| The photoacoustic effect was first discovered by Alexander Graham Bell in 1880.In recent years,it has become increasingly important in various fields,including photoacoustic communication,particle manipulation,and imaging.The fundamental principle of the photoacoustic effect is that a laser beam heats up the target material,causing it to vibrate and generate acoustic waves.One key technology for the research and application of the photoacoustic effect is the dynamic modulation of the photoacoustic field.In this thesis,several novel methods for generating different acoustic signals by dynamically modulating the photoacoustic field are proposed.These methods have the potential to advance the application of the photoacoustic effect in various areas,such as photoacoustic communication,shock wave generation,microparticle manipulation,and imaging.The theoretical and experimental validity of our research method is also demonstrated.The details are as follows.The current research and application of photoacoustic field manipulation are primarily constrained by several factors,including photoacoustic conversion efficiency,photoacoustic field interaction medium,photoacoustic energy absorption efficiency,and photoacoustic signal generation efficiency,among others.The photoacoustic effect refers to the absorption of energy from the manipulated optical field by a material,which is then converted into heat.This leads to periodic temperature changes within the material,causing transient thermal expansion and contraction,resulting in periodic pressure variations and generating acoustic wave signals.The frequency of the acoustic waves is modulated by the frequency of the laser used.There are two main types of lasers used for manipulation: continuous-wave lasers,which employ frequencydomain modulation methods,and pulsed lasers,which utilize time-domain modulation methods.By scanning continuous-wave laser beams at different rates and optical intensities in air,different acoustic wave signals can be generated based on the principles of photoacoustic effects.This enables spatial variations in the acoustic field,which finds applications in photoacoustic communication research.Similarly,when pulsed laser beams interact with water vapor in air,different phenomena such as acoustic waves or shock waves can be generated by adjusting the pulse frequency and energy.Building upon this foundation,photoacoustic fields can be further utilized to exert radiation force for manipulating micro-particles.Unlike optical or acoustic tweezers,this approach offers advantages for manipulating micro-and nano-robots within liquids and biological systems.Additionally,photoacoustic effects are a current focus in biomedical imaging,and improving photoacoustic efficiency is a key technology for enhancing the quality of photoacoustic imaging.Therefore,controlling the photoacoustic field is an important method for improving the quality of photoacoustic biomedical imaging.Based on the identified issues and proposed research methods,the key innovations of this doctoral thesis can be summarized as follows:1.A novel way of generating different acoustic signals by modulating the photoacoustic field through the frequency method is proposed.In this study,a periodically modulated Continuous Wave(CW)laser was used to generate acoustic signals by scanning the water vapor in air.Controlling the superposition effect of the generated acoustic signals in air is achieved by modulating the frequency and speed of the scanning mirrors.Two scanning mirrors were used to control two laser beams at different angles,creating complex superposition effects of multiple acoustic fields.Based on the principle of acoustic field superposition,the goal of enhancing the acoustic signals was ultimately achieved.The results have been published in the journal Physics of Fluids.2.A novel method based on the time domain method of generating different sound waves or shock waves by the interaction between ultrashort pulse laser and water vapor in air is proposed.The main method of this study is to modulate the frequency and pulse energy of a pulsed laser,which in turn controls the energy transfer and frequency of shock wave generation,achieving the production of different shock waves.The propagation speed and time of the shock wave were calculated and simulated,and then the emission frequency of the laser pulse was calculated to generate the desired shock waves.In this study,we found that the superposition effect of shock waves is related to the frequency of the laser pulse.When the pulse frequency is low,multiple shock wave superposition cannot be generated.However,when the frequency reaches a certain threshold,superposition effects can occur,thereby achieving the phenomenon of superimposed shock waves.The achievement has been authorized by the invention patent,an oral report was made at the LTO 2022 meeting.3.A novel method of controlling a pulsed laser to generate photoacoustic tweezers to control particles in liquid and air is proposed,respectively.Firstly,this study investigated the generation of a photoacoustic wave tweezer in liquids.Based on existing optical and acoustic tweezing methods,a vortex optical field phase was generated by coding modulation of a spatial light modulator(SLM)using MATLAB software.The vortex laser beam was then focused through an objective lens and acted on the opaque liquid,resulting in an acoustic radiation force similar to a circular laser-acoustic field that could drive the movement of particles in the liquid.Compared to direct manipulation by optical and acoustic tweezers,the photoacoustic wave tweezer method can not only reduce damage to the target medium caused by direct laser irradiation but also achieve more accurate manipulation of the target substance.Additionally,this study investigated the generation of a photoacoustic wave tweezer in air.The method involved controlling pulsed lasers to pass through micro-lenses,focusing them on a microfluidic channel to produce a superimposed optical-acoustic shock wave,and controlling the resulting wave to generate a traveling photoacoustic wave tweezer in air based on the principle of sound wave superposition.The existence of the light-sound wave tweezer in air was validated by manipulating particles in the microfluidic channel.This achievement currently has two patents in the process of substantive examination,and one paper has been published in the journal Optik.4.A novel research method for improving the imaging quality of biological tissue by modulating the photoacoustic field is proposed.Currently,improving the photoacoustic conversion efficiency is one of the main methods to enhance the reconstruction quality of photoacoustic imaging.This research method uses wavefront modulation of the incident light field to generate more efficient acoustic signals.Based on the traditional hemispherical array structure,a specific diffuser device is designed to modulate the intensity distribution of the light field,that is,modulating the wavefront of the incident Gaussian beam to increase the illumination area of the beam,thereby enhancing the conversion efficiency of the global photoacoustic effect and ultimately improving the quality of photoacoustic imaging.The results have been published in the journal Optics Express and in the 2022 Digital Holography and 3D imaging conference as an oral report and published in the conference proceedings.The entire research content of this thesis adopts a research method that combines theoretical research,simulation,and optical experiments.Based on the methods of manipulating the photoacoustic field to produce different sound waves in the laboratory,this research provides valuable utilization for social applications of photoacoustic effects in photoacoustic communication,particle manipulation,and imaging in the future. |
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