This PDF file contains the front matter associated with SPIE Proceedings Volume 10113, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

The seminal work of R.B. Wood (1902), who discovered anomalies in the reflection spectra of sub-wavelength metallic gratings, triggered the field of plasmonics, where ultra-thin metallic sheets laterally structured on a sub-wavelength scale, so called metallic meta-surface, are under operation. The goal of the field has extended considerably in the last decades and has aimed at arbitrary control over the amplitude, phase and polarization… of light waves at the sub-wavelength scale. All-dielectric meta-surfaces consisting in nano-structured thin films of high index dielectric material, are attracting much attention, owing to their capability to achieve the same goal as their metallic counterpart, yet with an enhanced efficiency (especially for the manipulation of strong optical resonances), being freed from significant energy dissipation as encountered in metallic nano-structures. All dielectric meta-surfaces have been around for quite a while, but were named differently (photonic crystal dielectric membranes or high index contrast gratings). Unless rare exceptions, the literature reports on structures with non-broken vertical symmetry. In the present contribution we emphasize that breaking the vertical symmetry of all-dielectric meta-surfaces provides a widely enhanced degree of freedom for the control of spatial routes and spectral characteristics of light, which depends, to an essential extent, on the local density of photonic states in the thin nano-structured dielectric film. As an enlightening illustration, we concentrate on a dielectric meta-surface formed by two super-imposed identical evanescently coupled gratings, with adjustable gap distance and lateral alignment. We show that this remarkably simple meta-surface can provide any local density of photonic states from zero (Dirac cone) to infinity (ultra-flat zero curvature dispersion characteristics), as well as any constant density over an adjustable spectral range. Exemplifying applications will illustrate the great potential of this new approach.

Structured light and structured matter are two fascinating branches of modern optics that recently started having a significant impact on each other. However, integrating structured light, which commonly is created using bulk optics, on miniaturized silicon chips represents a significant challenge. In this talk, we discuss fundamental optical phenomena at the interface of structured light and engineered optical structures, including theoretical and experimental studies of light-matter interactions of vector and singular optical beams in optical metamaterials and microcavities. The synergy of complex beams, such as the beams carrying an orbital angular momentum (OAM), with nanostructured “engineered” media is likely to bring new dimensions to the science and applications of structured light ranging from fundamentally new regimes of spin-orbit interaction to novel ways of information encoding for the future optical communication systems. We show that unique optical properties of engineered micro- and nanosctructures open unlimited prospects to “engineer” light itself. We discuss several approaches to ultra-compact structured light wavefront shaping using metal-dielectric and all-dielectric resonant metasurfaces. Moreover, by exploiting the emerging non-Hermitian photonics design at an exceptional point, we demonstrate a microring laser generating a single-mode OAM vortex lasing with the ability to precisely define the topological charge of the OAM mode. We show that the polarization associated with OAM lasing can be further manipulated on demand, creating a radially polarized vortex emission. Our OAM microlaser could find applications in the next generation of integrated optoelectronic devices for optical communications in both quantum and classical regimes.

Photonic crystal and plasmonic structures are the two main approaches used in nanophotonic for efficiently confining and enhancing the electromagnetic field at subwavelength scale. For these reasons, these two approaches have been both used for the optical trapping of nanometric particle. We present, here, experimental results showing that structures combining both photonic crystal and nanoantennas could lead to improved trapping performances. In previous theoretical papers [1, 2] we have shown that when the critical coupling between a photonic crystal and a nanoantenna is reached, a large Gaussian beam could be efficiently coupled to a single nanoantenna. In this way, it is possible to generate a nanometric hotspot in the nanoantenna leading to a very efficient optical trap. The experimental demonstration of this effect has been obtained on an SOI sample consisting in a gold nanoantenna located at the centre of a photonic crystal cavity. Stable trapping of 100 nm diameter nanoparticle has been observed using a 5mW laser at 1.31µm with a 5µm waist. The nanoparticle are trapped above the nanoantenna gap and a normalized trap stiffness of 0.3 fN.nm-1.mW-1 is measured. This result demonstrates the interest of this approach. We will discuss and compare it to the state of the art of nanotweezers. [1] A. El Eter et al. Opt. Express 22, 14464 (2014). [2] A. Belarouci et al. Opt. Express 18, A381 (2010).

