General design formalism for highly efficient flat optics for broadband applications.

The use of flat diffractive optical elements (DOEs) for broadband applications, e.g. conventional optical systems, requires DOEs that maintain high efficiencies across the required range of wavelengths, angles of incidence, and grating periods. Here we introduce a general framework for how dispersion engineering can be used to design DOEs that fulfill these requirements and use our approach to determine design rules for broadband DOEs. Our analysis shows that the key to making échelette-type gratings (EGs) suitable for broadband optical systems is the development of new optical materials with specific uncommon dispersion properties. Subsequently, we use our framework to design a representative range of prototype EGs, which allows us to link the specifications of an optical system to the requirements on the EGs' materials. Finally, we show that our design rules apply to all DOEs based on propagation delays including GRIN DOEs and metagratings. Our design rules therefore guide the way towards unlocking the full potential of DOEs for different kinds of broadband applications.

Analyzing the polarization response of a chiral metasurface stack by semi-analytic modeling.

We investigate a class of stacked metasurfaces where the interaction between layers is dominated by their respective far-field response. Using a semi-analytic scattering matrix approach, we exploit the Fabry-Perot-type response for different layer distances to show the spectral tunability of the resonant effect. This method presents a faster and more intuitive route to modeling Fabry-Perot-type effects than rigorous numerical simulations. The results are illustrated for a chiral metasurface stack that exhibits asymmetric transmission. Here, the effect of asymmetric transmission is highly sensitive to the layer distance, which is used as a free parameter in our model. To prove our theoretical findings we fabricate two variants of the stack with different layer distances and show that far-field interaction between layers is sufficient to generate the effect while being accessible by semi-analytic modeling. The analyticity of the approach is promising for designing sophisticated layered media containing stacks of arbitrary metasurfaces.

Design of a 2 diopter holographic progressive lens.

In this contribution, we investigate the use of holographic optical elements (HOEs) as progressive addition lenses (PALs). We design HOEs with high diffraction efficiency (DE) using the Fourier Modal Method (FMM) and optimize an optical system comprising two of these HOEs to fulfill the optical function of a 2 diopter (dpt) PAL. The resulting design is a holographic PAL (hPAL) exhibiting high DE and limited angular color error (CE) with a distribution of spherical power and astigmatism equivalent to its refractive counterpart. To our knowledge, our contribution is the first complete design of an hPAL. While our HOE design method is shown for PALs here, it has the potential to improve other applications of HOEs as well.

Plasmonic fano nanoantennas for on-chip separation of wavelength-encoded optical signals.

Here we suggest and realize an ultracompact plasmonic spectral-band demultiplexer for telecommunication wavelengths integrated onto an optical waveguide that couples two wavelength-encoded optical signals in the O- and the C-band in opposite directions of a silicon waveguide. In this way, we demonstrate a plasmonic key element for on-chip optical data processing that can also be used as a functional link between on- and off-chip optical signals.

Polarization-Independent Silicon Metadevices for Efficient Optical Wavefront Control.

We experimentally demonstrate a functional silicon metadevice at telecom wavelengths that can efficiently control the wavefront of optical beams by imprinting a spatially varying transmittance phase independent of the polarization of the incident beam. Near-unity transmittance efficiency and close to 0-2π phase coverage are enabled by utilizing the localized electric and magnetic Mie-type resonances of low-loss silicon nanoparticles tailored to behave as electromagnetically dual-symmetric scatterers. We apply this concept to realize a metadevice that converts a Gaussian beam into a vortex beam. The required spatial distribution of transmittance phases is achieved by a variation of the lattice spacing as a single geometric control parameter.

Enhanced third-harmonic generation in silicon nanoparticles driven by magnetic response.

We observe enhanced third-harmonic generation from silicon nanodisks exhibiting both electric and magnetic dipolar resonances. Experimental characterization of the nonlinear optical response through third-harmonic microscopy and spectroscopy reveals that the third-harmonic generation is significantly enhanced in the vicinity of the magnetic dipole resonances. The field localization at the magnetic resonance results in two orders of magnitude enhancement of the harmonic intensity with respect to unstructured bulk silicon with the conversion efficiency limited only by the two-photon absorption in the substrate.

Observation of Fano resonances in all-dielectric nanoparticle oligomers.

It is well-known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low-loss dielectric nanoparticles. Here, light scattering by all-dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically-fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically-induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically-induced magnetic Fano resonances in all-dielectric oligomers offer new opportunities for sensing and nonlinear applications.

Electro-optical switching by liquid-crystal controlled metasurfaces.

