This PDF file contains the front matter associated with SPIE Proceedings Volume 10928, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Recent progress of vertical-cavity surface-emitting lasers (VCSELs) using high contrast grating (HCG) and their applications will be reviewed. A typical electrically-pumped VCSEL consists of two oppositely doped distributed Bragg reflectors (DBRs) with a cavity layer in between. This conventional design requires epitaxy that is 6-8 micron thick having 0.1% precision in composition and thickness. Replacing the top DBR with a single layer, ultra-thin high contrast grating not only drastically increases the epitaxy and processing yield, but enables a wide and continuous wavelength swept. We will discuss recent progress of electrically-pumped, tunable VCSELs emitting at 850-nm, 940-nm, 1060-nm, 1300-nm and 1550-nm. New applications in optical coherence tomography, 3D sensing and LIDAR will be discussed.

Metamaterials---artificial electromagnetic media that are structured on the subwavelength scale---were initially suggested for the realisation of negative-index media, and later they became a paradigm for engineering electromagnetic space and control¬ling propagation of waves. However, applications of metamaterials in optics are limited due to inherent losses in metals employed for the realisation of artificial optical magnetism. Recently, we observe the emergence of a new field of all-dielectric resonant meta-optics aiming at the manipulation of strong optically-induced electric and magnetic Mie-type resonances in dielectric and semiconductor nanostructures with relatively high refractive index. Unique advantages of dielectric resonant nanostructures over their metallic counterparts are low dissipative losses and the enhancement of both electric and magnetic fields that provide competitive alternatives for plasmonic structures including optical nanoantennas, efficient biosensors, passive and active metasurfaces, and functional metadevices. This talk will summarize the most recent advances in all-dielectric Mie-resonant meta-optics including active nanophotonics as well as the recently emerged fields of topological photonics and nonlinear metasurfaces.

Subwavelength optical resonators and scatterers are dramatically expanding the toolset of the optical sciences and photonics engineering. By offering the opportunity to control and shape light waves in nanoscale volumes, recent developments using high-refractive-index dielectric scatterers gave rise to efficient flat-optical components such as lenses, polarizers, phase plates, color routers, and nonlinear elements with a subwavelength thickness. Here, we take a deeper look into the unique interaction of light with amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source—an aperture scanning near-field probe [1,2]. Scanning near-field optical microscopy (SNOM) is a powerful tool to image the near-field distribution of resonant optical modes supported by nanophotonic structures with sub-diffraction resolution. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe [3]. In stark contrast to the mostly uneventful far-field extinction response, a rich variety of distinct patterns of bright spots—corresponding to enhanced transmittance of the probe excitation—is observed in the near-field scans. Various transverse magnetic (TM) and transverse electric (TE) Fabry-Perot-like modes of different mode parities in a variety of nanostructure geometries can be revealed using full field 3D finite difference time domain simulations and group theory. [1] A. Frolov et al. Nano Letters 2017, 17 (12), 7629–7637 [2] A. Frolov et al. in preparation 2018 [3] D. Denkova et al., ACS Nano 2013, 7, 3168–3176

Traditionally, geometrical factors have not played an important role in determining the distribution of current across conducting boundaries. Typically, the classical skin depth expression is used to estimate currents within the volume. We have developed a novel geometry-based framework which describes current distributions within the volume of structures which allows us to engineer skin depth using boundary shapes. A more accurate knowledge of current densities is an important degree of freedom to design and analyze meta-structures and their interactions. Our approach is grounded in a rigorous analysis of electromagnetic wave scattering from shell structures for which the importance of geometrical parameters in the expressions for skin depth to accurately describe interactions has been confirmed. Starting from Maxwell’s equations, we have analyzed the temporal dynamics of electromagnetic interactions with meta-structures and their relationship to vector potentials. Individual wavelength or subwavelength sized meta-structures can be designed to localize the incident electromagnetic radiation and create a change in the local constitutive relations. Having an accurate determination of the current distribution within the volume of scattering structures plays an important role in designing and determining the effective constitutive parameters of 2D and 3D metamaterials. Combinations of materials with custom geometries suggest that this kind of skin depth engineering can lead to new families of linear and non-linear meta-atoms impacting imaging, harmonic generation, and the design of antennas and their shielding.

With signatures of high photon energy and short wavelength, ultraviolet (UV) light enables numerous applications such as high-resolution imaging, photolithography and sensing. In order to manipulate UV light, bulky optics are usually required and thereby do not meet the fast-growing requirements of integration in compact systems. Recently, metasurfaces, with subwavelength or wavelength thicknesses, have shown unprecedented control of light, enabling substantial miniaturization of photonic devices from Terahertz to visible regions. However, material limitations and fabrication challenges have hampered the realization of such functionalities at shorter wavelengths. Herein, we theoretically and experimentally demonstrate that metasurfaces, made of highly scattering silicon (Si) antennas, can be designed and fabricated to manipulate broadband UV light. The metasurface thickness is only one-tenth of the working wavelength, resulting in very small height-to-width aspect ratio (~ 1). Peak conversion efficiency reaches 15% and diffraction efficiency is up to 30%, which are comparable to plasmonic metasurface performances in infrared (IR). A double bar structure is proposed to further improve the metasurface’s diffraction efficiency to close to 100% in transmission mode over a broad UV band. Moreover, for the first time, we show photolithography enabled by metasurface-generated UV holograms. We attribute such performance enhancement to the high scattering cross-sections of Si antennas in the UV range, which is adequately modeled via a circuit. Our new platform will deepen our understanding of light-matter interactions and introduce even more material options to broadband metaphotonic applications, including those in integrated photonics and holographic lithography technologies.

Characteristic modes (c-modes) are a complete and orthogonal set of modes that can be used as a basis for the expansion of the waves scattered from electromagnetic and optical scatterers. In this talk, we present an introduction to the c-modes and discuss their applications in the analysis and design of 2D and 3D meta-structures. We present an equation for finding the c-modes and demonstrate that the c-modes and the natural (quasi-normal) resonances of meta-structures are related to each other. The relation between the c-modes and the natural mode leads to closed-form solutions for the transmission and reflection spectra of non-diffractive meta-structures that only depend on the complex-valued frequencies of the natural modes. As an example, we show that the wide bandwidth and high reflectivity of 1D high contrast gratings can be attributed to the alignment of two resonances associated with two different c-modes. In addition, using the c-modes concept, we present accurate expressions for the field enhancement and the Purcell factor in the presence of meta-structures and establish an upper bound on the number of degrees of freedom of meta-structures.

During the past decades, breakthroughs in nanophotonics and nanofabrication technologies have vigorously promoted the development of optical metastructures. With the help of precise design on metastructures, incident light can be effectively manipulated. However, the difficulty in finding high-index and low-loss dielectrics in visible range limits the application of all-dielectric metastructures for visible wavelengths. Besides, the edge and surface roughness of fabricated metastructure also have more significant effects on its performance. Here, we report the design of high contrast all-dielectric metastructure for visible range applications using the switchable all-dielectric metastructure an example. The physics behind the high contrast all dielectric metastructure is studied and analyzed. Based on this, the effect of edge and surface roughness on fabricated high contrast all-dielectric metastructure is explained. A method that can optimize the metastructure performance effectively is also proposed.

