Printing technology based on plasmonic structures has many advantages over pigment based color printing such as high resolution, ultra-compact size and low power consumption. However, due to high losses and broad resonance behavior of metals in the visible spectrum, it becomes challenging to produce well-defined colors. Here, we investigate cross-shaped dielectric nanoresonators which enable high quality resonance in the visible spectral regime and, hence, high quality colors. We numerically predict and experimentally demonstrate that the proposed all-dielectric nanostructures exhibit high quality colors with selective wavelengths, in particular, due to lower losses as compared to metal based plasmonic filters. This results in fundamental colors (RGB) with high hue and saturation. We further show that a large gamut of colors can be achieved by selecting the appropriate length and width of individual $Si$ nanoantennas. Moreover, the proposed all-dielectric metasurface based color filters can be integrated with the well matured fabrication technology of electronic devices.
While nanoscale color generations have been studied for years, the high performance transmission structural color, simultaneously equipped with large gamut, high resolution, and optical multiplexing abilities, still remains as a hanging issue. Here, a silicon metasurface embedded Fabry-Pérot cavity is demonstrated to address this problem. By changing the planar geometries of metasurface units, the cavities provide transmission colors with 194% sRGB gamut coverage and 141111 DPI resolution, along with more than 300% enhanced angular tolerance. Such high density allows two-dimensional color mixi…
We introduce a practical dielectric metasurface design for microwave frequencies. The metasurface is made of an array of dielectric resonators held together by dielectric connections thus avoiding the need of a mechanical support in the form of a dielectric slab and the spurious multiple reflections that such a slab would generate. The proposed design can be used either for broadband metasurface applications or monochromatic wave transformations. The capabilities of the concept to manipulate the transmission phase and amplitude of the metasurface are supported by numerical and experimental res…
Subwavelength dielectric resonators assembled into metasurfaces have become a versatile tool for miniaturising optical components approaching the nanoscale. An important class of metasurface functionalities is associated with asymmetry in both generation and transmission of light with respect to reversals of the positions of emitters and receivers. Nonlinear light-matter interaction in metasurfaces offers a promising pathway towards miniaturisation of the asymmetric control of light. Here we demonstrate asymmetric parametric generation of light in nonlinear metasurfaces. We assemble dissimilar…
Fully reconfigurable metasurfaces would enable new classes of optical devices that provide unprecedented control of electromagnetic beamforms. The principal challenge for achieving reconfigurability is the need to generate large tunability of subwavelength, low-Q metasurface resonators. Here, we demonstrate large refractive index tuning can be efficiently facilitated at mid-infrared wavelengths using novel temperature-dependent control over free-carrier refraction. In doped InSb we demonstrate nearly two-fold increase in the electron effective mass leading to a positive refractive index shift …
We present a new approach to dielectric metasurface design that relies on a single resonator per unit cell and produces robust, high quality-factor Fano resonances. Our approach utilizes symmetry breaking of highly symmetric resonator geoemetries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple "bright" dipole modes to "dark" dipole modes whose radiative decay is suppressed by local field effects in the array. Our approach is widely scalable from the near-infrared to radio frequencies. We first unravel the …
Metasurfaces exploit the ability to engineer the optical phase, amplitude and polarization at subwavelength dimensions providing unprecedented control of light. The realization of the all dielectric approach to metasurfaces has led to the demonstration of extensive flat optical elements and functionalities with low losses. However, to reach their ultimate potential, metasurfaces must move beyond static operation and incorporate active tunability and reconfigurable functions. The central challenge is achieving large tunability in subwavelength resonator elements which require large optical effe…
We develop a method to control the group delay of electromagnetic waves continuously using a doubly resonant metasurface. The method is based on the dependences of i) the group velocity in a medium featuring two resonance lines on the resonance linewidths and ii) the resonance linewidth of a metasurface composed of split-ring resonators on an incidence angle of electromagnetic wave. To verify this method for group-delay control, we design a terahertz metasurface composed of two split-ring resonators with different resonance frequencies and numerically analyze the transmission characteristic of…
A full (2$π$) phase modulation is critical for efficient wavefront manipulation. In this article, a metasurface based on graphene long/short-strip resonators is used to implement a dynamic 2$π$ phase modulation by applying different voltages to different graphene resonators. The configuration is found to have high reflection efficiency (minimum 56%) and has a full phase modulation in a wide frequency range. Terahertz (THz) beam steering as large as 120 degrees ($\pm60^\circ$) is demonstrated in a broad frequency range (1.2 to 1.9 THz) by changing the Fermi levels of different graphene resonato…
