Photon-phonon Interaction In The Microfiber Induced By Optical Forces

Photon-phonon coupling in nanoscale waveguides through guided-wave stimulated Brillouin scattering(SBS)has recently emerged as an important area of research.Stimulated Brillouin scattering via electrostrictive force is a fundamental interaction between light and sound which limits the power in conventional optical fibers.The emergence of optical microfibers with subwavelength diameter,ultralight mass and an intense light field,provides a new platform for photon-phonon coupling,resulting in the radiation pressure mediated contribution of SBS.This study examines the optomechanical system in cylindrical coordinates,reveals the theoretically radiation pressure and electrostrictive force.Then this study analyzes photon-phonon coupling via stimulated Brillouin scattering.The specific work is as following:1.We analyse the radiation pressure and electrostrictive force,which could drive phonon creation and cause SBS in microfibers,showing that the radiation pressure and electrostrictive force are comparable.These two forces are in opposite directions:the electrostrictive force tends to compress the microfiber while radiation pressure tends to extend the microfiber.The electrostrictive force is always larger than radiation pressure in a solid microfiber.Then,we change the structure of microfiber and analyse the hollow microfiber.In the hollow microfiber,the electrostrictive force can completely counteract radiation pressure when the radius is 0.7 ?m.The radiation pressure is larger than the electrostrictive force at a small size,which means the effect of the electrostrictive force can be counteracted by adjusting the size and structure of the microfiber.2.As is well-known,the phonon is driven by electrostriction in the process of SBS in microfibers.We can counteract the electrostrictive force with radiation pressure.So,SBS can be weakened or even completely inhibited in microfibers.In a solid microfiber,because the electrostrictive force and radiation pressure are in contrary directions,the two contributions weaken the photon-phonon coupling strength.In a hollow microfiber,The total photon-phonon overlap is about one fifth of that contributions by electrostriction.The photon-phonon coupling strength becomes zero because radiation pressure counteracts electrostrictive force while the thickness of silica ring is around 0.75 ?m.This means we completely cancel the Brillouin gain in the hollow microfiber!3.We analyze two fundamental acoustic modes in the microfiber.The theoretical calculation and simulation are done to analyze the two fundamental acoustic modes.Then we calculate the Brillouin gain spectrum using the two fundamental acoustic modes.We find the Brillouin frequency shift changes nonlinearly with the radius of the microfiber increases.In summary,this work can enhance or weaken optical forces in microfibers by changing the parameters and the structure of microfibers.Furthermore,photon-phonon coupling can be enhanced or decreased by elasto-optical effect,which is caused by the radiation pressure and electrostrictive force.These results thus show the potential of optical microfiber for low-threshold lasers and amplifiers.