In imaging lenses, various aberrations must be corrected through cascading many lenses made of different materials and curvatures. This causes imaging lenses bulky, heavy and complex. Here, we demonstrate a metasurface aberration corrector (meta-corrector), comprising only a layer of 600-nm-thick TiO2 nanostructures, capable of rendering a single refractive plano-convex lens achromatic and free of spherical aberration in a compact manner across the visible spectrum. Our approach is based on, for any incident polarization, compensating the glass dispersion and correcting the curvature error of the spherical lens by tailoring phase, group delay and group delay dispersion of each meta-corrector’s nanostructure. The resultant metasurface-refractive lens has a diameter and f-number of 1.5 mm and 6.6, respectively, and is diffraction-limited from wavelength of 460 to 700 nm. The generality of our design method can be applied to extremely complicated refractive optics. As an example, we show a meta-corrector that greatly increases the bandwidth of a state-of-the-art immersion objective from the violet to near-infrared wavelengths. The objective, consisting of 14 lenses and 7 different materials, has a numerical aperture of 1.45 and is commercially available from Zeiss. Metasurface-refractive optics combines the advantages of both technologies in terms of size scalability and complexity for many applications such as imaging, augmented reality and lithography.

Lenses with tunable focal length are crucial to the operation of many optical systems, as in photography, mixed reality, and microscopy. Various technologies exist that support this behavior, but they often entail high power consumption and rely on bulky and expensive optical components. With recent advances in metasurface optics in miniaturizing and augmenting traditional systems, these devices may enable the next generation of varifocal lenses. These devices are flat optical elements comprising arrays of subwavelength-spaced scatterers that can impart spatially varying phase, amplitude, and polarization changes on wavefronts. In recent years, this field has attracted substantial research interest and has produced several demonstrations of focal length adjustable metalenses. These techniques, however, often rely on high control voltages to apply a strain to a flexible substrate or depend on microelectromechanical actuators that require sophisticated fabrication and cannot scale to large area apertures. Here, we discuss our work developing and building a 1 cm aperture Alvarez lens metasurface system with which we demonstrate a focal length tuning range of 6 cm (>200% change) at 1550 nm wavelength. We also design 1 mm Alvarez lens-inspired higher order metasurfaces for full-color imaging when combined with post-capture deconvolution. Using both designs, we demonstrate varifocal zoom imaging.

I discuss our recent work on folded optics using metasurfaces and computational imaging.

In polarimetry, that is, a measurement of the four-component polarization Stokes vector, a measurement must either consist of four (or more) sequential intensity measurements, sacrificing time resolution, or contain four separate light paths each with separate polarization optics, increasing bulk, cost, and system complexity. Similar issues present difficulty across polarization optics technology. Metasurfaces, nanophotonic arrays of phase shifting elements, have emerged as a novel platform for polarization optics. These individual phase shifters can be designed with a characteristic anisotropy, and are thus imbued with tunable shape birefringence. A metasurface, then, can function as a subwavelength spaced array of nanoscale waveplates. I will describe how, through relatively simple optimization methods, a metasurface producing arbitrarily specified polarization states (when illuminated with light of a known polarization) can be designed. This functionality is equivalent to a traditional diffraction grating with individual waveplate optics on each order; here, all the necessary polarization optics can be integrated into a flat, ultrathin optical element. Moreover, such a metasurface can be used in a reverse configuration as a parallel snapshot polarimeter with no need for additional polarization optics (save for a single polarizer). I present a detailed experimental characterization of both concepts in the visible spectral region and a comparison of the performance of the metasurface to a commercially available rotating waveplate polarimeter. With no bulk birefringent crystal optics, a parallel, full-polarization state measurement can be made with an integrated, scalable, and inexpensive device. Given its diffractive nature, the design naturally extends to spectropolarimetry and polarization imaging.

Polarization is an important degree of freedom of light carrying information that is usually missing in other degrees of freedom. Polarimetric imaging is the process of measuring the state of polarization of light over an extended scene. It has several applications ranging from remote sensing to biological and medical imaging because it provides various pieces of information about the light source or the objects with which the light has interacted. So far polarization cameras have been made using polarization filters, and therefore suffer from two major drawbacks. First, there is a theoretical 50% upper limit on the efficiency of devices based on polarization filters. Second, to fully determine the state of polarization, multiple layers should be integrated in order to make polarization filters for circular or elliptical polarization states. Here, we present a polarization camera made using dielectric metasurfaces that operates based on separating and focusing orthogonal polarization states instead of polarization filtering. This allows for overcoming both drawbacks of current polarization camera designs. At the core of the design lies the capability of dielectric metasurfaces to fully control the polarization and phase of light. This enables designing and fabricating superpixels that separate and focus orthogonal polarization states of light on adjacent pixels on an image sensor over a single metasurface layer. Using this technique we have demonstrated full-Stokes polarization cameras with experimental efficiencies surpassing 60%, and superpixel dimensions reaching 4.8 µm×7.2 µm. We have also used this camera to form polarization images of custom-designed polarization targets.

One of the major challenges for emerging planar subwavelength micro lens/metasurfaces is the significant chromatic behavior due to phase mismatch of subwavelength phase shifters. In this work, the continuous achromatic micro lens covering the whole visible wavelength is demonstrated for the first time based on relatively low index contrast gratings. Based on the unique chromatic phase shift behavior of polymer nano structure, we have designed and fabricated a broadband continuous subwavelength achromatic microlens that can cover 250 nm of visible bandwidths (from 435 nm to 685 nm) with focal shift less than 5%. Our works represent the first time to design, fabricate and characterize micro scale lens (7 microns in size) promising for compact integrated nanophotonic devices on chip. There are many advantages of using a polymer based micro lens such as easy fabrication on flexible substrate and potential applications including imaging, spectroscopy, lithography, laser fabrication and future integrated wearable devices.

Conventional imaging systems are usually composed of bulky glass optics, and while they work well for many applications, they offer little functionality in applications where system size is a constraint. Optical metasurfaces provide a thin and light-weight alternative to conventional bulky optical elements by manipulating light scattering via resonant nanostructures. The inherent diffractive nature of metalenses induces severe chromatic aberrations when imaging under broadband illumination, which limits their potential applications where multi-color information is important. In this work, we present an alternative metalens plus computational design where the point spread function is engineered to be spectrally invariant to reduce chromatic aberrations and enables computational reconstruction of a measurement using a single digital filter. The created lenses have a numerical aperture of ~0.45 and generate in-focus images under whitelight illumination.

