Coherent Control Of Phonons In Optical Micro Systems

Phonons and photons are two typical bosons in physics.Photons are the basic components of the light waves whereas phonon is the quantum mechanical description of the lattice vibrational motion.A phonon is a quasi-particle which owns energy ?ω and momentum ?q.Phonons play a very important role in semiconductors,condensed matter physics and optics and so on,affecting physical processes such as thermal conduction,superconductivity and stimulated scattering.How to control the phases and amplitudes of the phonons are involved to key problems in the areas of photochemistry,ultrafast optics and thermal physics.Previously,a dual pump-probe method is used to control the phase difference between the phonons and then realize the constructive enhancement or destructive cancellation of phonons.However,under the circumstance of continuous light in micro/nano optical structures and optical integrated structures,this method is inappropriate.To solve this problem,we focus on the coherent control of phonons based on phase adjustment by means of three typical optical structures: acousto-optic modulator(AOM),polarization maintaining fiber(PMF)and high-quality optical microcavity.Firstly,we realized a phase-controlled two-wave mixing process in an AOM,which shows that the phase can indeed play an important role in the acousto-optic interaction.Next,we implemented the coherent control of the stimulated Brillouin phonon by adjusting the phases of the light waves in the PMF.Finally,in the high Q microcavity,we successfully realized the manipulation of the stimulated Brillouin phonon’s central frequency and intensity in a passive mode tuning method.In the traditional two-wave mixing process using the photorefractive effect,phase match condition is automatically satisfied.The phase difference between the interference pattern formed by the two waves and the resulting refractive index grating is π/2,making the energy exchange during the two-wave mixing possible.In this paper,the generation of the refractive index grating and the two-wave mixing process are processed separately.with the help of the refractive index dynamic grating caused by the acoustic wave in an external AOM,by adjusting the phase difference between the interference pattern of the two light waves and the dynamic grating,a phase controlled two-wave mixing process can be realized.This experiment directly demonstrates the important value of phase control in nonlinear optical processes.In optical fiber communications,the suppression of stimulated Brillouin scattering(SBS)is a core issue.In this paper,we implement the coherent control of stimulated Brillouin phonons in a polarization-maintaining fiber.The phonons can be constructively enhanced or destructively annihilated by adjusting the phases of the light waves propagating in the fast/slow axis of the PMF.As a result,the stimulated Brillouin scattering process can be effectively enhanced or suppressed by coherent control of Brillouin phonons.In this way,a quasi SBS-free transmission of pump beam can be achieved.When the Brillouin phonons are enhanced constructively,the pump light will be reflected back more strongly.In normal single mode fibers,the modulation of phonons in the optical fiber is closely related to the polarization state of the probe light,in addition to the phase difference between the two pump lights.In this paper,we have implemented a phase-sensitive stimulated Brillouin process to regulate pump light’s transmittance.This can be used to create phase or polarization sensitive SBS devices that control the transmission.In the optical whispering gallery mode(WGM)microcavity,The generation of stimulated Brillouin phonons in the cavity requires that pumping light,and the Stokes light and the acoustic wave satisfy the phase matching condition while in the corresponding WGMs.To meet these conditions,we propose a passive fine-tuning WGM method based on the thermo-optic effect,which enables the stimulated Brillouin phonons to be effectively generated.Using this method,we can not only control the center frequency of Brillouin phonons but also effectively control the intensity of the phonons.For instance,in the forward SBS process,the intensity of excited phonons can be enhanced by 43 dB.In summary,using acousto-optic modulators,polarization-maintaining fiber and high-quality factor microcavities,we have conducted theoretical and experimental research on the subject of coherent control of phonons.We believe that the method of coherently controlling phonons based on phase adjustment will have important applications and values in the fields of optical integrated structure,microcavity opto-mechanics,microcavity Brillouin laser and so on in the near future.