Radiative Cooling Properties of Portlandite and Tobermorite: Two Cementitious Minerals of Great Relevance in Concrete Science and Technology.

Although concrete and cement-based materials are the most engineered materials employed by mankind, their potential for use in daytime radiative cooling applications has yet to be fully explored. Due to its complex structure, which is composed of multiple phases and textural details, fine-tuning of concrete is impossible without first analyzing its most important ingredients. Here, the radiative cooling properties of Portlandite (Ca(OH)2) and Tobermorite (Ca5Si6O16(OH)2·4H2O) are studied due to their crucial relevance in cement and concrete science and technology. Our findings demonstrate that, in contrast to concrete (which is a strong infrared emitter but a poor sun reflector), both Portlandite and Tobermorite exhibit good radiative cooling capabilities. These results provide solid evidence that, with the correct optimization of composition and porosity, concrete can be transformed into a material suitable for daytime radiative cooling.

Review on the Scientific and Technological Breakthroughs in Thermal Emission Engineering.

The emission of thermal radiation is a physical process of fundamental and technological interest. From different approaches, thermal radiation can be regarded as one of the basic mechanisms of heat transfer, as a fundamental quantum phenomenon of photon production, or as the propagation of electromagnetic waves. However, unlike light emanating from conventional photonic sources, such as lasers or antennas, thermal radiation is characterized for being broadband, omnidirectional, and unpolarized. Due to these features, ultimately tied to its inherently incoherent nature, taming thermal radiation constitutes a challenging issue. Latest advances in the field of nanophotonics have led to a whole set of artificial platforms, ranging from spatially structured materials and, much more recently, to time-modulated media, offering promising avenues for enhancing the control and manipulation of electromagnetic waves, from far- to near-field regimes. Given the ongoing parallelism between the fields of nanophotonics and thermal emission, these recent developments have been harnessed to deal with radiative thermal processes, thereby forming the current basis of thermal emission engineering. In this review, we survey some of the main breakthroughs carried out in this burgeoning research field, from fundamental aspects to theoretical limits, the emergence of effects and phenomena, practical applications, challenges, and future prospects.

New Horizons in Near-Zero Refractive Index Photonics and Hyperbolic Metamaterials.

The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide efficient knobs to control light-matter interactions. This Perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular, light and thermal emission, nonlinear optics, sensing applications, and time-varying photonics.

Addressing the Impact of Surface Roughness on Epsilon-Near-Zero Silicon Carbide Substrates.

Epsilon-near-zero (ENZ) media have been very actively investigated due to their unconventional wave phenomena and strengthened nonlinear response. However, the technological impact of ENZ media will be determined by the quality of realistic ENZ materials, including material loss and surface roughness. Here, we provide a comprehensive experimental study of the impact of surface roughness on ENZ substrates. Using silicon carbide (SiC) substrates with artificially induced roughness, we analyze samples whose roughness ranges from a few to hundreds of nanometer size scales. It is concluded that ENZ substrates with roughness in the few nanometer scale are negatively affected by coupling to longitudinal phonons and strong ENZ fields normal to the surface. On the other hand, when the roughness is in the hundreds of nanometers scale, the ENZ band is found to be more robust than dielectric and surface phonon polariton (SPhP) bands.

Incandescent temporal metamaterials.

Regarded as a promising alternative to spatially shaping matter, time-varying media can be seized to control and manipulate wave phenomena, including thermal radiation. Here, based upon the framework of macroscopic quantum electrodynamics, we elaborate a comprehensive quantum theoretical formulation that lies the basis for investigating thermal emission effects in time-modulated media. Our theory unveils unique physical features brought about by time-varying media: nontrivial correlations between fluctuating electromagnetic currents at different frequencies and positions, thermal radiation overcoming the black-body spectrum, and quantum vacuum amplification effects at finite temperature. We illustrate how these features lead to striking phenomena and innovative thermal emitters, specifically, showing that the time-modulation releases strong field fluctuations confined within epsilon-near-zero (ENZ) bodies, and that, in turn, it enables a narrowband (partially coherent) emission spanning the whole range of wavevectors, from near to far-field regimes.

Effect of Epsilon-Near-Zero Modes on the Casimir Interaction between Ultrathin Films.

