials based on the finite-difference time-domain (FDTD) scheme. The method involves the time-evolution of electric and magnetic fields in response to a set of current sources, in a modified medium with frequency-independent conductivity. The advantage of this approach is that it allows one to exploit existing FDTD software, without modification, to compute Casimir forces. In this manuscript, part I, we focus on the derivation, implementation choices, and essential properties of the time-domain algorithm, considered both analytically and illustrated in the simplest parallel-plate geometry. Part II presents results for more complex two- and three-dimensional geometries.

quant-ph↗

arXiv2009

Ingredients of a Casimir analog computer

We present the basic ingredients of a technique to compute quantum Casimir forces at micrometer scales using antenna measurements at tabletop, e.g. centimeter, scales, forming a type of analog computer for the Casimir force. This technique relies on a correspondence that we derive between the contour integration of the Casimir force in the complex frequency plane and the electromagnetic response of a physical dissipative medium in a finite, real frequency bandwidth.

quant-ph↗

arXiv2008

Nonlinear photonic crystals near the supercollimation point

We uncover a strong coupling between nonlinearity and diffraction in a photonic crystal at the supercollimation point. We show this is modeled by a nonlinear diffraction term in a nonlinear schroedinger type equation, in which the properties of solitons are investigated. Linear stability analysis shows solitons are stable in an existence domain that obeys the Vakhitov-Kolokolov criterium. In addition, we investigate the influence of the nonlinear diffraction on soliton collision scenarios.

physics.optics↗