Periodic Reporting for period 1 - BEAMS (Multilayer photonic integration platform for free space optics)
Reporting period: 2022-09-01 to 2025-02-28
Laser light has revolutionized countless aspects of our societies and is at the foundation of a plethora of fields. In particular, communications heavily rely nowadays on the use of light to ensure a diffused access to high-speed connectivity. Whereas in the past the focus has mainly been on the development of fiber-optic communication networks, interest in free-space optical communications has grown again in recent years. Modulated light beams propagating in the free space instead of an optical fiber can be used to establish a high-capacity communication channel between two terminals with a simplified infrastructure and maintaining the link even if the terminals are moving. Beside many terrestrial applications, e.g. as a solution to connect rural and remote areas where optical fibers are difficult to lay, the use of free-space optical communications is also intensely studied in satellite-to-ground and satellite-to-satellite links. Despite this tremendous potential, the widespread application of free-space optical technologies is currently limited by the devices required to generate, shape, and point optical beams. These systems mostly rely on conventional optical components (e.g. lenses and mirrors) and mechanical assemblies. This makes them bulky, slow and costly. The solution envisioned by the project is to exploit photonic integration to realize these optical functionalities using a photonic chip. In particular, the original idea is to develop a new class of optical phased arrays, an on-chip device composed by many optical antennas, to generate, detect and point free-space laser beams. The project will exploit a multi-layer integration technology, metasurfaces and photonic circuits to achieve the high efficiency and large emission and detection areas that are required for long-distance communication links. Replacing motors, lenses and mirrors with a single, centimeter-scale chip through photonics integration will enable compact and lightweight devices and systems, reduced power consumption, novel functionalities, and robustness to mechanical vibrations, a groundbreaking achievement with extensive scientific, technical, and economic impact.
In the initial part of the project, the research mostly focused on developing the envisioned photonic integration technology that is required to achieve the ambitious objective of performing free-space optical communications over long distances. We successfully developed an initial multi-layer platform already offering low loss silicon and silicon nitride layers. We also started preparing critical components that we will use to building the full integrated system, especially inter-layer couplers and optical antennas. In particular, we experimentally demonstrated a micrometer-scale optical antenna based on a surface grating achieving the challenging objective of showing at the same time a good scattering efficiency and an extremely low dependence of the optical response to the polarization of light, an important aspect especially for light receivers where the incoming polarization could be unknown. We focused our attention also on the development of optical metasurfaces, which will be another component of the final system. We successfully developed a metalens with a large field of view and extremely low chromatic aberration to be used as a broadband, beam deflecting device. Regarding design tools, we worked on the development of advanced techniques to be used for the development of the photonic components. We focused in particular on including fabrication imperfections within the design cycle, with the objective of improving the device robustness to fabrication tolerances. This will be a crucial aspect to demonstrate large photonic systems (like antenna arrays) including hundreds of devices. The method that we developed combined dimensionality reduction and Gaussian process regression techniques to address multi-parameter, multi-objective stochastic optimization problems.
The research work performed so far already produced two intermediate results whose importance goes beyond the project objectives.
- A broadband and wide filed of view metalens. Demonstrating a flat metalens capable of achieving at the same time a large field of view (i.e. capable of focusing on the same image plane light coming from a wide range of angles) and no chromatic aberration over a large band is a challenging research question. Within the project, we achieved this goal experimentally demonstrating what it is, up to our knowledge, one of the best performing metalens in the two figures of merit (field of view and broadband operation). This result could be of great importance for example for imaging systems in the infrared as well as the visible wavelength range.
- Polarization insensitive antenna. Surface grating couplers are well-know devices to couple light from integrated waveguides to fibers or the free space. The efficiency and operation of these devices are however extremely sensitive to the polarization of light and no high efficiency solution was so far published in the literature. By incorporating in the grating structure a judiciously designed metamaterial we achieved state-of-the-art performance for our polarization insensitive grating antennas without resorting in complicated fabrication processes (e.g. we do not require substrate removal) but using a standard silicon photonic technology. This grating coupler could become a critical component in integrated systems relying on polarization diversity.
- A broadband and wide filed of view metalens. Demonstrating a flat metalens capable of achieving at the same time a large field of view (i.e. capable of focusing on the same image plane light coming from a wide range of angles) and no chromatic aberration over a large band is a challenging research question. Within the project, we achieved this goal experimentally demonstrating what it is, up to our knowledge, one of the best performing metalens in the two figures of merit (field of view and broadband operation). This result could be of great importance for example for imaging systems in the infrared as well as the visible wavelength range.
- Polarization insensitive antenna. Surface grating couplers are well-know devices to couple light from integrated waveguides to fibers or the free space. The efficiency and operation of these devices are however extremely sensitive to the polarization of light and no high efficiency solution was so far published in the literature. By incorporating in the grating structure a judiciously designed metamaterial we achieved state-of-the-art performance for our polarization insensitive grating antennas without resorting in complicated fabrication processes (e.g. we do not require substrate removal) but using a standard silicon photonic technology. This grating coupler could become a critical component in integrated systems relying on polarization diversity.