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 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.

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

We introduce a new structural cutoff beyond which subwavelength gratings cease to behave as homogeneous media and discuss its effects on the proper selection of the sampling periods of subwavelength diffractive elements. According to this analysis, a 3?-period blazed binary grating composed of square pillars is designed for He–Ne operation and is fabricated by etching of a TiO2 layer deposited upon a glass substrate. Its first-order measured diffraction efficiency is 12% larger than the theoretical efficiency of an ideal blazed echelette grating in glass with the same period.

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Blazed binary subwavelength gratings with efficiencies larger than those of conventional échelette gratings

With rapid advances in high-speed communication and computation, augmented reality (AR) and virtual reality (VR) are emerging as next-generation display platforms for deeper human-digital interactions. Nonetheless, to simultaneously match the exceptional performance of human vision and keep the near-eye display module compact and lightweight imposes unprecedented challenges on optical engineering. Fortunately, recent progress in holographic optical elements (HOEs) and lithography-enabled devices provide innovative ways to tackle these obstacles in AR and VR that are otherwise difficult with traditional optics. In this review, we begin with introducing the basic structures of AR and VR headsets, and then describing the operation principles of various HOEs and lithography-enabled devices. Their properties are analyzed in detail, including strong selectivity on wavelength and incident angle, and multiplexing ability of volume HOEs, polarization dependency and active switching of liquid crystal HOEs, device fabrication, and properties of micro-LEDs (light-emitting diodes), and large design freedoms of metasurfaces. Afterwards, we discuss how these devices help enhance the AR and VR performance, with detailed description and analysis of some state-of-the-art architectures. Finally, we cast a perspective on potential developments and research directions of these photonic devices for future AR and VR displays.

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Augmented reality and virtual reality displays: emerging technologies and future perspectives.

Recently, there has been an explosion of interest in metalenses for imaging. The interest is primarily based on their subwavelength thicknesses. Diffractive gratings have been used as thin optical elements since the late 19th century. Here, we show that multilevel diffractive lenses (MDLs), when designed properly, can exceed the performance of metalenses. Furthermore, MDLs can be designed and fabricated with larger constituent features, making them accessible to low-cost, large-area volume manufacturing, which is generally challenging for metalenses. The support substrate will dominate overall thickness for all flat optics. Therefore, the advantage of a slight decrease in thickness (from ∼2λ to ∼λ/2) afforded by metalenses may not be useful. We further elaborate on the differences between these approaches and clarify that metalenses have unique advantages when manipulating the electromagnetic fields, rather than intensity.

Imaging with flat optics: metalenses or diffractive lenses?

We propose the use of binary slanted surface-relief gratings with parallel-groove walls as input and output couplers in a planar optical interconnect. Parametric optimization of cascaded output couplers is employed to design an interconnect consisting of N output couplers producing a uniform intensity distribution with a high efficiency that may be realized in one lithographic etching step. The sensitivity of a N = 4 interconnect to various fabrication errors is analyzed. We demonstrate the operation of a slanted surface-relief grating manufactured with electron-beam lithography and reactive-ion etching for an operating wavelength of λ = 0.633 μm.

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Design and fabrication of binary slanted surface-relief gratings for a planar optical interconnection

We present the design, fabrication, and characterization of a subwavelength-pulse-width spatially modulated diffractive array illuminator for an operating wavelength of 0.633 μm. Electromagnetic and scalar diffraction theories are used to reduce manufacturing difficulties while yielding high diffraction efficiency coupled with low reconstruction error. We employ direct electron beam writing and reactive ion etching to realize a transmission-type three-beam array illuminator in photoresist.

Synthesis of a subwavelength-pulse-width spatially modulated array illuminator for 0.633 microm.

In the conception of a planar optical backplane interconnecting comprising of a uniform power distribution, we proposed to use cascaded high frequency diffractive optical elements, each with a different diffraction efficiency in the + 1 transmitted order. We introduced a new type of binary grating: the side walls are inclined angle. The gratings were optimized with a rigorous coupled wave algorithm, Here, we investigate various etching techniques in order to realize such slanted structures in fused silica. These methods include both ion beam milling and reactive sputter etching and their variants.

Design and fabrication of high-efficiency inclined binary high-frequency gratings

This paper describes the use of optics to enable collective communications between clusters of processors, within a `loosely synchronous' model of computation. In this model, phases of internal computation inside processors, alternate with phases of communication among processors. Collective communication, as in the new message passing interface (MPI) standard, means concerted action in a single command, e.g. shift of data from each processor to its right neighbor. Unlike previous work on general purpose optical implementation of given interconnects topology, that was concerned with providing only point-to-point communication, our work tries to provide services that improve programmability by the end- user. We are implementing a prototype with 3 clusters of 4 processors each, with an intra- cluster electronic crossbar. Inter-cluster communication is done via lasers transmitting message beams, which are suitably deflected in free-space to other clusters by means of acousto-optic devices. A typical collective communication comand is (multicast(source,Targetlist)), where the individual processors of the TargetList may reside in different clusters. Cluster addresses are encoded as acoustic wave input. Processor addresses are transmitted in the optical message headers, decoded at the detectors and redirected within the crossbar. The prototype itself and preliminary experiments are discussed.

Nicole de Beaucoudrey

Papers

Design and fabrication of binary slanted surface-relief gratings for a planar optical interconnection

Synthesis of a subwavelength-pulse-width spatially modulated array illuminator for 0.633 microm.

Design and fabrication of high-efficiency inclined binary high-frequency gratings

Optical implementation of collective communications