Brillouin silicon laser based on subwavelength optomechanic waveguides – BRIGHT

The main objectives for the initial 18 months of the project are:
- Explore the degrees of freedom released by subwavelength nanostructuration to develop Brillouin-active Si waveguides allowing flexible tailoring of both photonic and phononic modes. The goal of this task is to develop Si membrane waveguides that allow low propagation loss of the optical modes and tight confinement of the phononic modes (maximizing phonon lifetime). Different waveguide geometries will be designed considering typical minimum feature sizes and fabrication errors. Propagation loss and mechanical stability of the proposed geometries will be experimentally compared, providing key information for design optimization.
This task will start from the nanostructured membrane waveguide concepts recently developed by the Scientific Coordinator for applications in the mid-infrared. Such waveguides will be used for the early development of passive devices in task 1.2. Waveguide geometry will be adapted to efficiently confine both, photonic and phononic modes.

- Development of complete theatrical model for Brillouin in subwavelength waveguides: we have developed a complete model allowing accurate calculation of Brilluoin gain in 3D structures. Results validated with experimental data from literature.
- Design of optimized subwavelength structures for Brillouin gain: We have developed novel design strategies allowing the optimization of Brillouin gain in subwavelength structures. Results published in Optics Letters 2019.
- Demonstration of record mechanical frequency in silicon optomechanical resonators: Based on the thorough models and design strategies developed for subwavelength structures, we have fabricated Si optomechanical resonators in C2N cleanroom, showing record mechanical frequencies. Preprint in arXiv, submission in preparation for ACS Photonics.

In the first 18 months of the project, we have exploited subwavelength engineering to design high-efficiency Brillouin waveguides and demonstrate experimentally record high frequency optomechanical resonators in silicon. These results will serve as the basis to the work in the rest of the project, aiming at further optimizing the optomechanical Brillouin coupling to demonstrate the lasing effect.

1. J. Zhang, O. Ortiz, X. Le Roux, E. Cassan, L. Vivien, D. Marris-Morini, N. D. Lanzillotti-Kimura, C. Alonso-Ramos, “Subwavelength engineering for Brillouin gain optimization in silicon optomechanical waveguides,” Opt. Lett. 45, 3717, (2020).
2. T. T. D. Dinh, J. Zhang, D. Oser, X. Le Roux, M. Montesinos, C. Lafforgue, F. Mazeas, D. Pérez-Galacho, D. Benedikovic, E. Durán-Valdeiglesias, V. Vakarin, O. Ortiz, A. Rodirguez, O. Alibart, P. Cheben, S. Tanzilli, L. Labonté, D. Marris-Morini, E. Cassan, D. Kimura, L. Vivien, C. Alonso-Ramos, “Harnessing subwavelength nanostructuration to control the propagation of light and sound in silicon waveguides,” International Conference on Transparent Optical Networks (ICTON), May. 2020, Online (Invited).

The BRIGHT project is devoted to the emerging field of Brillouin-based optomechanics in silicon. Stimulated Brillouin scattering (SBS) has an immense potential for opto-acoustic signal processing which has no analogue in conventional electro-optic or all-optic approaches. Although SBS has been extensively developed in optical fibers, the large phonon leakage towards the silica cladding precluded SBS in silicon-on-insulator (SOI) waveguides. Recently, a new generation of Si optomechanic waveguides has overcome this barrier, revolutionizing the field and allowing the experimental demonstration of SBS nonlinearities surpassing Kerr and Raman effects (in 2013), complete phononic bandgap (in 2014), net amplification (in 2016), and Si Brillouin laser (in 2017).
Although very promising, the first demonstration of Si Brillouin laser exhibits moderate SBS gain. Thus, it requires few centimetres long waveguides, which compromise the compactness and robustness of the solution. The major limitation of SBS gain today is the trade-off between long photon lifetime and strong photon-phonon overlap in previously reported Si Brillouin waveguides.
This project will address a new route to overcome this limitation, enabling the realization of compact and high-efficiency Si Brillouin laser. The original idea is to exploit the unique degrees of freedom released by subwavelength nanostructuration to independently tailor phononic and photonic modes, maximizing the efficiency of the Brillouin effect. This technology will unlock new tools for Brillouin-based advanced on-chip signal processing and the study of fundamental light-sound interactions in nanometric Si waveguides.
Overcoming the trade-off between long phonon lifetime and large photon-phonon overlap and demonstrating compact and efficient Si Brillouin laser will constitute two major breakthroughs towards the development of a Si Brillouin optomechanics technology and will place the BRIGHT project and the Scientific Coordinator’s team at the forefront of the field.
Finally, the BRIGHT project will help the Scientific Coordinator, C. Alonso-Ramos (CRCN since October 2017), to develop his independent research career by providing the means to start building his research team, and open a new research line on light-sound interactions in subwavelength engineered Si photonic circuits. This independent research activity will prepare the Scientific Coordinator to apply for European Research Council (ERC) Starting grant. In addition, the multidisciplinary nature of the BRIGHT project, covering photonics, optomechanics and nanotechnology, will bring a great opportunity to link different departments and different activities in C2N to develop an emerging new research field with a clear complementarity with the current activities.