Vacuum fluctuation-induced interactions between macroscopic metallic objects result in an attractive force between them, a phenomenon known as the Casimir effect. This force is the result of both plasmonic and photonic modes. For very thin films, field penetration through the films will modify the allowed modes. Here, we theoretically investigate the Casimir interaction between ultrathin films from the perspective of force distribution over real frequencies for the first time. Pronounced repulsive contributions to the force are found due to the highly confined and nearly dispersion-free epsilon-near-zero (ENZ) modes that only exist in ultrathin films. These contributions persistently occur around the ENZ frequency of the film irrespective of the interfilm separation. We further associate the ENZ modes with a striking thickness dependence of a proposed figure of merit (FOM) for conductive thin films, suggesting that the motion of objects induced by Casimir interactions is boosted for deeply nanoscale sizes. Our results shed light on the correlation between special electromagnetic modes and the vacuum fluctuation-induced force as well as the resulting mechanical properties of ultrathin ENZ materials, which may create new opportunities for engineering the motion of ultrasmall objects in nanomechanical systems.

Noise Cancellation Effects in Integrated Photonics with Wilkinson Power Dividers.

Wilkinson power dividers (WPDs) are a popular element in RF and microwave technologies known for providing isolation capabilities. However, the benefits that WPDs could offer to integrated photonic systems are far less studied. Here, we investigate the thermal emission from and the noise performance of silicon-on-insulator (SOI) WPDs. We find that WPDs exhibit a noiseless port, with important implications for receiving systems and absorption-based quantum state transformations. At the same time, the thermal signals exiting noisy ports exhibit nontrivial correlations, opening the possibility for noise cancellation. We analyze passive and active networks containing WPDs showing how such nontrivial correlations can prevent the amplification of the thermal noise introduced by WPDs while benefiting from their isolation capabilities. Using this insight, we propose a modified ring-resonator amplifier that improves by N times the SNR in comparison with conventional traveling wave and ring-resonator amplifiers, with N being the number of inputs/outputs of the WPD. We believe that our results represent an important step forward in the implementation of SOI-WPDs and their integration in complex photonic networks, particularly for mid-IR and quantum photonics applications.

Suppressed-scattering spectral windows for radiative cooling applications.

The scattering of light by resonant nanoparticles is a key process for enhancing the solar reflectance in daylight radiative cooling. Here, we investigate the impact of material dispersion on the scattering performance of popular nanoparticles for radiative cooling applications. We show that, due to material dispersion, nanoparticles with a qualitatively similar response at visible frequencies exhibit fundamentally different scattering properties at infrared frequencies. It is found that dispersive nanoparticles exhibit suppressed-scattering windows, allowing for selective thermal emission within a highly reflective sample. The existence of suppressed-scattering windows solely depends on material dispersion, and they appear pinned to the same wavelength even in random composite materials and periodic metasurfaces. Finally, we investigate calcium-silicate-hydrate (CSH), the main phase of concrete, as an example of a dispersive host, illustrating that the co-design of nanoparticles and host allows for tuning of the suppressed-scattering windows. Our results indicate that controlled nanoporosities would enable concrete with daylight passive radiative cooling capabilities.

Spectrally stable thermal emitters enabled by material-based high-impedance surfaces.

Radiative thermal engineering with subwavelength metallic bodies is a key element for heat and energy management applications, communication and sensing. Here, we numerically and experimentally demonstrate metallic thermal emitters with narrowband but extremely stable emission spectra, whose resonant frequency does not shift with changes on the nanofilm thickness, the angle of observation and/or polarization. Our devices are based on epsilon-near-zero (ENZ) substrates acting as material-based high-impedance substrates. They do not require from complex nanofabrication processes, thus being compatible with large-area and low-cost applications.

Generalized approach to quantum interference in lossy N-port devices via a singular value decomposition.

Modeling quantum interference in the presence of dissipation is a critical aspect of quantum technologies. Including dissipation into the model of a linear device enables for assessing the detrimental impact of photon loss, as well as for studying dissipation-driven quantum state transformations. However, establishing the input-output relations characterizing quantum interference at a general lossy N-port network poses important theoretical challenges. Here, we propose a general procedure based on the singular value decomposition (SVD), which allows for the efficient calculation of the input-output relations for any arbitrary lossy linear device. In addition, we show how the SVD provides an intuitive description of the principle of operation of linear optical devices. We illustrate the applicability of our method by evaluating the input-output relations of popular reciprocal and nonreciprocal lossy linear devices, including devices with singular and nilpotent scattering matrices. Our method also enables the analysis of quantum interference in large lossy networks, as we exemplify with the study of an N-port epsilon-near-zero (ENZ) hub. We expect that our procedure will motivate future research on quantum interference in complex devices, as well as the realistic modelling of photon loss in linear lossy devices.

Direct observation of ideal electromagnetic fluids.

