Iñigo Liberal

Photonic doping of epsilon-near-zero media

Doped photonics Doping semiconductor materials with impurity atoms enables control of the optoelectronic properties that enhance functionality. Liberal et al. describe numerically and experimentally an analogous doping effect for a group of photonic materials. They introduced a dielectric into an otherwise nonmagnetic material, which produced a magnetic response. The generality of the method should allow the design of photonic materials with enhanced and controlled electromagnetic response. Science, this issue p. 1058 Photonic doping enables design of materials with enhanced electromagnetic response. Doping a semiconductor with foreign atoms enables the control of its electrical and optical properties. We transplant the concept of doping to macroscopic photonics, demonstrating that two-dimensional dielectric particles immersed in a two-dimensional epsilon-near-zero medium act as dopants that modify the medium’s effective permeability while keeping its effective permittivity near zero, independently of their positions within the host. The response of a large body can be tuned with a single impurity, including cases such as engineering perfect magnetic conductor and epsilon-and-mu-near-zero media with nonmagnetic constituents. This effect is experimentally demonstrated at microwave frequencies via the observation of geometry-independent tunneling. This methodology might provide a new pathway for engineering electromagnetic metamaterials and reconfigurable optical systems.

Geometry-invariant resonant cavities.

Resonant cavities are one of the basic building blocks in various disciplines of science and technology, with numerous applications ranging from abstract theoretical modelling to everyday life devices. The eigenfrequencies of conventional cavities are a function of their geometry, and, thus, the size and shape of a resonant cavity is selected to operate at a specific frequency. Here we demonstrate theoretically the existence of geometry-invariant resonant cavities, that is, resonators whose eigenfrequencies are invariant with respect to geometrical deformations of their external boundaries. This effect is obtained by exploiting the unusual properties of zero-index metamaterials, such as epsilon-near-zero media, which enable decoupling of the temporal and spatial field variations in the lossless limit. This new class of resonators may inspire alternative design concepts, and it might lead to the first generation of deformable resonant devices.

Nonradiating and radiating modes excited by quantum emitters in open epsilon-near-zero cavities

Quantum emitters embedded in arbitrarily shaped epsilon-near-zero cavities can selectively excite both nonradiating and radiating modes. Controlling the emission and interaction properties of quantum emitters (QEs) embedded within an optical cavity is a key technique in engineering light-matter interactions at the nanoscale, as well as in the development of quantum information processing. State-of-the-art optical cavities are based on high quality factor photonic crystals and dielectric resonators. However, wealthier responses might be attainable with cavities carved in more exotic materials. We theoretically investigate the emission and interaction properties of QEs embedded in open epsilon-near-zero (ENZ) cavities. Using analytical methods and numerical simulations, we demonstrate that open ENZ cavities present the unique property of supporting nonradiating modes independently of the geometry of the external boundary of the cavity (shape, size, topology, etc.). Moreover, the possibility of switching between radiating and nonradiating modes enables a dynamic control of the emission by, and the interaction between, QEs. These phenomena provide unprecedented degrees of freedom in controlling and trapping fields within optical cavities, as well as in the design of cavity opto- and acoustomechanical systems.

Waveguide metatronics: Lumped circuitry based on structural dispersion

A microwave test bed for metatronic “lumped” circuitry is introduced by exploiting structural dispersion in waveguides. Engineering optical nanocircuits by exploiting modularization concepts and methods inherited from electronics may lead to multiple innovations in optical information processing at the nanoscale. We introduce the concept of “waveguide metatronics,” an advanced form of optical metatronics that uses structural dispersion in waveguides to obtain the materials and structures required to construct this class of circuitry. Using numerical simulations, we demonstrate that the design of a metatronic circuit can be carried out by using a waveguide filled with materials with positive permittivity. This includes the implementation of all “lumped” circuit elements and their assembly in a single circuit board. In doing so, we extend the concepts of optical metatronics to frequency ranges where there are no natural plasmonic materials available. The proposed methodology could be exploited as a platform to experimentally validate optical metatronic circuits in other frequency regimes, such as microwave frequency setups, and/or to provide a new route to design optical nanocircuitry.

