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.
Extreme Thermal Materials
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.
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 integration of nanoscale electronics with conventional optical devices is restricted by the diffraction limit of light. Metals can confine light at the subwavelength scales needed, but they are lossy, while dielectric materials do not confine evanescent waves outside a waveguide or resonator, leading to cross talk between components. We show that light can be confined below the diffraction limit using completely transparent artificial media (metamaterials with ?>1, ?=1ε>1, μ=1). Our approach relies on controlling the optical momentum of evanescent waves—an important electromagnetic property overlooked in photonic devices. For practical applications, we propose a class of waveguides using this approach that outperforms the cross-talk performance by 1 order of magnitude as compared to any existing photonic structure. Our work overcomes a critical stumbling block for nanophotonics by completely averting the use of metals and can impact electromagnetic devices from the visible to microwave frequency ranges.
We give a detailed account of equilibrium and non-equilibrium fluctuational electrodynamics of hyperbolic metamaterials. We show the unifying aspects of two different approaches; one utilizes the second kind of fluctuation dissipation theorem and the other makes use of the scattering method. We analyze the near-field of hyperbolic media at finite temperatures and show that the lack of spatial coherence can be attributed to the multi-modal nature of super-Planckian thermal emission. We also adopt the analysis to phonon-polaritonic super-lattice metamaterials and describe the regimes suitable for experimental verification of our predicted effects. The results reveal that far-field thermal emission spectra are dominated by epsilon-near-zero and epsilon-near-pole responses as expected from Kirchoff's laws. Our work should aid both theorists and experimentalists to study complex media and engineer equilibrium and non-equilibrium fluctuations for applications in thermal photonics.
Nanoscale light-matter interaction in the weak-coupling regime has been achieved with unique hyperbolic metamaterial modes possessing a high density of states. Here, we show strong coupling between intersubband transitions (ISBTs) of a multiple quantum well (MQW) slab and the bulk polariton modes of a hyperbolic metamaterial (HMM). These HMM modes have large wave vectors (high-k modes) and are normally evanescent in conventional materials. We analyze a metal-dielectric practical multilayer HMM structure consisting of a highly doped semiconductor acting as a metallic layer and an active multiple quantum well dielectric slab. We observe delocalized metamaterial mode interaction with the active materials distributed throughout the structure. Strong coupling and characteristic anticrossing with a maximum Rabi splitting (RS) energy of up to 52 meV is predicted between the high-k mode of the HMM and the ISBT, a value approximately 10.5 times greater than the ISBT linewidth and 4.5 times greater than the material loss of the structure. The scalability and tunability of the RS energy in an active semiconductor metamaterial device have potential applications in quantum well infrared photodetectors and intersubband light-emitting devices.
Metamaterials are nano-engineered media with designed properties beyond those available in nature with applications in all aspects of materials science. In particular, metamaterials have shown promise for next generation optical materials with electromagnetic responses that cannot be obtained from conventional media. We review the fundamental properties of metamaterials with hyperbolic dispersion and present the various applications where such media offer potential for transformative impact. These artificial materials support unique bulk electromagnetic states which can tailor light-matter interaction at the nanoscale. We present a unified view of practical approaches to achieve hyperbolic dispersion using thin film and nanowire structures. We also review current research in the field of hyperbolic metamaterials such as sub-wavelength imaging and broadband photonic density of states engineering. The review introduces the concepts central to the theory of hyperbolic media as well as nanofabrication and characterization details essential to experimentalists. Finally, we outline the challenges in the area and offer a set of directions for future work.
We introduce the photonic analog of electronic van Hove singularities (VHS) in artificial media (metamaterials) with hyperbolic dispersion. Unlike photonic and electronic crystals, the VHS in metamaterials are unrelated to the underlying periodicity and occur due to slow-light modes in the structure. We show that the VHS characteristics are manifested in the near-field local density of optical states in spite of the losses, dispersion, and finite unit-cell size of the hyperbolic metamaterial. Finally, we show that this work should lead to quantum, thermal, nanolasing, and biosensing applications of van Hove singularities in hyperbolic metamaterials achievable by current fabrication technology.
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 propose an approach to enhance and direct the spontaneous emission from isolated emitters embedded inside hyperbolic metamaterials (HMMs) into single-photon beams. The approach rests on collective plasmonic Bloch modes of HMMs, which propagate in highly directional beams called quantum resonance cones. We propose a pumping scheme using the transparency window of the HMM that occurs near the topological transition. Finally, we address the challenge of outcoupling these broadband resonance cones into vacuum using a dielectric bullseye grating. We give a detailed analysis of quenching and design the metamaterial to have a huge Purcell factor in a broad bandwidth in spite of the losses in the metal. Our work should help motivate experiments in the development of single-photon sources for broadband emitters such as nitrogen vacancy centers in diamond.
