Periodic Reporting for period 1 - SUNRISE (Silicon Brillouin-assisted Optoelectronic Oscillator Based on Subwavelength Membranes)
Reporting period: 2022-07-01 to 2024-09-30
Photonic integrated circuits (PICs) are poised to bring a new revolution that will underpin the widespread deployment of light-enabled applications in the fields of communications, sensing, metrology and quantum. Silicon photonics holds the promise for large-scale, low-cost production of high-performance optoelectronic circuits leveraging existing complementary-metal-oxide semiconductor (CMOS) manufacturing infrastructure. However, addressing the needs of emerging applications requires the development of key building blocks that are not feasible in state-of-the-art silicon photonics technology. The Brillouin scattering (BS) process is especially important for realizing complex photonic circuits for optical communications, sensing, and quantum processing, but has been precluded in conventional silicon-on-insulator (SOI) due to strong phonon leakage towards the silica bottom cladding.
The SUNRISE project aims to develop a new generation of silicon active optomechanical devices by exploiting subwavelength structuring to tailor the optical and mechanical mode properties. Its main objective is to maximize optomechanical coupling in subwavelength-engineered waveguides or resonators to achieve high Brillouin gain, surpassing the state-of-the-art, and ultimately enabling advanced on-chip data processing functionalities. The development of this optomechanical devices will pave the way towards the generation of radiofrequency signals in the MHz to GHz range, with potential applications in microwave photonics, quantum signal processing and sensing.
The project has successfully addressed the development of novel subwavelength silicon membranes and suspended waveguides, auxiliary passive devices, the demonstration of stimulated BS gain in subwavelength optomechanical waveguides, and the development of resonant structures for BS enhancement. The achievement of these milestones has laid a solid foundation not only for the implementation of the optoelectronic oscillator, but also for future applications based on the developed technology.
The SUNRISE project aims to develop a new generation of silicon active optomechanical devices by exploiting subwavelength structuring to tailor the optical and mechanical mode properties. Its main objective is to maximize optomechanical coupling in subwavelength-engineered waveguides or resonators to achieve high Brillouin gain, surpassing the state-of-the-art, and ultimately enabling advanced on-chip data processing functionalities. The development of this optomechanical devices will pave the way towards the generation of radiofrequency signals in the MHz to GHz range, with potential applications in microwave photonics, quantum signal processing and sensing.
The project has successfully addressed the development of novel subwavelength silicon membranes and suspended waveguides, auxiliary passive devices, the demonstration of stimulated BS gain in subwavelength optomechanical waveguides, and the development of resonant structures for BS enhancement. The achievement of these milestones has laid a solid foundation not only for the implementation of the optoelectronic oscillator, but also for future applications based on the developed technology.
The first part of SUNRISE focused on developing various subwavelength silicon membranes and suspended waveguides to effectively confine photonic and phononic modes. Optical simulations of these devices were first conducted using finite difference eigenmode (FDE) and finite-difference time domain (FDTD) methods. Conversely, optomechanical simulations were carried out using multiphysics software based on finite element method. Subwavelength-structured silicon membrane waveguides for transverse-electric (TE) polarization and for transverse-magnetic (TM) polarization, and fully suspended silicon waveguides with subwavelength nanostructuring were designed.
Auxiliary passive devices were also developed to create a comprehensive set of novel building blocks capable of addressing the functionalities required to exploit Brillouin interactions in subwavelength silicon membranes and suspended waveguides. These devices included highly-efficient fiber-chip couplers, broadband power splitters and mode converters/multiplexers, and high-performance spatial and polarization splitters. The auxiliary devices were optimized using FDTD methods and subsequently fabricated using state-of-the-art cleanroom equipment (electron-beam lithography, resin development, reactive ion etching, and acid and plasma O2 cleaning).
The second phase of SUNRISE focused on the experimental demonstration of subwavelength silicon membranes. This involved exploring various configurations using the multiphysics solver to optimize membrane geometries and search for maximum Brillouin gain. Fabrication of these devices followed the same process as the auxiliary devices, although including an additional step for releasing the structures using hydrofluoric acid vapor. The subwavelength silicon membranes operating with TE-polarized optical mode showed higher optical mode propagation losses compared to subwavelength membranes operating with transverse-magnetic (TM) polarization, as expected. Nonetheless, the former demonstrated Brillouin gain for waveguide lengths on the order of only 6 millimeters.
