Investigations Of The Megahertz Ultrasonic Source And Terahertz Wave Detector Based On The Photo-Thermal-Acoustic Effect Of Graphene Foam

Graphene foam is a three-dimensional chemical cross-linked graphene-based macroscopic bulk material.The graphene foam not only retains the properties of graphene（e.g.,broad electromagnetic absorption spectrum,high optical absorptance,low heat capacity per unit area and high thermal conductivity）,but also has centimeter scale in three dimensions and can achieve self-standing,which makes it to be a highly efficient photo-thermal-acoustic conversion material.In recent years,photo-thermal-acoustic effect has attracted extensive attentions due to its important applications in detection,imaging,medical and other fields.For example,photo-thermal-acoustic effect has been applied to the detection of internal defects in materials,the detection of trace impurities in waste water,the imaging of dynamic electric field distribution in integrated circuits,high-resolution imaging of biological tissues,and photo-acoustic therapy.As a highly efficient photo-thermal-acoustic conversion material with a wide absorption bandwidth from THz to visible wavelength,graphene foam is expected to be an important candidate material for the next-generation megahertz（MHz）ultrasonic sources and novel terahertz（THz）wave detectors.Therefore,this thesis focuses on the photo-thermal-acoustic effect of graphene foam,and carries out research on the ultrasonic source of MHz and THz wave detector.The megahertz broadband ultrasonic source has important applications in medical therapy,biomedical imaging,and non-destructive testing due to its higher heating efficiency and shorter pulse duration.In this thesis,a megahertz broadband ultrasound source based on the photo-thermo-acoustic effect of graphene foam is proposed,which has the advantages of wide acoustic spectrum,no vibration components,anti-electromagnetic interference,high directivity,high robustness and remote control.The development of terahertz wave technology and applications largely depends on the performance of terahertz wave detectors,and most of the existing terahertz detectors cannot achieve room temperature,broadband and fast detection simultaneously.Also based on the photo-thermo-acoustic effect of graphene foam,this thesis proposes a method to measure terahertz waves using a microphone.The novel THz wave detector based on the photo-thermo-acoustic effect of graphene foam is a room temperature,broadband（THz-visible）and fast speed（>10 k Hz）THz detector with～90%THz energy utilization rate and without need of fabricating micro-electrode/antenna.The main contents and results of this thesis are as follows:（1）Combining the low heat capacity per unit volume of graphene foam with the ultrafast energy deposition of femtosecond laser,after irradiating the graphene foam with femtosecond laser with a central wavelength of 800 nm,broadband megahertz acoustic pulses with frequency band ranging from 50 k Hz to 1.8 MHz are excited in air.Combined with the photo-thermo-acoustic effect and the attenuation law of ultrasonic waves in air,the physical mechanisms limiting the frequency bandwidth of ultrasonic pulses are revealed.（2）The experimental results reveal that the megahertz ultrasonic source based on the graphene foam has a dipole-like emission directivity,the sound pressure of the ultrasonic pulse only depends on the incident laser energy,and is independent on the laser polarization,laser incident angle,laser wavelength（400 nm and 800 nm）,the temporal intensity distribution of the laser pulse,the laser pulse dispersion and other factors.Therefore,the graphene foam ultrasonic source has the advantages of good robustness,directionality and easy manipulation.The high directivity,laser energy controllability,and high robustness of the graphene foam megahertz ultrasonic source makes it have promising applications in directional information transfer and imaging.（3）In this thesis,a novel THz wave detection scheme is proposed in which the terahertz wave is detected by an audible microphone based on the photo-thermo-acoustic conversion in graphene foam.The experimental results show that at room temperature the graphene foam THz detector can achieve a detection speed of more than 10 k Hz,a responsivity of 1.52 V/W and a noise equivalent power of 345n W/Hz^0.5 when measuring the THz wave with frequency of 0.1 THz.According to the experimental results about the graphene foam ultrasonic source——graphene foam can still achieve photo-thermo-acoustic conversion under the visible/near infrared light（hundreds of THz）irradiation,it can be inferred that the detection bandwidth of graphene foam photo-thermo-acoustic THz wave detector can cover the range from0.1 THz to hundreds of THz.（4）The responsivity and the center acoustic frequency of the graphene foam photo-thermo-acoustic THz detector are optimized and the optimized detector is used to detect the broadband terahertz pulse generated by the dual-color femtosecond laser filament.By optimizing graphene foam thickness and annealing temperature,using hollow tubes and intrinsic silicon capping layer to enhance acoustic wave collection efficiency,and using programmable filtering,the responsivity of detector is improved from 1.52 V/W to 148 V/W.In addition,the measurement results of terahertz pulses irradiated by dual-color femtosecond laser filament using the graphene foam THz detector show that the spectrum of terahertz pulses generated by femtosecond laser filament covers 0.1-20 THz,which further verifies the broadband detection capability of the graphene foam terahertz detector.