Zero-index structures as an alternative platform for quantum optics

Significance Metamaterials provide additional venues for routing and shaping waves, as well as manipulating wave–matter interactions. In principle, similar concepts and techniques could be applied to engineer the properties of quantized fields. Here, we theoretically demonstrate that structures with a near-zero refractive index are capable of inhibiting and then selectively exciting electric field vacuum fluctuations. This effect might offer different pathways to manipulate quantized fields with metamaterials. We illustrate this point by investigating the decay dynamics of a quantum emitter embedded in a zero-index shell. For this particular example, we theoretically show how the distinctive features of zero-index media enable unique phenomena, such as a direct modulation of the vacuum Rabi frequency by deforming the zero-index shell. Vacuum fluctuations are one of the most distinctive aspects of quantum optics, being the trigger of multiple nonclassical phenomena. Thus, platforms like resonant cavities and photonic crystals that enable the inhibition and manipulation of vacuum fluctuations have been key to our ability to control light–matter interactions (e.g., the decay of quantum emitters). Here, we theoretically demonstrate that vacuum fluctuations may be naturally inhibited within bodies immersed in epsilon-and-mu-near-zero (EMNZ) media, while they can also be selectively excited via bound eigenmodes. Therefore, zero-index structures are proposed as an alternative platform to manipulate the decay of quantum emitters, possibly leading to the exploration of qualitatively different dynamics. For example, a direct modulation of the vacuum Rabi frequency is obtained by deforming the EMNZ region without detuning a bound eigenmode. Ideas for the possible implementation of these concepts using synthetic implementations based on structural dispersion are also proposed.

Zero-Index Platforms: Where Light Defies Geometry

Geometry-invariant effects in structures with near-zero permittivity or permeability could lead to deformable optical devices, and to new insights into unique light-matter interactions.

Metatronic analogues of the Wheatstone bridge

The Wheatstone bridge is a classical electrical circuit developed by Sir Charles Wheatstone in the middle of the 19th century. The operating principle of the Wheatstone bridge is based on the concept of a difference measurement, and it is one of the most popular circuits in the characterization of resistors. Here, we utilize the optical metatronic paradigm—metamaterial-inspired optical nanocircuitry—to extend the use of the Wheatstone bridge to different platforms and frequency regimes. Specifically, using numerical simulations we present three different designs in which the analogue of the Wheatstone bridge concept is implemented on all-optical metamaterial boards, microwave waveguides, and planar silicon photonic systems, operating in frequency bands ranging from microwaves to the visible. The proposed devices enrich the collection of available nanocircuitry, and it is shown that high accuracy and simplicity of the Wheatstone bridge can be transplanted and exploited in multiple nanophotonic applications including, e.g., the characterization of nanoparticles.

The rise of near-zero-index technologies

Materials with designed electromagnetic response have a wide range of exotic applications Since the beginning of metamaterial research, the electrodynamic properties of media with a refractive index near zero have attracted the interest of the scientific community because of the intriguing wave phenomena that they are expected to exhibit (1–3). As the refractive index approaches zero, the wavelength expands, and the spatial and temporal field variations effectively decouple (1, 3). This gives access to a new regime of wave dynamics in which geometry-invariant wave phenomena can take place. For example, waves can tunnel through deformed waveguides (2), resonators can preserve their resonance frequency independently of the geometry of their external boundary (4), and light can be trapped in small three-dimensional (3D) regions, even if open to an unbounded environment (5, 6). Recent experimental progress is also pushing forward the applied aspects of near-zero-index (NZI) media, leading to a generation of technologies with the potential to revolutionize different aspects of nanophotonics and other physical systems.

Dispersion synthesis with multi-ordered metatronic filters

We propose the synthesis of frequency dispersion of layered structures based on the design of multi-ordered optical filters using nanocircuit concepts. Following the well-known insertion loss method commonly employed in the design of electronic and microwave filters, here we theoretically show how we can tailor optical dispersion as we carry out the design of several low-pass, high-pass, band-pass and band-stop filters of different order with a (maximally flat) Butterworth response. We numerically demonstrate that these filters can be designed by combining metasurfaces made of one or two materials acting as optical lumped elements, and, hence, leading to simple, easy to apply, design rules. The theoretical results based on this circuital approach are validated with full-wave numerical simulations. The results presented here can be extended to virtually any frequency dispersion synthesis, filter design procedure and/or functionality, thus opening up exciting possibilities in the design of composite materials with on-demand dispersion and high-performance and compact optical filters using one or two materials.

Magnetic field concentration assisted by epsilon-near-zero media

Strengthening the magnetic response of matter at optical frequencies is of fundamental interest, as it provides additional information in spectroscopy, as well as alternative mechanisms to manipulate light at the nanoscale. Here, we demonstrate theoretically that epsilon-near-zero (ENZ) media can enhance the magnetic field concentration capabilities of dielectric resonators. We demonstrate that the magnetic field enhancement factor is unbounded in theory, and it diverges as the size of the ENZ host increases. In practice, the maximal enhancement factor is limited by dissipation losses in the host, and it is found via numerical simulations that ENZ hosts with moderate losses can enhance the performance of a circular dielectric rod resonator by around one order of magnitude. The physical mechanism behind this process is the strongly inhomogeneous magnetic field distributions induced by ENZ media in neighbouring dielectrics. We show that this is an intrinsic property of ENZ media, and that the occurrence of resonant enhancement is independent of the shape of the host. These results might find applications in spectroscopy, in sensing, in light emission and, in general, in investigating light–matter interactions beyond electric dipole transitions. This article is part of the themed issue ‘New horizons for nanophotonics’.