The miniaturization of current image sensors is primarily limited by the volume of optical elements. Using a subwavelength patterned quasi-periodic structure known as a metasurface, we can build planar optical elements based on the principle of diffraction. This platform allows us to mimic complex, asymmetric curvatures with ease and is ideal for the adaptation of freeform optics to the micron scale. The implementation of freeform optics on metasurfaces allows for extreme miniaturization of optical components. In our research we have demonstrated metasurface based optical elements such as lenses, vortex beam generators, and cubic phase plates near visible frequencies. Our fabricated lenses achieved beam spots of less than 1 μm with numerical apertures as high as ~ 0.75. We observed a transmission efficiency of 90% and focusing efficiency of 40% in the visible wavelengths. In addition, we have demonstrated a dynamic metasurface optical system called the Alvarez lens with a tunable focal length range of over 2.5 mm corresponding to a change in optical power of ~1600 diopters with 100 m of total mechanical displacement.

We present recent studies in which engineering the interaction between light and nanoscale materials has been pursued for applications in image sensors and in spectroscopy. In the first, we present recent work by the author and colleagues on the use of silicon [1-5] and germanium [6] nanowires for multispectral imaging. We show that their ability to support waveguide modes leads to spectrally-selective absorption properties [1]. We show that this in turn enables the nanowires to be used as filters or photodetectors for color and multispectral imaging [2-6]. [1] K. Seo et al, Nano Letters 11, 1851 (2011) [2] H. Park and K. B Crozier, Scientific Reports 3, 2460 (2012) [3] H. Park, Y. Dan, K. Seo, Y. J. Yu, P. K. Duane, M. Wober, K. B. Crozier, Nano Letters 14, 1804 (2014) [4] H. Park and K. B Crozier, Optics Express 23, 7209 (2015) [5] H. Park and K. B Crozier, ACS Photonics 2, 544 (2015) [6] A. Solanki and K. B. Crozier, Appl. Phys. Lett. 105, 191115 (2014)

Many conventional optoelectronic devices consist of thin, stacked films of metals and semiconductors. In this presentation, I will demonstrate how one can improve the performance of such devices by nano-structuring the constituent layers at length scales below the wavelength of light. The resulting metafilms and metasurfaces offer opportunities to dramatically modify the optical transmission, absorption, reflection, and refraction properties of device layers. This is accomplished by encoding the optical response of nanoscale resonant building blocks into the effective properties of the films and surfaces. To illustrate these points, I will show how nanopatterned metal and semiconductor layers may be used to enhance the performance of solar cells, photodetectors, and enable new imaging technologies. I will also demonstrate how the use of active nanoscale building blocks can facilitate the creation of active metafilm devices.

Zero-index metamaterials (ZIMs) offer exotic optical properties such as uniform spatial phase and infinite wavelength, as well as photonic applications including super-coupling and omnidirectional phase matching in nonlinear optics. Here we present an on-chip ZIM consisting of a square array of air-holes in a 220-nm-thick silicon-on-insulator (SOI) wafer. This design enables mass production of ZIM-based photonic devices at low cost and high fidelity using standard CMOS fabrication technology. To transition from the high-aspect ratio inverse case of silicon pillars under transverse magnetic (TM) polarization, our design is instead intended for a transverse electric (TE) polarization because of TE modes are, in general, better confined than TM modes for a given thin film. Furthermore, the larger volume fraction of silicon provided by the air-holes structure improves the confinement as compared with the silicon-pillars structure. We optimized the design to obtain a zero index corresponding to a finite impedance of 0.8 at 1550 nm. The bandstructure of the metamaterial shows a Dirac-cone dispersion at the center of the Brillouin zone at 1550 nm. These results indicate that this metamaterial possesses an impedance-matched, isotropic zero index at 1550 nm. To experimentally verify that the metamaterial has a zero index, we fabricated a right-triangular prism measuring twenty unit cells across. The measured effective index of this prism crosses zero linearly at 1630 nm and shows positive and negative indices at short and longer wavelengths, respectively, indicating a Dirac-cone induced zero index. This measurement is in excellent agreement with the result of full-wave simulation.

