Abstract:We develop the fluctuational electrodynamics of metamaterials with hyperbolic dispersion and show the existence of broadband thermal emission beyond the black body limit in the near field. This arises due to the thermal excitation of unique bulk metamaterial modes, which do not occur in conventional media. We consider a practical realization of the hyperbolic metamaterial and estimate that the effect will be observable using the characteristic dispersion (topological transitions) of the metamaterial states. Our work paves the way for engineering the near-field thermal emission using metamaterials.Engineering the black body thermal emission using artificial media promises to impact a variety of applications involving energy harvesting 1 , thermal management 2 and coherent thermal sources 3 . The usual upper limit to the black-body emission is not fundamental and arises since energy is carried to the far-field only by propagating waves emanating from the heated source. If one allows for energy transport in the near-field using evanescent waves, this limit can be overcome. Thus thermal emission beyond the black body limit is expected due to surface electromagnetic excitations 4 or at the edge of the bandgap in photonic crystals 5 where there is a large enhancement in the photonic density of states. Advances in near field scanning and probing techniques 6-8 have led to conclusive demonstrations of these effects.One limitation of the above mentioned approaches using photonic crystals or surface electromagnetic excitations is that the energy transfer beyond the black body limit (superplanckian 9,10 thermal emission) only occurs in a narrow bandwidth. In this paper, we show that artificial media (metamaterials) with engineered dielectric properties can overcome the limitation of super-planckian thermal emission at a single resonant frequency. Our work rests on the recently discovered singularity in the bulk density of states of metamaterials with hyperbolic dispersion [11][12][13] . The unique property which sets these hyperbolic metamaterials (HMMs) apart from conventional approaches of engineering the photonic density of states (PDOS) is the broad bandwidth in which the PDOS is enhanced.

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 review the properties of hyperbolic metamaterials and show that they are promising candidates as substrates for nano-imaging, nano-sensing, fluorescence engineering and for controlling thermal emission. Hyperbolic metamaterials can support unique bulk modes, tunable surface plasmon polaritons as well as surface hyperbolic states (Dyakonov plasmons) that can be used for a variety of applications. We compare the effective medium predictions with practical realizations of hyperbolic metamaterials to show their potential for radiative decay engineering, bio-imaging, sub-surface sensing, meta-plasmonics and super-Planckian thermal emission.

PEC cells demands the development of stable, economical, and efficient photoanodes. Stable, earth-abundant metal oxides such as TiO 2 , WO 3 , Fe 2 O 3 , and BiVO 4 are popular photoanode candidates. [2,3] However, these metal oxide photoanodes exhibit poor efficiency because they cannot achieve simultaneously high light absorption, charge separation, and charge transfer efficiencies. [4][5][6] One common strategy for improving these metal oxide photoanodes is to decorate them with various plasmonic metals, such as metal nanoparticles or nanorods, to introduce nearfield localized surface plasmon resonance (LSPR) and/or surface plasmon polaritons (SPP). [7][8][9][10][11][12][13][14][15][16][17][18] Many studies on these plasmonic metal nanostructures have focused on the light absorption enhancement effect from LSPR and SPP. [9,[19][20][21][22][23][24][25][26] In addition, LSPR can improve the performance of metal oxide photoanodes through plasmonic energy transfer through two mechanisms: direct electron transfer (DET) and plasmon-induced resonant energy transfer (PIRET). [9,21] DET refers to the hot-electrons injection from plasmonic metal nanoparticles to the conduction band of neighboring metal oxides, and it requires direct contact between metal and metal oxides. [9] PIRET was recently proposed by several pioneering studies. [9,21] PIRET utilizes the nonradiative dipole-dipole coupling between metals and metal oxides to Plasmonic metal nanostructures have been extensively investigated to improve the performance of metal oxide photoanodes for photoelectrochemical (PEC) solar water splitting cells. Most of these studies have focused on the effects of those metal nanostructures on enhancing light absorption and enabling direct energy transfer via hot electrons. However, several recent studies have shown that plasmonic metal nanostructures can improve the PEC performance of metal oxide photoanodes via another mechanism known as plasmon-induced resonant energy transfer (PIRET). However, this PIRET effect has not yet been tested for the molybdenum-doped bismuth vanadium oxide (Mo:BiVO 4 ), regarded as one of the best metal oxide photoanode candidates. Here, this study constructs a hybrid Au nanosphere/Mo:BiVO 4 photoanode interwoven in a hexagonal pattern to investigate the PIRET effect on the PEC performance of Mo:BiVO 4 . This study finds that the Au nanosphere array not only increases light absorption of the photoanode as expected, but also improves both its charge transport and charge transfer efficiencies via PIRET, as confirmed by time-correlated single photon counting and tran-

We consider the process of conversion between linear polarizations as light is reflected from a photonic crystal slab. We observe that, over a wide range of frequencies, complete polarization conversion can be found at isolated wave vectors. Moreover, such an effect is topological: the complex reflection coefficients have a nonzero winding number in the wave vector space. We also show that bound states in continuum in this system have their wave vectors lying on the critical coupling curve that defines the condition for complete polarization conversion. Our work points to the use of topological photonics concepts for the control of polarization, and suggests the exploration of topological properties of scattering matrices as a route towards creating robust optical devices.