Two-photon microscopy is a key imaging technique in biological sciences because of its superior deep tissue imaging capabilities in addition to high transverse and axial resolution. In recent years, development of low-weight miniature two-photon microscopes has been of great interest for in vivo imaging of brain activity. Limited by these mechanical constraints, most of the developed miniature two-photon microscopes utilize graded index objective lenses that usually have inferior optical characteristics compared to conventional refractive objective lenses. Dielectric metasurfaces, a recent category of diffractive optical elements with enhanced capabilities, have proven versatile in various applications ranging from lensing to holography and polarization control. Their ultrathin form factor and potentially extremely low-weight make them very attractive for applications with stringent size and weight constraints. However, despite their success in various types of microscopy and imaging applications, they have not been previously utilized for multi-photon fluorescence microscopy. The main barrier for using metasurface lenses in multi-photon microscopy arises from their large chromatic dispersion that effectively makes them single-wavelength. Here we will present a double-wavelength metasurface lens especially designed to have the same focal length at 820 and 605 nm, corresponding to the excitation and emission wavelengths of a certain fluorophore. After characterizing the poly-silicon metasurface lens at both wavelengths, we used it in a two-photon microscopy setup and demonstrated its capability to capture two-photon images qualitatively similar to images taken with a conventional objective lens. We will also discuss the effects of chromatic dispersion of the metasurface lens on its two-photon imaging performance.

The common way to manipulate light consists in using classical optical elements such as lenses and mirrors. Since few years, a new way to manipulate light with two dimensional optical components (metasurfaces) have been exploited to control light propagation using local phase discontinuities. Abrupt modifications of the fields across an interface can be engineered by depositing an array of sub-wavelength resonators specifically tailored to address local amplitude, phase and polarization changes [1]. Metasurfaces have been implemented to obtain various sorts of optical functionalities, ranging from the basic control of the transmission and reflection of light [1-2], to the control of the radiation patterns for comprehensive wavefront engineering and holography[3]. In this work, we will present our recent results on metasurfaces able to deflect and/or focus light at visible wavelength using an ensemble of spatially varying nano-ridges and nanopillars made of GaN semiconductor materials. The objective of our studies is to achieve controllable GaN nano-optoelectronic components. Classical nanofabrication techniques for realizing metasurfaces, such as reactive ion etching, considerably alter the electronic performance of nanostructures by the creation of surface edge states, typically sources for defects that greatly degrade device performances. The important penetration of plasma is critical for devices that become smaller that the electron elastic mean free path, modifying the transport and the luminescence of nanostructured materials. To circumvent this problem, we propose a new metasurface process based on selective area sublimation [4]. We will also discuss recent result on the integration of 2D metasurfaces on bottom emitting VCSELs for in-situ direct laser wavefront shaping. In the second part of this discussion, we will introduce the concept of conformal boundary optics and present our latest numerical tool to model and design free form optical components [6,7]. Illustrative examples such as absorbing metasurface, beam refractor, and curved lens have been explored, showing consistent results in agreement with fully theoretical predictions. This method turns into a powerful tool for accurately designing and predicting optical functionalities of conformal metasurfaces for new lightweight, small scaled, flexible and wearable optical devices. References [1] P. Genevet, F Capasso, F Aieta, M Khorasaninejad, R Devlin., “Recent advances in planar optics: from plasmonic to dielectric metasurfaces” Optica 4 (1), 139-152 (2017). [2] N. Yu, P. Genevet, M. Kats, J.P. Tetienne, F. Capasso, Z. Gaburro, Science 334,333 (2011) [3] P. Genevet and F. Capasso, “Reports of Progress in Physics”, 78, 024401 (2015) [4] Damilano, B., Vézian, S., Brault, J., Alloing, B., & Massies, J. (2016). Selective area sublimation: a simple top-down route for GaN-based nanowire fabrication. Nano letters, 16(3), 1863-1868. [5] Han, J. T. H., Wong, L.J., Molardi C. and Genevet P., “Controlling Electromagnetic Fields at Boundaries of Arbitrary Geometries”, PRA 94, 023820 (2016). [6] K. Achouri, M. A. Salem, and C. Caloz, IEEE Transactions on Antennas and Propagation, vol. 63, no. 7, pp. 2977–2991, 2014. [7] K. Wu, P. Coquet, Q. Wang and P. Genevet, “Free-form conformal Metasurfaces: modeling and design”, Nature Communications (in press, 2018) Acknowledgments: PG acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement no. 639109).

Metasurfaces based on dielectric nanoantennas are an attractive platform to control light with sub-wavelength resolution and high efficiency [1]. Using this platform, a whole range of metasurface devices with different functionalities have been demonstrated in recent, with ever increasing efficiencies that are approaching industry standards [2]. In most cases, however, the realized devices are static, i.e., once they are fabricated, their functionality is fixed. It would be very appealing if, on the contrary, a single device could be reconfigured to perform any functionality on demand. To do so, tuning mechanisms to dynamically control the optical response of the nanoantennas must be explored. One promising way to do so is embedding them in liquid crystals (LC), which have a large birefringence and can be electrically driven – an important requirement for practical devices. Moreover, their technology is very mature due to their extensive used in the display industry. In this talk, we will review our recent progress in electrical tuning of LC-embedded dielectric metasurfaces. We will show that the phase retardation experienced by a wave travelling through them can be controlled at will while maintaining high transmission levels, leading to highly efficient, tunable transmissive devices. We will show our experimental results operating at visible frequencies - around 660nm – by first showing tuning of metasurfaces as a whole to, subsequently, show that it is possible to address individual lines of nanoantennas to realize a tunable beam bending device. References: [1] A. I. Kuznetsov et al., “Optically resonant dielectric nanostructures”, Science 354, aag2472 (2016). [2] S. Kruk and Y. S. Kivshar, “Functional Meta-Optics and Nanophotonics Governed by Mie Resonances”, ACS Photonics 4, 2638–2649 (2017).

Using all-dielectric metasurfaces has be the interest of the scientific community recently. This is because the conventionally used plasmonic resonator-based metasurfaces have high ohmic losses in the optical domain. On the other hand, dielectric materials have minimal losses in the optical regime. Dielectric metasurfaces are based on dielectric resonators, periodic sub-wavelength structures that exhibit electric and magnetic resonance near the operation wavelength. In this work a novel all-dielectric metasurface design is studied using Electro-optic polymers. Applying an electric potential over the electro-optic polymer can change the steering angle of the metasurface. This study is done using finite difference time domain simulation for the optical behavior of this structure. This structure is CMOS compatible contrary to plasmonic metasurfaces.

