The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

All-dielectric metamaterials

Plasmonic metasurfaces have recently attracted much attention due to their ability to abruptly change the phase of light, allowing subwavelength optical elements for polarization and wavefront control. However, most previously demonstrated metasurface designs suffer from low coupling efficiency and are based on metallic resonators, leading to ohmic loss. Here, we present an alternative approach to plasmonic metasurfaces by replacing the metallic resonators with high-refractive-index silicon cut-wires in combination with a silver ground plane. We experimentally demonstrate that this meta-reflectarray can be used to realize linear polarization conversion with more than 98% conversion efficiency over a 200 nm bandwidth in the short-wavelength infrared band. We also demonstrate optical vortex beam generation using a meta-reflectarray with an azimuthally varied phase profile. The vortex beam generation is shown to have high efficiency over a wavelength range from 1500 to 1600 nm. The use of dielectric resonato...

Dielectric Meta-Reflectarray for Broadband Linear Polarization Conversion and Optical Vortex Generation

The so-called ‘flat optics’ that shape the amplitude and phase of light with high spatial resolution are presently receiving considerable attention. Numerous journal publications seemingly offer hope for great promises for ultra-flat metalenses with high efficiency, high numerical aperture, broadband operation… We temperate the expectation by referring to the current status of metalenses against their historical background, assessing the technical and scientific challenges recently solved and critically identifying those that still stand in the way.

Metalenses at visible wavelengths: past, present, perspectives

We have proposed, designed, manufactured and tested low loss dielectric micro-lenses for infrared (IR) radiation based on a dielectric metamaterial layer. This metamaterial layer was created by patterning a dielectric surface and etching to sub-micron depths. For a proof-of-concept lens demonstration, we have chosen a fine patterned array of nano-pillars with variable diameters. Gradient index (GRIN) properties were achieved by engineering the nano-pattern characteristics across the lens, so that the effective optical density of the dielectric metamaterial layer peaks around the lens center, and gradually drops at the lens periphery. A set of lens designs with reduced reflection and tailorable phase gradients have been developed and tested, demonstrating focal distances of a few hundred microns, beam area contraction ratio up to three, and insertion losses as low as 11%.

All-dielectric subwavelength metasurface focusing lens

Diffraction gratings disperse light in a rainbow of colors with the opposite order than refractive prisms, a phenomenon known as negative dispersion. While refractive dispersion can be controlled via material refractive index, diffractive dispersion is fundamentally an interference effect dictated by geometry. Here we show that this fundamental property can be altered using dielectric metasurfaces, and we experimentally demonstrate diffractive gratings and focusing mirrors with positive, zero, and hyper-negative dispersion. These optical elements are implemented using a reflective metasurface composed of dielectric nano-posts that provide simultaneous control over phase and its wavelength derivative. In addition, as a first practical application, we demonstrate a focusing mirror that exhibits a five-fold reduction in chromatic dispersion, and thus an almost three-times increase in operation bandwidth compared with a regular diffractive element. This concept challenges the generally accepted dispersive properties of diffractive optical devices and extends their applications and functionalities.

Controlling the sign of chromatic dispersion in diffractive optics with dielectric metasurfaces

The efficiency of conventional diffractive optical elements with echelette-type profiles drops rapidly as the illumination wavelength departs from the blaze wavelength. We use high dispersion of artificial materials to synthesize diffractive optical elements that are blazed over a broad spectral range (∼1 octave) or for two different wavelengths.

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Broadband blazing with artificial dielectrics.

Blazed diffractive elements are currently fabricated with continuous profiles or with staircase approximations. Blazed-binary gratings are more recent diffractive elements which implement continuous phase delays through a gradient-index artificial material with subwavelength binary etches. The performance of these diffractive elements in the resonance domain opens up new perspectives for manufacturing fast lenses. The objective of this paper is to review the main and unique properties of blazed-binary diffractive elements and to consider the consequences for monochromatic imaging systems.

https://iopscience.iop.org/article/10.1088/1464-4258/4/5/358/pdf

Imaging with blazed-binary diffractive elements

Blazed-binary gratings for which a blazed effect with binary etches is achieved under normal incidence offer first-order diffraction efficiencies larger than those of blazed-echelette gratings in the resonance domain [Opt. Lett.23, 1081 (1998)]. We provide further insight into the behavior of blazed-binary gratings and show that they operate efficiently under symmetrical mounting and over a wide field-angle interval. These properties are illustrated with theoretical and experimental results obtained for an ≈1000-line/mm grating at 633 nm.

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Wide-field-angle behavior of blazed-binary gratings in the resonance domain.

Hybrid refractive-diffractive optics are widely used in infrared systems, but their performance is limited by reduced diffraction efficiency away from the design wavelength Two techniques are currently being investigated to improve broadband efficiency; dual-layer blaze struct ures and blazed-binary optics This pa per discusses the design of dual-layer blaze structures in detail, and presents so me athermalised lenses which benefit fr om this approach A brief summary of using blazed-binary structures to im prove efficiency is presented Keywords: IR lenses, optical design, diffractive optics, blazed binary structures 1 INTRODUCTION Diffractive optics in the form of surface -relief blaze structures di amond-turned directly onto th e surface of conventional refractive lens elements are well-established and widely used in both mid-wave and long-wave infrared optical systems The diffractive surface in these so-called hybrid refractive-diffractive elements is used principally for the correction of chromatic aberration Hybrid optics enable a reduction in the number of lens elements and material types required in a particular design construction; this can have significant benefits to cost, mass, tolerance sensitivity and overall complexity of the optical system The principal disadvantage of employing these diffractive structures is that a proportion of the incident light is inevitably diffracted into spurious orders, degrading the image quality

Infrared hybrid optics with high broadband efficiency

We report on the design fabrication and characterization a 3l-period grating composed of subwavelength ridges of progressively varying widths for operation at 10.6 mm. The grating is blazed into the first transmitted order (an efficiency of 80% is measured) under TM po- larization and over a broad range of angles of incidence. The fabrication involves contact photolithography, reactive-ion etching, and an evapora- tion deposition over the etched structure. The result validates the use of photolithography, a low-cost technology, for the manufacture of efficient blazed binary diffractive elements for thermal imaging (the 8- to 12-mm IR band). © 2004 Society of Photo-Optical Instrumentation Engineers.

Mane-Si Laure Lee

Papers

Broadband blazing with artificial dielectrics.

Imaging with blazed-binary diffractive elements

Wide-field-angle behavior of blazed-binary gratings in the resonance domain.

Infrared hybrid optics with high broadband efficiency

Blazed binary diffractive gratings with antireflection coating for improved operation at 10.6 μm