Abstract
In our previous work, we have demonstrated that dielectric elastic grating can support Fabry–Perot modes and provide embedded optical interferometry to measure ultrasonic pressure. The Fabry–Perot modes inside the grating provide an enhancement in sensitivity and figure of merit compared to thin film-based Fabry–Perot structures. Here, in this paper, we propose a theoretical framework to explain that the elastic grating also supports dielectric waveguide grating mode, in which optical grating parameters control the excitation of the two modes. The optical properties of the two modes, including coupling conditions and loss mechanisms, are discussed. The proposed grating has the grating period in micron scale, which is shorter than the wavelength of the incident ultrasound leading to an ultrasonic scattering. The gap regions in the grating allow the elastic grating thickness to be compressed by the incident ultrasound and coupled to a surface acoustic wave mode. The thickness compression can be measured using an embedded interferometer through one of the optical guided modes. The dielectric waveguide grating is a narrow bandpass optical filter enabling an ultrasensitive mode to sense changes in optical displacement. This enhancement in mechanical and optical properties gives rise to a broader detectable pressure range and figure of merit in ultrasonic detection; the detectable pressure range and figure of merit can be enhanced by 2.7 times and 23 times, respectively, compared to conventional Fabry–Perot structures.
Highlights
Photoacoustic imaging (PI) has been of interest to the science and engineering community because of its complementary capability to optical imaging in measuring mechanical properties of samples, such as Young’s modulus and stiffness [1], rather than the optical properties, such as refractive index, reflectance, and transmittance
The incident ultrasound can significantly compress some gratings, and this is due to the surface acoustic wave (SAW) mode coupling [23], which depends on the grating parameters and the ultrasonic incident frequency
The relationship of grating parameters that can couple the incident ultrasound into the SAW mode is given in Equation (2)
Summary
Photoacoustic imaging (PI) has been of interest to the science and engineering community because of its complementary capability to optical imaging in measuring mechanical properties of samples, such as Young’s modulus and stiffness [1], rather than the optical properties, such as refractive index, reflectance, and transmittance. PI has proven to be applicable in a wide range of applications in several fields, including material science [2,3], biological science [4], and medical science [5]. PI’s current challenges to obtain high-resolution images are: (1) the generation of highfrequency ultrasound; the resolution depends on the bandwidth (∆f ) of the ultrasound. For the conventional 50 MHz sources, the ultrasonic image lateral resolution is around. 80 μm to 160 μm, and 20 μm to 100 μm in axial resolution [6,7]. To increase the resolution, there is a demand for the bandwidth of the ultrasound. Thermal expansion due to femtosecond laser illumination of materials [8] and a thin piezoelectric layer [9]
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