We propose a method for engineering thermally excited far field electromagnetic radiation using epsilon-near-zero metamaterials and introduce a new class of artificial media: epsilon-near-pole metamaterials. We also introduce the concept of high temperature plasmonics as conventional metamaterial building blocks have relatively poor thermal stability. Using our approach, the angular nature, spectral position, and width of the thermal emission and optical absorption can be finely tuned for a variety of applications. In particular, we show that these metamaterial emitters near 1500 K can be used as part of thermophotovoltaic devices to surpass the full concentration Shockley-Queisser limit of 41%. Our work paves the way for high temperature thermal engineering applications of metamaterials.
We review the properties of hyperbolic metamaterials and show that they are promising candidates as substrates for nanoimaging, nanosensing, fluorescence engineering, and controlling thermal emission. Hyperbolic metamaterials can support unique bulk modes, tunable surface plasmon polaritons, and 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, bioimaging, subsurface sensing, metaplasmonics, and super-Planckian thermal emission.
Engineering optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this paper, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials, using the broadband enhancement in the spontaneous emission to construct single-photon sources.
Light-matter interactions can be controlled by manipulating the photonic environment. We uncovered an optical topological transition in strongly anisotropic metamaterials that results in a dramatic increase in the photon density of states—an effect that can be used to engineer this interaction. We describe a transition in the topology of the iso-frequency surface from a closed ellipsoid to an open hyperboloid by use of artificially nanostructured metamaterials. We show that this topological transition manifests itself in increased rates of spontaneous emission of emitters positioned near the metamaterial. Altering the topology of the iso-frequency surface by using metamaterials provides a fundamentally new route to manipulating light-matter interactions.
Inverse design techniques in the context of nanophotonics have helped in discovery of compact and counter-intuitive structures/shapes. We introduce the concept of spectral domain inverse design to search through the optical trade-space (dispersive permittivity) of nanocomposite metamaterials. We develop a hybrid optimization technique that combines genetic algorithms and gradient descent methods. We utilize this technique to inverse design an ultra-thin thermophotovoltaic emitter coating material. Our work can lead to an efficient approach to search for new multi-functional optical/thermal metamaterials with desired complex permittivity.
The interplay of spin angular momentum and thermal radiation is a frontier area of interest to nanophotonics as well as topological physics. Here, we show that a thick planar slab of a nonreciprocal material, despite being at thermal equilibrium with its environment, can exhibit nonzero photon spin angular momentum and nonzero radiative heat flux in its vicinity. We identify them as the persistent thermal photon spin and the persistent planar heat current respectively. With a practical example system, we reveal that the fundamental origin of these phenomena is connected to the spin-momentum locking of thermally excited evanescent waves. We also discover spin magnetic moment of surface polaritons that further clarifies these features. We then propose an imaging experiment based on Brownian motion that allows one to witness these surprising features by directly looking at them using a lab microscope. We further demonstrate the universal behavior of these near-field thermal radiation phenomena through a comprehensive analysis of gyroelectric, gyromagnetic and magneto-electric nonreciprocal materials. Together, these results expose a surprisingly little explored research area of thermal spin photonics with prospects for new avenues related to non-Hermitian topological photonics and radiative heat transport.
High-temperature thermal photonics presents unique challenges for engineers as the database of materials that can withstand extreme environments are limited. In particular, ceramics with high temperature stability that can support coupled light-matter excitations, that is, polaritons, open new avenues for engineering radiative heat transfer. Hexagonal boron nitride (hBN) is an emerging ceramic 2D material that possesses low-loss polaritons in two spectrally distinct mid-infrared frequency bands. The hyperbolic nature of these frequency bands leads to a large local density of states (LDOS). In 2D form, these polaritonic states are dark modes, bound to the material. In cylindrical form, boron nitride nanotubes (BNNTs) create subwavelength particles capable of coupling these dark modes to radiative ones. In this study, we leverage the high-frequency optical phonons present in BNNTs to create strong mid-IR thermal antenna emitters at high temperatures (938 K). Through direct measurement of thermal emission of a disordered system of BNNTs, we confirm their radiative polaritonic modes and show that the antenna behavior can be observed even in a disordered system. These are among the highest-frequency optical phonon polaritons that exist and could be used as high-temperature mid-IR thermal nanoantenna sources.
Fast electrons interacting with matter have been instrumental for probing bulk and surface photonic excitations including Cherenkov radiation and plasmons. Additionally, fast electrons are ideal to investigate unique bulk and longitudinal photonic modes in hyperbolic materials at large wavevectors difficult to probe optically. Here, we use momentum-resolved electron energy loss spectroscopy (k-EELS) to perform the first experimental demonstration of high-k modes and hyperbolic Cherenkov radiation in the natural hyperbolic material Bi2Te3. This work establishes Bi2Te3 as one of the few viable natural hyperbolic materials in the visible and paves the way for k-EELS as a fundamental tool to probe hyperbolic media.