Strong light-matter coupling has recently been demonstrated in sub-wavelength volumes by coupling engineered optical transitions in semiconductor heterostructures (e.g., quantum wells) to metasurface resonances via near fields. It has also been shown that different resonator shapes may lead to different Rabi splittings, though this has not yet been well explained. In this paper, our aim is to understand the correlation between resonator shape and Rabi splitting, and in particular determine and quantify the physical parameters that affect strong coupling by developing an equivalent circuit netw…
Structural colors generated due to light scattering from static all-dielectric metasurfaces have successfully enabled high-resolution, high-saturation, and wide-gamut color printing applications. Despite recent advances, most demonstrations of these structure-dependent colors lack post-fabrication tunability. This hinders their applicability for front-end dynamic display technologies. Phase-change materials (PCMs), with significant contrast of their optical properties between their amorphous and crystalline states, have demonstrated promising potentials in reconfigurable nanophotonics. Herein,…
Structural colors generated due to light scattering from static all-dielectric metasurfaces have successfully enabled high-resolution, high-saturation and wide-gamut color printing applications. Despite recent advances, most demonstrations of these structure-dependent colors lack post-fabrication tunability that hinders their applicability for front-end dynamic display technologies. Phase-change materials (PCMs), with significant contrast of their optical properties between their amorphous and crystalline states, have demonstrated promising potentials in reconfigurable nanophotonics. Herein, w…
Light trapping in planar ultrathin-film solar cells is limited due to a small number of optical modes available in the thin-film slab. A nanostructured thin-film design could surpass this limit by providing broadband increase in the local density of states in a subwavelength volume and maintaining efficient coupling of light. Here we report a broadband metasurface design, enabling efficient and broadband absorption enhancement by direct coupling of incoming light to resonant modes of subwavelength-scale Mie nanoresonators defined in the thin-film active layer. Absorption was investigated both …
To enhance transmission efficiency of Pancharatnam-Berry (PB) phase metasurfaces, multilayer split-ring resonators were proposed to develop encoding sequences. As per the generalized Snell law, the deflection angle of the PB phase encoding metasurfaces depends on the metasurface period size. Therefore, it is impossible to design an infinitesimal metasurface unit.Consequently, the continuous transmission scattering angle cannot be obtained. In digital signal processing, this study introduces the Fourier convolution principle on encoding metasurface sequences to freely control the transmitted sc…
Subwavelength Mie resonators have enabled new classes of optical antennas1-4, photodetectors5,6, antireflection coatings7, and magnetic mirrors8 and can act as the basic meta-atoms constituents of low-loss dielectric metasurfaces2-4,8-10. In any application, tunable Mie resonances are key to achieving dynamic and reconfigurable operation11,12. Sub-linewidth tuning has been achieved via coupling to liquid crystals13, ultrafast free-carrier injection14,15,16,17, stretchable substrates18 and with phase change materials19. Here, we demonstrate ultra-wide dynamic tuning of PbTe Mie resonators fabri…
Metasurfaces are engineered thin surfaces comprising two dimensional (2D) arrays of sub-wavelength spaced and sub-wavelength sized resonators. Metasurfaces can locally manipulate the amplitude, phase and polarization of light with high spatial resolution. In this study, we report numerical and experimental results of a vortex-beam-generating metasurface fabricated specifically for infrared (IR) and terahertz (THz) wavelengths. The designed metasurface consisted of a 2D array of dielectric cross-shaped resonators with spatially varying length, thereby providing desired spatially varying phase s…
We present SERS-active metal holographic metasurfaces fabricated from metal periodical nanograting deposited on a dielectric substrate. The metasurface consists of a modulated dielectric, which is covered by a thin silver layer. The metasurface operates as an open plasmon resonator. The theory of plasmons excited in the open resonator formed by a metal nanograting is presented. The large local electromagnetic field is predicted for optical frequencies. The excitation of plasmons is experimentally demonstrated in the metasurface designed on a 4-inch Si wafer. The enhancement of the local electr…
Metasurfaces are an emerging platform for manipulating light on a two-dimensional plane. Existing metasurfaces comprise arrays of optical resonators such as plasmonic antennas or high-index nanorods. In this letter, we describe a new approach to realize metasurfaces based on electronic transitions in two-level systems (TLSs). These metasurfaces can reproduce all of the major results in conventional metasurfaces. In addition, since TLSs can be lossless, tunable, and orders of magnitude smaller than optical resonators, atomic metasurfaces can realize functions that are difficult to achieve with …