The use of a high-contrast grating (HCG) as the top mirror in a vertical-cavity surface-emitting laser (VCSEL) allows for setting the resonance wavelength by the grating parameters in a post-epitaxial growth fabrication process. Using this technique, we demonstrate electrically driven multi-wavelength VCSEL arrays at ~980 nm wavelength. The VCSELs are GaAs-based and the suspended GaAs HCGs were fabricated using electron-beam lithography, dry etching and selective removal of an InGaP sacrificial layer. The air-coupled cavity design enabled 4-channel arrays with 5 nm wavelength spacing and sub-mA threshold currents thanks to the high HCG reflectance.

In this work I present visually the results of a numerical analysis of the transition between classical High-Contrast Gratings (HCGs) and Monolithic High-Contrast Gratings (MHCGs) and I identify the source of the differences between the scatterless reflection peaks and those that either show strong scattering or do not occur in MHCGs. I show that the key property of MHCGs is the independence of the peak reflectivity wavelength on the substrate refractive index, which results from the modal interference inside the grating and the special form of its impedance/admittance matrix. This form of matrix can be obtained for any wavelength and in almost any material system by tuning the geometrical parameters of the grating—its pitch, fill-factor, and height.

A new material pairing is presented for the realisation of sub-wavelength graings in this work and has been used to realise high contrast gratings which operate at wavelengths of 10 μm and greater. The chosen material pairing overcomes the absorption issue which prevents the popular Si/SiO₂ pairing from being useful at wavelengths above 6 μm. The obstacles that exist with the currently used grating materials for this wavelength range are described and it is outlined how the chosen materials overcome these issues. It is numerically demonstrated that gratings utilising these materials are capable of wideband high reflectivity. The gratings were fabricated using standard optical photolithography only and it is shown experimentally that the spectral response of gratings which were fabricated show good agreement with theoretically predicted performance

We present results of computer simulations of vertical cavity surface emitting lasers (VCSELs) using novel, highreflectivity monolithic high refractive-index contrast grating (MHCG) mirrors and their more advanced version, partially covered by a thin metal layer - metallic MHCG (mMHCG) mirrors. The first experimental realization of this new class of mirrors is presented and discussed. We show that the metal layer does not deteriorate the high reflectivity of an mMHCG mirror, but in contrary, is a crucial element which allows high reflectivity and additionally opens a way for a more efficient electrical pumping of a VCSEL. Comparison of results of thermal-electrical-carrier-gain self-consistent simulations of both MHCG- and mMHCG-based VCSELs is presented and discussed. It is shown that using mHCG mirror as a top mirror of a VCSEL improves electrical characteristics and greatly decreases the differential resistance of the device.

So-called ‘flat optics’ control the phase of an illuminating wave in free space through shallow subwavelength structures which are also called metalenses, or more generally metasurfaces. The major steps in their development are presented in an historical perspective, showing that those components have made their way over the years from the microwave domain down to the visible. Recent work highlights the possible role of local resonance effects to reach the minimal possible thickness. In this contribution, we concentrate on the benefit of using low absorption index dielectric materials with the highest possible refractive index to maximize diffraction efficiency while keeping the thickness smaller than the vacuum wavelength, and discuss their design. In those nanostructured components, the effective index involves light being locally guided in the nanostructures. To achieve a high efficiency at large numerical apertures, and therefore at large deflection angles near the pupil edge, fine tuning at design stage is required to mitigate fine sampling for wavefront shape fidelity against independent guiding in neighboring structures.

We present a new platform that realizes high performance metasurfaces in the visible spectrum. This platform is based on atomic layer deposition of titanium dioxide and allows molding incident light wavefront to desired shapes including holographic images, optical vortices, and Bessel beams. The focus of this work will be on the design and demonstration of planar metalenses. We report on our recent experimental realization of high numerical aperture metalenses with efficiency as high as 86%. These metalenses can focus light into a diffraction-limited spot and can be employed for imaging purposes to provide sub-wavelength imaging resolution. In addition, by the judicious design of metalens building blocks, one can achieve a multispectral chiral metalens (MCML) within a single metasurface layer. The MCML can simultaneously resolve chiral and spectral information of an object without the requirement of additional optical components such as polarizers, wave-plates, or even gratings. Using this MCML, we map the chiroptical properties of a macroscopic chiral biological specimen across the visible range. Finally, since many applications require polarization insensitive planar lenses, we discuss the experimental realization of such metalenses with numerical apertures as high as NA=0.85. These metalenses can focus incident light to a spot as small as ~0.6lambda with efficiencies up to 70%. The straightforward and CMOS-compatible fabrication process of this platform is promising for a wide range of optics-based applications in multidisciplinary science and technology.

Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on developing multi-wavelength and dispersion engineered metasurfaces, as well as conformal, flexible, and tunable metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will introduce a vertical on-chip integration platform enabled by vertical stacking of multiple metasurfaces and active optoelectronic components. This vertical integration scheme introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.

Semiconducting nanostructures are promising as components in high performance metasurfaces. We show that single crystal silicon can be used to realize efficient metasurface devices across the entire visible spectrum, ranging from 480 to 700 nanometers. Alternative forms of silicon, such as polycrystalline and amorphous silicon, suffer from higher absorption losses and do not yield efficient metasurfaces across this wavelength range. To demonstrate, we theoretically and experimentally characterize the resonant scattering peaks of individual single crystal silicon nanoridges. In addition, we design high efficiency meta-gratings and lenses based on nanoridge arrays, operating at visible wavelengths, using a stochastic optimization approach. We find that at wavelengths where single crystal silicon is effectively lossless, devices based on high aspect ratio nanostructures are optimal. These devices possess efficiencies similar to those made of titanium oxide, which is an established material for high efficiency visible wavelength metasurfaces. At blue wavelengths, where single crystal silicon exhibits absorption losses, optimal devices are instead based on coupled low aspect ratio resonant nanostructures and are able to provide reasonably high efficiencies. We envision that crystalline silicon metasurfaces will enable compact optical systems spanning the full visible spectrum.

Diffractive optical devices based on dielectric metasurfaces have recently attracted significant attention. Small size, low weight, planar form factor, and potential for low-cost manufacturing using semiconductor fabrication techniques are some of the main features that make metasurfaces ideal candidates for implementation of low-cost miniaturized optical systems. However, to become competitive for practical applications, metasurfaces should also offer specifications (e.g. efficiency, bandwidth, and wavefront error) comparable to their refractive counterparts. We have recently demonstrated diffraction-limited metasurface lenses with high efficiency using high refractive index nano-posts. Low numerical aperture (NA) metasurface lenses have more than 90% focusing efficiency, but the efficiency of the lenses with NA>0.5 decreases with increasing NA and drops to ~40% for NA=0.9, thus resulting in a trade-off between the NA and efficiency. Here we identify the main physical origin of this trade-off as the low transmission of large diameter nano-posts for transverse-magnetic (TM) polarized light incident at large angles, and show that the low transmission is caused by the excitation of undesired high order modes in these nano-posts. To overcome this issues, we present a novel approach for evaluating different metasurface designs in implementation of high NA metasurface components. The approach is based on adiabatic approximation of aperiodic metasurfaces by periodic gratings, and considers the effect of large deflection angles. Using the proposed design approach, we experimentally demonstrate more than 75% focusing efficiency for metasurface lenses with NA=0.7, and more than 70% deflection efficiency for 50-degree beam deflectors for unpolarized light at 915 nm.

In the last years, much interest has grown around the concept of optical surfaces employing high contrast dielectric resonators. However, a systematic approach for the design of this optical surfaces under particular requirements has never been proposed. In this contribution, we describe this approach applied to the robust design of an array of microlenses characterized by a numerical aperture of NA=0.19 with a field of view of FOV = ±60 mrad in a bandwidth of 20 nm. Typically, dielectric resonators are engineered in such a way to have almost full transmissive surfaces with locally tunable phase. However, considering the multiple wavelengths and angles under which the lenses may work, it is difficult to get uniform transmission characteristics for all the dielectric resonators employed. The design strategy, here proposed, uses a particle swarm optimization routine to find the best resonator distribution able to meet the requirements considering the amplitude and phase dispersive characteristics of the resonators surfaces. In the optimization process, also the effects of possible manufacturing inaccuracies, such as variations of resonators radii, are taken into account, allowing a robust design of the structure, within the given manufacturing tolerances. Different designs, operating at 405 nm and 635 nm, are presented and their performances are discussed.

Two-dimensional metamaterials (metasurfaces) have led to many exciting phenomena both in linear and nonlinear optics. The ability to tailor optical modes and radiation patterns combined with low losses make all-dielectric metasurfaces an interesting platform for nonlinear optics and interactions with emitters. The combination of dielectric metasurfaces made from nanostructured III-V semiconductors results in very high second and third optical nonlinearities. Additionally, using epitaxial and heterostructured III-V semiconductors as the constituent material for such metasurfaces enables the inclusion of high quality quantum emitters. We will present recent results on the interaction between high Q modes present in dielectric metasurfaces and epitaxial quantum dots.

