The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.
We ask the question, what are the ideal characteristics of a near-field thermophotovoltaic cell? Our search leads us to a reformulation of near-field radiative heat transfer in terms of the joint density of electronic states of the emitter-absorber pair in the thermophotovoltaic system. This form reveals that semiconducting materials with narrowband absorption spectra are critical to the energy-conversion efficiency. This essential feature is unavailable in conventional bulk semiconductor cells but can be obtained using low-dimensional materials. Our results show that the presence of matched van Hove singularities resulting from quantum confinement in the emitter and absorber of a thermophotovoltaic cell boosts both the magnitude and spectral selectivity of radiative heat transfer, dramatically improving energy-conversion efficiency. We provide a model near-field thermophotovoltaic system design making use of this idea by employing the van Hove singularities present in carbon nanotubes. Shockley-Queisser analysis shows that the predicted heat transfer characteristics of this model device are fundamentally better than existing thermophotovoltaic designs. Our work paves the way for the use of quantum dots, quantum wells, two-dimensional semiconductors, semiconductor nanowires, and carbon nanotubes as future materials for thermophotovoltaic cells.
We report the use of epsilon near zero (ENZ) metamaterial to control spontaneous emission from Zinc-Oxide (ZnO) excitons. The ENZ material consists of alternating layers of silver and alumina with subwavelength thicknesses, resulting in an effective medium where one of the components of the dielectric constant approach zero between 370nm-440nm wavelength range. Bulk ZnO with photoluminescence maximum in the ENZ regime was deposited via atomic layer deposition to obtain a smooth film with near field coupling to the ENZ metamaterial. Preferential emission from the ZnO layer into the metamaterial with suppression of forward emission by 90% in comparison to ZnO on silicon is observed. We attribute this observation to the presence of dispersionless plasmonic modes in the ENZ regime as shown by the results of theoretical modeling presented here. Integration of ENZ metamaterials with light emitters is an attractive platform for realizing a low threshold subwavelength laser.
We report on the optical and physical characterization of metallic nanowire (NW) metamaterials fabricated by electrodeposition of ≈30 nm≈30 nm diameter gold nanowires in nanoporous anodic aluminum oxide. We observe a uniaxial anisotropic dielectric response for the NW metamaterials that displays both epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) resonances. We show that a fundamental difference in the behavior of NW metamaterials from metal-dielectric multilayer (ML) metamaterials is the differing directions of the ENZ and ENP dielectric responses relative to the optical axis of the effective dielectric tensor. In contrast to multilayer metamaterials, nanowire metamaterials exhibit an omnidirectional ENP and an angularly dependent ENZ. Also in contrast to ML metamaterials, the NW metamaterials exhibit ENP and ENZ resonances that are highly absorptive and can be effectively excited from free space. Our fabrication allows a large tunability of the epsilon-near-zero resonance in the visible and near-IR spectrum from 583 to 805 nm as the gold nanorod fill fraction changes from 26% to 10.5%. We support our fabrication process flow at each step with rigorous physical and optical characterization. Energy dispersive x-ray (EDX) and x-ray diffraction (XRD) analyses are used to ascertain the quality of electrochemically deposited Au nanowires prior to and after annealing. Our experimental results are in agreement with simulations of the periodic plasmonic crystal and also analytical calculations in the effective medium metamaterial limit. We also experimentally characterize the role of spatial dispersion at the ENZ resonance and show that the effect does not occur for the ENP resonance. The application of these materials to the fields of biosensing, quantum optics, and thermal devices shows considerable promise.
Recently, we proposed a paradigm shift in light confinement strategy showing how relaxed total internal reflection and photonic skin-depth engineering can lead to sub-diffraction waveguides without metal [Optica 1, 96 (2014) [CrossRef] ]. Here, we show that such extreme-skin-depth (e-skid) waveguides can counterintuitively confine light better than the best-case all-dielectric design of high index silicon waveguides surrounded by vacuum. We also establish analytically that figures of merit related to light confinement in dielectric waveguides are fundamentally tied to the skin depth of waves in the cladding, a quantity surprisingly overlooked in dielectric photonics. We contrast the propagation characteristics of the fundamental mode of e-skid waveguides and conventional waveguides to show that the decay constant in the cladding is dramatically larger in e-skid waveguides, which is the origin of sub-diffraction confinement. We also propose an approach to verify the reduced photonic skin depth in experiment using the decrease in the Goos–Hanschen phase shift. Finally, we provide a generalization of our work using concepts of transformation optics where the photonic skin-depth engineering can be interpreted as a transformation on the momentum of evanescent waves.
Hyperbolic metamaterials (HMMs) have recently garnered much attention because they possess the potential for broadband manipulation of the photonic density of states and subwavelength light confinement. These exceptional properties arise due to the excitation of electromagnetic states with high momentum (high-?k modes). However, a major hindrance to practical applications of HMMs is the difficulty in coupling light out of these modes because they become evanescent at the surface of the metamaterial. Here we report the first demonstration, to our knowledge, of simultaneous spontaneous emission enhancement and outcoupling of high-?k modes in an active HMM using a high-index-contrast bullseye grating. Quantum dots embedded inside the metamaterial are used for local excitation of high-?k modes. This demonstration could pave the way for the development of photonic devices such as single-photon sources, ultrafast LEDs, and true nanoscale lasers.
We observe unique absorption resonances in silver/silica multilayer-based epsilon-near-zero (ENZ) metamaterials that are related to radiative bulk plasmon-polariton states of thin-films originally studied by Ferrell (1958) and Berreman (1963). In the local effective medium, metamaterial description, the unique effect of the excitation of these microscopic modes is counterintuitive and captured within the complex propagation constant, not the effective dielectric permittivities. Theoretical analysis of the band structure for our metamaterials shows the existence of multiple Ferrell–Berreman branches with slow light characteristics. The demonstration that the propagation constant reveals subtle microscopic resonances can lead to the design of devices where Ferrell–Berreman modes can be exploited for practical applications ranging from plasmonic sensing to imaging and absorption enhancement.
We have studied angular distribution of emission of dye molecules deposited on lamellar metal/dielectric and Si/Ag nanowire based metamaterials with hyperbolic dispersion. In agreement with the theoretical prediction, the emission pattern of dye on top of lamellar metamaterial is similar to that on top of metal. At the same time, the effective medium model predicts the emission patterns of the nanowire array and the dye film deposited on glass to be nearly identical to each other. This is not the case of our experiment. We tentatively explain the nearly Lambertian (∝cosθ) angular distribution of emission of the nanowire based sample by a surface roughness.
The traditional approaches of exciting plasmons consist of either using electrons (e.g., electron energy loss spectroscopy) or light (Kretchman and Otto geometry) while more recently plasmons have been excited even by single photons. A different approach: thermal excitation of a plasmon resonance at high temperatures using alternate plasmonic media was proposed by S. Molesky et al. [Opt. Express 21, A96–A110 (2013)]. Here, we show how the long-standing search for a high temperature narrowband near-field emitter for thermophotovoltaics can be fulfilled by thermally exciting plasmons. We also describe a method to control Wein's displacement law in the near-field using high temperature epsilon-near-zero metamaterials. Finally, we show that our work opens up an interesting direction of research for the field of slow light: thermal emission control.