Metasurfaces are arrays of subwavelength anisotropic light scatters (optical antennas) that can produce abrupt changes in the phase, amplitude, or polarization of light. Within the last few years significant progress has been made in the design of metasurfaces that refract and focus light, enabling many unique properties and applications such as holograms, optical vortex generation/detection, ultrathin focusing lens, perfect absorber, etc.
This tutorial will cover the fundamental principles, advanced designs and technological applications of optical metasurfaces, particularly focusing on the topics of (1) the development of high-performance metasurface components and systems, (2) the advancement of nano-electro-mechanical systems (NEMS)-based tunable dielectric metasurfaces, (3) synthetic interfacial optics with metasurfaces and 2D materials, and (4) the numerical design methods for high-performance dielectric metasurfaces.
High-Performance Metasurface Flat Optics: From Components to Systems
Federico Capasso, Harvard University
Metasurfaces are leading to the emergence of new optical components that circumvent the limitations of standard refractive and diffractive ones by enabling dispersion engineering, which also leads to entirely new functionalities based on the local control of phase amplitude and polarization. The formulation of the generalized Snell’s law for metasurfaces and the use of dispersion engineering have led to the demonstration of metalenses with correction of monochromatic aberrations and to achromatic single lenses across the entire visible spectrum. The planarity of flat optics will lead to the unification of semiconductor manufacturing and lens-making, where the planar technology to manufacture computer chips will be used to make CMOS-compatible optical components for a wide range of applications. Instruments based on flat optics will be discussed such as ultracompact spectrometers, endoscopes for the detection of bronchial cancer, polarimeters and polarization sensitive cameras without moving parts and conventional polarization optics.
10:00 am BREAK
Creating Metasurfaces and Metadevices with Mie Resonators
Mark Brongersma, Stanford University
Semiconductor nanostructures are at the heart of electronic devices and systems. When properly sized and shaped, they can also support optical, Mie-type resonances that are capable of boosting light–matter interaction over bulk materials. By combining their desirable electronic and optical properties, it is possible to create new optoelectronic functionalities. In this tutorial talk, I will start with a discussion of the basic properties of Mie resonators and show how they have been harnessed to create a large variety of passive metasurfaces. One of the key application areas for passive metasurfaces is the realization of flat optical components that can replace more bulky traditional counterparts (e.g., lenses and gratings). One of the next frontiers is to realize active flat optics capable of dynamically shaping optical wavefronts to, for example, tune the focal length of a lens or to actively steer laser beams. I will illustrate different approaches to achieve such active functions. Another area of interest that I will highlight is the development of Mie-resonant antennas and metasurfaces that enable real-time measurement of the wavelength, polarization state, angle of incidence and angular momentum of light.
Synthetic Interfacial Optics with Metasurfaces and Transition-Metal Dichalcogenide Monolayer
Cheng-Wei Qiu, National University of Singapore
Metasurfaces and 2D materials have been developing as two important candidates in interfacial engineering, providing a plethora of new possibilities in novel optoelectronic functions and applications. The synergies between those two domains hold great promises in manipulating light–matter interaction. In this tutorial talk, I will review and report some of the most recent developments in this field of interfacial engineering, via the artificially constructed hybridized structures of ultrathin thickness compared to the wavelength. In particular, the low-dimension and high-frequency scaling may promise a lot more applications, while the challenges in design principle and fabrication capability will become critical limits. The atomic thickness of 2D monolayers provides many interesting physical properties, while it also limits the sufficient interaction with light. Hence, nano-patterned metasurfaces are deployed with 2D monolayers to modulate and structure novel light behavior. The following advanced functional optical devices, developed by our group, will be discussed: 3D meta-hologram, high-pixelated nanoprinting, dynamic OAM generation, and more interestingly, the 2D-material meta-lens of <1-nm thickness, significantly enhanced SHG, PL, and tunable structural colors, by the coordinated hybridization between those two parties. Our work paves a roadmap to design sophisticated and advanced optical devices with low dimension, miniaturization, randomness and scaled-up capability.
3:00 pm BREAK
Nonlinear Optical Metasurfaces: From Enhanced Light-Matter-Interaction to Full Phase Tailoring
Thomas Zentgraf, Universität Paderborn
For efficient nonlinear processes, the engineering of the nonlinear optical properties of media becomes an important task. The most well-known technique for spatially engineering nonlinear optical properties is the quasi-phase matching scheme for second-order processes like second-harmonic generation. However, the widely used technique of periodic polling of natural crystals only provides a binary state for the nonlinear material polarization, which is equivalent to a discrete phase change of π of the nonlinear polarization. The continuous tailoring of the phase of the nonlinear susceptibility would greatly enhance flexibility in the design and reduce parasitic effects.
In this tutorial, we will discuss nonlinear metamaterials with a continuously controllable phase of the local effective nonlinear polarizability. We will focus on plasmonic metasurfaces with various designs for the meta-atom geometry together with different polarization states of the light. In particular for circular polarization states, the controllable nonlinearity phase results from the phase accumulation due to the polarization change along the polarization path on the Poincaré Sphere (the so-called Pancharatnam–Berry phase) and depends therefore only on the spatial geometry of the metasurface. By using a fixed orientation of the meta-atom, the nonlinear phase can be spatially arbitrarily tailored over the entire range from 0 to 2π. In contrast to the quasi-phase matching scheme, the continuous phase engineering of the effective nonlinear polarizability enables complete control of the propagation of harmonic generation signals, and therefore, it seamlessly combines the generation and manipulation of the harmonic waves for highly compact nonlinear nanophotonic devices.
We will discuss the concepts of enhancing nonlinear processes with simultaneous phase engineering for the manipulation of second- and third-harmonic generation from metasurfaces and the restrictions with respect to symmetry and geometry of meta-atoms. Nonlinear metamaterials have fundamental significance in nonlinear optics and for tailored nonlinearities, as they provide a further degree of freedom in the design of nonlinear materials.