Gradient metasurfaces, or ultrathin optical components with engineered transverse impedance gradients along the surface, are able to locally control the phase and amplitude of the scattered fields over subwavelength scales, enabling a broad range of linear components in a flat, integrable platform^1–4. On the contrary, due to the weakness of their nonlinear optical responses, conventional nonlinear optical components are inherently bulky, with stringent requirements associated with phase matching and poor control over the phase and amplitude of the generated beam. Nonlinear metasurfaces have been recently proposed to enable frequency conversion in thin films without phase-matching constraints and subwavelength control of the local nonlinear phase^5–8. However, the associated optical nonlinearities are far too small to produce significant nonlinear conversion efficiency and compete with conventional nonlinear components for pump intensities below the materials damage threshold. Here, we report multi-quantum-well based gradient nonlinear metasurfaces with second-order nonlinear susceptibility over 10⁶ pm/V for second harmonic generation at a fundamental pump wavelength of 10 μm, 5-6 orders of magnitude larger than traditional crystals. Further, we demonstrate the efficacy of this approach to designing metasurfaces optimized for frequency conversion over a large range of wavelengths, by reporting multi-quantum-well and metasurface structures optimized for a pump wavelength of 6.7 μm. Finally, we demonstrate how the phase of this nonlinearly generated light can be locally controlled well below the diffraction limit using the Pancharatnam-Berry phase approach^5,7,9, opening a new paradigm for ultrathin, flat nonlinear optical components.

Diffractive optical devices have many applications in various fields of optics. A fundamental property of all diffractive devices is their negative chromatic dispersion: a diffractive grating always disperses light in the opposite order compared to a refractive prism made of a material with positive (normal) dispersion. Unlike refractive devices, chromatic dispersion in diffractive devices stems from geometrical features, and cannot be controlled via the intrinsic material dispersion. In addition to the always negative sign, the amplitude of diffractive chromatic dispersion is set only by the function of the device. For instance, the angular dispersion of a grating is always given by dθ/dλ=tan⁡(θ)/λ (where θ is the deflection angle and λ is wavelength), or the focal distance dispersion of a diffractive lens is given by df/dλ=-f/λ. Therefore, the chromatic dispersion of diffractive devices has always been set by their function (e.g. by the deflection angle for a grating or the focal distance for a lens), and could not be controlled separately. Here, we present our work on breaking this fundamental relation between the function and chromatic dispersion of diffractive devices using metasurfaces providing independent control over phase and group delays. We use a reflective dielectric metasurface to experimentally demonstrate gratings and lenses that have positive, zero, and extraordinary negative chromatic dispersion. Apart from its fundamental scientific value, this concept expands the applications of diffractive devices as it enables various types of chromatic dispersions. For instance, a special case would be a dispersionless lens operating over a wide bandwidth with the same focal distance.

Metasurfaces are two-dimensional arrangements of nano-scatterers that enable control of phase, amplitude, and polarization of light with high efficiency and subwavelength resolution. They have enabled diffractive optical elements with enhanced functionalities and performance. Nevertheless, metasurface diffractive optical elements share many of the properties of regular diffractive optical elements. One of these properties is the response of diffractive elements to changing the angle of illumination: if the beam incident on a grating is rotated by an angle, all diffraction orders will rotate by corresponding angles in the same direction. More precisely, because of the constant grating momentum, the change in the sine of all diffraction angles will be equal to the change in the sine of the illumination angle. Many optical devices of interest, however, do not require this type of behavior, which makes their implementation using metasurfaces very challenging. For instance retroreflectors, which reflect light incident from any angle to the same direction, or collimators, that deflect light coming from any angle to a single given direction, do not follow the regular diffractive optics angular response. We investigate properties of single-layer metasurfaces that enable devices like retroreflectors and collimators. We show that such metasurfaces should have the ability to control the phase, as well as the derivative of phase with respect to angle. We demonstrate designs that provide such control, and use them to show devices that defy the regular response of diffractive optical devices to changes in the illumination angle.

