Nanooptics and photonics 
Interferometric characterization of PVDF based ultrasonic cochlear transducer 
Ilchenko S.G.1,2, Lymarenko R.A.1, Ostroukh O.P. 1, Critello D.C. 3, 
Pullano S.A.3, Fiorillo A.S.3 
1 International Center “Institute of Applied Optics” National Academy of Science of Ukraine, 10G Kudryavska street, Kyiv, 04053, Ukraine.
E-mail: Svitlana-ilchenko@ukr.net
2 Institute of Physics NAS of Ukraine, Prospect Nauky 46, 03028, Kyiv, Ukraine
3 Department of Health Sciences, University “Magna Græcia” of Catanzaro, 88100, Italy

Recently piezopolymer sensor based on Polyvinylidene Fluoride (PVDF), capable of covering the wide frequency range of a bat’s biosonar were developed [1]. It has a curved form of logarithmic spiral as is present in biological models of the cochlea. The main task accomplished by biosonar is that of echolocation, which is the ability of certain mammals to locate specific targets based on the acoustic information they receive from echoes [2]. It was shown that both in transmission and reception, a spiral–shaped transducer can transmit and receive ultrasonic modulated signals in the range between 20–80 kHz. 
In this work we investigate the vibrational modalities of a spiral-shaped PVDF transducer. It was placed instead of one of the mirrors in Michelson interferometer configuration. A sinusoidal high voltage signal of excitation source was applied to the sensor. The amplitude of signal varied in range from 50V to 100V and frequency – from 20 to 100 kHz. 
We showed that with increasing of excitation frequency the amplitude of sensor vibrations decreases. At frequencies of 20-30 kHz, the fringes can exceed the wavelength and overmodulation is observed. Ultrasonic modes of sensor are investigated by measuring the amplitude of surface oscillation at several points of spiral–shaped transducer.

1. Fiorillo, A. S., Pullano, S. A., Critello, C. D. Spiral—Shaped Biologically—Inspired Ultrasonic Sensor. // IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 67, 3, 635–642, (2019). doi:10.1109/tuffc.2019.2948817
2. G. Jones, E.C. Teeling, The evolution of echolocation in bats. // Trends in Ecology & Evolution, 21, 3, 149-156, 2006