We study the optical response of a metamaterial surface created by a lattice of split-ring resonators covered with a nematic liquid crystal and demonstrate millisecond timescale switching between electric and magnetic resonances of the metasurface. This is achieved due to a high sensitivity of liquid-crystal molecular reorientation to the symmetry of the metasurface as well as to the presence of a bias electric field. Our experiments are complemented by numerical simulations of the liquid-crystal reorientation.

Near-field optical experiments on low-symmetry split-ring-resonator arrays.

Effective symmetric and antisymmetric eigenmodes of coupled plasmonic resonances play a crucial role in many photonic metamaterials. Recently, we discussed a particular arrangement of metallic split-ring resonators that is planar, hence enabling direct experimental access to the different eigenmodes via near-field optical microscopy. In this Letter, corresponding optical experiments are presented and compared with simple theoretical modeling, providing a direct confirmation of our previous, more indirect conclusions.

Chiral Bilayer All-Dielectric Metasurfaces.

Three-dimensional chiral plasmonic metasurfaces were demonstrated to offer enormous potential for ultrathin circular polarizers and applications in chiral sensing. However, the large absorption losses in the metallic systems generally limit their applicability for high-efficiency devices. In this work, we experimentally and numerically demonstrate three-dimensional chiral dielectric metasurfaces exhibiting multipolar resonances and examine their chiro-optical properties. In particular, we demonstrate that record high circular dichroism of 0.7 and optical activity of 2.67 × 105 degree/mm can be achieved based on the excitation of electric and magnetic dipolar resonances inside the chiral structures. These large values are facilitated by a small amount of dissipative loss present in the dielectric nanoresonator material and the formation of a chiral supermode in a 4-fold symmetric metasurface unit cell. Our results highlight the mechanisms for maximizing the chiral response of photonic nanostructures and offer important opportunities for high-efficiency, ultrathin polarizing elements, which can be used in miniaturized devices, for example, integrated circuits.

High-bit rate ultra-compact light routing with mode-selective on-chip nanoantennas.

Optical nanoantennas provide a promising pathway toward advanced manipulation of light waves, such as directional scattering, polarization conversion, and fluorescence enhancement. Although these functionalities were mainly studied for nanoantennas in free space or on homogeneous substrates, their integration with optical waveguides offers an important "wired" connection to other functional optical components. Taking advantage of the nanoantenna's versatility and unrivaled compactness, their imprinting onto optical waveguides would enable a marked enhancement of design freedom and integration density for optical on-chip devices. Several examples of this concept have been demonstrated recently. However, the important question of whether nanoantennas can fulfill functionalities for high-bit rate signal transmission without degradation, which is the core purpose of many integrated optical applications, has not yet been experimentally investigated. We introduce and investigate directional, polarization-selective, and mode-selective on-chip nanoantennas integrated with a silicon rib waveguide. We demonstrate that these nanoantennas can separate optical signals with different polarizations by coupling the different polarizations of light vertically to different waveguide modes propagating into opposite directions. As the central result of this work, we show the suitability of this concept for the control of optical signals with ASK (amplitude-shift keying) NRZ (nonreturn to zero) modulation [10 Gigabit/s (Gb/s)] without significant bit error rate impairments. Our results demonstrate that waveguide-integrated nanoantennas have the potential to be used as ultra-compact polarization-demultiplexing on-chip devices for high-bit rate telecommunication applications.

Refractive index sensing with Fano resonances in silicon oligomers.

We demonstrate experimentally refractive index sensing with localized Fano resonances in silicon oligomers, consisting of six disks surrounding a central one of slightly different diameter. Owing to the low absorption and narrow Fano-resonant spectral features appearing as a result of the interference of the modes of the outer and the central disks, we demonstrate refractive index sensitivity of more than 150 nm RIU-1 with a figure of merit of 3.8.This article is part of the themed issue 'New horizons for nanophotonics'.

Manipulation of photoluminescence of two-dimensional MoSe2 by gold nanoantennas.

Monolayer molybdenum diselenide (MoSe2), a member of the TMDCs family, is an appealing candidate for coupling to gold plasmonic nanostructures as it has smaller bandgap and higher electron mobility in comparison to frequently studied molybdenum disulfide (MoS2). The PL of MoSe2 occurs in the near-infrared spectral range where the emissive properties do not suffer from the enhanced dissipation in the gold due to inter-band transitions. Here, we study the interaction between monolayer MoSe2 and plasmonic dipolar antennas in resonance with the PL emission of MoSe2. By varying the thickness of the spacer between the MoSe2 layer and nanoantenna, we demonstrate manipulation of the PL intensity from nearly fourfold quenching to approximately threefold enhancement. Furthermore, we show that the coupled TMDC-nanoantenna system exhibits strong polarization-dependent PL, thus offering the possibility of polarization-based emission control. Our experimental results are supported by numerical simulations as well. To the best of our knowledge, this is the first study of Au-MoSe2 plasmonic hybrid structures realizing flexible PL manipulation.