Dielectric metasurfaces have proven to be able to manipulate the wavefront of incoming waves with high transmission efficiency. The important next question is: Can they enable enhanced interaction with the light to transform its colour or to be able to control one light beam with another? Here we show how an ultra-thin surface of subwavelength thickens can enable enhanced light matter interaction for efficient frequency conversion and ultra-fast light modulation. In particular, we show how designer dielectric metasurfaces can enhance second and third harmonic generation resulting in complete nonlinear control of directionality and polarisation state of the harmonics. Furthermore, we demonstrate how such enhanced light matter interactions can lead to optical switching with unprecedented speed. Our results open novel applications in ultra-thin light sources, light switches and modulators, ultra-fast displays, and other nonlinear optical metadevices based on low loss subwavelength dielectric resonant nanoparticles.

The study of nonlinear effects with high-index dielectric nanoparticles is emerging as a promising alternative to plasmonic systems usually utilized for nonlinear nanophotonics, due to negligible Ohmic losses and low heating in combination with multipolar radiation characteristics of both electric and magnetic nature. In this contribution, we discussed novel nonlinear-optical effects, such as enhanced second- and third-harmonic generation in silicon nanodisks excited in the spectral range close to the magnetic dipole resonance of the individual disk. Each of the nanodisks exhibits both electric and magnetic Mie-type resonances that are shown to affect significantly their nonlinear response. We have observed the third- harmonic radiation intensity that is comparable to that of a bulk silicon slab and demonstrated a pronounced reshaping of the third-harmonic spectra due to interference of the nonlinearly generated waves augmented by an interplay between the electric and the magnetic dipolar resonances. We have also demonstrated all-optical switching of femtosecond laser pulses passing through subwavelength silicon nanodisks at their magnetic dipolar resonance. In z-scan experiments, we have observed a modulation of up to 60% and a spectral resonance shift of 6 nm when pumping the nanostructure at picojoule-per-disk powers. Third-harmonic generation from silicon nanodisks arranged in the form of quadrumers or trimer oligomers with varying distance between the nanoparticles is studied.

Metasurfaces are planar arrangements of elements that are designed to present a particular response to an incident electromagnetic field. Due to their ability to control at will the phase, polarization and amplitude of the reflected and/or transmitted waves at a subwavelength scale they have gathered a great deal of attention among the research community. Although the first metasurface proposals were realized with plasmonic particles, the focus is now turning into all-dielectric approaches, in order to mitigate losses and increase the device efficiencies. Besides the obvious advantage of loss reduction, when high-index, subwavelength particles are considered a whole new family of resonant, magnetic-like modes is accessible. This new set of modes, which cannot be excited in simple metallic particles, brings additional functionalities for these metasurfaces, as will be shown in this talk. We will focus on the interesting effects that arise as a consequence of the far-field interference between electric and magnetic modes excited in the dielectric particles forming the metasurface and the strong modification of their scattering patterns as a consequence of this interference. In particular, we will show the possibility to realize so called ideal Huygens’ secondary sources to generate a perfectly transmissive metasurface with full phase control. We will also show that these metasurfaces support a generalized version of Brewster’s effect, in which the phenomenon is not restricted to a particular angle or polarization of incidence but can be tuned at will, and the different implications that this concept has.

Observations of anomalous far-field scattering patterns from non-perfectly conducting cylindrical shells have lead to an analysis including surface currents and waves. The concept of skin depth is analyzed for wavelength-scale scattering structures when the permeability of the material is low in terms of the magnetic vector potential. Conditions under which interactions between the incident field and material properties below the skin depth affect the far field scattering pattern are illustrated and explained. The possibility of exploiting this to better characterize material properties of meso and nanoscale structures at optical frequencies is presented.

In this talk, we will discuss our recent work in the area of wave manipulation with high-contrast metasurfaces, with special interest in manipulating optical wavefronts and thermal emission. In addition to our recent theoretical and experimental work in the area of gradient metasurfaces to manipulate the impinging light, we will discuss how similar concepts may be extended to thermal sources, to manipulate their emission features. Incandescent sources made of electrically-heated films suffer from low efficiencies and offer poor control over the directionality and spatial localization, as well as the spectral and polarization properties of the emitted light. We have recently demonstrated that, by nanostructuring a SiC surface, we can concentrate the thermal emission of a preselected spectral range into a well-defined location above the surface. Concentrating the thermal radiation can have direct impact on the design and operation of the future generation of thermo-photovoltaic cells in addition to providing the ability for local heat generation and moreover mitigate challenges associated with thermal management in low thermal budget devices. Our recent theoretical work suggests that gradient metasurface concepts may be suitably extended to tailor thermal emission control with a new degree of control.