In the last several years, metasurfaces have demonstrated promise to control constitutive properties of light via interaction with nanoscale elements. Unlike the passive metasurfaces developed to date, actively controlled metasurface properties can enable the realization of new electrically-tunable low-profile optical components with numerous applications such as dynamic holograms, convergent lenses with reconfigurable focal lengths, and beam steering arrays, which are key requirements for future chip-based light detection and ranging (LIDAR) systems. In this work, we report a gate-tunable reflectarray metasurface, which can act as a focusing lens with reconfigurable focal length or as a beam steering device. This active reflectarray metasurface is actively controlled by use of indium tin oxide (ITO) as a material with voltage-tunable complex permittivity at 1550 nm operating wavelength. First, we experimentally demonstrate electrical control of the reflection phase and amplitude for metasurface unit elements, and we show that the phase shift of the metasurface unit element can be actively tuned from 0° to 300°. Our design enables independent electrical control of each metasurface element via individual application of the DC voltage. We also show that the same metasurface can exhibit multiple functionalities, acting both as a reconfigurable lens and a beam steering device.

Optical metasurfaces based on dielectric Mie-resonators were established as an efficient platform for realizing a multitude of optical functionalities. Recently, tunable optical dielectric metasurfaces have attracted increased research interest, and various tunable dielectric metadevices have been demonstrated. Infiltrating dielectric metasurfaces with nematic liquid crystals (LCs) represents an efficient and convenient tuning approach [1], which is compatible with established LC industrial technologies. Here we demonstrate two electrically tunable LC-infiltrated dielectric metasurfaces working at near-infrared and visible wavelengths, respectively. We demonstrate that the metasurfaces can be electrically tuned into and out of the so-called Huygens’ regime of spectrally overlapping electric and magnetic dipolar resonances by application of an external voltage. For the first time to our knowledge, we have utilized a LC photoalignment material [2] to realize LC-tunable metasurface devices with drastic improvement of their tuning performance and reproducibility. In particular, we demonstrate tuning of the metasurface transmission from nearly opaque to nearly transparent at 1070 nm. Furthermore, we demonstrate a switchable silicon transmissive display with 53% contrast, operating in the visible spectral range. Finally, we propose a novel route toward phase-only tuning by applying simultaneous electrical and thermal stimuli to the LC-infiltrated dielectric Huygens’ metasurfaces. In our numerical simulations, we observe 178° phase modulation with a transmittance exceeding 64% over the entire tuning range at 1078 nm wavelength. [1] A. Komar et al., Appl. Phys. Lett. 110(7), 071109 (2017). [2] I. I. Rushnova et al., Opt. Commun. 413, 179-183 (2018).

Optical metasurfaces comprise of ultrathin two-dimensional (2D) metal/dielectric phase shifters with subwavelength features (also referring to meta-atoms). With rigorous design on the size, shape and location of meta-atoms in the reflect- or transmit-arrays, it can precisely tailor the scattering properties of light which is different from the incident beam and the characteristics of interaction between incident light and metasurfaces can be engineered to realize many functions of traditional bulky elements such as lenses, gratings, mirrors, waveplates and etc. Early works motivated by ultrathin metallic structures to resonant with the incidence of electromagnetic field to manipulate the light with resonant or geometric phase. However, the material loss significantly limits the performance of plasmonic metasurfaces, in particular in the transmission mode of single layer structures. Dielectric geometric phase based on half wave plate is able to shape wavefronts but their challenging issue lies in the difficulty of simultaneous realization of high efficiency and large deflection angle due to the coupling between the elements. The coupling between neighboring meta-atoms is also remarkable in the case of Huygens’ metasurfaces, which heavily degrades its performance. The drive to make highly efficient dielectric metasurfaces has been the mandatory prerequisite for the futuristic development of planar metadevices and systems. To address those challenging issues, I will introduce our latest development on high transmission efficiency metasurfaces operating in near UV range using a transition metal oxide of higher band gap energy than titanium dioxide and demonstrate its various applications.

A nanofabrication method of metasurfaces based on colloidal assembly and femtosecond ablation is proposed and demonstrated. The metasurfaces on microspheres own the advantages of large area, low cost, long-range periodicity, high light-scattering efficiency and compatibility with liquid crystal display (LCD) manufacturing. Here, the diffractionunlimited nano-ablation is realized on the gold layer hemispherically coated on the assembled silica microspheres. The radius of ablated spot and half-pitch of ablated slit achieved in the experiment are ~130 nm and ~30 nm, respectively. Through changing the incidence angle of the femtosecond laser beam, some complicated ablation nanopatterns consisting of spots and slits would be realized.

Recent years have witnessed the rapid development of flat optical elements (i.e., metasurfaces). With properly designed and arranged sub-wavelength structures over its plane, a metasurface is able to impart arbitrary, spatially variant amplitude, phase and polarization modulations on an incident electromagnetic wave. This highly customizable nature allows metasurface to simultaneously accomplish a variety of functions that have traditionally been fulfilled by a combination of different optics, such as gratings, lenses, beam splitters, and hologram plates, with a significantly reduced physical thickness compared to traditional optical elements. Researchers have demonstrated various high-performance metasurfaces operating in the infrared (IR) and visible regime. However, there has been a conspicuous lack of work in the ultraviolet (UV) region, which is a spectral range hosting important applications such as photolithography, DNA sequencing, sterilization, and medical imaging. Unfortunately, direct translation to the UV regime of implementation strategies for IR and visible-frequency metasurfaces will not work, so new constituent materials and designs need to be developed. Here, we report on high-efficiency, all-dielectric metasurfaces operating in the UV regime. The metasurfaces utilize wide bandgap (> 5.5 eV) dielectric materials, enabling the devices to operate over a broad-band UV range. We demonstrate metasurfaces working at three UV wavelengths (364, 325 and 266 nm), including hologram, lens, and self-accelerating beam devices. To further show the versatility of our design, we demonstrate metasurfaces producing spin-multiplexed holograms and self-accelerating beams at 364 nm, with operational efficiencies larger than 50%. Our work opens the door for high-performance and multi-functional UV flat optical elements.

Conventional wave plates for polarization control are based on birefringent materials or nanostructures that assign real-valued phases [1,2]. Recently, a new regime of complex-valued birefringence was suggested [3], extending the notion of real-valued birefringence through the specially introduced polarization-sensitive loss or gain. However, this theoretical approach was based on a complicated metamaterial with loss and gain, which remains inaccessible for fabrication. We develop and experimentally demonstrate a practical approach for optimal implementation of complex-birefringent wave plates with metasurfaces. We design dielectric metasurfaces incorporating pairs of nanoresonators, giving rise to engineered polarization-dependent diffraction, which effectively introduces only the minimally necessary amount of loss to achieve the desired unconventional polarization transformation. We fabricate such metasurfaces, characterize the transmission of various polarization states, and demonstrate two representative applications. First, we show that a metasurface can transforms a pair of nearly identical polarization states into orthogonally polarized ones, which can be used for improving polarization detection sensitivity and quantum state discrimination. In contrast, real-valued birefringence cannot change the angle between pairs of polarization states. Second, we develop a metasurface implementing polarization coupling with an unconventional relative phase, which can strongly affect the quantum interference of photons and control their coherent attenuation. The complex-birefringent metasurfaces can facilitate novel types of polarization manipulation and measurement with optical beams and quantum photon states. [1] A. Arbabi et al., Nat. Nanotechnol. 10, 937 (2015). [2] S. Kruk et al., APL Photonics 1, 030801 (2016). [3] A. Cerjan and S. H. Fan, Phys. Rev. Lett. 118, 253902 (2017).

