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

The rise in output power from rare-earth-doped fiber sources over the past decade, via the use of cladding-pumped fiber architectures, has been dramatic, leading to a range of fiber-based devices with outstanding performance in terms of output power, beam quality, overall efficiency, and flexibility with regard to operating wavelength and radiation format. This success in the high-power arena is largely due to the fiber’s geometry, which provides considerable resilience to the effects of heat generation in the core, and facilitates efficient conversion from relatively low-brightness diode pump radiation to high-brightness laser output. In this paper we review the current state of the art in terms of continuous-wave and pulsed performance of ytterbium-doped fiber lasers, the current fiber gain medium of choice, and by far the most developed in terms of high-power performance. We then review the current status and challenges of extending the technology to other rare-earth dopants and associated wavelengths of operation. Throughout we identify the key factors currently limiting fiber laser performance in different operating regimes—in particular thermal management, optical nonlinearity, and damage. Finally, we speculate as to the likely developments in pump laser technology, fiber design and fabrication, architectural approaches, and functionality that lie ahead in the coming decade and the implications they have on fiber laser performance and industrial/scientific adoption.

High power fiber lasers: current status and future perspectives [Invited]

A synthetic strategy for the creation of large scale chemical diversity. Solid-phase chemistry, photolabile protecting groups, and photolithography are used to achieve light-directed spatially-addressable parallel chemical synthesis. Binary masking techniques are utilized in one embodiment. A reactor system, photoremovable protective groups, and improved data collection and handling techniques are also disclosed. A technique for screening linker molecules is also provided.

Arrays of materials attached to a substrate

An imaging system for a vehicle includes an imaging device having a field of view exteriorly and forward of the vehicle in its direction of travel, and an image processor operable to process the captured images in accordance with an algorithm. The algorithm comprises a sign recognition routine and a character recognition routine. The image processor processes the image data captured by the imaging device to detect signs in the field of view of the imaging device and applies the sign recognition routine to determine a sign type of the detected sign. The image processor is operable to apply the character recognition routine to the image data to determine information on the detected sign. The image processor applies the character recognition routine to the captured images in response to an output of the sign recognition routine being indicative of the detected sign being a sign type of interest.

Imaging system for vehicle

A method and apparatus for preparation of a substrate containing a plurality of sequences. Photoremovable groups are attached to a surface of a substrate. Selected regions of the substrate are exposed to light so as to activate the selected areas. A monomer, also containing a photoremovable group, is provided to the substrate to bind at the selected areas. The process is repeated using a variety of monomers such as amino acids until sequences of a desired length are obtained. Detection methods and apparatus are also disclosed.

Very large scale immobilized polymer synthesis

The method utilizes high resolution lithography, mask aligning, and reactive ion etching. In particular, at least two binary amplitude masks are generated. A photoresist layer on an optical element substrate is exposed through the first mask and then etched. The process is then repeated for the second and subsequent masks to create a multistep configuration. The resulting optical element is highly efficient.

High-efficiency, multilevel, diffractive optical elements

Diffractive optical elements have the potential to improve the performance of infrared optical systems. An approach to constructing high quality diffractive elements has been developed using standard integrated circuit techniques. It is possible to implement arbitrary phase profiles since the elements are computer generated.

Diffractive optical elements for use in infrared systems

Binary optics are used in an illumination system to provide a method of imaging for a color projection display. In one embodiment a broad spectrum light source illuminates a multilevel optical phase element which disperses the broad spectrum light from the light source by diffraction. A display having a number of pixel elements, each capable of transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element so as to receive the light dispersed by the multilevel phase element.

Method of imaging using a liquid crystal display device

Binary phase diffraction gratings are shown to couple light coherently from a laser array into a single on-axis beam. The diffraction grating, designed to split a single beam into a specific number of equal intensity diffraction orders, is placed inside the cavity formed by the laser array and a common output mirror. The grating superimposes the light beams from the lasers in the array and produces a far-field pattern with the same divergence as that of a single laser. Six GaAlAs lasers from an antireflection-coated linear array were combined with a coupling efficiency of 68.4%. The far field of the combined GaAlAs lasers consisted of a single on-axis Gaussian beam.

Coherent laser addition using binary phase gratings

We describe a phase plate based on the fractional-Talbot effect that converts a single expanded laser beam into a regular array of uniformly illuminated apertures with virtually 100% efficiency. The size, spacing, and fill factor of the illuminated aperture grid can be freely chosen to interface with a variety of electro-optic devices. A binary-optics phase plate is demonstrated that converts uniform illumination into an array of square illumination cells with a fill factor of 1/16.

Gary J. Swanson

Papers

High-efficiency, multilevel, diffractive optical elements

Diffractive optical elements for use in infrared systems

Method of imaging using a liquid crystal display device

Coherent laser addition using binary phase gratings

Efficient array illuminator using binary-optics phase plates at fractional-Talbot planes