Timothy Amoah

High-Q optical cavities in hyperuniform disordered materials

We introduce designs for high-Q photonic cavities in slab architectures in hyperuniform disordered solids displaying isotropic band gaps. Despite their disordered character, hyperuniform disordered structures have the ability to tightly confine the transverse electric-polarized radiation in slab configurations that are readily fabricable. The architectures are based on carefully designed local modifications of otherwise unperturbed hyperuniform dielectric structures. We identify a wide range of confined cavity modes, which can be classified according to their approximate symmetry (monopole, dipole, quadrupole, etc.) of the confined electromagnetic wave pattern. We demonstrate that quality factors Q>109 can be achieved for purely two-dimensional structures, and that for three-dimensional finite-height photonic slabs, quality factors Q>20000 can be maintained.

Hyperuniform disordered phononic structures

We demonstrate the existence of large phononic band gaps in designed hyperuniform (isotropic) disordered two-dimensional (2D) phononic structures of Pb cylinders in epoxy matrix. The phononic band gaps in hyperuniform disordered phononic structures are comparable to band gaps of similar periodic structures, for both out-of-plane and in-plane polarizations. A large number of localized modes is identi ed near the band edges, as well as, di usive transmission throughout the rest of the frequency spectrum. Very high-Q cavity modes for both out-of-plane and in-plane polarizations are formed by selectively removing a single cylinder out of the structure. E cient waveguiding with almost 100% transmission trough waveguide structures with arbitrary bends is also presented. We expand our results to thin three-dimensional layers of such structures and demonstrate e ective band gaps related to the respective 2D band gaps. Moreover, the drop in the Q factor for the three-dimensional structures is not more than three orders of magnitude compared to the 2D ones.

Unfolding the band structure of non-crystalline photonic band gap materials

Non-crystalline photonic band gap (PBG) materials have received increasing attention, and sizeable PBGs have been reported in quasi-crystalline structures and, more recently, in disordered structures. Band structure calculations for periodic structures produce accurate dispersion relations, which determine group velocities, dispersion, density of states and iso-frequency surfaces, and are used to predict a wide-range of optical phenomena including light propagation, excited-state decay rates, temporal broadening or compression of ultrashort pulses and complex refraction phenomena. However, band calculations for non-periodic structures employ large super-cells of hundreds to thousands building blocks, and provide little useful information other than the PBG central frequency and width. Using stereolithography, we construct cm-scale disordered PBG materials and perform microwave transmission measurements, as well as finite-difference time-domain (FDTD) simulations. The photonic dispersion relations are reconstructed from the measured and simulated phase data. Our results demonstrate the existence of sizeable PBGs in these disordered structures and provide detailed information of the effective band diagrams, dispersion relation, iso-frequency contours, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study introduces a powerful tool to investigate photonic properties of non-crystalline structures and provides important effective dispersion information, otherwise difficult to obtain.

Hyperuniform disordered phononic structures (Conference Presentation)

Phononic crystals, artificial materials with periodically arranged scattering centers, were introduced more than two decades ago as the elastic waves analogue of photonic crystals. These materials, either in two or three dimensions, can exhibit large frequency regions of prohibited propagation of elastic waves, the so-called phononic band gaps (PBGs). On the other hand, typical elastic wave propagation in random structures is associated with diffusion, or in extreme situation with localization, and random structures do not exhibit band gaps. Here, we introduce a new class of structurally disordered phononic structures, hyperuniform disordered phononic structures (HDPS) that exhibit large elastic band gaps. These structures are created from initially arbitrary point patterns by imposing hyperuniform correlations among the points and finally decorating them with a specific scatterers, so that the structure factor becomes isotropic and vanishes for all k-vectors within a specific radius. The disorder can smoothly be tuned to produce structures ranging from totally random to fully periodic by adjusting a single parameter. Such amorphous structures exhibit large band gaps, comparable to the ones found in the periodic counterparts, ballistic and diffusive propagation depending on the modes frequency and a large number of localized modes near the band edges. We discuss the formation of high-Q cavity modes and waveguides with 100% transmission in these disordered structures in the GHz regime. Such phononic-circuit architectures are expected to have a direct impact on integrated micro-electro-mechanical filters/modulators for wireless communications and acoustic-optical sensing devices.

Hyperuniform photonic slabs for high-Q cavities and low-loss waveguides

Hyperuniform disordered photonic structures/solids (HUDS) are a new class of photonic solids, which display large, isotropic photonic band gaps (PBG) comparable in size to the ones found in photonic crystals (PC). The existence of large band gaps in HUDS contradicts the long-standing intuition that Bragg scattering and long- range translational order is required in PBG formation, and demonstrates that interactions between Mie-like local resonances and multiple scattering can induce on their own PBGs. HUDS combine advantages of both isotropy due to disorder (absence of long range two-point correlations) and controlled scattering properties from uniform local topology due to hyperuniformity (constrained disorder). In this paper we review the photonic properties of HUDS including the origin of PBGs and potential applications. We address technologically realisable designs of HUDS including localisation of light in point-defect-like optical cavities and the guiding of light in free-form PC waveguide analogues. We show that HUDS are a promising general-purpose design platform for integrated optical micro-circuitry, including active devices such as optical microcavity lasers and modulators.

Flexible cavity and waveguide light confinement in hyperuniform photonic slabs

We introduce novel planar hyperuniform-disordered architectures as potential general purpose platform for optical microcircuits. High-Q cavities and low-loss waveguides for TE-radiation are demonstrated using FDTD and band-structure simulations.

Hyperuniform disordered photonic band gap silicon devices for optical interconnects

We report experimental and simulation results for silicon waveguides and devices in hyperuniform disordered photonic solids. Our results demonstrate the ability of disordered photonic bandgap materials to serve as a platform for optical integrated circuits.

Isotropic band gaps, optical cavities, and freeform waveguides in hyperuniform disordered photonic solids

Hyperuniform disordered solids are a new class of designer photonic materials with large isotropic band gaps comparable to those found in photonic crystals. The hyperuniform disordered materials are statistically isotropic and possess a controllable constrained randomness. We have employed their unique properties to introduce novel architectures for optical cavities that achieve an ultimate isotropic confinement of radiation, and waveguides with arbitrary bending angles. Our experiments demonstrate low-loss waveguiding in submicron scale Si-based hyperuniform structures operating at infrared wavelengths and open the way for the realization of highly flexible, disorder-insensitive optical micro-circuit platforms.