Resonant metasurfaces present an excellent platform for a variety of mid-IR devices, ranging from linear passive to nonlinear active, with everything in between. The key attractions of such metasurfaces are: their ultra-thin (sub-wavelength) format, strong field enhancement, and high-Q response. I will describe three applications currently under development in our group. First, I will demonstrate how a multi-resonant Si metasurface can be used for making perfectly efficient diffraction gratings. We analytically show that at least four independent resonances are required. Experimental realizations of such gratings will be presented, and high-contrast between targeted and parasitic diffraction orders will be demonstrated. Second, I will present an experimental demonstration of a highly nonlinear high-Q Si metasurfaces whose optical properties rapidly change while the pulse is “trapped” by the structure. The metasurfaces were designed to exhibit sharp resonances in the 3–4 μm spectral range. Third harmonic generation spectroscopy and pump–probe spectroscopy revealed the enhancement of coherent nonlinearities and free-carrier-induced by orders of magnitude compared with unpatterned silicon film without compromising the bandwidth. We show that a phenomenon of photon acceleration (PA) manifests in tunable harmonics generation. The PA phenomenon paves the path towards high-efficiency broadband nonlinear photonics. Finally, I will discuss the possibility of making a efficient generator/modulator of the polarization state of light using thermally tunable high-Q metasurfaces. Our experimental results indicate that such photonic structures enable compact polarimeters and ellipsometers.

Ultrashort optical pulse characterisation typically requires gating a reference pulse with an unknown pulse in a nonlinear medium. Since nonlinear crystals are typically produced to enable a single nonlinear process, only a single pulse can be characterised at any one time. In this work, we explore the use of gold nanoparticles to produce a range of nonlinear processes simultaneously. We show that a single nanoparticle produces a multiple nonlinear responses without relying on conventional phase matching. This allows us to exploit Four Wave Mixing and Sum Frequency Generation, which are simultaneously present in our nonlinear signal, to characterise two near IR ultrafast pulses separated by about an octave in wavelength. Remarkably, this "double-blind" method does not require the use of a known reference pulse, since the pulse retrieval problem is specified by the two nonlinear mixing processes.

Asymmetric transport is an uneven physical response of counter-propagating signals that has significantly contributed to fundamental science and revolutionized advanced technology via a variety of significant devices including diodes and isolators in electronics, optics, acoustics, and heat transfer. Photonic metasurfaces are two-dimensional ultrathin arrays of engineered subwavelength meta-atoms, acting as local phase shifters, which unprecedentedly mold wavefronts at will with a virtually flat optical element. While such an architecture can be potentially harnessed to achieve two-way asymmetric response of free-space light at an optically thin flatland, asymmetric light transport cannot be fundamentally achieved by any linear system including linear metasurfaces. Here, we report asymmetric transport of free-space light at nonlinear metasurfaces, with harmonic generation, upon transmission and reflection. We also derive the nonlinear generalized Snell’s laws of reflection and refraction which were experimentally verified by angle-resolved anomalous refraction and reflection of the nonlinear light. The asymmetric transport at optically thin nonlinear interfaces is revealed by comparing the original path of light through the metasurface with its corresponding reversed propagation path. Such a two-way asymmetric response at metasurfaces opens a new paradigm for free-space ultrathin lightweight optical devices with one-way operation including unrivaled optical valves and diodes.

Eye tracking has been an indispensable analysis method in a wide range of research fields, including Psychology, Neurology, and ophthalmology. Recent developments in augmented reality are pushing for more compact, transparent eye trackers compatible with head-mounted display or heads-up display. Oblique half-mirror and holographic waveguide satisfy these criteria and now widely used in eye-controlled displays, auto-driving, and near-to-eye displays. However, these still require bulky supplementary optics, are poorly transparent, and produce rainbow images due to non-zero diffraction in the visible spectrum. Here, we demonstrate ultra-thin, rainbow-free eye tracking diffractive optical elements based on guided mode resonance that exhibits near-unity transmission. It consists of a 200-nm-thick Si3N4 slab waveguide sandwiched between a quartz substrate and a 100-nm-thick SiO2 capping layer designed for high transmission (>90%) over the whole visible spectrum. The insertion of 3-nm-thick Si grating layer at the interface between the slab waveguide and capping layer launches high-quality (Q~2,000), leaky guided modes in the slab waveguide at specific wavelengths for a fixed incident angle and polarization, which enables us to efficiently (13%) characterize resonant light diffraction at 870 nm. In the visible, on the other hand, the guided mode resonance becomes weak due to Si absorption, resulting in strongly suppressed rainbow-producing diffractions below 0.1% efficiency. By locating a single webcam at near-grazing angle, corresponding to the output diffracted order at 870 nm, the full anterior images of an artificial eyes are obtained. Our device opens a promising route toward ultra-compact, transparent, and non-obtrusive imaging for displays and optical switching applications.

Recently emerged metasurfaces, the two-dimensional (2D) counterpart of three dimensional (3D) metamaterials, gained significant attention in optics and photonics due to their less challenging fabrication requirements (compared to 3D metamaterials) and unique capabilities of wavefront manipulation by introducing abrupt phase shift. Realization of multiple functionalities in a single metasurface, is an intriguing perception to achieve further miniaturization and cost effectiveness. In this paper, we propose a polarization insensitive, highly efficient metasurfaces for the visible spectrum. For the design wavelength of 633nm, negligible absorption coefficient (k) and adequately large refractive index (n) of proposed hydrogenated amorphous silicon (a-Si:H) leads to considerably efficient and cost-effective solution towards metasurfaces design. Inherent property of cylindrical pillar to be polarization insensitive is exploited and 400 nm thick cylindrical nano–waveguide is opted as building block to construct the metasurface. A novel design strategy of achieving multiple functionalities from a single metasurface is proposed, where a combined effect of lensing and optical vortices with different topological charges at different focal planes is demonstrated for the proof of concept. Such unique design strategy of integrating multiple phases into a single device provides an innovative way of miniaturizing the optical devices and systems exhibiting versatile functionalities for on–chip applications.