Finally, resonant structures were developed to improve optomechanical interaction. This was particularly useful for fully suspended optomechanical waveguides. Since these waveguides are primarily suspended by their longitudinal ends, they have a very limited interaction length between optical and mechanical modes to enhance Brillouin gain. Optical quality factors between 5000 and 220000 and extinction ratios between 7 and 20 dB were obtained, in good agreement with simulations. Moreover, a novel type of non-suspended silicon optomechanical cavity was investigated.
Auxiliary passive devices were also developed to create a comprehensive set of novel building blocks capable of addressing the functionalities required to exploit Brillouin interactions in subwavelength silicon membranes and suspended waveguides. These devices included highly-efficient fiber-chip couplers, broadband power splitters and mode converters/multiplexers, and high-performance spatial and polarization splitters. The auxiliary devices were optimized using FDTD methods and subsequently fabricated using state-of-the-art cleanroom equipment (electron-beam lithography, resin development, reactive ion etching, and acid and plasma O2 cleaning).
The second phase of SUNRISE focused on the experimental demonstration of subwavelength silicon membranes. This involved exploring various configurations using the multiphysics solver to optimize membrane geometries and search for maximum Brillouin gain. Fabrication of these devices followed the same process as the auxiliary devices, although including an additional step for releasing the structures using hydrofluoric acid vapor. The subwavelength silicon membranes operating with TE-polarized optical mode showed higher optical mode propagation losses compared to subwavelength membranes operating with transverse-magnetic (TM) polarization, as expected. Nonetheless, the former demonstrated Brillouin gain for waveguide lengths on the order of only 6 millimeters.
Finally, resonant structures were developed to improve optomechanical interaction. This was particularly useful for fully suspended optomechanical waveguides. Since these waveguides are primarily suspended by their longitudinal ends, they have a very limited interaction length between optical and mechanical modes to enhance Brillouin gain. Optical quality factors between 5000 and 220000 and extinction ratios between 7 and 20 dB were obtained, in good agreement with simulations. Moreover, a novel type of non-suspended silicon optomechanical cavity was investigated.
Various geometries (both membrane and fully suspended configurations) have been proposed. These devices have demonstrated, to the best of our knowledge, one of the highest Brillouin gains reported to date for suspended silicon wires. Remarkably, subwavelength silicon membranes operating with TM-polarized optical modes show potential to achieve a high Stokes gain (>15 dB) with millimeter-long waveguides due to low propagation losses of the optical mode of <3 dB/cm. These promising results represent a size reduction of an order of magnitude with respect to the state of the art.
Fully suspended optomechanical waveguides were integrated into a Fabry-Pérot cavity, yielding self-sustained oscillation at frequencies of several MHz. By increasing the pump power, a frequency comb spanning several hundred MHz was achieved. The linewidth of the fundamental tone was ~20 kHz. Moreover, a new type of non-suspended optomechanical cavity based on SWGs was investigated. These waveguides effectively confined optical and mechanical modes without removing the BOX layer. This approach promises highly efficient Brillouin waveguides with a streamlined fabrication process, opening prospects for future integration with on-chip optoelectronic devices and for quantum processing.
The auxiliary building blocks developed in this project also represent a significant advance in performance compared to the state of the art. An ultra-broadband power splitter with low losses and broad operation bandwidth has been demonstrated. High-performance spatial and polarization splitters have also been demonstrated for the first time in a silicon photonic integrated circuit. These devices exhibit one of the lowest measured crosstalk (spatial splitter) and one of the highest extinction ratio (polarization splitter) within an ultra-broad bandwidth. Finally, a broadband mode exchanger that nearly doubles the operating bandwidth of its state-of-the-art counterparts was also demonstrated.
Fully suspended optomechanical waveguides were integrated into a Fabry-Pérot cavity, yielding self-sustained oscillation at frequencies of several MHz. By increasing the pump power, a frequency comb spanning several hundred MHz was achieved. The linewidth of the fundamental tone was ~20 kHz. Moreover, a new type of non-suspended optomechanical cavity based on SWGs was investigated. These waveguides effectively confined optical and mechanical modes without removing the BOX layer. This approach promises highly efficient Brillouin waveguides with a streamlined fabrication process, opening prospects for future integration with on-chip optoelectronic devices and for quantum processing.
The auxiliary building blocks developed in this project also represent a significant advance in performance compared to the state of the art. An ultra-broadband power splitter with low losses and broad operation bandwidth has been demonstrated. High-performance spatial and polarization splitters have also been demonstrated for the first time in a silicon photonic integrated circuit. These devices exhibit one of the lowest measured crosstalk (spatial splitter) and one of the highest extinction ratio (polarization splitter) within an ultra-broad bandwidth. Finally, a broadband mode exchanger that nearly doubles the operating bandwidth of its state-of-the-art counterparts was also demonstrated.