Mohamed A. K. Othman

Graphene-based tunable hyperbolic metamaterials and enhanced near-field absorption

We explore the near-field radiative thermal energy transfer properties of hyperbolic metamaterials. The presence of unique electromagnetic states in a broad bandwidth leads to super-planckian thermal energy transfer between metamaterials separated by a nano-gap. We consider practical phonon-polaritonic metamaterials for thermal engineering in the mid-infrared range and show that the effect exists in spite of the losses, absorption and finite unit cell size. For thermophotovoltaic energy conversion applications requiring energy transfer in the near-infrared range we introduce high temperature hyperbolic metamaterials based on plasmonic materials with a high melting point. Our work paves the way for practical high temperature radiative thermal energy transfer applications of hyperbolic metamaterials.We investigate a novel implementation of hyperbolic metamaterial (HM) at far-infrared frequencies composed of stacked graphene sheets separated by thin dielectric layers. Using the surface conductivity model of graphene, we derive the homogenization formula for the multilayer structure by treating graphene sheets as lumped layers with complex admittances. Homogenization results and limits are investigated by comparison with a transfer matrix formulation for the HM constituent layers. We show that infrared iso-frequency wavevector dispersion characteristics of the proposed HM can be tuned by varying the chemical potential of the graphene sheets via electrostatic biasing. Accordingly, reflection and transmission properties for a film made of graphene-dielectric multilayer are tunable at terahertz frequencies, and we investigate the limits in using the homogenized model compared to the more accurate transfer matrix model. We also propose to use graphene-based HM as a super absorber for near-fields generated at its surface. The power emitted by a dipole near the surface of a graphene-based HM is increased dramatically (up to 5 × 10(2) at 2 THz), furthermore we show that most of the scattered power is directed into the HM. The validity and limits of the homogenized HM model are assessed also for near-fields and show that in certain conditions it overestimates the dipole radiated power into the HM.

Graphene–dielectric composite metamaterials: evolution from elliptic to hyperbolic wavevector dispersion and the transverse epsilon-near-zero condition

Abstract. We investigated a multilayer graphene–dielectric composite material, comprising graphene sheets separated by subwavelength-thick dielectric spacer, and found it to exhibit hyperbolic isofrequency wavevector dispersion at far- and mid-infrared frequencies, allowing propagation of waves that would be otherwise evanescent in an isotropic dielectric. Electrostatic biasing was considered for tunable and controllable transition from hyperbolic to elliptic dispersion. We explored the validity and limitation of the effective medium approximation (EMA) for modeling wave propagation and cutoff of the propagating spatial spectrum due to the Brillouin zone edge. We reported that EMA is capable of predicting the transition of the isofrequency dispersion diagram under certain conditions. The graphene-based composite material allows propagation of backward waves under the hyperbolic dispersion regime and of forward waves under the elliptic regime. Transition from hyperbolic to elliptic dispersion regimes is governed by the transverse epsilon-near-zero (TENZ) condition, which implies a flatter and wider propagating spectrum with higher attenuation, when compared to the hyperbolic regime. We also investigated the wide-angle tunable transparency of the multilayer at that condition in contrast to other materials exhibiting ENZ phenomena.