One of the central challenges for practical applications of single-photon sources is the ability to efficiently extract light from a single quantum emitter. A useful single-photon source must emit into a well-defined direction because in practice one can collect light only in a finite solid angle. Here, we propose to harness the exceptional light molding capabilities of photonic metasurfaces to engineer the emission from quantum emitters and achieve highly directional emission. We have designed a phase gradient reflectarray metasurface, which efficiently collects spontaneous emission from a quantum emitter, located in the far-field (d~5 wavelengths), and redirects it back to the source. By controlling the phase imprinted by the metasurface on the incident light, we control the emission properties of the emitters. We apply this concept to design a metasurface for use with hexagonal boron nitride (hBN) single photon emitters operating at 620 nm. We have observed experimentally bright single photon emission at 620 nm with a remarkably narrow spectral width of zero-phonon line emission from multilayer hBN films synthesized by chemical vapor deposition. Simulations show that at a wavelength of 620 nm, the reflection efficiency of our metasurface is greater than 85%, and that the emission from these emitters are highly directional with deviation from emission in the surface-normal direction of 2∆θ ~ 20°. We will report on experimental measurements of hBN quantum emitters coupled to metasurfaces and describe metasurface designs for coupling of multiple quantum emitters.

Dielectric nanoantennas and metasurfaces have recently emerged as a new nanophotonic platform complimenting conventional plasmonics for light control at the nanoscale [1]. Due to their low losses, wide range of available electric and magnetic resonances and compatibility with conventional nanofabrication processes they provide a unique toolkit to achieve new functionalities and build highly-efficient nanophotonic devices. So far however, most of the demonstrated functionalities have been limited to passive light control while emission properties of the nanoantennas have rarely been studied. In this talk, I will review our recent research in the direction of light emitting semiconductor nanoantennas and metasurfaces. In particular I will show our first experimental demonstration of lasing action in active semiconductor 2D nanoantenna arrays based on bound state in the continuum [2]. I will also show how lasing can be achieved in 1D nanoantenna chains and even single nanoantennas using interference effects between different resonant modes. These results expand the toolkit of dielectric nanoantennas towards active functionalities providing a new platform for making chip-scale directional laser devices. References: 1) A. I. Kuznetsov et al., “Optically resonant dielectric nanostructures”, Science 354, aag2472 (2016); 2) S. T. Ha et al., “Directional lasing in resonant semiconductor nanoantenna arrays”, Nature Nanotech. (2018), in press.

We numerically and experimentally demonstrate that metasurfaces can be used to control the light emission from light emitting diodes (LED). This control provides a desired wavefront and functionality of the light emission in addition to enhancing light extraction efficiency. Simply placing the metasurface on top of the LED does not work as conventional metasurface designs require plane wave excitation, which LEDs cannot provide. To overcome this challenge we implement a novel concept using internal and external resonant cavities combined with the LED. Guided by our numerical simulations, we experimentally demonstrate this concept by fabricating Si and TiO2 metasurfaces on top of the resonant cavity LED structures. The integration of these metasurfaces with commercially available GaN and GaP LED devices show full wavefront control, beam deflection and beam collimation. Both the cavity and the metasurface enhance the LED radiation. Moreover, following the proposed principle, any random light emitting sources including fluorescent molecules and quantum dots can be integrated into a similar optical device to achieve focusing, beam deflecting, vortex beam generation and other capabilities.

In this work, we have designed, fabricated and characterized silicon nitride sub-wavelength gratings on glass substrate to enhance the fluorescence in the green-red wavelength range. Silicon nitride was chosen as the material to fabricate the gratings as it exhibits low absorption losses and negligible fluorescence at visible wavelengths. Due to lower refractive index contrast, the structures were designed such that the medium index contrast gratings still achieve good quality factor resonances by using higher duty cycles (~ 70%) which clearly distinguishes two-mode region from higher order diffraction regime. The designed structure (Duty cycle: ~70%, thickness: 290nm, pitch: 370nm) supports resonant modes at 542nm for TE and at 548nm and 568nm for TM polarization. Rhodamine B dye was attached to the grating through an intermediate polymer layer PAH (Polyallylamine hydrochloride) by dip coating method. Using a fluorescence microscope with suitable excitation (510-550nm) and emission (>590nm) filters, we observed fluorescence enhancement of 5.4x and 5.8x in TE and TM modes respectively.

High contrast gratings (HCGs) are an attractive alternative to distributed Bragg reflectors (DBRs) as highly reflective mirrors for VCSELs. In our previous work we proposed the use of monolithic HCGs (MHCGs) to reduce the vertical thickness and simplify the epitaxial structure of VCSELs. In this work we discuss the optimization and fabrication of MHCGs. We also analyze the impact of processing imperfections on the power reflectance of MHCGs.

Since the very first demonstration of a vertical-cavity surface-emitting laser (VCSEL) incorporating subwavelength high refractive index contrast grating (HCG) membrane mirror in 2007 by the group of Prof. Chang-Hasnain, numerous research groups around the world have presented devices based on the same concept emitting at wavelengths from ~400 to 1550 nm manufactured in gallium nitride (GaN), gallium arsenide (GaAs) and indium phosphide (InP) material systems. On one hand, an open access to a VCSEL cavity through an air gap combined with a very low inertia of an HCG mirror opened a way for a large range of emission wavelengths in MEMS tunable VCSELs. On the other hand, an air gap in a cavity generally hinders heat and current flow, while the potentially rather fragile HCG membrane is prone to mechanical instability. We present electrically-injected VCSELs incorporating monolithic HCG (MHCG) mirrors. An MHCG mirror being a special case of an HCG mirror, keeps the extraordinary features of an HCG such as scalability with wavelength, ultra-low thickness and very large power reflectance, but doesn't have to be surrounded by a low refractive index material and hence can be monolithically integrated with an all-semiconductor VCSEL cavity. We present an extensive analysis of the impact of the MHCG parameters on the modal properties and thermal stability of single- and double-mode devices, with various oxide apertures. We additionally compare MHCG VCSELs and generic distributed Bragg reflector VCSELs in terms of modal properties and temperature stability based on measured data and the results of computer simulations.

In this talk, we present and discuss our recent advances on meta-gratings. Graded metasurfaces, as ultrathin planar arrays of closely-located polarizable inclusions, have attracted significant interest due to their unprecedented control over the flow of light. It was recently shown that metasurfaces based on this approach suffer from fundamental limits on the overall conversion efficiency. In addition, this method typically requires deeply subwavelength fabrication resolution that is imposed by the need for discretization of continuous fastly-varying impedance profiles. To overcome this issue, we have introduced the concept of meta-gratings, formed by periodic arrays of carefully tailored bianisotropic inclusions. We show that the proposed concept enables wavefront engineering with unitary efficiency and significantly lower fabrication demands both in transmission and reflection. Beyond beam-steering, we show metagratings for focusing and lensing, and to impart linear operations on the optical wavefront in momentum space.

