Sub-WavelEngth Electro-optic sysTems – SWEET

The fast modulation of the wavefront of an optical beam is a key element of future photonic systems, with many potential applications. The most immediate ones are LIDAR type systems where this fast modulation will allow to control the scanning of the observed scene. Project SWEET proposes to demonstrate the interest of GaN-based transparent active metasurfaces for this type of application. The visible spectral range targeted by the project opens many additional application avenues such as confocal scanning microscopy or visible display and projection.
To achieve its goal, SWEET will address four main objectives: the optimization of the design of complex structures taking into account robustness against fabrication imperfections, the demonstration of fast index modulation in nanostructures, the demonstration of components allowing active control of metasurfaces, and finally the use of transparency to stack active optical functions.
The studied structures will be composed of interdigitated 1D gratings of sub-wavelength periods in GaN. Two channels will be pursued to realize an electrically controllable index modulation allowing to tune the beam phase along the grating, between 0 and 2 pi:
- A high refractive index variation route based on the impregnation of nematic type liquid crystals between the teeth of the n-doped GaN grating acting as an electrode. The extreme smallness of the air gap between the teeth should allow a frequency increase beyond that of conventional systems using liquid crystals;
- A low refractive index variation channel exploiting the Quantum Confinement Stark effect obtained by quantum well engineering. The achievable modulation frequencies will be very high (GHz), but the very low index variation obtained will have to be compensated by the use of optical resonators to reach the necessary 2 pi phase variation.
From these elementary components, interconnected active metasurfaces will be realized. We propose to exploit the nano-imprint technology to efficiently duplicate these nano-structured components on large active surfaces (~ 1mm²). The first demonstrator will be a 1D transmission liquid crystal phase modulator. The second demonstrator, which will benefit from the technological advances of the first one, especially in lithography, will be a 1D phase modulator based on GaN quantum wells operating in reflection. Finally, the stacking of the two previous modulators, with crossed scanning directions, will achieve a 2D phase modulator. This 2D scanning of the beam will be tested to evaluate the LIDAR performances.
The consortium includes three public laboratories and a deeptech SME, experts recognized by the community and having a number of world firsts to their credit. The fields of expertise are:
- Nano-fabrication (NAPA, LAAS) with the involvement of French network of high-end facilities in the field of micro & nanotechnology;
- Metasurfaces (CRHEA, LAAS) and the design of nanophotonic systems (INRIA, CRHEA);
- Epitaxial growth of nitrides, at the heart of the project (CRHEA)