Active tuning of all-dielectric metasurfaces.

All-dielectric metasurfaces provide a powerful platform for highly efficient flat optical devices, owing to their strong electric and magnetic dipolar response accompanied by negligible losses at near-infrared frequencies. Here we experimentally demonstrate dynamic tuning of electric and magnetic resonances in all-dielectric silicon nanodisk metasurfaces in the telecom spectral range based on the temperature-dependent refractive-index change of a nematic liquid crystal. We achieve a maximum resonance tuning range of 40 nm and a pronounced change in the transmittance intensity up to a factor of 5. Strongly different tuning rates are observed for the electric and the magnetic response, which allows for dynamically adjusting the spectral mode separation. Furthermore, we experimentally investigate the influence of the anisotropic (temperature-dependent) dielectric environment provided by the liquid crystal on both the electric and magnetic resonances. We demonstrate that the phase transition of the liquid crystal from its nematic to its isotropic phase can be used to break the symmetry of the optical metasurface response. As such, our approach allows for spectral tuning of electric and magnetic resonances of all-dielectric metasurfaces as well as switching of the anisotropy of the optical response of the device.

Tailoring directional scattering through magnetic and electric resonances in subwavelength silicon nanodisks.

Interference of optically induced electric and magnetic modes in high-index all-dielectric nanoparticles offers unique opportunities for tailoring directional scattering and engineering the flow of light. In this article we demonstrate theoretically and experimentally that the interference of electric and magnetic optically induced modes in individual subwavelength silicon nanodisks can lead to the suppression of resonant backscattering and to enhanced resonant forward scattering of light. To this end we spectrally tune the nanodisk's fundamental electric and magnetic resonances with respect to each other by a variation of the nanodisk aspect ratio. This ability to tune two modes of different character within the same nanoparticle provides direct control over their interference, and, in consequence, allows for engineering the particle's resonant and off-resonant scattering patterns. Most importantly, measured and numerically calculated transmittance spectra reveal that backward scattering can be suppressed and forward scattering can be enhanced at resonance for the particular case of overlapping electric and magnetic resonances. Our experimental results are in good agreement with calculations based on the discrete dipole approach as well as finite-integral frequency-domain simulations. Furthermore, we show useful applications of silicon nanodisks with tailored resonances as optical nanoantennas with strong unidirectional emission from a dipole source.

Dual-channel spontaneous emission of quantum dots in magnetic metamaterials.

Metamaterials, artificial electromagnetic media realized by subwavelength nano-structuring, have become a paradigm for engineering electromagnetic space, allowing for independent control of both electric and magnetic responses of the material. Whereas most metamaterials studied so far are limited to passive structures, the need for active metamaterials is rapidly growing. However, the fundamental question on how the energy of emitters is distributed between both (electric and magnetic) interaction channels of the metamaterial still remains open. Here we study simultaneous spontaneous emission of quantum dots into both of these channels and define the control parameters for tailoring the quantum-dot coupling to metamaterials. By superimposing two orthogonal modes of equal strength at the wavelength of quantum-dot photoluminescence, we demonstrate a sharp difference in their interaction with the magnetic and electric metamaterial modes. Our observations reveal the importance of mode engineering for spontaneous emission control in metamaterials, paving a way towards loss-compensated metamaterials and metamaterial nanolasers.

Coupling effects in low-symmetry planar split-ring resonator arrays.

We introduce a particular low-symmetry (point group of unit cell C(1)) planar periodic arrangement of magnetic split-ring resonators that acts as an effective optical wave plate. We show that this behavior specifically results from the in-plane interactions among the individual split-ring resonators. Measured normal-incidence transmittance and conversion spectra of gold-based samples fabricated via electron-beam lithography show fundamental resonances at around 235 THz frequency (1,275 nm wavelength) that are in good agreement with theory.

Gold helix photonic metamaterial as broadband circular polarizer.

We investigated propagation of light through a uniaxial photonic metamaterial composed of three-dimensional gold helices arranged on a two-dimensional square lattice. These nanostructures are fabricated via an approach based on direct laser writing into a positive-tone photoresist followed by electrochemical deposition of gold. For propagation of light along the helix axis, the structure blocks the circular polarization with the same handedness as the helices, whereas it transmits the other, for a frequency range exceeding one octave. The structure is scalable to other frequency ranges and can be used as a compact broadband circular polarizer.