Giant gain enhancement in photonic crystals with a degenerate band edge

Giant Gain Enhancement in Photonic Crystals with a Degenerate Band Edge Mohamed A. K. Othman 1 , Farshad Yazdi 1 , Alex Figotin 2 and Filippo Capolino 1 Department of Electrical Engineering and Computer Science, University of California, Irvine, Irvine, CA,.92697 USA Department of Mathematics, University of California, Irvine, Irvine, CA, 92697 USA arXiv:1411.0657v3 [physics.optics] 8 Dec. 2015 {mothman, fyazdi, afigotin, f.capolino}@uci.edu We propose a new approach leading to giant gain enhancement. It is based on unconventional slow wave resonance associated to a degenerate band edge (DBE) in the dispersion diagram for a special class of photonic crystals supporting two modes at each frequency. We show that the gain enhancement in a Fabry-Perot cavity (FPC) when operating at the DBE is several orders of magnitude stronger when compared to a cavity of the same length made of a standard photonic crystal with a regular band edge (RBE). The giant gain condition is explained by a significant increase in the photon lifetime and in the local density of states. We have demonstrated the existence of DBE operated special cavities that provide for superior gain conditions for solid-state lasers, quantum cascade lasers, traveling wave tubes, and distributed solid state amplifiers. We also report the possibility to achieve low-threshold lasing in FPC with DBE compared to RBE-based lasers. I. INTRODUCTION Light confinement using either mirrors or Bragg reflectors provides for high quality (Q)-factor in Fabry-Perot cavity (FPC) resonators and enhanced optical field intensity. Such cavities are commonly used for laser applications and spectroscopy. An important class of high Q-factor structures is formed by slow-wave resonators based on the regular band edge (RBE) of the wavenumber-frequency dispersion diagram relative to photonic crystals, whose simplest architecture is a periodic stack of dielectric layers, with one dimensional periodicity [1–3]. More elaborate designs of nanocavities adopted Silicon heterostructures [4], liquid crystals [5] technologies and demonstrated improved Q-factor compared to previously reported designs. The use of photonic crystals resulted in enhanced amplification properties for low-threshold lasing [2,6], enhanced directional- wave propagation through magneto-optical effects [7–9], nonlinear optics [10] and quantum processing [11]. Pursuing better performing photonic crystal cavities is essential to further advancement of photonic technology [12–15], and photonic integrated circuits [16–18] in particular. These advancement established a basis for a novel class of solar cell architecture with enhanced absorption [19–21], and other thin film applications [22], along with superior atomic interaction with strongly localized photons [23], and unconventional spontaneous emission dynamics [24,25]. Slow light in photonic crystals is yet another fundamental utility that can tailor the electromagnetic response and achieve superior performance through dispersion engineering [26–28]. Figotin and Vitebsky in [29–33], proposed FPC resonators made of unconventional photonic crystals composed by anisotropic dielectric layers. Those FPC resonators exhibit sharper transmission peaks, higher Q-factors, and better general performance in a vicinity of the photonic band edge frequency compare to conventional photonic crystal FPCs of the same size made of isotropic layers. The related field enhancement properties in those unconventional structures can be attributed to the degenerate band edge (DBE) conditions. This special DBE condition produces some four electromagnetic modes (EM) at the DBE frequency; that phenomenon does not occur in regular photonic crystals, i.e., conventional photonic crystals exhibiting an RBE providing a single EM mode operation. Consequently, it is important to acknowledge that the resonance characteristics in DBE cavities studied in this paper are fundamentally different from those in standard band-gap cavities [2,3,34,35]. Significant differences between DBE and RBE based FPCs are highlighted in Sec. II. The principal result of this paper that the DBE condition based on resonance properties discussed in [29–33] lead to giant power gain when an active

Demonstration of a Degenerate Band Edge in Periodically-Loaded Circular Waveguides

We demonstrate the existence of a special degeneracy condition, called degenerate band edge (DBE), between two Bloch modes in periodically-loaded circular all-metallic waveguides at microwave frequencies. The DBE condition has been associated with a dramatic reduction in group velocity and with some unique resonance properties, but it has not been shown in hollow waveguide structures yet. Hence, we show here its existence in two periodic waveguide examples. The unit cell of the first structure is composed of a circular waveguide loaded with two inner cylinders with elliptical irises with misaligned angles. The second structure is composed by loading the waveguide with elliptical rings. The demonstration of DBE in those waveguide is explained through a simple multi-transmission line approach where the conditions to obtain DBE are clarified, and suggests that the DBE can occur in several other analogous periodic waveguides. These structures can be potentially used to investigate unconventional gain schemes in traveling wave tubes or other kinds of distributed amplifiers, oscillators and novel pulse compressors.

Giant amplification in degenerate band edge slow-wave structures interacting with an electron beam

We propose a new amplification regime based on synchronous operation of four degenerate electromagnetic (EM) modes in a slow-wave structure and the electron beam, referred to as super synchronization. These four EM modes arise in a Fabry-Perot cavity (FPC) when degenerate band edge (DBE) condition is satisfied. The modes interact constructively with the electron beam resulting in superior amplification. In particular, much larger gains are achieved for smaller beam currents compared to conventional structures based on synchronization with only a single EM mode. We demonstrate giant gain scaling with respect to the length of the slow-wave structure compared to conventional Pierce type single mode traveling wave tube amplifiers. We construct a coupled transmission line (CTL) model for a loaded waveguide slow-wave structure exhibiting a DBE, and investigate the phenomenon of giant gain via super synchronization using the Pierce model generalized to multimode interaction.