We demonstrate a novel and simple approach to cloaking a scatterer on a ground plane. We use an extremely thin dielectric metasurface (λ/12) to reshape the wavefronts distorted by a scatterer in order to mimic the reflection pattern of a flat ground plane. To achieve such carpet cloaking, the reflection angle has to be equal to the incident angle everywhere on the scatterer. We use a graded metasurface and calculate the required phase gradient to achieve cloaking. Our metasurface locally provides additional phase to the wavefronts to compensate for the phase difference amongst light paths indu…
We conceptualized three-array scalable metasurfaces comprising only three thin strip grids and numerically demonstrated their characteristics in the mid-infrared spectral regime for switchable polarization manipulation and related asymmetric diodelike transmission (AT). A few or all of the grids were taken to be made of VO$_2$, a phase-change material. For each proposed metasurface, two effective structures and, therefore, two different functionalities exist, corresponding to the metallic and insulating phases of VO$_2$. The achieved scenarios of functionality switching that depend on the VO$_…
Digital imaging has been steadily improving over the past decades and we are moving towards a wide use of multi- and hyperspectral cameras. A key component of such imaging systems are color filter arrays, which define the spectrum of light detected by each camera pixel. Hence, it is essential to develop a variable, robust and scalable way for controlling the transmission of light. Nanostructured surfaces, also known as metasurfaces, offer a promising solution as their transmission spectra can be controlled by shaping the wavelength-dependent scattering properties of their constituting elements…
We studied a metasurface constituted as a periodic array of semiconductor split-ring resonators. The resonance frequencies of the metasurface excited by normally incident light were found to be continuously tunable in the terahertz regime through an external magnetostatic field of suitable orientation. As such metasurfaces can be assembled into 3D metamaterials, the foregoing conclusion also applies to metamaterials comprising semiconductor split-ring resonators.
In this paper we describe a methodology for tailoring the design of metamaterial dielectric resonators, which represent a promising path toward low-loss metamaterials at optical frequencies. We first describe a procedure to decompose the far field scattered by subwavelength resonators in terms of multipolar field components, providing explicit expressions for the multipolar far fields. We apply this formulation to confirm that an isolated high-permittivity cube resonator possesses frequency separated electric and magnetic dipole resonances, as well as a magnetic quadrupole resonance in close p…
Light concentration with strong temporal and spatial confinement is crucial for tailoring light-matter interaction. Electromagnetic cavity modes in photonic and plasmonic resonators provide platforms for optical field localization. Here, we propose a concept of quasi-bound states in the continuum gap cavity and reveal that ultra spatiotemporal confinements in free-space can be realized in a dielectric nanocavity metasurface. By introducing an asymmetric air slot in a nanodisk resonator, an ultra-high quality factor $\rm Q \sim 10^6$, accompanying an ultra-small effective mode volume, $\rm V_m …
Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently, allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced…
A switchable metasurface composed of plasmonic split ring resonators and a dye-doped liquid crystal is developed. The transmission of the metasurface in the infrared spectral range can be changed by illuminating the dye-doped liquid crystal with light in the visible spectral range. The effect is particularly efficient in the case of hybrid alignment of the liquid crystal, i. e. alignment of the director perpendicular to the surface on one substrate and parallel alignment on the counter substrate. This all-optical switching effect can be attributed to the behavior described in earlier works as …
Metasurfaces control optical wavefronts via arrays of nanoscale resonators laid out across a surface. When combined with III-V semiconductors with strong optical nonlinearities, a variety of nonlinear effects such as harmonic generation and all optical modulation can be enabled and enhanced at the nanoscale. This review presents our research on engineering and boosting nonlinear effects in ultrafast and nonlinear semiconductor metasurfaces fabricated at the Center for Integrated Nanotechnologies (CINT). We cover our recent works on parametric generation of harmonic light via direct and cascade…
A single-order transmission diffraction grating based on dispersion engineered all-dielectric metasurfaces is proposed and its wavelength discriminating properties have been theoretically described and confirmed using numerical simulations. The metasurface is designed using a 2D array of all-dielectric resonators, which emulates a Huygens source configuration to achieve a perfect match to free-space in broad bandwidth. Using a holey dielectric nanodisk structure as the unit cell, the resonant wavelength is tapered across the metasurface to engineer the wavelength dependent spatial phase gradie…
We propose and experimentally demonstrate a planar array of optical bandpass filters composed of low loss dielectric metasurface layers sandwiched between two distributed Bragg reflectors (DBRs). The two DBRs form a Fabry-Pérot resonator whose center wavelength is controlled by the design of the transmissive metasurface layer which functions as a phase shifting element. We demonstrate an array of bandpass filters with spatially varying center wavelengths covering a wide range of operation wavelengths of 250 nm around λ = 1550 nm (Δλ/λ = 16%). The center wavelengths of each filter are independe…
