Optimal Design Of Au-PDMS Composite Photoacoustic Transmitter

Laser ultrasound has been initially used in photoacoustic imaging and industrial flaw detection because of its advantages of non-contact,high precision,and wide frequency band.However,due to the low intensity of the ultrasound generated by the the sample to be tested,its detection sensitivity is poor.Therefore,the composite material with high photoacoustic conversion efficiency came into being.The material is composed of metal or carbon-based material and polydimethylsiloxane(PDMS)material.When the laser is irradiated to the composite photoacoustic transmitter,it could generate high-intensity ultrasonic waves.And then the transmitter is coupled to the sample to be measured.Finally high-intensity ultrasonic signal can be obtained.However,at present,scholars mainly construct new composite photoacoustic conversion materials through the combination of different interlayer materials,and there is little exploration of the thickness and structure of the materials.Therefore,this article focuses on key parameters of the composite photoacoustic tranamitter,such as thickness and structure,to carry out in-depth research for digging into the photoacoustic conversion efficiency and potential of the composite material.Aiming at typical AuPDMS composite photoacoustic transmitter,the research was conducted from the aspects of finite element simulation design and experiment.The innovations and research results obtained are as follows:(1)A systematic study was carried out about the influence of the photoacoustic conversion efficiency on the metal layer and the PDMS layer in the Au-PDMS photoacoustic transmitter to obtain their optimal key parameters of layer thickness.Based on the traditional thermal ratio formula,the difference in temperature rise between the metal layer and the PDMS layer was considered.The Drude-Lorentz model is introduced to obtain the law of light absorption rate varying with thickness.Then,based on the thermoelastic effect,a nanosecond laser ultrasonic model of the composite photoacoustic transmitter was built.Taking glass / Au / pdms as an example,as the thickness of the Au layer increases,the ultrasonic intensity increases first and then decreases.When the thickness of Au is 60 nm and the thickness of PDMS is 800 nm, the optimal acoustic conversion efficiency-3.17%is obtained.The conversion efficiency is significantly higher than that of similar structures.(2)After optimizing the layer thickness,the structure of glass / Au / pdms composite photoacoustic transmitter was optimized to further improve its photoacoustic conversion efficiency.With the help of the built nanosecond laser ultrasonic model,the influence of the glass substrate on the photoacoustic performance was deeply analyzed,and it was found that the heat of the glass substrate was mostly attenuated because of the low thermal expansion,which negatively affected the photoacoustic process.the photoacoustic conversion effect of the Au / pdms structure was better than glass / Au / pdms.The optimization simulation result shows that: when the thickness of Au is 30 nm,the photoacoustic conversion efficiency can reach 5.4%,which is optimal Then based on surface plasmon,a glass / Au / pdms / grid structure that can realize 200 nm broadband absorption with light absorption rate higher than 60% was designed.The optimized simulation results show that when the thickness of Au is 40 nm,the conversion efficiency,up to 6%,is optimal.(3)Aiming at the problem that the ultrasonic in the high frequency band of the picosecond laser ultrasound is easily attenuated and the intensity is low,a picosecond laser ultrasonic model of the composite photoacoustic transmitter was built,based on the thermoelastic effect and the theory of the dual temperature model in non-Fourier heat conduction.The picosecond laser ultrasound model of the material is optimized for layer thickness analysis to obtain high frequency and high intensity ultrasound.Taking glass / Au / pdms as an example,when the thickness of Au is 35 nm and the thickness of PDMS is 700 nm,high-frequency and high-intensity ultrasound with the center frequency of 49.4 GHz and the sound pressure of 19.1 MPa is obtained.In summary,this paper explores the layer thickness optimization and structural design of Au-PDMS composite photoacoustic transmitter,which provides ideas and methods for the optimized design of metal photoacoustic transmitter and the further improvement of their photoacoustic conversion efficiency.