Research On Broad Bandwidth Photoacoustic Sensing Based On Surface Plasmon Resonance

Photoacoustic sensing technology combines optical excitation and acoustic detection.After the short-pulse laser light is absorbed by pigment substance,the photoacoustic wave generated by the transient thermo-elastic effect enables specific observation of the optical absorption characteristics of the tissue.Traditional photoacoustic sensing usually uses piezoelectric transducers to detect photoacoustic signals.Due to the physical properties of the piezoelectric materials,the bandwidth of such transducers is limited at the order of tens of megahertz.There are many optical absorbers with different sizes and shapes in biological samples,such as cell nuclei with the size of sub-micrometers,capillaries with the diameter in micron level,and major blood vessels with 100-microns in diameter.The frequency components of the photoacoustic waves from these micro objects range from nearly DC to hundreds of megahertz.Obviously,traditional ultrasonic transducers cannot measure these broadband photoacoustic signals,making it difficult for traditional photoacoustic sensing technology to obtain physiological and pathological information that is closely correlated to the characteristics of the acoustic spectra.In order to solve these limitations,this work proposes an acoustic spectrum sensing technology based on surface plasmon resonance(SPR),demonstrating broad bandwidth photoacoustic detection.Relying on the characteristics of high locality of SPR field,short propagation distance,and fast response,a high numerical aperture TIRF objective is used to guide the laser to excite the local surface plasmons.This work achieves ultrasonic response bandwidth of up to 220 MHz.The work mainly includes the following parts.1)the principle of SPR response to acoustic signals is explained theoretically.We use Matlab and finite-difference time-domain modeling to simulate the effects of the polarization of the incident light,the angle of the incident beam,and the material and thickness of the metal film on the SPR field.Theoretical results show that only p-polarized incident light can excite SPR.When the incident light angle is 71.1 ° and the Au film thickness is 45 nm,SPR represents the most sensitive to the changes in refractive index.We use TIRF objective lens(with a numerical aperture of 1.49)to deliver the incident laser for the SPR excitation,which not only makes the incident light at an approximately parallel beam(71.1° incident angle),but also reduces the size of the SPR detection point.As a result,SPR sensing can improve the ultrasonic frequency response.2)we develop broadband SPR sensor.Guided by the above theory,we have established a combined optical system of the lens and the TIRF objective lens.By precisely adjusting the relative distance between the them and the lateral position of the focal point of the lens on the back focal plane of the TIRF objective,the incident laser beam realizes the optimal incident angle for the SPR excitation.Moreover,this operation effectively reduces the size of the SPR detection point.In addition,we introduce a polarization-differential optical detection method to record the difference in the light intensity of the p-polarized and s-polarized reflected beams,leading to the improvement in the pressure detection sensitivity by reducing the common mode noise.The system achieves the detection bandwidth of up to 219.8 MHz,and its noiseequivalent-pressure detection sensitivity is estimated at 3.5 k Pa.3)we perform the photoacoustic spectrum analysis.The photoacoustic characteristics of the sample are closely related to multiple factors,such as size,shape,and orientation.Relying on broadband SPR-based photoacoustic sensing system,we test a variety of phantoms with different thicknesses.The results show that the photoacoustic spectrum features(such as center frequency and bandwidth)can accurately reflect the thickness of the samples.In addition,photoacoustic image of a black tape phantom is obtained label-freely.This implies that the acoustic spectrum analysis capability of wideband SPR-based photoacoustic sensing technology is potentially valuable for many biomedical investigations.