Tunable structured light with flat optics

Abstract

Flat optics has emerged as a key player in the area of structured light and its applications, owing to its subwavelength resolution, ease of integration, and compact footprint. Although its first generation has revolutionized conventional lenses and enabled anomalous refraction, new classes of meta-optics can now shape light and dark features of an optical field with an unprecedented level of complexity and multifunctionality. Here, we review these efforts with a focus on metasurfaces that use different properties of input light—angle of incidence and direction, polarization, phase distribution, wavelength, and nonlinear behavior—as optical knobs for tuning the output response. We discuss ongoing advances in this area as well as future challenges and prospects. These recent developments indicate that optically tunable flat optics is poised to advance adaptive camera systems, microscopes, holograms, and portable and wearable devices and may suggest new possibilities in optical communications and sensing.

Actively structuring light

The development of metasurfaces has provided a route to replacing bulk optical components with thin layers of engineered materials. In a review, Dorrah and Capasso highlight some of the recent advances in wavefront shaping using multifunctional meta-optics. They focus on the ability to tune the response of the metasurface by simply tuning one or more degrees of freedom of incident light, for example, by varying its angle of incidence, polarization, wavelength, or phase. The key feature of these metasurfaces is that although they are static, they can produce a tunable response without the need for complex switching. These developments enable multifunctional and lightweight components for technologies such as augmented and virtual reality displays, drone-based sensing, and endoscopy. —ISO

Abstract

A review discusses methods to control the functionality of optical metasurfaces by the incident light.

Abstract

BACKGROUND

Structuring the degrees of freedom of light—including its phase, amplitude, and polarization—has opened new frontiers in science and technology alike. Adaptive cameras, microscopes, portable and wearable devices, optical communications, and laser machining are only a few of the domains that have evolved over the past decade owing to the advances in wavefront-shaping platforms. Flat optics composed of subwavelength-spaced optical scatterers—also known as metasurfaces or meta-optics—are key enabling tools for structured light not only for their compact footprint and complementary metal-oxide semiconductor (CMOS) compatibility but also because of their versatility and custom design. Although flat optics, or at least in its first generation, has led to the development of effects like anomalous refraction and diffraction-limited focusing, new classes of metasurfaces can now mold the flow of light in much more complex ways. Dispersion engineering and polarization optics are two prominent areas in which the metasurface’s ability to spatially manipulate each wavelength and/or polarization state, independently, cannot be paralleled using bulk optical components. At the heart of these developments is a carefully engineered light-matter interaction at the level of the meta-atom, which allows a passive metasurface to produce this complex response.

ADVANCES

The ability to manipulate light in different ways, depending on its properties, is intriguing because it allows a passive device to produce many functions without the need for active switching—that is, light itself can be used as an optical control knob. For example, the same flat optic may behave as a lens or a mirror depending on the incidence angle of light. Likewise, by changing light’s polarization, a metasurface can switch between different holograms or modify its focal length. In this spirit, a new generation of meta-optics can now perform parallel processing of the polarization of input light in the transverse plane or in 3D, reducing the function of many polarizers and waveplates into a single optical component that can be integrated in polarimeters and cellphone cameras. The spatial phase distribution of incoming light is another degree of freedom that can also be used as a switch, allowing a static flat optic to project different holograms by varying the helicity of the incident wavefront or its phase profile in general. Moreover, the ability to impart different phase and/or amplitude profiles on different wavelengths, independently, has enabled a wide class of versatile metalenses and compact pulse-shaping tools. Harnessing the nonlinear interaction of light with meta-atoms has also enabled multiwavelength holography on high harmonic-generated signals in addition to an asymmetric response. This versatility has made flat optics an ideal platform for the generation of structured light and has inspired many applications. The figure depicts the use of a metasurface as a multipurpose device (akin to a Swiss knife) that can mix and match output light by tuning the five abovementioned control knobs, without the need for any complex circuitry to drive the meta-atoms. Tunable behavior of this kind relies on an intricate light-matter interaction at the nanoscale, which is often difficult to replicate with other wavefront-shaping tools. With such tunability, many existing technologies can be dramatically miniaturized, enabling compact spectrometers, polarization-sensitive cameras, lightweight augmented reality and virtual reality headsets, and biomedical devices.

OUTLOOK

As the area of structured light matures, the quest for more sophisticated metasurfaces is also on the rise, aided by advanced nanofabrication, powerful computation capabilities for revealing new meta-atom libraries, and recent developments in actively tunable materials for time-varying control. Structured light with tunable metasurfaces is poised to reveal new functionalities and to replace conventional optical systems with on-chip photonic components. This includes integrating metasurfaces in laser cavities, Fabry-Perot resonators, fiber-based devices, and active wavefront-shaping tools. With the emerging trends in inverse design and topology optimization, new standardized protocols for large-scale multilayer metasurface fabrication and innovative material platforms will push the limits of multifunctional meta-optics and structured light from 2D to 3D and from static to animate, thus tackling the open challenges in this wide field of research and unlocking many new paths.