4:00pm - 4:30pmInvited
Flat and Conformal Optics with Dielectric Metasurfaces
California Institute of Technology, United States
Flat optical devices based on lithographically patterned sub-wavelength dielectric nano-structures provide precise control over optical wavefronts, and thus promise to revolutionize the field of free-space optics. I discuss our work on high contrast transmitarrays and reflectarrays composed of silicon nano-posts located on top of low index substrates like silica glass or transparent polymers. Complete control of both phase and polarization is achieved at the level of single nano-post, which enables control of the optical wavefront with sub-wavelength spatial resolution. Using this nano-post platform, we demonstrate lenses, waveplates, polarizers, arbitrary beam splitters and holograms. Devices that provide multiple functionalities, like simultaneous polarization beam splitting and focusing are implemented. By embedding the metasurfaces in flexible substrates, conformal optical devices that decouple the geometrical shape and optical function are shown. Multiple flat optical elements are integrated in optical systems such as planar retro-reflectors and Fourier lens systems with applications in ultra-compact imaging systems. Applications in microscopy and the prospects for tunable devices are discussed.
4:30pm - 4:45pmOral
An All-Dielectric Flat Lens with Near-Unity Numerical Aperture
Data Storage Institute, Agency for Science, Technology and Research (A*STAR), Singapore
The numerical aperture (NA) of a lens defines its ability to focus light and, by time reverse arguments, its resolving capabilities. Having a large NA, limited by the maximum acceptance angle of the lens, is thus a very desirable property, e.g. when one is interested in collecting as many photons as possible from a source located at its focus, or when tight light concentration is desired. While commercial lenses with large NA are readily available, these optical elements are rather expensive and quite bulky. Moreover, in practice, it is extremely difficult to exceed a NA of 0.95 for an air-embedded lens, setting an upper limit for the collection angle of around 72 degrees.
In the present work we will show that, employing the optical properties of resonant dielectric nanoantennas, it is possible to produce a flat lens with a numerical aperture approaching the ultimate, unity limit (NA > 0.99 at the operating wavelength, λ) and deep, sub-wavelength thickness (λ /4). The lens, with a collection angle exceeding 82 degrees, works on the basis of nanoantenna-mediated, energy redistribution in the periodic gratings defining the different Fresnel zones of the lens. Each nanoantenna is tailored to concentrate the incoming electromagnetic energy into certain desired diffracted order and to supress energy channelling into the other ones. This is achieved by careful design of the scattering patterns of the antennas which, in turn, are controlled by the set of multipoles (amplitude, phase and orientation) supported by them.
The experimental characterization of a lens operating at 720 nm wavelength, based on silicon inclusions, will be presented, together with the general principles and design rules governing this new class of devices.
4:45pm - 5:15pmInvited
Perfect Control of Reflection using Non-local Metasurfaces
1Aalto University, Finland; 2University of Massachusetts Amherst, United States
In this review presentation we explain how single-layer metasurfaces can be designed for perfect control of reflected fields, in the sense that the reflected power is sent into any desired direction or directions and no power is scattered into unwanted directions. In particular, we present examples of impenetrable metasurfaces for anomalous reflection and beam splitting. We show that passive perfectly performing anomalous reflectors must be non-local and that this property can be realized by proper engineering the reactive impedance of the metasurface.
5:15pm - 5:45pmKeynote
Quantum and Classical Anomalies in Extreme Photonic Platforms
University of Pennsylvania, United States
Motivated by the phenomenon of supercoupling in the epsilon-near-zero (ENZ) structures [M. Silveirinha & N. Engheta, Phys. Rev. Lett. 97, 157403 (2006)], we have been extensively exploring various features of light-matter interaction in epsilon-near-zero (ENZ), mu-near-zero (MNZ), and epsilon-and-mu-near-zero (EMNZ) structures. This category of materials, which we call extreme photonic platforms, has proven to exhibit unconventional characteristics when they interact with electromagnetic waves and fields. In such zero-index media, the connection between the frequency of the continuous wave and its wavelength is “loosened”, enabling unprecedented characteristics in wave dynamics and quantum optics [N. Engheta, Science, 340, 286-287 (2013); I. Liberal & N. Engheta, Optics and Photonics News (OPN), July/August issue (2016), I. Liberal & N. Engheta, Proceedings of the National Academy of Sciences (PNAS), published online January 17, 2017]. Since the wavelength in such structures is “stretched” for the frequency of operation, the physical sizes of obstacles located in them, while sizable physically, may appear to be small compared to this wavelength. This causes a variety of peculiar electromagnetic phenomena to occur. For example, we have shown that resonant cavities formed by such zero-index media may exhibit resonance frequencies independent of the shape of their external geometries [I. Liberal, A. Mahmoud, N. Engheta, Nature Communications, 7:10989 (2016)], providing a useful structure for flexible photonics. Such cavities may offer the photonic bound state in the continuum (BIC) [I. Liberal & N. Engheta, Science Advances, 2: e1600987 (2016)], a phenomenon with interesting quantum optical implications. In this talk, we will present some of our most recent findings in these topics.
5:45pm - 6:15pmInvited
Real and Imaginary Properties of Epsilon-near-Zero Materials
Georgia State University, United States
Following Ref. , we theoretically consider fundamental properties of Epsilon-near-Zero (ENZ) materials. The fundamental principle of causality dictates that any ENZ material with a very low (asymptotically zero) loss at the observation frequency has necessarily asymptotically zero group velocity at that frequency. Physically, this leads to enhanced scattering and dissipative losses as given by the diverging energy-loss function, L(ω)→0. Paradoxically, a reduction of the intrinsic loss, ε''→0, leads to an increase of energy-loss function and further deterioration of performance of reflectors and waveguides built from ENZ materials. Both analytically and numerically we have shown that a realistic ENZ material ITO at the bulk plasma frequency (the ENZ point) causes high reflection and propagation losses. The singular loss function is also responsible for anomalously strong optical damping of resonant systems (plasmonic nanoparticles, dye molecules, quantum dots, etc.) embedded into or positioned at the surfaces of ENZ materials. In contrast to plasmonic metals, there are no pronounced hot spots of local fields at rough ENZ surfaces. Structured dielectric media with practically zero loss in the optical region cannot function as true ENZ materials because of the singular response, L(ω)→0 ; they necessarily are diffractive photonic crystals, and not refractive effective media. Obviously, this anomalous loss of ENZ materials can be gainfully used in energy absorbers, which begets analogy with heating of plasmas at plasma frequency with charged particles or electromagnetic waves. These losses and singularities are fundamental, local properties of the ENZ media, which cannot be eliminated by micro- or nano-structuring.
 M. H. Javani and M. I. Stockman, Real and Imaginary Properties of Epsilon-near-Zero Materials, Phys. Rev. Lett. 117, 107404-1-6 (2016).