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Functional 2D metamaterials at visible wavelengths

Periodic Reporting for period 3 - FLATLIGHT (Functional 2D metamaterials at visible wavelengths)

Reporting period: 2018-11-01 to 2020-04-30

Leveraging on the recent concept of metaoptics, FLATLIGHT ERC starting grant main research objective consists in producing new types of passive and active ultrathin optical components for visible wavelengths. Our innovative approach for making wavelength-thick optical devices relies on nano-patterned interfaces made of mature nitride materials such as GaN and InGaN. We are addressing several problems related to the control of light at interfaces, ranging from the control of basic reflection and refraction of light to the generation of any arbitrary wavefronts and dynamical addressing of light properties at interfaces. Passive components of general interests such as metalenses and metasurfaces with advanced functionalities have already been fabricated. Polarization and dispersion controlled devices, which are working at visible and UV wavelengths have been designed, fabricated and are currently characterized in our laboratory. The successful realization of flat optical devices in III-N materials opens up new perspectives for their integration into commercial semiconductor-based optoelectronic components such as LED, CCD sensors and various optical systems for visible and UV light detection and ranging applications.
WP1 started by the implementation of new set of boundary conditions at interfaces of arbitrary shapes. This concept of conformal boundary optics is necessary to design any free-form optical devices. After understanding the underlying theoretical physics, we have elaborated a model and implemented numerically new boundary conditions of light at interfaces to design various sort of free-form metasurfaces. We have written a new simulation software, based on modified FDTD, for testing the conventional generalized sheet boundary conditions in some simple cases and propose new devices such as free form metalenses.
We have also started experimental research activities on III-Nitrides materials for metasurfaces in the passive regime, i.e. frequency below the band gap of the material, to deflect and/or focus light at visible wavelength. GaN metasurfaces consist of ensemble of spatially varying GaN nanoridges or nanopillars. Nanopillars arrays forming metasurfaces are fabricated using classical steps of nano-fabrication process. We succeeded to find all experimental parameters to process the devices as expected from numerical simulations. These initial results are fulfilling WP 1 requirements on the demonstration of GaN based passive metasurface in our laboratory. Carefully choosing the design of the nanostructures, we can design and realize components that work at visible and UV wavelengths down to 365nm.
We moved further to create achromatic optical devices by compensating the dispersion from conventional optical components using the dispersion properties of metasurfaces. Starting with the simple case of prism dispersion, constant phase gradient metasurfaces were fabricated to compensate normal material dispersion of the prism. Through this work, we demonstrate that metasurfaces can be combined to refractive materials to achieve achromatic behaviour at multiple wavelengths, ruling out some of the limitations of conventional refractive and diffractive optics.
In WP2, we are proposing the utilization of quantum confined stark effect to modulate the photonic response of optical metasurfaces. To integrate this effect and create our metasuface modulator, a Metal-Semiconductor -Metal (MSM) resonator structure has been designed by sandwiching thin semiconductor film into metallic layers. This fabrication is technically challenging and requires to fully remove all substrate on which the active layer are fabricated. We developed a specific fabrication process for this purpose and currently up to 1 mm * 1mm crack-free nitride films, on which we perform metasurface nanofabrication, have been successfully obtained. Currently, we are developing a feasible approach to make the electrical contacts onto these structures for electrical modulation of light at metasurfaces.
Another application to active devices consists in designing, fabricating and integrating metasurfaces directly onto vertical cavity emitting lasers, to act as microlenses and collimate laser emission. Metalens of specific optical properties have been directly integrated onto the back side (emitting side) of lasing devices, fully collimating laser beam emission from the lasers. The characterization of emitting performance from these devices are in progress but to compare with conventional bottom emitting VCSELs, we have already performed P-I-V electrical characterization of non-structured samples. Comparison with metalens VCSELs are ongoing.
FLATLIGHT research led to a new approach to control polariton properties in polariton lasers by controlling the photonic dispersion of guided slab modes in ZnO semiconductor. The control of emission is made possible by using metastructured interfaces on the top of the polariton waveguides.
Stacks of phase-discontinuity surfaces, operated in the nonlinear regime, are designed to achieve artificial nonlinear phase matching conditions for given beam profiles. Three types of structures have been designed, from the single grating to the m
Currently, our work focuses on new materials for metasurfaces and their advantages with respect to conventional dielectric materials. This concept of controlling light with nanostructured interfaces is still at its infancy, which therefore offers many opportunities to innovate beyond the state of the art. In particular, we have demonstrated a new fabrication process based on material selective sublimation to realize metasurfaces. With respect to traditional nanofabrication techniques, the semiconducting material is removed without using conventional reactive ion etching but rather due to the selective evaporation process of the crystal along well defined crystalline- axis. This method can only work for crystalline materials.
We are proposing compact solution to collimate the laser emission by including meta-optical components directly at the wafer. This method enables collimation of laser light without the utilization of any additional/external optical components.
Traditional nonlinear frequency conversion is achieved by periodically compensating the effect of dispersion adjusting exclusively the material nonlinear coefficient. We show that metasurfaces are interesting tools to address this problem. This technique could be exploited in particular for materials with intrinsically high nonlinearities but for which it is rather difficult (even impossible) to reversing the sign of their nonlinear coefficients.