The challenge

The momentum of silicon nitride in photonics is huge today. Thanks to the combination of the low propagation loss (below 0.1 dB/cm), high integration density, and ultra-wideband operation that it can offer, the silicon nitride is probably the platform with the highest potential for the development of complex photonic integrated circuits (PICs) that can sense and think with ultra-high sensitivity, precision and computation capacity. This oncoming generation of PICs has the potential to boost the use of photonics in existing application areas such as metrology, sensing, biosensing and microwave photonics, and enable the uptake of photonic solutions in emerging areas with huge interest such as neuromorphic computing, quantum communications and quantum computing.

Despite these excellent prospects and its unique potential in terms of passive waveguiding performance, the silicon nitride platform remains passive in nature. The hybrid solutions for emission, modulation, nonlinear processing and detection of light on this platform are still non-optimum or even totally absent if these solutions should offer, high photonic performance, wideband operation, and integration robustness at the same time. The potential of the silicon nitride platform is thus real and present, but its exploitation is still partial and challenging.

The vision

LOLIPOP is a photonic integration project that aims to fill this gap, and enable the silicon nitride platform to make the next step and fully flourish. To this end, LOLIPOP invests on the combination of the silicon nitride with the lithium niobate on insulator (LNOI) technology, while working in parallel on the integration of semiconducting materials on the silicon nitride platform using a disruptive set of methods. TriPleX, the silicon nitride variant of LXI serves in LOLIPOP as the motherboard of this new hybrid technology. Both this motherboard, the elements that are treated as daughterboards, and their integration methods have been picked out to serve a vision with three main axes related to the optical functionality, the spectral coverage, and the future production flow of the envisioned platform:

  • High-speed modulation and second order nonlinear processing on silicon nitride: The silicon nitride still lacks a modulation solution that can offer both high speed operation and high integration performance in terms of mechanical stability and potential for high-volume production. To bring this solution, LOLIPOP invests on the LNOI films, which is the most recent and promising form of the well-known lithium niobate (LN) material [2]. In this form, the LN has already proved that it can be a wonder material with intrinsic modulation bandwidth above 100 GHz, Vπ below 1.5 V and propagation loss below 0.25 dB/cm [2-4]. LOLIPOP will work on the transfer of LNOI films on TriPleX aiming at a hybrid LNOI-on-TriPleX platform with highest modulation and passive waveguiding performance. The result can be nothing but disruptive. Interestingly, thanks to the crystalline nature of the LN and the possibility to periodically pole the LNOI waveguides, the transfer of the LNOI films will also offer an unmatched potential for second order nonlinear effects, and support of optical parametric processes on TriPleX.
  • Cutting-edge performance from the deep blue up to 1600 nm on silicon nitride: The LN is by default a material that can match the ultra-wideband operation capacity of the silicon nitride from the blue part of the visible (Vis) spectrum up to 2 µm and above. The hybrid LNOI-on-TriPleX platform will be thus disruptive also in this regard. It will not be only able to have a cutting-edge performance, but also to deliver this performance at any wavelength within the spectrum of interest in this effort from 400 up to 1600 nm, supporting applications that cannot be effectively supported today by integrated photonics, especially in the Vis. To further work on the vision for an ultra-wideband photonic integration platform, LOLIPOP will develop a method for the heterogeneous integration of germanium photodiodes (Ge-PDs) inside the TriPleX structure, and will make use of them as high-performance photodetection elements over the entire spectrum of interest. Finally, although not possible to have a single active element for the entire spectrum from the Vis to 1600 nm, LOLIPOP will develop global methods for the integration of active elements, and the generation of high quality laser lines at the individual wavelength bands.
  • Prospect of wafer-scale integration processes and high-volume production: LOLIPOP will develop processes for the integration of the various materials and components on the TriPleX platform, which can be compatible with wafer scale integration, and thus with the prospect of high-volume production. These will include a novel micro-transfer-printing process for the transfer and stamping of LNOI films on TriPleX wafers, a process for the growth of Ge-PDs inside TriPleX wafers, and a process for the etching of recesses and the flip-chip bonding of active elements on the TriPleX platform. These processes will be extended to incorporate other processing steps that are part of the current TriPleX technology, and they will be consolidated as a single and cohesive process flow.