Optical metasurfaces are subwavelength-thick arrays of meta-atoms that have attracted significant attention due to their superior capabilities compared with conventional optical devices. Designing metasurfaces for practical applications requires system-level models that accurately predict their responses. The conventional approach for modeling metasurfaces is to ignore the coupling among the meta-atoms and to model metasurfaces as phase, amplitude, or polarization masks that are independent of the incident light’s wavefront, which is an inaccurate assumption for large incident angles. In this talk, we will introduce a novel technique for the modeling and design of metasurfaces based on the discrete-space impulse response (DSIR) concept. Because the waves propagating in free space are spatially band-limited, the incident, the transmitted, and the reflected waves can be represented using discrete-space signals that are obtained by sampling these waves at the Nyquist rate (at half a wavelength intervals). As a result, we can define discrete-space impulse responses for metasurfaces that relate the transmitted/reflected waves to the incident waves. We show that such impulse responses are local, accurately model the interactions among neighboring meta-atoms, and completely characterize the metasurfaces’ response to any incident waves. We also introduce a new approach for designing metasurfaces using the DSIR concept. As a proof-of-concept, we present the characterization results of a high numerical aperture meta-lens that is designed using the DSIR technique and show that its focusing efficiency is higher than that of a similar meta-lens designed using the conventional technique.

Metasurfaces are attractive options for the realization of on-chip optical systems because of their flat form factor and their ability to modify the wavefront, amplitude, and polarization of light with high efficiency. Several metasurface platforms have been reported that provide different levels of control over the polarization and phase of light, and it has been shown that a single layer birefringent metasurface can implement symmetric and unitary Jones matrices. Optical components with such Jones matrices can convert any arbitrary input polarization to any desired output polarization or perform independent wavefront transformations for two orthogonal polarizations while changing their handedness. However, the Jones matrices that describe the most general polarization and phase transformations are not symmetric, and this limits the range of possible devices that single layer birefringent metasurfaces can implement. For example, a single layer birefringent metasurface cannot impart two different phase shifts to x- and y-polarized light while simultaneously converting their polarizations to right- and left-handed circularly polarized. Here we show that bi-layer birefringent metasurfaces do not suffer from such limitations and can implement the most general form of Jones matrices that describe loss-less and reciprocal optical components. By using the Poincare sphere representation and closed-form relations, we identify the degrees of freedom in the design and present a procedure that allows for the design of large-scale devices based on bi-layer metasurfaces. As a proof-of-concept, we demonstrate a chiral bi-layer metasurface that focuses left- and right-handed polarized waves to two different points without changing their polarizations.

A physical structure constructed from stripes of a material with high refractive index that are separated with a low refractive index medium is called a high contrast grating (HCG). Here we present the simulations of long focal-length GaAs-based planar focusing monolithic HCG reflectors designed for 980 nm. We discuss how the focal spot size depends on the reflector size and how it is possible to improve the maximum value of the electric field intensity distribution.

Light confinement is of fundamental importance in science and technology. In recent years, different groups have investigated a unique approach to confine and trap light in open structures, based on the concept of bound states in the radiation continuum, or embedded eigenstates. While conventional bound states are forbidden from coupling to the radiation continuum by symmetry, momentum mismatch, or direct suppression of outgoing waves, embedded eigenstates are compatible with free-space radiation, but remain confined due to destructive interference between different radiation channels. More generally, embedded eigenstates correspond to non-radiating eigenmodes of an open system, namely, radiationless eigenmodal distributions of conduction/polarization currents. In our talk, we will discuss our recent efforts on this exciting topic [Doeleman, H. M., Monticone, F., den Hollander, W., Alù, A. and Koenderink, A. F., “Experimental observation of a polarization vortex at an optical bound state in the continuum,” Nat. Photonics 12(7), 397–401 (2018); Monticone, F., et al., “Trapping Light in Plain Sight: Embedded Photonic Eigenstates in Zero-Index Metamaterials,” Laser Photon. Rev. 12(5), 1700220 (2018)], with particular focus on our recent demonstration of topologically-protected embedded eigenstates in dielectric metasurfaces at optical frequencies. We show experimental evidence that the embedded eigenstate of this structure corresponds to a vortex of the polarization state in wavenumber space, characterized by a quantized topological charge, which determines an inherent robustness under continuous deformations. We also present a dipole model that explains the embedded eigenstate in terms of destructive interference between two radiation channels and fully accounts for its topological nature.

In this paper, we propose an anisotropic dual-band phase gradient metasurface in the infrared region based on tellurium material, which exhibits strong birefringence for z-cut. The refractive index of ordinary light no and extraordinary light ne are along the x- and y-direction of the metasurface, respectively. When a plane wave polarized along the x-direction is normally incident on the metasurface, the diffraction efficiency, which is defined as power of the deflected beam in the desired +1 diffraction order normalized to total transmission power, keeps higher than 95% within the wavelength from 7140nm to 7260nm, meanwhile the total transmission efficiency remains above 78%. On the other hand, the diffraction and total transmission efficiencies keep higher than 95% and 65% within wavelength range from 7840nm to 7940nm for the y-polarized illumination. The metasurface can be applied into polarization splitter, spectral beam splitter, and more in the future.

Metasurfaces have broad utility in spectroscopy, integrated optics, optical filtering, and holography. In this talk, I will discuss the development and utility of numerical tools for high performance design. For the first part, I will discuss the application of topology optimization to periodic and aperiodic metasurfaces. These methods are based on mathematical gradient descent, and they enable the discovery of new, freeform designs consisting of nonintuitive nanoscale patterns. For certain classes of devices, these metasurfaces have performance metrics (i.e., efficiencies and capabilities) that far exceed the performance of conventional metasurfaces based on phased arrays. Upon reverse-engineering these devices using coupled Bloch mode analysis, we find that the origins of high efficiency wavefront engineering is due to complex intramode and intermode coupling between the optical modes of the system. For the second part, I will discuss the potential of machine learning to learn features in high performance devices and its use in automated device design. Machine learning is applicable to metasurface design and nanophotonics more broadly because there are clear relationships between geometric structure and optical response. I will show that these non-linear correlations between geometric structure and optical response can be learned in a neural network, and that a properly trained network can produce devices beyond the parameter space of the training data set. This reported research sets the foundation for the future of metasurface and nanophotonic design: one where computers and algorithms identify new design regimes of light-matter interaction unattainable by designs based on human intuition.

Metasurface-based optical elements enable abrupt wavefront engineering by locally controlling the properties (amplitude, phase, etc.) of the incident illumination. They hold great potential to promote a new generation of wearable devices and thin optical systems for imaging and sensing. To date, most of the existing metasurface designs rely on highaspect-ratio nanostructures, with a thickness close to or even higher than the wavelength. There has been an increasing demand to reduce the metasurface thickness and nanostructure aspect-ratio, in order to facilitate the fabrication compatibility and integration with electronics and dynamic tunable platforms. Here we demonstrate ultrathin (~ 1/5 of the wavelength) transmissive metalenses for the visible light, using two different approaches of either amplitude or phase modulation. For amplitude modulation, we developed a digital transmission coding scheme that allows manipulation of multiple wavelengths without increasing the thickness or complexity of the structural elements. In order to improve the optical efficiency, phase modulation is necessary, but the design is more challenging. Because the nanoresonators are electromagnetically coupled with each other, compared with high-aspect-ratio nanostructures with wave-guiding confinement. To solve this problem, we developed an inverse design strategy using machine learning. We employ evolutionary algorithms interfaced with Finite-Difference Time-Domain solvers, which not only mimic natural selection in order to determine the optimal arrangement of nanoresonators to achieve the desired target optical functions, but also consider and benefit from the strong interactions between nanoresonators to improve the performance. The machine learning designs significantly improve the focusing efficiency, approximately double of the conventional human designs.

