Abstract
In this study, we report the first experimental realization of an ultrathin (0.14λ, λ = 1.482 mm means wavelength at 1 MHz in the water medium) subwavelength focusing acoustic lens that can surpass the Rayleigh diffraction limit (0.61λ/NA, NA means numerical aperture). It is termed a Super-Oscillatory Acoustic Lens (SOAL), and it operates in the megasonic range. The SOAL represents an interesting feature allowing the achievement of subwavelength focusing without the need to operate in close proximity to the object to be imaged. The optimal layout of the SOAL is obtained by utilizing a systematic design approach, referred to here as topology optimization. To this end, the optimization formulation is newly defined. The optimized SOAL is fabricated using a photo-etching process and its subwavelength focusing performance is verified experimentally via an acoustic intensity measurement system. From these measurements, we found that the proposed optimized SOAL can achieve superior focusing features with a Full Width at Half Maximum (FWHM) of ~0.40λ/NA ≃ 0.84 mm (for our SOAL, NA = 0.707) with the transmission efficiency of 26.5%.
Highlights
The achievement of the subwavelength focusing feature, which focuses acoustic power into a very small region, is a most important task in the medical ultrasound field, from diagnostics[1] to therapeutics[2]
As shown in this simple 1D demonstration, subwavelength focusing based on the super-oscillation phenomenon can be achieved by controlling both the amplitude (Fourier coefficient) and the phase (2πnr) of the wave diffracted from the micro-slit of the lens
To design the optimal layout of the Super-Oscillatory Acoustic Lens (SOAL) systematically, we applied an inverse design methodology known as topology optimization
Summary
The achievement of the subwavelength focusing feature, which focuses acoustic power into a very small region, is a most important task in the medical ultrasound field, from diagnostics[1] to therapeutics[2]. If one can realize an acoustic lens with a subwavelength focusing feature without using an evanescent wave, it will be a very powerful solution for clinical applications in practice Based on this need, a conventional refractive-type concave acoustic lens which is attached to the front of the source transducer is commonly used[13]. That study achieved the subwavelength focusing feature at a position 10λ away from the optical lens, with the configuration designed using a Particle Swarm Optimization (PSO) algorithm This was the first report of the concept of an SOL being applied to optical imaging as a practical solution for subwavelength focusing and imaging. To the best of our knowledge, this is the first study in which an optimized SOAL is experimentally realized and the subwavelength focusing feature is achieved in the megasonic range
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