Near-zero-index (NZI) media have been theoretically identified as media where electromagnetic radiations behave like ideal electromagnetic fluids. Within NZI media, the electromagnetic power flow obeys equations similar to those of motion for the velocity field in an ideal fluid, so that optical turbulence is intrinsically inhibited. Here, we experimentally observe the electromagnetic power flow distribution of such an ideal electromagnetic fluid propagating within a cutoff waveguide by a semi-analytical reconstruction technique. This technique provides direct proof of the inhibition of electromagnetic vorticity at the NZI frequency, even in the presence of complex obstacles and topological changes in the waveguide. Phase uniformity and spatially-static field distributions, essential characteristics of NZI materials, are also observed. Measurement of the same structure outside the NZI frequency range reveals existence of vortices in the power flow, as expected for conventional optical systems. Therefore, our results provide an important step forward in the development of ideal electromagnetic fluids, and introduce a tool to explore the subwavelength behavior of NZI media including fully vectorial and phase information.

Dispersion coding of ENZ media via multiple photonic dopants.

Epsilon-near-zero (ENZ) media are opening up exciting opportunities to observe exotic wave phenomena. In this work, we demonstrate that the ENZ medium comprising multiple dielectric photonic dopants would yield a comb-like dispersion of the effective permeability, with each magnetic resonance dominated by one specific dopant. Furthermore, at multiple frequencies of interest, the resonant supercouplings appearing or not can be controlled discretely via whether corresponding dopants are assigned or not. Importantly, the multiple dopants in the ENZ host at their magnetic resonances are demonstrated to be independent. Based on this platform, the concept of dispersion coding is proposed, where photonic dopants serve as "bits" to program the spectral response of the whole composite medium. As a proof of concept, a compact multi-doped ENZ cavity is fabricated and experimentally characterized, whose transmission spectrum is manifested as a multi-bit reconfigurable frequency comb. The dispersion coding is demonstrated to fuel a batch of innovative applications including dynamically tunable comb-like dispersion profiled filters, radio-frequency identification tags, etc.

Geometry-independent antenna based on Epsilon-near-zero medium.

It is well known that electromagnetic radiation from radiating elements (e.g., antennas, apertures, etc.) shows dependence on the element's geometry shape in terms of operating frequencies. This basic principle is ubiquitous in the design of radiators in multiple applications spanning from microwave, to optics and plasmonics. The emergence of epsilon-near-zero media exceptionally allows for an infinite wavelength of electromagnetic waves, manifesting exotic spatially-static wave dynamics which is not dependent on geometry. In this work, we analyze theoretically and verify experimentally such geometry-independent features for radiation, thus presenting a novel class of radiating resonators, i.e., antennas, with an operating frequency irrelevant to the geometry shape while only determined by the host material's dispersions. Despite being translated into different shapes and topologies, the designed epsilon-near-zero antenna resonates at a same frequency, while exhibiting very different far-field radiation patterns, with beams varying from wide to narrow, or even from single to multiple. Additionally, the photonic doping technique is employed to facilitate the high-efficiency radiation. The material-determined geometry-independent radiation may lead to numerous applications in flexible design and manufacturing for wireless communications, sensing, and wavefront engineering.

Momentum considerations inside near-zero index materials.

Near-zero index (NZI) materials, i.e., materials having a phase refractive index close to zero, are known to enhance or inhibit light-matter interactions. Most theoretical derivations of fundamental radiative processes rely on energetic considerations and detailed balance equations, but not on momentum considerations. Because momentum exchange should also be incorporated into theoretical models, we investigate momentum inside the three categories of NZI materials, i.e., inside epsilon-and-mu-near-zero (EMNZ), epsilon-near-zero (ENZ) and mu-near-zero (MNZ) materials. In the context of Abraham-Minkowski debate in dispersive materials, we show that Minkowski-canonical momentum of light is zero inside all categories of NZI materials while Abraham-kinetic momentum of light is zero in ENZ and MNZ materials but nonzero inside EMNZ materials. We theoretically demonstrate that momentum recoil, transfer momentum from the field to the atom and Doppler shift are inhibited in NZI materials. Fundamental radiative processes inhibition is also explained due to those momentum considerations inside three-dimensional NZI materials. Absence of diffraction pattern in slits experiments is seen as a consequence of zero Minkowski momentum. Lastly, consequence on Heisenberg inequality, microscopy applications and on the canonical momentum as generator of translations are discussed. Those findings are appealing for a better understanding of fundamental light-matter interactions at the nanoscale as well as for lasing applications.

Silicon carbide as a material-based high-impedance surface for enhanced absorption within ultra-thin metallic films.

