Metasurfaces are ultra-thin patterned photonic structures that emerged recently as planar metadevices capable of reshaping and controlling incident light. They are composed of resonant subwavelength elements distributed across a flat surface. Due to the resonant scattering, each element can alter the phase, amplitude and polarization of the incoming light. Many designs and functionalities of metasurfaces suggested so far are based on plasmonic planar structures, however most of these metasurfaces demonstrate low efficiencies in transmission due to losses in their metallic components. In contrast, all-dielectric resonant nanophotonic structures avoid absorption losses, and can drastically enhance the overall efficiency, especially in the transmission regime. Here we utilize this platform to create flat optical elements such as vector beam q-plates, holograms and quantum polarization tomography devices. Holograms, in particular, showcase a potential of the metasurface platform as they rely on a complex wavefront engineering. Metasurface platform enables a new way to create highly efficient holograms with single-step patterning. Here, we design and realize experimentally greyscale meta-holograms with superior transmission properties. Another promising area for implementation of all-dielectric metasurfaces is quantum optics. We suggest and develop experimentally a new concept of quantum-polarization measurements with a single all-dielectric resonant metasurface. A metasurface enables full reconstruction of the state of entangled photon pairs based on the photon correlations with single-photon detectors. The subwavelength thin structure provides an ultimate miniaturization, scalability to a larger number of entangled photons, and gives the possibility to study the dynamics of quantum states in real-time.
Metallic nanoantenna possess versatile scattering properties enabling to engineer the emission directionality at the nanoscale. However, due to their Ohmic losses and low heat resistance they cannot be practically applied in nonlinear optical processes for optical frequency conversion. Dielectric nanoparticles, e.g. silicon and germanium, are good candidates to overcome these limitations [1, 2]. Nevertheless, the centrosymmetric nature of these materials have voided the second-harmonic generation (SHG). Alternatively, the use of GaAs-based III-V semiconductors, with non-centrosymmetric structures, can overcome this difficulty [3,4]. However, fabrication of III-V semiconductor nanoantennas on low refractive index substrates remains very challenging, blocking the possibility to explore the SHG directionality in both forward and backward direction. Here, for the first time to our knowledge, we design and fabricate high-quality AlGaAs nanostructures on a glass substrate. Through this novel platform, we manage to excite, control and detect backward and forward nonlinear signals by SHG in AlGaAs nanodisks [5,6]. In particular, we observe that for certain size of nanoantenna, the SHG emission has a complex spatial distribution polarization state corresponding to radial polarization in the forward direction and a polarization state of a more general nature in the backward direction. Furthermore, we demonstrate an unprecedented SHG conversion efficiency of 10-4. Our breakthrough can open new avenues for enhancing the performance of photodetection, light emission and sensing.v
Optical metasurfaces have developed as a breakthrough concept for advanced wave-front. Key to these “designer metasurfaces” is that they provide full 360 degree phase coverage and that their local phase can be precisely controlled. The local control of phase, amplitude and polarization on an optically thin plane will lead to a new class of flat optical components in the areas of integrated optics, flat displays, energy harvesting and mid-infrared photonics, with increased performance and functionality. However, reflection and/or absorption losses as well as low polarization-conversion efficiencies pose a fundamental obstacle for achieving high transmission efficiencies that are required for practical applications.
A promising way to overcome these limitations is the use of metamaterial Huygens’ surfaces [2-4], i.e., reflection-less surfaces that can also provide full 360 degree phase coverage in transmission. Plasmonic implementations of Huygens’ surfaces for microwave  and the mid-infrared spectral range , where the intrinsic losses of the metals are negligible, have been suggested, however, these designs cannot be transferred to near-infrared or even visible frequencies because of the high dissipative losses of plasmonic structures at optical frequencies.
Here, we demonstrate the first holographic metasurface utilizing the concept of all-dielectric Huygens’ surfaces thereby achieving record transmission efficiencies of approximately 82% at 1477nm wavelength. Our low-loss Huygens’ surface is realized by two-dimensional subwavelength arrays of loss-less silicon nanodisks with both electric and magnetic dipole resonances . By controlling the intrinsic properties of the resonances, i.e. their relative electric and magnetic polarizabilities, quality factors and spectral position, we can design silicon nanodisks to behave as near-ideal Huygens’ particles. This allows us to realize all-dielectric Huygens’ surfaces providing full 360 degree phase coverage that lack dissipative losses and also suppress unwanted reflections without relying on cross-polarization schemes that additionally suffer from polarization-conversion losses.
We now use such Huygens’ surfaces in order to create a highly-efficient phase masks for the generation of optical holograms. By varying only one geometrical parameter, namely the lattice periodicity that can be controlled easily during the fabrication process we can effectively generate arbitrary hologram images from a 4-level phase discretization. In order to design the arrangement of the pixels in the metasurfaces, we calculate the phase mask required for a hologram generating the letters ‘hv’ in the hologram plane. In the next step the Huygens’ hologram is fabricated on a back-side polished SOI wafer by electron-beam lithography followed by a reactive-ion etching process. Then, we measure the phase of the generated hologram using a home-built Mach-Zehnder interferometer and perform a phase retrieval process to compare the experimental phase with the designed phase. Finally, we record the holographic image in the hologram plane and demonstrate that the device functionality is completely polarization insensitive with a transmission efficiency of 82%, in contrast to all the earlier works utilizing geometric phase.
 Yu et al., Nat. Mater. 13, 139 (2014).
 Pfeiffer et al., Phys. Rev. Lett. 110, 197401 (2013).
 Monticone et al., Phys. Rev. Lett. 110, 203903 (2013).
 Decker et al., Adv. Opt. Mater. 3, 813 (2015).