Theory and New Amplification Regime in Periodic Multimodal Slow Wave Structures With Degeneracy Interacting With an Electron Beam

We present the theory of a new amplification regime in traveling wave tubes (TWTs) composed of a slow wave periodic structure that supports multiple electromagnetic modes that can all be synchronized with the electron beam. The interaction between the multimodal slow wave structure and the electron beam is quantified using a multi-transmission line (MTL) approach based on a generalized Pierce model and transfer matrix analysis, leading to the identification of modes with complex Bloch wavenumber. In particular, we address a new possible operation condition for TWTs based on the supersynchronism between an electron beam and four modes exhibiting a degeneracy condition near a band edge of the periodic slow wave MTL. We show a phenomenological change in the band structure of a periodic MTL, where we observe at least two growing modal cooperating solutions as opposed to a uniform MTL, interacting with an electron beam where there is rigorously only one growing modal solution. We discuss the advantage of using such a degeneracy condition in TWTs that leads to larger gain conditions in amplifier regimes and also to very low starting beam current in high-power oscillators.

Low Starting Electron Beam Current in Degenerate Band Edge Oscillators

We propose a new principle of operation in vacuum electron-beam-based oscillators that leads to a low beam current for starting oscillations. The principle is based on supersynchronous operation of an electron beam interacting with four degenerate electromagnetic modes in a slow-wave structure (SWS). The four-mode supersynchronous regime is associated with a very special degeneracy condition in the dispersion diagram of a cold periodic SWS called degenerate band edge (DBE). This regime features a giant group delay in the finite-length SWS and low starting-oscillation beam current. The starting beam current is at least an order of magnitude smaller compared with a conventional backward-wave oscillator of the same length. As a representative example, we consider an SWS conceived by a periodically loaded metallic waveguide supporting a DBE and investigate starting-oscillation conditions using the Pierce theory generalized to coupled transmission lines. The proposed supersynchronism regime can be straightforwardly adapted to waveguide geometries others than the periodically loaded waveguide considered here since DBE is a general property that can be realized in a variety of structures.

Theory of coupled resonator optical waveguides exhibiting high-order exceptional points of degeneracy

We present a novel approach and a theoretical framework for generating high order exceptional points of degeneracy (EPD) in photonic structures based on periodic coupled resonators optical waveguides (CROWs). Such EPDs involve the coalescence of Floquet-Bloch eigenwaves in CROWs, without the presence of gain and loss, which is in contrast to the requirement of Parity-Time (PT) symmetry to develop exceptional points based on gain and loss balance. The EPDs arise here by introducing symmetry breaking in a conventional chain of coupled resonators through coupling the chain of resonators to an adjacent uniform optical waveguide, which leads to unique modal characteristics that cannot be realized in conventional CROWs. Such remarkable characteristics include high quality factors (Q-factor) and strong field enhancement, even without any mirrors at the two ends of a cavity. We show for the first time the capability of CROWs to exhibit EPDs of various order; including the degenerate band edge (DBE) and the stationary inflection point (SIP). The proposed CROW of finite length shows enhanced quality factor when operating near the DBE, and the Q-factor exhibits an anomalous scaling with the CROWs length. We develop the theory of EPDs in such unconventional CROW using coupled-wave equations, and we derive an analytical expression for the dispersion relation. The proposed unconventional CROW concepts have various potential applications including Q-switching, nonlinear devices, lasers, and extremely sensitive sensors.

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically Loaded Circular Waveguide

We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely, the degeneracy of four Bloch modes, in loaded circular metallic waveguides. The four modes forming the DBE represent a degeneracy of the fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. The DBE is associated with four Bloch eigenmodes representing wave propagation in the periodic structure that coalesce in both wavenumbers and eigenvectors (i.e., polarizations), at a single frequency. It leads to a very flat wavenumber-frequency dispersion relation, and the finite-length structure’s quality factor scales as

Theory of Double Ladder Lumped Circuits With Degenerate Band Edge

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