The absorption of infrared radiation within ultra-thin metallic films is technologically relevant for different thermal engineering applications and optoelectronic devices, as well as for fundamental research on sub-nanometer and atomically-thin materials. However, the maximal attainable absorption within an ultra-thin metallic film is intrinsically limited by both its geometry and material properties. Here, we demonstrate that material-based high-impedance surfaces enhance the absorptivity of the films, potentially leading to perfect absorption for optimal resistive layers, and a fourfold enhancement for films at deep nanometer scales. Moreover, material-based high-impedance surfaces do not suffer from spatial dispersion and the geometrical restrictions of their metamaterial counterparts. We provide a proof-of-concept experimental demonstration by using titanium nanofilms on top of a silicon carbide substrate.

Near-zero-index media as electromagnetic ideal fluids.

Near-zero-index (NZI) supercoupling, the transmission of electromagnetic waves inside a waveguide irrespective of its shape, is a counterintuitive wave effect that finds applications in optical interconnects and engineering light-matter interactions. However, there is a limited knowledge on the local properties of the electromagnetic power flow associated with supercoupling phenomena. Here, we theoretically demonstrate that the power flow in two-dimensional (2D) NZI media is fully analogous to that of an ideal fluid. This result opens an interesting connection between NZI electrodynamics and fluid dynamics. This connection is used to explain the robustness of supercoupling against any geometrical deformation, to enable the analysis of the electromagnetic power flow around complex geometries, and to examine the power flow when the medium is doped with dielectric particles. Finally, electromagnetic ideal fluids where the turbulence is intrinsically inhibited might offer interesting technological possibilities, e.g., in the design of optical forces and for optical systems operating under extreme mechanical conditions.

Soft surfaces and enhanced nonlinearity enabled via epsilon-near-zero media doped with zero-area perfect electric conductor inclusions.

Introducing a dielectric inclusion inside an epsilon-near-zero (ENZ) host has been shown to dramatically affect the effective permeability of the host for a TM-polarized incident wave, a concept coined as photonic doping [Science355, 1058 (2017)SCIEAS0036-807510.1126/science.aal2672]. Here, we theoretically study the prospect of doping the ENZ host with infinitesimally thin perfect electric conductor (PEC) inclusions, which we call "zero-area" PEC dopants. First, we theoretically demonstrate that zero-area PEC dopants enable the design of soft surfaces with an arbitrary cross-sectional geometry. Second, we illustrate the possibility of engineering the PEC dopants with the goal of transforming the electric field distribution inside the ENZ while maintaining a spatially invariant magnetic field. We exploit this property to enhance the effective nonlinearity of the ENZ host.

Structural dispersion-based reduction of loss in epsilon-near-zero and surface plasmon polariton waves.

The field of plasmonics has substantially affected the study of light-matter interactions at the subwavelength scale. However, dissipation losses still remain an inevitable obstacle in the development of plasmonic-based wave propagation. Although different materials with moderate losses are being extensively studied, absorption arguably continues to be the key challenge in the field. Here, we theoretically and numerically investigate a different route toward the reduction of loss in propagating plasmon waves. Rather than focusing on a material-based approach, we take advantage of structural dispersion in waveguides to manipulate effective material parameters, thus leading to smaller losses. The potential of this approach is illustrated with two examples: plane-wave propagation within a bulk epsilon-near-zero medium and surface plasmon polariton propagation at the interface of a medium with negative permittivity. We provide the recipe for a practical implementation at mid-infrared frequencies. Our results might represent an important step toward the development of low-loss plasmonic technologies.

Substrate-integrated photonic doping for near-zero-index devices.

Near-zero-index (NZI) media, a medium with near zero permittivity and/or permeability, exhibits unique wave phenomena and exciting potential for multiple applications. However, previous proof-of-concept realizations of NZI media based on bulky and expensive platforms are not easily compatible with low-cost and miniaturization demands. Here, we propose the method of substrate-integrated (SI) photonic doping, enabling the implementation of NZI media within a printed circuit board (PCB) integrated design. Additionally, the profile of the NZI device is reduced by half by using symmetries. We validate the concept experimentally by demonstrating NZI supercoupling in straight and curve substrate integrated waveguides, also validating properties of position-independent photonic doping, zero-phase advance and finite group delay. Based on this platform, we propose design of three NZI devices: a high-sensitivity dielectric sensor, an efficient acousto-microwave modulator, and an arbitrarily-curved 'electric fiber'. Our results represent an important step forward in the development of NZI technologies for microwave/terahertz applications.

Isotropic single-photon sources.

Classical antenna theory establishes that there are no coherent isotropic radiators. Thermal radiators can be isotropic, but they lack temporal coherence. Here, we demonstrate that quantum interference processes enable the design of isotropic and unpolarized single-photon sources that preserve temporal coherence properties. We illustrate how this effect can be realized with relatively simple multilevel emitters with degenerate ground states, and we discuss potential implementations based on atomic and solid-state single-photon sources.