Multifunctional metasurfaces perform different functions depending on the wavelength, polarization, or wavefront of the incident light. Designing such metasurfaces require more degrees of freedom (DOF) than what is available in a single layer metasurface, and stacking metasurface layers is one of the approaches for achieving the required DOF for realizing multifunctional metasurfaces. In the conventional metasurface design technique used for designing single layer metasurfaces, the couplings among the meta-atoms are ignored; however, the meta-atoms in multi-layer metasurfaces exhibit significant mutual couplings and multiple scattering phenomena are not negligible. As a result, multi-layer metasurfaces designed using the conventional techniques have low efficiencies. In this talk, we will present an inverse design technique that is suitable for designing efficient large-scale multi-layer metasurfaces. The method is based on a combination of the gradient descent optimization and the adjoint sensitivity techniques and is used to design efficient parametrized multifunctional metasurfaces. The design of multifunctional metasurfaces is cast as a multi-objective optimization problem and the optimal values of meta-atom geometrical parameters are found through an iterative approach. The sensitivities of the objective function and the metasurface response are computed using full-wave simulations; therefore, the mutual interactions and the multiple scattering effects are accurately considered. To demonstrate the effectiveness of the method, we present a bi-layer double-wavelength metasurface composed of more than 2,000 amorphous silicon nano-posts that are embedded in silicon dioxide and arranged in two stacked layers. The bi-layer metasurface projects two different patterns with more than 65% efficiency when illuminated with two different wavelengths.

In recent years, diffractive, discrete scatterer based optics such as metasurfaces have shown considerable promise in the realization of arbitrary optical functions. However, these optical elements are systems large numbers of tunable degrees of freedom that are impractical to tune using forward design methods. In parallel, there has been great progress in using computational inverse design methods to produce high quality nanophotonic elements. We show that this inverse design method is capable of handling the large scale of the three-dimensional electromagnetic scattering problem, and leads to a realistic path towards the computational design and optimization of these discrete scatterer based optics capable of performing arbitrary optical functions in the far field. Then, we present an experimental demonstration of an optical element at 1.55 μm that focuses light into a discrete helical pattern that is designed using an inverse method based on generalized Lorenz-Mie scattering theory. This optical function is realized by specifying a suitable figure of merit that encapsulates the performance of the optical element. The fabrication of these optical elements with such small length scales is done using the Nanoscribe GT two-photon lithography system.

Hyperbolic metamaterial (HMM) has paved the way for sub-diffraction focusing inside the HMM due to the propagation of large momentum wave vectors in the HMM. However, these high momentum K modes exponentially decay outside the HMM which results in decaying of the focusing resolution in the near field of the HMM. In this work, we introduce both a HMM and a hypergrating structures for sub-wavelength focusing in air. Hypergrating is a structure that combines a HMM with a grating surface. The proposed structure consists of upper metallic slit integrated on HMM based multilayer of doped/intrinsic InAs with lower intrinsic InAs grating surface. HMM based multilayer of doped/intrinsic InAs has the advantage of tuning the focusing wavelength in the mid-IR range. The proposed structure has reported sub-wavelength focusing in air with value reaching 0.08 λ. Hypergrating structure shows focusing resolution enhancement of 0.08λ as compared to 0.15λ for a HMM without lower grating, both at wavelength of 7.3μm. The focusing resolution outside the hypergrating structure is much higher than that is observed in the HMM only due to the introduced lower grating. This structure demonstrates a good candidate for sub -wavelength IR imaging application in air.

Highly sensitive optical sensor for magnetic field detection was experimentally demonstrated using a guided-mode resonance in waveguide with Ni nano-grating. The electromagnetic field distribution was calculated by finite-difference time-domain method in order to estimate the sensing performance of our device. The calculation results indicated that the optical characteristics of our sensor considerably varied with applying magnetic field. We fabricated the Ni-subwavelength grating/ Si₃N₄ waveguide structure on the optical glass substrate using electron beam lithography technique. The reflection peak resulting from the guided-mode in the waveguide was obtained with normal incident geometry. The peak intensity depended on static magnetic field applied to the structure, and the intensity changed by about 5 % for the magnetic field intensity of 39.4 mT. These experimental results suggest our sensor can sensitively detect magnetic field while avoiding use of the complex and expensive system, and our device is pretty suitable for the integration devices in internet of things society.

In the realm of intensive research on metamaterials, particularly, the two-dimensional analogue, known as metasurfaces have attracted researchers due to their lower losses, high efficiencies and low cost as compared to plasmonic metasurfaces. Dielectric metasurfaces (DMs) have been widely reported to experience magnetic and electric dipole Mie type resonances, in which, upon tuning these two resonances, dielectric metasurfaces can exhibit spatially varying optical responses, phases and polarizations of scattered fields. Recently, dielectric metasurfaces have been used for color printing application with very high color vibrancy. However, the fundamental building blocks essential for the realization of optical metasurfaces are designed with uniform dimension nano structures, resonating at particular wave length, thus printing image only with particular color. In order to be able to cover the whole optical regime, the metasurface needs to be designed with tunable optical response to be able to print images with multiple colors. In this work, we report a cubic TiO₂ metasurface which experience magnetic and electric dipole resonances in the optical regime. We are able to tune the reflection peak of both resonances using Nematic liquid crystal (LCs). LCs are anisotropic materials with controlled orientation based upon different applied voltages. Changing the orientation of the LC allows for tuning the resultant of the electric field component of the LC and thus the reflection peak of the metasurface can be tuned across the optical regime. We report a tunable DM for optical filters application using single dimension designed metausrfcae with efficiency close to 99 % covering three colors in the visible range: red, orange and green.

In this paper, we have proposed and numerically investigated an all-dielectric metasurface consisting of a 2-dimensional periodic array of convex facing silicon asymmetric split arcs on the silica substrate. Due to split asymmetric arcs configuration, the structure exhibits Fano resonance at the wavelength of 967 nm. The maximum achieved quality factor (Q-Factor) and the spectral contrast ratio of the Fano resonance are 213.4 and 99.91%, respectively. The proposed metasurface can be used as a refractive index sensor as the resonance wavelength is linearly dependent on the refractive index of the surrounding. With the increase in the refractive index, the resonance wavelength shows the red-shift. We found that the wavelength shift per refractive index unit (RIU) is 250 nm/RIU and the figure of merit of this sensor is 50.