Stimulated Brillouin scattering in integrated nano-scale waveguides and its applications using Aluminium Nitride

Abstract

In this work, the author theoretically and numerically studies the stimulated Brillouin scattering (SBS) in integrated nano-photonic waveguides. The author first reviews the progress in the field of SBS, including the discoveries of SBS, current progresses in the field of SBS and its applications. Then the author reviews and compares two mechanisms for the SBS in nano-scale waveguides. One of them describes the SBS in waveguides via optical forces induced by electrostriction and optical radiation on the waveguide boundary. The other one is understood from the acoustic wave’s modulation to the optical mode. The modulation is realized via photoelasticity and moving boundary. Both methods are validated by matching the simulation results from author’s models with reported results. In addition, the author analyses and compares the differences and links between these two mechanisms mathematically. To analyse the strength of the SBS (indicated by a parameter called the Brillouin gain), the quality factor (Q factor) of the mechanical mode is of great importance. This is because in order to generate sufficient SBS, it would require the simultaneous confinement of both optical mode and acoustic mode. While optical mode, in most designs, are well confined. The acoustic mode, on the other hand is not well confined and leaky. Q factor, reflecting how much energy is lost, is a crucial parameter in calculating the final SBS gain coefficient. The mechanical Q factor is decided by various resources. Among all the loss factors, anchor loss is believed to be the main loss in high frequency resonators. One method to obtain accurate Q factor is via perfectly matched layer (PML). However. one has to choose proper parameters of the PML to optimize its performance. Based on the mathematical analysis of PML and the well-researched beam resonator structure, the author proposes a novel method for setting up a well-behaved PML. The results show that the choice of a parameter defined as PML scale factor α via COMSOL is the key to obtain the accurate Q factor. This method is validated by matching the simulation results of substrate thickness’s impact on Q factor with the theoretical prediction for a beam resonator. Subsequently, the author proposes a III-V novel material Aluminium Nitride (AlN) as a candidate for the SBS application in visible wavelength range for its wide transparent window and unique piezoelectricity. A full-vectorial model for calculating SBS gain is first validated. Then the author studies the forward and backward SBS process in a partially suspended AlN waveguide for TE (transverse electric) mode and TM (transverse magnetic) mode, separately. The author obtains the value of Brillouin gain of 1311 W⁻¹m⁻¹ when Q factor is dominated by anchor loss. Apart from extending the SBS application into shorter wavelength range, another issue hindering the development of SBS is that most platforms requires partially or fully suspension of the core waveguide to stop phonon leakage into the substrate. The author proposes a silicon-AlN-sapphire platform for realizing large SBS gain without suspending the waveguide. The genetic algorithm (GA) is leveraged to optimize the waveguide structure. A simple structure is obtained with SBS gain of 2462 W⁻¹m⁻¹, which is 8 times larger than the recently reported result in unsuspended silicon waveguide. This platform can enable Brillouin-related phenomena in centimetre-scale waveguide and pave the way toward large-area unreleased opto-mechanics on silicon. Finally, the author presents fabrication results for realizing aforementioned platforms. A highly c-axis oriented AlN thin film is sputtering deposited on the top of silica via sputtering machine. The primary etching test proves that normal KOH- or TMAH based developer will react with AlN. These information will be helpful for the future work of realizing on-chip SBS laser. Conclusions and future perspectives for the development of on-chip SBS applications are presented in the final chapter.

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Identifiers

ORCID for Peng Li: orcid.org/0000-0003-1828-9597

ORCID for Jize Yan: orcid.org/0000-0002-2886-2847

ORCID for Stephen Beeby: orcid.org/0000-0002-0800-1759

ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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