We demonstrate that the metamaterial perfect absorber behaves as a meta-cavity bounded between a resonant metasurface and a metallic thin-film reflector. The perfect absorption is achieved by the Fabry-Perot cavity resonance via multiple reflections between the "quasi-open" boundary of resonator and the "closed" boundary of reflector. The characteristic features including angle independence, ultra-thin thickness and strong field localization can be well explained by this model. With this model, metamaterial perfect absorber can be redefined as a meta-cavity exhibiting high Q-factor, strong fie…
Nonlinear Pancharatnam-Berry phase metasurfaces facilitate the nontrivial phase modulation for frequency conversion processes by leveraging photon-spin dependent nonlinear geometric-phases. However, plasmonic metasurfaces show some severe limitation for nonlinear frequency conversion due to the intrinsic high ohmic loss and low damage threshold of plasmonic nanostructures. Here, we systematically study the nonlinear geometric-phases associated with the third-harmonic generation process occurring in all-dielectric metasurfaces, which are composed of silicon nanofins with different in-plane rota…
In the recent decade, the research field using arrays of high-index-contrast near-wavelength dieletric structures on flat surfaces, known as high-contrast metastructures (HCMs) or metasurfaces, has emerged and expanded rapidly. Although the HCMs and metasurfaces share great similarities in physical structures with photonic crystals (PhCs), i.e. periodic nanostructures, many differences exist in their design, analysis, operation conditions, and applications. In this paper, we provide a generalized theoretical understanding of the two subjects and show their intrinsic connections. We further dis…
An all-dielectric metasurface featuring resonant conditions of the trapped mode excitation is considered. It is composed of a lattice of subwavelength particles which are made of a high-refractive-index dielectric material structured in the form of disks. Each particle within the lattice behaves as an individual dielectric resonator supporting a set of electric and magnetic (Mie-type) modes. In order to access a trapped mode (which is the TE01 mode of the resonator), a round eccentric penetrating hole is made in the disk. In the lattice, the disks are arranged into clusters (unit super-cells) …
Since the discovery of the laser, optical parametric nonlinearities have been at the core of efficient light conversion sources. Typically, thick transparent crystals or quasi-phase matched waveguides, are utilized in conjunction with phase-matching techniques to select a single parametric process. In recent years, due to the rapid developments in artificially structured materials, optical frequency mixing has been achieved at the nanoscale in subwavelength resonators arrayed as metasurfaces. Phase matching becomes relaxed for these wavelength-scale structures, and all allowed nonlinear proces…
Frequency conversion processes, such as second- and third-harmonic generation, are one of the most common effects in nonlinear optics which offer many opportunities for photonics, chemistry, material science, characterization, and biosensing. Given the inherently weak nonlinear response of natural materials, one typically relies on optically large samples and complex phase-matching techniques to achieve nonlinear effects. A direct translation of these approaches to small dimensions, however, is extremely challenging, because nonlinear effects locally comparable to the linear response cannot be…
Most optical systems involve a combination of lenses separated by free-space regions where light acquires the required angle-dependent phase delay for a certain functionality. Very recently, flat-optics structures have been proposed to compress these large free-space volumes and miniaturize the overall optical system. However, these early designs can only replace free-space volumes of limited length, or operate in a very narrow angular range, or require a high-index background. These issues raise questions about the applicability of these devices in practical scenarios. Here, we first derive a…
Ultrathin metasurfaces have recently emerged as promising materials to enable novel, flat optical components and surface-confined, miniature photonic devices. However, experimental realization of high-performance metasurfaces at visible frequencies has been a significant challenge due to high plasmonic losses and difficulties in high-uniformity nanofabrication. Here, we propose a highly-efficient yet simple metasurface design comprising of single gradient antenna as unit cell. We demonstrate visible broadband (450 - 850 nm) anomalous reflection and spectrum splitting with 85% conversion effici…
Metamaterials comprising assemblies of dielectric resonators have attracted much attention due to their low intrinsic loss and isotropic optical response. In particular, metasurfaces made from silicon dielectric resonators have shown desirable behaviors such as efficient nonlinear optical conversion, spectral filtering and advanced wave-front engineering. To further explore the potential of dielectric metamaterials, we present all-dielectric metamaterials fabricated from epitaxially grown III-V semiconductors that can exploit the high second-order optical susceptibilities of III-V semiconducto…