The discovery of photonic hyperbolic dispersion surfaces in certain van der Waals bonded solids, such as hexagonal boron nitride and bismuth selenide (a topological insulator), offers intriguing possibilities for creating strongly modified light-matter interactions. However, open problems exist in quantifying electromagnetic field fluctuations in these media, complicating typical approaches for modeling photonic characteristics. Here, we address this issue by linking the identifying traits of hyperbolic response to a coupling between longitudinal and transverse fields that cannot occur in isotropic media. This description allows us to formulate a gauge theoretic description of the influence of hyperbolic response on electromagnetic fluctuations without explicitly imposing a characteristic size (model of nonlocality)—leading to formally bounded expressions so long as material absorption is included. We then apply this framework to two exemplary areas: the optical sum rule for modified spontaneous emission enhancement in a general uniaxial medium and thermal electromagnetic field fluctuations in hexagonal boron nitride and bismuth selenide. We find that while the sum rule is satisfied, it does not constrain the enhancement of light-matter interactions in either case. We also show that both hexagonal boron nitride and bismuth selenide possess broad spectral regions where the magnitude of electromagnetic field fluctuations are over 120 times larger, and over 800 times larger along specific angular directions, than they are in vacuum.
Silicon is widely used as the material of choice for semiconductor and insulator applications in nanoelectronics, micro-electro-mechanical systems, solar cells, and on-chip photonics. In stark contrast, in this paper, we explore silicon’s metallic properties and show that it can support propagating surface plasmons, collective charge oscillations, in the extreme ultraviolet (EUV) energy regime not possible with other plasmonic materials such as aluminum, silver, or gold. This is fundamentally different from conventional approaches, where doping semiconductors is considered necessary to observe plasmonic behavior. We experimentally map the photonic band structure of EUV surface and bulk plasmons in silicon using momentum-resolved electron energy loss spectroscopy. Our experimental observations are validated by macroscopic electrodynamic electron energy loss theory simulations as well as quantum density functional theory calculations. As an example of exploiting these EUV plasmons for applications, we propose a tunable and broadband thresholdless Cherenkov radiation source in the EUV using silicon plasmonic metamaterials. Our work can pave the way for the field of EUV plasmonics.
Dipole-dipole interactions (Vdd) between closely spaced atoms and molecules are related to real photon and virtual photon exchange between them and decrease in the near field connected with the characteristic Coulombic dipole field law. The control and modification of this marked scaling with distance have become a long-standing theme in quantum engineering since dipole-dipole interactions govern Van der Waals forces, collective Lamb shifts, atom blockade effects, and Förster resonance energy transfer. We show that metamaterials can fundamentally modify these interactions despite large physical separation between interacting quantum emitters. We demonstrate a two orders of magnitude increase in the near-field resonant dipole-dipole interactions at intermediate field distances (10 times the near field) and observe the distance scaling law consistent with a super-Coulombic interaction theory curtailed only by absorption and finite size effects of the metamaterial constituents. We develop a first-principles numerical approach of many-body dipole-dipole interactions in metamaterials to confirm our theoretical predictions and experimental observations. In marked distinction to existing approaches of engineering radiative interactions, our work paves the way for controlling long-range dipole-dipole interactions using hyperbolic metamaterials and natural hyperbolic two-dimensional materials.
An optical topological transition is defined as the change in the photonic iso-frequency surface around epsilon-near-zero (ENZ) frequencies which can considerably change the spontaneous emission of a quantum emitter placed near a metamaterial slab. Here, we show that due to the strong Kerr nonlinearity at ENZ frequencies, a high-power pulse can induce a sudden transition in the topology of the iso-frequency dispersion curve, leading to a significant change in the transmission of propagating as well as evanescent waves through the metamaterial slab. This evanescent wave switch effect allows for the control of spontaneous emission through modulation of the Purcell effect. We develop a theory of the enhanced nonlinear response of ENZ media to s and p polarized inputs and show that this nonlinear effect is stronger for p polarization and is almost independent of the incident angle. We perform finite-difference time-domain simulations to demonstrate the transient response of the metamaterial slab to an ultrafast pulse and fast switching of the Purcell effect at the sub-picosecond scale. The Purcell factor changes at ENZ by almost a factor of three which is an order of magnitude stronger than that away from ENZ. We also show that due to the inhomogeneous spatial field distribution inside the multilayer metal-dielectric super-lattice, a unique spatial topological transition metamaterial can be achieved by the control pulse induced nonlinearity. Our work can lead to ultra-fast control of quantum phenomena in ENZ metamaterials.
Thermal radiation from an unpatterned object is similar to that of a gray body. The thermal emission is insensitive to polarization, shows only Lambertian angular dependence, and is well modeled as the product of the blackbody distribution and a scalar emissivity over large frequency bands. Here, we design, fabricate and experimentally characterize the spectral, polarization, angular and temperature dependence of a microstructured SiC dual band thermal infrared source; achieving independent control of the frequency and polarization of thermal radiation in two spectral bands. The measured emission of the device in the Reststrahlen band (10.3–12.7 µm) selectively approaches that of a blackbody, peaking at an emissivity of 0.85 at λx=11.75μm and 0.81 at λy=12.25μm. This effect arises due to the thermally excited phonon polaritons in silicon carbide. The control of thermal emission properties exhibited by the design is well suited for applications requiring infrared sources, gas or temperature sensors and nanoscale heat transfer. Our work paves the way for future silicon carbide based thermal metasurfaces.