In the first half of the PHOENIX project, the consortium has worked on different activities.
New successful work on the integration of VO2 onto integrated photonic structures has been achieved using SiN/BTO waveguides. The integration of VO2 in the SiN/BTO platform for enabling ultra-compact amplitude switching devices offering a scalar multiplication functionality with multilevel operation has been investigated. An electronically reprogrammable switching device with a 5-bit amplitude encoding capability and an insertion loss of only 0.5 dB has been proposed. The device is built with a 9-µm-long VO2/SiN/BTO waveguide structure integrated with an efficient microheater using a transparent conducting oxide (TCO) to reduce energy consumption and achieve faster speeds.
In relation to the hybrid VO2/BTO device, which offers the possibility to build compact structures which can control the phase and the amplitude separately, the consortium has explored the deposition conditions for the growth of VO2 on BTO surfaces. We have developed a process to grow a material stack where VO2 exhibits the associated metal-to-insulator transition while preserving the BTO material integrity. The fabrication of the first hybrid devices is combined with the first tape-out, and will be completed after April 2024.
Related to the upscaling to larger platforms, we can report that the new MBE tool (DCA) was delivered to KU Leuven in December 2023. Testing and alignment of e.g. the wafer manipulation and of the e-beam evaporators are currently ongoing.
In parallel, we have been preparing further for the first PIC fabrication run. PHOENIX will work on two hardware platforms.
The first hardware platform has been designed for Fully Homomorphic Encryption (FHE) and 5G infrastructure. PHOENIX optical technology aims at accelerating the computations of FHE and improve its latency. For example, today one multiplication (with key switching) in CGGI programmable bootstrap takes about 5 to 15ms. Using our photonic devices, we aim to reduce these numbers by a factor 10 to 100. The requirement for the 5G demonstration focuses to use the FHE/5G system to demonstrate a 4096-point IFFT and FFT in under 4.46 ms with cyclic prefix.
The second hardware platform has been designed for inference of Deep Neural Networks (DNN) and training of DNN. In this platform a very compact tunable attenuation element can be built using the VOx absorbers. We also intend to demonstrate that we can obtain a tunable coupler by using reflection on a diffraction grating created in the BTO waveguides and using the photorefractive effect. The advantage of this approach is that the grating can be adjusted/trained optically and is non-volatile. Thus it does not need continuous electrical control to maintain the coefficient value.
In the first half of the project, the consortium has worked to define the specifications and architecture of the photonic devices, as well as the board/system hardware for those use cases.
In the first DNN fabrication run, we will produce structures to evaluate the photorefractive effect in BTO waveguides, and produce test versions of small optical crossbars that will be used in early experiments on DNN processing
In preparation of the first fabrication run based on BTO technology, the consortium has been working to design the circuits and building blocks (waveguides, splitters, bends, phase shifters, etc.) for each hardware platform as well as to build the setup used to confirm the presence of the photorefractive effect in BTO thin films The fabrication process started on December, 12th 2023, with a fab-out date expected to be April, 8th 2024.