Recently, metasurfaces and metalenses have become an important subject in the domain of novel optical devices. Numerous nanostructures, with different fundamental principles, materials, and topologies, have been proposed, but general design rules for optimization of their efficiency are not well established yet. Particularly attractive are metasurfaces consisting of Huygens resonators since they offer precise control of the intensity and phase of the transmitted light. In this paper, we demonstrate a Huygens metasurface capable of focusing visible light. We study the impact of the layout of the lens on their efficiency by comparing two metalenses designs: concentric annular regions of equal width and constant phase, with concentric rings defined by individual nano-resonators. The latter provides a better approximation to the ideal phase profile and as a consequence, a high-efficiency lens. Metalenses with both designs were fabricated by e-beam lithography and characterized with a custom-built setup. Experimental results demonstrate that the design with fine discretization improves the lens efficiency by a factor of 2.6.

Two-dimensional photonic crystal slabs (PCS) offer an appealing alternative to distributed Bragg reflectors or filters for various applications. Indeed, their scattering properties, governed by Fano-resonances, have been used in areas as diverse as optical wavelength and polarization filters, reflectors, semiconductor lasers, photodetectors, bio-sensors, or non-linear optical components. Suspended PCSs also find natural applications in the field of optomechanics, where the mechanical modes of a suspended slab interact via radiation pressure with the optical field of a high finesse cavity. The reflectivity and transmission properties of a defect-free suspended PCS around normal incidence can be used to couple out-of-plane mechanical modes to an optical field by integrating it in a free space cavity. We have demonstrated the successful implementation of a PCS reflector on a high-tensile stress Si3N4 nanomembrane. We could measure the photonic crystal band diagram with a spectrally, angular, and polarization resolved setup. Moreover, a cavity with a finesse as high as 12 000 was formed using the suspended membrane as end-mirror of a Fabry-Perot cavity. These achievements allow us to operate in the resolved sideband regime where the optical storage time exceeds the mechanical period of low-order mechanical drum modes. This condition is a prerequisite to achieve quantum control of the mechanical resonator with light.

Here, we surpass the diffraction limit of light by a new class of all-dielectric artificial materials that are lossless. This overcomes one of the fundamental challenges of light confinement in metamaterials and plasmonics: metallic loss. Our approach relies on controlling the optical momentum of evanescent waves as opposed to conventional photonic devices which manipulate propagating waves. This leads to a counterintuitive confinement strategy for electromagnetic waves across the entire spectrum. We introduce two distinct photonic design principles that can ideally lead to sub-diffraction light confinement without metal. They are i) relaxed total internal reflection and ii) photonic skin-depth engineering. We present initial experimental results on a CMOS compatible platform that prove the enhanced confinement of our all-dielectric metamaterial design.

Metamaterials, with the ability of tailoring optical properties of materials, have been applied to holograms recently, which has shown the priorities of switchable polarization and multicolor image comparing with the conventional holograms. However, the current metasurface based multicolor holograms have suffered the problems of narrow band and low efficiency in phase modulation for gold and silver when their feature dimensions are in few tens of nanometers. Interestingly, aluminum with higher plasma frequency could yield surface plasmon resonance across a broader range of the spectrum ranging from visible to UV. Metasurfaces incorporating with the aluminum offer the unique opportunity to extend the working wavelength to cover the entire visible spectrum for the generation of full color meta-holograms. Here we demonstrated a phase modulated multicolor meta-hologram that is polarization dependent and capable of producing images in red, green and blue colors. The metahologram is made of aluminum nanorods that are arranged in a two-dimensional array of pixels with surface plasmon resonance in the visible to UV range. The aluminum nanorod array is patterned on a 30 nm thick SIO2 spacer layer sputtered on top of a 130nm thick aluminum mirror. With proper design of the structure, we obtain resonances of narrow bandwidths to allow for implementation of multicolor scheme. Taking into account of the wavelength dependence of the diffraction angle, we can project images to specific locations with predetermined size and order. With tuning of aluminum nanorod size, we demonstrate that the image color can be continuously varied across the visible spectrum.

