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Functional 2D metamaterials at visible wavelengths

Periodic Reporting for period 4 - FLATLIGHT (Functional 2D metamaterials at visible wavelengths)

Berichtszeitraum: 2020-05-01 bis 2021-10-31

FLATLIGHT ERC starting grant aimed at producing new types of passive and active ultrathin optical components to manipulate the light at 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 semiconductors. We addressed 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 with interesting topological features and dynamical addressing of light properties at interfaces. Passive components of general interests such as metalenses and metasurfaces with advanced functionalities have been demonstrated. Polarization and dispersion controlled devices, operating at visible and UV wavelengths have been demonstrated. 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 light detection and ranging applications (LiDAR).
During the last decade including our ERC project, the realization of ultra-flat and ultralight optical components featuring on-demand optical wavefront addressability have significantly transform the optical design industry, expecting new business opportunities in the coming years. Metasurfaces of various sizes and functions have been demonstrated with relatively high optical performances in terms of wavefront modulation and efficiencies, which are thus readily impacting the optical industry. Metasurface designs and materials are becoming compatible in with those available for the processing of integrated circuits in semiconductors manufacturing facilities, offering perspectives for wafer-scale production of metasurface components leading to their deployment in the public domain.
Flatlight project started with experimental research activities on III-Nitrides materials for metasurfaces in the passive regime. We fulfilled WP 1 requirements on the demonstration of functional GaN based passive metasurfaces, operating at visible and UV wavelengths. Various scientific papers on this thematic have been published, including wide audience communications and high impact publications. Visible metasurfaces have been utilized in proof of concept ERC grant to realize disruptive LiDAR imaging systems and we are currently working on a maturation program to startup a company on metasurface-enhanced LiDAR technology.
Improving our nanofabrication capabilities enabled polarization dependent metaoptical components, leading to several high impact works and large audience press releases.

The second objective of Flatlight was to utilize quantum confined stark effect to modulate the photonic response of optical metasurfaces. Initially we started considering a Metal-Semiconductor -Metal structure by sandwiching thin semiconductor film into metallic layers. This fabrication appeared technically challenging, requiring full removal of substrate supporting the active layer. After unsuccessful fabrication attempts, we moved to 2D templated semiconductor films, which led to the demonstration of time-modulated photoluminescence by electrical doping of 2D materials.

System integration of metasurfaces directly onto lasing devices had also been realized for chip-scale collimation and laser wavefront engineering. The characterization of VCSEL emitting performances provided full wavefront collimation, directional laser emission and arbitrary laser wavefront emission.
The third objective of Flatlight was to achieve nonlinear frequency generation in a system using phase delaying metasurfaces. Stacks of phase-discontinuity surfaces have been designed to achieve artificial nonlinear phase matching conditions for given beam profiles. Three types of structures have been designed, from the guided effective mode structure to the multi-stacked metasurfaces. All of these methods exhibit high electric field enhancement of interest for nonlinear field generation, including second order nonlinear effect. Among the several unsuccessful attempts in fabricating various artificially phase matched materials for SHG fields, we succeeded using the cascaded metasurfaces technique proposed in WP3. We realized unconventional backward SHG using Quasi-Phase Matching by distributing phased metasurfaces along the optical path of the light, so as to rephase pump and second harmonic beams during propagation.
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.
The first progress realized beyond the state of the art and that helped us for design of metasurfaces used in all WPs is the utilization of numerical optimization tools to design efficient metasurfaces. Collaborative efforts with experienced researchers in optimization methods led to some of the first optimized metasurface design to improve optical efficiency of the components.
Beyond the state of the art in designing optical metasurfaces, we have implemented a 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 and aberration-reducing in curved metalenses.
We also reported an unexpected fabrication process for metasurfaces relying on material selective sublimation. With respect to traditional nanofabrication techniques, the semiconducting material is removed without using conventional reactive ion etching using selective evaporation process of the crystal along well defined crystalline-axis. This method works only for crystalline materials.
We also created achromatic optical devices by compensating the dispersion from conventional optical components using the dispersion properties of metasurfaces. With this work, we demonstrated how metasurfaces can be combined with refractive materials to achieve achromatic behaviour at multiple wavelengths, ruling out some of the limitations of conventional refractive and diffractive optics.
Our last discovery beyond the state of the art is a new phase addressing mechanism for metasurface design. We used concepts from topological physics and the extinction of the light beam corresponding to the presence of a singularity in the space of parameters defining the nanostructure final geometry. The amplitude of the light wave being zero, its phase is no longer defined. By staying in the vicinity of the singularity, it is then possible to draw antennas whose characteristics will give the wave the desired phase, between 0 and 2 pi. This total control of the wavefront makes it possible to develop a nanostructured interface reflecting a light ray at a well-determined angle, but also to obtain exceptional scattering effects, such as non-reflection behavior, perfect absorption on certain transmission or reflection channels.
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