The Study Of Photoacoustic Microscopy Based On Grapheme Surface Waves

At present,the optical resolution photoacoustic(PA)microscopy(OR-PAM)has achieved micron or even sub-micron lateral resolution.However,the PA waves is usually detected by piezoelectric transducer,whose limited bandwidth will lead to a much lower axial resolution than lateral resolution,the anisotropy of spatical resolution causes serious anamorphose of 3D image.In addition,the low sensitivity of transducers will reduce the quality of PA image.Therefore,it is crutial to develop a new method to realize high sensitivity and broadband detection for PA imaging.In PA imaging,the signal is produced by excitation laser with a character of low intensity as well as wide bandwidth.Based on graphene surface waves,the paper puts forward a new method for PA detection to accurately detect weak pressure wave.The surface wave of graphene has high sensitivity to the weak change of the refractive index of solution,which can improve the sensitivity of the PA detection.The characteristics of ultrafast response time and strong local optical field of surface wave can increase the PA detection bandwidth.Based on the principle,we have constructed a attenuated total reflectance(ATR)structure detector consisting of a prism/graphene/water solution for detecting transient PA waves.Firstly,we have studied the interaction between graphene surface waves and external pressure using the finite difference time domain(FDTD)numerical simulation method.The theoretical results show that the graphene surface wave electric field is very sensitive to the change of the solution refractive index induced by the sound pressure.The sensitivity is closely related to the polarization state and incident angle of incident light and the thickness of graphene.As a guide,we design a ATR structure of PA detected device,in which the incident angle is close to the critical angle(66.251 °),the polarization ratio of p:s ~ 1:4 and the thickness of the graphene is ~3.4 nm.At the same time,we adopt polarized optical difference detection method to eliminate the disturbance caused by unstable light source and environment vibration,so as to improve the sensitivity of optical ultrasound detection.Using a transducer as ultrasonic source,we determines the noise-equivalent-pressure(NEP)is ~550 Pa over an approximately linear pressure response from 11.0kPa to 55.0kPa,which is conform with theoretical simulation.Further,we rely on graphene surface waves to detect PA waves.With the help of λ=633nm He-Ne light source and nickel-based graphene supported by transparent tape,we achieved ~150MHz bandwidth,which is much broader than the bandwidth of traditional transducers.The broadband PA detected capability is mainly due to the superficial surface wave penetration depth of graphene.Through the spectrum analysis and the numerical shift-and-sum simulation method,we conclude that the vertical resolution of the graphene PA detector(GPAD)can reach ~8.3μm,which is close to the lateral resolution of majority of traditional OR-PAM.With the graphene-based PA detector(GPAD)as key component,we set up a PA imaging system.The 1-μm-diameter carbon filaments were used as the imaging targets.Matched with the theoretical resolution of 0.1 NA objective,the lateral resolution of the system is ~3.06 μm.From the strong optical absorption of inherent melanin,a high contrast 3D image of two human hairs was obtained to test the PA imaging capability.In addition,we carried out PA imaging experiments in vivo and obtained three-dimensional,microscale and unmarked images of the microvasculature of a mouse ear,including main blood vessels and abundant capillaries.