Studies On Cavity Optomechanical Systems With Ultralow Masses Based On Photonic Crystal Cavities

As the development of semiconductor processing technology,MEMS devices are shrinking in size into the nano-scale and the research and applications of NEMS devices have been started.Nanomechanical resonators,the core parts of NEMS devices,have ultralow masses.Therefore,they have extremely high detection sensitivity and are widely used in the fields of sensing and ultra-sensitive measurements.Cavity optomechanical coupling,the interaction of optical and mechanical degrees of freedom,has led to a variety of applications including gravitational wave detection,quantum ground-state cooling,highly sensitive detection and quantum information processing.Through integrating nanomechanical resonators into cavity optomechanical systems and detecting the mechanical movements by cavity optomechanical coupling,cavity optomechanical systems with ultralow masses can be formed.And as mass sensors,they have great significance of research and broad application prospects in the fields of biological and chemical detection and analysis.Silicon-based on-chip integrated cavity optomechanical systems with ultralow masses based on photonic crystal microcavities have the advantages of small size,on-chip integration,large-scale and low-cost production,making them one of the most promising device types.In this thesis,several novel silicon-based on-chip integrated cavity optomechanical systems with ultralow masses based on photonic crystal microcavities are proposed and experimentally demonstrated.The research accomplishments and innovations are summarized as follows:(1)Fabrication process of passive silicon-based optical devices is investigated.And electron beam lithography with the method of different precision stitching is developed,which greatly improves the exposure efficiency for the photonic crystal devices.Fabrication process of slots with high aspect ratio is developed and 60-nm-width slots are fabricated in the 220-nm-thickness silicon device layer of SOI substrates.These processes lay the foundation of the fabrication of the devices studied in this thesis.(2)A silicon-based on-chip integrated cavity optomechanical system based on a double-slot photonic crystal microcavity is designed,which has an ultralow effective mass of6.91 femtograms.Through designing the bandstructures of the photonic and phononic crystals,the optical resonant mode and the mechanical vibrant mode are simultaneously confined in the center of the device by the optical and acoustic bandgaps respectively.A value for the vacuum optomechanical coupling rate of 654.53 kHz is achieved.(3)Through embedding a nanomechanical resonator into a photonic crystal nanobeam cavity,a silicon-based on-chip integrated cavity optomechanical system is proposed and demonstrated.It has the lowest effective mass of 6.42 femtograms among the cavity optomechanical systems which consist of bulk materials and make from the semiconductor processing technology.With the method of cavity optomechanical coupling,the device achieves displacement detection sensitivity of 0.13 fm/(?) and a Q-factor of 1255 for the mechanical vibration at 3.928 GHz of the nanomechanical resonator in air and at room temperature.The device can be applied in the field of mass sensing.(4)A novel silicon-based air-mode photonic crystal microcavity is proposed and demonstrated.53.6%of the energy in the optical resonant mode is confined in the holes of the photonic crystals.Therefore,the light-matter interaction in the holes can be enhanced.An optical Q-factor of 3.70×10~5 is experimentally obtained.A nano-patterning process of suspended graphene is developed.With this process,a double clamped nanomechanical resonator made of single-layer graphene is fabricated in the center hole of the air-mode photonic crystal microcavity.And it can be efficiently coupled with the optical resonant mode.A silicon-based on-chip integrated cavity optomechanical system with an effective mass of 6.17 attograms is achieved.(5)A silicon-based on-chip integrated cavity optomechanical system,which works in water,is proposed and demonstrated.With the method of cavity optomechanical coupling,the device achieves displacement detection sensitivity of 9.33 am/(?) and a Q-factor of6.6 for the mechanical vibration at 5.251 GHz of the nanomechanical resonator in water.The device has an effective mass of 139 femtograms,and can achieve a mass sensitivity of 1.33ag/(?) with thermal excitation.By simulating the liquid-solid interaction between the mechanical vibration and the liquid,it is demonstrated that the mechanical energy losses mainly result from the inertial force of the liquid.As a mass sensor,the device can be applied in the field of mass sensing where liquid environment is needed.(6)A silicon-based on-chip integrated phononic circit device is proposed and demonstrated.In the cavity optomechanical systems based on optomechanical crystal nanobeam cavities,phonon at 5.5 GHz is produced by the self-sustained mechanical oscillation,which is driven by the radiation pressure of photon.And the phonon can be detected by the cavity optomechanical coupling.The acoustic waveguide consisting of phononic crystals guides the transmitting phonon between the two cavity optomechanical systems.Therefore a phononic circit device is formed.The generation,transmission and detection of the phonon in the phononic circit device are experimentally demonstrated.It provides research foundation for the information processing with phonon,and has application potential in the fields of RF and sensing.