Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

A light-emitting device package including: a package main body including a cavity and a lead frame including a mounting portion disposed in the cavity and a plurality of terminal portions; a light-emitting device chip mounted on the mounting portion; a plurality of bonding wires for electrically connecting the plurality of terminal portions and the light-emitting device chip; a light-transmitting encapsulation layer filled in the cavity; and a light-transmitting cap member disposed in the cavity and blocking the encapsulation layer to contact the plurality of bonding wires.

Light emitting device package

A light-emitting element of the present invention can have sufficiently high emission efficiency with a structure including a host material being able to remain chemically stable even if a phosphorescent compound having higher emission energy is used as a guest material. The relation between the relative emission intensity and the emission time of light emission obtained from the host material and the guest material contained in a light-emitting layer is represented by a multicomponent decay curve. The relative emission intensity of the slowest component of the multicomponent decay curve becomes 1/100 for a short time within a range where the slowest component is not interfered with by quenching of the host material (the emission time of the slowest component is preferably less than or equal to 15 μsec); thus, sufficiently high emission efficiency can be obtained.

Light emitting element, light emitting device, electronic device, and lighting device

We report here on the theoretical performance of blazed binary diffractive elements composed of pillars carefully arranged on a two-dimensional grid whose period is smaller than the structural cutoff. These diffractive elements operate under unpolarized light. For a given grating geometry, the structural cutoff is a period value above which the grating no longer behaves like a homogeneous thin film. Because the grid period is smaller than this value, effective-medium theories can be fully exploited for the design, and straightforward procedures are obtained. The theoretical performance of the blazed binary elements is investigated through electromagnetic theories. It is found that these elements substantially outperform standard blazed echelette diffractive elements in the resonance domain. The increase in efficiency is explained by a decrease of the shadowing effect and by an unexpected sampling effect. The theoretical analysis is confirmed by experimental evidence obtained for a 3λ-period prismlike grating operating at 633 nm and for a 20°-off-axis diffractive lens operating at 860 nm.

/pdf/design-and-fabrication-of-blazed-binary-diffractive-elements-558xsgkst7.pdf

Design and fabrication of blazed binary diffractive elements with sampling periods smaller than the structural cutoff

A laser writing system for the fabrication of continuous-relief micro-optical elements in photoresist is described. The technology enables a wide range of planar micro-optical elements to be fabricated and replicated into polymer film using Ni shims electroformed from the photo-resist originals. The advantages and limitations of laser writing technology for micro-optics fabrication are discussed. Examples of fabricated micro-optical elements include Fresnel microlenses and microlens arrays, kinoforms, and other continuous-relief phase elements.

Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists

The refractive and the diffractive properties of planar micro-optical elements are investigated. The transition between purely refractive and purely diffractive planar microlenses is numerically simulated for the example of differently designed phase-matched Fresnel elements. Results obtained from numerical simulations and experiments show that the refractive and diffractive types exhibit a distinctly different behavior in the presence of small fabrication errors or wavelength deviations. Based on these results, design rules for various applications, including low- and high-numerical-aperture lenses and hybrid refractive–diffractive elements, are derived. For a high-numerical-aperture (f/# = 1.0) lens the experimental characterization of the irradiance distribution in the image space is presented and shown to agree well with theoretical predictions.

/pdf/refractive-and-diffractive-properties-of-planar-micro-3huydfwphf.pdf

Refractive and diffractive properties of planar micro-optical elements.

Continuous surface-relief diffractive optical elements for two-dimensional array generation (fan-out) are designed and fabricated. Separable and nonseparable solutions for the two-dimensional element design are compared. The phase-grating microstructures are generated by laser-beam writing lithography in a single exposure step and converted to nickel shims by electroplating, enabling low-cost replicas to be produced by using laboratory and commercial replication processes. Results are presented for a 9 x 9 fan-out diffractive optical element with a measured efficiency of 94% and an overall uniformity within +/-8%; replicas in epoxy have the same efficiency and a uniformity of +/-15%.

https://doc.rero.ch/record/8761/files/Gale_Michael_T._-_Continous-relief_diffractive_optical_elements_20080213.pdf

Continuous-relief diffractive optical elements for two-dimensional array generation.

The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of μ > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-μm period to a value of 79%.

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

An opto-electronic module includes a detecting channel comprising a detecting member for detecting light and an emission channel comprising an emission member for emitting light generally detectable by said detecting member Therein, a radiation distribution characteristic for an emission of light from said emission channel is non rotationally symmetric; and/or a sensitivity distribution characteristic for a detection in said detecting channel of light incident on said detection channel is non rotationally symmetric; and/or a central or main emission direction for an emission of light from said emission channel and a central or main detection direction for a detection of light incident on said detection channel are aligned not parallel to each other; and/or at least a first one of the channels comprises one or more passive optical components

Markus Rossi

Papers

Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists

Refractive and diffractive properties of planar micro-optical elements.

Continuous-relief diffractive optical elements for two-dimensional array generation.

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

Opto-electronic module and devices comprising the same