Tamm plasmons are electromagnetic states located at the interface between a dielectric Bragg mirror and a metal [1]. Contrary to conventional surface plasmons, Tamm plasmons can exist in both TE and TM polarization and its parabolic dispersion lies above the light cone which allow a direct optical excitation at normal incidence. Besides, the Tamm mode confinement can be obtained by simply patterning the thin metallic film, such as microdisks [2,3] or microrectangles [4]. Here, we aim at obtaining ultimate confinement using photonic crystal periodic structures in the metallic layer. The samples are constituted by a DBR with 4 pairs of l/4n layers of Si and SiO2 above which periodic metallic patterns are defined using e-beam lithography and a 50nm gold deposition. Lift-off is performed at the end of the process. The period of the gratings is chosen to obtain a Tamm Bloch mode around 1.3micrometer. Microreflectivity experiments show that Tamm Bloch modes exist in such 1D periodic structures. Using an original design, we create a 1D photonic band gap as large as 140nm. Finally, we will present experimental results on cavity-confined Tamm Bloch modes. All results are in good agreement with numerical calculations. [1] M. Kaliteevski et al., Phys. Rev. B 76, 165415 (2007) [2] O. Gazzano et al., Phys. Rev. Lett. 107, 247402 (2011) [3] C. Symonds et al., Nanoletters, 13 (7), pp 3179–3184 (2013) [4] G. Lheureux et al., ACS Photonics 2 (7), pp 842–848 (2015)

In the past decade, subwavelength high contrast gratings (HCGs) have been developed and studied, which has led to many applications. The broadband reflectance in HCGs mainly comes from the contrast between the grating material and its surrounding environment, so high-index and low-loss materials are required for making HCGs. Compared with infrared (IR) ranges, building HCGs in visible or near-IR wavelength ranges is much harder due to the limitation of optical materials. In order to overcome the challenge of materials in making HCGs in visible to near-IR ranges, hybrid HCGs are proposed. The design of hybrid HCGs is a combination of low-loss and low-index materials and high-loss and high-index materials. In order to reduce the optical loss due to the incorporation of high-loss material, optical modes must be manipulated to be confined in the low-loss region. In our work, the structure and parameters for hybrid HCGs are optimized based on numerical study (both FDTD and RCWA). As a proof-of-principle demonstration, hybrid HCGs composed of amorphous silicon, silicon nitride and silicon dioxide are optimized. The optimal structure has a broadband reflectance (>90%) in visible to near-IR ranges. The design demonstrates a great fabrication tolerance to line width, dielectric thicknesses and sidewall verticality. The hybrid HCGs are patterned by nanoimprint lithography. Reactive ion etching at cryogenic temperature is optimized for the best etching profile. More details on design, fabrication and measurement will be presented at the conference.

The ability to tune the delay of an optical signal is a key component in photonics-based RF phased-array beamforming applications. Recent work has shown that high-contrast metastructure waveguides can be designed for a wide range of delay tuned by carrier injection or signal wavelength, enabling two-dimensional beam steering. In this work, we further explore the parameter space of these structures to maximize the delay change over optical wavelength while maintaining low insertion loss, with the goal of implementing phased-array beamforming in integrated photonic devices.

All-dielectric optical metasurfaces represent a new platform that is able to change electromagnetic field dramatically, while having thickness much smaller than the optical wavelength. The properties of metasurface depends critically on the geometry of its nanostructuring, therefore they are pre-set during the fabrication process. However, a number of practical application of such metasurfaces requires the dynamic change of their properties with operation. Embedding the metasurface in LC we can offer unique opportunities for such tuning and control of their properties. Here we show that by applying voltage across the LC or by varying its temperature we are able to control the spectral position of the metasurface’s electric and magnetic resonances. Using this method, we demonstrate experimentally the tuning of the properties of homogeneous metasurface consisted from array of equal elements as well as switch on and off different metasurface devices that use nanostructures with gradient geometries. In particular, we show that applying voltage across the LC cell (one substrate of which is a Si disk metasurface), we can change the spectral position of electric and magnetic resonances thus changing intensity and phase of transmitted electromagnetic wave. To demonstrate possibility of thermal controlling of metasurface with special geometry, we further utilize gradient metasurface that deflect light beams. By heating the LC to the critical temperature we can switch the transmitted beam from straight propagation to angular deflection. Overall, by combining developed LC technologies with the emerging field of nanostructured metasurfaces we show the potential for novel ultra-thin tunable optical devices.